path: root/doc/rfc/rfc6940.txt

Internet Engineering Task Force (IETF)                       C. Jennings
Request for Comments: 6940                                         Cisco
Category: Standards Track                               B. Lowekamp, Ed.
ISSN: 2070-1721                                                    Skype
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                                S. Baset
                                                          H. Schulzrinne
                                                     Columbia University
                                                            January 2014


         REsource LOcation And Discovery (RELOAD) Base Protocol

Abstract

   This specification defines REsource LOcation And Discovery (RELOAD),
   a peer-to-peer (P2P) signaling protocol for use on the Internet.  A
   P2P signaling protocol provides its clients with an abstract storage
   and messaging service between a set of cooperating peers that form
   the overlay network.  RELOAD is designed to support a P2P Session
   Initiation Protocol (P2PSIP) network, but can be utilized by other
   applications with similar requirements by defining new usages that
   specify the Kinds of data that need to be stored for a particular
   application.  RELOAD defines a security model based on a certificate
   enrollment service that provides unique identities.  NAT traversal is
   a fundamental service of the protocol.  RELOAD also allows access
   from "client" nodes that do not need to route traffic or store data
   for others.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6940.








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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.1.  Basic Setting . . . . . . . . . . . . . . . . . . . . . .   8
     1.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .  10
       1.2.1.  Usage Layer . . . . . . . . . . . . . . . . . . . . .  13
       1.2.2.  Message Transport . . . . . . . . . . . . . . . . . .  13
       1.2.3.  Storage . . . . . . . . . . . . . . . . . . . . . . .  14
       1.2.4.  Topology Plug-in  . . . . . . . . . . . . . . . . . .  15
       1.2.5.  Forwarding and Link Management Layer  . . . . . . . .  16
     1.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  16
     1.4.  Structure of This Document  . . . . . . . . . . . . . . .  17
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .  18
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  18
   4.  Overlay Management Overview . . . . . . . . . . . . . . . . .  21
     4.1.  Security and Identification . . . . . . . . . . . . . . .  21
       4.1.1.  Shared-Key Security . . . . . . . . . . . . . . . . .  23
     4.2.  Clients . . . . . . . . . . . . . . . . . . . . . . . . .  23
       4.2.1.  Client Routing  . . . . . . . . . . . . . . . . . . .  24
       4.2.2.  Minimum Functionality Requirements for Clients  . . .  25
     4.3.  Routing . . . . . . . . . . . . . . . . . . . . . . . . .  25



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     4.4.  Connectivity Management . . . . . . . . . . . . . . . . .  29
     4.5.  Overlay Algorithm Support . . . . . . . . . . . . . . . .  30
       4.5.1.  Support for Pluggable Overlay Algorithms  . . . . . .  30
       4.5.2.  Joining, Leaving, and Maintenance Overview  . . . . .  30
     4.6.  First-Time Setup  . . . . . . . . . . . . . . . . . . . .  32
       4.6.1.  Initial Configuration . . . . . . . . . . . . . . . .  32
       4.6.2.  Enrollment  . . . . . . . . . . . . . . . . . . . . .  32
       4.6.3.  Diagnostics . . . . . . . . . . . . . . . . . . . . .  33
   5.  Application Support Overview  . . . . . . . . . . . . . . . .  33
     5.1.  Data Storage  . . . . . . . . . . . . . . . . . . . . . .  33
       5.1.1.  Storage Permissions . . . . . . . . . . . . . . . . .  34
       5.1.2.  Replication . . . . . . . . . . . . . . . . . . . . .  35
     5.2.  Usages  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     5.3.  Service Discovery . . . . . . . . . . . . . . . . . . . .  36
     5.4.  Application Connectivity  . . . . . . . . . . . . . . . .  36
   6.  Overlay Management Protocol . . . . . . . . . . . . . . . . .  37
     6.1.  Message Receipt and Forwarding  . . . . . . . . . . . . .  37
       6.1.1.  Responsible ID  . . . . . . . . . . . . . . . . . . .  38
       6.1.2.  Other ID  . . . . . . . . . . . . . . . . . . . . . .  38
       6.1.3.  Opaque ID . . . . . . . . . . . . . . . . . . . . . .  40
     6.2.  Symmetric Recursive Routing . . . . . . . . . . . . . . .  41
       6.2.1.  Request Origination . . . . . . . . . . . . . . . . .  41
       6.2.2.  Response Origination  . . . . . . . . . . . . . . . .  42
     6.3.  Message Structure . . . . . . . . . . . . . . . . . . . .  42
       6.3.1.  Presentation Language . . . . . . . . . . . . . . . .  43
         6.3.1.1.  Common Definitions  . . . . . . . . . . . . . . .  44
       6.3.2.  Forwarding Header . . . . . . . . . . . . . . . . . .  46
         6.3.2.1.  Processing Configuration Sequence Numbers . . . .  49
         6.3.2.2.  Destination and Via Lists . . . . . . . . . . . .  50
         6.3.2.3.  Forwarding Option . . . . . . . . . . . . . . . .  52
       6.3.3.  Message Contents Format . . . . . . . . . . . . . . .  53
         6.3.3.1.  Response Codes and Response Errors  . . . . . . .  54
       6.3.4.  Security Block  . . . . . . . . . . . . . . . . . . .  57
     6.4.  Overlay Topology  . . . . . . . . . . . . . . . . . . . .  60
       6.4.1.  Topology Plug-in Requirements . . . . . . . . . . . .  60
       6.4.2.  Methods and Types for Use by Topology Plug-ins  . . .  61
         6.4.2.1.  Join  . . . . . . . . . . . . . . . . . . . . . .  61
         6.4.2.2.  Leave . . . . . . . . . . . . . . . . . . . . . .  62
         6.4.2.3.  Update  . . . . . . . . . . . . . . . . . . . . .  63
         6.4.2.4.  RouteQuery  . . . . . . . . . . . . . . . . . . .  63
         6.4.2.5.  Probe . . . . . . . . . . . . . . . . . . . . . .  65
     6.5.  Forwarding and Link Management Layer  . . . . . . . . . .  67
       6.5.1.  Attach  . . . . . . . . . . . . . . . . . . . . . . .  67
         6.5.1.1.  Request Definition  . . . . . . . . . . . . . . .  68
         6.5.1.2.  Response Definition . . . . . . . . . . . . . . .  70
         6.5.1.3.  Using ICE with RELOAD . . . . . . . . . . . . . .  71
         6.5.1.4.  Collecting STUN Servers . . . . . . . . . . . . .  71
         6.5.1.5.  Gathering Candidates  . . . . . . . . . . . . . .  72



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         6.5.1.6.  Prioritizing Candidates . . . . . . . . . . . . .  72
         6.5.1.7.  Encoding the Attach Message . . . . . . . . . . .  73
         6.5.1.8.  Verifying ICE Support . . . . . . . . . . . . . .  74
         6.5.1.9.  Role Determination  . . . . . . . . . . . . . . .  74
         6.5.1.10. Full ICE  . . . . . . . . . . . . . . . . . . . .  74
         6.5.1.11. No-ICE  . . . . . . . . . . . . . . . . . . . . .  75
         6.5.1.12. Subsequent Offers and Answers . . . . . . . . . .  75
         6.5.1.13. Sending Media . . . . . . . . . . . . . . . . . .  75
         6.5.1.14. Receiving Media . . . . . . . . . . . . . . . . .  75
       6.5.2.  AppAttach . . . . . . . . . . . . . . . . . . . . . .  75
         6.5.2.1.  Request Definition  . . . . . . . . . . . . . . .  76
         6.5.2.2.  Response Definition . . . . . . . . . . . . . . .  77
       6.5.3.  Ping  . . . . . . . . . . . . . . . . . . . . . . . .  77
         6.5.3.1.  Request Definition  . . . . . . . . . . . . . . .  77
         6.5.3.2.  Response Definition . . . . . . . . . . . . . . .  77
       6.5.4.  ConfigUpdate  . . . . . . . . . . . . . . . . . . . .  78
         6.5.4.1.  Request Definition  . . . . . . . . . . . . . . .  78
         6.5.4.2.  Response Definition . . . . . . . . . . . . . . .  79
     6.6.  Overlay Link Layer  . . . . . . . . . . . . . . . . . . .  80
       6.6.1.  Future Overlay Link Protocols . . . . . . . . . . . .  81
         6.6.1.1.  HIP . . . . . . . . . . . . . . . . . . . . . . .  82
         6.6.1.2.  ICE-TCP . . . . . . . . . . . . . . . . . . . . .  82
         6.6.1.3.  Message-Oriented Transports . . . . . . . . . . .  82
         6.6.1.4.  Tunneled Transports . . . . . . . . . . . . . . .  82
       6.6.2.  Framing Header  . . . . . . . . . . . . . . . . . . .  83
       6.6.3.  Simple Reliability  . . . . . . . . . . . . . . . . .  84
         6.6.3.1.  Stop and Wait Sender Algorithm  . . . . . . . . .  85
       6.6.4.  DTLS/UDP with SR  . . . . . . . . . . . . . . . . . .  86
       6.6.5.  TLS/TCP with FH, No-ICE . . . . . . . . . . . . . . .  86
       6.6.6.  DTLS/UDP with SR, No-ICE  . . . . . . . . . . . . . .  87
     6.7.  Fragmentation and Reassembly  . . . . . . . . . . . . . .  87
   7.  Data Storage Protocol . . . . . . . . . . . . . . . . . . . .  88
     7.1.  Data Signature Computation  . . . . . . . . . . . . . . .  90
     7.2.  Data Models . . . . . . . . . . . . . . . . . . . . . . .  91
       7.2.1.  Single Value  . . . . . . . . . . . . . . . . . . . .  91
       7.2.2.  Array . . . . . . . . . . . . . . . . . . . . . . . .  92
       7.2.3.  Dictionary  . . . . . . . . . . . . . . . . . . . . .  92
     7.3.  Access Control Policies . . . . . . . . . . . . . . . . .  93
       7.3.1.  USER-MATCH  . . . . . . . . . . . . . . . . . . . . .  93
       7.3.2.  NODE-MATCH  . . . . . . . . . . . . . . . . . . . . .  93
       7.3.3.  USER-NODE-MATCH . . . . . . . . . . . . . . . . . . .  93
       7.3.4.  NODE-MULTIPLE . . . . . . . . . . . . . . . . . . . .  94
     7.4.  Data Storage Methods  . . . . . . . . . . . . . . . . . .  94
       7.4.1.  Store . . . . . . . . . . . . . . . . . . . . . . . .  94
         7.4.1.1.  Request Definition  . . . . . . . . . . . . . . .  94
         7.4.1.2.  Response Definition . . . . . . . . . . . . . . . 100
         7.4.1.3.  Removing Values . . . . . . . . . . . . . . . . . 101




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       7.4.2.  Fetch . . . . . . . . . . . . . . . . . . . . . . . . 102
         7.4.2.1.  Request Definition  . . . . . . . . . . . . . . . 102
         7.4.2.2.  Response Definition . . . . . . . . . . . . . . . 104
       7.4.3.  Stat  . . . . . . . . . . . . . . . . . . . . . . . . 105
         7.4.3.1.  Request Definition  . . . . . . . . . . . . . . . 105
         7.4.3.2.  Response Definition . . . . . . . . . . . . . . . 106
       7.4.4.  Find  . . . . . . . . . . . . . . . . . . . . . . . . 107
         7.4.4.1.  Request Definition  . . . . . . . . . . . . . . . 108
         7.4.4.2.  Response Definition . . . . . . . . . . . . . . . 108
       7.4.5.  Defining New Kinds  . . . . . . . . . . . . . . . . . 109
   8.  Certificate Store Usage . . . . . . . . . . . . . . . . . . . 110
   9.  TURN Server Usage . . . . . . . . . . . . . . . . . . . . . . 110
   10. Chord Algorithm . . . . . . . . . . . . . . . . . . . . . . . 112
     10.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . 113
     10.2.  Hash Function  . . . . . . . . . . . . . . . . . . . . . 114
     10.3.  Routing  . . . . . . . . . . . . . . . . . . . . . . . . 114
     10.4.  Redundancy . . . . . . . . . . . . . . . . . . . . . . . 114
     10.5.  Joining  . . . . . . . . . . . . . . . . . . . . . . . . 115
     10.6.  Routing Attaches . . . . . . . . . . . . . . . . . . . . 116
     10.7.  Updates  . . . . . . . . . . . . . . . . . . . . . . . . 117
       10.7.1.  Handling Neighbor Failures . . . . . . . . . . . . . 118
       10.7.2.  Handling Finger Table Entry Failure  . . . . . . . . 119
       10.7.3.  Receiving Updates  . . . . . . . . . . . . . . . . . 119
       10.7.4.  Stabilization  . . . . . . . . . . . . . . . . . . . 120
         10.7.4.1.  Updating the Neighbor Table  . . . . . . . . . . 120
         10.7.4.2.  Refreshing the Finger Table  . . . . . . . . . . 121
         10.7.4.3.  Adjusting Finger Table Size  . . . . . . . . . . 122
         10.7.4.4.  Detecting Partitioning . . . . . . . . . . . . . 122
     10.8.  Route Query  . . . . . . . . . . . . . . . . . . . . . . 123
     10.9.  Leaving  . . . . . . . . . . . . . . . . . . . . . . . . 123
   11. Enrollment and Bootstrap  . . . . . . . . . . . . . . . . . . 124
     11.1.  Overlay Configuration  . . . . . . . . . . . . . . . . . 124
       11.1.1.  RELAX NG Grammar . . . . . . . . . . . . . . . . . . 132
     11.2.  Discovery through Configuration Server . . . . . . . . . 134
     11.3.  Credentials  . . . . . . . . . . . . . . . . . . . . . . 135
       11.3.1.  Self-Generated Credentials . . . . . . . . . . . . . 137
     11.4.  Contacting a Bootstrap Node  . . . . . . . . . . . . . . 138
   12. Message Flow Example  . . . . . . . . . . . . . . . . . . . . 138
   13. Security Considerations . . . . . . . . . . . . . . . . . . . 144
     13.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . 144
     13.2.  Attacks on P2P Overlays  . . . . . . . . . . . . . . . . 145
     13.3.  Certificate-Based Security . . . . . . . . . . . . . . . 145
     13.4.  Shared-Secret Security . . . . . . . . . . . . . . . . . 147
     13.5.  Storage Security . . . . . . . . . . . . . . . . . . . . 147
       13.5.1.  Authorization  . . . . . . . . . . . . . . . . . . . 147
       13.5.2.  Distributed Quota  . . . . . . . . . . . . . . . . . 148
       13.5.3.  Correctness  . . . . . . . . . . . . . . . . . . . . 148
       13.5.4.  Residual Attacks . . . . . . . . . . . . . . . . . . 149



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     13.6.  Routing Security . . . . . . . . . . . . . . . . . . . . 149
       13.6.1.  Background . . . . . . . . . . . . . . . . . . . . . 150
       13.6.2.  Admissions Control . . . . . . . . . . . . . . . . . 150
       13.6.3.  Peer Identification and Authentication . . . . . . . 151
       13.6.4.  Protecting the Signaling . . . . . . . . . . . . . . 151
       13.6.5.  Routing Loops and DoS Attacks  . . . . . . . . . . . 152
       13.6.6.  Residual Attacks . . . . . . . . . . . . . . . . . . 152
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 153
     14.1.  Well-Known URI Registration  . . . . . . . . . . . . . . 153
     14.2.  Port Registrations . . . . . . . . . . . . . . . . . . . 153
     14.3.  Overlay Algorithm Types  . . . . . . . . . . . . . . . . 154
     14.4.  Access Control Policies  . . . . . . . . . . . . . . . . 154
     14.5.  Application-ID . . . . . . . . . . . . . . . . . . . . . 155
     14.6.  Data Kind-ID . . . . . . . . . . . . . . . . . . . . . . 155
     14.7.  Data Model . . . . . . . . . . . . . . . . . . . . . . . 156
     14.8.  Message Codes  . . . . . . . . . . . . . . . . . . . . . 156
     14.9.  Error Codes  . . . . . . . . . . . . . . . . . . . . . . 158
     14.10. Overlay Link Types . . . . . . . . . . . . . . . . . . . 159
     14.11. Overlay Link Protocols . . . . . . . . . . . . . . . . . 159
     14.12. Forwarding Options . . . . . . . . . . . . . . . . . . . 160
     14.13. Probe Information Types  . . . . . . . . . . . . . . . . 160
     14.14. Message Extensions . . . . . . . . . . . . . . . . . . . 161
     14.15. Reload URI Scheme  . . . . . . . . . . . . . . . . . . . 161
       14.15.1.  URI Registration  . . . . . . . . . . . . . . . . . 162
     14.16. Media Type Registration  . . . . . . . . . . . . . . . . 162
     14.17. XML Namespace Registration . . . . . . . . . . . . . . . 163
       14.17.1.  Config URL  . . . . . . . . . . . . . . . . . . . . 164
       14.17.2.  Config Chord URL  . . . . . . . . . . . . . . . . . 164
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 164
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . . 165
     16.1.  Normative References . . . . . . . . . . . . . . . . . . 165
     16.2.  Informative References . . . . . . . . . . . . . . . . . 167
   Appendix A.  Routing Alternatives . . . . . . . . . . . . . . . . 171
     A.1.  Iterative vs. Recursive . . . . . . . . . . . . . . . . . 171
     A.2.  Symmetric vs. Forward Response  . . . . . . . . . . . . . 171
     A.3.  Direct Response . . . . . . . . . . . . . . . . . . . . . 172
     A.4.  Relay Peers . . . . . . . . . . . . . . . . . . . . . . . 173
     A.5.  Symmetric Route Stability . . . . . . . . . . . . . . . . 173
   Appendix B.  Why Clients? . . . . . . . . . . . . . . . . . . . . 174
     B.1.  Why Not Only Peers? . . . . . . . . . . . . . . . . . . . 174
     B.2.  Clients as Application-Level Agents . . . . . . . . . . . 175










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1.  Introduction

   This document defines REsource LOcation And Discovery (RELOAD), a
   peer-to-peer (P2P) signaling protocol for use on the Internet.
   RELOAD provides a generic, self-organizing overlay network service,
   allowing nodes to route messages to other nodes and to store and
   retrieve data in the overlay.  RELOAD provides several features that
   are critical for a successful P2P protocol for the Internet:

   Security Framework:  A P2P network will often be established among a
      set of peers that do not trust each other.  RELOAD leverages a
      central enrollment server to provide credentials for each peer,
      which can then be used to authenticate each operation.  This
      greatly reduces the possible attack surface.

   Usage Model:  RELOAD is designed to support a variety of
      applications, including P2P multimedia communications with the
      Session Initiation Protocol (SIP) [SIP-RELOAD].  RELOAD allows the
      definition of new application usages, each of which can define its
      own data types, along with the rules for their use.  This allows
      RELOAD to be used with new applications through a simple
      documentation process that supplies the details for each
      application.

   NAT Traversal:  RELOAD is designed to function in environments where
      many, if not most, of the nodes are behind NATs or firewalls.
      Operations for NAT traversal are part of the base design,
      including using Interactive Connectivity Establishment (ICE)
      [RFC5245] to establish new RELOAD or application protocol
      connections.

   Optimized Routing:  The very nature of overlay algorithms introduces
      a requirement that peers participating in the P2P network route
      requests on behalf of other peers in the network.  This introduces
      a load on those other peers in the form of bandwidth and
      processing power.  RELOAD has been defined with a simple,
      lightweight forwarding header, thus minimizing the amount of
      effort for intermediate peers.

   Pluggable Overlay Algorithms:  RELOAD has been designed with an
      abstract interface to the overlay layer to simplify implementing a
      variety of structured (e.g., distributed hash tables (DHTs)) and
      unstructured overlay algorithms.  The idea here is that RELOAD
      provides a generic structure that can fit most types of overlay
      topologies (ring, hyperspace, etc.).  To instantiate an actual
      network, you combine RELOAD with a specific overlay algorithm,
      which defines how to construct the overlay topology and route
      messages efficiently within it.  This specification also defines



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      how RELOAD is used with the Chord-based [Chord] DHT algorithm,
      which is mandatory to implement.  Specifying a default "mandatory-
      to-implement" overlay algorithm promotes interoperability, while
      extensibility allows selection of overlay algorithms optimized for
      a particular application.

   Support for Clients:  RELOAD clients differ from RELOAD peers
      primarily in that they do not store information on behalf of other
      nodes in the overlay.  Rather, they use the overlay only to locate
      users and resources, as well as to store information and to
      contact other nodes.

   These properties were designed specifically to meet the requirements
   for a P2P protocol to support SIP.  This document defines the base
   protocol for the distributed storage and location service, as well as
   critical usage for NAT traversal.  The SIP Usage itself is described
   separately in [SIP-RELOAD].  RELOAD is not limited to usage by SIP
   and could serve as a tool for supporting other P2P applications with
   similar needs.

1.1.  Basic Setting

   In this section, we provide a brief overview of the operational
   setting for RELOAD.  A RELOAD Overlay Instance consists of a set of
   nodes arranged in a partly connected graph.  Each node in the overlay
   is assigned a numeric Node-ID for the lifetime of the node, which,
   together with the specific overlay algorithm in use, determines its
   position in the graph and the set of nodes it connects to.  The
   Node-ID is also tightly coupled to the certificate (see
   Section 13.3).  The figure below shows a trivial example which isn't
   drawn from any particular overlay algorithm, but was chosen for
   convenience of representation.



















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      +--------+              +--------+              +--------+
      | Node 10|--------------| Node 20|--------------| Node 30|
      +--------+              +--------+              +--------+
          |                       |                       |
          |                       |                       |
      +--------+              +--------+              +--------+
      | Node 40|--------------| Node 50|--------------| Node 60|
      +--------+              +--------+              +--------+
          |                       |                       |
          |                       |                       |
      +--------+              +--------+              +--------+
      | Node 70|--------------| Node 80|--------------| Node 90|
      +--------+              +--------+              +--------+
                                  |
                                  |
                              +--------+
                              | Node 85|
                              |(Client)|
                              +--------+

   Because the graph is not fully connected, when a node wants to send a
   message to another node, it may need to route it through the network.
   For instance, Node 10 can talk directly to nodes 20 and 40, but not
   to Node 70.  In order to send a message to Node 70, it would first
   send it to Node 40, with instructions to pass it along to Node 70.
   Different overlay algorithms will have different connectivity graphs,
   but the general idea behind all of them is to allow any node in the
   graph to efficiently reach every other node within a small number of
   hops.

   The RELOAD network is not only a messaging network.  It is also a
   storage network, albeit one designed for small-scale transient
   storage rather than for bulk storage of large objects.  Records are
   stored under numeric addresses, called Resource-IDs, which occupy the
   same space as node identifiers.  Peers are responsible for storing
   the data associated with some set of addresses, as determined by
   their Node-ID.  For instance, we might say that every peer is
   responsible for storing any data value which has an address less than
   or equal to its own Node-ID, but greater than the next lowest
   Node-ID.  Thus, Node 20 would be responsible for storing values
   11-20.

   RELOAD also supports clients.  These are nodes which have Node-IDs
   but do not participate in routing or storage.  For instance, in the
   figure above, Node 85 is a client.  It can route to the rest of the
   RELOAD network via Node 80, but no other node will route through it,
   and Node 90 is still responsible for addresses in the range [81..90].
   We refer to non-client nodes as peers.



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   Other applications (for instance, SIP) can be defined on top of
   RELOAD and can use these two basic RELOAD services to provide their
   own services.

1.2.  Architecture

   RELOAD is fundamentally an overlay network.  The following figure
   shows the layered RELOAD architecture.

            Application

        +-------+  +-------+
        | SIP   |  | XMPP  |  ...
        | Usage |  | Usage |
        +-------+  +-------+
    ------------------------------------ Messaging Service Boundary
    +------------------+     +---------+
    |     Message      |<--->| Storage |
    |    Transport     |     +---------+
    +------------------+           ^
           ^       ^               |
           |       v               v
           |     +-------------------+
           |     |    Topology       |
           |     |    Plug-in        |
           |     +-------------------+
           |         ^
           v         v
        +------------------+
        |  Forwarding &    |
        | Link Management  |
        +------------------+
    ------------------------------------ Overlay Link Service Boundary
         +-------+  +-------+
         |TLS    |  |DTLS   |  ...
         |Overlay|  |Overlay|
         |Link   |  |Link   |
         +-------+  +-------+

   The major components of RELOAD are:

   Usage Layer:  Each application defines a RELOAD Usage, which is a set
      of data Kinds and behaviors which describe how to use the services
      provided by RELOAD.  These usages all talk to RELOAD through a
      common Message Transport Service.






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   Message Transport:  Handles end-to-end reliability, manages request
      state for the usages, and forwards Store and Fetch operations to
      the Storage component.  It delivers message responses to the
      component initiating the request.

   Storage:  The Storage component is responsible for processing
      messages relating to the storage and retrieval of data.  It talks
      directly to the Topology Plug-in to manage data replication and
      migration, and it talks to the Message Transport component to send
      and receive messages.

   Topology Plug-in:  The Topology Plug-in is responsible for
      implementing the specific overlay algorithm being used.  It uses
      the Message Transport component to send and receive overlay
      management messages, the Storage component to manage data
      replication, and the Forwarding Layer to control hop-by-hop
      message forwarding.  This component superficially parallels
      conventional routing algorithms, but is more tightly coupled to
      the Forwarding Layer, because there is no single "Routing Table"
      equivalent used by all overlay algorithms.  The Topology Plug-in
      has two functions: constructing the local forwarding instructions
      and selecting the operational topology (i.e., creating links by
      sending overlay management messages).

   Forwarding and Link Management Layer:  Stores and implements the
      Routing Table by providing packet forwarding services between
      nodes.  It also handles establishing new links between nodes,
      including setting up connections for overlay links across NATs
      using ICE.

   Overlay Link Layer:  Responsible for actually transporting traffic
      directly between nodes.  Transport Layer Security (TLS) [RFC5246]
      and Datagram Transport Layer Security (DTLS) [RFC6347] are the
      currently defined "overlay link layer" protocols used by RELOAD
      for hop-by-hop communication.  Each such protocol includes the
      appropriate provisions for per-hop framing and hop-by-hop ACKs
      needed by unreliable underlying transports.  New protocols can be
      defined, as described in Sections 6.6.1 and 11.1.  As this
      document defines only TLS and DTLS, we use those terms throughout
      the remainder of the document with the understanding that some
      future specification may add new overlay link layers.










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   To further clarify the roles of the various layers, the following
   figure parallels the architecture with each layer's role from an
   overlay perspective and implementation layer in the Internet:

    Internet    | Internet Model  |
    Model       |   Equivalent    |          Reload
                |   in Overlay    |       Architecture
   -------------+-----------------+------------------------------------
                |                 |    +-------+  +-------+
                |  Application    |    | SIP   |  | XMPP  |  ...
                |                 |    | Usage |  | Usage |
                |                 |    +-------+  +-------+
                |                 |  ----------------------------------
                |                 |+------------------+     +---------+
                |   Transport     ||     Message      |<--->| Storage |
                |                 ||    Transport     |     +---------+
                |                 |+------------------+           ^
                |                 |       ^       ^               |
                |                 |       |       v               v
   Application  |                 |       |     +-------------------+
                |   (Routing)     |       |     |     Topology      |
                |                 |       |     |     Plug-in       |
                |                 |       |     +-------------------+
                |                 |       |         ^
                |                 |       v         v
                |    Network      |    +------------------+
                |                 |    |  Forwarding &    |
                |                 |    | Link Management  |
                |                 |    +------------------+
                |                 |  ----------------------------------
   Transport    |      Link       |     +-------+  +------+
                |                 |     |TLS    |  |DTLS  |  ...
                |                 |     +-------+  +------+
   -------------+-----------------+------------------------------------
     Network    |
                |
       Link     |

   In addition to the above components, nodes may communicate with a
   central provisioning infrastructure (not shown) to get configuration
   information, authentication credentials, and the initial set of nodes
   to communicate with to join the overlay.









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1.2.1.  Usage Layer

   The top layer, called the Usage Layer, has application usages, such
   as the SIP Registration Usage [SIP-RELOAD], that use the abstract
   Message Transport Service provided by RELOAD.  The goal of this layer
   is to implement application-specific usages of the generic overlay
   services provided by RELOAD.  The Usage defines how a specific
   application maps its data into something that can be stored in the
   overlay, where to store the data, how to secure the data, and finally
   how applications can retrieve and use the data.

   The architecture diagram shows both a SIP Usage and an XMPP Usage.  A
   single application may require multiple usages; for example, a
   voicemail feature in a softphone application that stores links to the
   messages in the overlay would require a different usage than the type
   of rendezvous service of XMPP or SIP.  A usage may define multiple
   Kinds of data that are stored in the overlay and may also rely on
   Kinds originally defined by other usages.

   Because the security and storage policies for each Kind are dictated
   by the usage defining the Kind, the usages may be coupled with the
   Storage component to provide security policy enforcement and to
   implement appropriate storage strategies according to the needs of
   the usage.  The exact implementation of such an interface is outside
   the scope of this specification.

1.2.2.  Message Transport

   The Message Transport component provides a generic message routing
   service for the overlay.  The Message Transport layer is responsible
   for end-to-end message transactions.  Each peer is identified by its
   location in the overlay, as determined by its Node-ID.  A component
   that is a client of the Message Transport can perform two basic
   functions:

   o  Send a message to a given peer specified by Node-ID or to the peer
      responsible for a particular Resource-ID.

   o  Receive messages that other peers sent to a Node-ID or Resource-ID
      for which the receiving peer is responsible.

   All usages rely on the Message Transport component to send and
   receive messages from peers.  For instance, when a usage wants to
   store data, it does so by sending Store requests.  Note that the
   Storage component and the Topology Plug-in are themselves clients of
   the Message Transport, because they need to send and receive messages
   from other peers.




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   The Message Transport Service is responsible for end-to-end
   reliability, which is accomplished by timer-based retransmissions.
   Unlike the Internet transport layer, however, this layer does not
   provide congestion control.  RELOAD is a request-response protocol,
   with no more than two pairs of request-response messages used in
   typical transactions between pairs of nodes; therefore, there are no
   opportunities to observe and react to end-to-end congestion.  As with
   all Internet applications, implementers are strongly discouraged from
   writing applications that react to loss by immediately retrying the
   transaction.

   The Message Transport Service is similar to those described as
   providing "key-based routing" (KBR) [wikiKBR], although as RELOAD
   supports different overlay algorithms (including non-DHT overlay
   algorithms) that calculate keys (storage indices, not encryption
   keys) in different ways, the actual interface needs to accept
   Resource Names rather than actual keys.

   The Forwarding and Link Management layers are responsible for
   maintaining the overlay in the face of changes in the available nodes
   and underlying network supporting the overlay (the Internet).  They
   also handle congestion control between overlay neighbors, and
   exchange routing updates and data replicas in addition to forwarding
   end-to-end messages.

   Real-world experience has shown that a fixed timeout for the end-to-
   end retransmission timer is sufficient for practical overlay
   networks.  This timer is adjustable via the overlay configuration.
   As the overlay configuration can be rapidly updated, this value could
   be dynamically adjusted at coarse time scales, although algorithms
   for determining how to accomplish this are beyond the scope of this
   specification.  In many cases, however, other means of improving
   network performance, such as having the Topology Plug-in remove lossy
   links from use in overlay routing or reducing the overall hop count
   of end-to-end paths, will be more effective than simply increasing
   the retransmission timer.

1.2.3.  Storage

   One of the major functions of RELOAD is storage of data, that is,
   allowing nodes to store data in the overlay and to retrieve data
   stored by other nodes or by themselves.  The Storage component is
   responsible for processing data storage and retrieval messages.  For
   instance, the Storage component might receive a Store request for a
   given resource from the Message Transport.  It would then query the
   appropriate usage before storing the data value(s) in its local data
   store and sending a response to the Message Transport for delivery to
   the requesting node.  Typically, these messages will come from other



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   nodes, but depending on the overlay topology, a node might be
   responsible for storing data for itself as well, especially if the
   overlay is small.

   A peer's Node-ID determines the set of resources that it will be
   responsible for storing.  However, the exact mapping between these is
   determined by the overlay algorithm in use.  The Storage component
   will only receive a Store request from the Message Transport if this
   peer is responsible for that Resource-ID.  The Storage component is
   notified by the Topology Plug-in when the Resource-IDs for which it
   is responsible change, and the Storage component is then responsible
   for migrating resources to other peers.

1.2.4.  Topology Plug-in

   RELOAD is explicitly designed to work with a variety of overlay
   algorithms.  In order to facilitate this, the overlay algorithm
   implementation is provided by a Topology Plug-in so that each overlay
   can select an appropriate overlay algorithm that relies on the common
   RELOAD core protocols and code.

   The Topology Plug-in is responsible for maintaining the overlay
   algorithm Routing Table, which is consulted by the Forwarding and
   Link Management Layer before routing a message.  When connections are
   made or broken, the Forwarding and Link Management Layer notifies the
   Topology Plug-in, which adjusts the Routing Table as appropriate.
   The Topology Plug-in will also instruct the Forwarding and Link
   Management Layer to form new connections as dictated by the
   requirements of the overlay algorithm Topology.  The Topology Plug-in
   issues periodic update requests through Message Transport to maintain
   and update its Routing Table.

   As peers enter and leave, resources may be stored on different peers,
   so the Topology Plug-in also keeps track of which peers are
   responsible for which resources.  As peers join and leave, the
   Topology Plug-in instructs the Storage component to issue resource
   migration requests as appropriate, in order to ensure that other
   peers have whatever resources they are now responsible for.  The
   Topology Plug-in is also responsible for providing for redundant data
   storage to protect against loss of information in the event of a peer
   failure and to protect against compromised or subversive peers.










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1.2.5.  Forwarding and Link Management Layer

   The Forwarding and Link Management Layer is responsible for getting a
   message to the next peer, as determined by the Topology Plug-in.
   This layer establishes and maintains the network connections as
   needed by the Topology Plug-in.  This layer is also responsible for
   setting up connections to other peers through NATs and firewalls
   using ICE, and it can elect to forward traffic using relays for NAT
   and firewall traversal.

   Congestion control is implemented at this layer to protect the
   Internet paths used to form the link in the overlay.  Additionally,
   retransmission is performed to improve the reliability of end-to-end
   transactions.  The relation of this layer to the Message Transport
   Layer can be likened to the relation of the link-level congestion
   control and retransmission in modern wireless networks ` to Internet
   transport protocols.

   This layer provides a generic interface that allows the Topology
   Plug-in to control the overlay and resource operations and messages.
   Because each overlay algorithm is defined and functions differently,
   we generically refer to the table of other peers that the overlay
   algorithm maintains and uses to route requests as a Routing Table.
   The Topology Plug-in actually owns the Routing Table, and forwarding
   decisions are made by querying the Topology Plug-in for the next hop
   for a particular Node-ID or Resource-ID.  If this node is the
   destination of the message, the message is delivered to the Message
   Transport.

   This layer also utilizes a framing header to encapsulate messages as
   they are forwarded along each hop.  This header aids reliability
   congestion control, flow control, etc.  It has meaning only in the
   context of that individual link.

   The Forwarding and Link Management Layer sits on top of the Overlay
   Link Layer protocols that carry the actual traffic.  This
   specification defines how to use DTLS and TLS protocols to carry
   RELOAD messages.

1.3.  Security

   RELOAD's security model is based on each node having one or more
   public key certificates.  In general, these certificates will be
   assigned by a central server, which also assigns Node-IDs, although
   self-signed certificates can be used in closed networks.  These
   credentials can be leveraged to provide communications security for
   RELOAD messages.  RELOAD provides communications security at three
   levels:



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   Connection level:  Connections between nodes are secured with TLS,
      DTLS, or potentially some to-be-defined future protocol.

   Message level:  Each RELOAD message is signed.

   Object Level:  Stored objects are signed by the creating node.

   These three levels of security work together to allow nodes to verify
   the origin and correctness of data they receive from other nodes,
   even in the face of malicious activity by other nodes in the overlay.
   RELOAD also provides access control built on top of these
   communications security features.  Because the peer responsible for
   storing a piece of data can validate the signature on the data being
   stored, it can determine whether or not a given operation is
   permitted.

   RELOAD also provides an optional shared-secret-based admission
   control feature using shared secrets and TLS pre-shared keys (PSK) or
   TLS Secure Remote Password (SRP).  In order to form a TLS connection
   to any node in the overlay, a new node needs to know the shared
   overlay key, thus restricting access to authorized users only.  This
   feature is used together with certificate-based access control, not
   as a replacement for it.  It is typically used when self-signed
   certificates are being used but would generally not be used when the
   certificates were all signed by an enrollment server.

1.4.  Structure of This Document

   The remainder of this document is structured as follows.

   o  Section 3 provides definitions of terms used in this document.

   o  Section 4 provides an overview of the mechanisms used to establish
      and maintain the overlay.

   o  Section 5 provides an overview of the mechanism RELOAD provides to
      support other applications.

   o  Section 6 defines the protocol messages that RELOAD uses to
      establish and maintain the overlay.

   o  Section 7 defines the protocol messages that are used to store and
      retrieve data using RELOAD.

   o  Section 8 defines the Certificate Store Usages.

   o  Section 9 defines the TURN Server Usage needed to locate TURN
      (Traversal Using Relays around NAT) servers for NAT traversal.



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   o  Section 10 defines a specific Topology Plug-in using a Chord-based
      algorithm.

   o  Section 11 defines the mechanisms that new RELOAD nodes use to
      join the overlay for the first time.

   o  Section 12 provides an extended example.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Terminology

   Terms in this document are defined in-line when used and are also
   defined below for reference.  The definitions in this section use
   terminology and concepts that are not explained until later in the
   specification.

   Admitting Peer (AP):  A peer in the overlay which helps the Joining
      Node join the Overlay.

   Bootstrap Node:  A network node used by Joining Nodes to help locate
      the Admitting Peer.

   Client:  A host that is able to store data in and retrieve data from
      the overlay, but does not participate in routing or data storage
      for the overlay.

   Configuration Document:  An XML document containing all the Overlay
      Parameters for one overlay instance.

   Connection Table:  Contains connection information for the set of
      nodes to which a node is directly connected, which include nodes
      that are not yet available for routing.

   Destination List:  A list of Node-IDs, Resource-IDs, and Opaque IDs
      through which a message is to be routed, in strict order.  A
      single Node-ID, Resource-ID, or Opaque ID is a trivial form of
      Destination List.  When multiple Node-IDs are specified, a
      Destination List is a loose source route.  The list is reduced hop
      by hop, and does not include the source but does include the
      destination.






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   DHT:  A distributed hash table.  A DHT is an abstract storage service
      realized by storing the contents of the hash table across a set of
      peers.

   ID:  A generic term for any kind of identifiers in an Overlay.  This
      document specifies an ID as being an Application-ID, a Kind-ID, a
      Node-ID, a transaction ID, a component ID, a response ID, a
      Resource-ID, or an Opaque ID.

   Joining Node (JN):  A node that is attempting to become a peer in a
      particular Overlay.

   Kind:  A Kind defines a particular type of data that can be stored in
      the overlay.  Applications define new Kinds to store the data they
      use.  Each Kind is identified with a unique integer called a
      Kind-ID.

   Kind-ID:  A unique 32-bit value identifying a Kind.  Kind-IDs are
      either private or allocated by IANA (see Section 14.6).

   Maximum Request Lifetime:  The maximum time a request will wait for a
      response.  This value is equal to the value of the overlay
      reliability value (defined in Section 11.1) multiplied by the
      number of transmissions (defined in Section 6.2.1), and so
      defaults to 15 seconds.

   Node:  The term "node" refers to a host that may be either a peer or
      a client.  Because RELOAD uses the same protocol for both clients
      and peers, much of the text applies equally to both.  Therefore,
      we use "node" when the text applies to both clients and peers, and
      we use the more specific term (i.e., "client" or "peer") when the
      text applies only to clients or only to peers.

   Node-ID:  A value of fixed but configurable length that uniquely
      identifies a node.  Node-IDs of all 0s and all 1s are reserved.  A
      value of 0 is not used in the wire protocol, but can be used to
      indicate an invalid node in implementations and APIs.  The Node-ID
      of all 1s is used on the wire protocol as a wildcard.

   Overlay Algorithm:  An overlay algorithm defines the rules for
      determining which peers in an overlay store a particular piece of
      data and for determining a topology of interconnections amongst
      peers in order to find a piece of data.

   Overlay Instance:  A specific overlay algorithm and the collection of
      peers that are collaborating to provide read and write access to
      it.  Any number of overlay instances can be running in an IP
      network at a time, and each operates in isolation of the others.



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   Overlay Parameters:  A set of values that are shared among all nodes
      in an overlay.  The overlay parameters are distributed in an XML
      document called the Configuration Document.

   Peer:  A host that is participating in the overlay.  Peers are
      responsible for holding some portion of the data that has been
      stored in the overlay, and they are responsible for routing
      messages on behalf of other hosts as needed by the Overlay
      Algorithm.

   Peer Admission:  The act of admitting a node (the Joining Node) into
      an Overlay.  After the admission process is over, the Joining Node
      is a fully functional peer of the overlay.  During the admission
      process, the Joining Node may need to present credentials to prove
      that it has sufficient authority to join the overlay.

   Resource:  An object or group of objects stored in a P2P network.

   Resource-ID:  A value that identifies some resources and which is
      used as a key for storing and retrieving the resource.  Often this
      is not human friendly/readable.  One way to generate a Resource-ID
      is by applying a mapping function to some other unique name (e.g.,
      user name or service name) for the resource.  The Resource-ID is
      used by the distributed database algorithm to determine the peer
      or peers that are responsible for storing the data for the
      overlay.  In structured P2P networks, Resource-IDs are generally
      fixed length and are formed by hashing the Resource Name.  In
      unstructured networks, Resource Names may be used directly as
      Resource-IDs and may be of variable length.

   Resource Name:  The name by which a resource is identified.  In
      unstructured P2P networks, the Resource Name is sometimes used
      directly as a Resource-ID.  In structured P2P networks, the
      Resource Name is typically mapped into a Resource-ID by using the
      string as the input to hash function.  Structured and unstructured
      P2P networks are described in [RFC5694].  A SIP resource, for
      example, is often identified by its AOR (address-of-record), which
      is an example of a Resource Name.

   Responsible Peer:  The peer that is responsible for a specific
      resource, as defined by the Topology Plug-in algorithm.

   Routing Table:  The set of directly connected peers which a node can
      use to forward overlay messages.  In normal operation, these peers
      will all be in the Connection Table, but not vice versa, because
      some peers may not yet be available for routing.  Peers may send





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      messages directly to peers that are in their Connection Tables,
      but may forward messages to peers that are not in their Connection
      Table only through peers that are in the Routing Table.

   Successor Replacement Hold-Down Time:  The amount of time to wait
      before starting replication when a new successor is found; it
      defaults to 30 seconds.

   Transaction ID:  A randomly chosen identifier selected by the
      originator of a request that is used to correlate requests and
      responses.

   Usage:  The definition of a set of data structures (data Kinds) that
      an application wants to store in the overlay.  A usage may also
      define a set of network protocols (Application IDs) that can be
      tunneled over TLS or DTLS direct connections between nodes.  For
      example, the SIP Usage defines a SIP registration data Kind, which
      contains information on how to reach a SIP endpoint, and two
      Application IDs corresponding to the SIP and SIPS protocols.

   User:  A physical person identified by the certificates assigned to
      them.

   User Name:  A name identifying a user of the overlay, typically used
      as a Resource Name or as a label on a resource that identifies the
      user owning the resource.

4.  Overlay Management Overview

   The most basic function of RELOAD is as a generic overlay network.
   Nodes need to be able to join the overlay, form connections to other
   nodes, and route messages through the overlay to nodes to which they
   are not directly connected.  This section provides an overview of the
   mechanisms that perform these functions.

4.1.  Security and Identification

   The overlay parameters are specified in a Configuration Document.
   Because the parameters include security-critical information, such as
   the certificate signing trust anchors, the Configuration Document
   needs to be retrieved securely.  The initial Configuration Document
   is either initially fetched over HTTPS or manually provisioned.
   Subsequent Configuration Document updates are received either as a
   result of being refreshed periodically by the configuration server,
   or, more commonly, by being flood-filled through the overlay, which
   allows for fast propagation once an update is pushed.  In the latter
   case, updates are via digital signatures that trace back to the
   initial Configuration Document.



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   Every node in the RELOAD overlay is identified by a Node-ID.  The
   Node-ID is used for three major purposes:

   o  To address the node itself.

   o  To determine the node's position in the overlay topology (if the
      overlay is structured; overlays do not need to be structured).

   o  To determine the set of resources for which the node is
      responsible.

   Each node has a certificate [RFC5280] containing its Node-ID in a
   subjectAltName extension, which is unique within an overlay instance.

   The certificate serves multiple purposes:

   o  It entitles the user to store data at specific locations in the
      Overlay Instance.  Each data Kind defines the specific rules for
      determining which certificates can access each Resource-ID/Kind-ID
      pair.  For instance, some Kinds might allow anyone to write at a
      given location, whereas others might restrict writes to the owner
      of a single certificate.

   o  It entitles the user to operate a node that has a Node-ID found in
      the certificate.  When the node forms a connection to another
      peer, it uses this certificate so that a node connecting to it
      knows it is connected to the correct node.  (Technically, a TLS or
      DTLS association with client authentication is formed.)  In
      addition, the node can sign messages, thus providing integrity and
      authentication for messages which are sent from the node.

   o  It entitles the user to use the user name found in the
      certificate.

   If a user has more than one device, typically they would get one
   certificate for each device.  This allows each device to act as a
   separate peer.

   RELOAD supports multiple certificate issuance models.  The first is
   based on a central enrollment process, which allocates a unique name
   and Node-ID and puts them in a certificate for the user.  All peers
   in a particular Overlay Instance have the enrollment server as a
   trust anchor and so can verify any other peer's certificate.

   The second model is useful in settings, when a group of users want to
   set up an overlay network but are not concerned about attack by other
   users in the network.  For instance, users on a LAN might want to set
   up a short-term ad hoc network without going to the trouble of



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   setting up an enrollment server.  RELOAD supports the use of self-
   generated, self-signed certificates.  When self-signed certificates
   are used, the node also generates its own Node-ID and user name.  The
   Node-ID is computed as a digest of the public key, to prevent Node-ID
   theft.  Note that the relevant cryptographic property for the digest
   is partial preimage resistance.  Collision resistance is not needed,
   because an attacker who can create two nodes with the same Node-ID
   but a different public key obtains no advantage.  This model is still
   subject to a number of known attacks (most notably, Sybil attacks
   [Sybil]) and can be safely used only in closed networks where users
   are mutually trusting.  Another drawback of this approach is that the
   user's data is then tied to their key, so if a key is changed, any
   data stored under their Node-ID needs to be re-stored.  This is not
   an issue for centrally issued Node-IDs provided that the
   Certification Authority (CA) reissues the same Node-ID when a new
   certificate is generated.

   The general principle here is that the security mechanisms (TLS or
   DTLS at the data link layer and message signatures at the message
   transport layer) are always used, even if the certificates are self-
   signed.  This allows for a single set of code paths in the systems,
   with the only difference being whether certificate verification is
   used to chain to a single root of trust.

4.1.1.  Shared-Key Security

   RELOAD also provides an admission control system based on shared
   keys.  In this model, the peers all share a single key which is used
   to authenticate the peer-to-peer connections via TLS-PSK [RFC4279] or
   TLS-SRP [RFC5054].

4.2.  Clients

   RELOAD defines a single protocol that is used both as the peer
   protocol and as the client protocol for the overlay.  Having a single
   protocol simplifies implementation, particularly for devices that may
   act in either role, and allows clients to inject messages directly
   into the overlay.

   We use the term "peer" to identify a node in the overlay that routes
   messages for nodes other than those to which it is directly
   connected.  Peers also have storage responsibilities.  We use the
   term "client" to refer to nodes that do not have routing or storage
   responsibilities.  When text applies to both peers and clients, we
   will simply refer to such devices as "nodes".






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   RELOAD's client support allows nodes that are not participating in
   the overlay as peers to utilize the same implementation and to
   benefit from the same security mechanisms as the peers.  Clients
   possess and use certificates that authorize the user to store data at
   certain locations in the overlay.  The Node-ID in the certificate is
   used to identify the particular client as a member of the overlay and
   to authenticate its messages.

   In RELOAD, unlike some other designs, clients are not first-class
   entities.  From the perspective of a peer, a client is a node that
   has connected to the overlay, but that has not yet taken steps to
   insert itself into the overlay topology.  It might never do so (if
   it's a client), or it might eventually do so (if it's just a node
   that is taking a long time to join).  The routing and storage rules
   for RELOAD provide for correct behavior by peers regardless of
   whether other nodes attached to them are clients or peers.  Of
   course, a client implementation needs to know that it intends to be a
   client, but this localizes complexity only to that node.

   For more discussion about the motivation for RELOAD's client support,
   see Appendix B.

4.2.1.  Client Routing

   Clients may insert themselves in the overlay in two ways:

   o  Establish a connection to the peer responsible for the client's
      Node-ID in the overlay.  Then, requests may be sent from/to the
      client using its Node-ID in the same manner as if it were a peer,
      because the responsible peer in the overlay will handle the final
      step of routing to the client.  This may require a TURN [RFC5766]
      relay in cases where NATs or firewalls prevent a client from
      forming a direct connection with its responsible peer.  Note that
      clients that choose this option need to process Update messages
      from the peer (Section 6.4.2.3).  These updates can indicate that
      the peer is no longer responsible for the client's Node-ID.  The
      client would then need to form a connection to the appropriate
      peer.  Failure to do so will result in the client no longer
      receiving messages.

   o  Establish a connection with an arbitrary peer in the overlay
      (perhaps based on network proximity or an inability to establish a
      direct connection with the responsible peer).  In this case, the
      client will rely on RELOAD's Destination List feature
      (Section 6.3.2.2) to ensure reachability.  The client can initiate
      requests, and any node in the overlay that knows the Destination
      List to its current location can reach it, but the client is not
      directly reachable using only its Node-ID.  If the client is to



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      receive incoming requests from other members of the overlay, the
      Destination List needed to reach the client needs to be learnable
      via other mechanisms, such as being stored in the overlay by a
      usage.  A client connected this way using a certificate with only
      a single Node-ID can proceed to use the connection without
      performing an Attach (Section 6.5.1).  A client wishing to connect
      using this mechanism with a certificate with multiple Node-IDs can
      use a Ping (Section 6.5.3) to probe the Node-ID of the node to
      which it is connected before performing the Attach.

4.2.2.  Minimum Functionality Requirements for Clients

   A node may act as a client simply because it does not have the
   capacity or need to act as a peer in the overlay, or because it does
   not even have an implementation of the Topology Plug-in defined in
   Section 6.4.1, needed to act as a peer in the overlay.  In order to
   exchange RELOAD messages with a peer, a client needs to meet a
   minimum level of functionality.  Such a client will:

   o  Implement RELOAD's connection-management operations that are used
      to establish the connection with the peer.

   o  Implement RELOAD's data retrieval methods (with client
      functionality).

   o  Be able to calculate Resource-IDs used by the overlay.

   o  Possess security credentials needed by the overlay that it is
      implementing.

   A client speaks the same protocol as the peers, knows how to
   calculate Resource-IDs, and signs its requests in the same manner as
   peers.  While a client does not necessarily require a full
   implementation of the overlay algorithm, calculating the Resource-ID
   requires an implementation of an appropriate algorithm for the
   overlay.

4.3.  Routing

   This section discusses the capabilities of RELOAD's routing layer and
   the protocol features used to implement the capabilities, and
   provides a brief overview of how they are used.  Appendix A discusses
   some alternative designs and the trade-offs that would be necessary
   to support them.







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   RELOAD's routing provides the following capabilities:

   Resource-based Routing:   RELOAD supports routing messages based
      solely on the name of the resource.  Such messages are delivered
      to a node that is responsible for that resource.  Both structured
      and unstructured overlays are supported, so the route may not be
      deterministic for all Topology Plug-ins.

   Node-based Routing:   RELOAD supports routing messages to a specific
      node in the overlay.

   Clients:   RELOAD supports requests from and to clients that do not
      participate in overlay routing.  The clients are located via
      either of the mechanisms described above.

   NAT Traversal:   RELOAD supports establishing and using connections
      between nodes separated by one or more NATs, including locating
      peers behind NATs for those overlays allowing/requiring it.

   Low State:   RELOAD's routing algorithms do not require significant
      state (i.e., state linear or greater in the number of outstanding
      messages that have passed through it) to be stored on intermediate
      peers.

   Routability in Unstable Topologies:   Overlay topology changes
      constantly in an overlay of moderate size due to the failure of
      individual nodes and links in the system.  RELOAD's routing allows
      peers to reroute messages when a failure is detected, and replies
      can be returned to the requesting node as long as the peers that
      originally forwarded the successful request do not fail before the
      response is returned.

   RELOAD's routing utilizes three basic mechanisms:

   Destination Lists:   While, in principle, it is possible to just
      inject a message into the overlay with a single Node-ID as the
      destination, RELOAD provides a source-routing capability in the
      form of "Destination Lists".  A Destination List provides a list
      of the nodes through which a message flows in order (i.e., it is
      loose source routed).  The minimal Destination List contains just
      a single value.

   Via Lists:   In order to allow responses to follow the same path as
      requests, each message also contains a "Via List", which is
      appended to by each node a message traverses.  This Via List can
      then be inverted and used as a Destination List for the response.





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   RouteQuery:   The RouteQuery method allows a node to query a peer for
      the next hop it will use to route a message.  This method is
      useful for diagnostics and for iterative routing (see
      Section 6.4.2.4).

   The basic routing mechanism that RELOAD uses is symmetric recursive.
   We will first describe symmetric recursive routing and then discuss
   its advantages in terms of the requirements discussed above.

   Symmetric recursive routing requires that a request message follow a
   path through the overlay to the destination: each peer forwards the
   message closer to its destination.  The return path of the response
   goes through the same nodes as the request (though it may also go
   through some new intermediate nodes due to topology changes).  Note
   that a failure on the reverse path caused by a topology change after
   the request was sent will be handled by the end-to-end retransmission
   of the response as described in Section 6.2.1.  For example, the
   following figure shows a message following a route from A to Z
   through B and X:

   A         B         X         Z
   -------------------------------

   ---------->
   Dest=Z
            ---------->
            Via=A
            Dest=Z
                      ---------->
                      Via=A,B
                      Dest=Z

                      <----------
                       Dest=X,B,A
            <----------
               Dest=B,A
   <----------
        Dest=A

   Note that this figure does not indicate whether A is a client or
   peer.  A forwards its request to B, and the response is returned to A
   in the same manner regardless of A's role in the overlay.

   This figure shows use of full Via Lists by intermediate peers B and
   X.  However, if B and/or X are willing to store state, then they may
   elect to truncate the lists and save the truncated information
   internally using the transaction ID as a key to allow it to be
   retrieved later.  Later, when the response message arrives, the



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   transaction ID would be used to recover the truncated information and
   return the response message along the path from which the request
   arrived.  This option requires a greater amount of state to be stored
   on intermediate peers, but saves a small amount of bandwidth and
   reduces the need for modifying the message en route.  Selection of
   this mode of operation is a choice for the individual peer; the
   techniques are interoperable even on a single message.  The figure
   below shows B using full Via Lists, but X truncating them to X1 and
   saving the state internally.

   A         B         X         Z
   -------------------------------

   ---------->
   Dest=Z
            ---------->
            Via=A
            Dest=Z
                      ---------->
                      Via=X1
                      Dest=Z

                      <----------
                        Dest=X,X1
              <----------
                 Dest=B,A
   <----------
        Dest=A

   As before, when B receives the message, B creates a Via List
   consisting of [A].  However, instead of sending [A, B], X creates an
   opaque ID X1 which maps internally to [A, B] (perhaps by being an
   encryption of [A, B]) and then forwards to Z with only X1 as the Via
   List.  When the response arrives at X, it maps X1 back to [A, B],
   then inverts it to produce the new Destination List [B, A], and
   finally routes it to B.

   RELOAD also supports a basic iterative "routing" mode, in which the
   intermediate peers merely return a response indicating the next hop,
   but do not actually forward the message to that next hop themselves.
   Iterative routing is implemented using the RouteQuery method (see
   Section 6.4.2.4), which requests this behavior.  Note that iterative
   routing is selected only by the initiating node.








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4.4.  Connectivity Management

   In order to provide efficient routing, a peer needs to maintain a set
   of direct connections to other peers in the Overlay Instance.  Due to
   the presence of NATs, these connections often cannot be formed
   directly.  Instead, we use the Attach request to establish a
   connection.  Attach uses Interactive Connectivity Establishment (ICE)
   [RFC5245] to establish the connection.  It is assumed that the reader
   is familiar with ICE.

   Say that peer A wishes to form a direct connection to peer B, either
   to join the overlay or to add more connections in its Routing Table.
   It gathers ICE candidates and packages them up in an Attach request,
   which it sends to B through usual overlay routing procedures.  B does
   its own candidate gathering and sends back a response with its
   candidates.  A and B then do ICE connectivity checks on the candidate
   pairs.  The result is a connection between A and B.  At this point, A
   and B MAY send messages directly between themselves without going
   through other overlay peers.  In other words, A and B are in each
   other's Connection Tables.  They MAY then execute an Update process,
   resulting in additions to each other's Routing Tables, and may then
   become able to route messages through each other to other overlay
   nodes.

   There are two cases where Attach is not used.  The first is when a
   peer is joining the overlay and is not connected to any peers.  In
   order to support this case, a small number of bootstrap nodes
   typically need to be publicly accessible so that new peers can
   directly connect to them.  Section 11 contains more detail on this.
   The second case is when a client connects to a peer at an arbitrary
   IP address, rather than to its responsible peer, as described in the
   second bullet point of Section 4.2.1.

   In general, a peer needs to maintain connections to all of the peers
   near it in the Overlay Instance and to enough other peers to have
   efficient routing (the details on what "enough" and "near" mean
   depend on the specific overlay).  If a peer cannot form a connection
   to some other peer, this is not necessarily a disaster; overlays can
   route correctly even without fully connected links.  However, a peer
   needs to try to maintain the specified Routing Table defined by the
   Topology Plug-in algorithm and needs to form new connections if it
   detects that it has fewer direct connections than specified by the
   algorithm.  This also implies that peers, in accordance with the
   Topology Plug-in algorithm, need to periodically verify that the
   connected peers are still alive and, if not, need to try to re-form
   the connections or form alternate ones.  See Section 10.7.4.3 for an
   example on how a specific overlay algorithm implements these
   constraints.



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4.5.  Overlay Algorithm Support

   The Topology Plug-in allows RELOAD to support a variety of overlay
   algorithms.  This specification defines a DHT based on Chord, which
   is mandatory to implement, but the base RELOAD protocol is designed
   to support a variety of overlay algorithms.  The information needed
   to implement this DHT is fully contained in this specification, but
   it is easier to understand if you are familiar with Chord-based
   [Chord] DHTs.  A nice tutorial can be found at [wikiChord].

4.5.1.  Support for Pluggable Overlay Algorithms

   RELOAD defines three methods for overlay maintenance: Join, Update,
   and Leave.  However, the contents of these messages, when they are
   sent, and their precise semantics are specified by the actual overlay
   algorithm, which is specified by configuration for all nodes in the
   overlay and thus is known to nodes before they attempt to join the
   overlay.  RELOAD merely provides a framework of commonly needed
   methods that provide uniformity of notation (and ease of debugging)
   for a variety of overlay algorithms.

4.5.2.  Joining, Leaving, and Maintenance Overview

   When a new peer wishes to join the Overlay Instance, it will need a
   Node-ID that it is allowed to use and a set of credentials which
   match that Node-ID.  When an enrollment server is used, the Node-ID
   used is the one found in the certificate received from the enrollment
   server.  The details of the joining procedure are defined by the
   overlay algorithm, but the general steps for joining an Overlay
   Instance are:

   o  Form connections to some other peers.

   o  Acquire the data values this peer is responsible for storing.

   o  Inform the other peers which were previously responsible for that
      data that this peer has taken over responsibility.

   The first thing the peer needs to do is to form a connection to some
   bootstrap node.  Because this is the first connection the peer makes,
   these nodes will need public IP addresses so that they can be
   connected to directly.  Once a peer has connected to one or more
   bootstrap nodes, it can form connections in the usual way, by routing
   Attach messages through the overlay to other nodes.  After a peer has
   connected to the overlay for the first time, it can cache the set of
   past adjacencies which have public IP addresses and can attempt to
   use them as future bootstrap nodes.  Note that this requires some




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   notion of which addresses are likely to be public as discussed in
   Section 9.

   After a peer has connected to a bootstrap node, it then needs to take
   up its appropriate place in the overlay.  This requires two major
   operations:

   o  Form connections to other peers in the overlay to populate its
      Routing Table.

   o  Get a copy of the data it is now responsible for storing, and
      assume responsibility for that data.

   The second operation is performed by contacting the Admitting Peer
   (AP), the node which is currently responsible for the relevant
   section of the overlay.

   The details of this operation depend mostly on the overlay algorithm
   involved, but a typical case would be:

   1.  JN sends a Join request to AP announcing its intention to join.

   2.  AP sends a Join response.

   3.  AP does a sequence of Stores to JN to give it the data it will
       need.

   4.  AP does Updates to JN and to other peers to tell them about its
       own Routing Table.  At this point, both JN and AP consider JN
       responsible for some section of the Overlay Instance.

   5.  JN makes its own connections to the appropriate peers in the
       Overlay Instance.

   After this process completes, JN is a full member of the Overlay
   Instance and can process Store/Fetch requests.

   Note that the first node is a special case.  When ordinary nodes
   cannot form connections to the bootstrap nodes, then they are not
   part of the overlay.  However, the first node in the overlay can
   obviously not connect to other nodes.  In order to support this case,
   potential first nodes (which can also initially serve as bootstrap
   nodes) need to somehow be instructed that they are the entire
   overlay, rather than part of an existing overlay (e.g., by comparing
   their IP address to the bootstrap IP addresses in the configuration
   file).

   Note that clients do not perform either of these operations.



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4.6.  First-Time Setup

   Previous sections addressed how RELOAD works after a node has
   connected.  This section provides an overview of how users get
   connected to the overlay for the first time.  RELOAD is designed so
   that users can start with the name of the overlay they wish to join
   and perhaps an account name and password, and can leverage these into
   having a working peer with minimal user intervention.  This helps
   avoid the problems that have been experienced with conventional SIP
   clients in which users need to manually configure a large number of
   settings.

4.6.1.  Initial Configuration

   In the first phase of the setup process, the user starts with the
   name of the overlay and uses it to download an initial set of overlay
   configuration parameters.  The node does a DNS SRV [RFC2782] lookup
   on the overlay name to get the address of a configuration server.  It
   can then connect to this server with HTTPS [RFC2818] to download a
   Configuration Document which contains the basic overlay configuration
   parameters as well as a set of bootstrap nodes which can be used to
   join the overlay.  The details of the relationships between names in
   the HTTPS certificates and the overlay names are described in
   Section 11.2.

   If a node already has the valid Configuration Document that it
   received by an out-of-band method, this step can be skipped.  Note
   that this out-of-band method needs to provide authentication and
   integrity, because the Configuration Document contains the trust
   anchors used by the overlay.

4.6.2.  Enrollment

   If the overlay is using centralized enrollment, then a user needs to
   acquire a certificate before joining the overlay.  The certificate
   attests both to the user's name within the overlay and to the
   Node-IDs which they are permitted to operate.  In this case, the
   Configuration Document will contain the address of an enrollment
   server which can be used to obtain such a certificate and will also
   contain the trust anchor, so this document must be retrieved securely
   (see Section 11.2).  The enrollment server may (and probably will)
   require some sort of account name for the user and a password before
   issuing the certificate.  The enrollment server's ability to ensure
   attackers cannot get a large number of certificates for the overlay
   is one of the cornerstones of RELOAD's security.






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4.6.3.  Diagnostics

   Significant advice around managing a RELOAD overlay and extensions
   for diagnostics are described in [P2P-DIAGNOSTICS].

5.  Application Support Overview

   RELOAD is not intended to be used alone, but rather as a substrate
   for other applications.  These applications can use RELOAD for a
   variety of purposes:

   o  To store data in the overlay and to retrieve data stored by other
      nodes.

   o  As a discovery mechanism for services such as TURN.

   o  To form direct connections which can be used to transmit
      application-level messages without using the overlay.

   This section provides an overview of these services.

5.1.  Data Storage

   RELOAD provides operations to Store and Fetch data.  Each location in
   the Overlay Instance is referenced by a Resource-ID.  However, each
   location may contain data elements corresponding to multiple Kinds
   (e.g., certificate and SIP registration).  Similarly, there may be
   multiple elements of a given Kind, as shown below:

                      +--------------------------------+
                      |            Resource-ID         |
                      |                                |
                      | +------------+  +------------+ |
                      | |   Kind 1   |  |   Kind 2   | |
                      | |            |  |            | |
                      | | +--------+ |  | +--------+ | |
                      | | | Value  | |  | | Value  | | |
                      | | +--------+ |  | +--------+ | |
                      | |            |  |            | |
                      | | +--------+ |  | +--------+ | |
                      | | | Value  | |  | | Value  | | |
                      | | +--------+ |  | +--------+ | |
                      | |            |  +------------+ |
                      | | +--------+ |                 |
                      | | | Value  | |                 |
                      | | +--------+ |                 |
                      | +------------+                 |
                      +--------------------------------+



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   Each Kind is identified by a Kind-ID, which is a code point either
   assigned by IANA or allocated out of a private range.  As part of the
   Kind definition, protocol designers may define constraints (such as
   limits on size) on the values which may be stored.  For many Kinds,
   the set may be restricted to a single value, while some sets may be
   allowed to contain multiple identical items, and others may have only
   unique items.  Note that a Kind may be employed by multiple usages,
   and new usages are encouraged to use previously defined Kinds where
   possible.  We define the following data models in this document,
   although other usages can define their own structures:

   single value:  There can be at most one item in the set, and any
      value overwrites the previous item.

   array:  Many values can be stored and addressed by a numeric index.

   dictionary:  The values stored are indexed by a key.  Often, this key
      is one of the values from the certificate of the peer sending the
      Store request.

   In order to protect stored data from tampering by other nodes, each
   stored value is individually digitally signed by the node which
   created it.  When a value is retrieved, the digital signature can be
   verified to detect tampering.  If the certificate used to verify the
   stored value signature expires, the value can no longer be retrieved
   (although it may not be immediately garbage collected by the storing
   node), and the creating node will need to store the value again if it
   desires that the stored value continue to be available.

5.1.1.  Storage Permissions

   A major issue in peer-to-peer storage networks is minimizing the
   burden of becoming a peer and, in particular, minimizing the amount
   of data which any peer needs to store for other nodes.  RELOAD
   addresses this issue by allowing any given node to store data only at
   a small number of locations in the overlay, with those locations
   being determined by the node's certificate.  When a peer uses a Store
   request to place data at a location authorized by its certificate, it
   signs that data with the private key that corresponds to its
   certificate.  Then the peer responsible for storing the data is able
   to verify that the peer issuing the request is authorized to make
   that request.  Each data Kind defines the exact rules for determining
   what certificate is appropriate.

   The most natural rule is that a certificate authorizes a user to
   store data keyed with their user name X.  Thus, only a user with a
   certificate for "alice@example.org" could write to that location in




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   the overlay (see Section 11.3).  However, other usages can define any
   rules they choose, including publicly writable values.

   The digital signature over the data serves two purposes.  First, it
   allows the peer responsible for storing the data to verify that this
   Store is authorized.  Second, it provides integrity for the data.
   The signature is saved along with the data value (or values) so that
   any reader can verify the integrity of the data.  Of course, the
   responsible peer can "lose" the value, but it cannot undetectably
   modify it.

   The size requirements of the data being stored in the overlay are
   variable.  For instance, a SIP AOR and voicemail differ widely in the
   storage size.  RELOAD leaves it to the usage and overlay
   configuration to limit size imbalances of various Kinds.

5.1.2.  Replication

   Replication in P2P overlays can be used to provide:

   persistence:  if the responsible peer crashes and/or if the storing
      peer leaves the overlay

   security:  to guard against DoS attacks by the responsible peer or
      routing attacks to that responsible peer

   load balancing:  to balance the load of queries for popular resources

   A variety of schemes are used in P2P overlays to achieve some of
   these goals.  Common techniques include replicating on neighbors of
   the responsible peer, randomly locating replicas around the overlay,
   and replicating along the path to the responsible peer.

   The core RELOAD specification does not specify a particular
   replication strategy.  Instead, the first level of replication
   strategies is determined by the overlay algorithm, which can base the
   replication strategy on its particular topology.  For example, Chord
   places replicas on successor peers, which will take over
   responsibility if the responsible peer fails [Chord].

   If additional replication is needed, for example, if data persistence
   is particularly important for a particular usage, then that usage may
   specify additional replication, such as implementing random
   replications by inserting a different well-known constant into the
   Resource Name used to store each replicated copy of the resource.
   Such replication strategies can be added independently of the
   underlying algorithm, and their usage can be determined based on the
   needs of the particular usage.



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5.2.  Usages

   By itself, the distributed storage layer provides only the
   infrastructure on which applications are built.  In order to do
   anything useful, a usage needs to be defined.  Each usage needs to
   specify several things:

   o  Register Kind-ID code points for any Kinds that the usage defines
      (Section 14.6).

   o  Define the data structure for each of the Kinds (the value member
      in Section 7.2).  If the data structure contains character
      strings, conversion rules between characters and the binary
      storage need to be specified.

   o  Define access control rules for each of the Kinds (Section 7.3).

   o  Define how the Resource Name is used to form the Resource-ID where
      each Kind is stored.

   o  Describe how values will be merged when a network partition is
      being healed.

   The Kinds defined by a usage may also be applied to other usages.
   However, a need for different parameters, such as a different access
   control model, would imply the need to create a new Kind.

5.3.  Service Discovery

   RELOAD does not currently define a generic service discovery
   algorithm as part of the base protocol, although a simplistic TURN-
   specific discovery mechanism is provided.  A variety of service
   discovery algorithms can be implemented as extensions to the base
   protocol, such as the service discovery algorithm ReDIR
   [opendht-sigcomm05] and [REDIR-RELOAD].

5.4.  Application Connectivity

   There is no requirement that a RELOAD Usage needs to use RELOAD's
   primitives for establishing its own communication if it already
   possesses its own means of establishing connections.  For example,
   one could design a RELOAD-based resource discovery protocol which
   used HTTP to retrieve the actual data.

   For more common situations, however, it is the overlay itself --
   rather than an external authority such as DNS -- which is used to
   establish a connection.  RELOAD provides connectivity to applications
   using the AppAttach method.  For example, if a P2PSIP node wishes to



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   establish a SIP dialog with another P2PSIP node, it will use
   AppAttach to establish a direct connection with the other node.  This
   new connection is separate from the peer protocol connection.  It is
   a dedicated DTLS or TLS flow used only for the SIP dialog.

6.  Overlay Management Protocol

   This section defines the basic protocols used to create, maintain,
   and use the RELOAD overlay network.  We start by defining the basic
   concept of how message destinations are interpreted when routing
   messages.  We then describe the symmetric recursive routing model,
   which is RELOAD's default routing algorithm.  Finally, we define the
   message structure and the messages used to join and maintain the
   overlay.

6.1.  Message Receipt and Forwarding

   When a node receives a message, it first examines the overlay,
   version, and other header fields to determine whether the message is
   one it can process.  If any of these are incorrect, as defined in
   Section 6.3.2, it is an error and the message MUST be discarded.  The
   peer SHOULD generate an appropriate error, but local policy can
   override this and cause the message to be silently dropped.

   Once the peer has determined that the message is correctly formatted
   (note that this does not include signature-checking on intermediate
   nodes as the message may be fragmented), it examines the first entry
   on the Destination List.  There are three possible cases here:

   o  The first entry on the Destination List is an ID for which the
      peer is responsible.  A peer is always responsible for the
      wildcard Node-ID.  Handling of this case is described in
      Section 6.1.1.

   o  The first entry on the Destination List is an ID for which another
      peer is responsible.  Handling of this case is described in
      Section 6.1.2.

   o  The first entry on the Destination List is an opaque ID that is
      being used for Destination List compression.  Handling of this
      case is described in Section 6.1.3.  Note that opaque IDs can be
      distinguished from Node-IDs and Resource-IDs on the wire as
      described in Section 6.3.2.2.

   These cases are handled as discussed below.






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6.1.1.  Responsible ID

   If the first entry on the Destination List is an ID for which the
   peer is responsible, there are several (mutually exclusive) subcases
   to consider.

   o  If the entry is a Resource-ID, then it MUST be the only entry on
      the Destination List.  If there are other entries, the message
      MUST be silently dropped.  Otherwise, the message is destined for
      this node, so the node MUST verify the signature as described in
      Section 7.1 and MUST pass it to the upper layers.  "Upper layers"
      is used here to mean the components above the "Overlay Link
      Service Boundary" line in the figure in Section 1.2.

   o  If the entry is a Node-ID which equals this node's Node-ID, then
      the message is destined for this node.  If it is the only entry on
      the Destination List, the message is destined for this node and so
      the node passes it to the upper layers.  Otherwise, the node
      removes the entry from the Destination List and repeats the
      routing process with the next entry on the Destination List.  If
      the message is a response and list compression was used, then the
      node first modifies the Destination List to reinsert the saved
      state, e.g., by unpacking any opaque IDs.

   o  If the entry is the wildcard Node-ID (all "1"s), the message is
      destined for this node, and the node passes the message to the
      upper layers.  A message with a wildcard Node-ID as its first
      entry is never forwarded; it is consumed locally.

   o  If the entry is a Node-ID which is not equal to this node, then
      the node MUST drop the message silently unless the Node-ID
      corresponds to a node which is directly connected to this node
      (i.e., a client).  In the latter case, the node MUST attempt to
      forward the message to the destination node as described in the
      next section (though this may fail for connectivity reasons,
      because the TTL has expired, or because of some other error.)

   Note that this process implies that in order to address a message to
   "the peer that controls region X", a sender sends to Resource-ID X,
   not Node-ID X.

6.1.2.  Other ID

   If the first entry on the Destination List is neither an opaque ID
   nor an ID the peer is responsible for, then the peer MUST forward the
   message towards that entry.  This means that it MUST select one of
   the peers to which it is connected and which is most likely to be
   responsible (according to the Topology Plug-in) for the first entry



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   on the Destination List.  For the CHORD-RELOAD topology, the routing
   to the most likely responsible node is explained in Section 10.3.  If
   the first entry on the Destination List is in the peer's Connection
   Table, the peer MUST forward the message to that peer directly.
   Otherwise, the peer consults the Routing Table to forward the
   message.

   Any intermediate peer which forwards a RELOAD request MUST ensure
   that if it receives a response to that message, the response can be
   routed back through the set of nodes through which the request
   passed.  The peer selects one of these approaches:

   o  The peer can add an entry to the Via List in the forwarding header
      that will enable it to determine the correct node.  This is done
      by appending to the Via List the Node-ID of the node from which
      the request was received.

   o  The peer can keep per-transaction state which will allow it to
      determine the correct node.

   As an example of the first strategy, consider an example with nodes
   A, B, C, D, and E.  If node D receives a message from node C with Via
   List [A, B], then D would forward to the next node E with Via List
   [A, B, C].  Now, if E wants to respond to the message, it reverses
   the Via List to produce the Destination List, resulting in
   [D, C, B, A].  When D forwards the response to C, the Destination
   List will contain [C, B, A].

   As an example of the second strategy, if node D receives a message
   from node C with transaction ID X (as assigned by A) and Via List
   [A, B], it could store [X, C] in its state database and forward the
   message with the Via List unchanged.  When D receives the response,
   it consults its state database for transaction ID X, determines that
   the request came from C, and forwards the response to C.

   Intermediate peers which modify the Via List are not required to
   simply add entries.  The only requirement is that the peer MUST be
   able to reconstruct the correct Destination List on the return route.
   RELOAD provides explicit support for this functionality in the form
   of opaque IDs, which can replace any number of Via List entries.

   For instance, in the above example, Node D might send E a Via List
   containing only the opaque ID I.  E would then use the Destination
   List [D, I] to send its return message.  When D processes this
   Destination List, it would detect that I is an opaque ID, recover the
   Via List [A, B, C], and reverse that to produce the correct
   Destination List [C, B, A] before sending it to C.  This feature is
   called "list compression".  Possibilities for an opaque ID include a



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   compressed and/or encrypted version of the original Via List and an
   index into a state database containing the original Via List, but the
   details are a local matter.

   No matter what mechanism for storing Via List state is used, if an
   intermediate peer exits the overlay, then on the return trip the
   message cannot be forwarded and will be dropped.  The ordinary
   timeout and retransmission mechanisms provide stability over this
   type of failure.

   Note that if an intermediate peer retains per-transaction state
   instead of modifying the Via List, it needs some mechanism for timing
   out that state; otherwise, its state database will grow without
   bound.  Whatever algorithm is used, unless a FORWARD_CRITICAL
   forwarding option (Section 6.3.2.3) or an overlay configuration
   option explicitly indicates this state is not needed, the state MUST
   be maintained for at least the value of the overlay-reliability-timer
   configuration parameter and MAY be kept longer.  Future extensions,
   such as [P2PSIP-RELAY], may define mechanisms for determining when
   this state does not need to be retained.

   There is no requirement to ensure that a request issued after the
   receipt of a response follows the same path as the response.  As a
   consequence, there is no requirement to use either of the mechanisms
   described above (Via List or state retention) when processing a
   response message.

   A node receiving a request from another node MUST ensure that any
   response to that request exits that node with a Destination List
   equal to the concatenation of the Node-ID of the node from which the
   request was received with the Via List in the original request.  The
   intermediate node normally learns the Node-ID that the other node is
   using via an Attach, but a node using a certificate with a single
   Node-ID MAY elect not to send an Attach (see Section 4.2.1, bullet
   2).  If a node with a certificate with multiple Node-IDs attempts to
   route a message other than a Ping or Attach through a node without
   performing an Attach, the receiving node MUST reject the request with
   an Error_Forbidden error.  The node MUST implement support for
   returning responses to a Ping or Attach request made by a Joining
   Node Attaching to its responsible peer.

6.1.3.  Opaque ID

   If the first entry on the Destination List is an opaque ID (e.g., a
   compressed Via List), the peer MUST replace the entry with the
   original Via List that it replaced and then re-examine the
   Destination List to determine which of the three cases in Section 6.1
   now applies.



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6.2.  Symmetric Recursive Routing

   This section defines RELOAD's Symmetric Recursive Routing algorithm,
   which is the default algorithm used by nodes to route messages
   through the overlay.  All implementations MUST implement this routing
   algorithm.  An overlay MAY be configured to use alternative routing
   algorithms, and alternative routing algorithms MAY be selected on a
   per-message basis.  That is, a node in an overlay which supports
   Symmetric Recursive Routing and some other routing algorithm called
   XXX might use Symmetric Recursive Routing some of the time and XXX at
   other times.

6.2.1.  Request Origination

   In order to originate a message to a given Node-ID or Resource-ID, a
   node MUST construct an appropriate Destination List.  The simplest
   such Destination List is a single entry containing the Node-ID or
   Resource-ID.  The resulting message MUST be forwarded to its
   destination via the normal overlay routing mechanisms.  The node MAY
   also construct a more complicated Destination List for source
   routing.

   Once the message is constructed, the node sends the message to an
   adjacent peer.  If the first entry on the Destination List is
   directly connected, then the message MUST be routed down that
   connection.  Otherwise, the Topology Plug-in MUST be consulted to
   determine the appropriate next hop.

   Parallel requests for a resource are a common solution to improve
   reliability in the face of churn or subversive peers.  Parallel
   searches for usage-specified replicas are managed by the usage layer,
   for instance, by having the usage store data at multiple
   Resource-IDs, with the requesting node sending requests to each of
   those Resource-IDs.  However, a single request MAY also be routed
   through multiple adjacent peers, even when they are known to be
   suboptimal, to improve reliability [vulnerabilities-acsac04].  Such
   parallel searches MAY be specified by the Topology Plug-in, in which
   case it would return multiple next hops and the request would be
   routed to all of them.

   Because messages can be lost in transit through the overlay, RELOAD
   incorporates an end-to-end reliability mechanism.  When an
   originating node transmits a request, it MUST set a timer to the
   current overlay-reliability-timer.  If a response has not been
   received when the timer fires, the request MUST be retransmitted with
   the same transaction identifier.  The request MAY be retransmitted up
   to 4 times, for a total of 5 messages.  After the timer for the fifth
   transmission fires, the message MUST be considered to have failed.



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   Although the originating node will be doing both end-to-end and hop-
   by-hop retransmissions, the end-by-end retransmission procedure is
   not followed by intermediate nodes.  They follow the hop-by-hop
   reliability procedure described in Section 6.6.3.

   The above algorithm can result in multiple requests being delivered
   to a node.  Receiving nodes MUST generate semantically equivalent
   responses to retransmissions of the same request (this can be
   determined by the transaction ID) if the request is received within
   the maximum request lifetime (15 seconds).  For some requests (e.g.,
   Fetch), this can be accomplished merely by processing the request
   again.  For other requests (e.g., Store), it may be necessary to
   maintain state for the duration of the request lifetime.

6.2.2.  Response Origination

   When a peer sends a response to a request using this routing
   algorithm, it MUST construct the Destination List by reversing the
   order of the entries on the Via List.  This has the result that the
   response traverses the same peers as the request traversed, except in
   reverse order (symmetric routing) and possibly with extra nodes
   (loose routing).

6.3.  Message Structure

   RELOAD is a message-oriented request/response protocol.  The messages
   are encoded using binary fields.  All integers are represented in
   network byte order.  The general philosophy behind the design was to
   use Type, Length, Value (TLV) fields to allow for extensibility.
   However, for the parts of a structure that were required in all
   messages, we just define these in a fixed position, as adding a type
   and length for them is unnecessary and would only increase bandwidth
   and introduce new potential interoperability issues.

   Each message has three parts, which are concatenated, as shown below:

     +-------------------------+
     |    Forwarding Header    |
     +-------------------------+
     |    Message Contents     |
     +-------------------------+
     |     Security Block      |
     +-------------------------+








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   The contents of these parts are as follows:

   Forwarding Header:  Each message has a generic header which is used
      to forward the message between peers and to its final destination.
      This header is the only information that an intermediate peer
      (i.e., one that is not the target of a message) needs to examine.
      Section 6.3.2 describes the format of this part.

   Message Contents:  The message being delivered between the peers.
      From the perspective of the forwarding layer, the contents are
      opaque; however, they are interpreted by the higher layers.
      Section 6.3.3 describes the format of this part.

   Security Block:  A security block containing certificates and a
      digital signature over the "Message Contents" section.  Note that
      this signature can be computed without parsing the message
      contents.  All messages MUST be signed by their originator.
      Section 6.3.4 describes the format of this part.

6.3.1.  Presentation Language

   The structures defined in this document are defined using a C-like
   syntax based on the presentation language used to define TLS
   [RFC5246].  Advantages of this style include:

   o  It is familiar enough that most readers can grasp it quickly.

   o  The ability to define nested structures allows a separation
      between high-level and low-level message structures.

   o  It has a straightforward wire encoding that allows quick
      implementation, but the structures can be comprehended without
      knowing the encoding.

   o  It is possible to mechanically compile encoders and decoders.

   Several idiosyncrasies of this language are worth noting:

   o  All lengths are denoted in bytes, not objects.

   o  Variable-length values are denoted like arrays, with angle
      brackets.

   o  "select" is used to indicate variant structures.

   For instance, "uint16 array<0..2^8-2>;" represents up to 254 bytes,
   which corresponds to up to 127 values of two bytes (16 bits) each.




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   A repetitive structure member shares a common notation with a member
   containing a variable-length block of data.  The latter always starts
   with "opaque", whereas the former does not.  For instance, the
   following denotes a variable block of data:

                     opaque data<0..2^32-1>;

   whereas the following denotes a list of 0, 1, or more instances of
   the Name element:

                     Name names<0..2^32-1>;

6.3.1.1.  Common Definitions

   This section provides an introduction to the presentation language
   used throughout RELOAD.

   An enum represents an enumerated type.  The values associated with
   each possibility are represented in parentheses, and the maximum
   value is represented as a nameless value, for purposes of describing
   the width of the containing integral type.  For instance, Boolean
   represents a true or false:

         enum { false(0), true(1), (255) } Boolean;

   A boolean value is either a 1 or a 0.  The max value of 255 indicates
   that this is represented as a single byte on the wire.

   The NodeId, shown below, represents a single Node-ID.

             typedef opaque       NodeId[NodeIdLength];

   A NodeId is a fixed-length structure represented as a series of
   bytes, with the most significant byte first.  The length is set on a
   per-overlay basis within the range of 16-20 bytes (128 to 160 bits).
   (See Section 11.1 for how NodeIdLength is set.)  Note that the use of
   "typedef" here is an extension to the TLS language, but its meaning
   should be relatively obvious.  Also note that the [ size ] syntax
   defines a fixed-length element that does not include the length of
   the element in the on-the-wire encoding.

   A ResourceId, shown below, represents a single Resource-ID.

             typedef opaque       ResourceId<0..2^8-1>;

   Like a NodeId, a ResourceId is an opaque string of bytes, but unlike
   NodeIds, ResourceIds are variable length, up to 254 bytes (2040 bits)
   in length.  On the wire, each ResourceId is preceded by a single



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   length byte (allowing lengths up to 255 bytes).  Thus, the 3-byte
   value "FOO" would be encoded as: 03 46 4f 4f.  Note the < range >
   syntax defines a variable length element that includes the length of
   the element in the on-the-wire encoding.  The number of bytes to
   encode the length on the wire is derived by range; i.e., it is the
   minimum number of bytes which can encode the largest range value.

   A more complicated example is IpAddressPort, which represents a
   network address and can be used to carry either an IPv6 or IPv4
   address:

        enum { invalidAddressType(0), ipv4_address(1), ipv6_address(2),
             (255) } AddressType;

        struct {
          uint32                  addr;
          uint16                  port;
        } IPv4AddrPort;

        struct {
          uint128                 addr;
          uint16                  port;
        } IPv6AddrPort;

        struct {
          AddressType             type;
          uint8                   length;

          select (type) {
            case ipv4_address:
               IPv4AddrPort       v4addr_port;

            case ipv6_address:
               IPv6AddrPort       v6addr_port;

            /* This structure can be extended */
          };
        } IpAddressPort;

   The first two fields in the structure are the same no matter what
   kind of address is being represented:

   type:  The type of address (IPv4 or IPv6).

   length:  The length of the rest of the structure.






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   By having the type and the length appear at the beginning of the
   structure regardless of the kind of address being represented, an
   implementation which does not understand new address type X can still
   parse the IpAddressPort field and then discard it if it is not
   needed.

   The rest of the IpAddressPort structure is either an IPv4AddrPort or
   an IPv6AddrPort.  Both of these simply consist of an address
   represented as an integer and a 16-bit port.  As an example, here is
   the wire representation of the IPv4 address "192.0.2.1" with port
   "6084".

             01           ; type    = IPv4
             06           ; length  = 6
             c0 00 02 01  ; address = 192.0.2.1
             17 c4        ; port    = 6084

   Unless a given structure that uses a select explicitly allows for
   unknown types in the select, any unknown type SHOULD be treated as a
   parsing error, and the whole message SHOULD be discarded with no
   response.

6.3.2.  Forwarding Header

   The forwarding header is defined as a ForwardingHeader structure, as
   shown below.

        struct {
          uint32             relo_token;
          uint32             overlay;
          uint16             configuration_sequence;
          uint8              version;
          uint8              ttl;
          uint32             fragment;
          uint32             length;
          uint64             transaction_id;
          uint32             max_response_length;
          uint16             via_list_length;
          uint16             destination_list_length;
          uint16             options_length;
          Destination        via_list[via_list_length];
          Destination        destination_list
                               [destination_list_length];
          ForwardingOption   options[options_length];
        } ForwardingHeader;






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   The contents of the structure are:

   relo_token:  The first four bytes identify this message as a RELOAD
      message.  This field MUST contain the value 0xd2454c4f (the string
      "RELO" with the high bit of the first byte set).

   overlay:  The 32-bit checksum/hash of the overlay being used.  This
      MUST be formed by taking the lower 32 bits of the SHA-1 [RFC3174]
      hash of the overlay name.  The purpose of this field is to allow
      nodes to participate in multiple overlays and to detect accidental
      misconfiguration.  This is not a security-critical function.  The
      overlay name MUST consist of a sequence of characters that would
      be allowable as a DNS name.  Specifically, as it is used in a DNS
      lookup, it will need to be compliant with the grammar for the
      domain as specified in Section 2.3.1 of [RFC1035].

   configuration_sequence:  The sequence number of the configuration
      file.  See Section 6.3.2.1 for details.

   version:  The version of the RELOAD protocol being used times 10.
      RELOAD version numbers are fixed-point decimal numbers between
      fixed-point integer between 0.1 and 25.4.  This document describes
      version 1.0, with a value of 0x0a.  (Note that versions used prior
      to the publication of this RFC used version number 0.1.)  Nodes
      MUST reject messages with other versions.

   ttl:  An 8-bit field indicating the number of iterations, or hops, a
      message can experience before it is discarded.  The TTL (time-to-
      live) value MUST be decremented by one at every hop along the
      route the message traverses just before transmission.  If a
      received message has a TTL of 0 and the message is not destined
      for the receiving node, then the message MUST NOT be propagated
      further, and an Error_TTL_Exceeded error should be generated.  The
      initial value of the TTL SHOULD be 100 and MUST NOT exceed 100
      unless defined otherwise by the overlay configuration.
      Implementations which receive messages with a TTL greater than the
      current value of initial-ttl (or the default of 100) MUST discard
      the message and send an Error_TTL_Exceeded error.

   fragment:  This field is used to handle fragmentation.  The high bit
      (0x80000000) MUST be set for historical reasons.  If the next bit
      (0x40000000) is set to 1, it indicates that this is the last (or
      only) fragment.  The next six bits (0x20000000 through 0x01000000)
      are reserved and SHOULD be set to zero.  The remainder of the
      field is used to indicate the fragment offset; see Section 6.7 for
      details.





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   length:  The count in bytes of the size of the message, including the
      header, after the eventual fragmentation.

   transaction_id:  A unique 64-bit number that identifies this
      transaction and also allows receivers to disambiguate transactions
      which are otherwise identical.  In order to provide a high
      probability that transaction IDs are unique, they MUST be randomly
      generated.  Responses use the same transaction ID as the request
      to which they correspond.  Transaction IDs are also used for
      fragment reassembly.  See Section 6.7 for details.

   max_response_length:  The maximum size in bytes of a response.  This
      is used by requesting nodes to avoid receiving (unexpected) very
      large responses.  If this value is non-zero, responding peers MUST
      check that any response would not exceed it and if so generate an
      Error_Incompatible_with_Overlay value.  This value SHOULD be set
      to zero for responses.

   via_list_length:  The length of the Via List in bytes.  Note that in
      this field and the following two length fields, we depart from the
      usual variable-length convention of having the length immediately
      precede the value, in order to make it easier for hardware
      decoding engines to quickly determine the length of the header.

   destination_list_length:  The length of the Destination List in
      bytes.

   options_length:  The length of the header options in bytes.

   via_list:  The via_list contains the sequence of destinations through
      which the message has passed.  The via_list starts out empty and
      grows as the message traverses each peer.  In stateless cases, the
      previous hop that the message is from is appended to the Via List
      as specified in Section 6.1.2.

   destination_list:  The destination_list contains a sequence of
      destinations through which the message should pass.  The
      Destination List is constructed by the message originator.  The
      first element on the Destination List is where the message goes
      next.  Generally, the list shrinks as the message traverses each
      listed peer, though if list compression is used, this may not be
      true.

   options:  Contains a series of ForwardingOption entries.  See
      Section 6.3.2.3.






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6.3.2.1.  Processing Configuration Sequence Numbers

   In order to be part of the overlay, a node MUST have a copy of the
   overlay Configuration Document.  In order to allow for configuration
   document changes, each version of the Configuration Document MUST
   contain a sequence number which MUST be monotonically increasing mod
   65535.  Because the sequence number may, in principle, wrap, greater
   than or less than are interpreted by modulo arithmetic as in TCP.

   When a destination node receives a request, it MUST check that the
   configuration_sequence field is equal to its own configuration
   sequence number.  If they do not match, the node MUST generate an
   error, either Error_Config_Too_Old or Error_Config_Too_New.  In
   addition, if the configuration file in the request is too old, the
   node MUST generate a ConfigUpdate message to update the requesting
   node.  This allows new Configuration Documents to propagate quickly
   throughout the system.  The one exception to this rule is that if the
   configuration_sequence field is equal to 65535 and the message type
   is ConfigUpdate, then the message MUST be accepted regardless of the
   receiving node's configuration sequence number.  Since 65535 is a
   special value, peers sending a new configuration when the
   configuration sequence is currently 65534 MUST set the configuration
   sequence number to 0 when they send a new configuration.




























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6.3.2.2.  Destination and Via Lists

   The Destination List and Via List are sequences of Destination
   values:

     enum { invalidDestinationType(0), node(1), resource(2),
            opaque_id_type(3), /* 128-255 not allowed */ (255) }
          DestinationType;

     select (destination_type) {
      case node:
             NodeId               node_id;

      case resource:
             ResourceId           resource_id;

      case opaque_id_type:
             opaque               opaque_id<0..2^8-1>;

          /* This structure may be extended with new types */
     } DestinationData;

     struct {
        DestinationType         type;
        uint8                   length;
        DestinationData         destination_data;
     } Destination;

     struct {
        uint16               opaque_id; /* Top bit MUST be 1 */
     } Destination;

   If the destination structure is a 16-bit integer, then the first bit
   MUST be set to 1, and it MUST be treated as if it were a full
   structure with a DestinationType of opaque_id_type and an opaque_id
   that was 2 bytes long with the value of the 16-bit integer.  If the
   destination structure starts with DestinationType, then the first bit
   MUST be set to 0, and the destination structure must use a TLV
   structure with the following contents:

   type
      The type of the DestinationData Payload Data Unit (PDU).  It may
      be one of "node", "resource", or "opaque_id_type".

   length
      The length of the destination_data.





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   destination_data
      The destination value itself, which is an encoded DestinationData
      structure that depends on the value of "type".

   Note that the destination structure encodes a Type, Length, Value.
   The Length field specifies the length of the DestinationData values,
   which allows the addition of new DestinationTypes.  It also allows an
   implementation which does not understand a given DestinationType to
   skip over it.

   A DestinationData can be one of three types:

   node
      A Node-ID.

   opaque
      A compressed list of Node-IDs and an eventual Resource-ID.
      Because this value has been compressed by one of the peers, it is
      meaningful only to that peer and cannot be decoded by other peers.
      Thus, it is represented as an opaque string.

   resource
      The Resource-ID of the resource which is desired.  This type MUST
      appear only in the final location of a Destination List and MUST
      NOT appear in a Via List.  It is meaningless to try to route
      through a resource.

   One possible encoding of the 16-bit integer version as an opaque
   identifier is to encode an index into a Connection Table.  To avoid
   misrouting responses in the event a response is delayed and the
   Connection Table entry has changed, the identifier SHOULD be split
   between an index and a generation counter for that index.  When a
   Node first joins the overlay, the generation counters SHOULD be
   initialized to random values.  An implementation MAY use 12 bits for
   the Connection Table index and 3 bits for the generation counter.
   (Note that this does not suggest a 4096-entry Connection Table for
   every peer, only the ability to encode for a larger Connection
   Table.)  When a Connection Table slot is used for a new connection,
   the generation counter is incremented (with wrapping).  Connection
   Table slots are used on a rotating basis to maximize the time
   interval between uses of the same slot for different connections.
   When routing a message to an entry in the Destination List encoding a
   Connection Table entry, the peer MUST confirm that the generation
   counter matches the current generation counter of that index before
   forwarding the message.  If it does not match, the message MUST be
   silently dropped.





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6.3.2.3.  Forwarding Option

   The Forwarding header can be extended with forwarding header options,
   which are a series of ForwardingOption structures:

    enum { invalidForwardingOptionType(0), (255) }
      ForwardingOptionType;

    struct {
      ForwardingOptionType      type;
      uint8                     flags;
      uint16                    length;
      select (type) {
            /* This type may be extended */
      };
    } ForwardingOption;

   Each ForwardingOption consists of the following values:

   type
      The type of the option.  This structure allows for unknown options
      types.

   flags
      Three flags are defined: FORWARD_CRITICAL(0x01),
      DESTINATION_CRITICAL(0x02), and RESPONSE_COPY(0x04).  These flags
      MUST NOT be set in a response.  If the FORWARD_CRITICAL flag is
      set, any peer that would forward the message but does not
      understand this option MUST reject the request with an
      Error_Unsupported_Forwarding_Option error response.  If the
      DESTINATION_CRITICAL flag is set, any node that generates a
      response to the message but does not understand the forwarding
      option MUST reject the request with an
      Error_Unsupported_Forwarding_Option error response.  If the
      RESPONSE_COPY flag is set, any node generating a response MUST
      copy the option from the request to the response except that the
      RESPONSE_COPY, FORWARD_CRITICAL, and DESTINATION_CRITICAL flags
      MUST be cleared.

   length
      The length of the rest of the structure.  Note that a 0 length may
      be reasonable if the mere presence of the option is meaningful and
      no value is required.

   option
      The option value.





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6.3.3.  Message Contents Format

   The second major part of a RELOAD message is the contents part, which
   is defined by MessageContents:

   enum { invalidMessageExtensionType(0),
          (2^16-1) } MessageExtensionType;

   struct {
     MessageExtensionType  type;
     Boolean               critical;
     opaque                extension_contents<0..2^32-1>;
   } MessageExtension;

   struct {
     uint16                 message_code;
     opaque                 message_body<0..2^32-1>;
     MessageExtension       extensions<0..2^32-1>;
   } MessageContents;


   The contents of this structure are as follows:

   message_code
      This indicates the message that is being sent.  The code space is
      broken up as follows:

      0x0  Invalid Message Code.  This code will never be assigned.

      0x1 .. 0x7FFF  Requests and responses.  These code points are
         always paired, with requests being an odd value and the
         corresponding response being the request code plus 1.  Thus,
         "probe_request" (the Probe request) has the value 1 and
         "probe_answer" (the Probe response) has the value 2

      0x8000 .. 0xFFFE  Reserved

      0xFFFF  Error

      The message codes are defined in Section 14.8.

   message_body
      The message body itself, represented as a variable-length string
      of bytes.  The bytes themselves are dependent on the code value.
      See the sections describing the various RELOAD methods (Join,
      Update, Attach, Store, Fetch, etc.) for the definitions of the
      payload contents.




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   extensions
      Extensions to the message.  Currently no extensions are defined,
      but new extensions can be defined by the process described in
      Section 14.14.

   All extensions have the following form:

   type
      The extension type.

   critical
      Whether this extension needs to be understood in order to process
      the message.  If critical = True and the recipient does not
      understand the message, it MUST generate an
      Error_Unknown_Extension error.  If critical = False, the recipient
      MAY choose to process the message even if it does not understand
      the extension.

   extension_contents
      The contents of the extension (which are extension dependent).

   The subsections 6.4.2, 6.5, and 7 describe structures that are
   inserted inside the message_body member, depending on the value of
   the message_code value.  For example, a message_code value of
   join_req means that the structure named JoinReq is inserted inside
   message_body.  This document does not contain a mapping between
   message_code values and structure names, as the conversion between
   the two is obvious.

   Similarly, this document uses the name of the structure without the
   "Req" or "Ans" suffix to mean the execution of a transaction
   consisting of the matching request and answer.  For example, when the
   text says "perform an Attach", it must be understood as performing a
   transaction composed of an AttachReq and an AttachAns.

6.3.3.1.  Response Codes and Response Errors

   A node processing a request MUST return its status in the
   message_code field.  If the request was a success, then the message
   code MUST be set to the response code that matches the request (i.e.,
   the next code up).  The response payload is then as defined in the
   request/response descriptions.

   If the request has failed, then the message code MUST be set to
   0xffff (error) and the payload MUST be an error_response message, as
   shown below.





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   When the message code is 0xFFFF, the payload MUST be an
   ErrorResponse:

         public struct {
           uint16             error_code;
           opaque             error_info<0..2^16-1>;
         } ErrorResponse;

   The contents of this structure are as follows:

   error_code
      A numeric error code indicating the error that occurred.

   error_info
      An optional arbitrary byte string.  Unless otherwise specified,
      this will be a UTF-8 text string that provides further information
      about what went wrong.  Developers are encouraged to include
      enough diagnostic information to be useful in error_info.  The
      specific text to be used and any relevant language or encoding
      thereof is left to the implementation.

   The following error code values are defined.  The numeric values for
   these are defined in Section 14.9.

   Error_Forbidden
      The requesting node does not have permission to make this request.

   Error_Not_Found
      The resource or node cannot be found or does not exist.

   Error_Request_Timeout
      A response to the request has not been received in a suitable
      amount of time.  The requesting node MAY resend the request at a
      later time.

   Error_Data_Too_Old
      A store cannot be completed because the storage_time precedes the
      existing value.

   Error_Data_Too_Large
      A store cannot be completed because the requested object exceeds
      the size limits for that Kind.

   Error_Generation_Counter_Too_Low
      A store cannot be completed because the generation counter
      precedes the existing value.





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   Error_Incompatible_with_Overlay
      A peer receiving the request is using a different overlay, overlay
      algorithm, or hash algorithm, or some other parameter that is
      inconsistent with the overlay configuration.

   Error_Unsupported_Forwarding_Option
      A node received the request with a forwarding options flagged as
      critical, but the node does not support this option.  See
      Section 6.3.2.3.

   Error_TTL_Exceeded
      A peer received the request in which the TTL was decremented to
      zero.  See Section 6.3.2.

   Error_Message_Too_Large
      A peer received a request that was too large.  See Section 6.6.

   Error_Response_Too_Large
      A node would have generated a response that is too large per the
      max_response_length field.

   Error_Config_Too_Old
      A destination node received a request with a configuration
      sequence that is too old.  See Section 6.3.2.1.

   Error_Config_Too_New
      A destination node received a request with a configuration
      sequence that is too new.  See Section 6.3.2.1.

   Error_Unknown_Kind
      A destination peer received a request with an unknown Kind-ID.
      See Section 7.4.1.2.

   Error_In_Progress
      An Attach to this peer is already in progress.  See
      Section 6.5.1.2.

   Error_Unknown_Extension
      A destination node received a request with an unknown extension.

   Error_Invalid_Message
      Something about this message is invalid, but it does not fit the
      other error codes.  When this message is sent, implementations
      SHOULD provide some meaningful description in error_info to aid in
      debugging.






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   Error_Exp_A
      For the purposes of experimentation.  It is not meant for vendor-
      specific use of any sort and MUST NOT be used for operational
      deployments.

   Error_Exp_B
      For the purposes of experimentation.  It is not meant for vendor-
      specific use of any sort and MUST NOT be used for operational
      deployments.

6.3.4.  Security Block

   The third part of a RELOAD message is the security block.  The
   security block is represented by a SecurityBlock structure:

   struct {
      CertificateType     type;   // From RFC 6091
      opaque              certificate<0..2^16-1>;
   } GenericCertificate;

   struct {
      GenericCertificate certificates<0..2^16-1>;
      Signature          signature;
   } SecurityBlock;

   The contents of this structure are:

   certificates
      A bucket of certificates.

   signature
      A signature.

   The certificates bucket SHOULD contain all the certificates necessary
   to verify every signature in both the message and the internal
   message objects, except for those certificates in a root-cert element
   of the current configuration file.  This is the only location in the
   message which contains certificates, thus allowing only a single copy
   of each certificate to be sent.  In systems that have an alternative
   certificate distribution mechanism, some certificates MAY be omitted.
   However, unless an alternative mechanism for immediately generating
   certificates, such as shared secret security (Section 13.4) is used,
   implementers MUST include all referenced certificates.

   NOTE TO IMPLEMENTERS: This requirement implies that a peer storing
   data is obligated to retain certificates for the data that it holds.





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   Each certificate is represented by a GenericCertificate structure,
   which has the following contents:

   type
      The type of the certificate, as defined in [RFC6091].  Only the
      use of X.509 certificates is defined in this document.

   certificate
      The encoded version of the certificate.  For X.509 certificates,
      it is the Distinguished Encoding Rules (DER) form.

   The signature is computed over the payload and parts of the
   forwarding header.  In case of a Store, the payload MUST contain an
   additional signature computed as described in Section 7.1.  All
   signatures MUST be formatted using the Signature element.  This
   element is also used in other contexts where signatures are needed.
   The input structure to the signature computation MAY vary depending
   on the data element being signed.

     enum { invalidSignerIdentityType(0),
            cert_hash(1), cert_hash_node_id(2),
            none(3)
            (255) } SignerIdentityType;

     struct {
       select (identity_type) {

         case cert_hash;
           HashAlgorithm      hash_alg;              // From TLS
           opaque             certificate_hash<0..2^8-1>;

         case cert_hash_node_id:
           HashAlgorithm      hash_alg;              // From TLS
           opaque             certificate_node_id_hash<0..2^8-1>;

         case none:
           /* empty */
         /* This structure may be extended with new types if necessary*/
       };
     } SignerIdentityValue;

     struct {
       SignerIdentityType     identity_type;
       uint16                 length;
       SignerIdentityValue    identity[SignerIdentity.length];
     } SignerIdentity;





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     struct {
        SignatureAndHashAlgorithm     algorithm;   // From TLS
        SignerIdentity                identity;
        opaque                        signature_value<0..2^16-1>;
     } Signature;

   The Signature construct contains the following values:

   algorithm
      The signature algorithm in use.  The algorithm definitions are
      found in the IANA TLS SignatureAlgorithm and HashAlgorithm
      registries.  All implementations MUST support RSASSA-PKCS1-v1_5
      [RFC3447] signatures with SHA-256 hashes [RFC6234].

   identity
      The identity, as defined in the two paragraphs following this
      list, used to form the signature.

   signature_value
      The value of the signature.

      Note that storage operations allow for special values of algorithm
      and identity.  See the Store Request definition (Section 7.4.1.1)
      and the Fetch Response definition (Section 7.4.2.2).

   There are two permitted identity formats, one for a certificate with
   only one Node-ID and one for a certificate with multiple Node-IDs.
   In the first case, the cert_hash type MUST be used.  The hash_alg
   field is used to indicate the algorithm used to produce the hash.
   The certificate_hash contains the hash of the certificate object
   (i.e., the DER-encoded certificate).

   In the second case, the cert_hash_node_id type MUST be used.  The
   hash_alg is as in cert_hash, but the cert_hash_node_id is computed
   over the NodeId used to sign concatenated with the certificate; i.e.,
   H(NodeId || certificate).  The NodeId is represented without any
   framing or length fields, as simple raw bytes.  This is safe because
   NodeIds are a fixed length for a given overlay.

   For signatures over messages, the input to the signature is computed
   over:

      overlay || transaction_id || MessageContents || SignerIdentity

   where overlay and transaction_id come from the forwarding header and
   || indicates concatenation.





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   The input to signatures over data values is different and is
   described in Section 7.1.

   All RELOAD messages MUST be signed.  Intermediate nodes do not verify
   signatures.  Upon receipt (and fragment reassembly, if needed), the
   destination node MUST verify the signature and the authorizing
   certificate.  If the signature fails, the implementation SHOULD
   simply drop the message and MUST NOT process it.  This check provides
   a minimal level of assurance that the sending node is a valid part of
   the overlay, and it provides cryptographic authentication of the
   sending node.  In addition, responses MUST be checked as follows by
   the requesting node:

   1.  The response to a message sent to a Node-ID MUST have been sent
       by that Node-ID unless the response has been sent to the wildcard
       Node-ID.

   2.  The response to a message sent to a Resource-ID MUST have been
       sent by a Node-ID which is at least as close to the target
       Resource-ID as any node in the requesting node's Neighbor Table.

   The second condition serves as a primitive check for responses from
   wildly wrong nodes but is not a complete check.  Note that in periods
   of churn, it is possible for the requesting node to obtain a closer
   neighbor while the request is outstanding.  This will cause the
   response to be rejected and the request to be retransmitted.

   In addition, some methods (especially Store) have additional
   authentication requirements, which are described in the sections
   covering those methods.

6.4.  Overlay Topology

   As discussed in previous sections, RELOAD defines a default overlay
   topology (CHORD-RELOAD) but allows for other topologies through the
   use of Topology Plug-ins.  This section describes the requirements
   for new Topology Plug-ins and the methods that RELOAD provides for
   overlay topology maintenance.

6.4.1.  Topology Plug-in Requirements

   When specifying a new overlay algorithm, at least the following MUST
   be described:

   o  Joining procedures, including the contents of the Join message.






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   o  Stabilization procedures, including the contents of the Update
      message, the frequency of topology probes and keepalives, and the
      mechanism used to detect when peers have disconnected.

   o  Exit procedures, including the contents of the Leave message.

   o  The length of the Resource-IDs and for DHTs the hash algorithm to
      compute the hash of an identifier.

   o  The procedures that peers use to route messages.

   o  The replication strategy used to ensure data redundancy.

   All overlay algorithms MUST specify maintenance procedures that send
   Updates to clients and peers that have established connections to the
   peer responsible for a particular ID when the responsibility for that
   ID changes.  Because tracking this information is difficult, overlay
   algorithms MAY simply specify that an Update is sent to all members
   of the Connection Table whenever the range of IDs for which the peer
   is responsible changes.

6.4.2.  Methods and Types for Use by Topology Plug-ins

   This section describes the methods that Topology Plug-ins use to
   join, leave, and maintain the overlay.

6.4.2.1.  Join

   A new peer (which already has credentials) uses the JoinReq message
   to join the overlay.  The JoinReq is sent to the responsible peer
   depending on the routing mechanism described in the Topology Plug-in.
   This message notifies the responsible peer that the new peer is
   taking over some of the overlay and that it needs to synchronize its
   state.

         struct {
            NodeId                joining_peer_id;
            opaque                overlay_specific_data<0..2^16-1>;
         } JoinReq;

   The minimal JoinReq contains only the Node-ID which the sending peer
   wishes to assume.  Overlay algorithms MAY specify other data to
   appear in this request.  Receivers of the JoinReq MUST verify that
   the joining_peer_id field matches the Node-ID used to sign the
   message and, if not, the message MUST be rejected with an
   Error_Forbidden error.





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   Because joins may be executed only between nodes which are directly
   adjacent, receiving peers MUST verify that any JoinReq they receive
   arrives from a transport channel that is bound to the Node-ID to be
   assumed by the Joining Node.  Implementations MUST use DTLS
   anti-replay mechanisms, thus preventing replay attacks.

   If the request succeeds, the responding peer responds with a JoinAns
   message, as defined below:

         struct {
            opaque                overlay_specific_data<0..2^16-1>;
         } JoinAns;

   If the request succeeds, the responding peer MUST follow up by
   executing the right sequence of Stores and Updates to transfer the
   appropriate section of the overlay space to the Joining Node.  In
   addition, overlay algorithms MAY define data to appear in the
   response payload that provides additional information.

   Joining Nodes MUST verify that the signature on the JoinAns message
   matches the expected target (i.e., the adjacency over which they are
   joining).  If not, they MUST discard the message.

   In general, nodes which cannot form connections SHOULD report an
   error to the user.  However, implementations MUST provide some
   mechanism whereby nodes can determine that they are potentially the
   first node and can take responsibility for the overlay.  (The idea is
   to avoid having ordinary nodes try to become responsible for the
   entire overlay during a partition.)  This specification does not
   mandate any particular mechanism, but a configuration flag or setting
   seems appropriate.

6.4.2.2.  Leave

   The LeaveReq message is used to indicate that a node is exiting the
   overlay.  A node SHOULD send this message to each peer with which it
   is directly connected prior to exiting the overlay.

         struct {
            NodeId                leaving_peer_id;
            opaque                overlay_specific_data<0..2^16-1>;
         } LeaveReq;

   LeaveReq contains only the Node-ID of the leaving peer.  Overlay
   algorithms MAY specify other data to appear in this request.
   Receivers of the LeaveReq MUST verify that the leaving_peer_id field
   matches the Node-ID used to sign the message and, if not, the message
   MUST be rejected with an Error_Forbidden error.



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   Because leaves may be executed only between nodes which are directly
   adjacent, receiving peers MUST verify that any LeaveReq they receive
   arrives from a transport channel that is bound to the Node-ID to be
   assumed by the leaving peer.  This also prevents replay attacks,
   provided that DTLS anti-replay is used.

   Upon receiving a Leave request, a peer MUST update its own Routing
   Table and send the appropriate Store/Update sequences to re-stabilize
   the overlay.

   LeaveAns is an empty message.

6.4.2.3.  Update

   Update is the primary overlay-specific maintenance message.  It is
   used by the sender to notify the recipient of the sender's view of
   the current state of the overlay (that is, its routing state), and it
   is up to the recipient to take whatever actions are appropriate to
   deal with the state change.  In general, peers send Update messages
   to all their adjacencies whenever they detect a topology shift.

   When a peer receives an Attach request with the send_update flag set
   to True (Section 6.4.2.4.1), it MUST send an Update message back to
   the sender of the Attach request after completion of the
   corresponding ICE check and TLS connection.  Note that the sender of
   such an Attach request may not have joined the overlay yet.

   When a peer detects through an Update that it is no longer
   responsible for any data value it is storing, it MUST attempt to
   Store a copy to the correct node unless it knows the newly
   responsible node already has a copy of the data.  This prevents data
   loss during large-scale topology shifts, such as the merging of
   partitioned overlays.

   The contents of the UpdateReq message are completely overlay
   specific.  The UpdateAns response is expected to be either success or
   an error.

6.4.2.4.  RouteQuery

   The RouteQuery request allows the sender to ask a peer where they
   would route a message directed to a given destination.  In other
   words, a RouteQuery for a destination X requests the Node-ID for the
   node that the receiving peer would next route to in order to get to
   X.  A RouteQuery can also request that the receiving peer initiate an
   Update request to transfer the receiving peer's Routing Table.





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   One important use of the RouteQuery request is to support iterative
   routing.  The sender selects one of the peers in its Routing
   Table and sends it a RouteQuery message with the destination field
   set to the Node-ID or Resource-ID to which it wishes to route.  The
   receiving peer responds with information about the peers to which the
   request would be routed.  The sending peer MAY then use the Attach
   method to attach to that peer(s) and repeat the RouteQuery.
   Eventually, the sender gets a response from a peer that is closest to
   the identifier in the destination field as determined by the Topology
   Plug-in.  At that point, the sender can send messages directly to
   that peer.

6.4.2.4.1.  Request Definition

   A RouteQueryReq message indicates the peer or resource that the
   requesting node is interested in.  It also contains a "send_update"
   option that allows the requesting node to request a full copy of the
   other peer's Routing Table.

         struct {
           Boolean                send_update;
           Destination            destination;
           opaque                 overlay_specific_data<0..2^16-1>;
         } RouteQueryReq;

   The contents of the RouteQueryReq message are as follows:

   send_update
      A single byte.  This may be set to True to indicate that the
      requester wishes the responder to initiate an Update request
      immediately.  Otherwise, this value MUST be set to False.

   destination
      The destination which the requester is interested in.  This may be
      any valid destination object, including a Node-ID, opaque ID, or
      Resource-ID.
      Note: If implementations are using opaque IDs for privacy
      purposes, answering RouteQueryReqs for opaque IDs will allow the
      requester to translate an opaque ID.  Implementations MAY wish to
      consider limiting the use of RouteQuery for opaque IDs in such
      cases.

   overlay_specific_data
      Other data as appropriate for the overlay.







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6.4.2.4.2.  Response Definition

   A response to a successful RouteQueryReq request is a RouteQueryAns
   message.  This message is completely overlay specific.

6.4.2.5.  Probe

   Probe provides primitive "exploration" services: it allows a node to
   determine which resources another node is responsible for.  A probe
   can be addressed to a specific Node-ID or to the peer controlling a
   given location (by using a Resource-ID).  In either case, the target
   node responds with a simple response containing some status
   information.

6.4.2.5.1.  Request Definition

   The ProbeReq message contains a list (potentially empty) of the
   pieces of status information that the requester would like the
   responder to provide.

        enum { invalidProbeInformationType(0), responsible_set(1),
               num_resources(2), uptime(3), (255) }
             ProbeInformationType;

        struct {
          ProbeInformationType     requested_info<0..2^8-1>;
        } ProbeReq;

   The currently defined values for ProbeInformationType are:

   responsible_set
      Indicates that the peer should Respond with the fraction of the
      overlay for which the responding peer is responsible.

   num_resources
      Indicates that the peer should Respond with the number of
      resources currently being stored by the peer.  Note that multiple
      values under the same Resource-ID are counted only once.

   uptime
      Indicates that the peer should Respond with how long the peer has
      been up, in seconds.









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6.4.2.5.2.  Response Definition

   A successful ProbeAns response contains the information elements
   requested by the peer.

         struct {
           select (type) {
             case responsible_set:
               uint32             responsible_ppb;

             case num_resources:
               uint32             num_resources;

             case uptime:
               uint32             uptime;

             /* This type may be extended */
           };
         } ProbeInformationData;

         struct {
           ProbeInformationType    type;
           uint8                   length;
           ProbeInformationData    value;
         } ProbeInformation;

         struct {
           ProbeInformation        probe_info<0..2^16-1>;
         } ProbeAns;

   A ProbeAns message contains a sequence of ProbeInformation
   structures.  Each has a "length" indicating the length of the
   following value field.  This structure allows for unknown option
   types.

   Each of the current possible Probe information types is a 32-bit
   unsigned integer.  For type "responsible_ppb", it is the fraction of
   the overlay for which the peer is responsible, in parts per billion.
   For type "num_resources", it is the number of resources the peer is
   storing.  For the type "uptime", it is the number of seconds the peer
   has been up.

   The responding peer SHOULD include any values that the requesting
   node requested and that it recognizes.  They SHOULD be returned in
   the requested order.  Any other values MUST NOT be returned.






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6.5.  Forwarding and Link Management Layer

   Each node maintains connections to a set of other nodes defined by
   the Topology Plug-in.  This section defines the methods RELOAD uses
   to form and maintain connections between nodes in the overlay.  Three
   methods are defined:

   Attach
      Used to form RELOAD connections between nodes using ICE for NAT
      traversal.  When node A wants to connect to node B, it sends an
      Attach message to node B through the overlay.  The Attach contains
      A's ICE parameters.  B responds with its ICE parameters, and the
      two nodes perform ICE to form connection.  Attach also allows two
      nodes to connect via No-ICE instead of full ICE.

   AppAttach
      Used to form application-layer connections between nodes.

   Ping
      A simple request/response which is used to verify connectivity of
      the target peer.

6.5.1.  Attach

   A node sends an Attach request when it wishes to establish a direct
   Overlay Link connection to another node for the purpose of sending
   RELOAD messages.  A client that can establish a connection directly
   need not send an Attach, as described in the second bullet of
   Section 4.2.1.

   As described in Section 6.1, an Attach may be routed to either a
   Node-ID or a Resource-ID.  An Attach routed to a specific Node-ID
   will fail if that node is not reached.  An Attach routed to a
   Resource-ID will establish a connection with the peer currently
   responsible for that Resource-ID, which may be useful in establishing
   a direct connection to the responsible peer for use with frequent or
   large resource updates.

   An Attach, in and of itself, does not result in updating the Routing
   Table of either node.  That function is performed by Updates.  If
   node A has Attached to node B, but has not received any Updates from
   B, it MAY route messages which are directly addressed to B through
   that channel, but it MUST NOT route messages through B to other peers
   via that channel.  The process of Attaching is separate from the
   process of becoming a peer (using Join and Update), to prevent half-
   open states where a node has started to form connections but is not
   really ready to act as a peer.  Thus, clients (unlike peers) can
   simply Attach without sending Join or Update.



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6.5.1.1.  Request Definition

   An Attach request message contains the requesting node ICE connection
   parameters formatted into a binary structure.

        enum { invalidOverlayLinkType(0), DTLS-UDP-SR(1),
               DTLS-UDP-SR-NO-ICE(3), TLS-TCP-FH-NO-ICE(4),
               (255) } OverlayLinkType;

        enum { invalidCandType(0),
               host(1), srflx(2), /* RESERVED(3), */ relay(4),
               (255) } CandType;

        struct {
          opaque                name<0..2^16-1>;
          opaque                value<0..2^16-1>;
        } IceExtension;

        struct {
          IpAddressPort         addr_port;
          OverlayLinkType       overlay_link;
          opaque                foundation<0..255>;
          uint32                priority;
          CandType              type;
          select (type) {
            case host:
              ;          /* Empty */
            case srflx:
            case relay:
              IpAddressPort     rel_addr_port;
          };
          IceExtension          extensions<0..2^16-1>;
        } IceCandidate;

        struct {
          opaque                ufrag<0..2^8-1>;
          opaque                password<0..2^8-1>;
          opaque                role<0..2^8-1>;
          IceCandidate          candidates<0..2^16-1>;
          Boolean               send_update;
        } AttachReqAns;

   The values contained in AttachReqAns are:

   ufrag
      The username fragment (from ICE).





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   password
      The ICE password.

   role
      An active/passive/actpass attribute from RFC 4145 [RFC4145].  This
      value MUST be "passive" for the offerer (the peer sending the
      Attach request) and "active" for the answerer (the peer sending
      the Attach response).

   candidates
      One or more ICE candidate values, as described below.

   send_update
      Has the same meaning as the send_update field in RouteQueryReq.

   Each ICE candidate is represented as an IceCandidate structure, which
   is a direct translation of the information from the ICE string
   structures, with the exception of the component ID.  Since there is
   only one component, it is always 1, and thus left out of the
   structure.  The remaining values are specified as follows:

   addr_port
      Corresponds to the ICE connection-address and port productions.

   overlay_link
      Corresponds to the ICE transport production.  Overlay Link
      protocols used with No-ICE MUST specify "No-ICE" in their
      description.  Future overlay link values can be added by defining
      new OverlayLinkType values in the IANA registry as described in
      Section 14.10.  Future extensions to the encapsulation or framing
      that provide for backward compatibility with the previously
      specified encapsulation or framing values MUST use the same
      OverlayLinkType value that was previously defined.
      OverlayLinkType protocols are defined in Section 6.6

      A single AttachReqAns MUST NOT include both candidates whose
      OverlayLinkType protocols use ICE (the default) and candidates
      that specify "No-ICE".

   foundation
      Corresponds to the ICE foundation production.

   priority
      Corresponds to the ICE priority production.

   type
      Corresponds to the ICE cand-type production.




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   rel_addr_port
      Corresponds to the ICE rel-addr and rel-port productions.  It is
      present only for types "relay", "prfix", and "srflx".

   extensions
      ICE extensions.  The name and value fields correspond to binary
      translations of the equivalent fields in the ICE extensions.

   These values should be generated using the procedures described in
   Section 6.5.1.3.

6.5.1.2.  Response Definition

   If a peer receives an Attach request, it MUST determine how to
   process the request as follows:

   o  If the peer has not initiated an Attach request to the originating
      peer of this Attach request, it MUST process this request and
      SHOULD generate its own response with an AttachReqAns.  It should
      then begin ICE checks.

   o  If the peer has already sent an Attach request to and received the
      response from the originating peer of this Attach request and, as
      a result, an ICE check and TLS connection are in progress, then it
      SHOULD generate an Error_In_Progress error instead of an
      AttachReqAns.

   o  If the peer has already sent an Attach request to but not yet
      received the response from the originating peer of this Attach
      request, it SHOULD apply the following tie-breaker heuristic to
      determine how to handle this Attach request and the incomplete
      Attach request it has sent out:

      *  If the peer's own Node-ID is smaller when compared as big-
         endian unsigned integers, it MUST cancel retransmission of its
         own incomplete Attach request.  It MUST then process this
         Attach request, generate an AttachReqAns response, and proceed
         with the corresponding ICE check.

      *  If the peer's own Node-ID is larger when compared as big-endian
         unsigned integers, it MUST generate an Error_In_Progress error
         to this Attach request, and then proceed to wait for and
         complete the Attach and the corresponding ICE check it has
         originated.

   o  If the peer is overloaded or detects some other kind of error, it
      MAY generate an error instead of an AttachReqAns.




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   When a peer receives an Attach response, it SHOULD parse the response
   and begin its own ICE checks.

6.5.1.3.  Using ICE with RELOAD

   This section describes the profile of ICE that is used with RELOAD.
   RELOAD implementations MUST implement full ICE.

   In ICE, as defined by [RFC5245], the Session Description Protocol
   (SDP) is used to carry the ICE parameters.  In RELOAD, this function
   is performed by a binary encoding in the Attach method.  This
   encoding is more restricted than the SDP encoding because the RELOAD
   environment is simpler:

   o  Only a single media stream is supported.

   o  In this case, the "stream" refers not to RTP or other types of
      media, but rather to a connection for RELOAD itself or other
      application-layer protocols, such as SIP.

   o  RELOAD allows only for a single offer/answer exchange.  Unlike the
      usage of ICE within SIP, there is never a need to send a
      subsequent offer to update the default candidates to match the
      ones selected by ICE.

   An agent follows the ICE specification as described in [RFC5245] with
   the changes and additional procedures described in the subsections
   below.

6.5.1.4.  Collecting STUN Servers

   ICE relies on the node having one or more Session Traversal Utilities
   for NAT (STUN) servers to use.  In conventional ICE, it is assumed
   that nodes are configured with one or more STUN servers through some
   out-of-band mechanism.  This is still possible in RELOAD, but RELOAD
   also learns STUN servers as it connects to other peers.

   A peer on a well-provisioned wide-area overlay will be configured
   with one or more bootstrap nodes.  These nodes make an initial list
   of STUN servers.  However, as the peer forms connections with
   additional peers, it builds more peers that it can use like STUN
   servers.

   Because complicated NAT topologies are possible, a peer may need more
   than one STUN server.  Specifically, a peer that is behind a single
   NAT will typically observe only two IP addresses in its STUN checks:
   its local address and its server reflexive address from a STUN server
   outside its NAT.  However, if more NATs are involved, a peer may



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   learn additional server reflexive addresses (which vary based on
   where in the topology the STUN server is).  To maximize the chance of
   achieving a direct connection, a peer SHOULD group other peers by the
   peer-reflexive addresses it discovers through them.  It SHOULD then
   select one peer from each group to use as a STUN server for future
   connections.

   Only peers to which the peer currently has connections may be used.
   If the connection to that host is lost, it MUST be removed from the
   list of STUN servers, and a new server from the same group MUST be
   selected unless there are no others servers in the group, in which
   case some other peer MAY be used.

6.5.1.5.  Gathering Candidates

   When a node wishes to establish a connection for the purposes of
   RELOAD signaling or application signaling, it follows the process of
   gathering candidates as described in Section 4 of ICE [RFC5245].
   RELOAD utilizes a single component.  Consequently, gathering for
   these "streams" requires a single component.  In the case where a
   node has not yet found a TURN server, the agent would not include a
   relayed candidate.

   The ICE specification assumes that an ICE agent is configured with,
   or somehow knows of, TURN and STUN servers.  RELOAD provides a way
   for an agent to learn these by querying the overlay, as described in
   Sections 6.5.1.4 and 9.

   The default candidate selection described in Section 4.1.4 of ICE is
   ignored; defaults are not signaled or utilized by RELOAD.

   An alternative to using the full ICE supported by the Attach request
   is to use the No-ICE mechanism by providing candidates with "No-ICE"
   Overlay Link protocols.  Configuration for the overlay indicates
   whether or not these Overlay Link protocols can be used.  An overlay
   MUST be either all ICE or all No-ICE.

   No-ICE will not work in all the scenarios where ICE would work, but
   in some cases, particularly those with no NATs or firewalls, it will
   work.

6.5.1.6.  Prioritizing Candidates

   Standardization of additional protocols for use with ICE is expected,
   including TCP [RFC6544] and protocols such as the Stream Control
   Transmission Protocol (SCTP) [RFC4960] and Datagram Congestion
   Control Protocol (DCCP) [RFC4340].  UDP encapsulations for SCTP and
   DCCP would expand the Overlay Link protocols available for RELOAD.



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   When additional protocols are available, the following prioritization
   is RECOMMENDED:

   o  Highest priority is assigned to protocols that offer well-
      understood congestion and flow control without head-of-line
      blocking, for example, SCTP without message ordering, DCCP, and
      those protocols encapsulated using UDP.

   o  Second highest priority is assigned to protocols that offer well-
      understood congestion and flow control, but that have head-of-line
      blocking, such as TCP.

   o  Lowest priority is assigned to protocols encapsulated over UDP
      that do not implement well-established congestion control
      algorithms.  The DTLS/UDP with Simple Reliability (SR) overlay
      link protocol is an example of such a protocol.

   Head-of-line blocking is undesirable in an Overlay Link protocol,
   because the messages carried on a RELOAD link are independent, rather
   than stream-oriented.  Therefore, if message N on a link is lost,
   delaying message N+1 on that same link until N is successfully
   retransmitted does nothing other than increase the latency for the
   transaction of message N+1, as they are unrelated to each other.
   Therefore, while the high quality, performance, and availability of
   modern TCP implementations makes them very attractive, their
   performance as Overlay Link protocols is not optimal.

   Note that none of the protocols defined in this document meets these
   conditions, but it is expected that new Overlay Link protocols
   defined in the future will fill this gap.

6.5.1.7.  Encoding the Attach Message

   Section 4.3 of ICE describes procedures for encoding the SDP for
   conveying RELOAD candidates.  Instead of actually encoding an SDP
   message, the candidate information (IP address and port and transport
   protocol, priority, foundation, type, and related address) is carried
   within the attributes of the Attach request or its response.
   Similarly, the username fragment and password are carried in the
   Attach message or its response.  Section 6.5.1 describes the detailed
   attribute encoding for Attach.  The Attach request and its response
   do not contain any default candidates or the ice-lite attribute, as
   these features of ICE are not used by RELOAD.

   Since the Attach request contains the candidate information and short
   term credentials, it is considered as an offer for a single media
   stream that happens to be encoded in a format different than SDP, but
   is otherwise considered a valid offer for the purposes of following



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   the ICE specification.  Similarly, the Attach response is considered
   a valid answer for the purposes of following the ICE specification.

6.5.1.8.  Verifying ICE Support

   An agent MUST skip the verification procedures in Sections 5.1 and
   6.1 of ICE.  Since RELOAD requires full ICE from all agents, this
   check is not required.

6.5.1.9.  Role Determination

   The roles of controlling and controlled, as described in Section 5.2
   of ICE, are still utilized with RELOAD.  However, the offerer (the
   entity sending the Attach request) will always be controlling, and
   the answerer (the entity sending the Attach response) will always be
   controlled.  The connectivity checks MUST still contain the ICE-
   CONTROLLED and ICE-CONTROLLING attributes, however, even though the
   role reversal capability for which they are defined will never be
   needed with RELOAD.  This is to allow for a common codebase between
   ICE for RELOAD and ICE for SDP.

6.5.1.10.  Full ICE

   When the overlay uses ICE, connectivity checks and nominations are
   used as in regular ICE.

6.5.1.10.1.  Connectivity Checks

   The processes of forming check lists in Section 5.7 of ICE,
   scheduling checks in Section 5.8, and checking connectivity checks in
   Section 7 are used with RELOAD without change.

6.5.1.10.2.  Concluding ICE

   The procedures in Section 8 of ICE are followed to conclude ICE, with
   the following exceptions:

   o  The controlling agent MUST NOT attempt to send an updated offer
      once the state of its single media stream reaches Completed.

   o  Once the state of ICE reaches Completed, the agent can immediately
      free all unused candidates.  This is because RELOAD does not have
      the concept of forking, and thus the three-second delay in
      Section 8.3 of ICE does not apply.







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6.5.1.10.3.  Media Keepalives

   STUN MUST be utilized for the keepalives described in Section 10 of
   ICE.

6.5.1.11.  No-ICE

   No-ICE is selected when either side has provided "no ICE" Overlay
   Link candidates.  STUN is not used for connectivity checks when doing
   No-ICE; instead, the DTLS or TLS handshake (or similar security layer
   of future overlay link protocols) forms the connectivity check.  The
   certificate exchanged during the TLS or DTLS handshake MUST match the
   node which sent the AttachReqAns, and if it does not, the connection
   MUST be closed.

6.5.1.12.  Subsequent Offers and Answers

   An agent MUST NOT send a subsequent offer or answer.  Thus, the
   procedures in Section 9 of ICE MUST be ignored.

6.5.1.13.  Sending Media

   The procedures of Section 11 of ICE apply to RELOAD as well.
   However, in this case, the "media" takes the form of application-
   layer protocols (e.g., RELOAD) over TLS or DTLS.  Consequently, once
   ICE processing completes, the agent will begin TLS or DTLS procedures
   to establish a secure connection.  The node that sent the Attach
   request MUST be the TLS server.  The other node MUST be the TLS
   client.  The server MUST request TLS client authentication.  The
   nodes MUST verify that the certificate presented in the handshake
   matches the identity of the other peer as found in the Attach
   message.  Once the TLS or DTLS signaling is complete, the application
   protocol is free to use the connection.

   The concept of a previous selected pair for a component does not
   apply to RELOAD, since ICE restarts are not possible with RELOAD.

6.5.1.14.  Receiving Media

   An agent MUST be prepared to receive packets for the application
   protocol (TLS or DTLS carrying RELOAD) at any time.  The jitter and
   RTP considerations in Section 11 of ICE do not apply to RELOAD.

6.5.2.  AppAttach

   A node sends an AppAttach request when it wishes to establish a
   direct connection to another node for the purposes of sending
   application-layer messages.  AppAttach is nearly identical to Attach,



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   except for the purpose of the connection: it is used to transport
   non-RELOAD "media".  A separate request is used to avoid implementer
   confusion between the two methods (this was found to be a real
   problem with initial implementations).  The AppAttach request and its
   response contain an application attribute, which indicates what
   protocol is to be run over the connection.

6.5.2.1.  Request Definition

   An AppAttachReq message contains the requesting node's ICE connection
   parameters formatted into a binary structure.

        struct {
          opaque                  ufrag<0..2^8-1>;
          opaque                  password<0..2^8-1>;
          uint16                  application;
          opaque                  role<0..2^8-1>;
          IceCandidate            candidates<0..2^16-1>;
        } AppAttachReq;


   The values contained in AppAttachReq and AppAttachAns are:

   ufrag
      The username fragment (from ICE).

   password
      The ICE password.

   application
      A 16-bit Application-ID, as defined in the Section 14.5.  This
      number represents the IANA-registered application that is going to
      send data on this connection.

   role
      An active/passive/actpass attribute from RFC 4145 [RFC4145].

   candidates
      One or more ICE candidate values.

   The application using the connection that is set up with this request
   is responsible for providing traffic of sufficient frequency to keep
   the NAT and Firewall binding alive.  Applications will often send
   traffic every 25 seconds to ensure this.







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6.5.2.2.  Response Definition

   If a peer receives an AppAttach request, it SHOULD process the
   request and generate its own response with a AppAttachAns.  It should
   then begin ICE checks.  When a peer receives an AppAttach response,
   it SHOULD parse the response and begin its own ICE checks.  If the
   Application ID is not supported, the peer MUST reply with an
   Error_Not_Found error.

        struct {
          opaque                  ufrag<0..2^8-1>;
          opaque                  password<0..2^8-1>;
          uint16                  application;
          opaque                  role<0..2^8-1>;
          IceCandidate            candidates<0..2^16-1>;
        } AppAttachAns;


   The meaning of the fields is the same as in the AppAttachReq.

6.5.3.  Ping

   Ping is used to test connectivity along a path.  A ping can be
   addressed to a specific Node-ID, to the peer controlling a given
   location (by using a Resource-ID), or to the wildcard Node-ID.

6.5.3.1.  Request Definition

   The PingReq structure is used to make a Ping request.

        struct {
          opaque<0..2^16-1> padding;
        } PingReq;


   The Ping request is empty of meaningful contents.  However, it may
   contain up to 65535 bytes of padding to facilitate the discovery of
   overlay maximum packet sizes.

6.5.3.2.  Response Definition

   A successful PingAns response contains the information elements
   requested by the peer.

         struct {
           uint64                 response_id;
           uint64                 time;
         } PingAns;



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   A PingAns message contains the following elements:

   response_id
      A randomly generated 64-bit response ID.  This is used to
      distinguish Ping responses.

   time
      The time when the Ping response was created, represented in the
      same way as storage_time, defined in Section 7.

6.5.4.  ConfigUpdate

   The ConfigUpdate method is used to push updated configuration data
   across the overlay.  Whenever a node detects that another node has
   old configuration data, it MUST generate a ConfigUpdate request.  The
   ConfigUpdate request allows updating of two kinds of data: the
   configuration data (Section 6.3.2.1) and the Kind information
   (Section 7.4.1.1).

6.5.4.1.  Request Definition

   The ConfigUpdateReq structure is used to provide updated
   configuration information.

        enum { invalidConfigUpdateType(0), config(1), kind(2), (255) }
             ConfigUpdateType;

        typedef uint32           KindId;
        typedef opaque           KindDescription<0..2^16-1>;

        struct {
          ConfigUpdateType       type;
          uint32                 length;

          select (type) {
            case config:
                        opaque             config_data<0..2^24-1>;

            case kind:
                        KindDescription    kinds<0..2^24-1>;

            /* This structure may be extended with new types */
          };
        } ConfigUpdateReq;







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   The ConfigUpdateReq message contains the following elements:

   type
      The type of the contents of the message.  This structure allows
      for unknown content types.

   length
      The length of the remainder of the message.  This is included to
      preserve backward compatibility and is 32 bits instead of 24 to
      facilitate easy conversion between network and host byte order.

   config_data (type==config)
      The contents of the Configuration Document.

   kinds (type==kind)
      One or more XML kind-block productions (see Section 11.1).  These
      MUST be encoded with UTF-8 and assume a default namespace of
      "urn:ietf:params:xml:ns:p2p:config-base".

6.5.4.2.  Response Definition

   The ConfigUpdateAns structure is used to respond to a ConfigUpdateReq
   request.

        struct {
        } ConfigUpdateAns;

   If the ConfigUpdateReq is of type "config", it MUST be processed only
   if all the following are true:

   o  The sequence number in the document is greater than the current
      configuration sequence number.

   o  The Configuration Document is correctly digitally signed (see
      Section 11 for details on signatures).

   Otherwise, appropriate errors MUST be generated.

   If the ConfigUpdateReq is of type "kind", it MUST be processed only
   if it is correctly digitally signed by an acceptable Kind signer
   (i.e., one listed in the current configuration file).  Details on the
   kind-signer field in the configuration file are described in
   Section 11.1.  In addition, if the Kind update conflicts with an
   existing known Kind (i.e., it is signed by a different signer), then
   it should be rejected with an Error_Forbidden error.  This should not
   happen in correctly functioning overlays.





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   If the update is acceptable, then the node MUST reconfigure itself to
   match the new information.  This may include adding permissions for
   new Kinds, deleting old Kinds, or even, in extreme circumstances,
   exiting and re-entering the overlay, if, for instance, the DHT
   algorithm has changed.

   If an implementation misses enough ConfigUpdates that include key
   changes, it is possible that it will no longer be able to verify new
   valid ConfigUpdates.  In this case, the only available recovery
   mechanism is to attempt to retrieve a new Configuration Document,
   typically by the mechanisms used for initial bootstrapping.  It is up
   to implementers whether or how to decide to employ this sort of
   recovery mechanism.

   The response for ConfigUpdate is empty.

6.6.  Overlay Link Layer

   RELOAD can use multiple Overlay Link protocols to send its messages.
   Because ICE is used to establish connections (see Section 6.5.1.3),
   RELOAD nodes are able to detect which Overlay Link protocols are
   offered by other nodes and establish connections between them.  Any
   link protocol needs to be able to establish a secure, authenticated
   connection and to provide data origin authentication and message
   integrity for individual data elements.  RELOAD currently supports
   three Overlay Link protocols:

   o  DTLS [RFC6347] over UDP with Simple Reliability (SR)
      (OverlayLinkType=DTLS-UDP-SR)

   o  TLS [RFC5246] over TCP with Framing Header, No-ICE
      (OverlayLinkType=TLS-TCP-FH-NO-ICE)

   o  DTLS [RFC6347] over UDP with SR, No-ICE
      (OverlayLinkType=DTLS-UDP-SR-NO-ICE)

   Note that although UDP does not properly have "connections", both TLS
   and DTLS have a handshake that establishes a similar, stateful
   association.  We refer to these as "connections" for the purposes of
   this document.

   If a peer receives a message that is larger than the value of max-
   message-size defined in the overlay configuration, the peer SHOULD
   send an Error_Message_Too_Large error and then close the TLS or DTLS
   session from which the message was received.  Note that this error
   can be sent and the session closed before the peer receives the
   complete message.  If the forwarding header is larger than the max-




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   message-size, the receiver SHOULD close the TLS or DTLS session
   without sending an error.

   The RELOAD mechanism requires that failed links be quickly removed
   from the Routing Table so end-to-end retransmission can handle lost
   messages.  Overlay Link protocols MUST be designed with a mechanism
   that quickly signals a likely failure, and implementations SHOULD
   quickly act to remove a failed link from the Routing Table when
   receiving this signal.  The entry can be restored if it proves to
   resume functioning, or it can be replaced at some point in the future
   if necessary.  Section 10.7.2 contains more details specific to the
   CHORD-RELOAD Topology Plug-in.

   The Framing Header (FH) is used to frame messages and provide timing
   when used on a reliable stream-based transport protocol.  Simple
   Reliability (SR) uses the FH to provide congestion control and
   partial reliability when using unreliable message-oriented transport
   protocols.  We will first define each of these algorithms in Sections
   6.6.2 and 6.6.3, and then define Overlay Link protocols that use them
   in Sections 6.6.4, 6.6.5, and 6.6.6.

   Note: We expect future Overlay Link protocols to define replacements
   for all components of these protocols, including the Framing Header.
   The three protocols that we will discuss have been chosen for
   simplicity of implementation and reasonable performance.

6.6.1.  Future Overlay Link Protocols

   It is possible to define new link-layer protocols and apply them to a
   new overlay using the "overlay-link-protocol" configuration directive
   (see Section 11.1.).  However, any new protocols MUST meet the
   following requirements:

   Endpoint authentication:  When a node forms an association with
      another endpoint, it MUST be possible to cryptographically verify
      that the endpoint has a given Node-ID.

   Traffic origin authentication and integrity:  When a node receives
      traffic from another endpoint, it MUST be possible to
      cryptographically verify that the traffic came from a given
      association and that it has not been modified in transit from the
      other endpoint in the association.  The overlay link protocol MUST
      also provide replay prevention/detection.

   Traffic confidentiality:  When a node sends traffic to another
      endpoint, it MUST NOT be possible for a third party that is not
      involved in the association to determine the contents of that
      traffic.



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   Any new overlay protocol MUST be defined via Standards Action
   [RFC5226].  See Section 14.11.

6.6.1.1.  HIP

   In a Host Identity Protocol Based Overlay Networking Environment (HIP
   BONE) [RFC6079], HIP [RFC5201] provides connection management (e.g.,
   NAT traversal and mobility) and security for the overlay network.
   The P2PSIP Working Group has expressed interest in supporting a HIP-
   based link protocol.  Such support would require specifying such
   details as:

   o  How to issue certificates which provide identities meaningful to
      the HIP base exchange.  We anticipate that this would require a
      mapping between Overlay Routable Cryptographic Hash Identifiers
      (ORCHIDs) and NodeIds.

   o  How to carry the HIP I1 and I2 messages.

   o  How to carry RELOAD messages over HIP.

   [HIP-RELOAD] documents work in progress on using RELOAD with the HIP
   BONE.

6.6.1.2.  ICE-TCP

   The ICE-TCP RFC [RFC6544] allows TCP to be supported as an Overlay
   Link protocol that can be added using ICE.

6.6.1.3.  Message-Oriented Transports

   Modern message-oriented transports offer high performance and good
   congestion control, and they avoid head-of-line blocking in case of
   lost data.  These characteristics make them preferable as underlying
   transport protocols for RELOAD links.  SCTP without message ordering
   and DCCP are two examples of such protocols.  However, currently they
   are not well-supported by commonly available NATs, and specifications
   for ICE session establishment are not available.

6.6.1.4.  Tunneled Transports

   As of the time of this writing, there is significant interest in the
   IETF community in tunneling other transports over UDP, which is
   motivated by the situation that UDP is well-supported by modern NAT
   hardware and by the fact that performance similar to a native
   implementation can be achieved.  Currently, SCTP, DCCP, and a generic
   tunneling extension are being proposed for message-oriented
   protocols.  Once ICE traversal has been specified for these tunneled



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   protocols, they should be straightforward to support as overlay link
   protocols.

6.6.2.  Framing Header

   In order to support unreliable links and to allow for quick detection
   of link failures when using reliable end-to-end transports, each
   message is wrapped in a very simple framing layer (FramedMessage),
   which is used only for each hop.  This layer contains a sequence
   number which can then be used for ACKs.  The same header is used for
   both reliable and unreliable transports for simplicity of
   implementation.

   The definition of FramedMessage is:

        enum { data(128), ack(129), (255) } FramedMessageType;

        struct {
          FramedMessageType       type;

          select (type) {
            case data:
              uint32              sequence;
              opaque              message<0..2^24-1>;

            case ack:
              uint32              ack_sequence;
              uint32              received;
          };
        } FramedMessage;

   The type field of the PDU is set to indicate whether the message is
   data or an acknowledgement.

   If the message is of type "data", then the remainder of the PDU is as
   follows:

   sequence
      The sequence number.  This increments by one for each framed
      message sent over this transport session.

   message
      The message that is being transmitted.

   Each connection has it own sequence number space.  Initially, the
   value is zero, and it increments by exactly one for each message sent
   over that connection.




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   When the receiver receives a message, it SHOULD immediately send an
   ACK message.  The receiver MUST keep track of the 32 most recent
   sequence numbers received on this association in order to generate
   the appropriate ACK.

   If the PDU is of type "ack", the contents are as follows:

   ack_sequence
      The sequence number of the message being acknowledged.

   received
      A bitmask indicating if each of the previous 32 sequence numbers
      before this packet has been among the 32 packets most recently
      received on this connection.  When a packet is received with a
      sequence number N, the receiver looks at the sequence number of
      the 32 previously received packets on this connection.  We call
      the previously received packet number M.  For each of the previous
      32 packets, if the sequence number M is less than N but greater
      than N-32, the N-M bit of the received bitmask is set to one;
      otherwise, it is set to zero.  Note that a bit being set to one
      indicates positively that a particular packet was received, but a
      bit being set to zero means only that it is unknown whether or not
      the packet has been received, because it might have been received
      before the 32 most recently received packets.

   The received field bits in the ACK provide a high degree of
   redundancy so that the sender can figure out which packets the
   receiver has received and can then estimate packet loss rates.  If
   the sender also keeps track of the time at which recent sequence
   numbers have been sent, the RTT (round-trip time) can be estimated.

   Note that because retransmissions receive new sequence numbers,
   multiple ACKs may be received for the same message.  This approach
   provides more information than traditional TCP sequence numbers, but
   care must be taken when applying algorithms designed based on TCP's
   stream-oriented sequence number.

6.6.3.  Simple Reliability

   When RELOAD is carried over DTLS or another unreliable link protocol,
   it needs to be used with a reliability and congestion control
   mechanism, which is provided on a hop-by-hop basis.  The basic
   principle is that each message, regardless of whether or not it
   carries a request or response, will get an ACK and be reliably
   retransmitted.  The receiver's job is very simple, and is limited to
   just sending ACKs.  All the complexity is at the sender side.  This
   allows the sending implementation to trade off performance versus
   implementation complexity without affecting the wire protocol.



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   Because the receiver's role is limited to providing packet
   acknowledgements, a wide variety of congestion control algorithms can
   be implemented on the sender side while using the same basic wire
   protocol.  The sender algorithm used MUST meet the requirements of
   [RFC5405].

6.6.3.1.  Stop and Wait Sender Algorithm

   This section describes one possible implementation of a sender
   algorithm for Simple Reliability.  It is adequate for overlays
   running on underlying networks with low latency and loss (LANs) or
   low-traffic overlays on the Internet.

   A node MUST NOT have more than one unacknowledged message on the DTLS
   connection at a time.  Note that because retransmissions of the same
   message are given new sequence numbers, there may be multiple
   unacknowledged sequence numbers in use.

   The RTO (Retransmission TimeOut) is based on an estimate of the RTT.
   The value for RTO is calculated separately for each DTLS session.
   Implementations can use a static value for RTO or a dynamic estimate,
   which will result in better performance.  For implementations that
   use a static value, the default value for RTO is 500 ms.  Nodes MAY
   use smaller values of RTO if it is known that all nodes are within
   the local network.  The default RTO MAY be set to a larger value,
   which is RECOMMENDED if it is known in advance (such as on high-
   latency access links) that the RTT is larger.

   Implementations that use a dynamic estimate to compute the RTO MUST
   use the algorithm described in RFC 6298 [RFC6298], with the exception
   that the value of RTO SHOULD NOT be rounded up to the nearest second,
   but instead rounded up to the nearest millisecond.  The RTT of a
   successful STUN transaction from the ICE stage is used as the initial
   measurement for formula 2.2 of RFC 6298.  The sender keeps track of
   the time each message was sent for all recently sent messages.  Any
   time an ACK is received, the sender can compute the RTT for that
   message by looking at the time the ACK was received and the time when
   the message was sent.  This is used as a subsequent RTT measurement
   for formula 2.3 of RFC 6298 to update the RTO estimate.  (Note that
   because retransmissions receive new sequence numbers, all received
   ACKs are used.)

   An initiating node SHOULD retransmit a message if it has not received
   an ACK after an interval of RTO (transit nodes do not retransmit at
   this layer).  The node MUST double the time to wait after each
   retransmission.  For each retransmission, the sequence number MUST be
   incremented.




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   Retransmissions continue until a response is received, until a total
   of 5 requests have been sent, until there has been a hard ICMP error
   [RFC1122], or until a TLS alert indicating the end of the connection
   has been sent or received.  The sender knows a response was received
   when it receives an ACK with a sequence number that indicates it is a
   response to one of the transmissions of this message.  For example,
   assuming an RTO of 500 ms, requests would be sent at times 0 ms, 500
   ms, 1500 ms, 3500 ms, and 7500 ms.  If all retransmissions for a
   message fail, then the sending node SHOULD close the connection
   routing the message.

   To determine when a link might be failing without waiting for the
   final timeout, observe when no ACKs have been received for an entire
   RTO interval, and then wait for three retransmissions to occur beyond
   that point.  If no ACKs have been received by the time the third
   retransmission occurs, it is RECOMMENDED that the link be removed
   from the Routing Table.  The link MAY be restored to the Routing
   Table if ACKs resume before the connection is closed, as described
   above.

   A sender MUST wait 10 ms between receipt of an ACK and transmission
   of the next message.

6.6.4.  DTLS/UDP with SR

   This overlay link protocol consists of DTLS over UDP while
   implementing the SR protocol.  STUN connectivity checks and
   keepalives are used.  Any compliant sender algorithm may be used.

6.6.5.  TLS/TCP with FH, No-ICE

   This overlay link protocol consists of TLS over TCP with the framing
   header.  Because ICE is not used, STUN connectivity checks are not
   used upon establishing the TCP connection, nor are they used for
   keepalives.

   Because the TCP layer's application-level timeout is too slow to be
   useful for overlay routing, the Overlay Link implementation MUST use
   the framing header to measure the RTT of the connection and calculate
   an RTO as specified in Section 2 of [RFC6298].  The resulting RTO is
   not used for retransmissions, but rather as a timeout to indicate
   when the link SHOULD be removed from the Routing Table.  It is
   RECOMMENDED that such a connection be retained for 30 seconds to
   determine if the failure was transient before concluding the link has
   failed permanently.

   When sending candidates for TLS/TCP with FH, No-ICE, a passive
   candidate MUST be provided.



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6.6.6.  DTLS/UDP with SR, No-ICE

   This overlay link protocol consists of DTLS over UDP while
   implementing the Simple Reliability protocol.  Because ICE is not
   used, no STUN connectivity checks or keepalives are used.

6.7.  Fragmentation and Reassembly

   In order to allow transmission over datagram protocols such as DTLS,
   RELOAD messages may be fragmented.

   Any node along the path can fragment the message, but only the final
   destination reassembles the fragments.  When a node takes a packet
   and fragments it, each fragment has a full copy of the forwarding
   header, but the data after the forwarding header is broken up into
   appropriately sized chunks.  The size of the payload chunks needs to
   take into account space to allow the Via and Destination Lists to
   grow.  Each fragment MUST contain a full copy of the Via List,
   Destination List, and ForwardingOptions and MUST contain at least 256
   bytes of the message body.  If these elements cannot fit within the
   MTU of the underlying datagram protocol, RELOAD fragmentation is not
   performed, and IP-layer fragmentation is allowed to occur.  The
   length field MUST contain the size of the message after
   fragmentation.  When a message MUST be fragmented, it SHOULD be split
   into equal-sized fragments that are no larger than the Path MTU
   (PMTU) of the next overlay link minus 32 bytes.  This is to allow the
   Via List to grow before further fragmentation is required.

   Note that this fragmentation is not optimal for the end-to-end
   path -- a message may be refragmented multiple times as it traverses
   the overlay, but it is assembled only at the final destination.  This
   option has been chosen as it is far easier to implement than end-to-
   end (e2e) PMTU discovery across an ever-changing overlay and it
   effectively addresses the reliability issues of relying on IP-layer
   fragmentation.  However, Ping can be used to allow e2e PMTU discovery
   to be implemented if desired.

   Upon receipt of a fragmented message by the intended peer, the peer
   holds the fragments in a holding buffer until the entire message has
   been received.  The message is then reassembled into a single message
   and processed.  In order to mitigate denial-of-service (DoS) attacks,
   receivers SHOULD time out incomplete fragments after the maximum
   request lifetime (15 seconds).  This time was derived from looking at
   the end-to-end retransmission time and saving fragments long enough
   for the full end-to-end retransmissions to take place.  Ideally, the
   receiver would have enough buffer space to deal with as many
   fragments as can arrive in the maximum request lifetime.  However, if




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   the receiver runs out of buffer space to reassemble a message, it
   MUST drop the message.

   The fragment field of the forwarding header is used to encode
   fragmentation information.  The offset is the number of bytes between
   the end of the forwarding header and the start of the data.  The
   first fragment therefore has an offset of 0.  The last fragment
   indicator MUST be appropriately set.  If the message is not
   fragmented, it is simply treated as if it is the only fragment: the
   last fragment bit is set and the offset is 0, resulting in a fragment
   value of 0xC0000000.

   Note: The reason for this definition of the fragment field is that
   originally, the high bit was defined in part of the specification as
   "is fragmented", so there was some specification ambiguity about how
   to encode messages with only one fragment.  This ambiguity was
   resolved in favor of always encoding as the "last" fragment with
   offset 0, thus simplifying the receiver code path, but resulting in
   the high bit being redundant.  Because messages MUST be set with the
   high bit set to 1, implementations SHOULD discard any message with it
   set to 0.  Implementations (presumably legacy ones) which choose to
   accept such messages MUST either ignore the remaining bits or ensure
   that they are 0.  They MUST NOT try to interpret as fragmented
   messages with the high bit set low.

7.  Data Storage Protocol

   RELOAD provides a set of generic mechanisms for storing and
   retrieving data in the Overlay Instance.  These mechanisms can be
   used for new applications simply by defining new code points and a
   small set of rules.  No new protocol mechanisms are required.

   The basic unit of stored data is a single StoredData structure:

        struct {
          uint32                  length;
          uint64                  storage_time;
          uint32                  lifetime;
          StoredDataValue         value;
          Signature               signature;
        } StoredData;

   The contents of this structure are as follows:

   length
      The size of the StoredData structure, in bytes, excluding the size
      of length itself.




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   storage_time
      The time when the data was stored, represented as the number of
      milliseconds elapsed since midnight Jan 1, 1970 UTC, not counting
      leap seconds.  This will have the same values for seconds as
      standard UNIX or POSIX time.  More information can be found at
      [UnixTime].  Any attempt to store a data value with a storage time
      before that of a value already stored at this location MUST
      generate an Error_Data_Too_Old error.  This prevents rollback
      attacks.  The node SHOULD make a best-effort attempt to use a
      correct clock to determine this number.  However, the protocol
      does not require synchronized clocks: the receiving peer uses the
      storage time in the previous store, not its own clock.  Clock
      values are used so that when clocks are generally synchronized,
      data may be stored in a single transaction, rather than querying
      for the value of a counter before the actual store.

      If a node attempting to store new data in response to a user
      request (rather than as an overlay maintenance operation such as
      occurs when healing the overlay from a partition) is rejected with
      an Error_Data_Too_Old error, the node MAY elect to perform its
      store using a storage_time that increments the value used with the
      previous store (this may be obtained by doing a Fetch).  This
      situation may occur when the clocks of nodes storing to this
      location are not properly synchronized.

   lifetime
      The validity period for the data, in seconds, starting from the
      time the peer receives the StoreReq.

   value
      The data value itself, as described in Section 7.2.

   signature
      A signature, as defined in Section 7.1.

   Each Resource-ID specifies a single location in the Overlay Instance.
   However, each location may contain multiple StoredData values,
   distinguished by Kind-ID.  The definition of a Kind describes both
   the data values which may be stored and the data model of the data.
   Some data models allow multiple values to be stored under the same
   Kind-ID.  Section 7.2 describes the available data models.  Thus, for
   instance, a given Resource-ID might contain a single-value element
   stored under Kind-ID X and an array containing multiple values stored
   under Kind-ID Y.







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7.1.  Data Signature Computation

   Each StoredData element is individually signed.  However, the
   signature also must be self-contained and must cover the Kind-ID and
   Resource-ID, even though they are not present in the StoredData
   structure.  The input to the signature algorithm is:

      resource_id || kind || storage_time || StoredDataValue ||
      SignerIdentity

   where || indicates concatenation and where these values are:

   resource_id
      The Resource-ID where this data is stored.

   kind
      The Kind-ID for this data.

   storage_time
      The contents of the storage_time data value.

   StoredDataValue
      The contents of the stored data value, as described in the
      previous sections.

   SignerIdentity
      The signer identity, as defined in Section 6.3.4.

   Once the signature has been computed, the signature is represented
   using a signature element, as described in Section 6.3.4.

   Note that there is no necessary relationship between the validity
   window of a certificate and the expiry of the data it is
   authenticating.  When signatures are verified, the current time MUST
   be compared to the certificate validity period.  Stored data MAY be
   set to expire after the signing certificate's validity period.  Such
   signatures are not considered valid after the signing certificate
   expires.  Implementations may "garbage collect" such data at their
   convenience, either by purging it automatically (perhaps by setting
   the upper bound on data storage to the lifetime of the signing
   certificate) or by simply leaving it in place until it expires
   naturally and relying on users of that data to notice the expired
   signing certificate.








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7.2.  Data Models

   The protocol currently defines the following data models:

   o  single value

   o  array

   o  dictionary

   These are represented with the StoredDataValue structure.  The actual
   data model is known from the Kind being stored.

        struct {
          Boolean                exists;
          opaque                 value<0..2^32-1>;
        } DataValue;

        struct {
          select (DataModel) {
            case single_value:
              DataValue             single_value_entry;

            case array:
              ArrayEntry            array_entry;

            case dictionary:
              DictionaryEntry       dictionary_entry;

            /* This structure may be extended */
          };
        } StoredDataValue;

   The following sections discuss the properties of each data model.

7.2.1.  Single Value

   A single-value element is a simple sequence of bytes.  There may be
   only one single-value element for each Resource-ID, Kind-ID pair.

   A single value element is represented as a DataValue, which contains
   the following two elements:

   exists
      This value indicates whether the value exists at all.  If it is
      set to False, it means that no value is present.  If it is True,
      this means that a value is present.  This gives the protocol a
      mechanism for indicating nonexistence as opposed to emptiness.



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   value
      The stored data.

7.2.2.  Array

   An array is a set of opaque values addressed by an integer index.
   Arrays are zero based.  Note that arrays can be sparse.  For
   instance, a Store of "X" at index 2 in an empty array produces an
   array with the values [ NA, NA, "X"].  Future attempts to fetch
   elements at index 0 or 1 will return values with "exists" set to
   False.

   An array element is represented as an ArrayEntry:

         struct {
           uint32                  index;
           DataValue               value;
         } ArrayEntry;

   The contents of this structure are:

   index
      The index of the data element in the array.

   value
      The stored data.

7.2.3.  Dictionary

   A dictionary is a set of opaque values indexed by an opaque key, with
   one value for each key.  A single dictionary entry is represented as
   a DictionaryEntry:

         typedef opaque           DictionaryKey<0..2^16-1>;

         struct {
           DictionaryKey          key;
           DataValue              value;
         } DictionaryEntry;

   The contents of this structure are:

   key
      The dictionary key for this value.

   value
      The stored data.




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7.3.  Access Control Policies

   Every Kind which is storable in an overlay MUST be associated with an
   access control policy.  This policy defines whether a request from a
   given node to operate on a given value should succeed or fail.  It is
   anticipated that only a small number of generic access control
   policies are required.  To that end, this section describes a small
   set of such policies, and Section 14.4 establishes a registry for new
   policies, if required.  Each policy has a short string identifier
   which is used to reference it in the Configuration Document.

   In the following policies, the term "signer" refers to the signer of
   the StoredValue object and, in the case of non-replica stores, to the
   signer of the StoreReq message.  That is, in a non-replica store,
   both the signer of the StoredValue and the signer of the StoreReq
   MUST conform to the policy.  In the case of a replica store, the
   signer of the StoredValue MUST conform to the policy, and the
   StoreReq itself MUST be checked as described in Section 7.4.1.1.

7.3.1.  USER-MATCH

   In the USER-MATCH policy, a given value MUST be written (or
   overwritten) if and only if the signer's certificate has a user name
   which hashes (using the hash function for the overlay) to the
   Resource-ID for the resource.  Recall that the certificate may,
   depending on the overlay configuration, be self-signed.

7.3.2.  NODE-MATCH

   In the NODE-MATCH policy, a given value MUST be written (or
   overwritten) if and only if the signer's certificate has a specified
   Node-ID which hashes (using the hash function for the overlay) to the
   Resource-ID for the resource and that Node-ID is the one indicated in
   the SignerIdentity value cert_hash.

7.3.3.  USER-NODE-MATCH

   The USER-NODE-MATCH policy may be used only with dictionary types.
   In the USER-NODE-MATCH policy, a given value MUST be written (or
   overwritten) if and only if the signer's certificate has a user name
   which hashes (using the hash function for the overlay) to the
   Resource-ID for the resource.  In addition, the dictionary key MUST
   be equal to the Node-ID in the certificate, and that Node-ID MUST be
   the one indicated in the SignerIdentity value cert_hash.







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7.3.4.  NODE-MULTIPLE

   In the NODE-MULTIPLE policy, a given value MUST be written (or
   overwritten) if and only if the signer's certificate contains a
   Node-ID such that H(Node-ID || i) is equal to the Resource-ID for
   some small integer value of i and that Node-ID is the one indicated
   in the SignerIdentity value cert_hash.  When this policy is in use,
   the maximum value of i MUST be specified in the Kind definition.

   Note that because i is not carried on the wire, the verifier MUST
   iterate through potential i values, up to the maximum value, to
   determine whether a store is acceptable.

7.4.  Data Storage Methods

   RELOAD provides several methods for storing and retrieving data:

   o  Store values in the overlay.

   o  Fetch values from the overlay.

   o  Stat: Get metadata about values in the overlay.

   o  Find the values stored at an individual peer.

   These methods are described in the following sections.

7.4.1.  Store

   The Store method is used to store data in the overlay.  The format of
   the Store request depends on the data model, which is determined by
   the Kind.

7.4.1.1.  Request Definition

   A StoreReq message is a sequence of StoreKindData values, each of
   which represents a sequence of stored values for a given Kind.  The
   same Kind-ID MUST NOT be used twice in a given store request.  Each
   value is then processed in turn.  These operations MUST be atomic.
   If any operation fails, the state MUST be rolled back to what it was
   before the request was received.










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   The store request is defined by the StoreReq structure:

       struct {
           KindId                 kind;
           uint64                 generation_counter;
           StoredData             values<0..2^32-1>;
       } StoreKindData;

       struct {
           ResourceId             resource;
           uint8                  replica_number;
           StoreKindData          kind_data<0..2^32-1>;
       } StoreReq;

   A single Store request stores data of a number of Kinds to a single
   resource location.  The contents of the structure are:

   resource
      The resource at which to store.

   replica_number
      The number of this replica.  When a storing peer saves replicas to
      other peers, each peer is assigned a replica number, starting from
      1, that is sent in the Store message.  This field is set to 0 when
      a node is storing its own data.  This allows peers to distinguish
      replica writes from original writes.  Different topologies may
      choose to allocate or interpret the replica number differently
      (see Section 10.4).

   kind_data
      A series of elements, one for each Kind of data to be stored.

   The peer MUST check that it is responsible for the resource if the
   replica number is zero; if it is not, the peer must reject the
   request.  The peer MUST check that it expects to be a replica for the
   resource and that the request sender is consistent with being the
   responsible node (i.e., that the receiving peer does not know of a
   better node) if the replica number is nonzero; if the request sender
   is not consistent, it should reject the request.

   Each StoreKindData element represents the data to be stored for a
   single Kind-ID.  The contents of the element are:

   kind
      The Kind-ID.  Implementations MUST reject requests corresponding
      to unknown Kinds.





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   generation_counter
      The expected current state of the generation counter
      (approximately the number of times that this object has been
      written; see below for details).

   values
      The value or values to be stored.  This may contain one or more
      stored_data values, depending on the data model associated with
      each Kind.

   The peer MUST perform the following checks:

   o  The Kind-ID is known and supported.

   o  The signatures over each individual data element, if any, are
      valid.  If this check fails, the request MUST be rejected with an
      Error_Forbidden error.

   o  Each element is signed by a credential which is authorized to
      write this Kind at this Resource-ID.  If this check fails, the
      request MUST be rejected with an Error_Forbidden error.

   o  For original (non-replica) stores, the StoreReq is signed by a
      credential which is authorized to write this Kind at this
      Resource-ID.  If this check fails, the request MUST be rejected
      with an Error_Forbidden error.

   o  For replica stores, the StoreReq is signed by a Node-ID which is a
      plausible node to either have originally stored the value or have
      been in the replica set.  What this means is overlay specific, but
      in the case of the Chord-based DHT defined in this specification,
      replica StoreReqs MUST come from nodes which are either in the
      known replica set for a given resource or which are closer than
      some node in the replica set.  If this check fails, the request
      MUST be rejected with an Error_Forbidden error.

   o  For original (non-replica) stores, the peer MUST check that if the
      generation counter is nonzero, it equals the current value of the
      generation counter for this Kind.  This feature allows the
      generation counter to be used in a way similar to the HTTP ETag
      feature.

   o  For replica Stores, the peer MUST set the generation counter to
      match the generation counter in the message and MUST NOT check the
      generation counter against the current value.  Replica Stores MUST
      NOT use a generation counter of 0.





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   o  The storage time values are greater than that of any values which
      would be replaced by this Store.

   o  The size and number of the stored values are consistent with the
      limits specified in the overlay configuration.

   o  If the data is signed with identity_type set to "none" and/or
      SignatureAndHashAlgorithm values set to {0, 0} ("anonymous" and
      "none"), the StoreReq MUST be rejected with an Error_forbidden
      error.  Only synthesized data returned by the storage can use
      these values (see Section 7.4.2.2)

   If all these checks succeed, the peer MUST attempt to store the data
   values.  For non-replica stores, if the store succeeds and the data
   is changed, then the peer MUST increase the generation counter by at
   least 1.  If there are multiple stored values in a single
   StoreKindData, it is permissible for the peer to increase the
   generation counter by only 1 for the entire Kind-ID or by 1 or more
   than 1 for each value.  Accordingly, all stored data values MUST have
   a generation counter of 1 or greater. 0 is used in the Store request
   to indicate that the generation counter should be ignored for
   processing this request.  However, the responsible peer should
   increase the stored generation counter and should return the correct
   generation counter in the response.

   When a peer stores data previously stored by another node (e.g., for
   replicas or topology shifts), it MUST adjust the lifetime value
   downward to reflect the amount of time the value was stored at the
   peer.  The adjustment SHOULD be implemented by an algorithm
   equivalent to the following: at the time the peer initially receives
   the StoreReq, it notes the local time T.  When it then attempts to do
   a StoreReq to another node, it should decrement the lifetime value by
   the difference between the current local time and T.

   Unless otherwise specified by the usage, if a peer attempts to store
   data previously stored by another node (e.g., for replicas or
   topology shifts) and that store fails with either an
   Error_Generation_Counter_Too_Low or an Error_Data_Too_Old error, the
   peer MUST fetch the newer data from the peer generating the error and
   use that to replace its own copy.  This rule allows resynchronization
   after partitions heal.

   When a network partition is being healed and unless otherwise
   specified, the default merging rule is to act as if all the values
   that need to be merged were stored and as if the order they were
   stored in corresponds to the stored time values associated with (and
   carried in) their values.  Because the stored time values are those
   associated with the peer which did the writing, clock skew is



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   generally not an issue.  If two nodes are on different partitions,
   write to the same location, and have clock skew, this can create
   merge conflicts.  However, because RELOAD deliberately segregates
   storage so that data from different users and peers is stored in
   different locations, and a single peer will typically only be in a
   single network partition, this case will generally not arise.

   The properties of stores for each data model are as follows:

   single-value:  A store of a new single-value element creates the
      element if it does not exist and overwrites any existing value
      with the new value.

   array:  A store of an array entry replaces (or inserts) the given
      value at the location specified by the index.  Because arrays are
      sparse, a store past the end of the array extends it with
      nonexistent values (exists = False) as required.  A store at index
      0xffffffff places the new value at the end of the array,
      regardless of the length of the array.  The resulting StoredData
      has the correct index value when it is subsequently fetched.

   dictionary:  A store of a dictionary entry replaces (or inserts) the
      given value at the location specified by the dictionary key.




























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   The following figure shows the relationship between these structures
   for an example store which stores the following values at resource
   "1234":

   o  The value "abc" is in the single-value location for Kind X.

   o  The value "foo" at index 0 is in the array for Kind Y.

   o  The value "bar" at index 1 is in the array for Kind Y.

                                     Store
                                resource=1234
                              replica_number = 0
                                   /      \
                                  /        \
                      StoreKindData        StoreKindData
                  kind=X (Single-Value)    kind=Y (Array)
                generation_counter = 99    generation_counter = 107
                           |                    /\
                           |                   /  \
                       StoredData             /    \
             storage_time = xxxxxxx          /      \
                   lifetime = 86400         /        \
                   signature = XXXX        /          \
                           |               |           |
                           |        StoredData       StoredData
                           |    storage_time =       storage_time =
                           |          yyyyyyyy       zzzzzzz
                           |  lifetime = 86400       lifetime = 33200
                           |  signature = YYYY       signature = ZZZZ
                           |               |           |
                    StoredDataValue        |           |
                     value="abc"           |           |
                                           |           |
                                  StoredDataValue  StoredDataValue
                                        index=0      index=1
                                     value="foo"    value="bar"














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7.4.1.2.  Response Definition

   In response to a successful Store request, the peer MUST return a
   StoreAns message containing a series of StoreKindResponse elements,
   which contains the current value of the generation counter for each
   Kind-ID, as well as a list of the peers where the data will be
   replicated by the node processing the request.

        struct {
          KindId                  kind;
          uint64                  generation_counter;
          NodeId                  replicas<0..2^16-1>;
        } StoreKindResponse;

        struct {
          StoreKindResponse       kind_responses<0..2^16-1>;
        } StoreAns;

   The contents of each StoreKindResponse are:

   kind
      The Kind-ID being represented.

   generation_counter
      The current value of the generation counter for that Kind-ID.

   replicas
      The list of other peers at which the data was/will be replicated.
      In overlays and applications where the responsible peer is
      intended to store redundant copies, this allows the storing node
      to independently verify that the replicas have in fact been
      stored.  It does this verification by using the Stat method (see
      Section 7.4.3).  Note that the storing node is not required to
      perform this verification.

   The response itself is just StoreKindResponse values packed end to
   end.

   If any of the generation counters in the request precede the
   corresponding stored generation counter, then the peer MUST fail the
   entire request and respond with an Error_Generation_Counter_Too_Low
   error.  The error_info in the ErrorResponse MUST be a StoreAns
   response containing the correct generation counter for each Kind and
   the replica list, which will be empty.  For original (non-replica)
   stores, a node which receives such an error SHOULD attempt to fetch
   the data and, if the storage_time value is newer, replace its own
   data with that newer data.  This rule improves data consistency in
   the case of partitions and merges.



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   If the data being stored is too large for the allowed limit by the
   given usage, then the peer MUST fail the request and generate an
   Error_Data_Too_Large error.

   If any type of request tries to access a data Kind that the peer does
   not know about, the peer MUST fail the request and generate an
   Error_Unknown_Kind error.  The error_info in the Error_Response is:

              KindId        unknown_kinds<0..2^8-1>;

   which lists all the Kinds that were unrecognized.  A node which
   receives this error MUST generate a ConfigUpdate message which
   contains the appropriate Kind definition (assuming which, in fact, a
   Kind which was defined in the configuration document was used).

7.4.1.3.  Removing Values

   RELOAD does not have an explicit Remove operation.  Rather, values
   are Removed by storing "nonexistent" values in their place.  Each
   DataValue contains a boolean value called "exists" which indicates
   whether a value is present at that location.  In order to effectively
   remove a value, the owner stores a new DataValue with "exists" set to
   False:

      exists = False

      value = {} (0 length)

   The owner SHOULD use a lifetime for the nonexistent value that is at
   least as long as the remainder of the lifetime of the value it is
   replacing.  Otherwise, it is possible for the original value to be
   accidentally or maliciously re-stored after the storing node has
   expired it.  Note that a window of vulnerability for replay attack
   still exists after the original lifetime has expired (as with any
   store).  This attack can be mitigated by doing a nonexistent store
   with a very long lifetime.

   Storing nodes MUST treat these nonexistent values the same way they
   treat any other stored value, including overwriting the existing
   value, replicating them, and aging them out as necessary when the
   lifetime expires.  When a stored nonexistent value's lifetime
   expires, it is simply removed from the storing node, as happens when
   any other stored value expires.

   Note that in the case of arrays and dictionaries, expiration may
   create an implicit, unsigned "nonexistent" value to represent a gap
   in the data structure, as might happen when any value is aged out.




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   However, this value isn't persistent, nor is it replicated.  It is
   simply synthesized by the storing node.

7.4.2.  Fetch

   The Fetch request retrieves one or more data elements stored at a
   given Resource-ID.  A single Fetch request can retrieve multiple
   different Kinds.

7.4.2.1.  Request Definition

   Fetch requests are defined by the FetchReq structure:

        struct {
          int32            first;
          int32            last;
        } ArrayRange;

        struct {
          KindId                  kind;
          uint64                  generation;
          uint16                  length;

          select (DataModel) {
            case single_value: ;    /* Empty */

            case array:
                 ArrayRange       indices<0..2^16-1>;

            case dictionary:
                 DictionaryKey    keys<0..2^16-1>;

            /* This structure may be extended */

          } model_specifier;
        } StoredDataSpecifier;

        struct {
          ResourceId              resource;
          StoredDataSpecifier     specifiers<0..2^16-1>;
        } FetchReq;

   The contents of the Fetch requests are as follows:

   resource
      The Resource-ID to fetch from.





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   specifiers
      A sequence of StoredDataSpecifier values, each specifying some of
      the data values to retrieve.

   Each StoredDataSpecifier specifies a single Kind of data to retrieve
   and, if appropriate, the subset of values that are to be retrieved.
   The contents of the StoredDataSpecifier structure are as follows:

   kind
      The Kind-ID of the data being fetched.  Implementations SHOULD
      reject requests corresponding to unknown Kinds unless specifically
      configured otherwise.

   DataModel
      The data model of the data.  This is not transmitted on the wire,
      but comes from the definition of the Kind.

   generation
      The last generation counter that the requesting node saw.  This
      may be used to avoid unnecessary fetches, or it may be set to
      zero.

   length
      The length of the rest of the structure, thus allowing
      extensibility.

   model_specifier
      A reference to the data value being requested within the data
      model specified for the Kind.  For instance, if the data model is
      "array", it might specify some subset of the values.

   The model_specifier is as follows:

   o  If the data model is single value, the specifier is empty.

   o  If the data model is array, the specifier contains a list of
      ArrayRange elements, each of which contains two integers.  The
      first integer is the beginning of the range, and the second is the
      end of the range.  0 is used to indicate the first element, and
      0xffffffff is used to indicate the final element.  The first
      integer MUST be less than or equal to the second.  While multiple
      ranges MAY be specified, they MUST NOT overlap.

   o  If the data model is dictionary, then the specifier contains a
      list of the dictionary keys being requested.  If no keys are
      specified, then this is a wildcard fetch and all key-value pairs
      are returned.




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   The generation counter is used to indicate the requester's expected
   state of the storing peer.  If the generation counter in the request
   matches the stored counter, then the storing peer returns a response
   with no StoredData values.

7.4.2.2.  Response Definition

   The response to a successful Fetch request is a FetchAns message
   containing the data requested by the requester.

         struct {
           KindId                 kind;
           uint64                 generation;
           StoredData             values<0..2^32-1>;
         } FetchKindResponse;

         struct {
           FetchKindResponse      kind_responses<0..2^32-1>;
         } FetchAns;

   The FetchAns structure contains a series of FetchKindResponse
   structures.  There MUST be one FetchKindResponse element for each
   Kind-ID in the request.

   The contents of the FetchKindResponse structure are as follows:

   kind
      The Kind that this structure is for.

   generation
      The generation counter for this Kind.

   values
      The relevant values.  If the generation counter in the request
      matches the generation counter in the stored data, then no
      StoredData values are returned.  Otherwise, all relevant data
      values MUST be returned.  A nonexistent value (i.e., one which the
      node has no knowledge of) is represented by a synthetic value with
      "exists" set to False and has an empty signature.  Specifically,
      the identity_type is set to "none", the SignatureAndHashAlgorithm
      values are set to {0, 0} ("anonymous" and "none", respectively),
      and the signature value is of zero length.  This removes the need
      for the responding node to do signatures for values which do not
      exist.  These signatures are unnecessary, as the entire response
      is signed by that node.  Note that entries which have been removed
      by the procedure given in Section 7.4.1.3 and which have not yet
      expired also have exists = False, but have valid signatures from
      the node which did the store.



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   Upon receipt of a FetchAns message, nodes MUST verify the signatures
   on all the received values.  Any values with invalid signatures
   (including expired certificates) MUST be discarded.  Note that this
   implies that implementations which wish to store data for long
   periods of time must have certificates with appropriate expiration
   dates or must re-store periodically.  Implementations MAY return the
   subset of values with valid signatures, but in that case, they SHOULD
   somehow signal to the application that a partial response was
   received.

   There is one subtle point about signature computation on arrays.  If
   the storing node uses the append feature (where the
   index=0xffffffff), then the index in the StoredData that is returned
   will not match that used by the storing node, which would break the
   signature.  In order to avoid this issue, the index value in the
   array is set to zero before the signature is computed.  This implies
   that malicious storing nodes can reorder array entries without being
   detected.

7.4.3.  Stat

   The Stat request is used to get metadata (length, generation counter,
   digest, etc.) for a stored element without retrieving the element
   itself.  The name is from the UNIX stat(2) system call, which
   performs a similar function for files in a file system.  It also
   allows the requesting node to get a list of matching elements without
   requesting the entire element.

7.4.3.1.  Request Definition

   The Stat request is identical to the Fetch request.  It simply
   specifies the elements to get metadata about.

        struct {
          ResourceId              resource;
          StoredDataSpecifier     specifiers<0..2^16-1>;
        } StatReq;














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7.4.3.2.  Response Definition

   The Stat response contains the same sort of entries that a Fetch
   response would contain.  However, instead of containing the element
   data, it contains metadata.

        struct {
          Boolean                exists;
          uint32                 value_length;
          HashAlgorithm          hash_algorithm;
          opaque                 hash_value<0..255>;
        } MetaData;

        struct {
          uint32                 index;
          MetaData               value;
        } ArrayEntryMeta;

        struct {
          DictionaryKey          key;
          MetaData               value;
        } DictionaryEntryMeta;

        struct {
          select (DataModel) {
            case single_value:
              MetaData              single_value_entry;

            case array:
              ArrayEntryMeta        array_entry;

            case dictionary:
              DictionaryEntryMeta   dictionary_entry;

            /* This structure may be extended */
          };
        } MetaDataValue;

        struct {
          uint32                  value_length;
          uint64                  storage_time;
          uint32                  lifetime;
          MetaDataValue           metadata;
        } StoredMetaData;







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        struct {
          KindId                 kind;
          uint64                 generation;
          StoredMetaData         values<0..2^32-1>;
        } StatKindResponse;

        struct {
          StatKindResponse      kind_responses<0..2^32-1>;
        } StatAns;

   The structures used in StatAns parallel those used in FetchAns: a
   response consists of multiple StatKindResponse values, one for each
   Kind that was in the request.  The contents of the StatKindResponse
   are the same as those in the FetchKindResponse, except that the
   values list contains StoredMetaData entries instead of StoredData
   entries.

   The contents of the StoredMetaData structure are the same as the
   corresponding fields in StoredData, except that there is no signature
   field and the value is a MetaDataValue rather than a StoredDataValue.

   A MetaDataValue is a variant structure, like a StoredDataValue,
   except for the types of each arm, which replace DataValue with
   MetaData.

   The only new structure is MetaData, which has the following contents:

   exists
      Same as in DataValue.

   value_length
      The length of the stored value.

   hash_algorithm
      The hash algorithm used to perform the digest of the value.

   hash_value
      A digest using hash_algorithm on the value field of the DataValue,
      including its 4 leading length bytes.

7.4.4.  Find

   The Find request can be used to explore the Overlay Instance.  A Find
   request for a Resource-ID R and a Kind-ID T retrieves the
   Resource-ID, if any, of the resource of Kind T known to the target
   peer which is closest to R.  This method can be used to walk the
   Overlay Instance by iteratively fetching R_n+1=nearest(1 + R_n).




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7.4.4.1.  Request Definition

   The FindReq message contains a Resource-ID and a series of Kind-IDs
   identifying the resource the peer is interested in.

     struct {
       ResourceId                 resource;
       KindId                     kinds<0..2^8-1>;
     } FindReq;

   The request contains a list of Kind-IDs which the Find is for, as
   indicated below:

   resource
      The desired Resource-ID.

   kinds
      The desired Kind-IDs.  Each value MUST appear only once.
      Otherwise, the request MUST be rejected with an error.

7.4.4.2.  Response Definition

   A response to a successful Find request is a FindAns message
   containing the closest Resource-ID on the peer for each Kind
   specified in the request.

    struct {
      KindId                      kind;
      ResourceId                  closest;
    } FindKindData;

    struct {
      FindKindData                results<0..2^16-1>;
    } FindAns;

   If the processing peer is not responsible for the specified
   Resource-ID, it SHOULD return an Error_Not_Found error code.

   For each Kind-ID in the request, the response MUST contain a
   FindKindData indicating the closest Resource-ID for that Kind-ID,
   unless the Kind is not allowed to be used with Find, in which case a
   FindKindData for that Kind-ID MUST NOT be included in the response.
   If a Kind-ID is not known, then the corresponding Resource-ID MUST be
   0.  Note that different Kind-IDs may have different closest
   Resource-IDs.






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   The response is simply a series of FindKindData elements, one per
   Kind, concatenated end to end.  The contents of each element are:

   kind
      The Kind-ID.

   closest
      The closest Resource-ID to the specified Resource-ID.  It is 0 if
      no Resource-ID is known.

   Note that the response does not contain the contents of the data
   stored at these Resource-IDs.  If the requester wants this, it must
   retrieve it using Fetch.

7.4.5.  Defining New Kinds

   There are two ways to define a new Kind.  The first is by writing a
   document and registering the Kind-ID with IANA.  This is the
   preferred method for Kinds which may be widely used and reused.  The
   second method is to simply define the Kind and its parameters in the
   Configuration Document using the section of Kind-ID space set aside
   for private use.  This method MAY be used to define ad hoc Kinds in
   new overlays.

   However a Kind is defined, the definition MUST include:

   o  The meaning of the data to be stored (in some textual form).

   o  The Kind-ID.

   o  The data model (single value, array, dictionary, etc.).

   o  The access control model.

   In addition, when Kinds are registered with IANA, each Kind is
   assigned a short string name which is used to refer to it in
   Configuration Documents.

   While each Kind needs to define what data model is used for its data,
   this does not mean that it must define new data models.  Where
   practical, Kinds should use the existing data models.  The intention
   is that the basic data model set be sufficient for most applications/
   usages.








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8.  Certificate Store Usage

   The Certificate Store Usage allows a node to store its certificate in
   the overlay.

   A user/node MUST store its certificate at Resource-IDs derived from
   two Resource Names:

   o  The user name in the certificate.

   o  The Node-ID in the certificate.

   Note that in the second case, the certificate for a peer is not
   stored at its Node-ID but rather at a hash of its Node-ID.  The
   intention here (as is common throughout RELOAD) is to avoid making a
   peer responsible for its own data.

   New certificates are stored at the end of the list.  This structure
   allows users to store an old and a new certificate that both have the
   same Node-ID, which allows for migration of certificates when they
   are renewed.

   This usage defines the following Kinds:

   Name:  CERTIFICATE_BY_NODE

   Data Model:  The data model for CERTIFICATE_BY_NODE data is array.

   Access Control:  NODE-MATCH

   Name:  CERTIFICATE_BY_USER

   Data Model:  The data model for CERTIFICATE_BY_USER data is array.

   Access Control:  USER-MATCH

9.  TURN Server Usage

   The TURN Server Usage allows a RELOAD peer to advertise that it is
   prepared to be a TURN server, as defined in [RFC5766].  When a node
   starts up, it joins the overlay network and forms several connections
   in the process.  If the ICE stage in any of these connections returns
   a reflexive address that is not the same as the peer's perceived
   address, then the peer is behind a NAT and SHOULD NOT be a candidate
   for a TURN server.  Additionally, if the peer's IP address is in the
   private address space range as defined by [RFC1918], then it is also





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   SHOULD NOT be a candidate for a TURN server.  Otherwise, the peer
   SHOULD assume that it is a potential TURN server and follow the
   procedures below.

   If the node is a candidate for a TURN server, it will insert some
   pointers in the overlay so that other peers can find it.  The overlay
   configuration file specifies a turn-density parameter that indicates
   how many times each TURN server SHOULD record itself in the overlay.
   Typically, this should be set to the reciprocal of the estimate of
   what percentage of peers will act as TURN servers.  If the turn-
   density is not set to zero, for each value, called d, between 1 and
   turn-density, the peer forms a Resource Name by concatenating its
   Node-ID and the value d.  This Resource Name is hashed to form a
   Resource-ID.  The address of the peer is stored at that Resource-ID
   using type TURN-SERVICE and the TurnServer object:

        struct {
          uint8                   iteration;
          IpAddressPort           server_address;
        } TurnServer;

   The contents of this structure are as follows:

   iteration
      The d value.

   server_address
      The address at which the TURN server can be contacted.

   Note:  Correct functioning of this algorithm depends on having turn-
      density be a reasonable estimate of the reciprocal of the
      proportion of nodes in the overlay that can act as TURN servers.
      If the turn-density value in the configuration file is too low,
      the process of finding TURN servers becomes more expensive, as
      multiple candidate Resource-IDs must be probed to find a TURN
      server.

   Peers that provide this service need to support the TURN extensions
   to STUN for media relay, as defined in [RFC5766].

   This usage defines the following Kind to indicate that a peer is
   willing to act as a TURN server:

   Name:  TURN-SERVICE

   Data Model:  The TURN-SERVICE Kind stores a single value for each
      Resource-ID.




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   Access Control:  NODE-MULTIPLE, with a maximum iteration of counter
      20.

   Peers MAY find other servers by selecting a random Resource-ID and
   then doing a Find request for the appropriate Kind-ID with that
   Resource-ID.  The Find request gets routed to a random peer based on
   the Resource-ID.  If that peer knows of any servers, they will be
   returned.  The returned response may be empty if the peer does not
   know of any servers, in which case the process gets repeated with
   some other random Resource-ID.  As long as the ratio of servers
   relative to peers is not too low, this approach will result in
   finding a server relatively quickly.

   Note to implementers: The certificates used by TurnServer entries
   need to be retained, as described in Section 6.3.4.

10.  Chord Algorithm

   This algorithm is assigned the name CHORD-RELOAD to indicate that it
   is an adaptation of the basic Chord-based DHT algorithm.

   This algorithm differs from the Chord algorithm that was originally
   presented in [Chord].  It has been updated based on more recent
   research results and implementation experiences, and to adapt it to
   the RELOAD protocol.  Here is a short list of differences:

   o  The original Chord algorithm specified that a single predecessor
      and a successor list be stored.  The CHORD-RELOAD algorithm
      attempts to have more than one predecessor and successor.  The
      predecessor sets help other neighbors learn their successor list.

   o  The original Chord specification and analysis called for iterative
      routing.  RELOAD specifies recursive routing.  In addition to the
      performance implications, the cost of NAT traversal dictates
      recursive routing.

   o  Finger Table entries are indexed in the opposite order.  Original
      Chord specifies finger[0] as the immediate successor of the peer.
      CHORD-RELOAD specifies finger[0] as the peer 180 degrees around
      the ring from the peer.  This change was made to simplify
      discussion and implementation of variable-sized Finger Tables.
      However, with either approach, no more than O(log N) entries
      should typically be stored in a Finger Table.

   o  The stabilize() and fix_fingers() algorithms in the original Chord
      algorithm are merged into a single periodic process.
      Stabilization is implemented slightly differently because of the
      larger neighborhood, and fix_fingers is not as aggressive to



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      reduce load, nor does it search for optimal matches of the Finger
      Table entries.

   o  RELOAD allows for a 128-bit hash instead of a 160-bit hash, as
      RELOAD is not designed to be used in networks with close to or
      more than 2^128 nodes or objects (and it is hard to see how one
      would assemble such a network).

   o  RELOAD uses randomized finger entries, as described in
      Section 10.7.4.2.

   o  The CHORD-RELOAD algorithm allows the use of either reactive or
      periodic recovery.  The original Chord paper used periodic
      recovery.  Reactive recovery provides better performance in small
      overlays, but is believed to be unstable in large overlays
      (greater than 1000) with high levels of churn
      [handling-churn-usenix04].  The overlay configuration file
      specifies a "chord-reactive" element that indicates whether
      reactive recovery should be used.

10.1.  Overview

   The algorithm described here, CHORD-RELOAD, is a modified version of
   the Chord algorithm.  In Chord (and in the algorithm described here),
   nodes are arranged in a ring, with node n being adjacent to nodes n-1
   and n+1 and with all arithmetic being done modulo 2^{k}, where k is
   the length of the Node-ID in bits, so that node 2^{k} - 1 is directly
   before node 0.

   Each peer keeps track of a Finger Table and a Neighbor Table.  The
   Neighbor Table contains at least the three peers before and after
   this peer in the DHT ring.  There may not be three entries in all
   cases, such as small rings or while the ring topology is changing.
   The first entry in the Finger Table contains the peer halfway around
   the ring from this peer, the second entry contains the peer that is
   1/4th of the way around, the third entry contains the peer that is
   1/8th of the way around, and so on.  Fundamentally, the Chord DHT can
   be thought of as a doubly linked list formed by knowing the
   successors and predecessor peers in the Neighbor Table, sorted by the
   Node-ID.  As long as the successor peers are correct, the DHT will
   return the correct result.  The pointers to the prior peers are kept
   to enable the insertion of new peers into the list structure.
   Keeping multiple predecessor and successor pointers makes it possible
   to maintain the integrity of the data structure even when consecutive
   peers simultaneously fail.  The Finger Table forms a skip list
   [wikiSkiplist] so that entries in the linked list can be found in
   O(log(N)) time instead of the typical O(N) time that a linked list
   would provide, where N represents the number of nodes in the DHT.



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   The Neighbor Table and Finger Table entries contain logical Node-IDs
   as values, but the actual mapping of an IP level addressing
   information to reach that Node-ID is kept in the Connection Table.

   A peer, x, is responsible for a particular Resource-ID, k, if k is
   less than or equal to x and k is greater than p, where p is the
   Node-ID of the previous peer in the Neighbor Table.  Care must be
   taken when computing to note that all math is modulo 2^128.

10.2.  Hash Function

   For this Chord-based Topology Plug-in, the size of the Resource-ID is
   128 bits.  The hash of a Resource-ID MUST be computed using SHA-1
   [RFC3174], and then the SHA-1 result MUST be truncated to the most
   significant 128 bits.

10.3.  Routing

   The Routing Table is conceptually the union of the Neighbor Table and
   the Finger Table.

   If a peer is not responsible for a Resource-ID k, but is directly
   connected to a node with Node-ID k, then it MUST route the message to
   that node.  Otherwise, it MUST route the request to the peer in the
   Routing Table that has the largest Node-ID that is in the interval
   between the peer and k. If no such node is found, the peer finds the
   smallest Node-ID that is greater than k and MUST route the message to
   that node.

10.4.  Redundancy

   When a peer receives a Store request for Resource-ID k and it is
   responsible for Resource-ID k, it MUST store the data and return a
   success response.  It MUST then send a Store request to its successor
   in the Neighbor Table and to that peer's successor, incrementing the
   replica number for each successor.  Note that these Store requests
   are addressed to those specific peers, even though the Resource-ID
   they are being asked to store is outside the range that they are
   responsible for.  The peers receiving these SHOULD check that they
   came from an appropriate predecessor in their Neighbor Table and that
   they are in a range that this predecessor is responsible for.  Then,
   they MUST store the data.  They do not themselves perform further
   Stores, because they can determine that they are not responsible for
   the Resource-ID.

   Note that this Topology Plug-in does not use the replica number for
   purposes other than knowing the difference between a replica and a
   non-replica.



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   Managing replicas as the overlay changes is described in
   Section 10.7.3.

   The sequential replicas used in this overlay algorithm protect
   against peer failure but not against malicious peers.  Additional
   replication from the Usage is required to protect resources from such
   attacks, as discussed in Section 13.5.4.

10.5.  Joining

   The join process for a Joining Node (JN) with Node-ID n is as
   follows:

   1.  JN MUST connect to its chosen bootstrap node, as specified in
       Section 11.4.

   2.  JN SHOULD send an Attach request to the Admitting Peer (AP) for
       Resource-ID n+1.  The "send_update" flag can be used to acquire
       the Routing Table of AP.

   3.  JN SHOULD send Attach requests to initiate connections to each of
       the peers in the Neighbor Table as well as to the desired peers
       in the Finger Table.  Note that this does not populate their
       Routing Tables, but only their Connection Tables, so JN will not
       get messages that it is expected to route to other nodes.

   4.  JN MUST enter into its Routing Table all the peers that it has
       successfully contacted.

   5.  JN MUST send a Join to AP.  The AP MUST send the response to the
       Join.

   6.  AP MUST do a series of Store requests to JN to store the data
       that JN will be responsible for.

   7.  AP MUST send JN an Update explicitly labeling JN as its
       predecessor.  At this point, JN is part of the ring and is
       responsible for a section of the overlay.  AP MAY now forget any
       data which is assigned to JN and not AP.  AP SHOULD NOT forget
       any data where AP is the replica set for the data.

   8.  The AP MUST send an Update to all of its neighbors (including JN)
       with the new values of its neighbor set (including JN).

   9.  JN MUST send Updates to all of the peers in its Neighbor Table.






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   If JN sends an Attach to AP with send_update, it immediately knows
   most of its expected neighbors from AP's Routing Table update and MAY
   directly connect to them.  This is the RECOMMENDED procedure.

   If for some reason JN does not get AP's Routing Table, it MAY still
   populate its Neighbor Table incrementally.  It SHOULD send a Ping
   directed at Resource-ID n+1 (directly after its own Resource-ID).
   This allows JN to discover its own successor.  Call that node p0.  JN
   then SHOULD send a Ping to p0+1 to discover its successor (p1).  This
   process MAY be repeated to discover as many successors as desired.
   The values for the two peers before p will be found at a later stage,
   when n receives an Update.  An alternate procedure is to send
   Attaches to those nodes rather than Pings, which form the connections
   immediately, but may be slower if the nodes need to collect ICE
   candidates.

   In order to set up its i'th Finger Table entry, JN MUST send an
   Attach to peer n+2^(128-i).  This will be routed to a peer in
   approximately the right location around the ring.  (Note that the
   first entry in the Finger Table has i=1 and not i=0 in this
   formulation.)

   The Joining Node MUST NOT send any Update message placing itself in
   the overlay until it has successfully completed an Attach with each
   peer that should be in its Neighbor Table.

10.6.  Routing Attaches

   When a peer needs to Attach to a new peer in its Neighbor Table, it
   MUST source-route the Attach request through the peer from which it
   learned the new peer's Node-ID.  Source-routing these requests allows
   the overlay to recover from instability.

   All other Attach requests, such as those for new Finger
   Table entries, are routed conventionally through the overlay.
















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10.7.  Updates

   An Update for this DHT is defined as:

        enum { invalidChordUpdateType(0),
               peer_ready(1), neighbors(2), full(3), (255) }
             ChordUpdateType;

        struct {
           uint32                 uptime;
           ChordUpdateType        type;
           select (type){
            case peer_ready:                   /* Empty */
              ;

            case neighbors:
              NodeId              predecessors<0..2^16-1>;
              NodeId              successors<0..2^16-1>;

            case full:
              NodeId              predecessors<0..2^16-1>;
              NodeId              successors<0..2^16-1>;
              NodeId              fingers<0..2^16-1>;
          };
        } ChordUpdate;

   The "uptime" field contains the time this peer has been up in
   seconds.

   The "type" field contains the type of the update, which depends on
   the reason the update was sent.

   peer_ready
      This peer is ready to receive messages.  This message is used to
      indicate that a node which has Attached is a peer and can be
      routed through.  It is also used as a connectivity check to non-
      neighbor peers.

   neighbors
      This version is sent to members of the Chord Neighbor Table.

   full
      This version is sent to peers which request an Update with a
      RouteQueryReq.







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   If the message is of type "neighbors", then the contents of the
   message will be:

   predecessors
      The predecessor set of the Updating peer.

   successors
      The successor set of the Updating peer.

   If the message is of type "full", then the contents of the message
   will be:

   predecessors
      The predecessor set of the Updating peer.

   successors
      The successor set of the Updating peer.

   fingers
      The Finger Table of the Updating peer, in numerically ascending
      order.

   A peer MUST maintain an association (via Attach) to every member of
   its neighbor set.  A peer MUST attempt to maintain at least three
   predecessors and three successors, even though this will not be
   possible if the ring is very small.  It is RECOMMENDED that O(log(N))
   predecessors and successors be maintained in the neighbor set.  There
   are many ways to estimate N, some of which are discussed in
   [DHT-RELOAD].

10.7.1.  Handling Neighbor Failures

   Every time a connection to a peer in the Neighbor Table is lost (as
   determined by connectivity pings or the failure of some request), the
   peer MUST remove the entry from its Neighbor Table and replace it
   with the best match it has from the other peers in its Routing Table.
   If using reactive recovery, the peer MUST send an immediate Update to
   all nodes in its Neighbor Table.  The update will contain all the
   Node-IDs of the current entries of the table (after the failed one
   has been removed).  Note that when replacing a successor, the peer
   SHOULD delay the creation of new replicas for the successor
   replacement hold-down time (30 seconds) after removing the failed
   entry from its Neighbor Table in order to allow a triggered update to
   inform it of a better match for its Neighbor Table.

   If the neighbor failure affects the peer's range of responsible IDs,
   then the Update MUST be sent to all nodes in its Connection Table.




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   A peer MAY attempt to reestablish connectivity with a lost neighbor
   either by waiting additional time to see if connectivity returns or
   by actively routing a new Attach to the lost peer.  Details for these
   procedures are beyond the scope of this document.  In the case of an
   attempt to reestablish connectivity with a lost neighbor, the peer
   MUST be removed from the Neighbor Table.  Such a peer is returned to
   the Neighbor Table once connectivity is reestablished.

   If connectivity is lost to all successor peers in the Neighbor Table,
   then this peer SHOULD behave as if it is joining the network and MUST
   use Pings to find a peer and send it a Join.  If connectivity is lost
   to all the peers in the Finger Table, this peer SHOULD assume that it
   has been disconnected from the rest of the network, and it SHOULD
   periodically try to join the DHT.

10.7.2.  Handling Finger Table Entry Failure

   If a Finger Table entry is found to have failed (as determined by
   connectivity pings or the failure of some request), all references to
   the failed peer MUST be removed from the Finger Table and replaced
   with the closest preceding peer from the Finger Table or Neighbor
   Table.

   If using reactive recovery, the peer MUST initiate a search for a new
   Finger Table entry, as described below.

10.7.3.  Receiving Updates

   When a peer x receives an Update request, it examines the Node-IDs in
   the UpdateReq and at its Neighbor Table and decides if this UpdateReq
   would change its Neighbor Table.  This is done by taking the set of
   peers currently in the Neighbor Table and comparing them to the peers
   in the Update request.  There are two major cases:

   o  The UpdateReq contains peers that match x's Neighbor Table, so no
      change is needed to the neighbor set.

   o  The UpdateReq contains peers that x does not know about that
      should be in x's Neighbor Table; i.e., they are closer than
      entries in the Neighbor Table.

   In the first case, no change is needed.

   In the second case, x MUST attempt to Attach to the new peers, and if
   it is successful, it MUST adjust its neighbor set accordingly.  Note
   that x can maintain the now inferior peers as neighbors, but it MUST
   remember the closer ones.




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   After any Pings and Attaches are done, if the Neighbor Table changes
   and the peer is using reactive recovery, the peer MUST send an Update
   request to each member of its Connection Table.  These Update
   requests are what end up filling in the predecessor/successor tables
   of peers that this peer is a neighbor to.  A peer MUST NOT enter
   itself in its successor or predecessor table and instead should leave
   the entries empty.

   If peer x is responsible for a Resource-ID R and x discovers that the
   replica set for R (the next two nodes in its successor set) has
   changed, it MUST send a Store for any data associated with R to any
   new node in the replica set.  It SHOULD NOT delete data from peers
   which have left the replica set.

   When peer x detects that it is no longer in the replica set for a
   resource R (i.e., there are three predecessors between x and R), it
   SHOULD delete all data associated with R from its local store.

   When a peer discovers that its range of responsible IDs has changed,
   it MUST send an Update to all entries in its Connection Table.

10.7.4.  Stabilization

   There are four components to stabilization:

   1.  Exchange Updates with all peers in its Neighbor Table to exchange
       state.

   2.  Search for better peers to place in its Finger Table.

   3.  Search to determine if the current Finger Table size is
       sufficiently large.

   4.  Search to determine if the overlay has partitioned and needs to
       recover.

10.7.4.1.  Updating the Neighbor Table

   A peer MUST periodically send an Update request to every peer in its
   Neighbor Table.  The purpose of this is to keep the predecessor and
   successor lists up to date and to detect failed peers.  The default
   time is about every ten minutes, but the configuration server SHOULD
   set this in the Configuration Document using the "chord-update-
   interval" element (denominated in seconds).  A peer SHOULD randomly
   offset these Update requests so they do not occur all at once.






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10.7.4.2.  Refreshing the Finger Table

   A peer MUST periodically search for new peers to replace invalid
   entries in the Finger Table.  For peer x, the i'th Finger Table entry
   is valid if it is in the range [ x+2^( 128-i ),
   x+2^( 128-(i-1) )-1 ].  Invalid entries occur in the Finger
   Table when a previous Finger Table entry has failed or when no peer
   has been found in that range.

   Two possible methods for searching for new peers for the Finger
   Table entries are presented:

   Alternative 1: A peer selects one entry in the Finger Table from
   among the invalid entries.  It pings for a new peer for that Finger
   Table entry.  The selection SHOULD be exponentially weighted to
   attempt to replace earlier (lower i) entries in the Finger Table.  A
   simple way to implement this selection is to search through the
   Finger Table entries from i=1, and each time an invalid entry is
   encountered, send a Ping to replace that entry with probability 0.5.

   Alternative 2: A peer monitors the Update messages received from its
   connections to observe when an Update indicates a peer that would be
   used to replace an invalid Finger Table entry, i, and flags that
   entry in the Finger Table.  Every "chord-ping-interval" seconds, the
   peer selects from among those flagged candidates using an
   exponentially weighted probability, as above.

   When searching for a better entry, the peer SHOULD send the Ping to a
   Node-ID selected randomly from that range.  Random selection is
   preferred over a search for strictly spaced entries to minimize the
   effect of churn on overlay routing [minimizing-churn-sigcomm06].  An
   implementation or subsequent specification MAY choose a method for
   selecting Finger Table entries other than choosing randomly within
   the range.  Any such alternate methods SHOULD be employed only on
   Finger Table stabilization and not for the selection of initial
   Finger Table entries unless the alternative method is faster and
   imposes less overhead on the overlay.

   A peer SHOULD NOT send Ping requests looking for new finger table
   entries more often than the configuration element "chord-ping-
   interval", which defaults to 3600 seconds (one per hour).

   A peer MAY choose to keep connections to multiple peers that can act
   for a given Finger Table entry.







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10.7.4.3.  Adjusting Finger Table Size

   If the Finger Table has fewer than 16 entries, the node SHOULD
   attempt to discover more fingers to grow the size of the table to 16.
   The value 16 was chosen to ensure high odds of a node maintaining
   connectivity to the overlay even with strange network partitions.

   For many overlays, 16 Finger Table entries will be enough, but as an
   overlay grows very large, more than 16 entries may be required in the
   Finger Table for efficient routing.  An implementation SHOULD be
   capable of increasing the number of entries in the Finger Table to
   128 entries.

   Although log(N) entries are all that are required for optimal
   performance, careful implementation of stabilization will result in
   no additional traffic being generated when maintaining a Finger
   Table larger than log(N) entries.  Implementers are encouraged to
   make use of RouteQuery and algorithms for determining where new
   Finger Table entries may be found.  Complete details of possible
   implementations are outside the scope of this specification.

   A simple approach to sizing the Finger Table is to ensure that the
   Finger Table is large enough to contain at least the final successor
   in the peer's Neighbor Table.

10.7.4.4.  Detecting Partitioning

   To detect that a partitioning has occurred and to heal the overlay, a
   peer P MUST periodically repeat the discovery process used in the
   initial join for the overlay to locate an appropriate bootstrap node,
   B.  P SHOULD then send a Ping for its own Node-ID routed through B.
   If a response is received from peer S', which is not P's successor,
   then the overlay is partitioned and P SHOULD send an Attach to S'
   routed through B, followed by an Update sent to S'.  (Note that S'
   may not be in P's Neighbor Table once the overlay is healed, but the
   connection will allow S' to discover appropriate neighbor entries for
   itself via its own stabilization.)

   Future specifications may describe alternative mechanisms for
   determining when to repeat the discovery process.











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10.8.  Route Query f.in 3

       For CHORD-RELOAD, the RouteQueryReq contains no additional
       information.  The RouteQueryAns contains the single Node-ID of
       the next peer to which the responding peer would have routed the
       request message in recursive routing:

      struct {
         NodeId                  next_peer;
      } ChordRouteQueryAns;

   The contents of this structure are as follows:

   next_peer
      The peer to which the responding peer would route the message in
      order to deliver it to the destination listed in the request.

   If the requester has set the send_update flag, the responder SHOULD
   initiate an Update immediately after sending the RouteQueryAns.

10.9.  Leaving

   To support extensions, such as [DHT-RELOAD], peers SHOULD send a
   Leave request to all members of their Neighbor Table before exiting
   the Overlay Instance.  The overlay_specific_data field MUST contain
   the ChordLeaveData structure, defined below:

              enum { invalidChordLeaveType(0),
                      from_succ(1), from_pred(2), (255) }
                    ChordLeaveType;

               struct {
                 ChordLeaveType         type;

                  select (type) {
                    case from_succ:
                      NodeId            successors<0..2^16-1>;

                    case from_pred:
                      NodeId           predecessors<0..2^16-1>;
                  };
               } ChordLeaveData;









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   The "type" field indicates whether the Leave request was sent by a
   predecessor or a successor of the recipient:

   from_succ
      The Leave request was sent by a successor.

   from_pred
      The Leave request was sent by a predecessor.

   If the type of the request is "from_succ", the contents will be:

   successors
      The sender's successor list.

   If the type of the request is "from_pred", the contents will be:

   predecessors
      The sender's predecessor list.

   Any peer which receives a Leave for a peer n in its neighbor set MUST
   follow procedures as if it had detected a peer failure as described
   in Section 10.7.1.

11.  Enrollment and Bootstrap

   The section defines the format of the configuration data as well the
   process to join a new overlay.

11.1.  Overlay Configuration

   This specification defines a new content type
   "application/p2p-overlay+xml" for a MIME entity that contains overlay
   information.  An example document is shown below:

   <?xml version="1.0" encoding="UTF-8"?>
   <overlay xmlns="urn:ietf:params:xml:ns:p2p:config-base"
      xmlns:ext="urn:ietf:params:xml:ns:p2p:config-ext1"
      xmlns:chord="urn:ietf:params:xml:ns:p2p:config-chord">
      <configuration instance-name="overlay.example.org" sequence="22"
          expiration="2002-10-10T07:00:00Z" ext:ext-example="stuff" >
          <topology-plugin> CHORD-RELOAD </topology-plugin>
          <node-id-length>16</node-id-length>
          <root-cert>
   MIIDJDCCAo2gAwIBAgIBADANBgkqhkiG9w0BAQUFADBwMQswCQYDVQQGEwJVUzET
   MBEGA1UECBMKQ2FsaWZvcm5pYTERMA8GA1UEBxMIU2FuIEpvc2UxDjAMBgNVBAoT
   BXNpcGl0MSkwJwYDVQQLEyBTaXBpdCBUZXN0IENlcnRpZmljYXRlIEF1dGhvcml0
   eTAeFw0wMzA3MTgxMjIxNTJaFw0xMzA3MTUxMjIxNTJaMHAxCzAJBgNVBAYTAlVT
   MRMwEQYDVQQIEwpDYWxpZm9ybmlhMREwDwYDVQQHEwhTYW4gSm9zZTEOMAwGA1UE



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   ChMFc2lwaXQxKTAnBgNVBAsTIFNpcGl0IFRlc3QgQ2VydGlmaWNhdGUgQXV0aG9y
   aXR5MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDDIh6DkcUDLDyK9BEUxkud
   +nJ4xrCVGKfgjHm6XaSuHiEtnfELHM+9WymzkBNzZpJu30yzsxwfKoIKugdNUrD4
   N3viCicwcN35LgP/KnbN34cavXHr4ZlqxH+OdKB3hQTpQa38A7YXdaoz6goW2ft5
   Mi74z03GNKP/G9BoKOGd5QIDAQABo4HNMIHKMB0GA1UdDgQWBBRrRhcU6pR2JYBU
   bhNU2qHjVBShtjCBmgYDVR0jBIGSMIGPgBRrRhcU6pR2JYBUbhNU2qHjVBShtqF0
   pHIwcDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCkNhbGlmb3JuaWExETAPBgNVBAcT
   CFNhbiBKb3NlMQ4wDAYDVQQKEwVzaXBpdDEpMCcGA1UECxMgU2lwaXQgVGVzdCBD
   ZXJ0aWZpY2F0ZSBBdXRob3JpdHmCAQAwDAYDVR0TBAUwAwEB/zANBgkqhkiG9w0B
   AQUFAAOBgQCWbRvv1ZGTRXxbH8/EqkdSCzSoUPrs+rQqR0xdQac9wNY/nlZbkR3O
   qAezG6Sfmklvf+DOg5RxQq/+Y6I03LRepc7KeVDpaplMFGnpfKsibETMipwzayNQ
   QgUf4cKBiF+65Ue7hZuDJa2EMv8qW4twEhGDYclpFU9YozyS1OhvUg==
          </root-cert>
          <root-cert> YmFkIGNlcnQK </root-cert>
          <enrollment-server>https://example.org</enrollment-server>
          <enrollment-server>https://example.net</enrollment-server>
          <self-signed-permitted
                    digest="sha1">false</self-signed-permitted>
          <bootstrap-node address="192.0.0.1" port="6084" />
          <bootstrap-node address="192.0.2.2" port="6084" />
          <bootstrap-node address="2001:DB8::1" port="6084" />
          <turn-density> 20 </turn-density>
          <clients-permitted> false </clients-permitted>
          <no-ice> false </no-ice>
          <chord:chord-update-interval>
              400</chord:chord-update-interval>
          <chord:chord-ping-interval>30</chord:chord-ping-interval>
          <chord:chord-reactive> true </chord:chord-reactive>
          <shared-secret> password </shared-secret>
          <max-message-size>4000</max-message-size>
          <initial-ttl> 30 </initial-ttl>
          <overlay-reliability-timer> 3000 </overlay-reliability-timer>
          <overlay-link-protocol>TLS</overlay-link-protocol>
          <configuration-signer>47112162e84c69ba</configuration-signer>
          <kind-signer> 47112162e84c69ba </kind-signer>
          <kind-signer> 6eba45d31a900c06 </kind-signer>
          <bad-node> 6ebc45d31a900c06 </bad-node>
          <bad-node> 6ebc45d31a900ca6 </bad-node>

          <ext:example-extension> foo </ext:example-extension>

          <mandatory-extension>
              urn:ietf:params:xml:ns:p2p:config-ext1
          </mandatory-extension>

          <required-kinds>
            <kind-block>
              <kind name="SIP-REGISTRATION">



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                  <data-model>SINGLE</data-model>
                  <access-control>USER-MATCH</access-control>
                  <max-count>1</max-count>
                  <max-size>100</max-size>
              </kind>
              <kind-signature>
                   VGhpcyBpcyBub3QgcmlnaHQhCg==
              </kind-signature>
            </kind-block>
            <kind-block>
              <kind id="2000">
                  <data-model>ARRAY</data-model>
                  <access-control>NODE-MULTIPLE</access-control>
                  <max-node-multiple>3</max-node-multiple>
                  <max-count>22</max-count>
                  <max-size>4</max-size>
                  <ext:example-kind-extension> 1
                          </ext:example-kind-extension>
              </kind>
              <kind-signature>
                 VGhpcyBpcyBub3QgcmlnaHQhCg==
              </kind-signature>
            </kind-block>
          </required-kinds>
      </configuration>
      <signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>

      <configuration instance-name="other.example.net">
      </configuration>
      <signature> VGhpcyBpcyBub3QgcmlnaHQhCg== </signature>

    </overlay>

   The file MUST be a well-formed XML document, and it SHOULD contain an
   encoding declaration in the XML declaration.  The file MUST use the
   UTF-8 character encoding.  The namespaces for the elements defined in
   this specification are urn:ietf:params:xml:ns:p2p:config-base and
   urn:ietf:params:xml:ns:p2p:config-chord.

   Note that elements or attributes that are defined as type xsd:boolean
   in the RELAX NG schema (Section 11.1.1) have two lexical
   representations, "1" or "true" for the concept true, and "0" or
   "false" for the concept false.  Whitespace and case processing
   follows the rules of [OASIS.relax_ng] and XML Schema Datatypes
   [W3C.REC-xmlschema-2-20041028].






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   The file MAY contain multiple "configuration" elements, where each
   one contains the configuration information for a different overlay.
   Each configuration element MAY be followed by signature elements that
   provide a signature over the preceding configuration element.  Each
   configuration element has the following attributes:

   instance-name
      The name of the overlay (referred to as "overlay name" in this
      specification)

   expiration
      Time in the future at which this overlay configuration is no
      longer valid.  The node SHOULD retrieve a new copy of the
      configuration at a randomly selected time that is before the
      expiration time.  Note that if the certificates expire before a
      new configuration is retried, the node will not be able to
      validate the configuration file.  All times MUST conform to the
      Internet date/time format defined in [RFC3339] and be specified
      using UTC.

   sequence
      A monotonically increasing sequence number between 0 and 2^16-2.

   Inside each overlay element, the following elements can occur:

   topology-plug-in
      This element defines the overlay algorithm being used.  If
      missing, the default is "CHORD-RELOAD".

   node-id-length
      This element contains the length of a NodeId (NodeIdLength), in
      bytes.  This value MUST be between 16 (128 bits) and 20 (160
      bits).  If this element is not present, the default of 16 is used.

   root-cert
      This element contains a base-64-encoded X.509v3 certificate that
      is a root trust anchor used to sign all certificates in this
      overlay.  There can be more than one root-cert element.

   enrollment-server
      This element contains the URL at which the enrollment server can
      be reached in a "url" element.  This URL MUST be of type "https:".
      More than one enrollment-server element MAY be present.  Note that
      there is no necessary relationship between the overlay name/
      configuration server name and the enrollment server name.






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   self-signed-permitted
      This element indicates whether self-signed certificates are
      permitted.  If it is set to "true", then self-signed certificates
      are allowed, in which case the enrollment-server and root-cert
      elements MAY be absent.  Otherwise, it SHOULD be absent, but MAY
      be set to "false".  This element also contains an attribute
      "digest", which indicates the digest to be used to compute the
      Node-ID.  Valid values for this parameter are "sha1" and "sha256",
      representing SHA-1 [RFC3174] and SHA-256 [RFC6234], respectively.
      Implementations MUST support both of these algorithms.

   bootstrap-node
      This element represents the address of one of the bootstrap nodes.
      It has an attribute called "address" that represents the IP
      address (either IPv4 or IPv6, since they can be distinguished) and
      an optional attribute called "port" that represents the port and
      defaults to 6084.  The IPv6 address is in typical hexadecimal form
      using standard period and colon separators as specified in
      [RFC5952].  More than one bootstrap-node element MAY be present.

   turn-density
      This element is a positive integer that represents the approximate
      reciprocal of density of nodes that can act as TURN servers.  For
      example, if 5% of the nodes can act as TURN servers, this element
      would be set to 20.  If it is not present, the default value is 1.
      If there are no TURN servers in the overlay, it is set to zero.

   clients-permitted
      This element represents whether clients are permitted or whether
      all nodes must be peers.  If clients are permitted, the element
      MUST be set to "true" or be absent.  If the nodes are not allowed
      to remain clients after the initial join, the element MUST be set
      to "false".  There is currently no way for the overlay to enforce
      this.

   no-ice
      This element represents whether nodes are REQUIRED to use the
      "No-ICE" Overlay Link protocols in this overlay.  If it is absent,
      it is treated as if it were set to "false".

   chord-update-interval
      The update frequency for the CHORD-RELOAD Topology Plug-in (see
      Section 10).

   chord-ping-interval
      The Ping frequency for the CHORD-RELOAD Topology Plug-in (see
      Section 10).




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   chord-reactive
      Whether reactive recovery SHOULD be used for this overlay.  It is
      set to "true" or "false".  If missing, the default is "true" (see
      Section 10).

   shared-secret
      If shared secret mode is used, this element contains the shared
      secret.  The security guarantee here is that any agent which is
      able to access the Configuration Document (presumably protected by
      some sort of HTTP access control or network topology) is able to
      recover the shared secret and hence join the overlay.

   max-message-size
      Maximum size, in bytes, of any message in the overlay.  If this
      value is not present, the default is 5000.

   initial-ttl
      Initial default TTL for messages (see Section 6.3.2).  If this
      value is not present, the default is 100.

   overlay-reliability-timer
      Default value for the end-to-end retransmission timer for
      messages, in milliseconds.  If not present, the default value is
      3000.  The value MUST be at least 200 milliseconds, which means
      the minimum time delay before dropping a link is 1000
      milliseconds.

   overlay-link-protocol
      Indicates a permissible overlay link protocol (see Section 6.6.1
      for requirements for such protocols).  An arbitrary number of
      these elements may appear.  If none appear, then this implies the
      default value, "TLS", which refers to the use of TLS and DTLS.  If
      one or more elements appear, then no default value applies.

   kind-signer
      This contains a single Node-ID in hexadecimal and indicates that
      the certificate with this Node-ID is allowed to sign Kinds.
      Identifying kind-signer by Node-ID instead of certificate allows
      the use of short-lived certificates without constantly having to
      provide an updated configuration file.

   configuration-signer
      This contains a single Node-ID in hexadecimal and indicates that
      the certificate with this Node-ID is allowed to sign
      configurations for this instance-name.  Identifying the signer by
      Node-ID instead of certificate allows the use of short-lived
      certificates without constantly having to provide an updated
      configuration file.



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   bad-node
      This contains a single Node-ID in hexadecimal and indicates that
      the certificate with this Node-ID MUST NOT be considered valid.
      This allows certificate revocation.  An arbitrary number of these
      elements can be provided.  Note that because certificates may
      expire, bad-node entries need be present only for the lifetime of
      the certificate.  Technically speaking, bad Node-IDs may be reused
      after their certificates have expired.  The requirement for
      Node-IDs to be pseudorandomly generated gives this event a
      vanishing probability.

   mandatory-extension
      This element contains the name of an XML namespace that a node
      joining the overlay MUST support.  The presence of a mandatory-
      extension element does not require the extension to be used in the
      current configuration file, but can indicate that it may be used
      in the future.  Note that the namespace is case-sensitive, as
      specified in Section 2.3 of [w3c-xml-namespaces].  More than one
      mandatory-extension element MAY be present.

   Inside each configuration element, the required-kinds element MAY
   also occur.  This element indicates the Kinds that members MUST
   support and contains multiple kind-block elements that each define a
   single Kind that MUST be supported by nodes in the overlay.  Each
   kind-block consists of a single kind element and a kind-signature.
   The kind element defines the Kind.  The kind-signature is the
   signature computed over the kind element.

   Each kind element has either an id attribute or a name attribute.
   The name attribute is a string representing the Kind (the name
   registered to IANA), while the id is an integer Kind-ID allocated out
   of private space.

   In addition, the kind element MUST contain the following elements:

   max-count
      The maximum number of values which members of the overlay must
      support.

   data-model
      The data model to be used.

   max-size
      The maximum size of individual values.

   access-control
      The access control model to be used.




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   The kind element MAY also contain the following element:

   max-node-multiple
      If the access control is NODE-MULTIPLE, this element MUST be
      included.  This indicates the maximum value for the i counter.  It
      MUST be an integer greater than 0.

   All of the non-optional values MUST be provided.  If the Kind is
   registered with IANA, the data-model and access-control elements MUST
   match those in the Kind registration, and clients MUST ignore them in
   favor of the IANA versions.  Multiple kind-block elements MAY be
   present.

   The kind-block element also MUST contain a "kind-signature" element.
   This signature is computed across the kind element from the beginning
   of the first < of the kind element to the end of the last > of the
   kind element in the same way as the signature element described later
   in this section. kind-block elements MUST be signed by a node listed
   in the kind-signers block of the current configuration.  Receivers
   MUST verify the signature prior to accepting a kind-block.

   The configuration element MUST be treated as a binary blob that
   cannot be changed -- including any whitespace changes -- or the
   signature will break.  The signature MUST be computed by taking each
   configuration element and starting from, and including, the first <
   at the start of <configuration> up to and including the > in </
   configuration> and treating this as a binary blob that MUST be signed
   using the standard SecurityBlock defined in Section 6.3.4.  The
   SecurityBlock MUST be base-64 encoded using the base64 alphabet from
   [RFC4648] and MUST be put in the signature element following the
   configuration object in the configuration file.  Any configuration
   file MUST be signed by one of the configuration-signer elements from
   the previous extant configuration.  Recipients MUST verify the
   signature prior to accepting the configuration file.

   When a node receives a new configuration file, it MUST change its
   configuration to meet the new requirements.  This may require the
   node to exit the DHT and rejoin.  If a node is not capable of
   supporting the new requirements, it MUST exit the overlay.  If some
   information about a particular Kind changes from what the node
   previously knew about the Kind (for example, the max size), the new
   information in the configuration files overrides any previously
   learned information.  If any Kind data was signed by a node that is
   no longer allowed to sign Kinds, that Kind MUST be discarded along
   with any stored information of that Kind.  Note that forcing an
   avalanche restart of the overlay with a configuration change that
   requires rejoining the overlay may result in serious performance
   problems, including total collapse of the network if configuration



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   parameters are not properly considered.  Such an event may be
   necessary in case of a compromised CA or similar problem, but for
   large overlays, it should be avoided in almost all circumstances.

11.1.1.  RELAX NG Grammar

   The grammar for the configuration data is:

   namespace chord = "urn:ietf:params:xml:ns:p2p:config-chord"
   namespace local = ""
   default namespace p2pcf = "urn:ietf:params:xml:ns:p2p:config-base"
   namespace rng = "http://relaxng.org/ns/structure/1.0"

   anything =
       (element * { anything }
        | attribute * { text }
        | text)*

   foreign-elements = element * - (p2pcf:* | local:* | chord:*)
                      { anything }*
   foreign-attributes = attribute * - (p2pcf:*|local:*|chord:*)
                        { text }*
   foreign-nodes = (foreign-attributes | foreign-elements)*

   start =  element p2pcf:overlay {
         overlay-element
   }

   overlay-element &=  element configuration {
               attribute instance-name { xsd:string },
               attribute expiration { xsd:dateTime }?,
               attribute sequence { xsd:long }?,
               foreign-attributes*,
               parameter
           }+
   overlay-element &= element signature {
               attribute algorithm { signature-algorithm-type }?,
               xsd:base64Binary
           }*

   signature-algorithm-type |= "rsa-sha1"
   signature-algorithm-type |=  xsd:string # signature alg extensions

   parameter &= element topology-plugin { topology-plugin-type }?
   topology-plugin-type |= xsd:string # topo plugin extensions
   parameter &= element max-message-size { xsd:unsignedInt }?
   parameter &= element initial-ttl { xsd:int }?
   parameter &= element root-cert { xsd:base64Binary }*



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   parameter &= element required-kinds { kind-block* }?
   parameter &= element enrollment-server { xsd:anyURI }*
   parameter &= element kind-signer {  xsd:string }*
   parameter &= element configuration-signer {  xsd:string }*
   parameter &= element bad-node {  xsd:string }*
   parameter &= element no-ice { xsd:boolean }?
   parameter &= element shared-secret { xsd:string }?
   parameter &= element overlay-link-protocol { xsd:string }*
   parameter &= element clients-permitted { xsd:boolean }?
   parameter &= element turn-density { xsd:unsignedByte }?
   parameter &= element node-id-length { xsd:int }?
   parameter &= element mandatory-extension { xsd:string }*
   parameter &= foreign-elements*

   parameter &=
       element self-signed-permitted {
           attribute digest { self-signed-digest-type },
           xsd:boolean
       }?
   self-signed-digest-type |= "sha1"
   self-signed-digest-type |=  xsd:string # signature digest extensions

   parameter &= element bootstrap-node {
                   attribute address { xsd:string },
                   attribute port { xsd:int }?
                }*

   kind-block = element kind-block {
       element kind {
           (  attribute name { kind-names }
              | attribute id { xsd:unsignedInt } ),
           kind-parameter
       } &
       element kind-signature  {
           attribute algorithm { signature-algorithm-type }?,
           xsd:base64Binary
       }?
   }

   kind-parameter &= element max-count { xsd:int }
   kind-parameter &= element max-size { xsd:int }
   kind-parameter &= element max-node-multiple { xsd:int }?

   kind-parameter &= element data-model { data-model-type }
   data-model-type |= "SINGLE"
   data-model-type |= "ARRAY"
   data-model-type |= "DICTIONARY"
   data-model-type |=  xsd:string # data model extensions



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   kind-parameter &= element access-control { access-control-type }
   access-control-type |= "USER-MATCH"
   access-control-type |= "NODE-MATCH"
   access-control-type |= "USER-NODE-MATCH"
   access-control-type |= "NODE-MULTIPLE"
   access-control-type |= xsd:string # access control extensions

   kind-parameter &= foreign-elements*

   kind-names |= "TURN-SERVICE"
   kind-names |= "CERTIFICATE_BY_NODE"
   kind-names |= "CERTIFICATE_BY_USER"
   kind-names |= xsd:string # kind extensions

   # Chord specific parameters
   topology-plugin-type |= "CHORD-RELOAD"
   parameter &= element chord:chord-ping-interval { xsd:int }?
   parameter &= element chord:chord-update-interval { xsd:int }?
   parameter &= element chord:chord-reactive { xsd:boolean }?

11.2.  Discovery through Configuration Server

   When a node first enrolls in a new overlay, it starts with a
   discovery process to find a configuration server.

   The node MAY start by determining the overlay name.  This value MUST
   be provided by the user or some other out-of-band provisioning
   mechanism.  The out-of-band mechanism MAY also provide an optional
   URL for the configuration server.  If a URL for the configuration
   server is not provided, the node MUST do a DNS SRV query using a
   Service name of "reload-config" and a protocol of TCP to find a
   configuration server and form the URL by appending a path of
   "/.well-known/reload-config" to the overlay name.  This uses the
   "well-known URI" framework defined in [RFC5785].  For example, if the
   overlay name was example.com, the URL would be
   "https://example.com/.well-known/reload-config".

   Once an address and URL for the configuration server are determined,
   the peer MUST form an HTTPS connection to that IP address.  If an
   optional URL for the configuration server was provided, the
   certificate MUST match the domain name from the URL as described in
   [RFC2818]; otherwise, the certificate MUST match the overlay name as
   described in [RFC2818].  If the HTTPS certificates pass the name
   matching, the node MUST fetch a new copy of the configuration file.
   To do this, the peer performs a GET to the URL.  The result of the
   HTTP GET is an XML configuration file described above.  If the XML is
   not valid or the instance-name attribute of the overlay-element in
   the XML does not match the overlay name, this configurations file



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   SHOULD be discarded.  Otherwise, the new configuration MUST replace
   any previously learned configuration file for this overlay.

   For overlays that do not use a configuration server, nodes MUST
   obtain the configuration information needed to join the overlay
   through some out-of-band approach, such as an XML configuration file
   sent over email.

11.3.  Credentials

   If the Configuration Document contains an enrollment-server element,
   credentials are REQUIRED to join the Overlay Instance.  A peer which
   does not yet have credentials MUST contact the enrollment server to
   acquire them.

   RELOAD defines its own trivial certificate request protocol.  We
   would have liked to have used an existing protocol, but were
   concerned about the implementation burden of even the simplest of
   those protocols, such as [RFC5272] and [RFC5273].  The objective was
   to have a protocol which could be easily implemented in a Web server
   which the operator did not control (e.g., in a hosted service) and
   which was compatible with the existing certificate-handling tooling
   as used with the Web certificate infrastructure.  This means
   accepting bare PKCS#10 requests and returning a single bare X.509
   certificate.  Although the MIME types for these objects are defined,
   none of the existing protocols support exactly this model.

   The certificate request protocol MUST be performed over HTTPS.  The
   server certificate MUST match the overlay name as described in
   [RFC2818].  The request MUST be an HTTP POST with the parameters
   encoded as described in [RFC2388] and with the following properties:

   o  If authentication is required, there MUST be form parameters of
      "password" and "username" containing the user's account name and
      password in the clear (hence the need for HTTPS).  The username
      and password strings MUST be UTF-8 strings compared as binary
      objects.  Applications using RELOAD SHOULD define any needed
      string preparation as per [RFC4013] or its successor documents.

   o  If more than one Node-ID is required, there MUST be a form
      parameter of "nodeids" containing the number of Node-IDs required.

   o  There MUST be a form parameter of "csr" with a content type of
      "application/pkcs10", as defined in [RFC2311], that contains the
      certificate signing request (CSR).

   o  The Accept header MUST contain the type "application/pkix-cert",
      indicating the type that is expected in the response.



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   The enrollment server MUST authenticate the request using the
   provided account name and password.  The reason for using the RFC
   2388 "multipart/form-data" encoding is so that the password parameter
   will not be encoded in the URL, to reduce the chance of accidental
   leakage of the password.  If the authentication succeeds and the
   requested user name in the CSR is acceptable, the server MUST
   generate and return a certificate for the CSR in the "csr" parameter
   of the request.  The SubjectAltName field in the certificate MUST
   contain the following values:

   o  One or more Node-IDs which MUST be cryptographically random
      [RFC4086].  Each MUST be chosen by the enrollment server in such a
      way that it is unpredictable to the requesting user.  For example,
      the user MUST NOT be informed of potential (random) Node-IDs prior
      to authenticating.  Each is placed in the subjectAltName using the
      uniformResourceIdentifier type, each MUST contain RELOAD URI, as
      described in Section 14.15, and each MUST contain a Destination
      List with a single entry of type "node_id".  The enrollment server
      SHOULD maintain a mapping of users to Node-IDs and if the same
      user returns (e.g., to have their certificate re-issued), the
      enrollment server should return the same Node-IDs, thus avoiding
      the need for implementations to re-store all their data when their
      certificates expire.

   o  A single name (the "user name") that this user is allowed to use
      in the overlay, using type rfc822Name.  Enrollment servers SHOULD
      take care to allow only legal characters in the name (e.g., no
      embedded NULs), rather than simply accepting any name provided by

      the user.  In some usages, the right side of the user name will
      match the overlay name, but there is no requirement for this match
      in this specification.  Applications using this specification MAY
      define such a requirement or MAY otherwise limit the allowed range
      of allowed user names.

   The SubjectAltName field in the certificate MUST NOT contain any
   identities other than those listed above.  The subject distinguished
   name in the certificate MUST be empty.

   The certificate MUST be returned as type "application/pkix-cert", as
   defined in [RFC2585], with an HTTP status code of 200 OK.










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   Certificate processing errors SHOULD result in an HTTP return code of
   403 Forbidden, along with a body of type "text/plain" and body that
   consists of one of the tokens defined in the following list:

   failed_authentication
      The account name and password combination used in the HTTPS
      request was not valid.

   username_not_available
      The requested user name in the CSR was not acceptable.

   Node-IDs_not_available
      The number of Node-IDs requested was not acceptable.

   bad_CSR
      There was some other problem with the CSR.

   If the client receives an unknown token in the body, it SHOULD treat
   it as a failure for an unknown reason.

   The client MUST check that the returned certificate chains back to
   one of the certificates received in the "root-cert" list of the
   overlay configuration data (including PKIX BasicConstraints checks).
   The node then reads the certificate to find the Node-ID it can use.

11.3.1.  Self-Generated Credentials

   If the "self-signed-permitted" element is present in the
   configuration and is set to "true", then a node MUST generate its own
   self-signed certificate to join the overlay.  The self-signed
   certificate MAY contain any user name of the user's choice.

   For self-signed certificates containing only one Node-ID, the Node-ID
   MUST be computed by applying the digest specified in the self-signed-
   permitted element to the DER representation of the user's public key
   (more specifically, the subjectPublicKeyInfo) and taking the high-
   order bits.  For self-signed certificates containing multiple
   Node-IDs, the index of the Node-ID (from 1 to the number of Node-IDs
   needed) must be prepended as a 4-byte big-endian integer to the DER
   representation of the user's public key and taking the high-order
   bits.  When accepting a self-signed certificate, nodes MUST check
   that the Node-ID and public keys match.  This prevents Node-ID theft.

   Once the node has constructed a self-signed certificate, it MAY join
   the overlay.  It MUST store its certificate in the overlay
   (Section 8), but SHOULD look to see if the user name is already taken
   and, if so, choose another user name.  Note that this provides
   protection only against accidental name collisions.  Name theft is



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   still possible.  If protection against name theft is desired, then
   the enrollment service MUST be used.

11.4.  Contacting a Bootstrap Node

   In order to join the overlay, the Joining Node MUST contact a node in
   the overlay.  Typically this means contacting the bootstrap nodes,
   since they are reachable by the local peer or have public IP
   addresses.  If the Joining Node has cached a list of peers that it
   has previously been connected with in this overlay, as an
   optimization it MAY attempt to use one or more of them as bootstrap
   nodes before falling back to the bootstrap nodes listed in the
   configuration file.

   When contacting a bootstrap node, the Joining Node MUST first form
   the DTLS or TLS connection to the bootstrap node and then send an
   Attach request over this connection with the destination Resource-ID
   set to the Joining Node's Node-ID plus 1.

   When the requester node finally does receive a response from some
   responding node, it MUST use the Node-ID in the response to start
   sending requests to join the Overlay Instance as described in
   Section 6.4.

   After a node has successfully joined the overlay network, it will
   have direct connections to several peers.  Some MAY be added to the
   cached bootstrap nodes list and used in future boots.  Peers that are
   not directly connected MUST NOT be cached.  The suggested number of
   peers to cache is 10.  Algorithms for determining which peers to
   cache are beyond the scope of this specification.

12.  Message Flow Example

   The following abbreviations are used in the message flow diagrams:
   JN = Joining Node, AP = Admitting Peer, NP = next peer after the AP,
   NNP = next next peer which is the peer after NP, PP = previous peer
   before the AP, PPP = previous previous peer which is the peer before
   the PP, BP = bootstrap node.

   In the following example, we assume that JN has formed a connection
   to one of the bootstrap nodes.  JN then sends an Attach through that
   peer to a Resource-ID of itself plus 1 (JN+1).  It gets routed to the
   AP, because JN is not yet part of the overlay.  When AP responds, JN
   and the AP use ICE to set up a connection and then set up DTLS.  Once
   AP has connected to JN, AP sends to JN an Update to populate its
   Routing Table.  The following example shows the Update happening
   after the DTLS connection is formed, but it could also happen before,
   in which case the Update would often be routed through other nodes.



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       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachReq Dest=JN+1|         |         |         |         |
        |---------------------------------------------------------->|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachReq Dest=JN+1|         |
        |         |         |         |<----------------------------|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachAns          |         |
        |         |         |         |---------------------------->|
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns          |         |         |         |         |
        |<----------------------------------------------------------|
        |         |         |         |         |         |         |
        |ICE      |         |         |         |         |         |
        |<===========================>|         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |<...........................>|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

                                 Figure 1













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   The JN then forms connections to the appropriate neighbors, such as
   NP, by sending an Attach which gets routed via other nodes.  When NP
   responds, JN and NP use ICE and DTLS to set up a connection.

       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachReq NP       |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachReq NP       |         |
        |         |         |         |-------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |AttachAns|         |         |
        |         |         |         |<--------|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |ICE      |         |         |         |         |         |
        |<=====================================>|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |<.....................................>|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |


                                 Figure 2














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   The JN also needs to populate its Finger Table (for the Chord-based
   DHT).  It issues an Attach to a variety of locations around the
   overlay.  The diagram below shows JN sending an Attach halfway around
   the Chord ring to the JN + 2^127.

       JN        NP        XX        TP
        |         |         |         |
        |         |         |         |
        |         |         |         |
        |AttachReq JN+2<<126|         |
        |-------->|         |         |
        |         |         |         |
        |         |         |         |
        |         |AttachReq JN+2<<126|
        |         |-------->|         |
        |         |         |         |
        |         |         |         |
        |         |         |AttachReq JN+2<<126
        |         |         |-------->|
        |         |         |         |
        |         |         |         |
        |         |         |AttachAns|
        |         |         |<--------|
        |         |         |         |
        |         |         |         |
        |         |AttachAns|         |
        |         |<--------|         |
        |         |         |         |
        |         |         |         |
        |AttachAns|         |         |
        |<--------|         |         |
        |         |         |         |
        |ICE      |         |         |
        |<===========================>|
        |         |         |         |
        |TLS      |         |         |
        |<...........................>|
        |         |         |         |
        |         |         |         |

                                 Figure 3










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   Once JN has a reasonable set of connections, it is ready to take its
   place in the DHT.  It does this by sending a Join to AP.  AP sends a
   series of Store requests to JN to store the data that JN will be
   responsible for.  AP then sends JN an Update that explicitly labels
   JN as its predecessor.  At this point, JN is part of the ring and is
   responsible for a section of the overlay.  AP can now forget any data
   which is assigned to JN and not to AP.

       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |JoinReq  |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |JoinAns  |         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreReq Data A    |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreAns |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreReq Data B    |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |StoreAns |         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

                                 Figure 4



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   In Chord, JN's Neighbor Table needs to contain its own predecessors.
   It couldn't connect to them previously, because it did not yet know
   their addresses.  However, now that it has received an Update from
   AP, as in the previous diagram, it has AP's predecessors, which are
   also its own, so it sends Attaches to them.  Below, it is shown
   connecting only to AP's closest predecessor, PP.

       JN        PPP       PP        AP        NP        NNP       BP
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachReq Dest=PP  |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachReq Dest=PP  |         |         |
        |         |         |<--------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |AttachAns|         |         |         |
        |         |         |-------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |AttachAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |TLS      |         |         |         |         |         |
        |...................|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |------------------>|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<------------------|         |         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |
        |---------------------------->|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<----------------------------|         |         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateReq|         |         |         |         |         |



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        |-------------------------------------->|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |
        |UpdateAns|         |         |         |         |         |
        |<--------------------------------------|         |         |
        |         |         |         |         |         |         |
        |         |         |         |         |         |         |

                                 Figure 5

   Finally, now that JN has a copy of all the data and is ready to route
   messages and receive requests, it sends Updates to everyone in its
   Routing Table to tell them it is ready to go.  Below, it is shown
   sending such an update to TP.


           JN        NP        XX        TP
            |         |         |         |
            |         |         |         |
            |         |         |         |
            |UpdateReq|         |         |
            |---------------------------->|
            |         |         |         |
            |         |         |         |
            |UpdateAns|         |         |
            |<----------------------------|
            |         |         |         |
            |         |         |         |
            |         |         |         |
            |         |         |         |

                                 Figure 6

13.  Security Considerations

13.1.  Overview

   RELOAD provides a generic storage service, albeit one designed to be
   useful for P2PSIP.  In this section, we discuss security issues that
   are likely to be relevant to any usage of RELOAD.  More background
   information can be found in [RFC5765].

   In any Overlay Instance, any given user depends on a number of peers
   with which they have no well-defined relationship except that they
   are fellow members of the Overlay Instance.  In practice, these other
   nodes may be friendly, lazy, curious, or outright malicious.  No
   security system can provide complete protection in an environment
   where most nodes are malicious.  The goal of security in RELOAD is to



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   provide strong security guarantees of some properties even in the
   face of a large number of malicious nodes and to allow the overlay to
   function correctly in the face of a modest number of malicious nodes.

   P2PSIP deployments require the ability to authenticate both peers and
   resources (users) without the active presence of a trusted entity in
   the system.  We describe two mechanisms.  The first mechanism is
   based on public key certificates and is suitable for general
   deployments.  The second is an admission control mechanism based on
   an overlay-wide shared symmetric key.

13.2.  Attacks on P2P Overlays

   The two basic functions provided by overlay nodes are storage and
   routing: some peer is responsible for storing a node's data and for
   allowing a third node to fetch this stored data, while other peers
   are responsible for routing messages to and from the storing nodes.
   Each of these issues is covered in the following sections.

   P2P overlays are subject to attacks by subversive nodes that may
   attempt to disrupt routing, corrupt or remove user registrations, or
   eavesdrop on signaling.  The certificate-based security algorithms we
   describe in this specification are intended to protect overlay
   routing and user registration information in RELOAD messages.

   To protect the signaling from attackers pretending to be valid nodes
   (or nodes other than themselves), the first requirement is to ensure
   that all messages are received from authorized members of the
   overlay.  For this reason, RELOAD MUST transport all messages over a
   secure channel (TLS and DTLS are defined in this document) which
   provides message integrity and authentication of the directly
   communicating peer.  In addition, messages and data MUST be digitally
   signed with the sender's private key, providing end-to-end security
   for communications.

13.3.  Certificate-Based Security

   This specification stores users' registrations and possibly other
   data in an overlay network.  This requires a solution both to
   securing this data and to securing, as well as possible, the routing
   in the overlay.  Both types of security are based on requiring that
   every entity in the system (whether user or peer) authenticate
   cryptographically using an asymmetric key pair tied to a certificate.

   When a user enrolls in the Overlay Instance, they request or are
   assigned a unique name, such as "alice@dht.example.net".  These names
   MUST be unique and are meant to be chosen and used by humans much
   like a SIP address-of-record (AOR) or an email address.  The user



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   MUST also be assigned one or more Node-IDs by the central enrollment
   authority.  Both the name and the Node-IDs are placed in the
   certificate, along with the user's public key.

   Each certificate enables an entity to act in two sorts of roles:

   o  As a user, storing data at specific Resource-IDs in the Overlay
      Instance corresponding to the user name.

   o  As a overlay peer with the Node-IDs listed in the certificate.

   Note that since only users of this Overlay Instance need to validate
   a certificate, this usage does not require a global Public Key
   Infrastructure (PKI).  Instead, certificates MUST be signed by a
   central enrollment authority which acts as the certificate authority
   for the Overlay Instance.  This authority signs each node's
   certificate.  Because each node possesses the CA's certificate (which
   they receive upon enrollment), they can verify the certificates of
   the other entities in the overlay without further communication.
   Because the certificates contain the user's/node's public key,
   communications from the user/node can, in turn, be verified.

   If self-signed certificates are used, then the security provided is
   significantly decreased, since attackers can mount Sybil attacks.  In
   addition, attackers cannot trust the user names in certificates
   (although they can trust the Node-IDs, because they are
   cryptographically verifiable).  This scheme may be appropriate for
   some small deployments, such as a small office or an ad hoc overlay
   set up among participants in a meeting where all hosts on the network
   are trusted.  Some additional security can be provided by using the
   shared secret admission control scheme as well.

   Because all stored data is signed by the owner of the data, the
   storing node can verify that the storer is authorized to perform a
   store at that Resource-ID and also can allow any consumer of the data
   to verify the provenance and integrity of the data when it retrieves
   it.

   Note that RELOAD does not itself provide a revocation/status
   mechanism (although certificates may, of course, include Online
   Certificate Status Protocol [OCSP] responder information).  Thus,
   certificate lifetimes SHOULD be chosen to balance the compromise
   window versus the cost of certificate renewal.  Because RELOAD is
   already designed to operate in the face of some fraction of malicious
   nodes, this form of compromise is not fatal.

   All implementations MUST implement certificate-based security.




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13.4.  Shared-Secret Security

   RELOAD also supports a shared secret admission control scheme that
   relies on a single key that is shared among all members of the
   overlay.  It is appropriate for small groups that wish to form a
   private network without complexity.  In shared secret mode, all the
   peers MUST share a single symmetric key which is used to key TLS-PSK
   or TLS-SRP mode.  A peer which does not know the key cannot form TLS
   connections with any other peer and therefore cannot join the
   overlay.

   One natural approach to a shared-secret scheme is to use a user-
   entered password as the key.  The difficulty with this is that in
   TLS-PSK mode, such keys are very susceptible to dictionary attacks.
   If passwords are used as the source of shared keys, then TLS-SRP is a
   superior choice, because it is not subject to dictionary attacks.

13.5.  Storage Security

   When certificate-based security is used in RELOAD, any given
   Resource-ID/Kind-ID pair is bound to some small set of certificates.
   In order to write data, the writer must prove possession of the
   private key for one of those certificates.  Moreover, all data is
   stored, signed with the same private key that was used to authorize
   the storage.  This set of rules makes questions of authorization and
   data integrity, which have historically been thorny for overlays,
   relatively simple.

13.5.1.  Authorization

   When a node wants to store some value, it MUST first digitally sign
   the value with its own private key.  It then sends a Store request
   that contains both the value and the signature towards the storing
   peer (which is defined by the Resource Name construction algorithm
   for that particular Kind of value).

   When the storing peer receives the request, it MUST determine whether
   the storing node is authorized to store at this Resource-ID/Kind-ID
   pair.  Determining this requires comparing the user's identity to the
   requirements of the access control model (see Section 7.3).  If it
   satisfies those requirements, the user is authorized to write,
   pending quota checks, as described in the next section.

   For example, consider a certificate with the following properties:

          User name: alice@dht.example.com
          Node-ID:   013456789abcdef
          Serial:    1234



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   If Alice wishes to Store a value of the "SIP Location" Kind, the
   Resource Name will be the SIP AOR "sip:alice@dht.example.com".  The
   Resource-ID will be determined by hashing the Resource Name.  Because
   SIP Location uses the USER-NODE-MATCH policy, it first verifies that
   the user name in the certificate hashes to the requested Resource-ID.
   It then verifies that the Node-ID in the certificate matches the
   dictionary key being used for the store.  If both of these checks
   succeed, the Store is authorized.  Note that because the access
   control model is different for different Kinds, the exact set of
   checks will vary.

13.5.2.  Distributed Quota

   Being a peer in an Overlay Instance carries with it the
   responsibility to store data for a given region of the Overlay
   Instance.  However, allowing nodes to store unlimited amounts of data
   would create unacceptable burdens on peers and would also enable
   trivial denial-of-service (DoS) attacks.  RELOAD addresses this issue
   by requiring configurations to define maximum sizes for each Kind of
   stored data.  Attempts to store values exceeding this size MUST be
   rejected.  (If peers are inconsistent about this, then strange
   artifacts will happen when the zone of responsibility shifts and a
   different peer becomes responsible for overlarge data.)  Because each
   Resource-ID/Kind-ID pair is bound to a small set of certificates,
   these size restrictions also create a distributed quota mechanism,
   with the quotas administered by the central configuration server.

   Allowing different Kinds of data to have different size restrictions
   allows new usages the flexibility to define limits that fit their
   needs without requiring all usages to have expansive limits.

13.5.3.  Correctness

   Because each stored value is signed, it is trivial for any retrieving
   node to verify the integrity of the stored value.  More care needs to
   be taken to prevent version rollback attacks.  Rollback attacks on
   storage are prevented by the use of store times and lifetime values
   in each store.  A lifetime represents the latest time at which the
   data is valid and thus limits (although does not completely prevent)
   the ability of the storing node to perform a rollback attack on
   retrievers.  In order to prevent a rollback attack at the time of the
   Store request, it is REQUIRED that storage times be monotonically
   increasing.  Storing peers MUST reject Store requests with storage
   times smaller than or equal to those that they are currently storing.
   In addition, a fetching node which receives a data value with a
   storage time older than the result of the previous fetch knows that a
   rollback has occurred.




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13.5.4.  Residual Attacks

   The mechanisms described here provide a high degree of security, but
   some attacks remain possible.  Most simply, it is possible for
   storing peers to refuse to store a value (i.e., they reject any
   request).  In addition, a storing peer can deny knowledge of values
   which it has previously accepted.  To some extent, these attacks can
   be ameliorated by attempting to store to and retrieve from replicas,
   but a retrieving node does not know whether or not it should try
   this, as there is a cost to doing so.

   The certificate-based authentication scheme prevents a single peer
   from being able to forge data owned by other peers.  Furthermore,
   although a subversive peer can refuse to return data resources for
   which it is responsible, it cannot return forged data, because it
   cannot provide authentication for such registrations.  Therefore,
   parallel searches for redundant registrations can mitigate most of
   the effects of a compromised peer.  The ultimate reliability of such
   an overlay is a statistical question based on the replication factor
   and the percentage of compromised peers.

   In addition, when a Kind is multivalued (e.g., an array data model),
   the storing peer can return only some subset of the values, thus
   biasing its responses.  This can be countered by using single values
   rather than sets, but that makes coordination between multiple
   storing agents much more difficult.  This is a trade-off that must be
   made when designing any usage.

13.6.  Routing Security

   Because the storage security system guarantees (within limits) the
   integrity of the stored data, routing security focuses on stopping
   the attacker from performing a DoS attack that misroutes requests in
   the overlay.  There are a few obvious observations to make about
   this.  First, it is easy to ensure that an attacker is at least a
   valid node in the Overlay Instance.  Second, this is a DoS attack
   only.  Third, if a large percentage of the nodes on the Overlay
   Instance are controlled by the attacker, it is probably impossible to
   perfectly secure against this.












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13.6.1.  Background

   In general, attacks on DHT routing are mounted by the attacker
   arranging to route traffic through one or two nodes that it controls.
   In the Eclipse attack [Eclipse], the attacker tampers with messages
   to and from nodes for which it is on-path with respect to a given
   victim node.  This allows it to pretend to be all the nodes that are
   reachable through it.  In the Sybil attack [Sybil], the attacker
   registers a large number of nodes and is therefore able to capture a
   large amount of the traffic through the DHT.

   Both the Eclipse and Sybil attacks require the attacker to be able to
   exercise control over her Node-IDs.  The Sybil attack requires the
   creation of a large number of peers.  The Eclipse attack requires
   that the attacker be able to impersonate specific peers.  In both
   cases, RELOAD attempts to mitigate these attacks by the use of
   centralized, certificate-based admission control.

13.6.2.  Admissions Control

   Admission to a RELOAD Overlay Instance is controlled by requiring
   that each peer have a certificate containing its Node-ID.  The
   requirement to have a certificate is enforced by using certificate-
   based mutual authentication on each connection.  (Note: the following
   applies only when self-signed certificates are not used.)  Whenever a
   peer connects to another peer, each side automatically checks that
   the other has a suitable certificate.  These Node-IDs MUST be
   randomly assigned by the central enrollment server.  This has two
   benefits:

   o  It allows the enrollment server to limit the number of Node-IDs
      issued to any individual user.

   o  It prevents the attacker from choosing specific Node-IDs.

   The first property allows protection against Sybil attacks (provided
   that the enrollment server uses strict rate-limiting policies).  The
   second property deters but does not completely prevent Eclipse
   attacks.  Because an Eclipse attacker must impersonate peers on the
   other side of the attacker, the attacker must have a certificate for
   suitable Node-IDs, which requires him to repeatedly query the
   enrollment server for new certificates, which will match only by
   chance.  From the attacker's perspective, the difficulty is that if
   the attacker has only a small number of certificates, the region of
   the Overlay Instance he is impersonating appears to be very sparsely
   populated by comparison to the victim's local region.





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13.6.3.  Peer Identification and Authentication

   In general, whenever a peer engages in overlay activity that might
   affect the Routing Table, it must establish its identity.  This
   happens in two ways.  First, whenever a peer establishes a direct
   connection to another peer, it authenticates via certificate-based
   mutual authentication.  All messages between peers are sent over this
   protected channel, and therefore the peers can verify the data origin
   of the last-hop peer for requests and responses without further
   cryptography.

   In some situations, however, it is desirable to be able to establish
   the identity of a peer with whom one is not directly connected.  The
   most natural case is when a peer Updates its state.  At this point,
   other peers may need to update their view of the overlay structure,
   but they need to verify that the Update message came from the actual
   peer rather than from an attacker.  To prevent having a peer accept
   Update messages from an attacker, all overlay routing messages are
   signed by the peer that generated them.

   For messages that impact the topology of the overlay, replay is
   typically prevented by having the information come directly from, or
   be verified by, the nodes that claimed to have generated the update.
   Data storage replay detection is done by signing the time of the node
   that generated the signature on the Store request, thus providing a
   time-based replay protection, but the time synchronization is needed
   only between peers that can write to the same location.

13.6.4.  Protecting the Signaling

   The goal here is to stop an attacker from knowing who is signaling
   what to whom.  An attacker is unlikely to be able to observe the
   activities of a specific individual, given the randomization of IDs
   and routing based on the present peers discussed above.  Furthermore,
   because messages can be routed using only the header information, the
   actual body of the RELOAD message can be encrypted during
   transmission.

   There are two lines of defense here.  The first is the use of TLS or
   DTLS for each communications link between peers.  This provides
   protection against attackers who are not members of the overlay.  The
   second line of defense is to digitally sign each message.  This
   prevents adversarial peers from modifying messages in flight, even if
   they are on the routing path.







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13.6.5.  Routing Loops and DoS Attacks

   Source-routing mechanisms are known to create the possibility for DoS
   amplification, especially by the induction of routing loops
   [RFC5095].  In order to limit amplification, the initial-ttl value in
   the configuration file SHOULD be set to a value slightly larger than
   the longest expected path through the network.  For Chord, experience
   has shown that log(2) of the number of nodes in the network + 5 is a
   safe bound.  Because nodes are required to enforce the initial-ttl as
   the maximum value, an attacker cannot achieve an amplification factor
   greater than initial-ttl, thus limiting the additional capabilities
   provided by source routing.

   In order to prevent the use of loops for targeted implementation
   attacks, implementations SHOULD check the Destination List for
   duplicate entries and discard such records with an
   "Error_Invalid_Message" error.  This does not completely prevent
   loops, but it does require that at least one attacker node be part of
   the loop.

13.6.6.  Residual Attacks

   The routing security mechanisms in RELOAD are designed to contain
   rather than eliminate attacks on routing.  It is still possible for
   an attacker to mount a variety of attacks.  In particular, if an
   attacker is able to take up a position on the overlay routing between
   A and B, it can make it appear as if B does not exist or is
   disconnected.  It can also advertise false network metrics in an
   attempt to reroute traffic.  However, these are primarily DoS
   attacks.

   The certificate-based security scheme secures the namespace, but if
   an individual peer is compromised or if an attacker obtains a
   certificate from the CA, then a number of subversive peers can still
   appear in the overlay.  While these peers cannot falsify responses to
   resource queries, they can respond with error messages, effecting a
   DoS attack on the resource registration.  They can also subvert
   routing to other compromised peers.  To defend against such attacks,
   a resource search must still consist of parallel searches for
   replicated registrations.











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14.  IANA Considerations

   This section contains the new code points registered by this
   document.

14.1.  Well-Known URI Registration

   IANA has registered a "well-known URI" as described in [RFC5785]:

           +----------------------------+----------------------+
           | URI suffix:                | reload-config        |
           | Change controller:         | IETF <iesg@ietf.org> |
           | Specification document(s): | RFC 6940             |
           | Related information:       | None                 |
           +----------------------------+----------------------+

14.2.  Port Registrations

   IANA has already allocated a TCP port for the main peer-to-peer
   protocol.  This port had the name p2psip-enroll and the port number
   of 6084.  Per this document, IANA has updated this registration to
   change the service name to reload-config.

   IANA has made the following port registration:

   +-----------------------------+-------------------------------------+
   | Registration Technical      | IETF Chair <chair@ietf.org>         |
   | Contact                     |                                     |
   | Registration Owner          | IETF <iesg@ietf.org>                |
   | Transport Protocol          | TCP                                 |
   | Port Number                 | 6084                                |
   | Service Name                | reload-config                       |
   | Description                 | Peer-to-Peer Infrastructure         |
   |                             | Configuration                       |
   +-----------------------------+-------------------------------------+
















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14.3.  Overlay Algorithm Types

   IANA has created a "RELOAD Overlay Algorithm Types" Registry.
   Entries in this registry are strings denoting the names of overlay
   algorithms, as described in Section 11.1 of [RFC6940].  The
   registration policy for this registry is "IETF Review" [RFC522].  The
   initial contents of this registry are:

                      +----------------+-----------+
                      | Algorithm Name | Reference |
                      +----------------+-----------+
                      | CHORD-RELOAD   |  RFC 6940 |
                      | EXP-OVERLAY    |  RFC 6940 |
                      +----------------+-----------+

   The value EXP-OVERLAY has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.

14.4.  Access Control Policies

   IANA has created a "RELOAD Access Control Policies" Registry.
   Entries in this registry are strings denoting access control
   policies, as described in Section 7.3 of [RFC6940].  New entries in
   this registry SHALL be registered via Standards Action [RFC5226].
   The initial contents of this registry are:

                      +-----------------+-----------+
                      | Access Policy   | Reference |
                      +-----------------+-----------+
                      | USER-MATCH      |  RFC 6940 |
                      | NODE-MATCH      |  RFC 6940 |
                      | USER-NODE-MATCH |  RFC 6940 |
                      | NODE-MULTIPLE   |  RFC 6940 |
                      | EXP-MATCH       |  RFC 6940 |
                      +-----------------+-----------+

   The value EXP-MATCH has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.











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14.5.  Application-ID

   IANA has created a "RELOAD Application-ID" Registry.  Entries in this
   registry are 16-bit integers denoting Application-IDs, as described
   in Section 6.5.2 of [RFC6940].  Code points in the range 1 to 32767
   SHALL be registered via Standards Action [RFC5226].  Code points in
   the range 32768 to 61440 SHALL be registered via Expert Review
   [RFC5226].  Code points in the range 61441 to 65534 are reserved for
   private use.  The initial contents of this registry are:

     +-------------+----------------+-------------------------------+
     | Application | Application-ID |                 Specification |
     +-------------+----------------+-------------------------------+
     | INVALID     |              0 |                      RFC 6940 |
     | SIP         |           5060 | Reserved for use by SIP Usage |
     | SIP         |           5061 | Reserved for use by SIP Usage |
     | Reserved    |          65535 |                      RFC 6940 |
     +-------------+----------------+-------------------------------+

14.6.  Data Kind-ID

   IANA has created a "RELOAD Data Kind-ID" registry.  Entries in this
   registry are 32-bit integers denoting data Kinds, as described in
   Section 5.2 of [RFC6940].  Code points in the range 0x00000001 to
   0x7FFFFFFF SHALL be registered via Standards Action [RFC5226].  Code
   points in the range 0x8000000 to 0xF0000000 SHALL be registered via
   Expert Review [RFC5226].  Code points in the range 0xF0000001 to
   0xFFFFFFFE are reserved for private use via the Kind description
   mechanism described in Section 11 of [RFC6940].  The initial contents
   of this registry are:

             +---------------------+------------+-----------+
             | Kind                |    Kind-ID | Reference |
             +---------------------+------------+-----------+
             | INVALID             |        0x0 |  RFC 6940 |
             | TURN-SERVICE        |        0x2 |  RFC 6940 |
             | CERTIFICATE_BY_NODE |        0x3 |  RFC 6940 |
             | CERTIFICATE_BY_USER |       0x10 |  RFC 6940 |
             | Reserved            | 0x7fffffff |  RFC 6940 |
             | Reserved            | 0xfffffffe |  RFC 6940 |
             +---------------------+------------+-----------+










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14.7.  Data Model

   IANA has created a "RELOAD Data Model" registry.  Entries in this
   registry are strings denoting data models, as described in
   Section 7.2 of [RFC6940].  New entries in this registry SHALL be
   registered via Standards Action [RFC5226].  The initial contents of
   this registry are:

                        +------------+-----------+
                        | Data Model | Reference |
                        +------------+-----------+
                        | INVALID    |  RFC 6940 |
                        | SINGLE     |  RFC 6940 |
                        | ARRAY      |  RFC 6940 |
                        | DICTIONARY |  RFC 6940 |
                        | EXP-DATA   |  RFC 6940 |
                        | RESERVED   |  RFC 6940 |
                        +------------+-----------+

   The value EXP-DATA has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.

14.8.  Message Codes

   IANA has created a "RELOAD Message Codes" registry.  Entries in this
   registry are 16-bit integers denoting method codes, as described in
   Section 6.3.3 of [RFC6940].  These codes SHALL be registered via
   Standards Action [RFC5226].  The initial contents of this registry
   are:





















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   +-------------------------------------+----------------+-----------+
   | Message Code Name                   |     Code Value | Reference |
   +-------------------------------------+----------------+-----------+
   | invalidMessageCode                  |            0x0 |  RFC 6940 |
   | probe_req                           |            0x1 |  RFC 6940 |
   | probe_ans                           |            0x2 |  RFC 6940 |
   | attach_req                          |            0x3 |  RFC 6940 |
   | attach_ans                          |            0x4 |  RFC 6940 |
   | Unassigned                          |            0x5 |           |
   | Unassigned                          |            0x6 |           |
   | store_req                           |            0x7 |  RFC 6940 |
   | store_ans                           |            0x8 |  RFC 6940 |
   | fetch_req                           |            0x9 |  RFC 6940 |
   | fetch_ans                           |            0xA |  RFC 6940 |
   | Unassigned (was remove_req)         |            0xB |  RFC 6940 |
   | Unassigned (was remove_ans)         |            0xC |  RFC 6940 |
   | find_req                            |            0xD |  RFC 6940 |
   | find_ans                            |            0xE |  RFC 6940 |
   | join_req                            |            0xF |  RFC 6940 |
   | join_ans                            |           0x10 |  RFC 6940 |
   | leave_req                           |           0x11 |  RFC 6940 |
   | leave_ans                           |           0x12 |  RFC 6940 |
   | update_req                          |           0x13 |  RFC 6940 |
   | update_ans                          |           0x14 |  RFC 6940 |
   | route_query_req                     |           0x15 |  RFC 6940 |
   | route_query_ans                     |           0x16 |  RFC 6940 |
   | ping_req                            |           0x17 |  RFC 6940 |
   | ping_ans                            |           0x18 |  RFC 6940 |
   | stat_req                            |           0x19 |  RFC 6940 |
   | stat_ans                            |           0x1A |  RFC 6940 |
   | Unassigned (was attachlite_req)     |           0x1B |  RFC 6940 |
   | Unassigned (was attachlite_ans)     |           0x1C |  RFC 6940 |
   | app_attach_req                      |           0x1D |  RFC 6940 |
   | app_attach_ans                      |           0x1E |  RFC 6940 |
   | Unassigned (was app_attachlite_req) |           0x1F |  RFC 6940 |
   | Unassigned (was app_attachlite_ans) |           0x20 |  RFC 6940 |
   | config_update_req                   |           0x21 |  RFC 6940 |
   | config_update_ans                   |           0x22 |  RFC 6940 |
   | exp_a_req                           |           0x23 |  RFC 6940 |
   | exp_a_ans                           |           0x24 |  RFC 6940 |
   | exp_b_req                           |           0x25 |  RFC 6940 |
   | exp_b_ans                           |           0x26 |  RFC 6940 |
   | Reserved                            | 0x8000..0xFFFE |  RFC 6940 |
   | error                               |         0xFFFF |  RFC 6940 |
   +-------------------------------------+----------------+-----------+






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   The values exp_a_req, exp_a_ans, exp_b_req, and exp_b_ans have been
   made available for the purposes of experimentation.  These values are
   not meant for vendor-specific use of any sort, and they MUST NOT be
   used for operational deployments.

14.9.  Error Codes

   IANA has created a "RELOAD Error Code" registry.  Entries in this
   registry are 16-bit integers denoting error codes, as described in
   Section 6.3.3.1 of [RFC6940].  New entries SHALL be defined via
   Standards Action [RFC5226].  The initial contents of this registry
   are:

   +-------------------------------------+----------------+-----------+
   | Error Code Name                     |     Code Value | Reference |
   +-------------------------------------+----------------+-----------+
   | invalidErrorCode                    |            0x0 |  RFC 6940 |
   | Unassigned                          |            0x1 |           |
   | Error_Forbidden                     |            0x2 |  RFC 6940 |
   | Error_Not_Found                     |            0x3 |  RFC 6940 |
   | Error_Request_Timeout               |            0x4 |  RFC 6940 |
   | Error_Generation_Counter_Too_Low    |            0x5 |  RFC 6940 |
   | Error_Incompatible_with_Overlay     |            0x6 |  RFC 6940 |
   | Error_Unsupported_Forwarding_Option |            0x7 |  RFC 6940 |
   | Error_Data_Too_Large                |            0x8 |  RFC 6940 |
   | Error_Data_Too_Old                  |            0x9 |  RFC 6940 |
   | Error_TTL_Exceeded                  |            0xA |  RFC 6940 |
   | Error_Message_Too_Large             |            0xB |  RFC 6940 |
   | Error_Unknown_Kind                  |            0xC |  RFC 6940 |
   | Error_Unknown_Extension             |            0xD |  RFC 6940 |
   | Error_Response_Too_Large            |            0xE |  RFC 6940 |
   | Error_Config_Too_Old                |            0xF |  RFC 6940 |
   | Error_Config_Too_New                |           0x10 |  RFC 6940 |
   | Error_In_Progress                   |           0x11 |  RFC 6940 |
   | Error_Exp_A                         |           0x12 |  RFC 6940 |
   | Error_Exp_B                         |           0x13 |  RFC 6940 |
   | Error_Invalid_Message               |           0x14 |  RFC 6940 |
   | Reserved                            | 0x8000..0xFFFE |  RFC 6940 |
   +-------------------------------------+----------------+-----------+

   The values Error_Exp_A and Error_Exp_B have been made available for
   the purposes of experimentation.  These values are not meant for
   vendor-specific use of any sort, and they MUST NOT be used for
   operational deployments.







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14.10.  Overlay Link Types

   IANA has created a "RELOAD Overlay Link Registry".  Entries in this
   registry are 8-bit integers, as described in Section 6.5.1.1 of
   [RFC6940].  For more information on the link types defined here, see
   Section 6.6 of [RFC6940].  New entries SHALL be defined via Standards
   Action [RFC5226].  This registry has been initially populated with
   the following values:

                 +--------------------+------+-----------+
                 | Protocol           | Code | Reference |
                 +--------------------+------+-----------+
                 | INVALID-PROTOCOL   |    0 |  RFC 6940 |
                 | DTLS-UDP-SR        |    1 |  RFC 6940 |
                 | DTLS-UDP-SR-NO-ICE |    3 |  RFC 6940 |
                 | TLS-TCP-FH-NO-ICE  |    4 |  RFC 6940 |
                 | EXP-LINK           |    5 |  RFC 6940 |
                 | Reserved           |  255 |  RFC 6940 |
                 +--------------------+------+-----------+

   The value EXP-LINK has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.

14.11.  Overlay Link Protocols

   IANA has created a "RELOAD Overlay Link Protocol Registry".  Entries
   in this registry are strings denoting protocols as described in
   Section 11.1 of this document and SHALL be defined via Standards
   Action [RFC5226].  This registry has been initially populated with
   the following values:

                       +---------------+-----------+
                       | Link Protocol | Reference |
                       +---------------+-----------+
                       | TLS           |  RFC 6940 |
                       | EXP-PROTOCOL  |  RFC 6940 |
                       +---------------+-----------+

   The value EXP-PROTOCOL has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.









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14.12.  Forwarding Options

   IANA has created a "RELOAD Forwarding Option Registry".  Entries in
   this registry are 8-bit integers denoting options, as described in
   Section 6.3.2.3 of [RFC6940].  Values between 1 and 127 SHALL be
   defined via Standards Action [RFC5226].  Entries in this registry
   between 128 and 254 SHALL be defined via Specification Required
   [RFC5226].  This registry has been initially populated with the
   following values:

              +-------------------------+------+-----------+
              | Forwarding Option       | Code | Reference |
              +-------------------------+------+-----------+
              | invalidForwardingOption |    0 |  RFC 6940 |
              | exp-forward             |    1 |  RFC 6940 |
              | Reserved                |  255 |  RFC 6940 |
              +-------------------------+------+-----------+

   The value exp-forward has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.

14.13.  Probe Information Types

   IANA has created a "RELOAD Probe Information Type Registry".  Entries
   are 8-bit integers denoting types as described in Section 6.4.2.5.1
   of [RFC6940] and SHALL be defined via Standards Action [RFC5226].
   This registry has been initially populated with the following values:

                 +--------------------+------+-----------+
                 | Probe Option       | Code | Reference |
                 +--------------------+------+-----------+
                 | invalidProbeOption |    0 |  RFC 6940 |
                 | responsible_set    |    1 |  RFC 6940 |
                 | num_resources      |    2 |  RFC 6940 |
                 | uptime             |    3 |  RFC 6940 |
                 | exp-probe          |    4 |  RFC 6940 |
                 | Reserved           |  255 |  RFC 6940 |
                 +--------------------+------+-----------+

   The value exp-probe has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.








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14.14.  Message Extensions

   IANA has created a "RELOAD Extensions Registry".  Entries in this
   registry are 8-bit integers denoting extensions as described in
   Section 6.3.3 of [RFC6940] and SHALL be defined via Specification
   Required [RFC5226].  This registry has been initially populated with
   the following values:

           +-----------------------------+--------+-----------+
           | Extensions Name             |   Code | Reference |
           +-----------------------------+--------+-----------+
           | invalidMessageExtensionType |    0x0 |  RFC 6940 |
           | exp-ext                     |    0x1 |  RFC 6940 |
           | Reserved                    | 0xFFFF |  RFC 6940 |
           +-----------------------------+--------+-----------+

   The value exp-ext has been made available for the purposes of
   experimentation.  This value is not meant for vendor-specific use of
   any sort, and it MUST NOT be used for operational deployments.

14.15.  Reload URI Scheme

   This section describes the scheme for a reload URI, which can be used
   to refer to either:

   o  A peer, e.g., as used in a certificate (see Section 11.3 of
      [RFC6940]).

   o  A resource inside a peer.

   The reload URI is defined using a subset of the URI schema specified
   in Appendix A of RFC 3986 [RFC3986] and the associated URI Guidelines
   [RFC4395] per the following ABNF syntax:

      RELOAD-URI = "reload://" destination "@" overlay "/"
               [specifier]
      destination = 1*HEXDIG
      overlay = reg-name
      specifier = 1*HEXDIG

   The definitions of these productions are as follows:

   destination
      A hexadecimal-encoded Destination List object (i.e., multiple
      concatenated Destination objects with no length prefix prior to
      the object as a whole).





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   overlay
      The name of the overlay.

   specifier
      A hexadecimal-encoded StoredDataSpecifier indicating the data
      element.

   If no specifier is present, this URI addresses the peer which can be
   reached via the indicated Destination List at the indicated overlay
   name.  If a specifier is present, the URI addresses the data value.

14.15.1.  URI Registration

   The following summarizes the information necessary to register the
   reload URI.

   URI Scheme Name:  reload

   Status:   permanent

   URI Scheme Syntax:  see Section 14.15 of RFC 6940

   URI Scheme Semantics:  The reload URI is intended to be used as a
      reference to a RELOAD peer or resource.

   Encoding Considerations:  The reload URI is not intended to be human-
      readable text, so it is encoded entirely in US-ASCII.

   Applications/protocols that Use this URI Scheme:  The RELOAD protocol
      described in RFC 6940.

   Interoperability Considerations:  See RFC 6940.

   Security Considerations:  See RFC 6940

   Contact:  Cullen Jennings <fluffy@cisco.com>

   Author/Change Controller:  IESG

   References:  RFC 6940

14.16.  Media Type Registration

   Type Name: application

   Subtype Name: p2p-overlay+xml

   Required Parameters: none



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   Optional Parameters: none

   Encoding Considerations: Must be binary encoded.

   Security Considerations: This media type is typically not used to
   transport information that needs to be kept confidential.  However,
   there are cases where it is integrity of the information is
   important.  For these cases, using a digital signature is
   RECOMMENDED.  One way of doing this is specified in RFC 6940.  In the
   case when the media includes a shared-secret element, the contents of
   the file MUST be kept confidential or else anyone who can see the
   shared secret can affect the RELOAD overlay network.

   Interoperability Considerations: No known interoperability
   consideration beyond those identified for application/xml in
   [RFC3023].

   Published Specification: RFC 6940

   Applications that Use this Media Type: The type is used to configure
   the peer-to-peer overlay networks defined in RFC 6940.

   Additional Information: The syntax for this media type is specified
   in Section 11.1 of [RFC6940].  The contents MUST be valid XML that is
   compliant with the RELAX NG grammar specified in RFC 6940 and that
   use the UTF-8[RFC3629] character encoding.

      Magic Number(s): none

      File Extension(s): relo

      Macintosh File Type Code(s): none

   Person & Email Address to Contact for Further Information: Cullen
   Jennings <fluffy@cisco.com>

   Intended Usage: COMMON

   Restrictions on Usage: None

   Author: Cullen Jennings <fluffy@cisco.com>

   Change Controller: IESG

14.17.  XML Namespace Registration

   This document registers two URIs for the config and config-chord XML
   namespaces in the IETF XML registry defined in [RFC3688].



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14.17.1.  Config URL

   URI: urn:ietf:params:xml:ns:p2p:config-base

   Registrant Contact: IESG.

   XML: N/A, the requested URIs are XML namespaces

14.17.2.  Config Chord URL

   URI: urn:ietf:params:xml:ns:p2p:config-chord

   Registrant Contact: The IESG.

   XML: N/A, the requested URIs are XML namespaces

15.  Acknowledgments

   This specification is a merge of the "REsource LOcation And Discovery
   (RELOAD)" document by David A. Bryan, Marcia Zangrilli, and Bruce B.
   Lowekamp; the "Address Settlement by Peer to Peer" document by Cullen
   Jennings, Jonathan Rosenberg, and Eric Rescorla; the "Security
   Extensions for RELOAD" document by Bruce B. Lowekamp and James
   Deverick; the "A Chord-based DHT for Resource Lookup in P2PSIP" by
   Marcia Zangrilli and David A. Bryan; and the Peer-to-Peer Protocol
   (P2PP) document by Salman A. Baset, Henning Schulzrinne, and Marcin
   Matuszewski.  Thanks to the authors of [RFC5389] for text included
   from that document.  Vidya Narayanan provided many comments and
   improvements.

   The ideas and text for the Chord-specific extension data to the Leave
   mechanisms were provided by Jouni Maenpaa, Gonzalo Camarillo, and
   Jani Hautakorpi.

   Thanks to the many people who contributed, including Ted Hardie,
   Michael Chen, Dan York, Das Saumitra, Lyndsay Campbell, Brian Rosen,
   David Bryan, Dave Craig, and Julian Cain.  Extensive last call
   comments were provided by Jouni Maenpaa, Roni Even, Gonzalo
   Camarillo, Ari Keranen, John Buford, Michael Chen, Frederic-Philippe
   Met, Mary Barnes, Roland Bless, David Bryan, and Polina Goltsman.
   Special thanks to Marc Petit-Huguenin, who provided an amazing amount
   of detailed review.

   Dean Willis and Marc Petit-Huguenin helped resolve and provided text
   to fix many comments received during the IESG review.






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16.  References

16.1.  Normative References

   [OASIS.relax_ng]
              Bray, T. and M. Murata, "RELAX NG Specification", December
              2001.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2388]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 2388, August 1998.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP", RFC
              2585, May 1999.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the
              Internet: Timestamps", RFC 3339, July 2002.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.




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   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279, December
              2005.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35, RFC
              4395, February 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, June 2008.

   [RFC5273]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC): Transport Protocols", RFC 5273, June 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, November
              2008.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

   [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
              for Transport Layer Security (TLS) Authentication", RFC
              6091, February 2011.




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   [RFC6234]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298, June
              2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [W3C.REC-xmlschema-2-20041028]
              Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
              Second Edition", World Wide Web Consortium Recommendation
              REC-xmlschema-2-20041028, October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

   [w3c-xml-namespaces]
              Bray, T., Hollander, D., Layman, A., Tobin, R., and
              University of Edinburgh and W3C, "Namespaces in XML 1.0
              (Third Edition)", December 2008.

16.2.  Informative References

   [Chord]    Stoica, I., Morris, R., Liben-Nowell, D., Karger, D.,
              Kaashoek, M., Dabek, F., and H. Balakrishnan, "Chord: A
              Scalable Peer-to-peer Lookup Protocol for Internet
              Applications", IEEE/ACM Transactions on Networking Volume
              11, Issue 1, 17-32, Feb 2003, 2001.

   [DHT-RELOAD]
              Maenpaa, J. and G. Camarillo, "A Self-tuning Distributed
              Hash Table (DHT) for REsource LOcation And Discovery
              (RELOAD)", Work in Progress, August 2013.

   [Eclipse]  Singh, A., Ngan, T., Druschel, T., and D. Wallach,
              "Eclipse Attacks on Overlay Networks: Threats and
              Defenses", INFOCOM 2006, April 2006.

   [P2P-DIAGNOSTICS]
              Song, H., Jiang, X., Even, R., and D. Bryan, "P2P Overlay
              Diagnostics", Work in Progress, August 2013.

   [P2PSIP-RELAY]
              Zong, N., Jiang, X., Even, R., and Y. Zhang, "An extension
              to RELOAD to support Relay Peer Routing", Work in
              Progress, October 2013.





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   [REDIR-RELOAD]
              Maenpaa, J. and G. Camarillo, "Service Discovery Usage for
              REsource LOcation And Discovery (RELOAD)", Work in
              Progress, August 2013.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1122]  Braden, R., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989.

   [RFC2311]  Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
              L. Repka, "S/MIME Version 2 Message Specification", RFC
              2311, March 1998.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC4013]  Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, November 2007.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095, December
              2007.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.



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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5694]  Camarillo, G., Ed., and IAB, "Peer-to-Peer (P2P)
              Architecture: Definition, Taxonomies, Examples, and
              Applicability", RFC 5694, November 2009.

   [RFC5765]  Schulzrinne, H., Marocco, E., and E. Ivov, "Security
              Issues and Solutions in Peer-to-Peer Systems for Realtime
              Communications", RFC 5765, February 2010.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785, April
              2010.

   [RFC6079]  Camarillo, G., Nikander, P., Hautakorpi, J., Keranen, A.,
              and A. Johnston, "HIP BONE: Host Identity Protocol (HIP)
              Based Overlay Networking Environment (BONE)", RFC 6079,
              January 2011.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", RFC 6544, March 2012.

   [RFC7086]  Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
              Protocol-Based Overlay Networking Environment (HIP BONE)
              Instance Specification for REsource LOcation And Discovery
              (RELOAD)", RFC 7086, January 2014.

   [SIP-RELOAD]
              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
              Schulzrinne, H., and T. Schmidt, "A SIP Usage for RELOAD",
              Work in Progress, July 2013.

   [Sybil]    Douceur, J., "The Sybil Attack", IPTPS 02, March 2002.

   [UnixTime] Wikipedia, "Unix Time", 2013, <http://en.wikipedia.org/w/
              index.php?title=Unix_time&oldid=551527446>.

   [bryan-design-hotp2p08]
              Bryan, D., Lowekamp, B., and M. Zangrilli, "The Design of
              a Versatile, Secure P2PSIP Communications Architecture for
              the Public Internet", Hot-P2P'08, 2008.






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   [handling-churn-usenix04]
              Rhea, S., Geels, D., Roscoe, T., and J. Kubiatowicz,
              "Handling Churn in a DHT", In Proc. of the USENIX Annual
              Technical Conference June 2004 USENIX 2004, 2004.

   [lookups-churn-p2p06]
              Wu, D., Tian, Y., and K. Ng, "Analytical Study on
              Improving DHT Lookup Performance under Churn", IEEE
              P2P'06, 2006.

   [minimizing-churn-sigcomm06]
              Godfrey, P., Shenker, S., and I. Stoica, "Minimizing Churn
              in Distributed Systems", SIGCOMM 2006, 2006.

   [non-transitive-dhts-worlds05]
              Freedman, M., Lakshminarayanan, K., Rhea, S., and I.
              Stoica, "Non-Transitive Connectivity and DHTs", WORLDS'05,
              2005.

   [opendht-sigcomm05]
              Rhea, S., Godfrey, B., Karp, B., Kubiatowicz, J.,
              Ratnasamy, S., Shenker, S., Stoica, I., and H. Yu,
              "OpenDHT: A Public DHT and its Uses", SIGCOMM'05, 2005.

   [vulnerabilities-acsac04]
              Srivatsa, M. and L. Liu, "Vulnerabilities and Security
              Threats in Structured Peer-to-Peer Systems: A Quantitative
              Analysis", ACSAC 2004, 2004.

   [wikiChord]
              Wikipedia, "Chord (peer-to-peer)", 2013,
              <http://en.wikipedia.org/w/
              index.php?title=Chord_%28peer-to-peer%29&oldid=549516287>.

   [wikiKBR]  Wikipedia, "Key-based routing", 2013, <en.wikipedia.org/w/
              index.php?title=Key-based_routing&oldid=543850833>.

   [wikiSkiplist]
              Wikipedia, "Skip list", 2013, <http://en.wikipedia.org/w/
              index.php?title=Skip_list&oldid=551304213>.











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Appendix A.  Routing Alternatives

   Significant discussion has been focused on the selection of a routing
   algorithm for P2PSIP.  This section discusses the motivations for
   selecting symmetric recursive routing for RELOAD and describes the
   extensions that would be required to support additional routing
   algorithms.

A.1.  Iterative vs. Recursive

   Iterative routing has a number of advantages.  It is easier to debug,
   consumes fewer resources on intermediate peers, and allows the
   querying peer to identify and route around misbehaving peers
   [non-transitive-dhts-worlds05].  However, in the presence of NATs,
   iterative routing is intolerably expensive, because a new connection
   must be established for each hop (using ICE) [bryan-design-hotp2p08].

   Iterative routing is supported through the RouteQuery mechanism and
   is primarily intended for debugging.  It also allows the querying
   peer to evaluate the routing decisions made by the peers at each hop,
   consider alternatives, and perhaps detect at what point the
   forwarding path fails.

A.2.  Symmetric vs. Forward Response

   An alternative to the symmetric recursive routing method used by
   RELOAD is forward-only routing, where the response is routed to the
   requester as if it were a new message initiated by the responder.
   (In the previous example, Z sends the response to A as if it were
   sending a request.)  Forward-only routing requires no state in either
   the message or intermediate peers.

   The drawback of forward-only routing is that it does not work when
   the overlay is unstable.  For example, if A is in the process of
   joining the overlay and is sending a Join request to Z, it is not yet
   reachable via forward-only routing.  Even if it is established in the
   overlay, if network failures produce temporary instability, A may not
   be reachable (and may be trying to stabilize its network connectivity
   via Attach messages).

   Furthermore, forward-only responses are less likely to reach the
   querying peer than symmetric recursive ones are, because the forward
   path is more likely to have a failed peer than is the request path
   (which was just tested to route the request)
   [non-transitive-dhts-worlds05].






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   An extension to RELOAD that supports forward-only routing but relies
   on symmetric responses as a fallback would be possible, but due to
   the complexities of determining when to use forward-only routing and
   when to fallback to symmetric routing, we have chosen not to include
   it as an option at this point.

A.3.  Direct Response

   Another routing option is direct response routing, in which the
   response is returned directly to the querying node.  In the previous
   example, if A encodes its IP address in the request, then Z can
   simply deliver the response directly to A.  In the absence of NATs or
   other connectivity issues, this is the optimal routing technique.

   The challenge of implementing direct response routing is the presence
   of NATs.  There are a number of complexities that must be addressed.
   In this discussion, we will continue our assumption that A issued the
   request and Z is generating the response.

   o  The IP address listed by A may be unreachable, either due to NAT
      or firewall rules.  Therefore, a direct response technique must
      fallback to symmetric response [non-transitive-dhts-worlds05].
      The hop-by-hop ACKs used by RELOAD allow Z to determine when A has
      received the message (and the TLS negotiation will provide earlier
      confirmation that A is reachable), but this fallback requires a
      timeout that will increase the response latency whenever A is not
      reachable from Z.

   o  Whenever A is behind a NAT it, will have multiple candidate IP
      addresses, each of which must be advertised to ensure
      connectivity.  Therefore, Z will need to attempt multiple
      connections to deliver the response.

   o  One (or all) of A's candidate addresses may route from Z to a
      different device on the Internet.  In the worst case, these nodes
      may actually be running RELOAD on the same port.  Therefore, it is
      absolutely necessary to establish a secure connection to
      authenticate A before delivering the response.  This step
      diminishes the efficiency of direct response routing, because
      multiple round-trips are required before the message can be
      delivered.

   o  If A is behind a NAT and does not have a connection already
      established with Z, there are only two ways the direct response
      will work.  The first is that A and Z must both be behind the same
      NAT, in which case the NAT is not involved.  In the more common
      case, when Z is outside A's NAT, the response will be received
      only if A's NAT implements endpoint-independent filtering.  As the



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      choice of filtering mode conflates application transparency with
      security [RFC4787] and no clear recommendation is available, the
      prevalence of this feature in future devices remains unclear.

   An extension to RELOAD that supports direct response routing but
   relies on symmetric responses as a fallback would be possible, but
   due to the complexities of determining when to use direct response
   routing and when to fallback to symmetric routing, and the reduced
   performance for responses to peers behind restrictive NATs, we have
   chosen not to include it as an option at this point.

A.4.  Relay Peers

   [P2PSIP-RELAY] has proposed implementing a form of direct response by
   having A identify a peer, Q, that will be directly reachable by any
   other peer.  A uses Attach to establish a connection with Q and
   advertises Q's IP address in the request sent to Z.  Z sends the
   response to Q, which relays it to A.  This then reduces the latency
   to two hops, and Z is negotiating a secure connection to Q.

   This technique relies on the relative population of nodes such as A
   that require relay peers and peers such as Q that are capable of
   serving as a relay peer.  It also requires nodes to be able to
   identify which category they are in.  This identification problem has
   turned out to be hard to solve and is still an open area of
   exploration.

   An extension to RELOAD that supports relay peers is possible, but due
   to the complexities of implementing such an alternative, we have not
   added such a feature to RELOAD at this point.

   A concept similar to relay peers, essentially choosing a relay peer
   at random, has previously been suggested to solve problems of pair-
   wise non-transitivity [non-transitive-dhts-worlds05], but
   deterministic filtering provided by NATs makes random relay peers no
   more likely to work than the responding peer.

A.5.  Symmetric Route Stability

   A common concern about symmetric recursive routing has been that one
   or more peers along the request path may fail before the response is
   received.  The significance of this problem essentially depends on
   the response latency of the overlay.  An overlay that produces slow
   responses will be vulnerable to churn, whereas responses that are
   delivered very quickly are vulnerable only to failures that occur
   over that small interval.





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   The other aspect of this issue is whether the request itself can be
   successfully delivered.  Assuming typical connection maintenance
   intervals, the time period between the last maintenance and the
   request being sent will be orders of magnitude greater than the delay
   between the request being forwarded and the response being received.
   Therefore, if the path was stable enough to be available to route the
   request, it is almost certainly going to remain available to route
   the response.

   An overlay that is unstable enough to suffer this type of failure
   frequently is unlikely to be able to support reliable functionality
   regardless of the routing mechanism.  However, regardless of the
   stability of the return path, studies show that in the event of high
   churn, iterative routing is a better solution to ensure request
   completion [lookups-churn-p2p06] [non-transitive-dhts-worlds05]

   Finally, because RELOAD retries the end-to-end request, that retry
   will address the issues of churn that remain.

Appendix B.  Why Clients?

   There are a wide variety of reasons a node may act as a client rather
   than as a peer.  This section outlines some of those scenarios and
   how the client's behavior changes based on its capabilities.

B.1.  Why Not Only Peers?

   For a number of reasons, a particular node may be forced to act as a
   client even though it is willing to act as a peer.  These include:

   o  The node does not have appropriate network connectivity, typically
      because it has a low-bandwidth network connection.

   o  The node may not have sufficient resources, such as computing
      power, storage space, or battery power.

   o  The overlay algorithm may dictate specific requirements for peer
      selection.  These may include participating in the overlay to
      determine trustworthiness, controlling the number of peers in the
      overlay to reduce overly long routing paths, and ensuring minimum
      application uptime before a node can join as a peer.

   The ultimate criteria for a node to become a peer are determined by
   the overlay algorithm and specific deployment.  A node acting as a
   client that has a full implementation of RELOAD and the appropriate
   overlay algorithm is capable of locating its responsible peer in the
   overlay and using Attach to establish a direct connection to that
   peer.  In that way, it may elect to be reachable under either of the



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   routing approaches listed above.  Particularly for overlay algorithms
   that elect nodes to serve as peers based on trustworthiness or
   population, the overlay algorithm may require such a client to locate
   itself at a particular place in the overlay.

B.2.  Clients as Application-Level Agents

   SIP defines an extensive protocol for registration and security
   between a client and its registrar/proxy server(s).  Any SIP device
   can act as a client of a RELOAD-based P2PSIP overlay if it contacts a
   peer that implements the server-side functionality required by the
   SIP protocol.  In this case, the peer would be acting as if it were
   the user's peer and would need the appropriate credentials for that
   user.

   Application-level support for clients is defined by a usage.  A usage
   offering support for application-level clients should specify how the
   security of the system is maintained when the data is moved between
   the application and RELOAD layers.
































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Authors' Addresses

   Cullen Jennings
   Cisco
   400 3rd Avenue SW, Suite 350
   Calgary
   Canada

   EMail: fluffy@cisco.com


   Bruce B. Lowekamp (editor)
   Skype
   Palo Alto, CA
   USA

   EMail: bbl@lowekamp.net


   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Phone: +1 650 678 2350
   EMail: ekr@rtfm.com


   Salman A. Baset
   Columbia University
   1214 Amsterdam Avenue
   New York, NY
   USA

   EMail: salman@cs.columbia.edu


   Henning Schulzrinne
   Columbia University
   1214 Amsterdam Avenue
   New York, NY
   USA

   EMail: hgs@cs.columbia.edu






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