path: root/doc/rfc/rfc5973.txt

Internet Engineering Task Force (IETF)                    M. Stiemerling
Request for Comments: 5973                                           NEC
Category: Experimental                                     H. Tschofenig
ISSN: 2070-1721                                   Nokia Siemens Networks
                                                                 C. Aoun
                                                              Consultant
                                                               E. Davies
                                                        Folly Consulting
                                                            October 2010


           NAT/Firewall NSIS Signaling Layer Protocol (NSLP)

Abstract

   This memo defines the NSIS Signaling Layer Protocol (NSLP) for
   Network Address Translators (NATs) and firewalls.  This NSLP allows
   hosts to signal on the data path for NATs and firewalls to be
   configured according to the needs of the application data flows.  For
   instance, it enables hosts behind NATs to obtain a publicly reachable
   address and hosts behind firewalls to receive data traffic.  The
   overall architecture is given by the framework and requirements
   defined by the Next Steps in Signaling (NSIS) working group.  The
   network scenarios, the protocol itself, and examples for path-coupled
   signaling are given in this memo.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see 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/rfc5973.








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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Without obtaining an adequate license from the person(s) controlling
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   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.

























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Scope and Background . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology and Abbreviations  . . . . . . . . . . . . . .  8
     1.3.  Notes on the Experimental Status . . . . . . . . . . . . . 10
     1.4.  Middleboxes  . . . . . . . . . . . . . . . . . . . . . . . 10
     1.5.  General Scenario for NATFW Traversal . . . . . . . . . . . 11
   2.  Network Deployment Scenarios Using the NATFW NSLP  . . . . . . 13
     2.1.  Firewall Traversal . . . . . . . . . . . . . . . . . . . . 13
     2.2.  NAT with Two Private Networks  . . . . . . . . . . . . . . 14
     2.3.  NAT with Private Network on Sender Side  . . . . . . . . . 15
     2.4.  NAT with Private Network on Receiver Side Scenario . . . . 15
     2.5.  Both End Hosts behind Twice-NATs . . . . . . . . . . . . . 16
     2.6.  Both End Hosts behind Same NAT . . . . . . . . . . . . . . 17
     2.7.  Multihomed Network with NAT  . . . . . . . . . . . . . . . 18
     2.8.  Multihomed Network with Firewall . . . . . . . . . . . . . 18
   3.  Protocol Description . . . . . . . . . . . . . . . . . . . . . 19
     3.1.  Policy Rules . . . . . . . . . . . . . . . . . . . . . . . 19
     3.2.  Basic Protocol Overview  . . . . . . . . . . . . . . . . . 20
       3.2.1.  Signaling for Outbound Traffic . . . . . . . . . . . . 20
       3.2.2.  Signaling for Inbound Traffic  . . . . . . . . . . . . 22
       3.2.3.  Signaling for Proxy Mode . . . . . . . . . . . . . . . 23
       3.2.4.  Blocking Traffic . . . . . . . . . . . . . . . . . . . 24
       3.2.5.  State and Error Maintenance  . . . . . . . . . . . . . 24
       3.2.6.  Message Types  . . . . . . . . . . . . . . . . . . . . 25
       3.2.7.  Classification of RESPONSE Messages  . . . . . . . . . 25
       3.2.8.  NATFW NSLP Signaling Sessions  . . . . . . . . . . . . 26
     3.3.  Basic Message Processing . . . . . . . . . . . . . . . . . 27
     3.4.  Calculation of Signaling Session Lifetime  . . . . . . . . 27
     3.5.  Message Sequencing . . . . . . . . . . . . . . . . . . . . 31
     3.6.  Authentication, Authorization, and Policy Decisions  . . . 32
     3.7.  Protocol Operations  . . . . . . . . . . . . . . . . . . . 32
       3.7.1.  Creating Signaling Sessions  . . . . . . . . . . . . . 32
       3.7.2.  Reserving External Addresses . . . . . . . . . . . . . 35
       3.7.3.  NATFW NSLP Signaling Session Refresh . . . . . . . . . 43
       3.7.4.  Deleting Signaling Sessions  . . . . . . . . . . . . . 45
       3.7.5.  Reporting Asynchronous Events  . . . . . . . . . . . . 46
       3.7.6.  Proxy Mode of Operation  . . . . . . . . . . . . . . . 48
     3.8.  Demultiplexing at NATs . . . . . . . . . . . . . . . . . . 53
     3.9.  Reacting to Route Changes  . . . . . . . . . . . . . . . . 54
     3.10. Updating Policy Rules  . . . . . . . . . . . . . . . . . . 55
   4.  NATFW NSLP Message Components  . . . . . . . . . . . . . . . . 55
     4.1.  NSLP Header  . . . . . . . . . . . . . . . . . . . . . . . 56
     4.2.  NSLP Objects . . . . . . . . . . . . . . . . . . . . . . . 57
       4.2.1.  Signaling Session Lifetime Object  . . . . . . . . . . 58
       4.2.2.  External Address Object  . . . . . . . . . . . . . . . 58
       4.2.3.  External Binding Address Object  . . . . . . . . . . . 59



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       4.2.4.  Extended Flow Information Object . . . . . . . . . . . 59
       4.2.5.  Information Code Object  . . . . . . . . . . . . . . . 60
       4.2.6.  Nonce Object . . . . . . . . . . . . . . . . . . . . . 64
       4.2.7.  Message Sequence Number Object . . . . . . . . . . . . 64
       4.2.8.  Data Terminal Information Object . . . . . . . . . . . 64
       4.2.9.  ICMP Types Object  . . . . . . . . . . . . . . . . . . 66
     4.3.  Message Formats  . . . . . . . . . . . . . . . . . . . . . 67
       4.3.1.  CREATE . . . . . . . . . . . . . . . . . . . . . . . . 67
       4.3.2.  EXTERNAL . . . . . . . . . . . . . . . . . . . . . . . 68
       4.3.3.  RESPONSE . . . . . . . . . . . . . . . . . . . . . . . 68
       4.3.4.  NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . 69
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 69
     5.1.  Authorization Framework  . . . . . . . . . . . . . . . . . 70
       5.1.1.  Peer-to-Peer Relationship  . . . . . . . . . . . . . . 70
       5.1.2.  Intra-Domain Relationship  . . . . . . . . . . . . . . 71
       5.1.3.  End-to-Middle Relationship . . . . . . . . . . . . . . 72
     5.2.  Security Framework for the NAT/Firewall NSLP . . . . . . . 73
       5.2.1.  Security Protection between Neighboring NATFW NSLP
               Nodes  . . . . . . . . . . . . . . . . . . . . . . . . 73
       5.2.2.  Security Protection between Non-Neighboring NATFW
               NSLP Nodes . . . . . . . . . . . . . . . . . . . . . . 74
     5.3.  Implementation of NATFW NSLP Security  . . . . . . . . . . 75
   6.  IAB Considerations on UNSAF  . . . . . . . . . . . . . . . . . 76
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 77
     7.1.  NATFW NSLP Message Type Registry . . . . . . . . . . . . . 77
     7.2.  NATFW NSLP Header Flag Registry  . . . . . . . . . . . . . 77
     7.3.  NSLP Message Object Registry . . . . . . . . . . . . . . . 78
     7.4.  NSLP Response Code Registry  . . . . . . . . . . . . . . . 78
     7.5.  NSLP IDs and Router Alert Option Values  . . . . . . . . . 78
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 78
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 79
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 79
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 79
   Appendix A.  Selecting Signaling Destination Addresses for
                EXTERNAL  . . . . . . . . . . . . . . . . . . . . . . 81
   Appendix B.  Usage of External Binding Addresses . . . . . . . . . 82
   Appendix C.  Applicability Statement on Data Receivers behind
                Firewalls . . . . . . . . . . . . . . . . . . . . . . 83
   Appendix D.  Firewall and NAT Resources  . . . . . . . . . . . . . 84
     D.1.  Wildcarding of Policy Rules  . . . . . . . . . . . . . . . 84
     D.2.  Mapping to Firewall Rules  . . . . . . . . . . . . . . . . 84
     D.3.  Mapping to NAT Bindings  . . . . . . . . . . . . . . . . . 85
     D.4.  NSLP Handling of Twice-NAT . . . . . . . . . . . . . . . . 85
   Appendix E.  Example for Receiver Proxy Case . . . . . . . . . . . 86







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1.  Introduction

1.1.  Scope and Background

   Firewalls and Network Address Translators (NATs) have both been used
   throughout the Internet for many years, and they will remain present
   for the foreseeable future.  Firewalls are used to protect networks
   against certain types of attacks from internal networks and the
   Internet, whereas NATs provide a virtual extension of the IP address
   space.  Both types of devices may be obstacles to some applications,
   since they only allow traffic created by a limited set of
   applications to traverse them, typically those that use protocols
   with relatively predetermined and static properties (e.g., most HTTP
   traffic, and other client/server applications).  Other applications,
   such as IP telephony and most other peer-to-peer applications, which
   have more dynamic properties, create traffic that is unable to
   traverse NATs and firewalls without assistance.  In practice, the
   traffic of many applications cannot traverse autonomous firewalls or
   NATs, even when they have additional functionality that attempts to
   restore the transparency of the network.

   Several solutions to enable applications to traverse such entities
   have been proposed and are currently in use.  Typically, application-
   level gateways (ALGs) have been integrated with the firewall or NAT
   to configure the firewall or NAT dynamically.  Another approach is
   middlebox communication (MIDCOM).  In this approach, ALGs external to
   the firewall or NAT configure the corresponding entity via the MIDCOM
   protocol [RFC3303].  Several other work-around solutions are
   available, such as Session Traversal Utilities for NAT (STUN)
   [RFC5389].  However, all of these approaches introduce other problems
   that are generally hard to solve, such as dependencies on the type of
   NAT implementation (full-cone, symmetric, etc.), or dependencies on
   certain network topologies.

   NAT and firewall (NATFW) signaling shares a property with Quality-of-
   Service (QoS) signaling -- each must reach any device that is on the
   data path and is involved in (respectively) NATFW or QoS treatment of
   data packets.  This means that for both NATFW and QoS it is
   convenient if signaling travels path-coupled, i.e., the signaling
   messages follow exactly the same path that the data packets take.
   The Resource Reservation Protocol (RSVP) [RFC2205] is an example of a
   current QoS signaling protocol that is path-coupled. [rsvp-firewall]
   proposes the use of RSVP as a firewall signaling protocol but does
   not include NATs.

   This memo defines a path-coupled signaling protocol for NAT and
   firewall configuration within the framework of NSIS, called the NATFW
   NSIS Signaling Layer Protocol (NSLP).  The general requirements for



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   NSIS are defined in [RFC3726] and the general framework of NSIS is
   outlined in [RFC4080].  It introduces the split between an NSIS
   transport layer and an NSIS signaling layer.  The transport of NSLP
   messages is handled by an NSIS Network Transport Layer Protocol
   (NTLP, with General Internet Signaling Transport (GIST) [RFC5971]
   being the implementation of the abstract NTLP).  The signaling logic
   for QoS and NATFW signaling is implemented in the different NSLPs.
   The QoS NSLP is defined in [RFC5974].

   The NATFW NSLP is designed to request the dynamic configuration of
   NATs and/or firewalls along the data path.  Dynamic configuration
   includes enabling data flows to traverse these devices without being
   obstructed, as well as blocking of particular data flows at inbound
   firewalls.  Enabling data flows requires the loading of firewall
   rules with an action that allows the data flow packets to be
   forwarded and NAT bindings to be created.  The blocking of data flows
   requires the loading of firewall rules with an action that will deny
   forwarding of the data flow packets.  A simplified example for
   enabling data flows: a source host sends a NATFW NSLP signaling
   message towards its data destination.  This message follows the data
   path.  Every NATFW NSLP-enabled NAT/firewall along the data path
   intercepts this message, processes it, and configures itself
   accordingly.  Thereafter, the actual data flow can traverse all these
   configured firewalls/NATs.

   It is necessary to distinguish between two different basic scenarios
   when operating the NATFW NSLP, independent of the type of the
   middleboxes to be configured.

   1.  Both the data sender and data receiver are NSIS NATFW NSLP aware.
       This includes the cases in which the data sender is logically
       decomposed from the initiator of the NSIS signaling (the so-
       called NSIS initiator) or the data receiver logically decomposed
       from the receiver of the NSIS signaling (the so-called NSIS
       receiver), but both sides support NSIS.  This scenario assumes
       deployment of NSIS all over the Internet, or at least at all NATs
       and firewalls.  This scenario is used as a base assumption, if
       not otherwise noted.

   2.  Only one end host or region of the network is NSIS NATFW NSLP
       aware, either the data receiver or data sender.  This scenario is
       referred to as proxy mode.

   The NATFW NSLP has two basic signaling messages that are sufficient
   to cope with the various possible scenarios likely to be encountered
   before and after widespread deployment of NSIS:





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      CREATE message: Sent by the data sender for configuring a path
      outbound from a data sender to a data receiver.

      EXTERNAL message: Used by a data receiver to locate inbound NATs/
      firewalls and prime them to expect inbound signaling and used at
      NATs to pre-allocate a public address.  This is used for data
      receivers behind these devices to enable their reachability.

   CREATE and EXTERNAL messages are sent by the NSIS initiator (NI)
   towards the NSIS responder (NR).  Both types of message are
   acknowledged by a subsequent RESPONSE message.  This RESPONSE message
   is generated by the NR if the requested configuration can be
   established; otherwise, the NR or any of the NSLP forwarders (NFs)
   can also generate such a message if an error occurs.  NFs and the NR
   can also generate asynchronous messages to notify the NI, the so-
   called NOTIFY messages.

   If the data receiver resides in a private addressing realm or behind
   a firewall, and it needs to preconfigure the edge-NAT/edge-firewall
   to provide a (publicly) reachable address for use by the data sender,
   a combination of EXTERNAL and CREATE messages is used.

   During the introduction of NSIS, it is likely that one or the other
   of the data sender and receiver will not be NSIS aware.  In these
   cases, the NATFW NSLP can utilize NSIS-aware middleboxes on the path
   between the data sender and data receiver to provide proxy NATFW NSLP
   services (i.e., the proxy mode).  Typically, these boxes will be at
   the boundaries of the realms in which the end hosts are located.

   The CREATE and EXTERNAL messages create NATFW NSLP and NTLP state in
   NSIS entities.  NTLP state allows signaling messages to travel in the
   forward (outbound) and the reverse (inbound) direction along the path
   between a NAT/firewall NSLP sender and a corresponding receiver.
   This state is managed using a soft-state mechanism, i.e., it expires
   unless it is refreshed from time to time.  The NAT bindings and
   firewall rules being installed during the state setup are bound to
   the particular signaling session.  However, the exact local
   implementation of the NAT bindings and firewall rules are NAT/
   firewall specific and it is out of the scope of this memo.

   This memo is structured as follows.  Section 2 describes the network
   environment for NATFW NSLP signaling.  Section 3 defines the NATFW
   signaling protocol and Section 4 defines the message components and
   the overall messages used in the protocol.  The remaining parts of
   the main body of the document cover security considerations
   Section 5, IAB considerations on UNilateral Self-Address Fixing





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   (UNSAF) [RFC3424] in Section 6, and IANA considerations in Section 7.
   Please note that readers familiar with firewalls and NATs and their
   possible location within networks can safely skip Section 2.

1.2.  Terminology and Abbreviations

   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 [RFC2119].

   This document uses a number of terms defined in [RFC3726] and
   [RFC4080].  The following additional terms are used:

   o  Policy rule: A policy rule is "a basic building block of a policy-
      based system.  It is the binding of a set of actions to a set of
      conditions - where the conditions are evaluated to determine
      whether the actions are performed" [RFC3198].  In the context of
      NSIS NATFW NSLP, the conditions are the specification of a set of
      packets to which the rule is applied.  The set of actions always
      contains just a single element per rule, and is limited to either
      action "deny" or action "allow".

   o  Reserved policy rule: A policy rule stored at NATs or firewalls
      for activation by a later, different signaling exchange.  This
      type of policy rule is kept in the NATFW NSLP and is not loaded
      into the firewall or NAT engine, i.e., it does not affect the data
      flow handling.

   o  Installed policy rule: A policy rule in operation at NATs or
      firewalls.  This type of rule is kept in the NATFW NSLP and is
      loaded into the firewall or NAT engine, i.e., it is affecting the
      data flow.

   o  Remembered policy rule: A policy rule stored at NATs and firewalls
      for immediate use, as soon as the signaling exchange is
      successfully completed.

   o  Firewall: A packet filtering device that matches packets against a
      set of policy rules and applies the actions.

   o  Network Address Translator: Network Address Translation is a
      method by which IP addresses are mapped from one IP address realm
      to another, in an attempt to provide transparent routing between
      hosts (see [RFC2663]).  Network Address Translators are devices
      that perform this work by modifying packets passing through them.

   o  Data Receiver (DR): The node in the network that is receiving the
      data packets of a flow.



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   o  Data Sender (DS): The node in the network that is sending the data
      packets of a flow.

   o  NATFW NSLP peer (or simply "peer"): An NSIS NATFW NSLP node with
      which an NTLP adjacency has been created as defined in [RFC5971].

   o  NATFW NSLP signaling session (or simply "signaling session"): A
      signaling session defines an association between the NI, NFs, and
      the NR related to a data flow.  All the NATFW NSLP peers on the
      path, including the NI and the NR, use the same identifier to
      refer to the state stored for the association.  The same NI and NR
      may have more than one signaling session active at any time.  The
      state for the NATFW NSLP consists of NSLP state and associated
      policy rules at a middlebox.

   o  Edge-NAT: An edge-NAT is a NAT device with a globally routable IP
      address that is reachable from the public Internet.

   o  Edge-firewall: An edge-firewall is a firewall device that is
      located on the borderline of an administrative domain.

   o  Public Network: "A Global or Public Network is an address realm
      with unique network addresses assigned by Internet Assigned
      Numbers Authority (IANA) or an equivalent address registry.  This
      network is also referred as external network during NAT
      discussions" [RFC2663].

   o  Private/Local Network: "A private network is an address realm
      independent of external network addresses.  Private network may
      also be referred alternately as Local Network.  Transparent
      routing between hosts in private realm and external realm is
      facilitated by a NAT router" [RFC2663].

   o  Public/Global IP address: An IP address located in the public
      network according to Section 2.7 of [RFC2663].

   o  Private/Local IP address: An IP address located in the private
      network according to Section 2.8 of [RFC2663].

   o  Signaling Destination Address (SDA): An IP address generally taken
      from the public/global IP address range, although, the SDA may, in
      certain circumstances, be part of the private/local IP address
      range.  This address is used in EXTERNAL signaling message
      exchanges, if the data receiver's IP address is unknown.







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1.3.  Notes on the Experimental Status

   The same deployment issues and extensibility considerations described
   in [RFC5971] and [RFC5978] also apply to this document.

1.4.  Middleboxes

   The term "middlebox" covers a range of devices and is well-defined in
   [RFC3234]: "A middlebox is defined as any intermediary device
   performing functions other than the normal, standard functions of an
   IP router on the datagram path between a source host and a
   destination host".  As such, middleboxes fall into a number of
   categories with a wide range of functionality, not all of which is
   pertinent to the NATFW NSLP.  Middlebox categories in the scope of
   this memo are firewalls that filter data packets against a set of
   filter rules, and NATs that translate packet addresses from one
   address realm to another address realm.  Other categories of
   middleboxes, such as QoS traffic shapers, are out of the scope of
   this memo.

   The term "NAT" used in this document is a placeholder for a range of
   different NAT flavors.  We consider the following types of NATs:

   o  Traditional NAT (basic NAT and NAPT)

   o  Bi-directional NAT

   o  Twice-NAT

   o  Multihomed NAT

   For definitions and a detailed discussion about the characteristics
   of each NAT type, please see [RFC2663].

   All types of middleboxes under consideration here use policy rules to
   make a decision on data packet treatment.  Policy rules consist of a
   flow identifier that selects the packets to which the policy applies
   and an associated action; data packets matching the flow identifier
   are subjected to the policy rule action.  A typical flow identifier
   is the 5-tuple selector that matches the following fields of a packet
   to configured values:

   o  Source and destination IP addresses

   o  Transport protocol number

   o  Transport source and destination port numbers




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   Actions for firewalls are usually one or more of:

   o  Allow: forward data packet

   o  Deny: block data packet and discard it

   o  Other actions such as logging, diverting, duplicating, etc.

   Actions for NATs include (amongst many others):

   o  Change source IP address and transport port number to a globally
      routable IP address and associated port number.

   o  Change destination IP address and transport port number to a
      private IP address and associated port number.

   It should be noted that a middlebox may contain two logical
   representations of the policy rule.  The policy rule has a
   representation within the NATFW NSLP, comprising the message routing
   information (MRI) of the NTLP and NSLP information (such as the rule
   action).  The other representation is the implementation of the NATFW
   NSLP policy rule within the NAT and firewall engine of the particular
   device.  Refer to Appendix D for further details.

1.5.  General Scenario for NATFW Traversal

   The purpose of NSIS NATFW signaling is to enable communication
   between endpoints across networks, even in the presence of NAT and
   firewall middleboxes that have not been specially engineered to
   facilitate communication with the application protocols used.  This
   removes the need to create and maintain application layer gateways
   for specific protocols that have been commonly used to provide
   transparency in previous generations of NAT and firewall middleboxes.
   It is assumed that these middleboxes will be statically configured in
   such a way that NSIS NATFW signaling messages themselves are allowed
   to reach the locally installed NATFW NSLP daemon.  NSIS NATFW NSLP
   signaling is used to dynamically install additional policy rules in
   all NATFW middleboxes along the data path that will allow
   transmission of the application data flow(s).  Firewalls are
   configured to forward data packets matching the policy rule provided
   by the NSLP signaling.  NATs are configured to translate data packets
   matching the policy rule provided by the NSLP signaling.  An
   additional capability, that is an exception to the primary goal of
   NSIS NATFW signaling, is that the NATFW nodes can request blocking of
   particular data flows instead of enabling these flows at inbound
   firewalls.





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   The basic high-level picture of NSIS usage is that end hosts are
   located behind middleboxes, meaning that there is at least one
   middlebox on the data path from the end host in a private network to
   the external network (NATFW in Figure 1).  Applications located at
   these end hosts try to establish communication with corresponding
   applications on other such end hosts.  This communication
   establishment may require that the applications contact an
   application server that serves as a rendezvous point between both
   parties to exchange their IP address and port(s).  The local
   applications trigger the NSIS entity at the local host to control
   provisioning for middlebox traversal along the prospective data path
   (e.g., via an API call).  The NSIS entity, in turn, uses NSIS NATFW
   NSLP signaling to establish policy rules along the data path,
   allowing the data to travel from the sender to the receiver without
   obstruction.

   Application          Application Server (0, 1, or more)   Application

   +----+                        +----+                        +----+
   |    +------------------------+    +------------------------+    |
   +-+--+                        +----+                        +-+--+
     |                                                           |
     |         NSIS Entities                      NSIS Entities  |
   +-+--+        +----+                            +-----+     +-+--+
   |    +--------+    +----------------------------+     +-----+    |
   +-+--+        +-+--+                            +--+--+     +-+--+
     |             |               ------             |          |
     |             |           ////      \\\\\        |          |
   +-+--+        +-+--+      |/               |     +-+--+     +-+--+
   |    |        |    |     |     Internet     |    |    |     |    |
   |    +--------+    +-----+                  +----+    +-----+    |
   +----+        +----+      |\               |     +----+     +----+
                               \\\\      /////
   sender    NATFW (1+)            ------          NATFW (1+) receiver

   Note that 1+ refers to one or more NATFW nodes.

         Figure 1: Generic View of NSIS with NATs and/or Firewalls

   For end-to-end NATFW signaling, it is necessary that each firewall
   and each NAT along the path between the data sender and the data
   receiver implements the NSIS NATFW NSLP.  There might be several NATs
   and FWs in various possible combinations on a path between two hosts.
   Section 2 presents a number of likely scenarios with different
   combinations of NATs and firewalls.  However, the scenarios given in
   the following sections are only examples and should not be treated as
   limiting the scope of the NATFW NSLP.




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2.  Network Deployment Scenarios Using the NATFW NSLP

   This section introduces several scenarios for middlebox placement
   within IP networks.  Middleboxes are typically found at various
   different locations, including at enterprise network borders, within
   enterprise networks, as mobile phone network gateways, etc.  Usually,
   middleboxes are placed more towards the edge of networks than in
   network cores.  Firewalls and NATs may be found at these locations
   either alone or combined; other categories of middleboxes may also be
   found at such locations, possibly combined with the NATs and/or
   firewalls.

   NSIS initiators (NI) send NSIS NATFW NSLP signaling messages via the
   regular data path to the NSIS responder (NR).  On the data path,
   NATFW NSLP signaling messages reach different NSIS nodes that
   implement the NATFW NSLP.  Each NATFW NSLP node processes the
   signaling messages according to Section 3 and, if necessary, installs
   policy rules for subsequent data packets.

   Each of the following sub-sections introduces a different scenario
   for a different set of middleboxes and their ordering within the
   topology.  It is assumed that each middlebox implements the NSIS
   NATFW NSLP signaling protocol.

2.1.  Firewall Traversal

   This section describes a scenario with firewalls only; NATs are not
   involved.  Each end host is behind a firewall.  The firewalls are
   connected via the public Internet.  Figure 2 shows the topology.  The
   part labeled "public" is the Internet connecting both firewalls.

                  +----+    //----\\       +----+
          NI -----| FW |---|        |------| FW |--- NR
                  +----+    \\----//       +----+

                 private     public        private

             FW: Firewall
             NI: NSIS Initiator
             NR: NSIS Responder

                   Figure 2: Firewall Traversal Scenario

   Each firewall on the data path must provide traversal service for
   NATFW NSLP in order to permit the NSIS message to reach the other end
   host.  All firewalls process NSIS signaling and establish appropriate
   policy rules, so that the required data packet flow can traverse
   them.



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   There are several very different ways to place firewalls in a network
   topology.  To distinguish firewalls located at network borders, such
   as administrative domains, from others located internally, the term
   edge-firewall is used.  A similar distinction can be made for NATs,
   with an edge-NAT fulfilling the equivalent role.

2.2.  NAT with Two Private Networks

   Figure 3 shows a scenario with NATs at both ends of the network.
   Therefore, each application instance, the NSIS initiator and the NSIS
   responder, are behind NATs.  The outermost NAT, known as the edge-NAT
   (MB2 and MB3), at each side is connected to the public Internet.  The
   NATs are generically labeled as MBX (for middlebox No. X), since
   those devices certainly implement NAT functionality, but can
   implement firewall functionality as well.

   Only two middleboxes (MBs) are shown in Figure 3 at each side, but in
   general, any number of MBs on each side must be considered.

           +----+     +----+    //----\\    +----+     +----+
      NI --| MB1|-----| MB2|---|        |---| MB3|-----| MB4|--- NR
           +----+     +----+    \\----//    +----+     +----+

                private          public          private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

             Figure 3: NAT with two Private Networks Scenario

   Signaling traffic from the NI to the NR has to traverse all the
   middleboxes on the path (MB1 to MB4, in this order), and all the
   middleboxes must be configured properly to allow NSIS signaling to
   traverse them.  The NATFW signaling must configure all middleboxes
   and consider any address translation that will result from this
   configuration in further signaling.  The sender (NI) has to know the
   IP address of the receiver (NR) in advance, otherwise it will not be
   possible to send any NSIS signaling messages towards the responder.
   Note that this IP address is not the private IP address of the
   responder but the NAT's public IP address (here MB3's IP address).
   Instead, a NAT binding (including a public IP address) has to be
   previously installed on the NAT MB3.  This NAT binding subsequently
   allows packets reaching the NAT to be forwarded to the receiver
   within the private address realm.  The receiver might have a number
   of ways to learn its public IP address and port number (including the
   NATFW NSLP) and might need to signal this information to the sender
   using an application-level signaling protocol.



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2.3.  NAT with Private Network on Sender Side

   This scenario shows an application instance at the sending node that
   is behind one or more NATs (shown as generic MB, see discussion in
   Section 2.2).  The receiver is located in the public Internet.

             +----+     +----+    //----\\
        NI --| MB |-----| MB |---|        |--- NR
             +----+     +----+    \\----//

                  private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

             Figure 4: NAT with Private Network on Sender Side

   The traffic from NI to NR has to traverse middleboxes only on the
   sender's side.  The receiver has a public IP address.  The NI sends
   its signaling message directly to the address of the NSIS responder.
   Middleboxes along the path intercept the signaling messages and
   configure accordingly.

   The data sender does not necessarily know whether or not the receiver
   is behind a NAT; hence, it is the receiving side that has to detect
   whether or not it is behind a NAT.

2.4.  NAT with Private Network on Receiver Side Scenario

   The application instance receiving data is behind one or more NATs
   shown as MB (see discussion in Section 2.2).

               //----\\    +----+     +----+
        NI ---|        |---| MB |-----| MB |--- NR
               \\----//    +----+     +----+

                public          private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

          Figure 5: NAT with Private Network on Receiver Scenario

   Initially, the NSIS responder must determine its publicly reachable
   IP address at the external middlebox and notify the NSIS initiator
   about this address.  One possibility is that an application-level



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   protocol is used, meaning that the public IP address is signaled via
   this protocol to the NI.  Afterwards, the NI can start its signaling
   towards the NR and therefore establish the path via the middleboxes
   in the receiver side private network.

   This scenario describes the use case for the EXTERNAL message of the
   NATFW NSLP.

2.5.  Both End Hosts behind Twice-NATs

   This is a special case, where the main problem arises from the need
   to detect that both end hosts are logically within the same address
   space, but are also in two partitions of the address realm on either
   side of a twice-NAT (see [RFC2663] for a discussion of twice-NAT
   functionality).

   Sender and receiver are both within a single private address realm,
   but the two partitions potentially have overlapping IP address
   ranges.  Figure 6 shows the arrangement of NATs.

                                   public

             +----+     +----+    //----\\
        NI --| MB |--+--| MB |---|        |
             +----+  |  +----+    \\----//
                     |
                     |  +----+
                     +--| MB |------------ NR
                        +----+

                   private

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

     Figure 6: NAT to Public, Sender and Receiver on Either Side of a
                            Twice-NAT Scenario

   The middleboxes shown in Figure 6 are twice-NATs, i.e., they map IP
   addresses and port numbers on both sides, meaning the mapping of
   source and destination IP addresses at the private and public
   interfaces.

   This scenario requires the assistance of application-level entities,
   such as a DNS server.  The application-level entities must handle
   requests that are based on symbolic names and configure the
   middleboxes so that data packets are correctly forwarded from NI to



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   NR.  The configuration of those middleboxes may require other
   middlebox communication protocols, such as MIDCOM [RFC3303].  NSIS
   signaling is not required in the twice-NAT only case, since
   middleboxes of the twice-NAT type are normally configured by other
   means.  Nevertheless, NSIS signaling might be useful when there are
   also firewalls on the path.  In this case, NSIS will not configure
   any policy rule at twice-NATs, but will configure policy rules at the
   firewalls on the path.  The NSIS signaling protocol must be at least
   robust enough to survive this scenario.  This requires that twice-
   NATs must implement the NATFW NSLP also and participate in NATFW
   signaling sessions, but they do not change the configuration of the
   NAT, i.e., they only read the address mapping information out of the
   NAT and translate the Message Routing Information (MRI, [RFC5971])
   within the NSLP and NTLP accordingly.  For more information, see
   Appendix D.4.

2.6.  Both End Hosts behind Same NAT

   When the NSIS initiator and NSIS responder are behind the same NAT
   (thus, being in the same address realm, see Figure 7), they are most
   likely not aware of this fact.  As in Section 2.4, the NSIS responder
   must determine its public IP address in advance and transfer it to
   the NSIS initiator.  Afterwards, the NSIS initiator can start sending
   the signaling messages to the responder's public IP address.  During
   this process, a public IP address will be allocated for the NSIS
   initiator at the same middlebox as for the responder.  Now, the NSIS
   signaling and the subsequent data packets will traverse the NAT
   twice: from initiator to public IP address of responder (first time)
   and from public IP address of responder to responder (second time).

               NI              public
                \  +----+     //----\\
                 +-| MB |----|        |
                /  +----+     \\----//
               NR
                   private


             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

            Figure 7: NAT to Public, Both Hosts behind Same NAT








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2.7.  Multihomed Network with NAT

   The previous sub-sections sketched network topologies where several
   NATs and/or firewalls are ordered sequentially on the path.  This
   section describes a multihomed scenario with two NATs placed on
   alternative paths to the public network.

             +----+    //---\\
   NI -------| MB |---|       |
      \      +----+    \\-+-//
       \                  |
        \                 +----- NR
         \                |
          \  +----+    //-+-\\
           --| MB |---|       |
             +----+    \\---//

        private          public

             MB: Middlebox
             NI: NSIS Initiator
             NR: NSIS Responder

                Figure 8: Multihomed Network with Two NATs

   Depending on the destination, either one or the other middlebox is
   used for the data flow.  Which middlebox is used, depends on local
   policy or routing decisions.  NATFW NSLP must be able to handle this
   situation properly, see Section 3.7.2 for an extended discussion of
   this topic with respect to NATs.

2.8.  Multihomed Network with Firewall

   This section describes a multihomed scenario with two firewalls
   placed on alternative paths to the public network (Figure 9).  The
   routing in the private and public networks decides which firewall is
   being taken for data flows.  Depending on the data flow's direction,
   either outbound or inbound, a different firewall could be traversed.
   This is a challenge for the EXTERNAL message of the NATFW NSLP where
   the NSIS responder is located behind these firewalls within the
   private network.  The EXTERNAL message is used to block a particular
   data flow on an inbound firewall.  NSIS must route the EXTERNAL
   message inbound from NR to NI probably without knowing which path the
   data traffic will take from NI to NR (see also Appendix C).







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             +----+
   NR -------| FW |\
       \     +----+ \  //---\\
        \            -|       |-- NI
         \             \\---//
          \  +----+       |
           --| FW |-------+
             +----+
             private

        private          public

             FW: Firewall
             NI: NSIS Initiator
             NR: NSIS Responder

              Figure 9: Multihomed Network with Two Firewalls

3.  Protocol Description

   This section defines messages, objects, and protocol semantics for
   the NATFW NSLP.

3.1.  Policy Rules

   Policy rules, bound to a NATFW NSLP signaling session, are the
   building blocks of middlebox devices considered in the NATFW NSLP.
   For firewalls, the policy rule usually consists of a 5-tuple and an
   action such as allow or deny.  The information contained in the tuple
   includes source/destination IP addresses, transport protocol, and
   source/destination port numbers.  For NATs, the policy rule consists
   of the action 'translate this address' and further mapping
   information, that might be, in the simplest case, internal IP address
   and external IP address.

   The NATFW NSLP carries, in conjunction with the NTLP's Message
   Routing Information (MRI), the policy rules to be installed at NATFW
   peers.  This policy rule is an abstraction with respect to the real
   policy rule to be installed at the respective firewall or NAT.  It
   conveys the initiator's request and must be mapped to the possible
   configuration on the particular used NAT and/or firewall in use.  For
   pure firewalls, one or more filter rules must be created, and for
   pure NATs, one or more NAT bindings must be created.  In mixed
   firewall and NAT boxes, the policy rule must be mapped to filter
   rules and bindings observing the ordering of the firewall and NAT
   engine.  Depending on the ordering, NAT before firewall or vice





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   versa, the firewall rules must carry public or private IP addresses.
   However, the exact mapping depends on the implementation of the
   firewall or NAT that is possibly different for each implementation.

   The policy rule at the NATFW NSLP level comprises the message routing
   information (MRI) part, carried in the NTLP, and the information
   available in the NATFW NSLP.  The information provided by the NSLP is
   stored in the 'extend flow information' (NATFW_EFI) and 'data
   terminal information' (NATFW_DTINFO) objects, and the message type.
   Additional information, such as the external IP address and port
   number, stored in the NAT or firewall, will be used as well.  The MRI
   carries the filter part of the NAT/firewall-level policy rule that is
   to be installed.

   The NATFW NSLP specifies two actions for the policy rules: deny and
   allow.  A policy rule with action set to deny will result in all
   packets matching this rule to be dropped.  A policy rule with action
   set to allow will result in all packets matching this rule to be
   forwarded.

3.2.  Basic Protocol Overview

   The NSIS NATFW NSLP is carried over the General Internet Signaling
   Transport (GIST, the implementation of the NTLP) defined in
   [RFC5971].  NATFW NSLP messages are initiated by the NSIS initiator
   (NI), handled by NSLP forwarders (NFs) and received by the NSIS
   responder (NR).  It is required that at least NI and NR implement
   this NSLP, intermediate NFs only implement this NSLP when they
   provide relevant middlebox functions.  NSLP forwarders that do not
   have any NATFW NSLP functions just forward these packets as they have
   no interest in them.

3.2.1.  Signaling for Outbound Traffic

   A data sender (DS), intending to send data to a data receiver (DR),
   has to start NATFW NSLP signaling.  This causes the NI associated
   with the DS to launch NSLP signaling towards the address of the DR
   (see Figure 10).  Although it is expected that the DS and the NATFW
   NSLP NI will usually reside on the same host, this specification does
   not rule out scenarios where the DS and NI reside on different hosts,
   the so-called proxy mode (see Section 3.7.6).










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             +-------+    +-------+    +-------+    +-------+
             | DS/NI |<~~~| MB1/  |<~~~| MB2/  |<~~~| DR/NR |
             |       |--->| NF1   |--->| NF2   |--->|       |
             +-------+    +-------+    +-------+    +-------+

                 ========================================>
                    Data Traffic Direction (outbound)


                  --->  : NATFW NSLP request signaling
                  ~~~>  : NATFW NSLP response signaling
                  DS/NI : Data sender and NSIS initiator
                  DR/NR : Data receiver and NSIS responder
                  MB1   : Middlebox 1 and NSLP forwarder 1
                  MB2   : Middlebox 2 and NSLP forwarder 2

                     Figure 10: General NSIS Signaling

   The following list shows the normal sequence of NSLP events without
   detailing the interaction with the NTLP and the interactions on the
   NTLP level.

   o  NSIS initiators generate request messages (which are either CREATE
      or EXTERNAL messages) and send these towards the NSIS responder.
      This request message is the initial message that creates a new
      NATFW NSLP signaling session.  The NI and the NR will most likely
      already share an application session before they start the NATFW
      NSLP signaling session.  Note well the difference between both
      sessions.

   o  NSLP request messages are processed each time an NF with NATFW
      NSLP support is traversed.  Each NF that is intercepting a request
      message and is accepting it for further treatment is joining the
      particular NATFW NSLP signaling session.  These nodes process the
      message, check local policies for authorization and
      authentication, possibly create policy rules, and forward the
      signaling message to the next NSIS node.  The request message is
      forwarded until it reaches the NSIS responder.

   o  NSIS responders will check received messages and process them if
      applicable.  NSIS responders generate RESPONSE messages and send
      them hop-by-hop back to the NI via the same chain of NFs
      (traversal of the same NF chain is guaranteed through the
      established reverse message routing state in the NTLP).  The NR is
      also joining the NATFW NSLP signaling session if the request
      message is accepted.





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   o  The RESPONSE message is processed at each NF that has been
      included in the prior NATFW NSLP signaling session setup.

   o  If the NI has received a successful RESPONSE message and if the
      signaling NATFW NSLP session started with a CREATE message, the
      data sender can start sending its data flow to the data receiver.
      If the NI has received a successful RESPONSE message and if the
      signaling NATFW NSLP session started with an EXTERNAL message, the
      data receiver is ready to receive further CREATE messages.

   Because NATFW NSLP signaling follows the data path from DS to DR,
   this immediately enables communication between both hosts for
   scenarios with only firewalls on the data path or NATs on the sender
   side.  For scenarios with NATs on the receiver side, certain problems
   arise, as described in Section 2.4.

3.2.2.  Signaling for Inbound Traffic

   When the NR and the NI are located in different address realms and
   the NR is located behind a NAT, the NI cannot signal to the NR
   address directly.  The DR/NR is not reachable from other NIs using
   the private address of the NR and thus NATFW signaling messages
   cannot be sent to the NR/DR's address.  Therefore, the NR must first
   obtain a NAT binding that provides an address that is reachable for
   the NI.  Once the NR has acquired a public IP address, it forwards
   this information to the DS via a separate protocol.  This
   application-layer signaling, which is out of the scope of the NATFW
   NSLP, may involve third parties that assist in exchanging these
   messages.

   The same holds partially true for NRs located behind firewalls that
   block all traffic by default.  In this case, NR must tell its inbound
   firewalls of inbound NATFW NSLP signaling and corresponding data
   traffic.  Once the NR has informed the inbound firewalls, it can
   start its application-level signaling to initiate communication with
   the NI.  This mechanism can be used by machines hosting services
   behind firewalls as well.  In this case, the NR informs the inbound
   firewalls as described, but does not need to communicate this to the
   NIs.

   NATFW NSLP signaling supports this scenario by using the EXTERNAL
   message.

   1.  The DR acquires a public address by signaling on the reverse path
       (DR towards DS) and thus making itself available to other hosts.
       This process of acquiring public addresses is called reservation.
       During this process the DR reserves publicly reachable addresses
       and ports suitable for further usage in application-level



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       signaling and the publicly reachable address for further NATFW
       NSLP signaling.  However, the data traffic will not be allowed to
       use this address/port initially (see next point).  In the process
       of reservation, the DR becomes the NI for the messages necessary
       to obtain the publicly reachable IP address, i.e., the NI for
       this specific NATFW NSLP signaling session.

   2.  Now on the side of the DS, the NI creates a new NATFW NSLP
       signaling session and signals directly to the public IP address
       of the DR.  This public IP address is used as NR's address, as
       the NI would do if there is no NAT in between, and creates policy
       rules at middleboxes.  Note, that the reservation will only allow
       forwarding of signaling messages, but not data flow packets.
       Policy rules allowing forwarding of data flow packets set up by
       the prior EXTERNAL message signaling will be activated when the
       signaling from NI towards NR is confirmed with a positive
       RESPONSE message.  The EXTERNAL message is described in
       Section 3.7.2.

3.2.3.  Signaling for Proxy Mode

                    administrative domain
               ----------------------------------\
                                                 |
             +-------+    +-------+    +-------+ |  +-------+
             | DS/NI |<~~~| MB1/  |<~~~| MB2/  | |  |   DR  |
             |       |--->| NF1   |--->| NR    | |  |       |
             +-------+    +-------+    +-------+ |  +-------+
                                                 |
               ----------------------------------/

                 ========================================>
                    Data Traffic Direction (outbound)


                  --->  : NATFW NSLP request signaling
                  ~~~>  : NATFW NSLP response signaling
                  DS/NI : Data sender and NSIS initiator
                  DR/NR : Data receiver and NSIS responder
                  MB1   : Middlebox 1 and NSLP forwarder 1
                  MB2   : Middlebox 2 and NSLP responder

              Figure 11: Proxy Mode Signaling for Data Sender

   The above usage assumes that both ends of a communication support
   NSIS, but fails when NSIS is only deployed at one end of the path.
   In this case, only one of the sending side (see Figure 11) or
   receiving side (see Figure 12) is NSIS aware and not both at the same



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   time.  NATFW NSLP supports both scenarios (i.e., either the DS or DR
   does not support NSIS) by using a proxy mode, as described in
   Section 3.7.6.

                               administrative domain
                        / ----------------------------------
                        |
             +-------+  | +-------+    +-------+    +-------+
             |   DS  |  | | MB2/  |~~~>|  MB1/ |~~~>|   DR  |
             |       |  | | NR    |<---|  NF1  |<---|       |
             +-------+  | +-------+    +-------+    +-------+
                        |
                        \----------------------------------

                 ========================================>
                    Data Traffic Direction (inbound)


                  --->  : NATFW NSLP request signaling
                  ~~~>  : NATFW NSLP response signaling
                  DS/NI : Data sender and NSIS initiator
                  DR/NR : Data receiver and NSIS responder
                  MB1   : Middlebox 1 and NSLP forwarder 1
                  MB2   : Middlebox 2 and NSLP responder

             Figure 12: Proxy Mode Signaling for Data Receiver

3.2.4.  Blocking Traffic

   The basic functionality of the NATFW NSLP provides for opening
   firewall pin holes and creating NAT bindings to enable data flows to
   traverse these devices.  Firewalls are normally expected to work on a
   "deny-all" policy, meaning that traffic not explicitly matching any
   firewall filter rule will be blocked.  Similarly, the normal behavior
   of NATs is to block all traffic that does not match any already
   configured/installed binding or NATFW NSLP session.  However, some
   scenarios require support of firewalls having "allow-all" policies,
   allowing data traffic to traverse the firewall unless it is blocked
   explicitly.  Data receivers can utilize NATFW NSLP's EXTERNAL message
   with action set to "deny" to install policy rules at inbound
   firewalls to block unwanted traffic.

3.2.5.  State and Error Maintenance

   The protocol works on a soft-state basis, meaning that whatever state
   is installed or reserved on a middlebox will expire, and thus be
   uninstalled or forgotten after a certain period of time.  To prevent
   premature removal of state that is needed for ongoing communication,



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   the NATFW NI involved will have to specifically request a NATFW NSLP
   signaling session extension.  An explicit NATFW NSLP state deletion
   capability is also provided by the protocol.

   If the actions requested by a NATFW NSLP message cannot be carried
   out, NFs and the NR must return a failure, such that appropriate
   actions can be taken.  They can do this either during the request
   message handling (synchronously) by sending an error RESPONSE message
   or at any time (asynchronously) by sending a NOTIFY notification
   message.

   The next sections define the NATFW NSLP message types and formats,
   protocol operations, and policy rule operations.

3.2.6.  Message Types

   The protocol uses four messages types:

   o  CREATE: a request message used for creating, changing, refreshing,
      and deleting NATFW NSLP signaling sessions, i.e., open the data
      path from DS to DR.

   o  EXTERNAL: a request message used for reserving, changing,
      refreshing, and deleting EXTERNAL NATFW NSLP signaling sessions.
      EXTERNAL messages are forwarded to the edge-NAT or edge-firewall
      and allow inbound CREATE messages to be forwarded to the NR.
      Additionally, EXTERNAL messages reserve an external address and,
      if applicable, port number at an edge-NAT.

   o  NOTIFY: an asynchronous message used by NATFW peers to alert other
      NATFW peers about specific events (especially failures).

   o  RESPONSE: used as a response to CREATE and EXTERNAL request
      messages.

3.2.7.  Classification of RESPONSE Messages

   RESPONSE messages will be generated synchronously to CREATE and
   EXTERNAL messages by NSLP forwarders and responders to report success
   or failure of operations or some information relating to the NATFW
   NSLP signaling session or a node.  RESPONSE messages MUST NOT be
   generated for any other message, such as NOTIFY and RESPONSE.

   All RESPONSE messages MUST carry a NATFW_INFO object that contains an
   error class code and a response code (see Section 4.2.5).  This
   section defines terms for groups of RESPONSE messages depending on
   the error class.




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   o  Successful RESPONSE: Messages carrying NATFW_INFO with error class
      'Success' (2).

   o  Informational RESPONSE: Messages carrying NATFW_INFO with error
      class 'Informational' (1) (only used with NOTIFY messages).

   o  Error RESPONSE: Messages carrying NATFW_INFO with error class
      other than 'Success' or 'Informational'.

3.2.8.  NATFW NSLP Signaling Sessions

   A NATFW NSLP signaling session defines an association between the NI,
   NFs, and the NR related to a data flow.  This association is created
   when the initial CREATE or EXTERNAL message is successfully received
   at the NFs or the NR.  There is signaling NATFW NSLP session state
   stored at the NTLP layer and at the NATFW NSLP level.  The NATFW NSLP
   signaling session state for the NATFW NSLP comprises NSLP state and
   the associated policy rules at a middlebox.

   The NATFW NSLP signaling session is identified by the session ID
   (plus other information at the NTLP level).  The session ID is
   generated by the NI before the initial CREATE or EXTERNAL message is
   sent.  The value of the session ID MUST be generated as a
   cryptographically random number (see [RFC4086]) by the NI, i.e., the
   output MUST NOT be easily guessable by third parties.  The session ID
   is not stored in any NATFW NSLP message but passed on to the NTLP.

   A NATFW NSLP signaling session has several conceptual states that
   describe in what state a signaling session is at a given time.  The
   signaling session can have these states at a node:

   o  Pending: The NATFW NSLP signaling session has been created and the
      node is waiting for a RESPONSE message to the CREATE or EXTERNAL
      message.  A NATFW NSLP signaling session in state 'Pending' MUST
      be marked as 'Dead' if no corresponding RESPONSE message has been
      received within the time of the locally granted NATFW NSLP
      signaling session lifetime of the forwarded CREATE or EXTERNAL
      message (as described in Section 3.4).

   o  Established: The NATFW NSLP signaling session is established, i.e,
      the signaling has been successfully performed and the lifetime of
      NATFW NSLP signaling session is counted from now on.  A NATFW NSLP
      signaling session in state 'Established' MUST be marked as 'Dead'
      if no refresh message has been received within the time of the
      locally granted NATFW NSLP signaling session lifetime of the
      RESPONSE message (as described in Section 3.4).





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   o  Dead: Either the NATFW NSLP signaling session is timed out or the
      node has received an error RESPONSE message for the NATFW NSLP
      signaling session and the NATFW NSLP signaling session can be
      deleted.

   o  Transitory: The node has received an asynchronous message, i.e., a
      NOTIFY, and can delete the NATFW NSLP signaling session if needed
      after some time.  When a node has received a NOTIFY message, it
      marks the signaling session as 'Transitory'.  This signaling
      session SHOULD NOT be deleted before a minimum hold time of 30
      seconds, i.e., it can be removed after 30 seconds or more.  This
      hold time ensures that the existing signaling session can be
      reused by the NI, e.g., a part of a signaling session that is not
      affected by the route change can be reused once the updating
      request message is received.

3.3.  Basic Message Processing

   All NATFW messages are subject to some basic message processing when
   received at a node, independent of the message type.  Initially, the
   syntax of the NSLP message is checked and a RESPONSE message with an
   appropriate error of class 'Protocol error' (3) code is generated if
   a non-recoverable syntax error is detected.  A recoverable error is,
   for instance, when a node receives a message with reserved flags set
   to values other than zero.  This also refers to unknown NSLP objects
   and their handling, according to Section 4.2.  If a message is
   delivered to the NATFW NSLP, this implies that the NTLP layer has
   been able to correlate it with the session ID (SID) and MRI entries
   in its database.  There is therefore enough information to identify
   the source of the message and routing information to route the
   message back to the NI through an established chain of NTLP messaging
   associations.  The message is not further forwarded if any error in
   the syntax is detected.  The specific response codes stemming from
   the processing of objects are described in the respective object
   definition section (see Section 4).  After passing this check, the
   NATFW NSLP node performs authentication- and authorization-related
   checks, described in Section 3.6.  Further processing is executed
   only if these tests have been successfully passed; otherwise, the
   processing stops and an error RESPONSE is returned.

   Further message processing stops whenever an error RESPONSE message
   is generated, and the EXTERNAL or CREATE message is discarded.

3.4.  Calculation of Signaling Session Lifetime

   NATFW NSLP signaling sessions, and the corresponding policy rules
   that may have been installed, are maintained via a soft-state
   mechanism.  Each signaling session is assigned a signaling session



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   lifetime and the signaling session is kept alive as long as the
   lifetime is valid.  After the expiration of the signaling session
   lifetime, signaling sessions and policy rules MUST be removed
   automatically and resources bound to them MUST be freed as well.
   Signaling session lifetime is handled at every NATFW NSLP node.  The
   NSLP forwarders and NSLP responder MUST NOT trigger signaling session
   lifetime extension refresh messages (see Section 3.7.3): this is the
   task of the NSIS initiator.

   The NSIS initiator MUST choose a NATFW NSLP signaling session
   lifetime value (expressed in seconds) before sending any message,
   including the initial message that creates the NATFW NSLP signaling
   session, to other NSLP nodes.  It is RECOMMENDED that the NATFW NSLP
   signaling session lifetime value is calculated based on:

   o  the number of lost refresh messages with which NFs should cope;

   o  the end-to-end delay between the NI and NR;

   o  network vulnerability due to NATFW NSLP signaling session
      hijacking ([RFC4081]), NATFW NSLP signaling session hijacking is
      made easier when the NI does not explicitly remove the NATFW NSLP
      signaling session;

   o  the user application's data exchange duration, in terms of time
      and networking needs.  This duration is modeled as R, with R the
      message refresh period (in seconds);

   o  the load on the signaling plane.  Short lifetimes imply more
      frequent signaling messages;

   o  the acceptable time for a NATFW NSLP signaling session to be
      present after it is no longer actually needed.  For example, if
      the existence of the NATFW NSLP signaling session implies a
      monetary cost and teardown cannot be guaranteed, shorter lifetimes
      would be preferable;

   o  the lease time of the NI's IP address.  The lease time of the IP
      address must be longer than the chosen NATFW NSLP signaling
      session lifetime; otherwise, the IP address can be re-assigned to
      a different node.  This node may receive unwanted traffic,
      although it never has requested a NAT/firewall configuration,
      which might be an issue in environments with mobile hosts.

   The RSVP specification [RFC2205] provides an appropriate algorithm
   for calculating the NATFW NSLP signaling session lifetime as well as
   a means to avoid refresh message synchronization between NATFW NSLP
   signaling sessions.  [RFC2205] recommends:



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   1.  The refresh message timer to be randomly set to a value in the
       range [0.5R, 1.5R].

   2.  To avoid premature loss of state, lt (with lt being the NATFW
       NSLP signaling session lifetime) must satisfy lt >= (K +
       0.5)*1.5*R, where K is a small integer.  Then, in the worst case,
       K-1 successive messages may be lost without state being deleted.
       Currently, K = 3 is suggested as the default.  However, it may be
       necessary to set a larger K value for hops with high loss rate.
       Other algorithms could be used to define the relation between the
       NATFW NSLP signaling session lifetime and the refresh message
       period; the algorithm provided is only given as an example.

   It is RECOMMENDED to use a refresh timer of 300 s (5 minutes), unless
   the NI or the requesting application at the NI has other requirements
   (e.g., flows lasting a very short time).

   This requested NATFW NSLP signaling session lifetime value lt is
   stored in the NATFW_LT object of the NSLP message.

   NSLP forwarders and the NSLP responder can execute the following
   behavior with respect to the requested lifetime handling:

   Requested signaling session lifetime acceptable:

      No changes to the NATFW NSLP signaling session lifetime values are
      needed.  The CREATE or EXTERNAL message is forwarded, if
      applicable.


   Signaling session lifetime can be lowered:

      An NSLP forwarded or the NSLP responder MAY also lower the
      requested NATFW NSLP signaling session lifetime to an acceptable
      value (based on its local policies).  If an NF changes the NATFW
      NSLP signaling session lifetime value, it MUST store the new value
      in the NATFW_LT object.  The CREATE or EXTERNAL message is
      forwarded.


   Requested signaling session lifetime is too big:

      An NSLP forwarded or the NSLP responder MAY reject the requested
      NATFW NSLP signaling session lifetime value as being too big and
      MUST generate an error RESPONSE message of class 'Signaling
      session failure' (7) with response code 'Requested lifetime is too
      big' (0x02) upon rejection.  Lowering the lifetime is preferred
      instead of generating an error message.



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   Requested signaling session lifetime is too small:

      An NSLP forwarded or the NSLP responder MAY reject the requested
      NATFW NSLP signaling session lifetime value as being to small and
      MUST generate an error RESPONSE message of class 'Signaling
      session failure' (7) with response code 'Requested lifetime is too
      small' (0x10) upon rejection.

   NFs or the NR MUST NOT increase the NATFW NSLP signaling session
   lifetime value.  Messages can be rejected on the basis of the NATFW
   NSLP signaling session lifetime being too long when a NATFW NSLP
   signaling session is first created and also on refreshes.

   The NSLP responder generates a successful RESPONSE for the received
   CREATE or EXTERNAL message, sets the NATFW NSLP signaling session
   lifetime value in the NATFW_LT object to the above granted lifetime
   and sends the message back towards NSLP initiator.

   Each NSLP forwarder processes the RESPONSE message and reads and
   stores the granted NATFW NSLP signaling session lifetime value.  The
   forwarders MUST accept the granted NATFW NSLP signaling session
   lifetime, if the lifetime value is within the acceptable range.  The
   acceptable value refers to the value accepted by the NSLP forwarder
   when processing the CREATE or EXTERNAL message.  For received values
   greater than the acceptable value, NSLP forwarders MUST generate a
   RESPONSE message of class 'Signaling session failure' (7) with
   response code 'Modified lifetime is too big' (0x11), including a
   Signaling Session Lifetime object that carries the maximum acceptable
   signaling session lifetime for this node.  For received values lower
   than the values acceptable by the node local policy, NSLP forwarders
   MUST generate a RESPONSE message of class 'Signaling session failure'
   (7) with response code 'Modified lifetime is too small' (0x12),
   including a Signaling Session Lifetime object that carries the
   minimum acceptable signaling session lifetime for this node.  In both
   cases, either 'Modified lifetime is too big' (0x11) or 'Modified
   lifetime is too small' (0x12), the NF MUST generate a NOTIFY message
   and send it outbound with the error class set to 'Informational' (1)
   and with the response code set to 'NATFW signaling session
   terminated' (0x05).

   Figure 13 shows the procedure with an example, where an initiator
   requests 60 seconds lifetime in the CREATE message and the lifetime
   is shortened along the path by the forwarder to 20 seconds and by the
   responder to 15 seconds.  When the NSLP forwarder receives the
   RESPONSE message with a NATFW NSLP signaling session lifetime value
   of 15 seconds it checks whether this value is lower or equal to the
   acceptable value.




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   +-------+ CREATE(lt=60s)  +-------------+ CREATE(lt=20s)  +--------+
   |       |---------------->|     NSLP    |---------------->|        |
   |  NI   |                 |  forwarder  |                 |  NR    |
   |       |<----------------| check 15<20 |<----------------|        |
   +-------+ RESPONSE(lt=15s)+-------------+ RESPONSE(lt=15s)+--------+

      lt  = lifetime

           Figure 13: Signaling Session Lifetime Setting Example

3.5.  Message Sequencing

   NATFW NSLP messages need to carry an identifier so that all nodes
   along the path can distinguish messages sent at different points in
   time.  Messages can be lost along the path or duplicated.  So, all
   NATFW NSLP nodes should be able to identify messages that have been
   received before (duplicated) or lost before (loss).  For message
   replay protection, it is necessary to keep information about messages
   that have already been received and requires every NATFW NSLP message
   to carry a message sequence number (MSN), see also Section 4.2.7.

   The MSN MUST be set by the NI and MUST NOT be set or modified by any
   other node.  The initial value for the MSN MUST be generated randomly
   and MUST be unique only within the NATFW NSLP signaling session for
   which it is used.  The NI MUST increment the MSN by one for every
   message sent.  Once the MSN has reached the maximum value, the next
   value it takes is zero.  All NATFW NSLP nodes MUST use the algorithm
   defined in [RFC1982] to detect MSN wrap-arounds.

   NSLP forwarders and the responder store the MSN from the initial
   CREATE or EXTERNAL packet that creates the NATFW NSLP signaling
   session as the start value for the NATFW NSLP signaling session.  NFs
   and NRs MUST include the received MSN value in the corresponding
   RESPONSE message that they generate.

   When receiving a CREATE or EXTERNAL message, a NATFW NSLP node uses
   the MSN given in the message to determine whether the state being
   requested is different from the state already installed.  The message
   MUST be discarded if the received MSN value is equal to or lower than
   the stored MSN value.  Such a received MSN value can indicate a
   duplicated and delayed message or replayed message.  If the received
   MSN value is greater than the already stored MSN value, the NATFW
   NSLP MUST update its stored state accordingly, if permitted by all
   security checks (see Section 3.6), and store the updated MSN value
   accordingly.






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3.6.  Authentication, Authorization, and Policy Decisions

   NATFW NSLP nodes receiving signaling messages MUST first check
   whether this message is authenticated and authorized to perform the
   requested action.  NATFW NSLP nodes requiring more information than
   provided MUST generate an error RESPONSE of class 'Permanent failure'
   (0x5) with response code 'Authentication failed' (0x01) or with
   response code 'Authorization failed' (0x02).

   The NATFW NSLP is expected to run in various environments, such as
   IP-based telephone systems, enterprise networks, home networks, etc.
   The requirements on authentication and authorization are quite
   different between these use cases.  While a home gateway, or an
   Internet cafe, using NSIS may well be happy with a "NATFW signaling
   coming from inside the network" policy for authorization of
   signaling, enterprise networks are likely to require more strongly
   authenticated/authorized signaling.  This enterprise scenario may
   require the use of an infrastructure and administratively assigned
   identities to operate the NATFW NSLP.

   Once the NI is authenticated and authorized, another step is
   performed.  The requested policy rule for the NATFW NSLP signaling
   session is checked against a set of policy rules, i.e., whether the
   requesting NI is allowed to request the policy rule to be loaded in
   the device.  If this fails, the NF or NR must send an error RESPONSE
   of class 'Permanent failure' (5) and with response code
   'Authorization failed' (0x02).

3.7.  Protocol Operations

   This section defines the protocol operations including how to create
   NATFW NSLP signaling sessions, maintain them, delete them, and how to
   reserve addresses.

   This section requires a good knowledge of the NTLP [RFC5971] and the
   message routing method mechanism and the associated message routing
   information (MRI).  The NATFW NSLP uses information from the MRI,
   e.g., the destination and source ports, and the NATFW NSLP to
   construct the policy rules used on the NATFW NSLP level.  See also
   Appendix D for further information about this.

3.7.1.  Creating Signaling Sessions

   Allowing two hosts to exchange data even in the presence of
   middleboxes is realized in the NATFW NSLP by the use of the CREATE
   message.  The NI (either the data sender or a proxy) generates a
   CREATE message as defined in Section 4.3.1 and hands it to the NTLP.
   The NTLP forwards the whole message on the basis of the message



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   routing information (MRI) towards the NR.  Each NSLP forwarder along
   the path that implements NATFW NSLP processes the NSLP message.
   Forwarding is done hop-by-hop but may pass transparently through NSLP
   forwarders that do not contain NATFW NSLP functionality and non-NSIS-
   aware routers between NSLP hop way points.  When the message reaches
   the NR, the NR can accept the request or reject it.  The NR generates
   a response to CREATE and this response is transported hop-by-hop
   towards the NI.  NATFW NSLP forwarders may reject requests at any
   time.  Figure 14 sketches the message flow between the NI (DS in this
   example), an NF (e.g., NAT), and an NR (DR in this example).

       NI      Private Network        NF    Public Internet        NR
       |                              |                            |
       | CREATE                       |                            |
       |----------------------------->|                            |
       |                              |                            |
       |                              |                            |
       |                              | CREATE                     |
       |                              |--------------------------->|
       |                              |                            |
       |                              | RESPONSE                   |
       |    RESPONSE                  |<---------------------------|
       |<-----------------------------|                            |
       |                              |                            |
       |                              |                            |

           Figure 14: CREATE Message Flow with Success RESPONSE

   There are several processing rules for a NATFW peer when generating
   and receiving CREATE messages, since this message type is used for
   creating new NATFW NSLP signaling sessions, updating existing ones,
   and extending the lifetime and deleting NATFW NSLP signaling
   sessions.  The three latter functions operate in the same way for all
   kinds of CREATE messages, and are therefore described in separate
   sections:

   o  Extending the lifetime of NATFW NSLP signaling sessions is
      described in Section 3.7.3.

   o  Deleting NATFW NSLP signaling sessions is described in
      Section 3.7.4.

   o  Updating policy rules is described in Section 3.10.

   For an initial CREATE message creating a new NATFW NSLP signaling
   session, the processing of CREATE messages is different for every
   NATFW node type:




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   o  NSLP initiator: An NI only generates CREATE messages and hands
      them over to the NTLP.  The NI should never receive CREATE
      messages and MUST discard them.

   o  NATFW NSLP forwarder: NFs that are unable to forward the CREATE
      message to the next hop MUST generate an error RESPONSE of class
      'Permanent failure' (5) with response code 'Did not reach the NR'
      (0x07).  This case may occur if the NTLP layer cannot find a NATFW
      NSLP peer, either another NF or the NR, and returns an error via
      the GIST API (a timeout error reported by GIST).  The NSLP message
      processing at the NFs depends on the middlebox type:

      *  NAT: When the initial CREATE message is received at the public
         side of the NAT, it looks for a reservation made in advance, by
         using an EXTERNAL message (see Section 3.7.2).  The matching
         process considers the received MRI information and the stored
         MRI information, as described in Section 3.8.  If no matching
         reservation can be found, i.e., no reservation has been made in
         advance, the NSLP MUST return an error RESPONSE of class
         'Signaling session failure' (7) with response code 'No
         reservation found matching the MRI of the CREATE request'
         (0x03).  If there is a matching reservation, the NSLP stores
         the data sender's address (and if applicable port number) as
         part of the source IP address of the policy rule ('the
         remembered policy rule') to be loaded, and forwards the message
         with the destination IP address set to the internal (private in
         most cases) address of the NR.  When the initial CREATE message
         is received at the private side, the NAT binding is allocated,
         but not activated (see also Appendix D.3).  An error RESPONSE
         message is generated, if the requested policy rule cannot be
         reserved right away, of class 'Signaling session failure' (7)
         with response code 'Requested policy rule denied due to policy
         conflict' (0x4).  The MRI information is updated to reflect the
         address, and if applicable port, translation.  The NSLP message
         is forwarded towards the NR with source IP address set to the
         NAT's external address from the newly remembered binding.

      *  Firewall: When the initial CREATE message is received, the NSLP
         just remembers the requested policy rule, but does not install
         any policy rule.  Afterwards, the message is forwarded towards
         the NR.  If the requested policy rule cannot be reserved right
         away, an error RESPONSE message is generated, of class
         'Signaling session failure' (7) with response code 'Requested
         policy rule denied due to policy conflict' (0x4).

      *  Combined NAT and firewall: Processing at combined firewall and
         NAT middleboxes is the same as in the NAT case.  No policy
         rules are installed.  Implementations MUST take into account



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         the order of packet processing in the firewall and NAT
         functions within the device.  This will be referred to as
         "order of functions" and is generally different depending on
         whether the packet arrives at the external or internal side of
         the middlebox.

   o  NSLP receiver: NRs receiving initial CREATE messages MUST reply
      with a success RESPONSE of class 'Success' (2) with response code
      set to 'All successfully processed' (0x01), if they accept the
      CREATE message.  Otherwise, they MUST generate a RESPONSE message
      with a suitable response code.  RESPONSE messages are sent back
      NSLP hop-by-hop towards the NI, irrespective of the response
      codes, either success or error.

   Remembered policy rules at middleboxes MUST be only installed upon
   receiving a corresponding successful RESPONSE message with the same
   SID as the CREATE message that caused them to be remembered.  This is
   a countermeasure to several problems, for example, wastage of
   resources due to loading policy rules at intermediate NFs when the
   CREATE message does not reach the final NR for some reason.

   Processing of a RESPONSE message is different for every NSIS node
   type:

   o  NSLP initiator: After receiving a successful RESPONSE, the data
      path is configured and the DS can start sending its data to the
      DR.  After receiving an error RESPONSE message, the NI MAY try to
      generate the CREATE message again or give up and report the
      failure to the application, depending on the error condition.

   o  NSLP forwarder: NFs install the remembered policy rules, if a
      successful RESPONSE message with matching SID is received.  If an
      ERROR RESPONSE message with matching SID is received, the NATFW
      NSLP session is marked as 'Dead', no policy rule is installed and
      the remembered rule is discarded.

   o  NSIS responder: The NR should never receive RESPONSE messages and
      MUST silently drop any such messages received.

   NFs and the NR can also tear down the CREATE session at any time by
   generating a NOTIFY message with the appropriate response code set.

3.7.2.  Reserving External Addresses

   NSIS signaling is intended to travel end-to-end, even in the presence
   of NATs and firewalls on-path.  This works well in cases where the
   data sender is itself behind a NAT or a firewall as described in
   Section 3.7.1.  For scenarios where the data receiver is located



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   behind a NAT or a firewall and it needs to receive data flows from
   outside its own network (usually referred to as inbound flows, see
   Figure 5), the problem is more troublesome.

   NSIS signaling, as well as subsequent data flows, are directed to a
   particular destination IP address that must be known in advance and
   reachable.  Data receivers must tell the local NSIS infrastructure
   (i.e., the inbound firewalls/NATs) about incoming NATFW NSLP
   signaling and data flows before they can receive these flows.  It is
   necessary to differentiate between data receivers behind NATs and
   behind firewalls to understand the further NATFW procedures.  Data
   receivers that are only behind firewalls already have a public IP
   address and they need only to be reachable for NATFW signaling.
   Unlike data receivers that are only behind firewalls, data receivers
   behind NATs do not have public IP addresses; consequently, they are
   not reachable for NATFW signaling by entities outside their
   addressing realm.

   The preceding discussion addresses the situation where a DR node that
   wants to be reachable is unreachable because the NAT lacks a suitable
   rule with the 'allow' action that would forward inbound data.
   However, in certain scenarios, a node situated behind inbound
   firewalls that do not block inbound data traffic (firewalls with
   "default to allow") unless requested might wish to prevent traffic
   being sent to it from specified addresses.  In this case, NSIS NATFW
   signaling can be used to achieve this by installing a policy rule
   with its action set to 'deny' using the same mechanisms as for
   'allow' rules.

   The required result is obtained by sending an EXTERNAL message in the
   inbound direction of the intended data flow.  When using this
   functionality, the NSIS initiator for the 'Reserve External Address'
   signaling is typically the node that will become the DR for the
   eventual data flow.  To distinguish this initiator from the usual
   case where the NI is associated with the DS, the NI is denoted by NI+
   and the NSIS responder is similarly denoted by NR+.















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       Public Internet                Private Address
                                           Space

                    Edge
    NI(DS)         NAT/FW                  NAT                   NR(DR)
    NR+                                                          NI+

    |               |                       |                       |
    |               |                       |                       |
    |               |                       |                       |
    |               |  EXTERNAL[(DTInfo)]   |  EXTERNAL[(DTInfo)]   |
    |               |<----------------------|<----------------------|
    |               |                       |                       |
    |               |RESPONSE[Success/Error]|RESPONSE[Success/Error]|
    |               |---------------------->|---------------------->|
    |               |                       |                       |
    |               |                       |                       |

      ============================================================>
                        Data Traffic Direction

     Figure 15: Reservation Message Flow for DR behind NAT or Firewall

   Figure 15 shows the EXTERNAL message flow for enabling inbound NATFW
   NSLP signaling messages.  In this case, the roles of the different
   NSIS entities are:

   o  The data receiver (DR) for the anticipated data traffic is the
      NSIS initiator (NI+) for the EXTERNAL message, but becomes the
      NSIS responder (NR) for following CREATE messages.

   o  The actual data sender (DS) will be the NSIS initiator (NI) for
      later CREATE messages and may be the NSIS target of the signaling
      (NR+).

   o  It may be necessary to use a signaling destination address (SDA)
      as the actual target of the EXTERNAL message (NR+) if the DR is
      located behind a NAT and the address of the DS is unknown.  The
      SDA is an arbitrary address in the outermost address realm on the
      other side of the NAT from the DR.  Typically, this will be a
      suitable public IP address when the 'outside' realm is the public
      Internet.  This choice of address causes the EXTERNAL message to
      be routed through the NATs towards the outermost realm and would
      force interception of the message by the outermost NAT in the
      network at the boundary between the private address and the public
      address realm (the edge-NAT).  It may also be intercepted by other
      NATs and firewalls on the path to the edge-NAT.




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   Basically, there are two different signaling scenarios.  Either

   1.  the DR behind the NAT/firewall knows the IP address of the DS in
       advance, or

   2.  the address of the DS is not known in advance.

   Case 1 requires the NATFW NSLP to request the path-coupled message
   routing method (PC-MRM) from the NTLP.  The EXTERNAL message MUST be
   sent with PC-MRM (see Section 5.8.1 in [RFC5971]) with the direction
   set to 'upstream' (inbound).  The handling of case 2 depends on the
   situation of the DR: if the DR is solely located behind a firewall,
   the EXTERNAL message MUST be sent with the PC-MRM, direction
   'upstream' (inbound), and the data flow source IP address set to
   'wildcard'.  If the DR is located behind a NAT, the EXTERNAL message
   MUST be sent with the loose-end message routing method (LE-MRM, see
   Section 5.8.2 in [RFC5971]), the destination-address set to the
   signaling destination IP address (SDA, see also Appendix A).  For
   scenarios with the DR behind a firewall, special conditions apply
   (see applicability statement in Appendix C).  The data receiver is
   challenged to determine whether it is solely located behind firewalls
   or NATs in order to choose the right message routing method.  This
   decision can depend on a local configuration parameter, possibly
   given through DHCP, or it could be discovered through other non-NSLP
   related testing of the network configuration.  The use of the PC-MRM
   with the known data sender's IP address is RECOMMENDED.  This gives
   GIST the best possible handle to route the message 'upstream'
   (outbound).  The use of the LE-MRM, if and only if the data sender's
   IP address is not known and the data receiver is behind a NAT, is
   RECOMMENDED.

   For case 2 with NAT, the NI+ (which could be on the data receiver DR
   or on any other host within the private network) sends the EXTERNAL
   message targeted to the signaling destination IP address.  The
   message routing for the EXTERNAL message is in the reverse direction
   of the normal message routing used for path-coupled signaling where
   the signaling is sent outbound (as opposed to inbound in this case).
   When establishing NAT bindings (and a NATFW NSLP signaling session),
   the signaling direction does not matter since the data path is
   modified through route pinning due to the external IP address at the
   NAT.  Subsequent NSIS messages (and also data traffic) will travel
   through the same NAT boxes.  However, this is only valid for the NAT
   boxes, but not for any intermediate firewall.  That is the reason for
   having a separate CREATE message enabling the reservations made with
   EXTERNAL at the NATs and either enabling prior reservations or
   creating new pinholes at the firewalls that are encountered on the
   outbound path depending on whether the inbound and outbound routes
   coincide.



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   The EXTERNAL signaling message creates an NSIS NATFW signaling
   session at any intermediate NSIS NATFW peer(s) encountered,
   independent of the message routing method used.  Furthermore, it has
   to be ensured that the edge-NAT or edge-firewall device is discovered
   as part of this process.  The end host cannot be assumed to know this
   device -- instead the NAT or firewall box itself is assumed to know
   that it is located at the outer perimeter of the network.  Forwarding
   of the EXTERNAL message beyond this entity is not necessary, and MUST
   be prohibited as it may provide information on the capabilities of
   internal hosts.  It should be noted, that it is the outermost NAT or
   firewall that is the edge-device that must be found during this
   discovery process.  For instance, when there are a NAT and
   (afterwards) a firewall on the outbound path at the network border,
   the firewall is the edge-firewall.  All messages must be forwarded to
   the topology-wise outermost edge-device to ensure that this device
   knows about the NATFW NSLP signaling sessions for incoming CREATE
   messages.  However, the NAT is still the edge-NAT because it has a
   public globally routable IP address on its public side: this is not
   affected by any firewall between the edge-NAT and the public network.

   Possible edge arrangements are:

          Public Net   -----------------  Private net  --------------

        | Public Net|--|Edge-FW|--|FW|...|FW|--|DR|

        | Public Net|--|Edge-FW|--|Edge-NAT|...|NAT or FW|--|DR|

        | Public Net|--|Edge-NAT|--|NAT or FW|...|NAT or FW|--|DR|

   The edge-NAT or edge-firewall device closest to the public realm
   responds to the EXTERNAL request message with a successful RESPONSE
   message.  An edge-NAT includes a NATFW_EXTERNAL_IP object (see
   Section 4.2.2), carrying the publicly reachable IP address, and if
   applicable, a port number.

   The NI+ can request each intermediate NAT (i.e., a NAT that is not
   the edge-NAT) to include the external binding address (and if
   applicable port number) in the external binding address object.  The
   external binding address object stores the external IP address (and
   port) at the particular NAT.  The NI+ has to include the external
   binding address (see Section 4.2.3) object in the request message, if
   it wishes to obtain the information.

   There are several processing rules for a NATFW peer when generating
   and receiving EXTERNAL messages, since this message type is used for
   creating new reserve NATFW NSLP signaling sessions, updating
   existing, extending the lifetime, and deleting NATFW NSLP signaling



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   session.  The three latter functions operate in the same way for all
   kinds of CREATE and EXTERNAL messages, and are therefore described in
   separate sections:

   o  Extending the lifetime of NATFW NSLP signaling sessions is
      described in Section 3.7.3.

   o  Deleting NATFW NSLP signaling sessions is described in
      Section 3.7.4.

   o  Updating policy rules is described in Section 3.10.

   The NI+ MUST always include a NATFW_DTINFO object in the EXTERNAL
   message.  Especially, the LE-MRM does not include enough information
   for some types of NATs (basically, those NATs that also translate
   port numbers) to perform the address translation.  This information
   is provided in the NATFW_DTINFO (see Section 4.2.8).  This
   information MUST include at least the 'dst port number' and
   'protocol' fields, in the NATFW_DTINFO object as these may be
   required by NATs that are en route, depending on the type of the NAT.
   All other fields MAY be set by the NI+ to restrict the set of
   possible NIs.  An edge-NAT will use the information provided in the
   NATFW_DTINFO object to allow only a NATFW CREATE message with a
   matching MRI to be forwarded.  The MRI of the NATFW CREATE message
   has to use the parameters set in NATFW_DTINFO object ('src IPv4/v6
   address', 'src port number', 'protocol') as the source IP address/
   port of the flow from DS to DR.  A NAT requiring information carried
   in the NATFW_DTINFO can generate a number of error RESPONSE messages
   of class 'Signaling session failure' (7):

   o  'Requested policy rule denied due to policy conflict' (0x04)

   o  'Unknown policy rule action' (0x05)

   o  'Requested rule action not applicable' (0x06)

   o  'NATFW_DTINFO object is required' (0x07)

   o  'Requested value in sub_ports field in NATFW_EFI not permitted'
      (0x08)

   o  'Requested IP protocol not supported' (0x09)

   o  'Plain IP policy rules not permitted -- need transport layer
      information' (0x0A)

   o  'Source IP address range is too large' (0x0C)




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   o  'Destination IP address range is too large' (0x0D)

   o  'Source L4-port range is too large' (0x0E)

   o  'Destination L4-port range is too large' (0x0F)

   Processing of EXTERNAL messages is specific to the NSIS node type:

   o  NSLP initiator: NI+ only generate EXTERNAL messages.  When the
      data sender's address information is known in advance, the NI+ can
      include a NATFW_DTINFO object in the EXTERNAL message, if not
      anyway required to do so (see above).  When the data sender's IP
      address is not known, the NI+ MUST NOT include an IP address in
      the NATFW_DTINFO object.  The NI should never receive EXTERNAL
      messages and MUST silently discard it.

   o  NSLP forwarder: The NSLP message processing at NFs depends on the
      middlebox type:

      *  NAT: NATs check whether the message is received at the external
         (public in most cases) address or at the internal (private)
         address.  If received at the external address, an NF MUST
         generate an error RESPONSE of class 'Protocol error' (3) with
         response code 'Received EXTERNAL request message on external
         side' (0x0B).  If received at the internal (private address)
         and the NATFW_EFI object contains the action 'deny', an error
         RESPONSE of class 'Protocol error' (3) with response code
         'Requested rule action not applicable' (0x06) MUST be
         generated.  If received at the internal address, an IP address,
         and if applicable, one or more ports, are reserved.  If the
         NATFW_EXTERNAL_BINDING object is present in the message, any
         NAT that is not an edge-NAT MUST include the allocated external
         IP address, and if applicable one or more ports, (the external
         binding address) in the NATFW_EXTERNAL_BINDING object.  If it
         is an edge-NAT and there is no edge-firewall beyond, the NSLP
         message is not forwarded any further and a successful RESPONSE
         message is generated containing a NATFW_EXTERNAL_IP object
         holding the translated address, and if applicable, port
         information from the binding reserved as a result of the
         EXTERNAL message.  The edge-NAT MUST copy the
         NATFW_EXTERNAL_BINDING object to response message, if the
         object is included in the EXTERNAL message.  The RESPONSE
         message is sent back towards the NI+.  If it is not an edge-
         NAT, the NSLP message is forwarded further using the translated
         IP address as signaling source IP address in the LE-MRM and
         translated port in the NATFW_DTINFO object in the field 'DR
         port number', i.e., the NATFW_DTINFO object is updated to
         reflect the translated port number.  The edge-NAT or any other



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         NAT MUST reject EXTERNAL messages not carrying a NATFW_DTINFO
         object or if the address information within this object is
         invalid or is not compliant with local policies (e.g., the
         information provided relates to a range of addresses
         ('wildcarded') but the edge-NAT requires exact information
         about DS's IP address and port) with the above mentioned
         response codes.

      *  Firewall: Non edge-firewalls remember the requested policy
         rule, keep NATFW NSLP signaling session state, and forward the
         message.  Edge-firewalls stop forwarding the EXTERNAL message.
         The policy rule is immediately loaded if the action in the
         NATFW_EFI object is set to 'deny' and the node is an edge-
         firewall.  The policy rule is remembered, but not activated, if
         the action in the NATFW_EFI object is set to 'allow'.  In both
         cases, a successful RESPONSE message is generated.  If the
         action is 'allow', and the NATFW_DTINFO object is included, and
         the MRM is set to LE-MRM in the request, additionally a
         NATFW_EXTERNAL_IP object is included in the RESPONSE message,
         holding the translated address, and if applicable port,
         information.  This information is obtained from the
         NATFW_DTINFO object's 'DR port number' and the source-address
         of the LE-MRM.  The edge-firewall MUST copy the
         NATFW_EXTERNAL_BINDING object to response message, if the
         object is included in the EXTERNAL message.

      *  Combined NAT and firewall: Processing at combined firewall and
         NAT middleboxes is the same as in the NAT case.

   o  NSLP receiver: This type of message should never be received by
      any NR+, and it MUST generate an error RESPONSE message of class
      'Permanent failure' (5) with response code 'No edge-device here'
      (0x06).

   Processing of a RESPONSE message is different for every NSIS node
   type:

   o  NSLP initiator: Upon receiving a successful RESPONSE message, the
      NI+ can rely on the requested configuration for future inbound
      NATFW NSLP signaling sessions.  If the response contains a
      NATFW_EXTERNAL_IP object, the NI can use IP address and port pairs
      carried for further application signaling.  After receiving an
      error RESPONSE message, the NI+ MAY try to generate the EXTERNAL
      message again or give up and report the failure to the
      application, depending on the error condition.






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   o  NSLP forwarder: NFs simply forward this message as long as they
      keep state for the requested reservation, if the RESPONSE message
      contains NATFW_INFO object with class set to 'Success' (2).  If
      the RESPONSE message contains NATFW_INFO object with class set not
      to 'Success' (2), the NATFW NSLP signaling session is marked as
      'Dead'.

   o  NSIS responder: This type of message should never be received by
      any NR+.  The NF should never receive response messages and MUST
      silently discard it.

   NFs and the NR can also tear down the EXTERNAL session at any time by
   generating a NOTIFY message with the appropriate response code set.

   Reservations with action 'allow' made with EXTERNAL MUST be enabled
   by a subsequent CREATE message.  A reservation made with EXTERNAL
   (independent of selected action) is kept alive as long as the NI+
   refreshes the particular NATFW NSLP signaling session and it can be
   reused for multiple, different CREATE messages.  An NI+ may decide to
   tear down a reservation immediately after receiving a CREATE message.
   This implies that a new NATFW NSLP signaling session must be created
   for each new CREATE message.  The CREATE message does not re-use the
   NATFW NSLP signaling session created by EXTERNAL.

   Without using CREATE (see Section 3.7.1) or EXTERNAL in proxy mode
   (see Section 3.7.6) no data traffic will be forwarded to the DR
   beyond the edge-NAT or edge-firewall.  The only function of EXTERNAL
   is to ensure that subsequent CREATE messages traveling towards the NR
   will be forwarded across the public-private boundary towards the DR.
   Correlation of incoming CREATE messages to EXTERNAL reservation
   states is described in Section 3.8.

3.7.3.  NATFW NSLP Signaling Session Refresh

   NATFW NSLP signaling sessions are maintained on a soft-state basis.
   After a specified timeout, sessions and corresponding policy rules
   are removed automatically by the middlebox, if they are not
   refreshed.  Soft-state is created by CREATE and EXTERNAL and the
   maintenance of this state must be done by these messages.  State
   created by CREATE must be maintained by CREATE, state created by
   EXTERNAL must be maintained by EXTERNAL.  Refresh messages, are
   messages carrying the same session ID as the initial message and a
   NATFW_LT lifetime object with a lifetime greater than zero.  Messages
   with the same SID but which carry a different MRI are treated as
   updates of the policy rules and are processed as defined in
   Section 3.10.  Every refresh CREATE or EXTERNAL message MUST be
   acknowledged by an appropriate response message generated by the NR.
   Upon reception by each NSLP forwarder, the state for the given



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   session ID is extended by the NATFW NSLP signaling session refresh
   period, a period of time calculated based on a proposed refresh
   message period.  The new (extended) lifetime of a NATFW NSLP
   signaling session is calculated as current local time plus proposed
   lifetime value (NATFW NSLP signaling session refresh period).
   Section 3.4 defines the process of calculating lifetimes in detail.

   NI      Public Internet        NAT    Private address       NR

      |                              |          space             |
      | CREATE[lifetime > 0]         |                            |

      |----------------------------->|                            |
      |                              |                            |
      |                              |                            |
      |                              |  CREATE[lifetime > 0]      |
      |                              |--------------------------->|
      |                              |                            |
      |                              |   RESPONSE[Success/Error]  |
      |   RESPONSE[Success/Error]    |<---------------------------|
      |<-----------------------------|                            |
      |                              |                            |
      |                              |                            |

       Figure 16: Successful Refresh Message Flow, CREATE as Example

   Processing of NATFW NSLP signaling session refresh CREATE and
   EXTERNAL messages is different for every NSIS node type:

   o  NSLP initiator: The NI/NI+ can generate NATFW NSLP signaling
      session refresh CREATE/EXTERNAL messages before the NATFW NSLP
      signaling session times out.  The rate at which the refresh
      CREATE/EXTERNAL messages are sent and their relation to the NATFW
      NSLP signaling session state lifetime is discussed further in
      Section 3.4.

   o  NSLP forwarder: Processing of this message is independent of the
      middlebox type and is as described in Section 3.4.

   o  NSLP responder: NRs accepting a NATFW NSLP signaling session
      refresh CREATE/EXTERNAL message generate a successful RESPONSE
      message, including the granted lifetime value of Section 3.4 in a
      NATFW_LT object.








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3.7.4.  Deleting Signaling Sessions

   NATFW NSLP signaling sessions can be deleted at any time.  NSLP
   initiators can trigger this deletion by using a CREATE or EXTERNAL
   messages with a lifetime value set to 0, as shown in Figure 17.
   Whether a CREATE or EXTERNAL message type is use depends on how the
   NATFW NSLP signaling session was created.

      NI      Public Internet        NAT    Private address       NR

      |                              |          space             |
      |    CREATE[lifetime=0]        |                            |
      |----------------------------->|                            |
      |                              |                            |
      |                              | CREATE[lifetime=0]         |
      |                              |--------------------------->|
      |                              |                            |

             Figure 17: Delete message flow, CREATE as Example

   NSLP nodes receiving this message delete the NATFW NSLP signaling
   session immediately.  Policy rules associated with this particular
   NATFW NSLP signaling session MUST be also deleted immediately.  This
   message is forwarded until it reaches the final NR.  The CREATE/
   EXTERNAL message with a lifetime value of 0, does not generate any
   response, either positive or negative, since there is no NSIS state
   left at the nodes along the path.

   NSIS initiators can use CREATE/EXTERNAL message with lifetime set to
   zero in an aggregated way, such that a single CREATE or EXTERNAL
   message is terminating multiple NATFW NSLP signaling sessions.  NIs
   can follow this procedure if they like to aggregate NATFW NSLP
   signaling session deletion requests: the NI uses the CREATE or
   EXTERNAL message with the session ID set to zero and the MRI's
   source-address set to its used IP address.  All other fields of the
   respective NATFW NSLP signaling sessions to be terminated are set as
   well; otherwise, these fields are completely wildcarded.  The NSLP
   message is transferred to the NTLP requesting 'explicit routing' as
   described in Sections 5.2.1 and 7.1.4. in [RFC5971].

   The outbound NF receiving such an aggregated CREATE or EXTERNAL
   message MUST reject it with an error RESPONSE of class 'Permanent
   failure' (5) with response code 'Authentication failed' (0x01) if the
   authentication fails and with an error RESPONSE of class 'Permanent
   failure' (5) with response code 'Authorization failed' (0x02) if the
   authorization fails.  Proof of ownership of NATFW NSLP signaling
   sessions, as it is defined in this memo (see Section 5.2.1), is not
   possible when using this aggregation for multiple session



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   termination.  However, the outbound NF can use the relationship
   between the information of the received CREATE or EXTERNAL message
   and the GIST messaging association where the request has been
   received.  The outbound NF MUST only accept this aggregated CREATE or
   EXTERNAL message through already established GIST messaging
   associations with the NI.  The outbound NF MUST NOT propagate this
   aggregated CREATE or EXTERNAL message but it MAY generate and forward
   per NATFW NSLP signaling session CREATE or EXTERNAL messages.

3.7.5.  Reporting Asynchronous Events

   NATFW NSLP forwarders and NATFW NSLP responders must have the ability
   to report asynchronous events to other NATFW NSLP nodes, especially
   to allow reporting back to the NATFW NSLP initiator.  Such
   asynchronous events may be premature NATFW NSLP signaling session
   termination, changes in local policies, route change or any other
   reason that indicates change of the NATFW NSLP signaling session
   state.

   NFs and NRs may generate NOTIFY messages upon asynchronous events,
   with a NATFW_INFO object indicating the reason for event.  These
   reasons can be carried in the NATFW_INFO object (class MUST be set to
   'Informational' (1)) within the NOTIFY message.  This list shows the
   response codes and the associated actions to take at NFs and the NI:

   o  'Route change: Possible route change on the outbound path' (0x01):
      Follow instructions in Section 3.9.  This MUST be sent inbound and
      outbound, if the signaling session is any state except
      'Transitory'.  The NOTIFY message for signaling sessions in state
      Transitory MUST be discarded, as the signaling session is anyhow
      Transitory.  The outbound NOTIFY message MUST be sent with
      explicit routing by providing the SII-Handle to the NTLP.

   o  'Re-authentication required' (0x02): The NI should re-send the
      authentication.  This MUST be sent inbound.

   o  'NATFW node is going down soon' (0x03): The NI and other NFs
      should be prepared for a service interruption at any time.  This
      message MAY be sent inbound and outbound.

   o  'NATFW signaling session lifetime expired' (0x04): The NATFW
      signaling session has expired and the signaling session is invalid
      now.  NFs MUST mark the signaling session as 'Dead'.  This message
      MAY be sent inbound and outbound.







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   o  'NATFW signaling session terminated' (0x05): The NATFW signaling
      session has been terminated for some reason and the signaling
      session is invalid now.  NFs MUST mark the signaling session as
      'Dead'.  This message MAY be sent inbound and outbound.

   NOTIFY messages are always sent hop-by-hop inbound towards NI until
   they reach NI or outbound towards the NR as indicated in the list
   above.

   The initial processing when receiving a NOTIFY message is the same
   for all NATFW nodes: NATFW nodes MUST only accept NOTIFY messages
   through already established NTLP messaging associations.  The further
   processing is different for each NATFW NSLP node type and depends on
   the events notified:

   o  NSLP initiator: NIs analyze the notified event and behave
      appropriately based on the event type.  NIs MUST NOT generate
      NOTIFY messages.

   o  NSLP forwarder: NFs analyze the notified event and behave based on
      the above description per response code.  NFs SHOULD generate
      NOTIFY messages upon asynchronous events and forward them inbound
      towards the NI or outbound towards the NR, depending on the
      received direction, i.e., inbound messages MUST be forwarded
      further inbound and outbound messages MUST be forwarded further
      outbound.  NFs MUST silently discard NOTIFY messages that have
      been received outbound but are only allowed to be sent inbound,
      e.g., 'Re-authentication required' (0x02).

   o  NSLP responder: NRs SHOULD generate NOTIFY messages upon
      asynchronous events including a response code based on the
      reported event.  The NR MUST silently discard NOTIFY messages that
      have been received outbound but are only allowed to be sent
      inbound, e.g., 'Re-authentication required' (0x02).

   NATFW NSLP forwarders, keeping multiple NATFW NSLP signaling sessions
   at the same time, can experience problems when shutting down service
   suddenly.  This sudden shutdown can be as a result of local node
   failure, for instance, due to a hardware failure.  This NF generates
   NOTIFY messages for each of the NATFW NSLP signaling sessions and
   tries to send them inbound.  Due to the number of NOTIFY messages to
   be sent, the shutdown of the node may be unnecessarily prolonged,
   since not all messages can be sent at the same time.  This case can
   be described as a NOTIFY storm, if a multitude of NATFW NSLP
   signaling sessions is involved.






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   To avoid the need for generating per NATFW NSLP signaling session
   NOTIFY messages in such a scenario described or similar cases, NFs
   SHOULD follow this procedure: the NF uses the NOTIFY message with the
   session ID in the NTLP set to zero, with the MRI completely
   wildcarded, using the 'explicit routing' as described in Sections
   5.2.1 and 7.1.4 of [RFC5971].  The inbound NF receiving this type of
   NOTIFY immediately regards all NATFW NSLP signaling sessions from
   that peer matching the MRI as void.  This message will typically
   result in multiple NOTIFY messages at the inbound NF, i.e., the NF
   can generate per terminated NATFW NSLP signaling session a NOTIFY
   message.  However, an NF MAY also aggregate the NOTIFY messages as
   described here.

3.7.6.  Proxy Mode of Operation

   Some migration scenarios need specialized support to cope with cases
   where NSIS is only deployed in some areas of the Internet.  End-to-
   end signaling is going to fail without NSIS support at or near both
   data sender and data receiver terminals.  A proxy mode of operation
   is needed.  This proxy mode of operation must terminate the NATFW
   NSLP signaling topologically-wise as close as possible to the
   terminal for which it is proxying and proxy all messages.  This NATFW
   NSLP node doing the proxying of the signaling messages becomes either
   the NI or the NR for the particular NATFW NSLP signaling session,
   depending on whether it is the DS or DR that does not support NSIS.
   Typically, the edge-NAT or the edge-firewall would be used to proxy
   NATFW NSLP messages.

   This proxy mode operation does not require any new CREATE or EXTERNAL
   message type, but relies on extended CREATE and EXTERNAL message
   types.  They are called, respectively, CREATE-PROXY and EXTERNAL-
   PROXY and are distinguished by setting the P flag in the NSLP header
   to P=1.  This flag instructs edge-NATs and edge-firewalls receiving
   them to operate in proxy mode for the NATFW NSLP signaling session in
   question.  The semantics of the CREATE and EXTERNAL message types are
   not changed and the behavior of the various node types is as defined
   in Sections 3.7.1 and 3.7.2, except for the proxying node.  The
   following paragraphs describe the proxy mode operation for data
   receivers behind middleboxes and data senders behind middleboxes.

3.7.6.1.  Proxying for a Data Sender

   The NATFW NSLP gives the NR the ability to install state on the
   inbound path towards the data sender for outbound data packets, even
   when only the receiving side is running NSIS (as shown in Figure 18).
   The goal of the method described is to trigger the edge-NAT/
   edge-firewall to generate a CREATE message on behalf of the data
   receiver.  In this case, an NR can signal towards the network border



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   as it is performed in the standard EXTERNAL message handling scenario
   as in Section 3.7.2.  The message is forwarded until the edge-NAT/
   edge-firewall is reached.  A public IP address and port number is
   reserved at an edge-NAT/edge-firewall.  As shown in Figure 18, unlike
   the standard EXTERNAL message handling case, the edge-NAT/
   edge-firewall is triggered to send a CREATE message on a new reverse
   path that traverse several firewalls or NATs.  The new reverse path
   for CREATE is necessary to handle routing asymmetries between the
   edge-NAT/edge-firewall and the DR.  It must be stressed that the
   semantics of the CREATE and EXTERNAL messages are not changed, i.e.,
   each is processed as described earlier.

      DS       Public Internet     NAT/FW    Private address      DR
     No NI                            NF         space            NR
      NR+                                                         NI+

      |                               |  EXTERNAL-PROXY[(DTInfo)] |
      |                               |<------------------------- |
      |                               |  RESPONSE[Error/Success]  |
      |                               | ---------------------- >  |
      |                               |   CREATE                  |
      |                               | ------------------------> |
      |                               |  RESPONSE[Error/Success]  |
      |                               | <----------------------   |
      |                               |                           |

         Figure 18: EXTERNAL Triggering Sending of CREATE Message

   A NATFW_NONCE object, carried in the EXTERNAL and CREATE message, is
   used to build the relationship between received CREATEs at the
   message initiator.  An NI+ uses the presence of the NATFW_NONCE
   object to correlate it to the particular EXTERNAL-PROXY.  The absence
   of a NONCE object indicates a CREATE initiated by the DS and not by
   the edge-NAT.  The two signaling sessions, i.e., the session for
   EXTERNAL-PROXY and the session for CREATE, are not independent.  The
   primary session is the EXTERNAL-PROXY session.  The CREATE session is
   secondary to the EXTERNAL-PROXY session, i.e., the CREATE session is
   valid as long as the EXTERNAL-PROXY session is the signaling states
   'Established' or 'Transitory'.  There is no CREATE session in any
   other signaling state of the EXTERNAL-PROXY, i.e., 'Pending' or
   'Dead'.  This ensures fate-sharing between the two signaling
   sessions.

   These processing rules of EXTERNAL-PROXY messages are added to the
   regular EXTERNAL processing:






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   o  NSLP initiator (NI+): The NI+ MUST take the session ID (SID) value
      of the EXTERNAL-PROXY session as the nonce value of the
      NATFW_NONCE object.

   o  NSLP forwarder being either edge-NAT or edge-firewall: When the NF
      accepts an EXTERNAL-PROXY message, it generates a successful
      RESPONSE message as if it were the NR, and it generates a CREATE
      message as defined in Section 3.7.1 and includes a NATFW_NONCE
      object having the same value as of the received NATFW_NONCE
      object.  The NF MUST NOT generate a CREATE-PROXY message.  The NF
      MUST refresh the CREATE message signaling session only if an
      EXTERNAL-PROXY refresh message has been received first.  This also
      includes tearing down signaling sessions, i.e., the NF must tear
      down the CREATE signaling session only if an EXTERNAL-PROXY
      message with lifetime set to 0 has been received first.

   The scenario described in this section challenges the data receiver
   because it must make a correct assumption about the data sender's
   ability to use NSIS NATFW NSLP signaling.  It is possible for the DR
   to make the wrong assumption in two different ways:

      a) the DS is NSIS unaware but the DR assumes the DS to be NSIS
         aware, and

      b) the DS is NSIS aware but the DR assumes the DS to be NSIS
         unaware.

   Case a) will result in middleboxes blocking the data traffic, since
   the DS will never send the expected CREATE message.  Case b) will
   result in the DR successfully requesting proxy mode support by the
   edge-NAT or edge-firewall.  The edge-NAT/edge-firewall will send
   CREATE messages and DS will send CREATE messages as well.  Both
   CREATE messages are handled as separated NATFW NSLP signaling
   sessions and therefore the common rules per NATFW NSLP signaling
   session apply; the NATFW_NONCE object is used to differentiate CREATE
   messages generated by the edge-NAT/edge-firewall from the NI-
   initiated CREATE messages.  It is the NR's responsibility to decide
   whether to tear down the EXTERNAL-PROXY signaling sessions in the
   case where the data sender's side is NSIS aware, but was incorrectly
   assumed not to be so by the DR.  It is RECOMMENDED that a DR behind
   NATs use the proxy mode of operation by default, unless the DR knows
   that the DS is NSIS aware.  The DR MAY cache information about data
   senders that it has found to be NSIS aware in past NATFW NSLP
   signaling sessions.







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   There is a possible race condition between the RESPONSE message to
   the EXTERNAL-PROXY and the CREATE message generated by the edge-NAT.
   The CREATE message can arrive earlier than the RESPONSE message.  An
   NI+ MUST accept CREATE messages generated by the edge-NAT even if the
   RESPONSE message to the EXTERNAL-PROXY was not received.

3.7.6.2.  Proxying for a Data Receiver

   As with data receivers behind middleboxes, data senders behind
   middleboxes can require proxy mode support.  The issue here is that
   there is no NSIS support at the data receiver's side and, by default,
   there will be no response to CREATE messages.  This scenario requires
   the last NSIS NATFW NSLP-aware node to terminate the forwarding and
   to proxy the response to the CREATE message, meaning that this node
   is generating RESPONSE messages.  This last node may be an edge-NAT/
   edge-firewall, or any other NATFW NSLP peer, that detects that there
   is no NR available (probably as a result of GIST timeouts but there
   may be other triggers).

      DS       Private Address      NAT/FW   Public Internet      NR
      NI           Space              NF                         no NR

      |                               |                           |
      |         CREATE-PROXY          |                           |
      |------------------------------>|                           |
      |                               |                           |
      |   RESPONSE[SUCCESS/ERROR]     |                           |
      |<------------------------------|                           |
      |                               |                           |

                 Figure 19: Proxy Mode CREATE Message Flow

   The processing of CREATE-PROXY messages and RESPONSE messages is
   similar to Section 3.7.1, except that forwarding is stopped at the
   edge-NAT/edge-firewall.  The edge-NAT/edge-firewall responds back to
   NI according to the situation (error/success) and will be the NR for
   future NATFW NSLP communication.

   The NI can choose the proxy mode of operation although the DR is NSIS
   aware.  The CREATE-PROXY mode would not configure all NATs and
   firewalls along the data path, since it is terminated at the edge-
   device.  Any device beyond this point will never receive any NATFW
   NSLP signaling for this flow.








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3.7.6.3.  Incremental Deployment Using the Proxy Mode

   The above sections described the proxy mode for cases where the NATFW
   NSLP is solely deployed at the network edges.  However, the NATFW
   NSLP might be incrementally deployed first in some network edges, but
   later on also in other parts of the network.  Using the proxy mode
   only would prevent the NI from determining whether the other parts of
   the network have also been upgraded to use the NATFW NSLP.  One way
   of determining whether the path from the NI to the NR is NATFW-NSLP-
   capable is to use the regular CREATE message and to wait for a
   successful response or an error response.  This will lead to extra
   messages being sent, as a CREATE message, in addition to the CREATE-
   PROXY message (which is required anyhow), is sent from the NI.

   The NATFW NSLP allows the usage of the proxy-mode and a further
   probing of the path by the edge-NAT or edge-firewall.  The NI can
   request proxy-mode handling as described, and can set the E flag (see
   Figure 20) to request the edge-NAT or edge-firewall to probe the
   further path for NATFW NSLP enabled NFs or an NR.

   The edge-NAT or edge-firewall MUST continue to send the CREATE-PROXY
   or EXTERNAL-proxy towards the NR, if the received proxy-mode message
   has the E flag set, in addition to the regular proxy mode handling.
   The edge-NAT or edge-firewall relies on NTLP measures to determine
   whether or not there is another NATFW NSLP reachable towards the NR.
   A failed attempt to forward the request message to the NR will be
   silently discarded.  A successful attempt of forwarding the request
   message to the NR will be acknowledged by the NR with a successful
   response message, which is subject to the regular behavior described
   in the proxy-mode sections.

3.7.6.4.  Deployment Considerations for Edge-Devices

   The proxy mode assumes that the edge-NAT or edge-firewall are
   properly configured by network operator, i.e., the edge-device is
   really the final NAT or firewall of that particular network.  There
   is currently no known way of letting the NATFW NSLP automatically
   detect which of the NAT or firewalls are the actual edge of a
   network.  Therefore, it is important for the network operator to
   configure the edge-NAT or edge-firewall and also to re-configure
   these devices if they are not at the edge anymore.  For instance, an
   edge-NAT is located within an ISP and the ISP chooses to place
   another NAT in front of this edge-NAT.  In this case, the ISP needs
   to reconfigure the old edge-NAT to be a regular NATFW NLSP NAT and to
   configure the newly installed NAT to be the edge-NAT.






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3.8.  Demultiplexing at NATs

   Section 3.7.2 describes how NSIS nodes behind NATs can obtain a
   publicly reachable IP address and port number at a NAT and how the
   resulting mapping rule can be activated by using CREATE messages (see
   Section 3.7.1).  The information about the public IP address/port
   number can be transmitted via an application-level signaling protocol
   and/or third party to the communication partner that would like to
   send data toward the host behind the NAT.  However, NSIS signaling
   flows are sent towards the address of the NAT at which this
   particular IP address and port number is allocated and not directly
   to the allocated IP address and port number.  The NATFW NSLP
   forwarder at this NAT needs to know how the incoming NSLP CREATE
   messages are related to reserved addresses, meaning how to
   demultiplex incoming NSIS CREATE messages.

   The demultiplexing method uses information stored at the local NATFW
   NSLP node and in the policy rule.  The policy rule uses the LE-MRM
   MRI source-address (see [RFC5971]) as the flow destination IP address
   and the network-layer-version (IP-ver) as IP version.  The external
   IP address at the NAT is stored as the external flow destination IP
   address.  All other parameters of the policy rule other than the flow
   destination IP address are wildcarded if no NATFW_DTINFO object is
   included in the EXTERNAL message.  The LE-MRM MRI destination-address
   MUST NOT be used in the policy rule, since it is solely a signaling
   destination address.

   If the NATFW_DTINFO object is included in the EXTERNAL message, the
   policy rule is filled with further information.  The 'dst port
   number' field of the NATFW_DTINFO is stored as the flow destination
   port number.  The 'protocol' field is stored as the flow protocol.
   The 'src port number' field is stored as the flow source port number.
   The 'data sender's IPv4 address' is stored as the flow source IP
   address.  Note that some of these fields can contain wildcards.

   When receiving a CREATE message at the NATFW NSLP, the NATFW NSLP
   uses the flow information stored in the MRI to do the matching
   process.  This table shows the parameters to be compared against each
   other.  Note that not all parameters need be present in an MRI at the
   same time.











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    +-------------------------------+--------------------------------+
    |  Flow parameter (Policy Rule) | MRI parameter (CREATE message) |
    +-------------------------------+--------------------------------+
    |           IP version          |      network-layer-version     |
    |            Protocol           |           IP-protocol          |
    |     source IP address (w)     |       source-address (w)       |
    |      external IP address      |       destination-address      |
    |  destination IP address (n/u) |               N/A              |
    |     source port number (w)    |       L4-source-port (w)       |
    |    external port number (w)   |     L4-destination-port (w)    |
    | destination port number (n/u) |               N/A              |
    |           IPsec-SPI           |            ipsec-SPI           |
    +-------------------------------+--------------------------------+

            Table entries marked with (w) can be wildcarded and
         entries marked with (n/u) are not used for the matching.

                                  Table 1

   It should be noted that the Protocol/IP-protocol entries in Table 1
   refer to the 'Protocol' field in the IPv4 header or the 'next header'
   entry in the IPv6 header.

3.9.  Reacting to Route Changes

   The NATFW NSLP needs to react to route changes in the data path.
   This assumes the capability to detect route changes, to perform NAT
   and firewall configuration on the new path and possibly to tear down
   NATFW NSLP signaling session state on the old path.  The detection of
   route changes is described in Section 7 of [RFC5971], and the NATFW
   NSLP relies on notifications about route changes by the NTLP.  This
   notification will be conveyed by the API between NTLP and NSLP, which
   is out of the scope of this memo.

   A NATFW NSLP node other than the NI or NI+ detecting a route change,
   by means described in the NTLP specification or others, generates a
   NOTIFY message indicating this change and sends this inbound towards
   NI and outbound towards the NR (see also Section 3.7.5).
   Intermediate NFs on the way to the NI can use this information to
   decide later if their NATFW NSLP signaling session can be deleted
   locally, if they do not receive an update within a certain time
   period, as described in Section 3.2.8.  It is important to consider
   the transport limitations of NOTIFY messages as mandated in
   Section 3.7.5.

   The NI receiving this NOTIFY message MAY generate a new CREATE or
   EXTERNAL message and send it towards the NATFW NSLP signaling
   session's NI as for the initial message using the same session ID.



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   All the remaining processing and message forwarding, such as NSLP
   next-hop discovery, is subject to regular NSLP processing as
   described in the particular sections.  Normal routing will guide the
   new CREATE or EXTERNAL message to the correct NFs along the changed
   route.  NFs that were on the original path receiving these new CREATE
   or EXTERNAL messages (see also Section 3.10), can use the session ID
   to update the existing NATFW NSLP signaling session; whereas NFs that
   were not on the original path will create new state for this NATFW
   NSLP signaling session.  The next section describes how policy rules
   are updated.

3.10.  Updating Policy Rules

   NSIS initiators can request an update of the installed/reserved
   policy rules at any time within a NATFW NSLP signaling session.
   Updates to policy rules can be required due to node mobility (NI is
   moving from one IP address to another), route changes (this can
   result in a different NAT mapping at a different NAT device), or the
   wish of the NI to simply change the rule.  NIs can update policy
   rules in existing NATFW NSLP signaling sessions by sending an
   appropriate CREATE or EXTERNAL message (similar to Section 3.4) with
   modified message routing information (MRI) as compared with that
   installed previously, but using the existing session ID to identify
   the intended target of the update.  With respect to authorization and
   authentication, this update CREATE or EXTERNAL message is treated in
   exactly the same way as any initial message.  Therefore, any node
   along in the NATFW NSLP signaling session can reject the update with
   an error RESPONSE message, as defined in the previous sections.

   The message processing and forwarding is executed as defined in the
   particular sections.  An NF or the NR receiving an update simply
   replaces the installed policy rules installed in the firewall/NAT.
   The local procedures on how to update the MRI in the firewall/NAT is
   out of the scope of this memo.

4.  NATFW NSLP Message Components

   A NATFW NSLP message consists of an NSLP header and one or more
   objects following the header.  The NSLP header is carried in all
   NATFW NSLP messages and objects are Type-Length-Value (TLV) encoded
   using big endian (network ordered) binary data representations.
   Header and objects are aligned to 32-bit boundaries and object
   lengths that are not multiples of 32 bits must be padded to the next
   higher 32-bit multiple.

   The whole NSLP message is carried as payload of a NTLP message.

   Note that the notation 0x is used to indicate hexadecimal numbers.



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4.1.  NSLP Header

   All GIST NSLP-Data objects for the NATFW NSLP MUST contain this
   common header as the first 32 bits of the object (this is not the
   same as the GIST Common Header).  It contains two fields, the NSLP
   message type and the P Flag, plus two reserved fields.  The total
   length is 32 bits.  The layout of the NSLP header is defined by
   Figure 20.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Message type  |P|E| reserved  |       reserved                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 20: Common NSLP Header

   The reserved field MUST be set to zero in the NATFW NSLP header
   before sending and MUST be ignored during processing of the header.

   The defined messages types are:

   o  0x1: CREATE

   o  0x2: EXTERNAL

   o  0x3: RESPONSE

   o  0x4: NOTIFY

   If a message with another type is received, an error RESPONSE of
   class 'Protocol error' (3) with response code 'Illegal message type'
   (0x01) MUST be generated.

   The P flag indicates the usage of proxy mode.  If the proxy mode is
   used, it MUST be set to 1.  Proxy mode MUST only be used in
   combination with the message types CREATE and EXTERNAL.  The P flag
   MUST be ignored when processing messages with type RESPONSE or
   NOTIFY.

   The E flag indicates, in proxy mode, whether the edge-NAT or edge-
   firewall MUST continue sending the message to the NR, i.e., end-to-
   end.  The E flag can only be set to 1 if the P flag is set;
   otherwise, it MUST be ignored.  The E flag MUST only be used in
   combination with the message types CREATE and EXTERNAL.  The E flag
   MUST be ignored when processing messages with type RESPONSE or
   NOTIFY.




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4.2.  NSLP Objects

   NATFW NSLP objects use a common header format defined by Figure 21.
   The object header contains these fields: two flags, two reserved
   bits, the NSLP object type, a reserved field of 4 bits, and the
   object length.  Its total length is 32 bits.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |A|B|r|r|   Object Type         |r|r|r|r|   Object Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 21: Common NSLP Object Header

   The object length field contains the total length of the object
   without the object header.  The unit is a word, consisting of 4
   octets.  The particular values of type and length for each NSLP
   object are listed in the subsequent sections that define the NSLP
   objects.  An error RESPONSE of class 'Protocol error' (3) with
   response code 'Wrong object length' (0x07) MUST be generated if the
   length given in the object header is inconsistent with the type of
   object specified or the message is shorter than implied by the object
   length.  The two leading bits of the NSLP object header are used to
   signal the desired treatment for objects whose treatment has not been
   defined in this memo (see [RFC5971], Appendix A.2.1), i.e., the
   Object Type has not been defined.  NATFW NSLP uses a subset of the
   categories defined in GIST:

   o  AB=00 ("Mandatory"): If the object is not understood, the entire
      message containing it MUST be rejected with an error RESPONSE of
      class 'Protocol error' (3) with response code 'Unknown object
      present' (0x06).

   o  AB=01 ("Optional"): If the object is not understood, it should be
      deleted and then the rest of the message processed as usual.

   o  AB=10 ("Forward"): If the object is not understood, it should be
      retained unchanged in any message forwarded as a result of message
      processing, but not stored locally.

   The combination AB=11 MUST NOT be used and an error RESPONSE of class
   'Protocol error' (3) with response code 'Invalid Flag-Field
   combination' (0x09) MUST be generated.

   The following sections do not repeat the common NSLP object header,
   they just list the type and the length.




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4.2.1.  Signaling Session Lifetime Object

   The signaling session lifetime object carries the requested or
   granted lifetime of a NATFW NSLP signaling session measured in
   seconds.

      Type: NATFW_LT (0x00C)

      Length: 1

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          NATFW NSLP signaling session lifetime                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 22: Signaling Session Lifetime Object

4.2.2.  External Address Object

   The external address object can be included in RESPONSE messages
   (Section 4.3.3) only.  It carries the publicly reachable IP address,
   and if applicable port number, at an edge-NAT.

      Type: NATFW_EXTERNAL_IP (0x00D)

      Length: 2

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         port number           |IP-Ver |   reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IPv4 address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 23: External Address Object for IPv4 Addresses

   Please note that the field 'port number' MUST be set to 0 if only an
   IP address has been reserved, for instance, by a traditional NAT.  A
   port number of 0 MUST be ignored in processing this object.

   IP-Ver (4 bits): The IP version number.  This field MUST be set to 4.








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4.2.3.  External Binding Address Object

   The external binding address object can be included in RESPONSE
   messages (Section 4.3.3) and EXTERNAL (Section 3.7.2) messages.  It
   carries one or multiple external binding addresses, and if applicable
   port number, for multi-level NAT deployments (for an example, see
   Section 2.5).  The utilization of the information carried in this
   object is described in Appendix B.

      Type: NATFW_EXTERNAL_BINDING (0x00E)

      Length: 1 + number of IPv4 addresses

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         port number           |IP-Ver |   reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IPv4 address #1                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                           . . .                             //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           IPv4 address  #n                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 24: External Binding Address Object

   Please note that the field 'port number' MUST be set to 0 if only an
   IP address has been reserved, for instance, by a traditional NAT.  A
   port number of 0 MUST be ignored in processing this object.

   IP-Ver (4 bits): The IP version number.  This field MUST be set to 4.

4.2.4.  Extended Flow Information Object

   In general, flow information is kept in the message routing
   information (MRI) of the NTLP.  Nevertheless, some additional
   information may be required for NSLP operations.  The 'extended flow
   information' object carries this additional information about the
   action of the policy rule for firewalls/NATs and a potential
   contiguous port.

      Type: NATFW_EFI (0x00F)

      Length: 1






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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           rule action         |           sub_ports           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 25: Extended Flow Information

   This object has two fields, 'rule action' and 'sub_ports'.  The 'rule
   action' field has these meanings:

   o  0x0001: Allow: A policy rule with this action allows data traffic
      to traverse the middlebox and the NATFW NSLP MUST allow NSLP
      signaling to be forwarded.

   o  0x0002: Deny: A policy rule with this action blocks data traffic
      from traversing the middlebox and the NATFW NSLP MUST NOT allow
      NSLP signaling to be forwarded.

   If the 'rule action' field contains neither 0x0001 nor 0x0002, an
   error RESPONSE of class 'Signaling session failure' (7) with response
   code 'Unknown policy rule action' (0x05) MUST be generated.

   The 'sub_ports' field contains the number of contiguous transport
   layer ports to which this rule applies.  The default value of this
   field is 0, i.e., only the port specified in the NTLP's MRM or
   NATFW_DTINFO object is used for the policy rule.  A value of 1
   indicates that additionally to the port specified in the NTLP's MRM
   (port1), a second port (port2) is used.  This value of port 2 is
   calculated as: port2 = port1 + 1.  Other values than 0 or 1 MUST NOT
   be used in this field and an error RESPONSE of class 'Signaling
   session failure' (7) with response code 'Requested value in sub_ports
   field in NATFW_EFI not permitted' (0x08) MUST be generated.  These
   two contiguous numbered ports can be used by legacy voice over IP
   equipment.  This legacy equipment assumes two adjacent port numbers
   for its RTP/RTCP flows, respectively.

4.2.5.  Information Code Object

   This object carries the response code in RESPONSE messages, which
   indicates either a successful or failed CREATE or EXTERNAL message
   depending on the value of the 'response code' field.  This object is
   also carried in a NOTIFY message to specify the reason for the
   notification.

      Type: NATFW_INFO (0x010)

      Length: 1



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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Resv. | Class | Response Code |r|r|r|r|     Object Type       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 26: Information Code Object

   The field 'resv.' is reserved for future extensions and MUST be set
   to zero when generating such an object and MUST be ignored when
   receiving.  The 'Object Type' field contains the type of the object
   causing the error.  The value of 'Object Type' is set to 0, if no
   object is concerned.  The leading fours bits marked with 'r' are
   always set to zero and ignored.  The 4-bit class field contains the
   error class.  The following classes are defined:

   o  0: Reserved

   o  1: Informational (NOTIFY only)

   o  2: Success

   o  3: Protocol error

   o  4: Transient failure

   o  5: Permanent failure

   o  7: Signaling session failure

   Within each error class a number of responses codes are defined as
   follows.

   o  Informational:

      *  0x01: Route change: possible route change on the outbound path.

      *  0x02: Re-authentication required.

      *  0x03: NATFW node is going down soon.

      *  0x04: NATFW signaling session lifetime expired.

      *  0x05: NATFW signaling session terminated.

   o  Success:

      *  0x01: All successfully processed.



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   o  Protocol error:

      *  0x01: Illegal message type: the type given in the Message Type
         field of the NSLP header is unknown.

      *  0x02: Wrong message length: the length given for the message in
         the NSLP header does not match the length of the message data.

      *  0x03: Bad flags value: an undefined flag or combination of
         flags was set in the NSLP header.

      *  0x04: Mandatory object missing: an object required in a message
         of this type was missing.

      *  0x05: Illegal object present: an object was present that must
         not be used in a message of this type.

      *  0x06: Unknown object present: an object of an unknown type was
         present in the message.

      *  0x07: Wrong object length: the length given for the object in
         the object header did not match the length of the object data
         present.

      *  0x08: Unknown object field value: a field in an object had an
         unknown value.

      *  0x09: Invalid Flag-Field combination: An object contains an
         invalid combination of flags and/or fields.

      *  0x0A: Duplicate object present.

      *  0x0B: Received EXTERNAL request message on external side.

   o  Transient failure:

      *  0x01: Requested resources temporarily not available.

   o  Permanent failure:

      *  0x01: Authentication failed.

      *  0x02: Authorization failed.

      *  0x04: Internal or system error.

      *  0x06: No edge-device here.




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      *  0x07: Did not reach the NR.

   o  Signaling session failure:

      *  0x01: Session terminated asynchronously.

      *  0x02: Requested lifetime is too big.

      *  0x03: No reservation found matching the MRI of the CREATE
         request.

      *  0x04: Requested policy rule denied due to policy conflict.

      *  0x05: Unknown policy rule action.

      *  0x06: Requested rule action not applicable.

      *  0x07: NATFW_DTINFO object is required.

      *  0x08: Requested value in sub_ports field in NATFW_EFI not
         permitted.

      *  0x09: Requested IP protocol not supported.

      *  0x0A: Plain IP policy rules not permitted -- need transport
         layer information.

      *  0x0B: ICMP type value not permitted.

      *  0x0C: Source IP address range is too large.

      *  0x0D: Destination IP address range is too large.

      *  0x0E: Source L4-port range is too large.

      *  0x0F: Destination L4-port range is too large.

      *  0x10: Requested lifetime is too small.

      *  0x11: Modified lifetime is too big.

      *  0x12: Modified lifetime is too small.









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4.2.6.  Nonce Object

   This object carries the nonce value as described in Section 3.7.6.

      Type: NATFW_NONCE (0x011)

      Length: 1

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         nonce                                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 27: Nonce Object

4.2.7.  Message Sequence Number Object

   This object carries the MSN value as described in Section 3.5.

      Type: NATFW_MSN (0x012)

      Length: 1

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    message sequence number                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 28: Message Sequence Number Object

4.2.8.  Data Terminal Information Object

   The 'data terminal information' object carries additional information
   that MUST be included the EXTERNAL message.  EXTERNAL messages are
   transported by the NTLP using the Loose-End message routing method
   (LE-MRM).  The LE-MRM contains only the DR's IP address and a
   signaling destination address (destination IP address).  This
   destination IP address is used for message routing only and is not
   necessarily reflecting the address of the data sender.  This object
   contains information about (if applicable) DR's port number (the
   destination port number), the DS's port number (the source port
   number), the used transport protocol, the prefix length of the IP
   address, and DS's IP address.

      Type: NATFW_DTINFO (0x013)




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      Length: variable.  Maximum 3.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |I|P|S|    reserved             | sender prefix |    protocol   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :      DR port number           |       DS port number          :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                            IPsec-SPI                          :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  data sender's IPv4 address                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 29: Data Terminal IPv4 Address Object

   The flags are:

   o  I: I=1 means that 'protocol' should be interpreted.

   o  P: P=1 means that 'dst port number' and 'src port number' are
      present and should be interpreted.

   o  S: S=1 means that SPI is present and should be interpreted.

   The SPI field is only present if S is set.  The port numbers are only
   present if P is set.  The flags P and S MUST NOT be set at the same
   time.  An error RESPONSE of class 'Protocol error' (3) with response
   code 'Invalid Flag-Field combination' (0x09) MUST be generated if
   they are both set.  If either P or S is set, I MUST be set as well
   and the protocol field MUST carry the particular protocol.  An error
   RESPONSE of class 'Protocol error' (3) with response code 'Invalid
   Flag-Field combination' (0x09) MUST be generated if S or P is set but
   I is not set.

   The fields MUST be interpreted according to these rules:

   o  (data) sender prefix: This parameter indicates the prefix length
      of the 'data sender's IP address' in bits.  For instance, a full
      IPv4 address requires 'sender prefix' to be set to 32.  A value of
      0 indicates an IP address wildcard.

   o  protocol: The IP protocol field.  This field MUST be interpreted
      if I=1; otherwise, it MUST be set to 0 and MUST be ignored.







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   o  DR port number: The port number at the data receiver (DR), i.e.,
      the destination port.  A value of 0 indicates a port wildcard,
      i.e., the destination port number is not known.  Any other value
      indicates the destination port number.

   o  DS port number: The port number at the data sender (DS), i.e., the
      source port.  A value of 0 indicates a port wildcard, i.e., the
      source port number is not known.  Any other value indicates the
      source port number.

   o  data sender's IPv4 address: The source IP address of the data
      sender.  This field MUST be set to zero if no IP address is
      provided, i.e., a complete wildcard is desired (see the dest
      prefix field above).

4.2.9.  ICMP Types Object

   The 'ICMP types' object contains additional information needed to
   configure a NAT of firewall with rules to control ICMP traffic.  The
   object contains a number of values of the ICMP Type field for which a
   filter action should be set up:

      Type: NATFW_ICMP_TYPES (0x014)

      Length: Variable = ((Number of Types carried + 1) + 3) DIV 4

   Where DIV is an integer division.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Count      |     Type      |      Type     |    ........   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       ................                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    ........   |     Type      |           (Padding)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 30: ICMP Types Object

   The fields MUST be interpreted according to these rules:

      count: 8-bit integer specifying the number of 'Type' entries in
      the object.

      type: 8-bit field specifying an ICMP Type value to which this rule
      applies.




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      padding: Sufficient 0 bits to pad out the last word so that the
      total size of the object is an even multiple of words.  Ignored on
      reception.

4.3.  Message Formats

   This section defines the content of each NATFW NSLP message type.
   The message types are defined in Section 4.1.

   Basically, each message is constructed of an NSLP header and one or
   more NSLP objects.  The order of objects is not defined, meaning that
   objects may occur in any sequence.  Objects are marked either with
   mandatory (M) or optional (O).  Where (M) implies that this
   particular object MUST be included within the message and where (O)
   implies that this particular object is OPTIONAL within the message.
   Objects defined in this memo always carry the flag combination AB=00
   in the NSLP object header.  An error RESPONSE message of class
   'Protocol error' (3) with response code 'Mandatory object missing'
   (0x04) MUST be generated if a mandatory declared object is missing.
   An error RESPONSE message of class 'Protocol error' (3) with response
   code 'Illegal object present' (0x05) MUST be generated if an object
   was present that must not be used in a message of this type.  An
   error RESPONSE message of class 'Protocol error' (3) with response
   code 'Duplicate object present' (0x0A) MUST be generated if an object
   appears more than once in a message.

   Each section elaborates the required settings and parameters to be
   set by the NSLP for the NTLP, for instance, how the message routing
   information is set.

4.3.1.  CREATE

   The CREATE message is used to create NATFW NSLP signaling sessions
   and to create policy rules.  Furthermore, CREATE messages are used to
   refresh NATFW NSLP signaling sessions and to delete them.

   The CREATE message carries these objects:

   o  Signaling Session Lifetime object (M)

   o  Extended flow information object (M)

   o  Message sequence number object (M)

   o  Nonce object (M) if P flag set to 1 in the NSLP header, otherwise
      (O)

   o  ICMP Types Object (O)



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   The message routing information in the NTLP MUST be set to DS as
   source IP address and DR as destination IP address.  All other
   parameters MUST be set according to the required policy rule.  CREATE
   messages MUST be transported by using the path-coupled MRM with the
   direction set to 'downstream' (outbound).

4.3.2.  EXTERNAL

   The EXTERNAL message is used to a) reserve an external IP address/
   port at NATs, b) to notify firewalls about NSIS capable DRs, or c) to
   block incoming data traffic at inbound firewalls.

   The EXTERNAL message carries these objects:

   o  Signaling Session Lifetime object (M)

   o  Message sequence number object (M)

   o  Extended flow information object (M)

   o  Data terminal information object (M)

   o  Nonce object (M) if P flag set to 1 in the NSLP header, otherwise
      (O)

   o  ICMP Types Object (O)

   o  External binding address object (O)

   The selected message routing method of the EXTERNAL message depends
   on a number of considerations.  Section 3.7.2 describes exhaustively
   how to select the correct method.  EXTERNAL messages can be
   transported via the path-coupled message routing method (PC-MRM) or
   via the loose-end message routing method (LE-MRM).  In the case of
   PC-MRM, the source-address is set to the DS's address and the
   destination-address is set to the DR's address, the direction is set
   to inbound.  In the case of LE-MRM, the destination-address is set to
   the DR's address or to the signaling destination IP address.  The
   source-address is set to the DS's address.

4.3.3.  RESPONSE

   RESPONSE messages are responses to CREATE and EXTERNAL messages.
   RESPONSE messages MUST NOT be generated for any other message, such
   as NOTIFY and RESPONSE.

   The RESPONSE message for the class 'Success' (2) carries these
   objects:



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   o  Signaling Session Lifetime object (M)

   o  Message sequence number object (M)

   o  Information code object (M)

   o  External address object (O)

   o  External binding address object (O)

   The RESPONSE message for other classes than 'Success' (2) carries
   these objects:

   o  Message sequence number object (M)

   o  Information code object (M)

   o  Signaling Session Lifetime object (O)

   This message is routed towards the NI hop-by-hop, using existing NTLP
   messaging associations.  The MRM used for this message MUST be the
   same as MRM used by the corresponding CREATE or EXTERNAL message.

4.3.4.  NOTIFY

   The NOTIFY messages is used to report asynchronous events happening
   along the signaled path to other NATFW NSLP nodes.

   The NOTIFY message carries this object:

   o  Information code object (M)

   The NOTIFY message is routed towards the next NF, NI, or NR hop-by-
   hop using the existing inbound or outbound node messaging association
   entry within the node's Message Routing State table.  The MRM used
   for this message MUST be the same as MRM used by the corresponding
   CREATE or EXTERNAL message.

5.  Security Considerations

   Security is of major concern particularly in the case of firewall
   traversal.  This section provides security considerations for the
   NAT/firewall traversal and is organized as follows.

   In Section 5.1, we describe how the participating entities relate to
   each other from a security point of view.  That subsection also
   motivates a particular authorization model.




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   Security threats that focus on NSIS in general are described in
   [RFC4081] and they are applicable to this document as well.

   Finally, we illustrate how the security requirements that were
   created based on the security threats can be fulfilled by specific
   security mechanisms.  These aspects will be elaborated in
   Section 5.2.

5.1.  Authorization Framework

   The NATFW NSLP is a protocol that may involve a number of NSIS nodes
   and is, as such, not a two-party protocol.  Figures 1 and 2 of
   [RFC4081] already depict the possible set of communication patterns.
   In this section, we will re-evaluate these communication patterns
   with respect to the NATFW NSLP protocol interaction.

   The security solutions for providing authorization have a direct
   impact on the treatment of different NSLPs.  As it can be seen from
   the QoS NSLP [RFC5974] and the corresponding Diameter QoS work
   [RFC5866], accounting and charging seems to play an important role
   for QoS reservations, whereas monetary aspects might only indirectly
   effect authorization decisions for NAT and firewall signaling.
   Hence, there are differences in the semantics of authorization
   handling between QoS and NATFW signaling.  A NATFW-aware node will
   most likely want to authorize the entity (e.g., user or machine)
   requesting the establishment of pinholes or NAT bindings.  The
   outcome of the authorization decision is either allowed or
   disallowed, whereas a QoS authorization decision might indicate that
   a different set of QoS parameters is authorized (see [RFC5866] as an
   example).

5.1.1.  Peer-to-Peer Relationship

   Starting with the simplest scenario, it is assumed that neighboring
   nodes are able to authenticate each other and to establish keying
   material to protect the signaling message communication.  The nodes
   will have to authorize each other, additionally to the
   authentication.  We use the term 'Security Context' as a placeholder
   for referring to the entire security procedure, the necessary
   infrastructure that needs to be in place in order for this to work
   (e.g., key management) and the established security-related state.
   The required long-term keys (symmetric or asymmetric keys) used for
   authentication either are made available using an out-of-band
   mechanism between the two NSIS NATFW nodes or are dynamically
   established using mechanisms not further specified in this document.
   Note that the deployment environment will most likely have an impact
   on the choice of credentials being used.  The choice of these
   credentials used is also outside the scope of this document.



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   +------------------------+              +-------------------------+
   |Network A               |              |                Network B|
   |              +---------+              +---------+               |
   |        +-///-+ Middle- +---///////----+ Middle- +-///-+         |
   |        |     |  box 1  | Security     |  box 2  |     |         |
   |        |     +---------+ Context      +---------+     |         |
   |        | Security      |              |  Security     |         |
   |        | Context       |              |  Context      |         |
   |        |               |              |               |         |
   |     +--+---+           |              |            +--+---+     |
   |     | Host |           |              |            | Host |     |
   |     |  A   |           |              |            |  B   |     |
   |     +------+           |              |            +------+     |
   +------------------------+              +-------------------------+

                   Figure 31: Peer-to-Peer Relationship

   Figure 31 shows a possible relationship between participating NSIS-
   aware nodes.  Host A might be, for example, a host in an enterprise
   network that has keying material established (e.g., a shared secret)
   with the company's firewall (Middlebox 1).  The network administrator
   of Network A (company network) has created access control lists for
   Host A (or whatever identifiers a particular company wants to use).
   Exactly the same procedure might also be used between Host B and
   Middlebox 2 in Network B.  For the communication between Middlebox 1
   and Middlebox 2 a security context is also assumed in order to allow
   authentication, authorization, and signaling message protection to be
   successful.

5.1.2.  Intra-Domain Relationship

   In larger corporations, for example, a middlebox is used to protect
   individual departments.  In many cases, the entire enterprise is
   controlled by a single (or a small number of) security department(s),
   which give instructions to the department administrators.  In such a
   scenario, the previously discussed peer-to-peer relationship might be
   prevalent.  Sometimes it might be necessary to preserve
   authentication and authorization information within the network.  As
   a possible solution, a centralized approach could be used, whereby an
   interaction between the individual middleboxes and a central entity
   (for example, a policy decision point - PDP) takes place.  As an
   alternative, individual middleboxes exchange the authorization
   decision with another middlebox within the same trust domain.
   Individual middleboxes within an administrative domain may exploit
   their relationship instead of requesting authentication and
   authorization of the signaling initiator again and again.  Figure 32
   illustrates a network structure that uses a centralized entity.




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       +-----------------------------------------------------------+
       |                                               Network A   |
       |                      +---------+                +---------+
       |      +----///--------+ Middle- +------///------++ Middle- +---
       |      | Security      |  box 2  | Security       |  box 2  |
       |      | Context       +----+----+ Context        +----+----+
       | +----+----+               |                          |    |
       | | Middle- +--------+      +---------+                |    |
       | |  box 1  |        |                |                |    |
       | +----+----+        |                |                |    |
       |      | Security    |           +----+-----+          |    |
       |      | Context     |           | Policy   |          |    |
       |   +--+---+         +-----------+ Decision +----------+    |
       |   | Host |                     | Point    |               |
       |   |  A   |                     +----------+               |
       |   +------+                                                |
       +-----------------------------------------------------------+

                   Figure 32: Intra-Domain Relationship

   The interaction between individual middleboxes and a policy decision
   point (or AAA server) is outside the scope of this document.

5.1.3.  End-to-Middle Relationship

   The peer-to-peer relationship between neighboring NSIS NATFW NSLP
   nodes might not always be sufficient.  Network B might require
   additional authorization of the signaling message initiator (in
   addition to the authorization of the neighboring node).  If
   authentication and authorization information is not attached to the
   initial signaling message then the signaling message arriving at
   Middlebox 2 would result in an error message being created, which
   indicates the additional authorization requirement.  In many cases,
   the signaling message initiator might already be aware of the
   additionally required authorization before the signaling message
   exchange is executed.

   Figure 33 shows this scenario.













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       +--------------------+              +---------------------+
       |          Network A |              |Network B            |
       |                    |   Security   |                     |
       |          +---------+   Context    +---------+           |
       |    +-///-+ Middle- +---///////----+ Middle- +-///-+     |
       |    |     |  box 1  |      +-------+  box 2  |     |     |
       |    |     +---------+      |       +---------+     |     |
       |    |Security       |      |       | Security      |     |
       |    |Context        |      |       | Context       |
       |    |               |      |       |               |     |
       | +--+---+           |      |       |            +--+---+ |
       | | Host +----///----+------+       |            | Host | |
       | |  A   |           |   Security   |            |  B   | |
       | +------+           |   Context    |            +------+ |
       +--------------------+              +---------------------+

                   Figure 33: End-to-Middle Relationship

5.2.  Security Framework for the NAT/Firewall NSLP

   The following list of security requirements has been created to
   ensure proper secure operation of the NATFW NSLP.

5.2.1.  Security Protection between Neighboring NATFW NSLP Nodes

   Based on the analyzed threats, it is RECOMMENDED to provide, between
   neighboring NATFW NSLP nodes, the following mechanisms:

   o  data origin authentication,

   o  replay protection,

   o  integrity protection, and,

   o  optionally, confidentiality protection

   It is RECOMMENDED to use the authentication and key exchange security
   mechanisms provided in [RFC5971] between neighboring nodes when
   sending NATFW signaling messages.  The proposed security mechanisms
   of GIST provide support for authentication and key exchange in
   addition to denial-of-service protection.  Depending on the chosen
   security protocol, support for multiple authentication protocols
   might be provided.  If security between neighboring nodes is desired,
   then the usage of C-MODE with a secure transport protocol for the
   delivery of most NSIS messages with the usage of D-MODE only to
   discover the next NATFW NSLP-aware node along the path is highly
   RECOMMENDED.  See [RFC5971] for the definitions of C-MODE and D-MODE.
   Almost all security threats at the NATFW NSLP-layer can be prevented



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   by using a mutually authenticated Transport Layer secured connection
   and by relying on authorization by the neighboring NATFW NSLP
   entities.

   The NATFW NSLP relies on an established security association between
   neighboring peers to prevent unauthorized nodes from modifying or
   deleting installed state.  Between non-neighboring nodes the session
   ID (SID) carried in the NTLP is used to show ownership of a NATFW
   NSLP signaling session.  The session ID MUST be generated in a random
   way and thereby prevents an off-path adversary from mounting targeted
   attacks.  Hence, an adversary would have to learn the randomly
   generated session ID to perform an attack.  In a mobility environment
   a former on-path node that is now off-path can perform an attack.
   Messages for a particular NATFW NSLP signaling session are handled by
   the NTLP to the NATFW NSLP for further processing.  Messages carrying
   a different session ID not associated with any NATFW NSLP are subject
   to the regular processing for new NATFW NSLP signaling sessions.

5.2.2.  Security Protection between Non-Neighboring NATFW NSLP Nodes

   Based on the security threats and the listed requirements, it was
   noted that some threats also demand authentication and authorization
   of a NATFW signaling entity (including the initiator) towards a non-
   neighboring node.  This mechanism mainly demands entity
   authentication.  The most important information exchanged at the
   NATFW NSLP is information related to the establishment for firewall
   pinholes and NAT bindings.  This information can, however, not be
   protected over multiple NSIS NATFW NSLP hops since this information
   might change depending on the capability of each individual NATFW
   NSLP node.

   Some scenarios might also benefit from the usage of authorization
   tokens.  Their purpose is to associate two different signaling
   protocols (e.g., SIP and NSIS) and their authorization decision.
   These tokens are obtained by non-NSIS protocols, such as SIP or as
   part of network access authentication.  When a NAT or firewall along
   the path receives the token it might be verified locally or passed to
   the AAA infrastructure.  Examples of authorization tokens can be
   found in RFC 3520 [RFC3520] and RFC 3521 [RFC3521].  Figure 34 shows
   an example of this protocol interaction.

   An authorization token is provided by the SIP proxy, which acts as
   the assertion generating entity and gets delivered to the end host
   with proper authentication and authorization.  When the NATFW
   signaling message is transmitted towards the network, the
   authorization token is attached to the signaling messages to refer to
   the previous authorization decision.  The assertion-verifying entity
   needs to process the token or it might be necessary to interact with



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   the assertion-granting entity using HTTP (or other protocols).  As a
   result of a successfully authorization by a NATFW NSLP node, the
   requested action is executed and later a RESPONSE message is
   generated.

    +----------------+   Trust Relationship    +----------------+
    | +------------+ |<.......................>| +------------+ |
    | | Protocol   | |                         | | Assertion  | |
    | | requesting | |    HTTP, SIP Request    | | Granting   | |
    | | authz      | |------------------------>| | Entity     | |
    | | assertions | |<------------------------| +------------+ |
    | +------------+ |    Artifact/Assertion   |  Entity Cecil  |
    |       ^        |                         +----------------+
    |       |        |                          ^     ^|
    |       |        |                          .     || HTTP,
    |       |        |              Trust       .     || other
    |   API Access   |              Relationship.     || protocols
    |       |        |                          .     ||
    |       |        |                          .     ||
    |       |        |                          v     |v
    |       v        |                         +----------------+
    | +------------+ |                         | +------------+ |
    | | Protocol   | |  NSIS NATFW CREATE +    | | Assertion  | |
    | | using authz| |  Assertion/Artifact     | | Verifying  | |
    | | assertion  | | ----------------------- | | Entity     | |
    | +------------+ |                         | +------------+ |
    |  Entity Alice  | <---------------------- |  Entity Bob    |
    +----------------+   RESPONSE              +----------------+

                   Figure 34: Authorization Token Usage

   Threats against the usage of authorization tokens have been mentioned
   in [RFC4081].  Hence, it is required to provide confidentiality
   protection to avoid allowing an eavesdropper to learn the token and
   to use it in another NATFW NSLP signaling session (replay attack).
   The token itself also needs to be protected against tempering.

5.3.  Implementation of NATFW NSLP Security

   The prior sections describe how to secure the NATFW NSLP in the
   presence of established trust between the various players and the
   particular relationships (e.g., intra-domain, end-to-middle, or peer-
   to-peer).  However, in typical Internet deployments there is no
   established trust, other than granting access to a network, but not
   between various sites in the Internet.  Furthermore, the NATFW NSLP
   may be incrementally deployed with a widely varying ability to be
   able to use authentication and authorization services.




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   The NATFW NSLP offers a way to keep the authentication and
   authorization at the "edge" of the network.  The local edge network
   can deploy and use any type of Authentication and Authorization (AA)
   scheme without the need to have AA technology match with other edges
   in the Internet (assuming that firewalls and NATs are deployed at the
   edges of the network and not somewhere in the cores).

   Each network edge that has the NATFW NSLP deployed can use the
   EXTERNAL request message to allow a secure access to the network.
   Using the EXTERNAL request message does allow the DR to open the
   firewall/NAT on the receiver's side.  However, the edge-devices
   should not allow the firewall/NAT to be opened up completely (i.e.,
   should not apply an allow-all policy), but should let DRs reserve
   very specific policies.  For instance, a DR can request reservation
   of an 'allow' policy rule for an incoming TCP connection for a Jabber
   file transfer.  This reserved policy (see Figure 15) rule must be
   activated by matching the CREATE request message (see Figure 15).
   This mechanism allows for the authentication and authorization issues
   to be managed locally at the particular edge-network.  In the reverse
   direction, the CREATE request message can be handled independently on
   the DS side with respect to authentication and authorization.

   The usage described in the above paragraph is further simplified for
   an incremental deployment: there is no requirement to activate a
   reserved policy rule with a CREATE request message.  This is
   completely handled by the EXTERNAL-PROXY request message and the
   associated CREATE request message.  Both of them are handled by the
   local authentication and authorization scheme.

6.  IAB Considerations on UNSAF

   UNilateral Self-Address Fixing (UNSAF) is described in [RFC3424] as a
   process at originating endpoints that attempts to determine or fix
   the address (and port) by which they are known to another endpoint.
   UNSAF proposals, such as STUN [RFC5389] are considered as a general
   class of workarounds for NAT traversal and as solutions for scenarios
   with no middlebox communication.

   This memo specifies a path-coupled middlebox communication protocol,
   i.e., the NSIS NATFW NSLP.  NSIS in general and the NATFW NSLP are
   not intended as a short-term workaround, but more as a long-term
   solution for middlebox communication.  In NSIS, endpoints are
   involved in allocating, maintaining, and deleting addresses and ports
   at the middlebox.  However, the full control of addresses and ports
   at the middlebox is at the NATFW NSLP daemon located at the
   respective NAT.





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   Therefore, this document addresses the UNSAF considerations in
   [RFC3424] by proposing a long-term alternative solution.

7.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to the
   NATFW NSLP, in accordance with BCP 26, RFC 5226 [RFC5226].

   The NATFW NSLP requires IANA to create a number of new registries:

   o  NATFW NSLP Message Types

   o  NATFW NSLP Header Flags

   o  NSLP Response Codes

   It also requires registration of new values in a number of
   registries:

   o  NSLP Message Objects

   o  NSLP Identifiers (under GIST Parameters)

   o  Router Alert Option Values (IPv4 and IPv6)

7.1.  NATFW NSLP Message Type Registry

   The NATFW NSLP Message Type is an 8-bit value.  The allocation of
   values for new message types requires IETF Review.  Updates and
   deletion of values from the registry are not possible.  This
   specification defines four NATFW NSLP message types, which form the
   initial contents of this registry.  IANA has added these four NATFW
   NSLP Message Types: CREATE (0x1), EXTERNAL (0x2), RESPONSE (0x3), and
   NOTIFY (0x4). 0x0 is Reserved.  Each registry entry consists of
   value, description, and reference.

7.2.  NATFW NSLP Header Flag Registry

   NATFW NSLP messages have a message-specific 8-bit flags/reserved
   field in their header.  The registration of flags is subject to IANA
   registration.  The allocation of values for flag types requires IETF
   Review.  Updates and deletion of values from the registry are not
   possible.  This specification defines only two flags in Section 4.1,
   the P flag (bit 8) and the E flag (bit 9).  Each registry entry
   consists of value, bit position, description (containing the section
   number), and reference.




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7.3.  NSLP Message Object Registry

   In Section 4.2 this document defines 9 objects for the NATFW NSLP:
   NATFW_LT, NATFW_EXTERNAL_IP, NATFW_EXTERNAL_BINDING, NATFW_EFI,
   NATFW_INFO, NATFW_NONCE, NATFW_MSN, NATFW_DTINFO, NATFW_ICMP_TYPES.
   IANA has assigned values for them from the NSLP Message Objects
   registry.

7.4.  NSLP Response Code Registry

   In addition, this document defines a number of Response Codes for the
   NATFW NSLP.  These can be found in Section 4.2.5 and have been
   assigned values from the NSLP Response Code registry.  The allocation
   of new values for Response Codes requires IETF Review.  IANA has
   assigned values for them as given in Section 4.2.5 for the error
   class and also for the number of responses values per error class.
   Each registry entry consists of response code, value, description,
   and reference.

7.5.  NSLP IDs and Router Alert Option Values

   GIST NSLPID

   This specification defines an NSLP for use with GIST and thus
   requires an assigned NSLP identifier.  IANA has added one new value
   (33) to the NSLP Identifiers (NSLPID) registry defined in [RFC5971]
   for the NATFW NSLP.

   IPv4 and IPv6 Router Alert Option (RAO) value

   The GIST specification also requires that each NSLP-ID be associated
   with specific Router Alert Option (RAO) value.  For the purposes of
   the NATFW NSLP, a single IPv4 RAO value (65) and a single IPv6 RAO
   value (68) have been allocated.

8.  Acknowledgments

   We would like to thank the following individuals for their
   contributions to this document at different stages:

   o  Marcus Brunner and Henning Schulzrinne for their work on IETF
      documents that led us to start with this document;

   o  Miquel Martin for his large contribution on the initial version of
      this document and one of the first prototype implementations;

   o  Srinath Thiruvengadam and Ali Fessi work for their work on the
      NAT/firewall threats document;



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   o  Henning Peters for his comments and suggestions;

   o  Ben Campbell as Gen-ART reviewer;

   o  and the NSIS working group.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5971]  Schulzrinne, H. and R. Hancock, "GIST: General Internet
              Signalling Transport", RFC 5971, October 2010.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              August 1996.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

9.2.  Informative References

   [RFC4080]  Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
              Bosch, "Next Steps in Signaling (NSIS): Framework",
              RFC 4080, June 2005.

   [RFC3726]  Brunner, M., "Requirements for Signaling Protocols",
              RFC 3726, April 2004.

   [RFC5974]  Manner, J., Karagiannis, G., and A. McDonald, "NSIS
              Signaling Layer Protocol (NSLP) for Quality-of-Service
              Signaling", RFC 5974, October 2010.

   [RFC5866]  Sun, D., McCann, P., Tschofenig, H., Tsou, T., Doria, A.,
              and G. Zorn, "Diameter Quality-of-Service Application",
              RFC 5866, May 2010.

   [RFC5978]  Manner, J., Bless, R., Loughney, J., and E. Davies, "Using
              and Extending the NSIS Protocol Family", RFC 5978,
              October 2010.

   [RFC3303]  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
              A. Rayhan, "Middlebox communication architecture and
              framework", RFC 3303, August 2002.





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   [RFC4081]  Tschofenig, H. and D. Kroeselberg, "Security Threats for
              Next Steps in Signaling (NSIS)", RFC 4081, June 2005.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, February 2002.

   [RFC2205]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, September 1997.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC3198]  Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
              M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
              J., and S. Waldbusser, "Terminology for Policy-Based
              Management", RFC 3198, November 2001.

   [RFC3520]  Hamer, L-N., Gage, B., Kosinski, B., and H. Shieh,
              "Session Authorization Policy Element", RFC 3520,
              April 2003.

   [RFC3521]  Hamer, L-N., Gage, B., and H. Shieh, "Framework for
              Session Set-up with Media Authorization", RFC 3521,
              April 2003.

   [rsvp-firewall]
              Roedig, U., Goertz, M., Karten, M., and R. Steinmetz,
              "RSVP as firewall Signalling Protocol", Proceedings of the
              6th IEEE Symposium on Computers and Communications,
              Hammamet, Tunisia, pp. 57 to 62, IEEE Computer Society
              Press, July 2001.






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Appendix A.  Selecting Signaling Destination Addresses for EXTERNAL

   As with all other message types, EXTERNAL messages need a reachable
   IP address of the data sender on the GIST level.  For the path-
   coupled MRM, the source-address of GIST is the reachable IP address
   (i.e., the real IP address of the data sender, or a wildcard).  While
   this is straightforward, it is not necessarily so for the loose-end
   MRM.  Many applications do not provide the IP address of the
   communication counterpart, i.e., either the data sender or both a
   data sender and receiver.  For the EXTERNAL messages, the case of
   data sender is of interest only.  The rest of this section gives
   informational guidance about determining a good destination-address
   of the LE-MRM in GIST for EXTERNAL messages.

   This signaling destination address (SDA, the destination-address in
   GIST) can be the data sender, but for applications that do not
   provide an address upfront, the destination IP address has to be
   chosen independently, as it is unknown at the time when the NATFW
   NSLP signaling has to start.  Choosing the 'correct' destination IP
   address may be difficult and it is possible that there is no 'right
   answer' for all applications relying on the NATFW NSLP.

   Whenever possible, it is RECOMMENDED to chose the data sender's IP
   address as the SDA.  It is necessary to differentiate between the
   received IP addresses on the data sender.  Some application-level
   signaling protocols (e.g., SIP) have the ability to transfer multiple
   contact IP addresses of the data sender.  For instance, private IP
   addresses, public IP addresses at a NAT, and public IP addresses at a
   relay.  It is RECOMMENDED to use all non-private IP addresses as
   SDAs.

   A different SDA must be chosen, if the IP address of the data sender
   is unknown.  This can have multiple reasons: the application-level
   signaling protocol cannot determine any data sender IP address at
   this point in time or the data receiver is server behind a NAT, i.e.,
   accepting inbound packets from any host.  In this case, the NATFW
   NSLP can be instructed to use the public IP address of an application
   server or any other node.  Choosing the SDA in this case is out of
   the scope of the NATFW NSLP and depends on the application's choice.
   The local network can provide a network-SDA, i.e., an SDA that is
   only meaningful to the local network.  This will ensure that GIST
   packets with destination-address set to this network-SDA are going to
   be routed to an edge-NAT or edge-firewall.








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Appendix B.  Usage of External Binding Addresses

   The NATFW_EXTERNAL_BINDING object carries information, which has a
   different utility to the information carried within the
   NATFW_EXTERNAL_IP object.  The NATFW_EXTERNAL_IP object has the
   public IP address and potentially port numbers that can be used by
   the application at the NI to be reachable via the public Internet.
   However, there are cases in which various NIs are located behind the
   same public NAT, but are subject to a multi-level NAT deployment, as
   shown in Figure 35.  They can use their public IP address port
   assigned to them to communicate between each other (e.g., NI with NR1
   and NR2) but they are forced to send their traffic through the edge-
   NAT, even though there is a shorter way possible.

       NI --192.168.0/24-- NAT1--10.0.0.0/8--NAT2 Internet (public IP)
                                |
       NR1--192.168.0/24-- NAT3--
                                |
                                NR2 10.1.2.3

                    Figure 35: Multi-Level NAT Scenario

   Figure 35 shows an example that is explored here:

   1.  NI -> NR1: Both NI and NR1 send EXTERNAL messages towards NAT2
       and get an external address+port binding.  Then, they exchange
       that external binding and all traffic gets pinned to NAT2 instead
       of taking the shortest path by NAT1 to NAT3 directly.  However,
       to do that, NR1 and NI both need to be aware that they also have
       the address on the external side of NAT1 and NAT3, respectively.
       If ICE is deployed and there is actually a STUN server in the
       10/8 network configured, it is possible to get the shorter path
       to work.  The NATFW NSLP provides all external addresses in the
       NATFW_EXTERNAL_BINDING towards the public network it could allow
       for optimizations.

   2.  For the case NI -> NR2 is even more obvious.  Pinning this to
       NAT2 is an important fallback, but allowing for trying for a
       direct path between NAT1 and NAT3 might be worth it.

   Please note that if there are overlapping address domains between NR
   and the public Internet, the regular routing will not necessary allow
   sending the packet to the right domain.








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Appendix C.  Applicability Statement on Data Receivers behind Firewalls

   Section 3.7.2 describes how data receivers behind middleboxes can
   instruct inbound firewalls/NATs to forward NATFW NSLP signaling
   towards them.  Finding an inbound edge-NAT in an address environment
   with NAT'ed addresses is quite easy.  It is only required to find
   some edge-NAT, as the data traffic will be route-pinned to the NAT.
   Locating the appropriate edge-firewall with the PC-MRM sent inbound
   is difficult.  For cases with a single, symmetric route from the
   Internet to the data receiver, it is quite easy; simply follow the
   default route in the inbound direction.

                             +------+                  Data Flow
                     +-------| EFW1 +----------+     <===========
                     |       +------+       ,--+--.
                  +--+--+                  /       \
          NI+-----| FW1 |                 (Internet )----NR+/NI/DS
          NR      +--+--+                  \       /
                     |       +------+       `--+--'
                     +-------| EFW2 +----------+
                             +------+

           ~~~~~~~~~~~~~~~~~~~~~>
             Signaling Flow

            Figure 36: Data Receiver behind Multiple Firewalls
                            Located in Parallel

   When a data receiver, and thus NR, is located in a network site that
   is multihomed with several independently firewalled connections to
   the public Internet (as shown in Figure 36), the specific firewall
   through which the data traffic will be routed has to be ascertained.
   NATFW NSLP signaling messages sent from the NI+/NR during the
   EXTERNAL message exchange towards the NR+ must be routed by the NTLP
   to the edge-firewall that will be passed by the data traffic as well.
   The NTLP would need to be aware about the routing within the Internet
   to determine the path between the DS and DR.  Out of this, the NTLP
   could determine which of the edge-firewalls, either EFW1 or EFW2,
   must be selected to forward the NATFW NSLP signaling.  Signaling to
   the wrong edge-firewall, as shown in Figure 36, would install the
   NATFW NSLP policy rules at the wrong device.  This causes either a
   blocked data flow (when the policy rule is 'allow') or an ongoing
   attack (when the policy rule is 'deny').  Requiring the NTLP to know
   all about the routing within the Internet is definitely a tough
   challenge and usually not possible.  In a case as described, the NTLP
   must basically give up and return an error to the NSLP level,
   indicating that the next hop discovery is not possible.




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Appendix D.  Firewall and NAT Resources

   This section gives some examples on how NATFW NSLP policy rules could
   be mapped to real firewall or NAT resources.  The firewall rules and
   NAT bindings are described in a natural way, i.e., in a way that one
   will find in common implementations.

D.1.  Wildcarding of Policy Rules

   The policy rule/MRI to be installed can be wildcarded to some degree.
   Wildcarding applies to IP address, transport layer port numbers, and
   the IP payload (or next header in IPv6).  Processing of wildcarding
   splits into the NTLP and the NATFW NSLP layer.  The processing at the
   NTLP layer is independent of the NSLP layer processing and per-layer
   constraints apply.  For wildcarding in the NTLP, see Section 5.8 of
   [RFC5971].

   Wildcarding at the NATFW NSLP level is always a node local policy
   decision.  A signaling message carrying a wildcarded MRI (and thus
   policy rule) arriving at an NSLP node can be rejected if the local
   policy does not allow the request.  For instance, take an MRI with IP
   addresses set (not wildcarded), transport protocol TCP, and TCP port
   numbers completely wildcarded.  If the local policy allows only
   requests for TCP with all ports set and not wildcarded, the request
   is going to be rejected.

D.2.  Mapping to Firewall Rules

   This section describes how a NSLP policy rule signaled with a CREATE
   message is mapped to a firewall rule.  The MRI is set as follows:

   o  network-layer-version=IPv4

   o  source-address=192.0.2.100, prefix-length=32

   o  destination-address=192.0.50.5, prefix-length=32

   o  IP-protocol=UDP

   o  L4-source-port=34543, L4-destination-port=23198

   The NATFW_EFI object is set to action=allow and sub_ports=0.

   The resulting policy rule (firewall rule) to be installed might look
   like: allow udp from 192.0.2.100 port=34543 to 192.0.50.5 port=23198.






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D.3.  Mapping to NAT Bindings

   This section describes how a NSLP policy rule signaled with an
   EXTERNAL message is mapped to a NAT binding.  It is assumed that the
   EXTERNAL message is sent by a NI+ located behind a NAT and does
   contain a NATFW_DTINFO object.  The MRI is set following using the
   signaling destination address, since the IP address of the real data
   sender is not known:

   o  network-layer-version=IPv4

   o  source-address= 192.168.5.100

   o  destination-address=SDA

   o  IP-protocol=UDP

   The NATFW_EFI object is set to action=allow and sub_ports=0.  The
   NATFW_DTINFO object contains these parameters:

   o  P=1

   o  dest prefix=0

   o  protocol=UDP

   o  dst port number = 20230, src port number=0

   o  src IP=0.0.0.0

   The edge-NAT allocates the external IP 192.0.2.79 and port 45000.

   The resulting policy rule (NAT binding) to be installed could look
   like: translate udp from any to 192.0.2.79 port=45000 to
   192.168.5.100 port=20230.

D.4.  NSLP Handling of Twice-NAT

   The dynamic configuration of twice-NATs requires application-level
   support, as stated in Section 2.5.  The NATFW NSLP cannot be used for
   configuring twice-NATs if application-level support is needed.
   Assuming application-level support performing the configuration of
   the twice-NAT and the NATFW NSLP being installed at this devices, the
   NATFW NSLP must be able to traverse it.  The NSLP is probably able to
   traverse the twice-NAT, as is any other data traffic, but the flow
   information stored in the NTLP's MRI will be invalidated through the
   translation of source and destination IP addresses.  The NATFW NSLP
   implementation on the twice-NAT MUST intercept NATFW NSLP and NTLP



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   signaling messages as any other NATFW NSLP node does.  For the given
   signaling flow, the NATFW NSLP node MUST look up the corresponding IP
   address translation and modify the NTLP/NSLP signaling accordingly.
   The modification results in an updated MRI with respect to the source
   and destination IP addresses.

Appendix E.  Example for Receiver Proxy Case

   This section gives an example on how to use the NATFW NLSP for a
   receiver behind a NAT, where only the receiving side is NATFW NSLP
   enabled.  We assume FTP as the application to show a working example.
   An FTP server is located behind a NAT, as shown in Figure 5, and uses
   the NATFW NSLP to allocate NAT bindings for the control and data
   channel of the FTP protocol.  The information about where to reach
   the server is communicated by a separate protocol (e.g., email, chat)
   to the DS side.



































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                   Public Internet                 Private Address
                                                        Space
      FTP Client                                            FTP Server

       DS                          NAT                         NI+
       |                           |                            |
       |                           |  EXTERNAL                  |
       |                           |<---------------------------|(1)
       |                           |                            |
       |                           |RESPONSE[Success]           |
       |                           |--------------------------->|(2)
       |                           |CREATE                      |
       |                           |--------------------------->|(3)
       |                           |RESPONSE[Success]           |
       |                           |<---------------------------|(4)
       |                           |                            |
       |                           | <Use port=XYZ, IP=a.b.c.d> |
       |<=======================================================|(5)
       |FTP control port=XYZ       | FTP control port=21        |
       |~~~~~~~~~~~~~~~~~~~~~~~~~~>|~~~~~~~~~~~~~~~~~~~~~~~~~~~>|(6)
       |                           |                            |
       |  FTP control/get X        |   FTP control/get X        |
       |~~~~~~~~~~~~~~~~~~~~~~~~~~>|~~~~~~~~~~~~~~~~~~~~~~~~~~~>|(7)
       |                           |  EXTERNAL                  |
       |                           |<---------------------------|(8)
       |                           |                            |
       |                           |RESPONSE[Success]           |
       |                           |--------------------------->|(9)
       |                           |CREATE                      |
       |                           |--------------------------->|(10)
       |                           |RESPONSE[Success]           |
       |                           |<---------------------------|(11)
       |                           |                            |
       | Use port=FOO, IP=a.b.c.d  |  Use port=FOO, IP=a.b.c.d  |
       |<~~~~~~~~~~~~~~~~~~~~~~~~~~|<~~~~~~~~~~~~~~~~~~~~~~~~~~~|(12)
       |                           |                            |
       |FTP data to port=FOO       | FTP data to port=20        |
       |~~~~~~~~~~~~~~~~~~~~~~~~~~>|~~~~~~~~~~~~~~~~~~~~~~~~~~~>|(13)


                           Figure 37: Flow Chart










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   1.   EXTERNAL request message sent to NAT, with these objects:
        signaling session lifetime, extended flow information object
        (rule action=allow, sub_ports=0), message sequence number
        object, nonce object (carrying nonce for CREATE), and the data
        terminal information object (I/P-flags set, sender prefix=0,
        protocol=TCP, DR port number = 21, DS's IP address=0); using the
        LE-MRM.  This is used to allocate the external binding for the
        FTP control channel (TCP, port 21).

   2.   Successful RESPONSE sent to NI+, with these objects: signaling
        session lifetime, message sequence number object, information
        code object ('Success':2), external address object (port=XYZ,
        IPv4 addr=a.b.c.d).

   3.   The NAT sends a CREATE towards NI+, with these objects:
        signaling session lifetime, extended flow information object
        (rule action=allow, sub_ports=0), message sequence number
        object, nonce object (with copied value from (1)); using the PC-
        MRM (src-IP=a.b.c.d, src-port=XYZ, dst-IP=NI+, dst-port=21,
        downstream).

   4.   Successful RESPONSE sent to NAT, with these objects: signaling
        session lifetime, message sequence number object, information
        code object ('Success':2).

   5.   The application at NI+ sends external NAT binding information to
        the other end, i.e., the FTP client at the DS.

   6.   The FTP client connects the FTP control channel to port=XYZ,
        IP=a.b.c.d.

   7.   The FTP client sends a get command for file X.

   8.   EXTERNAL request message sent to NAT, with these objects:
        signaling session lifetime, extended flow information object
        (rule action=allow, sub_ports=0), message sequence number
        object, nonce object (carrying nonce for CREATE), and the data
        terminal information object (I/P-flags set, sender prefix=32,
        protocol=TCP, DR port number = 20, DS's IP address=DS-IP); using
        the LE-MRM.  This is used to allocate the external binding for
        the FTP data channel (TCP, port 22).

   9.   Successful RESPONSE sent to NI+, with these objects: signaling
        session lifetime, message sequence number object, information
        code object ('Success':2), external address object (port=FOO,
        IPv4 addr=a.b.c.d).





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   10.  The NAT sends a CREATE towards NI+, with these objects:
        signaling session lifetime, extended flow information object
        (rule action=allow, sub_ports=0), message sequence number
        object, nonce object (with copied value from (1)); using the PC-
        MRM (src-IP=a.b.c.d, src-port=FOO, dst-IP=NI+, dst-port=20,
        downstream).

   11.  Successful RESPONSE sent to NAT, with these objects: signaling
        session lifetime, message sequence number object, information
        code object ('Success':2).

   12.  The FTP server responses with port=FOO and IP=a.b.c.d.

   13.  The FTP clients connects the data channel to port=FOO and
        IP=a.b.c.d.




































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Authors' Addresses

   Martin Stiemerling
   NEC Europe Ltd. and University of Goettingen
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Phone: +49 (0) 6221 4342 113
   EMail: Martin.Stiemerling@neclab.eu
   URI:   http://www.stiemerling.org


   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   EMail: Hannes.Tschofenig@nsn.com
   URI:   http://www.tschofenig.priv.at


   Cedric Aoun
   Consultant
   Paris, France

   EMail: cedaoun@yahoo.fr


   Elwyn Davies
   Folly Consulting
   Soham
   UK

   Phone: +44 7889 488 335
   EMail: elwynd@dial.pipex.com













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