path: root/doc/rfc/rfc7285.txt

Internet Engineering Task Force (IETF)                     R. Alimi, Ed.
Request for Comments: 7285                                        Google
Category: Standards Track                                  R. Penno, Ed.
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                            Y. Yang, Ed.
                                                         Yale University
                                                               S. Kiesel
                                                 University of Stuttgart
                                                              S. Previdi
                                                     Cisco Systems, Inc.
                                                                W. Roome
                                                          Alcatel-Lucent
                                                             S. Shalunov
                                                             Open Garden
                                                               R. Woundy
                                                                 Comcast
                                                          September 2014


         Application-Layer Traffic Optimization (ALTO) Protocol

Abstract

   Applications using the Internet already have access to some topology
   information of Internet Service Provider (ISP) networks.  For
   example, views to Internet routing tables at Looking Glass servers
   are available and can be practically downloaded to many network
   application clients.  What is missing is knowledge of the underlying
   network topologies from the point of view of ISPs.  In other words,
   what an ISP prefers in terms of traffic optimization -- and a way to
   distribute it.

   The Application-Layer Traffic Optimization (ALTO) services defined in
   this document provide network information (e.g., basic network
   location structure and preferences of network paths) with the goal of
   modifying network resource consumption patterns while maintaining or
   improving application performance.  The basic information of ALTO is
   based on abstract maps of a network.  These maps provide a simplified
   view, yet enough information about a network for applications to
   effectively utilize them.  Additional services are built on top of
   the maps.

   This document describes a protocol implementing the ALTO services.
   Although the ALTO services would primarily be provided by ISPs, other
   entities, such as content service providers, could also provide ALTO
   services.  Applications that could use the ALTO services are those
   that have a choice to which end points to connect.  Examples of such
   applications are peer-to-peer (P2P) and content delivery networks.



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Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7285.

Copyright Notice

   Copyright (c) 2014 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................6
      1.1. Problem Statement ..........................................6
           1.1.1. Requirements Language ...............................7
      1.2. Design Overview ............................................7
   2. Terminology .....................................................7
      2.1. Endpoint ...................................................8
      2.2. Endpoint Address ...........................................8
      2.3. Network Location ...........................................8
      2.4. ALTO Information ...........................................8
      2.5. ALTO Information Base ......................................8
   3. Architecture ....................................................8
      3.1. ALTO Services and Protocol Scope ...........................9
      3.2. ALTO Information Reuse and Redistribution .................11
   4. ALTO Information Service Framework .............................11
      4.1. ALTO Information Services .................................12
           4.1.1. Map Service ........................................12
           4.1.2. Map-Filtering Service ..............................12



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           4.1.3. Endpoint Property Service ..........................12
           4.1.4. Endpoint Cost Service ..............................13
   5. Network Map ....................................................13
      5.1. Provider-Defined Identifier (PID) .........................13
      5.2. Endpoint Addresses ........................................14
      5.3. Example Network Map .......................................14
   6. Cost Map .......................................................15
      6.1. Cost Types ................................................16
           6.1.1. Cost Metric ........................................16
           6.1.2. Cost Mode ..........................................17
      6.2. Cost Map Structure ........................................18
      6.3. Network Map and Cost Map Dependency .......................18
      6.4. Cost Map Update ...........................................19
   7. Endpoint Properties ............................................19
      7.1. Endpoint Property Type ....................................19
           7.1.1. Endpoint Property Type: pid ........................19
   8. Protocol Specification: General Processing .....................19
      8.1. Overall Design ............................................19
      8.2. Notation ..................................................20
      8.3. Basic Operations ..........................................21
           8.3.1. Client Discovering Information Resources ...........21
           8.3.2. Client Requesting Information Resources ............22
           8.3.3. Server Responding to Information Resource Request ..22
           8.3.4. Client Handling Server Response ....................23
           8.3.5. Authentication and Encryption ......................23
           8.3.6. Information Refreshing .............................24
           8.3.7. Parsing of Unknown Fields ..........................24
      8.4. Server Response Encoding ..................................24
           8.4.1. Meta Information ...................................24
           8.4.2. Data Information ...................................25
      8.5. Protocol Errors ...........................................25
           8.5.1. Media Type .........................................25
           8.5.2. Response Format and Error Codes ....................25
           8.5.3. Overload Conditions and Server Unavailability ......28
   9. Protocol Specification: Information Resource Directory .........28
      9.1. Information Resource Attributes ...........................29
           9.1.1. Resource ID ........................................29
           9.1.2. Media Type .........................................29
           9.1.3. Capabilities .......................................29
           9.1.4. Accepts Input Parameters ...........................29
           9.1.5. Dependent Resources ................................30
      9.2. Information Resource Directory (IRD) ......................30
           9.2.1. Media Type .........................................30
           9.2.2. Encoding ...........................................30
           9.2.3. Example ............................................32
           9.2.4. Delegation Using IRD ...............................35
           9.2.5. Considerations of Using IRD ........................37
   10. Protocol Specification: Basic Data Types ......................38



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      10.1. PID Name .................................................38
      10.2. Resource ID ..............................................38
      10.3. Version Tag ..............................................38
      10.4. Endpoints ................................................39
           10.4.1. Typed Endpoint Addresses ..........................39
           10.4.2. Address Type ......................................39
           10.4.3. Endpoint Address ..................................40
           10.4.4. Endpoint Prefixes .................................40
           10.4.5. Endpoint Address Group ............................41
      10.5. Cost Mode ................................................41
      10.6. Cost Metric ..............................................42
      10.7. Cost Type ................................................42
      10.8. Endpoint Property ........................................42
           10.8.1. Resource-Specific Endpoint Properties .............43
           10.8.2. Global Endpoint Properties ........................43
   11. Protocol Specification: Service Information Resources .........43
      11.1. Meta Information .........................................43
      11.2. Map Service ..............................................43
           11.2.1. Network Map .......................................44
           11.2.2. Mapping IP Addresses to PIDs for
                   'ipv4'/'ipv6' Network Maps ........................46
           11.2.3. Cost Map ..........................................47
      11.3. Map-Filtering Service ....................................50
           11.3.1. Filtered Network Map ..............................50
           11.3.2. Filtered Cost Map .................................53
      11.4. Endpoint Property Service ................................57
           11.4.1. Endpoint Property .................................58
      11.5. Endpoint Cost Service ....................................61
           11.5.1. Endpoint Cost .....................................61
   12. Use Cases .....................................................64
      12.1. ALTO Client Embedded in P2P Tracker ......................65
      12.2. ALTO Client Embedded in P2P Client: Numerical Costs ......66
      12.3. ALTO Client Embedded in P2P Client: Ranking ..............67
   13. Discussions ...................................................68
      13.1. Discovery ................................................68
      13.2. Hosts with Multiple Endpoint Addresses ...................68
      13.3. Network Address Translation Considerations ...............69
      13.4. Endpoint and Path Properties .............................69
   14. IANA Considerations ...........................................70
      14.1. application/alto-* Media Types ...........................70
      14.2. ALTO Cost Metric Registry ................................71
      14.3. ALTO Endpoint Property Type Registry .....................73
      14.4. ALTO Address Type Registry ...............................75
      14.5. ALTO Error Code Registry .................................76
   15. Security Considerations .......................................76
      15.1. Authenticity and Integrity of ALTO Information ...........77
           15.1.1. Risk Scenarios ....................................77
           15.1.2. Protection Strategies .............................77



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           15.1.3. Limitations .......................................77
      15.2. Potential Undesirable Guidance from Authenticated ALTO
            Information ..............................................78
           15.2.1. Risk Scenarios ....................................78
           15.2.2. Protection Strategies .............................78
      15.3. Confidentiality of ALTO Information ......................79
           15.3.1. Risk Scenarios ....................................79
           15.3.2. Protection Strategies .............................79
           15.3.3. Limitations .......................................80
      15.4. Privacy for ALTO Users ...................................80
           15.4.1. Risk Scenarios ....................................80
           15.4.2. Protection Strategies .............................80
      15.5. Availability of ALTO Services ............................81
           15.5.1. Risk Scenarios ....................................81
           15.5.2. Protection Strategies .............................81
   16. Manageability Considerations ..................................81
      16.1. Operations ...............................................82
           16.1.1. Installation and Initial Setup ....................82
           16.1.2. Migration Path ....................................82
           16.1.3. Dependencies on Other Protocols and
                   Functional Components .............................83
           16.1.4. Impact and Observation on Network Operation .......83
      16.2. Management ...............................................84
           16.2.1. Management Interoperability .......................84
           16.2.2. Management Information ............................84
           16.2.3. Fault Management ..................................84
           16.2.4. Configuration Management ..........................84
           16.2.5. Performance Management ............................85
           16.2.6. Security Management ...............................85
   17. References ....................................................85
      17.1. Normative References .....................................85
      17.2. Informative References ...................................86
   Appendix A. Acknowledgments .......................................89
   Appendix B. Design History and Merged Proposals ...................90

















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1.  Introduction

1.1.  Problem Statement

   This document defines the ALTO Protocol, which provides a solution
   for the problem stated in [RFC5693].  Specifically, in today's
   networks, network information such as network topologies, link
   availability, routing policies, and path costs are hidden from the
   application layer, and many applications benefited from such hiding
   of network complexity.  However, new applications, such as
   application-layer overlays, can benefit from information about the
   underlying network infrastructure.  In particular, these new network
   applications can be adaptive; hence, they can become more network
   efficient (e.g., reduce network resource consumption) and achieve
   better application performance (e.g., accelerated download rate), by
   leveraging network-provided information.

   At a high level, the ALTO Protocol specified in this document is an
   information-publishing interface that allows a network to publish its
   network information such as network locations, costs between them at
   configurable granularities, and endhost properties to network
   applications.  The information published by the ALTO Protocol should
   benefit both the network and the applications (i.e., the consumers of
   the information).  Either the operator of the network or a third
   party (e.g., an information aggregator) can retrieve or derive
   related information of the network and publish it using the ALTO
   Protocol.

   To allow better understanding of the goal of the ALTO Protocol, this
   document provides a short, non-normative overview of the benefits of
   ALTO to both networks and applications:

   o  A network that provides ALTO information can achieve better
      utilization of its networking infrastructure.  For example, by
      using ALTO as a tool to interact with applications, a network is
      able to provide network information to applications so that the
      applications can better manage traffic on more expensive or
      difficult-to-provision links such as long-distance, transit, or
      backup links.  During the interaction, the network can choose to
      protect its sensitive and confidential network state information,
      by abstracting real metric values into non-real numerical scores
      or ordinal ranking.

   o  An application that uses ALTO information can benefit from better
      knowledge of the network to avoid network bottlenecks.  For
      example, an overlay application can use information provided by
      the ALTO services to avoid selecting peers connected via high-
      delay links (e.g., some intercontinental links).  Using ALTO to



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      initialize each node with promising ("better-than-random") peers,
      an adaptive peer-to-peer overlay may achieve faster, better
      convergence.

1.1.1.  Requirements Language

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

1.2.  Design Overview

   The ALTO Protocol specified in this document meets the ALTO
   requirements specified in [RFC5693], and unifies multiple protocols
   previously designed with similar intentions.  See Appendix A for a
   list of people and Appendix B for a list of proposals that have made
   significant contributions to this effort.

   The ALTO Protocol uses a REST-ful (Representational State Transfer
   (REST)) design [Fielding-Thesis], and encodes its requests and
   responses using JSON [RFC7159].  These designs are chosen because of
   their flexibility and extensibility.  In addition, these designs make
   it possible for ALTO to be deployed at scale by leveraging existing
   HTTP [RFC7230] implementations, infrastructures and deployment
   experience.

   The ALTO Protocol uses a modular design by dividing ALTO information
   publication into multiple ALTO services (e.g., the Map service, the
   Map-Filtering Service, the Endpoint Property Service, and the
   Endpoint Cost Service).  Each ALTO service provides a given set of
   functionalities and is realized by a set of information resources,
   which are announced by information resource directories, to guide
   ALTO clients.

2.  Terminology

   This document uses the following terms defined in [RFC5693]:
   Application, Overlay Network, Peer, Resource, Resource Identifier,
   Resource Provider, Resource Consumer, Resource Directory, Transport
   Address, ALTO Server, ALTO Client, ALTO Query, ALTO Response, ALTO
   Transaction, Local Traffic, Peering Traffic, and Transit Traffic.

   This document extends the term "ALTO Service" defined in [RFC5693].
   In particular, by adopting a modular design, this document allows the
   ALTO Protocol to provide multiple ALTO services.






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   This document also uses the following additional terms: Endpoint
   Address, Network Location, ALTO Information, and ALTO Information
   Base.

2.1.  Endpoint

   An endpoint is an application or host that is capable of
   communicating (sending and/or receiving messages) on a network.

   An endpoint is typically either a resource provider or a resource
   consumer.

2.2.  Endpoint Address

   An endpoint address represents the communication address of an
   endpoint.  Common forms of endpoint addresses include IP addresses,
   Media Access Control (MAC) addresses, and overlay IDs.  An endpoint
   address can be network-attachment based (e.g., IP address) or
   network-attachment agnostic (e.g., MAC address).

   Each endpoint address has an associated address type, which indicates
   both its syntax and semantics.

2.3.  Network Location

   This document uses network location as a generic term to denote a
   single endpoint or a group of endpoints.  For instance, it can be a
   single IPv4 or IPv6 address, an IPv4 or IPv6 prefix, or a set of
   prefixes.

2.4.  ALTO Information

   This document uses ALTO information as a generic term to refer to the
   network information provided by an ALTO server.

2.5.  ALTO Information Base

   This document uses the term ALTO information base to refer to the
   internal representation of ALTO information maintained by an ALTO
   server.  Note that the structure of this internal representation is
   not defined by this document.

3.  Architecture

   This section defines the ALTO architecture and the ALTO Protocol's
   place in the overall architecture.





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3.1.  ALTO Services and Protocol Scope

   Each network region in the global Internet can provide its ALTO
   services, which convey network information from the perspective of
   that network region.  A network region in this context can be an
   Autonomous System (AS), an ISP, a region smaller than an AS or ISP,
   or a set of ISPs.  The specific network region that an ALTO service
   represents will depend on the ALTO deployment scenario and ALTO
   service discovery mechanism.

   The ALTO services specified in this document define network endpoints
   (and aggregations thereof) and generic costs amongst them from the
   region's perspective.  The network endpoints may include all
   endpoints in the global Internet.  We say that the network
   information provided by the ALTO services of a network region
   represents the "my-Internet view" of the network region.

   The "my-Internet view" defined in this document does not specify the
   internal topology of a network, and hence, it is said to provide a
   "single-node" abstract topology.  Extensions to this document may
   provide topology details in "my-Internet view".

   Figure 1 provides an overall picture of ALTO's system architecture,
   so that one can better understand the ALTO services and the role of
   the ALTO Protocol.  In this architecture, an ALTO server prepares
   ALTO information, an ALTO client uses ALTO service discovery to
   identify an appropriate ALTO server, and the ALTO client requests
   available ALTO information from the ALTO server using the ALTO
   Protocol.

   The ALTO information provided by the ALTO server can be updated
   dynamically based on network conditions, or they can be seen as a
   policy that is updated on a longer time scale.


















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   +-------------------------------------------------------------------+
   |                         Network Region                            |
   |                                                                   |
   |                    +-----------+                                  |
   |                    | Routing   |                                  |
   |  +--------------+  | Protocols |                                  |
   |  | Provisioning |  +-----------+                                  |
   |  | Policy       |        |                                        |
   |  +--------------+\       |                                        |
   |                   \      |                                        |
   |                    \     |                                        |
   |  +-----------+      \+---------+                      +--------+  |
   |  |Dynamic    |       | ALTO    | ALTO Protocol        | ALTO   |  |
   |  |Network    |.......| Server  | ==================== | Client |  |
   |  |Information|       +---------+                      +--------+  |
   |  +-----------+      /                                /            |
   |                    /         ALTO SD Query/Response /             |
   |                   /                                /              |
   |          +----------+                  +----------------+         |
   |          | External |                  | ALTO Service   |         |
   |          | Interface|                  | Discovery (SD) |         |
   |          +----------+                  +----------------+         |
   |               |                                                   |
   +-------------------------------------------------------------------+
                   |
         +------------------+
         | Third Parties    |
         |                  |
         | Content Providers|
         +------------------+

                     Figure 1: Basic ALTO Architecture

   Figure 1 illustrates that the ALTO information provided by an ALTO
   server may be influenced (at the service provider's discretion) by
   other systems.  In particular, the ALTO server can aggregate
   information from multiple systems to provide an abstract and unified
   view that can be more useful to applications.  Examples of other
   systems include (but are not limited to) static network configuration
   databases, dynamic network information, routing protocols,
   provisioning policies, and interfaces to outside parties.  These
   components are shown in the figure for completeness but are outside
   the scope of this specification.  Recall that while the ALTO Protocol
   may convey dynamic network information, it is not intended to replace
   near-real-time congestion control protocols.






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   It may also be possible for an ALTO server to exchange network
   information with other ALTO servers (either within the same
   administrative domain or another administrative domain with the
   consent of both parties) in order to adjust exported ALTO
   information.  Such a protocol is also outside the scope of this
   specification.

3.2.  ALTO Information Reuse and Redistribution

   ALTO information may be useful to a large number of applications and
   users.  At the same time, distributing ALTO information must be
   efficient and not become a bottleneck.

   The design of the ALTO Protocol allows integration with the existing
   HTTP caching infrastructure to redistribute ALTO information.  If
   caching or redistribution is used, the response message to an ALTO
   client may be returned from a third party.

   Application-dependent mechanisms, such as P2P Distributed Hash Tables
   (DHTs) or P2P file sharing, may be used to cache and redistribute
   ALTO information.  This document does not define particular
   mechanisms for such redistribution.

   Additional protocol mechanisms (e.g., expiration times and digital
   signatures for returned ALTO information) are left for future
   investigation.

4.  ALTO Information Service Framework

   The ALTO Protocol conveys network information through ALTO
   information services (services for short), where each service defines
   a set of related functionalities.  An ALTO client can request each
   service individually.  All of the services defined in ALTO are said
   to form the ALTO service framework and are provided through a common
   transport protocol; messaging structure and encoding; and transaction
   model.  Functionalities offered in different services can overlap.

   The goals of the ALTO information services defined in this document
   are to convey (1) network locations, which denote the locations of
   endpoints at a network, (2) provider-defined costs for paths between
   pairs of network locations, and (3) network-related properties of
   endpoints.  The aforementioned goals are achieved by defining the Map
   Service, which provides the core ALTO information to clients, and
   three additional information services: the Map-Filtering Service, the
   Endpoint Property Service (EPS), and the Endpoint Cost Service (ECS).
   Additional information services can be defined in companion
   documents.  Figure 2 gives an overview of the information services.
   Details of the services are presented in subsequent sections.



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        .-----------------------------------------.
        | ALTO Information Services               |
        | .-----------. .----------. .----------. |
        | |    Map-   | | Endpoint | | Endpoint | |
        | | Filtering | | Property | |   Cost   | |
        | |  Service  | | Service  | | Service  | |
        | `-----------' `----------' `----------' |
        | .-------------------------------------. |
        | |  Map Service                        | |
        | |  .-------------.  .--------------.  | |
        | |  | Network Map |  |  Cost Map    |  | |
        | |  `-------------'  `--------------'  | |
        | `-------------------------------------' |
        `-----------------------------------------'

      Figure 2: ALTO Information Service Framework

4.1.  ALTO Information Services

4.1.1.  Map Service

   The Map Service provides batch information to ALTO clients in the
   forms of ALTO network maps (network maps for short) and ALTO cost
   maps (cost maps for short).  An ALTO network map (See Section 5)
   provides a full set of network location groupings defined by the ALTO
   server and the endpoints contained within each grouping.  An ALTO
   cost map (see Section 6) provides costs between defined groupings.

   These two maps can be thought of (and implemented) as simple files
   with appropriate encoding provided by the ALTO server.

4.1.2.  Map-Filtering Service

   Resource-constrained ALTO clients may benefit from the filtering of
   query results at the ALTO server.  This avoids the situation in which
   an ALTO client first spends network bandwidth and CPU cycles to
   collect results and then performs client-side filtering.  The Map-
   Filtering Service allows ALTO clients to query an ALTO server on ALTO
   network maps and/or cost maps based on additional parameters.

4.1.3.  Endpoint Property Service

   This service allows ALTO clients to look up properties for individual
   endpoints.  An example property of an endpoint is its network
   location (i.e., its grouping defined by the ALTO server).  Another
   example property is its connectivity type such as ADSL (Asymmetric
   Digital Subscriber Line), Cable, or FTTH (Fiber To The Home).




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4.1.4.  Endpoint Cost Service

   Some ALTO clients may also benefit from querying for costs and
   rankings based on endpoints.  The Endpoint Cost Service allows an
   ALTO server to return costs directly amongst endpoints.

5.  Network Map

   An ALTO network map defines a grouping of network endpoints.  This
   document uses ALTO network map to refer to the syntax and semantics
   of how an ALTO server defines the grouping.  This document does not
   discuss the internal representation of this data structure within an
   ALTO server.

   The definition of ALTO network maps is based on the observation that,
   in reality, many endpoints are near by to one another in terms of
   network connectivity.  By treating a group of nearby endpoints
   together as a single entity, an ALTO server indicates aggregation of
   these endpoints due to their proximity.  This aggregation can also
   lead to greater scalability without losing critical information when
   conveying other network information (e.g., when defining cost maps).

5.1.  Provider-Defined Identifier (PID)

   One issue is that proximity varies depending on the granularity of
   the ALTO information configured by the provider.  In one deployment,
   endpoints on the same subnet may be considered close; while in
   another deployment, endpoints connected to the same Point of Presence
   (POP) may be considered close.

   ALTO introduces provider-defined network location identifiers called
   Provider-defined Identifiers (PIDs) to provide an indirect and
   network-agnostic way to specify an aggregation of network endpoints
   that may be treated similarly, based on network topology, type, or
   other properties.  Specifically, a PID is a string of type PIDName
   (see Section 10.1) and its associated set of endpoint addresses.  As
   discussed above, there can be many different ways of grouping the
   endpoints and assigning PIDs.  For example, a PID may denote a
   subnet, a set of subnets, a metropolitan area, a POP, an autonomous
   system, or a set of autonomous systems.  Interpreting the PIDs
   defined in an ALTO network map using the "single-node" abstraction,
   one can consider that each PID represents an abstract port (POP) that
   connects a set of endpoints.

   A key use case of PIDs is to specify network preferences (costs)
   between PIDs instead of individual endpoints.  This allows cost
   information to be more compactly represented and updated at a faster
   time scale than the network aggregations themselves.  For example, an



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   ISP may prefer that endpoints associated with the same POP in a P2P
   application communicate locally instead of communicating with
   endpoints in other POPs.  The ISP may aggregate endpoints within a
   POP into a single PID in a network map.  The cost may be encoded to
   indicate that network locations within the same PID are preferred;
   for example, cost(PID_i, PID_i) == c and cost(PID_i, PID_j) > c for i
   != j.  Section 6 provides further details on using PIDs to represent
   costs in an ALTO cost map.

5.2.  Endpoint Addresses

   The endpoints aggregated into a PID are denoted by endpoint
   addresses.  There are many types of addresses, such as IP addresses,
   MAC addresses, or overlay IDs.  This document specifies (in
   Section 10.4) how to specify IPv4/IPv6 addresses or prefixes.
   Extension documents may define further address types; Section 14.4 of
   this document provides an IANA registry for endpoint address types.

5.3.  Example Network Map

   This document uses the ALTO network map shown in Figure 3 in most
   examples.





























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       .------------------------------------------------------------.
       | An ALTO Network Map                                        |
       |                                                            |
       |  .-----------------------------------.  .----------------. |
       |  | NetLoc: PID-1                     |  | NetLoc: PID-3  | |
       |  |  .------------------------------. |  |                | |
       |  |  | 192.0.2.0/24                 | |  |  .-----------. | |
       |  |  | .--------------------------. | |  |  | 0.0.0.0/0 | | |
       |  |  | | Endpoint: 192.0.2.34     | | |  |  `-----------` | |
       |  |  | `--------------------------` | |  |                | |
       |  |  `------------------------------` |  |                | |
       |  |  .------------------------------. |  |                | |
       |  |  | 198.51.100.0/25              | |  |                | |
       |  |  | .--------------------------. | |  |                | |
       |  |  | | Endpoint: 198.51.100.100 | | |  |                | |
       |  |  | `--------------------------` | |  |                | |
       |  |  `------------------------------` |  |                | |
       |  `-----------------------------------`  |                | |
       |                                         |                | |
       |  .-----------------------------------.  |                | |
       |  | NetLoc: PID-2                     |  |                | |
       |  |  .------------------------------. |  |                | |
       |  |  | 198.51.100.128/25            | |  |                | |
       |  |  `------------------------------` |  |                | |
       |  `-----------------------------------`  `----------------` |
       `------------------------------------------------------------`

                       Figure 3: Example Network Map

6.  Cost Map

   An ALTO server indicates preferences amongst network locations in the
   form of path costs.  Path costs are generic costs and can be
   internally computed by a network provider according to its own
   policy.

   For a given ALTO network map, an ALTO cost map defines path costs
   pairwise amongst the set of source and destination network locations
   defined by the PIDs contained in the network map.  Each path cost is
   the end-to-end cost when a unit of traffic goes from the source to
   the destination.

   Since cost is directional from the source to the destination, an
   application, when using ALTO information, may independently determine
   how the resource consumer and resource provider are designated as the
   source or destination in an ALTO query and, hence, how to utilize the
   path cost provided by ALTO information.  For example, if the cost is




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   expected to be correlated with throughput, a typical application
   concerned with bulk data retrieval may use the resource provider as
   the source and the resource consumer as the destination.

   One advantage of separating ALTO information into network maps and
   cost maps is that the two types of maps can be updated at different
   time scales.  For example, network maps may be stable for a longer
   time while cost maps may be updated to reflect more dynamic network
   conditions.

   As used in this document, an ALTO cost map refers to the syntax and
   semantics of the information distributed by the ALTO server.  This
   document does not discuss the internal representation of this data
   structure within the ALTO server.

6.1.  Cost Types

   Path costs have attributes:

   o  Cost Metric: identifies what the costs represent;

   o  Cost Mode: identifies how the costs should be interpreted.

   The combination of a cost metric and a cost mode defines an ALTO cost
   type.  Certain queries for ALTO cost maps allow the ALTO client to
   indicate the desired cost type.  For a given ALTO server, the
   combination of cost type and network map defines a key.  In other
   words, an ALTO server MUST NOT define two ALTO cost maps with the
   same cost type \ network map pair.

6.1.1.  Cost Metric

   The cost metric attribute indicates what the cost represents.  For
   example, an ALTO server could define costs representing air miles,
   hop-counts, or generic routing costs.

   Cost metrics are indicated in protocol messages as strings.

6.1.1.1.  Cost Metric: routingcost

   An ALTO server MUST offer the "routingcost" cost metric.

   This cost metric conveys a generic measure for the cost of routing
   traffic from a source to a destination.  A lower value indicates a
   higher preference for traffic to be sent from a source to a
   destination.





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   Note that an ISP may internally compute routing cost using any method
   that it chooses (e.g., air miles or hop-count) as long as it conforms
   to the semantics.

6.1.2.  Cost Mode

   The cost mode attribute indicates how costs should be interpreted.
   Specifically, the cost mode attribute indicates whether returned
   costs should be interpreted as numerical values or ordinal rankings.

   It is important to communicate such information to ALTO clients, as
   certain operations may not be valid on certain costs returned by an
   ALTO server.  For example, it is possible for an ALTO server to
   return a set of IP addresses with costs indicating a ranking of the
   IP addresses.  Arithmetic operations that would make sense for
   numerical values, do not make sense for ordinal rankings.  ALTO
   clients may handle such costs differently.

   Cost modes are indicated in protocol messages as strings.

   An ALTO server MUST support at least one of the following modes:
   numerical and ordinal.  An ALTO client needs to be cognizant of
   operations when its desired cost mode is not supported.
   Specifically, an ALTO client desiring numerical costs MAY adjust its
   behaviors if only the ordinal cost mode is available.  Alternatively,
   an ALTO client desiring ordinal costs MAY construct ordinal costs
   from retrieved numerical values, if only the numerical cost mode is
   available.

6.1.2.1.  Cost Mode: numerical

   This cost mode is indicated by the string "numerical".  This mode
   indicates that it is safe to perform numerical operations (e.g.,
   normalization or computing ratios for weighted load-balancing) on the
   returned costs.  The values are floating-point numbers.

6.1.2.2.  Cost Mode: ordinal

   This cost mode is indicated by the string "ordinal".  This mode
   indicates that the cost values in a cost map represent ranking
   (relative to all other values in a cost map), not actual costs.  The
   values are non-negative integers, with a lower value indicating a
   higher preference.  Ordinal cost values in a cost map need not be
   unique or contiguous.  In particular, it is possible that two entries
   in a cost map have an identical rank (ordinal cost value).  This
   document does not specify any behavior by an ALTO client in this
   case; an ALTO client may decide to break ties by random selection,
   other application knowledge, or some other means.



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6.2.  Cost Map Structure

   A request for an ALTO cost map will either explicitly or implicitly
   include a list of source network locations and a list of destination
   network locations.  (Recall that a network location can be an
   endpoint address or a PID.)

   Specifically, assume that a request specifies a list of source
   network locations, say [Src_1, Src_2, ..., Src_m], and a list of
   destination network locations, say [Dst_1, Dst_2, ..., Dst_n].

   The ALTO server will return the path cost for each of the m*n
   communicating pairs (i.e., Src_1 -> Dst_1, ..., Src_1 -> Dst_n, ...,
   Src_m -> Dst_1, ..., Src_m -> Dst_n).  If the ALTO server does not
   define the path cost for a particular pair, that cost may be omitted.
   This document refers to this structure as a cost map.

   If the cost mode is ordinal, the path cost of each communicating pair
   is relative to the m*n entries.

6.3.  Network Map and Cost Map Dependency

   An ALTO cost map gives path costs between the PIDs defined in an ALTO
   network map.  An ALTO server may modify an ALTO network map at any
   time, say by adding or deleting PIDs, or even redefining them.
   Hence, to effectively use an instance of an ALTO cost map, an ALTO
   client must know which version of the network map defined the PIDs in
   that cost map.  Version tags allow an ALTO client to correlate cost
   map instances with the corresponding versions of the network maps.

   Specifically, a version tag is a tuple of (1) an ID for the resource
   (e.g., an ALTO network map) and (2) a tag (an opaque string)
   associated with the version of that resource.  An ALTO network map
   distributed by an ALTO server includes its version tag.  An ALTO cost
   map referring to PIDs also includes the version tag for the network
   map on which it is based.

   Two ALTO network maps are the same if they have the same version tag.
   Whenever the content of an ALTO network map maintained by an ALTO
   server changes, the tag MUST also be changed.  Possibilities of
   setting the tag component include the last-modified timestamp for the
   network map, or a hash of its contents, where the collision
   probability is considered zero in practical deployment scenarios.








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6.4.  Cost Map Update

   An ALTO server can update an ALTO cost map at any time.  Hence, the
   same cost map retrieved from the same ALTO server but from different
   requests can be inconsistent.

7.  Endpoint Properties

   An endpoint property defines a network-aware property of an endpoint.

7.1.  Endpoint Property Type

   For each endpoint and an endpoint property type, there can be a value
   for the property.  The type of an endpoint property is indicated in
   protocol messages as a string.  The value depends on the specific
   property.  For example, for a property such as whether an endpoint is
   metered, the value is a true or false value.  See Section 10.8 for
   more details on specifying endpoint properties.

7.1.1.  Endpoint Property Type: pid

   An ALTO server MUST define the "pid" endpoint property type for each
   ALTO network map that it provides.  Specifically, each ALTO network
   map defines multiple PIDs.  For an "ipv4"/"ipv6" network map, given
   an endpoint's IP address, the ALTO server uses the algorithm
   specified in Section 11.2.2 to look up the PID of the endpoint.  This
   PID is the "pid" property of the endpoint for the network map.  See
   Section 11.4.1.7 for an example.

8.  Protocol Specification: General Processing

   This section first specifies general client and server processing.
   The details of specific services will be covered in the following
   sections.

8.1.  Overall Design

   The ALTO Protocol uses a REST-ful design.  There are two primary
   components to this design:

   o  Information Resources: Each ALTO service is realized by a set of
      network information resources.  Each information resource has a
      media type [RFC2046].  An ALTO client may construct an HTTP
      request for a particular information resource (including any
      parameters, if necessary), and the ALTO server returns the
      requested information resource in an HTTP response.





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   o  Information Resource Directory (IRD): An ALTO server uses an IRD
      to inform an ALTO client about a list of available information
      resources and the URI at which each can be accessed.  ALTO clients
      consult the IRDs to determine the services provided by ALTO
      servers.

8.2.  Notation

   This document uses JSONString, JSONNumber, and JSONBool to indicate
   the JSON string, number, and boolean types, respectively.  The type
   JSONValue indicates a JSON value, as specified in Section 3 of
   [RFC7159].

   This document uses an adaptation of the C-style struct notation to
   define JSON objects.  A JSON object consists of name/value pairs.
   This document refers to each pair as a field.  In some context, this
   document also refers to a field as an attribute.  The name of a
   field/attribute may be referred to as the key.  An optional field is
   enclosed by [ ].  In the definitions, the JSON names of the fields
   are case sensitive.  An array is indicated by two numbers in angle
   brackets, <m..n>, where m indicates the minimal number of values and
   n is the maximum.  When this document uses * for n, it means no upper
   bound.

   For example, the definition below defines a new type Type4, with
   three fields named "name1", "name2", and "name3", respectively.  The
   field named "name3" is optional, and the field named "name2" is an
   array of at least one value.

    object { Type1 name1; Type2 name2<1..*>; [Type3 name3;]
             } Type4;

   This document also defines dictionary maps (or maps for short) from
   strings to JSON values.  For example, the definition below defines a
   Type3 object as a map.  Type1 must be defined as string, and Type2
   can be defined as any type.

    object-map { Type1 -> Type2; } Type3;

   This document uses subtyping to denote that one type is derived from
   another type.  The example below denotes that TypeDerived is derived
   from TypeBase.  TypeDerived includes all fields defined in TypeBase.
   If TypeBase does not have a field named "name1", TypeDerived will
   have a new field named "name1".  If TypeBase already has a field
   named "name1" but with a different type, TypeDerived will have a
   field named "name1" with the type defined in TypeDerived (i.e., Type1
   in the example).




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    object { Type1 name1; } TypeDerived : TypeBase;

   Note that, despite the notation, no standard, machine-readable
   interface definition or schema is provided in this document.
   Extension documents may describe these as necessary.

8.3.  Basic Operations

   The ALTO Protocol employs standard HTTP [RFC7230].  It is used for
   discovering available information resources at an ALTO server and
   retrieving Information Resources.  ALTO clients and ALTO servers use
   HTTP requests and responses carrying ALTO-specific content with
   encoding as specified in this document, and they MUST be compliant
   with [RFC7230].

   Instead of specifying the generic application/json media type for all
   ALTO request parameters (if any) and responses, ALTO clients and
   servers use multiple, specific JSON-based media types (e.g.,
   application/alto-networkmap+json, application/alto-costmap+json) to
   indicate content types; see Table 2 for a list of media types defined
   in this document.  This allows easy extensibility while maintaining
   clear semantics and versioning.  For example, a new version of a
   component of the ALTO Protocol (e.g., a new version of ALTO network
   maps) can be defined by simply introducing a new media type (e.g.,
   application/alto-networkmap-v2+json).

8.3.1.  Client Discovering Information Resources

   To discover available information resources provided by an ALTO
   server, an ALTO client requests its IRD(s).

   Specifically, using an ALTO service discovery protocol, an ALTO
   client obtains a URI through which it can request an information
   resource directory (IRD).  This document refers to this IRD as the
   Root IRD of the ALTO client.  Each entry in an IRD indicates a URI at
   which an ALTO server accepts requests, and returns either an
   information resource or an information resource directory that
   references additional information resources.  Beginning with its Root
   IRD and following links to IRDs recursively, an ALTO client can
   discover all information resources available to it.  This set of
   information resources is referred to as the information resource
   closure of the ALTO client.  By inspecting its information resource
   closure, an ALTO client can determine whether an ALTO server supports
   the desired information resource, and if it is supported, the URI at
   which it is available.

   See Section 9.2 for a detailed specification of IRDs.




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8.3.2.  Client Requesting Information Resources

   Where possible, the ALTO Protocol uses the HTTP GET method to request
   resources.  However, some ALTO services provide information resources
   that are the function of one or more input parameters.  Input
   parameters are encoded in the HTTP request's entity body, and the
   ALTO client MUST use the HTTP POST method to send the parameters.

   When requesting an ALTO information resource that requires input
   parameters specified in a HTTP POST request, an ALTO client MUST set
   the Content-Type HTTP header to the media type corresponding to the
   format of the supplied input parameters.

   An ALTO client MUST NOT assume that the HTTP GET and POST methods are
   interchangeable.  In particular, for an information resource that
   uses the HTTP GET method, an ALTO client MUST NOT assume that the
   information resource will accept a POST request as equivalent to a
   GET request.

8.3.3.  Server Responding to Information Resource Request

   Upon receiving a request for an information resource that the ALTO
   server can provide, the ALTO server normally returns the requested
   information resource.  In other cases, to be more informative
   ([RFC7231]), the ALTO server either provides the ALTO client with an
   information resource directory indicating how to reach the desired
   information resource, or it returns an ALTO error object; see
   Section 8.5 for more details on ALTO error handling.

   It is possible for an ALTO server to leverage caching HTTP
   intermediaries to respond to both GET and POST requests by including
   explicit freshness information (see Section 14 of [RFC7230]).
   Caching of POST requests is not widely implemented by HTTP
   intermediaries; however, an alternative approach is for an ALTO
   server, in response to POST requests, to return an HTTP 303 status
   code ("See Other") indicating to the ALTO client that the resulting
   information resource is available via a GET request to an alternate
   URL.  HTTP intermediaries that do not support caching of POST
   requests could then cache the response to the GET request from the
   ALTO client following the alternate URL in the 303 response if the
   response to the subsequent GET request contains explicit freshness
   information.

   The ALTO server MUST indicate the type of its response using a media
   type (i.e., the Content-Type HTTP header of the response).






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8.3.4.  Client Handling Server Response

8.3.4.1.  Using Information Resources

   This specification does not indicate any required actions taken by
   ALTO clients upon successfully receiving an information resource from
   an ALTO server.  Although ALTO clients are suggested to interpret the
   received ALTO information and adapt application behavior, ALTO
   clients are not required to do so.

8.3.4.2.  Handling Server Response and IRD

   After receiving an information resource directory, the client can
   consult it to determine if any of the offered URIs contain the
   desired information resource.  However, an ALTO client MUST NOT
   assume that the media type returned by the ALTO server for a request
   to a URI is the media type advertised in the IRD or specified in its
   request (i.e., the client must still check the Content-Type header).
   The expectation is that the media type returned should normally be
   the media type advertised and requested, but, in some cases, it may
   legitimately not be so.

   In particular, it is possible for an ALTO client to receive an
   information resource directory from an ALTO server as a response to
   its request for a specific information resource.  In this case, the
   ALTO client may ignore the response or still parse the response.  To
   indicate that an ALTO client will always check if a response is an
   information resource directory, the ALTO client can indicate in the
   "Accept" header of a HTTP request that it can accept information
   resource directory; see Section 9.2.1 for the media type.

8.3.4.3.  Handling Error Conditions

   If an ALTO client does not successfully receive a desired information
   resource from a particular ALTO server (i.e., server response
   indicates error or there is no response), the client can either
   choose another server (if one is available) or fall back to a default
   behavior (e.g., perform peer selection without the use of ALTO
   information, when used in a peer-to-peer system).

8.3.5.  Authentication and Encryption

   ALTO server implementations as well as ALTO client implementations
   MUST support the "https" URI scheme [RFC2818] and Transport Layer
   Security (TLS) [RFC5246].  See Section 15.1.2 for security
   considerations and Section 16 for manageability considerations
   regarding the usage of HTTPS/TLS.




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   For deployment scenarios where client authentication is desired, HTTP
   Digest Authentication MUST be supported.  TLS Client Authentication
   is the preferred mechanism if it is available.

8.3.6.  Information Refreshing

   An ALTO client can determine the frequency at which ALTO information
   is refreshed based on information made available via HTTP.

8.3.7.  Parsing of Unknown Fields

   This document only details object fields used by this specification.
   Extensions may include additional fields within JSON objects defined
   in this document.  ALTO implementations MUST ignore unknown fields
   when processing ALTO messages.

8.4.  Server Response Encoding

   Though each type of ALTO server response (i.e., an information
   resource directory, an individual information resource, or an error
   message) has its distinct syntax and, hence, its unique media type,
   they are designed to have a similar structure: a field named "meta"
   to provide meta definitions, and another field named "data" to
   contain the data, if needed.

   Specifically, this document defines the base type of each ALTO server
   response as ResponseEntityBase:

    object { ResponseMeta meta; } ResponseEntityBase;

   with field:

   meta:  meta information pertaining to the response.

8.4.1.  Meta Information

   Meta information is encoded as a map object for flexibility.
   Specifically, ResponseMeta is defined as:

    object-map { JSONString -> JSONValue } ResponseMeta;











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8.4.2.  Data Information

   The data component of the response encodes the response-specific
   data.  This document derives five types from ResponseEntityBase to
   add different types of data component: InfoResourceDirectory
   (Section 9.2.2), InfoResourceNetworkMap (Section 11.2.1.6),
   InfoResourceCostMap (Section 11.2.3.6),
   InfoResourceEndpointProperties (Section 11.4.1.6), and
   InfoResourceEndpointCostMap (Section 11.5.1.6).

8.5.  Protocol Errors

   If an ALTO server encounters an error while processing a request, the
   ALTO server SHOULD return additional ALTO-layer information, if it is
   available, in the form of an ALTO error resource encoded in the HTTP
   response' entity body.  If no ALTO-layer information is available, an
   ALTO server may omit the ALTO error resource from the response.

   With or without additional ALTO-layer error information, an ALTO
   server MUST set an appropriate HTTP status code.  It is important to
   note that the HTTP status code and ALTO error resource have distinct
   roles.  An ALTO error resource provides detailed information about
   why a particular request for an ALTO information resource was not
   successful.  The HTTP status code, on the other hand, indicates to
   HTTP processing elements (e.g., intermediaries and clients) how the
   response should be treated.

8.5.1.  Media Type

   The media type for an ALTO error response is "application/
   alto-error+json".

8.5.2.  Response Format and Error Codes

   An ALTO error response MUST include a field named "code" in the
   "meta" field of the response.  The value MUST be an ALTO error code,
   encoded in string, defined in Table 1.  Note that the ALTO error
   codes defined in Table 1 are limited to support the error conditions
   needed for purposes of this document.  Additional status codes may be
   defined in companion or extension documents.











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   +-----------------------+-------------------------------------------+
   | ALTO Error Code       | Description                               |
   +-----------------------+-------------------------------------------+
   | E_SYNTAX              | Parsing error in request (including       |
   |                       | identifiers)                              |
   | E_MISSING_FIELD       | A required JSON field is missing          |
   | E_INVALID_FIELD_TYPE  | The type of the value of a JSON field is  |
   |                       | invalid                                   |
   | E_INVALID_FIELD_VALUE | The value of a JSON field is invalid      |
   +-----------------------+-------------------------------------------+

                     Table 1: Defined ALTO Error Codes

   After an ALTO server receives a request, it needs to verify the
   syntactic and semantic validity of the request.  The following
   paragraphs in this section are intended to illustrate the usage of
   the error codes defined above during the verification.  An individual
   implementation may define its message processing in a different
   order.

   In the first step after an ALTO server receives a request, it checks
   the syntax of the request body (i.e., whether the JSON structure can
   be parsed), and indicates a syntax error using the error code
   E_SYNTAX.  For an E_SYNTAX error, the ALTO server MAY provide an
   optional field named "syntax-error" in the "meta" field of the error
   response.  The objective of providing "syntax-error" is to provide
   technical debugging information to developers, not end users.  Hence,
   it should be a human-readable, free-form text describing the syntax
   error.  If possible, the text should include position information
   about the syntax error, such as line number and offset within the
   line.  If nothing else, the value of the field named "syntax-error"
   could include just the position.  If a syntax error occurs in a
   production environment, the ALTO client could inform the end user
   that there was an error communicating with the ALTO server, and
   suggest that the user submit the error information, which includes
   "syntax-error", to the developers.

   A request without syntax errors may still be invalid.  An error case
   is that the request misses a required field.  The server indicates
   such an error using the error code E_MISSING_FIELD.  This document
   defines required fields for Filtered Network Map (Section 11.3.1.3),










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   Filtered Cost Map (Section 11.3.2.3), Endpoint Properties
   (Section 11.4.1.3), and Endpoint Cost (Section 11.5.1.3) services.
   For an E_MISSING_FIELD error, the server may include an optional
   field named "field" in the "meta" field of the error response, to
   indicate the missing field. "field" should be a JSONString indicating
   the full path of the missing field.  For example, assume that a
   Filtered Cost Map request (see Section 11.3.2.3) omits the "cost-
   metric" field.  The error response from the ALTO server may specify
   the value of "field" as "cost-type/cost-metric".

   A request with the correct fields might use a wrong type for the
   value of a field.  For example, the value of a field could be a
   JSONString when a JSONNumber is expected.  The server indicates such
   an error using the error code E_INVALID_FIELD_TYPE.  The server may
   include an optional field named "field" in the "meta" field of the
   response, to indicate the field that contains the wrong type.

   A request with the correct fields and types of values for the fields
   may specify a wrong value for a field.  For example, a Filtered Cost
   Map request may specify a wrong value for CostMode in the "cost-type"
   field (Section 11.3.2.3).  The server indicates such an error with
   the error code E_INVALID_FIELD_VALUE.  For an E_INVALID_FIELD_VALUE
   error, the server may include an optional field named "field" in the
   "meta" field of the response, to indicate the field that contains the
   wrong value.  The server may also include an optional field named
   "value" in the "meta" field of the response to indicate the wrong
   value that triggered the error.  If the "value" field is specified,
   the "field" field MUST be specified.  The "value" field MUST have a
   JSONString value.  If the invalid value is not a string, the ALTO
   server MUST convert it to a string.  Below are the rules to specify
   the "value" key:

   o  If the invalid value is a string, "value" is that string;

   o  If the invalid value is a number, "value" must be the invalid
      number as a string;

   o  If the invalid value is a subfield, the server must set the
      "field" key to the full path of the field name and "value" to the
      invalid subfield value, converting it to a string if needed.  For
      example, if the "cost-mode" subfield of the "cost-type" field is
      an invalid mode "foo", the server should set "value" to "foo", and
      "field" to "cost-mode/cost-type";

   o  If an element of a JSON array has an invalid value, the server
      sets "value" to the value of the invalid element, as a string, and
      "field" to the name of the array.  An array element of the wrong
      type (e.g., a number in what is supposed to be an array of



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      strings) is an invalid value error, not an invalid type error.
      The server sets "value" to the string version of the incorrect
      element, and "field" to the name of the array.

   If multiple errors are present in a single request (e.g., a request
   uses a JSONString when a JSONNumber is expected and a required field
   is missing), then the ALTO server MUST return exactly one of the
   detected errors.  However, the reported error is implementation
   defined, since specifying a particular order for message processing
   encroaches needlessly on implementation techniques.

8.5.3.  Overload Conditions and Server Unavailability

   If an ALTO server detects that it cannot handle a request from an
   ALTO client due to excessive load, technical problems, or system
   maintenance, it SHOULD do one of the following:

   o  Return an HTTP 503 ("Service Unavailable") status code to the ALTO
      client.  As indicated by [RFC7230], the Retry-After HTTP header
      may be used to indicate when the ALTO client should retry the
      request.

   o  Return an HTTP 307 ("Temporary Redirect") status code indicating
      an alternate ALTO server that may be able to satisfy the request.
      Using Temporary Redirect may generate infinite redirection loops.
      Although [RFC7231] Section 6.4 specifies that an HTTP client
      SHOULD detect infinite redirection loops, it is more desirable
      that multiple ALTO servers be configured not to form redirection
      loops.

   The ALTO server MAY also terminate the connection with the ALTO
   client.

   The particular policy applied by an ALTO server to determine that it
   cannot service a request is outside of the scope of this document.

9.  Protocol Specification: Information Resource Directory

   As already discussed, an ALTO client starts by retrieving an
   information resource directory, which specifies the attributes of
   individual information resources that an ALTO server provides.










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9.1.  Information Resource Attributes

   In this document, each information resource has up to five attributes
   associated with it, including its assigned ID, its response format,
   its capabilities, its accepted input parameters, and other resources
   on which it may depend.  The function of an information resource
   directory is to publishes these attributes.

9.1.1.  Resource ID

   Each information resource that an ALTO client can request MUST be
   assigned a resource ID attribute that is unique amongst all
   information resources in the information resource closure of the
   client.  The resource ID SHOULD remain stable even when the data
   provided by that resource changes.  For example, even though the
   number of PIDs in an ALTO network map may be adjusted, its resource
   ID should remain the same.  Similarly, if the entries in an ALTO cost
   map are updated, its resource ID should remain the same.  IDs SHOULD
   NOT be reused for different resources over time.

9.1.2.  Media Type

   ALTO uses media types [RFC2046] to uniquely indicate the data format
   used to encode the content to be transmitted between an ALTO server
   and an ALTO client in the HTTP entity body.

9.1.3.  Capabilities

   The Capabilities attribute of an information resource indicates
   specific capabilities that the server can provide.  For example, if
   an ALTO server allows an ALTO client to specify cost constraints when
   the client requests a cost map information resource, then the server
   advertises the "cost-constraints" capability of the cost map
   information resource.

9.1.4.  Accepts Input Parameters

   An ALTO server may allow an ALTO client to supply input parameters
   when requesting certain information resources.  The associated
   "accepts" attribute of such an information resource specifies a media
   type, which indicates how the client specifies the input parameters
   as contained in the entity body of the HTTP POST request.









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9.1.5.  Dependent Resources

   The information provided in an information resource may use
   information provided in some other resources (e.g., a cost map uses
   the PIDs defined in a network map).  The "uses" attribute conveys
   such information.

9.2.  Information Resource Directory (IRD)

   An ALTO server uses the information resource directory to publish
   available information resources and their aforementioned attributes.
   Since resource selection happens after consumption of the information
   resource directory, the format of the information resource directory
   is designed to be simple with the intention of future ALTO Protocol
   versions maintaining backwards compatibility.  Future extensions or
   versions of the ALTO Protocol SHOULD be accomplished by extending
   existing media types or adding new media types but retaining the same
   format for the Information Resource Directory.

   An ALTO server MUST make one information resource directory available
   via the HTTP GET method to a URI discoverable by an ALTO client.
   Discovery of this URI is out of scope of this document, but it could
   be accomplished by manual configuration or by returning the URI of an
   information resource directory from the ALTO Discovery Protocol
   [ALTO-SERVER-DISC].  For recommendations on what the URI may look
   like, see [ALTO-SERVER-DISC].

9.2.1.  Media Type

   The media type to indicate an information resource directory is
   "application/alto-directory+json".

9.2.2.  Encoding

   An information resource directory response may include in the "meta"
   field the "cost-types" field, whose value is of type IRDMetaCostTypes
   defined below, where CostType is defined in Section 10.7:


       object-map {
         JSONString -> CostType;
       } IRDMetaCostTypes;


   The function of "cost-types" is to assign names to a set of CostTypes
   that can be used in one or more "resources" entries in the IRD to
   simplify specification.  The names defined in "cost-types" in an IRD
   are local to the IRD.



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   For a Root IRD, "meta" MUST include a field named "default-alto-
   network-map", which value specifies the resource ID of an ALTO
   network map.  When there are multiple network maps defined in an IRD
   (e.g., with different levels of granularity), the "default-alto-
   network-map" field provides a guideline to simple clients that use
   only one network map.

   The data component of an information resource directory response is
   named "resources", which is a JSON object of type IRDResourceEntries:

       object {
         IRDResourceEntries resources;
       } InfoResourceDirectory : ResponseEntityBase;


       object-map {
         ResourceID  -> IRDResourceEntry;
       } IRDResourceEntries;

       object {
         JSONString      uri;
         JSONString      media-type;
         [JSONString     accepts;]
         [Capabilities   capabilities;]
         [ResourceID     uses<0..*>;]
       } IRDResourceEntry;



       object {
         ...
       } Capabilities;

   An IRDResourceEntries object is a dictionary map keyed by
   ResourceIDs, where ResourceID is defined in Section 10.2.  The value
   of each entry specifies:

   uri:           A URI at which the ALTO server provides one or more
                  information resources, or an information resource
                  directory indicating additional information resources.
                  URIs can be relative to the URI of the IRD and MUST be
                  resolved according to Section 5 of [RFC3986].

   media-type:    The media type of the information resource (see
                  Section 9.1.2) available via GET or POST requests to
                  the corresponding URI.  A value of "application/
                  alto-directory+json" indicates that the response for a




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                  request to the URI will be an information resource
                  directory defining additional information resources in
                  the information resource closure.

   accepts:       The media type of input parameters (see Section 9.1.4)
                  accepted by POST requests to the corresponding URI.
                  If this field is not present, it MUST be assumed to be
                  empty.

   capabilities:  A JSON object enumerating capabilities of an ALTO
                  server in providing the information resource at the
                  corresponding URI and information resources
                  discoverable via the URI.  If this field is not
                  present, it MUST be assumed to be an empty object.  If
                  a capability for one of the offered information
                  resources is not explicitly listed here, an ALTO
                  client may either issue an OPTIONS HTTP request to the
                  corresponding URI to determine if the capability is
                  supported or assume its default value documented in
                  this specification or an extension document describing
                  the capability.

   uses:          A list of resource IDs, defined in the same IRD, that
                  define the resources on which this resource directly
                  depends.  An ALTO server SHOULD include in this list
                  any resources that the ALTO client would need to
                  retrieve in order to interpret the contents of this
                  resource.  For example, an ALTO cost map resource
                  should include in this list the network map on which
                  it depends.  ALTO clients may wish to consult this
                  list in order to pre-fetch necessary resources.

   If an entry has an empty list for "accepts", then the corresponding
   URI MUST support GET requests.  If an entry has a non-empty
   "accepts", then the corresponding URI MUST support POST requests.  If
   an ALTO server wishes to support both GET and POST on a single URI,
   it MUST specify two entries in the information resource directory.

9.2.3.  Example

   The following is an example information resource directory returned
   by an ALTO server to an ALTO client.  Assume it is the Root IRD of
   the client.








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     GET /directory HTTP/1.1
     Host: alto.example.com
     Accept: application/alto-directory+json,application/alto-error+json

      HTTP/1.1 200 OK
      Content-Length: 2333
      Content-Type: application/alto-directory+json

      {
        "meta" : {
           "cost-types": {
              "num-routing": {
                 "cost-mode"  : "numerical",
                 "cost-metric": "routingcost",
                 "description": "My default"
              },
              "num-hop":     {
                 "cost-mode"  : "numerical",
                 "cost-metric": "hopcount"
              },
              "ord-routing": {
                 "cost-mode"  : "ordinal",
                 "cost-metric": "routingcost"
              },
              "ord-hop":     {
                 "cost-mode"  : "ordinal",
                 "cost-metric": "hopcount"
              }
           },
           "default-alto-network-map" : "my-default-network-map"
        },
        "resources" : {
           "my-default-network-map" : {
              "uri" : "http://alto.example.com/networkmap",
              "media-type" : "application/alto-networkmap+json"
           },
           "numerical-routing-cost-map" : {
              "uri" : "http://alto.example.com/costmap/num/routingcost",
              "media-type" : "application/alto-costmap+json",
              "capabilities" : {
                 "cost-type-names" : [ "num-routing" ]
              },
              "uses": [ "my-default-network-map" ]
           },
           "numerical-hopcount-cost-map" : {
              "uri" : "http://alto.example.com/costmap/num/hopcount",
              "media-type" : "application/alto-costmap+json",
              "capabilities" : {



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                 "cost-type-names" : [ "num-hop" ]
              },
              "uses": [ "my-default-network-map" ]
           },
           "custom-maps-resources" : {
              "uri" : "http://custom.alto.example.com/maps",
              "media-type" : "application/alto-directory+json"
           },
           "endpoint-property" : {
              "uri" : "http://alto.example.com/endpointprop/lookup",
              "media-type" : "application/alto-endpointprop+json",
              "accepts" : "application/alto-endpointpropparams+json",
              "capabilities" : {
                "prop-types" : [ "my-default-network-map.pid",
                                 "priv:ietf-example-prop" ]
              },
           },
           "endpoint-cost" : {
              "uri" : "http://alto.example.com/endpointcost/lookup",
              "media-type" : "application/alto-endpointcost+json",
              "accepts" : "application/alto-endpointcostparams+json",
              "capabilities" : {
                 "cost-constraints" : true,
                 "cost-type-names" : [ "num-routing", "num-hop",
                                       "ord-routing", "ord-hop"]
              }
           }
        }
      }


   Specifically, the "cost-types" field of "meta" of the example IRD
   defines names for four cost types in this IRD.  For example,
   "num-routing" in the example is the name that refers to a cost type
   with cost mode being "numerical" and cost metric being "routingcost".
   This name is used in the second entry of "resources", which defines a
   cost map.  In particular, the "cost-type-names" of its "capabilities"
   specifies that this resource supports a cost type named as
   "num-routing".  The ALTO client looks up the name "num-routing" in
   "cost-types" of the IRD to obtain the cost type named as
   "num-routing".  The last entry of "resources" uses all four names
   defined in "cost-types".

   Another field defined in "meta" of the example IRD is
   "default-alto-network-map", which has value "my-default-network-map",
   which is the resource ID of an ALTO network map that will be defined
   in "resources".




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   The "resources" field of the example IRD defines six information
   resources.  For example, the second entry, which is assigned a
   resource ID "numerical-routing-cost-map", provides a cost map, as
   indicated by the media-type "application/alto-costmap+json".  The
   cost map is based on the network map defined with resource ID
   "my-default-network-map".  As another example, the last entry, which
   is assigned resource ID "endpoint-cost", provides the Endpoint Cost
   Service, which is indicated by the media-type "application/
   alto-endpointcost+json".  An ALTO client should use uri
   "http://alto.example.com/endpointcost/lookup" to access the service.

   The ALTO client should format its request body to be the
   "application/alto-endpointcostparams+json" media type, as specified
   by the "accepts" attribute of the information resource.  The "cost-
   type-names" field of the "capabilities" attribute of the information
   resource includes four defined cost types specified in the "cost-
   types" field of "meta" of the IRD.  Hence, an ALTO client can verify
   that the Endpoint Cost information resource supports both cost
   metrics "routingcost" and "hopcount", each available for both
   "numerical" and "ordinal" cost modes.  When requesting the
   information resource, an ALTO client can specify cost constraints, as
   indicated by the "cost-constraints" field of the "capabilities"
   attribute.

9.2.4.  Delegation Using IRD

   ALTO IRDs provide the flexibility to define a set of information
   resources that are provided by ALTO servers running in multiple
   domains.  Consider the preceding example.  Assume that the ALTO
   server running at alto.example.com wants to delegate some information
   resources to a separate subdomain: "custom.alto.example.com".  In
   particular, assume that the maps available via this subdomain are
   filtered network maps, filtered cost maps, and some pre-generated
   maps for the "hopcount" and "routingcost" cost metrics in the
   "ordinal" cost mode.  The fourth entry of "resources" in the
   preceding example IRD implements the delegation.  The entry has a
   media-type of "application/alto-directory+json", and an ALTO client
   can discover the information resources available at
   "custom.alto.example.com" if its request to
   "http://custom.alto.example.com/maps" is successful:











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     GET /maps HTTP/1.1
     Host: custom.alto.example.com
     Accept: application/alto-directory+json,application/alto-error+json

   HTTP/1.1 200 OK
   Content-Length: 1900
   Content-Type: application/alto-directory+json

   {
     "meta" : {
        "cost-types": {
           "num-routing": {
              "cost-mode"  : "numerical",
              "cost-metric": "routingcost",
              "description": "My default"
           },
           "num-hop":     {
              "cost-mode"  : "numerical",
              "cost-metric": "hopcount"
           },
           "ord-routing": {
              "cost-mode"  : "ordinal",
              "cost-metric": "routingcost"
           },
           "ord-hop":     {
              "cost-mode"  : "ordinal",
              "cost-metric": "hopcount"
           }
        }
     },
     "resources" : {
        "filtered-network-map" : {
           "uri" : "http://custom.alto.example.com/networkmap/filtered",
           "media-type" : "application/alto-networkmap+json",
           "accepts" : "application/alto-networkmapfilter+json",
           "uses": [ "my-default-network-map" ]
        },
        "filtered-cost-map" : {
           "uri" : "http://custom.alto.example.com/costmap/filtered",
           "media-type" : "application/alto-costmap+json",
           "accepts" : "application/alto-costmapfilter+json",
           "capabilities" : {
              "cost-constraints" : true,
              "cost-type-names"  : [ "num-routing", "num-hop",
                                     "ord-routing", "ord-hop" ]
           },
           "uses": [ "my-default-network-map" ]
        },



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        "ordinal-routing-cost-map" : {
           "uri" : "http://custom.alto.example.com/ord/routingcost",
           "media-type" : "application/alto-costmap+json",
           "capabilities" : {
              "cost-type-names" : [ "ord-routing" ]
           },
           "uses": [ "my-default-network-map" ]
        },
        "ordinal-hopcount-cost-map" : {
           "uri" : "http://custom.alto.example.com/ord/hopcount",
           "media-type" : "application/alto-costmap+json",
           "capabilities" : {
              "cost-type-names" : [ "ord-hop" ]
           },
           "uses": [ "my-default-network-map" ]
        }
     }
   }

   Note that the subdomain does not define any network maps, and uses
   the network map with resource ID "my-default-network-map" defined in
   the Root IRD.

9.2.5.  Considerations of Using IRD

9.2.5.1.  ALTO client

   This document specifies no requirements or constraints on ALTO
   clients with regard to how they process an information resource
   directory to identify the URI corresponding to a desired information
   resource.  However, some advice is provided for implementers.

   It is possible that multiple entries in the directory match a desired
   information resource.  For instance, in the example in Section 9.2.3,
   a full cost map with the "numerical" cost mode and the "routingcost"
   cost metric could be retrieved via a GET request to
   "http://alto.example.com/costmap/num/routingcost" or via a POST
   request to "http://custom.alto.example.com/costmap/filtered".

   In general, it is preferred for ALTO clients to use GET requests
   where appropriate, since it is more likely for responses to be
   cacheable.  However, an ALTO client may need to use POST, for
   example, to get ALTO costs or properties that are for a restricted
   set of PIDs or endpoints or to update cached information previously
   acquired via GET requests.






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9.2.5.2.  ALTO server

   This document indicates that an ALTO server may or may not provide
   the information resources specified in the Map-Filtering Service.  If
   these resources are not provided, it is indicated to an ALTO client
   by the absence of a network map or cost map with any media types
   listed under "accepts".

10.  Protocol Specification: Basic Data Types

   This section details the format of basic data types.

10.1.  PID Name

   A PID Name is encoded as a JSON string.  The string MUST be no more
   than 64 characters, and it MUST NOT contain characters other than US-
   ASCII alphanumeric characters (U+0030-U+0039, U+0041-U+005A, and
   U+0061-U+007A), the hyphen ('-', U+002D), the colon (':', U+003A),
   the at sign ('@', code point U+0040), the low line ('_', U+005F), or
   the '.' separator (U+002E).  The '.' separator is reserved for future
   use and MUST NOT be used unless specifically indicated in this
   document, or an extension document.

   The type PIDName is used in this document to indicate a string of
   this format.

10.2.  Resource ID

   A resource ID uniquely identifies a particular resource (e.g., an
   ALTO network map) within an ALTO server (see Section 9.2).

   A resource ID is encoded as a JSON string with the same format as
   that of the type PIDName.

   The type ResourceID is used in this document to indicate a string of
   this format.

10.3.  Version Tag

   A version tag is defined as:


       object {
         ResourceID resource-id;
         JSONString tag;
       } VersionTag;





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   As described in Section 6.3, the "resource-id" field provides the
   resource ID of a resource (e.g., a network map) defined in the
   information resource directory, and "tag" provides an identifier
   string.

   Two version tags are equal if and only if both the "resource-id"
   fields are byte-for-byte equal and the "tag" fields are byte-for-byte
   equal.

   A string representing the "tag" field MUST be no more than 64
   characters, and it MUST NOT contain any character below U+0021 or
   above U+007E.  It is RECOMMENDED that the "tag" string have a low
   collision probability with other tags.  One suggested mechanism is to
   compute it using a hash of the data contents of the resource.

10.4.  Endpoints

   This section defines formats used to encode addresses for endpoints.
   In a case that multiple textual representations encode the same
   endpoint address or prefix (within the guidelines outlined in this
   document), the ALTO Protocol does not require ALTO clients or ALTO
   servers to use a particular textual representation, nor does it
   require that ALTO servers reply to requests using the same textual
   representation used by requesting ALTO clients.  ALTO clients must be
   cognizant of this.

10.4.1.  Typed Endpoint Addresses

   When an endpoint address is used, an ALTO implementation must be able
   to determine its type.  For this purpose, the ALTO Protocol allows
   endpoint addresses to also explicitly indicate their types.  This
   document refers to such addresses as "Typed Endpoint Addresses".

   Typed endpoint addresses are encoded as strings of the format
   AddressType:EndpointAddr, with the ':' character as a separator.  The
   type TypedEndpointAddr is used to indicate a string of this format.

10.4.2.  Address Type

   The AddressType component of TypedEndPointAddr is defined as a string
   consisting of only US-ASCII alphanumeric characters (U+0030-U+0039,
   U+0041-U+005A, and U+0061-U+007A).  The type AddressType is used in
   this document to indicate a string of this format.








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   This document defines two values for AddressType: "ipv4" to refer to
   IPv4 addresses and "ipv6" to refer to IPv6 addresses.  All
   AddressType identifiers appearing in an HTTP request or response with
   an "application/alto-*" media type MUST be registered in the "ALTO
   Address Type Registry" (see Section 14.4).

10.4.3.  Endpoint Address

   The EndpointAddr component of TypedEndPointAddr is also encoded as a
   string.  The exact characters and format depend on AddressType.  This
   document defines EndpointAddr when AddressType is "ipv4" or "ipv6".

10.4.3.1.  IPv4

   IPv4 Endpoint Addresses are encoded as specified by the IPv4address
   rule in Section 3.2.2 of [RFC3986].

10.4.3.2.  IPv6

   IPv6 endpoint addresses are encoded as specified in Section 4 of
   [RFC5952].

10.4.4.  Endpoint Prefixes

   For efficiency, it is useful to denote a set of endpoint addresses
   using a special notation (if one exists).  This specification makes
   use of the prefix notations for both IPv4 and IPv6 for this purpose.

   Endpoint prefixes are encoded as strings.  The exact characters and
   format depend on the type of endpoint address.

   The type EndpointPrefix is used in this document to indicate a string
   of this format.

10.4.4.1.  IPv4

   IPv4 endpoint prefixes are encoded as specified in Section 3.1 of
   [RFC4632].

10.4.4.2.  IPv6

   IPv6 endpoint prefixes are encoded as specified in Section 7 of
   [RFC5952].








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10.4.5.  Endpoint Address Group

   The ALTO Protocol includes messages that specify potentially large
   sets of endpoint addresses.  Endpoint address groups provide a more
   efficient way to encode such sets, even when the set contains
   endpoint addresses of different types.

   An endpoint address group is defined as:


       object-map {
         AddressType -> EndpointPrefix<0..*>;
       } EndpointAddrGroup;

   In particular, an endpoint address group is a JSON object
   representing a map, where each key is the string corresponding to an
   address type, and the corresponding value is an array listing
   prefixes of addresses of that type.

   The following is an example with both IPv4 and IPv6 endpoint
   addresses:


       {
         "ipv4": [
           "192.0.2.0/24",
           "198.51.100.0/25"
         ],
         "ipv6": [
           "2001:db8:0:1::/64",
           "2001:db8:0:2::/64"
         ]
       }

10.5.  Cost Mode

   A cost mode is encoded as a string.  The string MUST have a value of
   either "numerical" or "ordinal".

   The type CostMode is used in this document to indicate a string of
   this format.










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10.6.  Cost Metric

   A cost metric is encoded as a string.  The string MUST be no more
   than 32 characters, and it MUST NOT contain characters other than US-
   ASCII alphanumeric characters (U+0030-U+0039, U+0041-U+005A, and
   U+0061-U+007A), the hyphen ('-', U+002D), the colon (':', U+003A),
   the low line ('_', U+005F), or the '.' separator (U+002E).  The '.'
   separator is reserved for future use and MUST NOT be used unless
   specifically indicated by a companion or extension document.

   Identifiers prefixed with "priv:" are reserved for Private Use
   [RFC5226] without a need to register with IANA.  All other
   identifiers that appear in an HTTP request or response with an
   "application/alto-*" media type and indicate cost metrics MUST be
   registered in the "ALTO Cost Metric Registry" Section 14.2.  For an
   identifier with the "priv:" prefix, an additional string (e.g.,
   company identifier or random string) MUST follow (i.e., "priv:" only
   is not a valid identifier) to reduce potential collisions.

   The type CostMetric is used in this document to indicate a string of
   this format.

10.7.  Cost Type

   The combination of CostMetric and CostMode defines the type CostType:

       object {
         CostMetric cost-metric;
         CostMode   cost-mode;
         [JSONString description;]
       } CostType;


   The "description" field, if present, MUST provide a string value with
   a human-readable description of the cost-metric and cost-mode.  An
   ALTO client MAY present this string to a developer, as part of a
   discovery process; however, the field is not intended to be
   interpreted by an ALTO client.

10.8.  Endpoint Property

   This document distinguishes two types of endpoint properties:
   resource-specific endpoint properties and global endpoint properties.
   The type EndpointPropertyType is used in this document to indicate a
   string denoting either a resource-specific endpoint property or a
   global endpoint property.





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10.8.1.  Resource-Specific Endpoint Properties

   The name of resource-specific endpoint property MUST follow this
   format: a resource ID, followed by the '.' separator (U+002E),
   followed by a name obeying the same rules as for global endpoint
   property names (Section 10.8.2).

   This document defines only one resource-specific endpoint property:
   pid.  An example is "my-default-networkmap.pid".

10.8.2.  Global Endpoint Properties

   A global endpoint property is encoded as a string.  The string MUST
   be no more than 32 characters, and it MUST NOT contain characters
   other than US-ASCII alphanumeric characters (U+0030-U+0039,
   U+0041-U+005A, and U+0061-U+007A), the hyphen ('-', U+002D), the
   colon (':', U+003A), or the low line ('_', U+005F).  Note that the
   '.' separator is not allowed so that there is no ambiguity on whether
   an endpoint property is global or resource specific.

   Identifiers prefixed with "priv:" are reserved for Private Use
   [RFC5226] without a need to register with IANA.  All other
   identifiers for endpoint properties appearing in an HTTP request or
   response with an "application/alto-*" media type MUST be registered
   in the "ALTO Endpoint Property Type Registry" Section 14.3.  For an
   endpoint property identifier with the "priv:" prefix, an additional
   string (e.g., company identifier or random string) MUST follow (i.e.,
   "priv:" only is not a valid endpoint property identifier) to reduce
   potential collisions.

11.  Protocol Specification: Service Information Resources

   This section documents the individual information resources defined
   to provide the services defined in this document.

11.1.  Meta Information

   For the "meta" field of the response to an individual information
   resource, this document defines two generic fields: the "vtag" field,
   which provides the version tag (see Section 10.3) of the current
   information resource, and the "dependent-vtags" field, which is an
   array of version tags, to indicate the version tags of the resources
   on which this resource depends.

11.2.  Map Service

   The Map Service provides batch information to ALTO clients in the
   form of two types of maps: ALTO network maps and ALTO cost maps.



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11.2.1.  Network Map

   An ALTO network map information resource defines a set of PIDs, and
   for each PID, lists the network locations (endpoints) within the PID.
   An ALTO server MUST provide at least one network map.

11.2.1.1.  Media Type

   The media type of ALTO network maps is "application/alto-
   networkmap+json".

11.2.1.2.  HTTP Method

   An ALTO network map resource is requested using the HTTP GET method.

11.2.1.3.  Accept Input Parameters

   None.

11.2.1.4.  Capabilities

   None.

11.2.1.5.  Uses

   None.

11.2.1.6.  Response

   The "meta" field of an ALTO network map response MUST include the
   "vtag" field, which provides the version tag of the retrieved network
   map.

   The data component of an ALTO network map response is named "network-
   map", which is a JSON object of type NetworkMapData:


       object {
         NetworkMapData network-map;
       } InfoResourceNetworkMap : ResponseEntityBase;

       object-map {
         PIDName -> EndpointAddrGroup;
       } NetworkMapData;







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   Specifically, a NetworkMapData object is a dictionary map keyed by
   PIDs.  The value of each PID is the associated set of endpoint
   addresses for the PID.

   The returned network map MUST include all PIDs known to the ALTO
   server.

11.2.1.7.  Example

    GET /networkmap HTTP/1.1
    Host: alto.example.com
    Accept: application/alto-networkmap+json,application/alto-error+json

       HTTP/1.1 200 OK
       Content-Length: 449
       Content-Type: application/alto-networkmap+json

       {
         "meta" : {
           "vtag": {
             "resource-id": "my-default-network-map",
              "tag": "da65eca2eb7a10ce8b059740b0b2e3f8eb1d4785"
           }
         },
         "network-map" : {
           "PID1" : {
             "ipv4" : [
               "192.0.2.0/24",
               "198.51.100.0/25"
             ]
           },
           "PID2" : {
             "ipv4" : [
               "198.51.100.128/25"
             ]
           },
           "PID3" : {
             "ipv4" : [
               "0.0.0.0/0"
             ],
             "ipv6" : [
               "::/0"
             ]
           }
         }
       }





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   When parsing an ALTO network map, an ALTO client MUST ignore any
   EndpointAddressGroup whose address type it does not recognize.  If as
   a result a PID does not have any address types known to the client,
   the client still MUST recognize that PID name as valid, even though
   the PID then contains no endpoints.

   Note that the encoding of an ALTO network map response was chosen for
   readability and compactness.  If lookup efficiency at runtime is
   crucial, then the returned network map can be transformed into data
   structures offering more efficient lookup.  For example, one may
   store an ALTO network map as a trie-based data structure, which may
   allow efficient longest-prefix matching of IP addresses.

11.2.2.  Mapping IP Addresses to PIDs for 'ipv4'/'ipv6' Network Maps

   A key usage of an ALTO network map is to map endpoint addresses to
   PIDs.  For network maps containing the "ipv4" and "ipv6" address
   types defined in this document, when either an ALTO client or an ALTO
   server needs to compute the mapping from IP addresses to PIDs, the
   longest-prefix matching algorithm (Longest Match in Section 5.2.4.3
   of [RFC1812]) MUST be used.

   To ensure that the longest-prefix matching algorithm yields one and
   only one PID, an ALTO network map containing the "ipv4"/"ipv6"
   address types MUST satisfy the following two requirements.

   First, such a network map MUST define a PID for each possible address
   in the IP address space for all of the address types contained in the
   map.  This is defined as the completeness property of an ALTO network
   map.  A RECOMMENDED way to satisfy this property is to define a PID
   with the shortest enclosing prefix of the addresses provided in the
   map.  For a map with full IPv4 reachability, this would mean
   including the 0.0.0.0/0 prefix in a PID; for full IPv6 reachability,
   this would be the ::/0 prefix.

   Second, such a network map MUST NOT define two or more PIDs that
   contain an identical IP prefix, in order to ensure that the longest-
   prefix matching algorithm maps each IP addresses into exactly one
   PID.  This is defined as the non-overlapping property of an ALTO
   network map.  Specifically, to map an IP address to its PID in a non-
   overlapping network map, one considers the set S, which consists of
   all prefixes defined in the network map, applies the longest-prefix
   mapping algorithm to S to identify the longest prefix containing the
   IP address and assigns that prefix the IP address belonging to the
   PID containing the identified longest prefix.






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   The following example shows a complete and non-overlapping ALTO
   network map:

       "network-map" : {
         "PID0" : { "ipv6" : [ "::/0" ] },
         "PID1" : { "ipv4" : [ "0.0.0.0/0" ] },
         "PID2" : { "ipv4" : [ "192.0.2.0/24", "198.51.100.0/24" ] },
         "PID3" : { "ipv4" : [ "192.0.2.0/25", "192.0.2.128/25" ] }
       }

   The IP address 192.0.2.1 should be mapped to PID3.

   If, however, the two adjacent prefixes in PID3 were combined as a
   single prefix, then PID3 was changed to:

         "PID3" : { "ipv4" : [ "192.0.2.0/24" ] }

   The new map is no longer non-overlapping, and 192.0.2.1 could no
   longer be mapped unambiguously to a PID by means of longest-prefix
   matching.

   Extension documents may define techniques to allow a single IP
   address being mapped to multiple PIDs, when a need is identified.

11.2.3.  Cost Map

   An ALTO cost map resource lists the path cost for each pair of
   source/destination PIDs defined by the ALTO server for a given cost
   metric and cost mode.  This resource MUST be provided for at least
   the "routingcost" cost metric.

11.2.3.1.  Media Type

   The media type of ALTO cost maps is "application/alto-costmap+json".

11.2.3.2.  HTTP Method

   An ALTO cost map resource is requested using the HTTP GET method.

11.2.3.3.  Accept Input Parameters

   None.









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11.2.3.4.  Capabilities

   The capabilities of an ALTO server URI providing an unfiltered cost
   map is a JSON object of type CostMapCapabilities:

       object {
         JSONString cost-type-names<1..1>;
       } CostMapCapabilities;

   with field:

   cost-type-names:  Note that the array MUST include a single CostType
      name defined by the "cost-types" field in the "meta" field of the
      IRD.  This is because an unfiltered cost map (accept == "") is
      requested via an HTTP GET that accepts no input parameters.  As a
      contrast, for filtered cost maps (see Section 11.3.2), the array
      can have multiple elements.

11.2.3.5.  Uses

   The resource ID of the network map based on which the cost map will
   be defined.  Recall (Section 6) that the combination of a network map
   and a cost type defines a key.  In other words, an ALTO server MUST
   NOT define two cost maps with the same cost type / network map pair.

11.2.3.6.  Response

   The "meta" field of a cost map response MUST include the "dependent-
   vtags" field, whose value is a single-element array to indicate the
   version tag of the network map used, where the network map is
   specified in "uses" of the IRD.  The "meta" MUST also include the
   "cost-type" field, whose value indicates the cost type (Section 10.7)
   of the cost map.


















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   The data component of a cost map response is named "cost-map", which
   is a JSON object of type CostMapData:

       object {
         CostMapData cost-map;
       } InfoResourceCostMap : ResponseEntityBase;

       object-map {
         PIDName -> DstCosts;
       } CostMapData;

       object-map {
         PIDName -> JSONValue;
       } DstCosts;

   Specifically, a CostMapData object is a dictionary map object, with
   each key being the PIDName string identifying the corresponding
   source PID, and value being a type of DstCosts, which denotes the
   associated costs from the source PID to a set of destination PIDs
   (Section 6.2).  An implementation of the protocol in this document
   SHOULD assume that the cost is a JSONNumber and fail to parse if it
   is not, unless the implementation is using an extension to this
   document that indicates when and how costs of other data types are
   signaled.

   The returned cost map MUST include the path cost for each (source
   PID, destination PID) pair for which a path cost is defined.  An ALTO
   server MAY omit entries for which path costs are not defined (e.g.,
   either the source or the destination PIDs contain addresses outside
   of the network provider's administrative domain).

   Similar to the encoding of ALTO network maps, the encoding of ALTO
   cost maps was chosen for readability and compactness.  If lookup
   efficiency at runtime is crucial, then the returned cost map can be
   transformed into data structures offering more efficient lookup.  For
   example, one may store a cost map as a matrix.















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11.2.3.7.  Example

       GET /costmap/num/routingcost HTTP/1.1
       Host: alto.example.com
       Accept: application/alto-costmap+json,application/alto-error+json

          HTTP/1.1 200 OK
          Content-Length: 435
          Content-Type: application/alto-costmap+json

          {
            "meta" : {
              "dependent-vtags" : [
                {"resource-id": "my-default-network-map",
                 "tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
                }
              ],
              "cost-type" : {"cost-mode"  : "numerical",
                             "cost-metric": "routingcost"
              }
            },
            "cost-map" : {
              "PID1": { "PID1": 1,  "PID2": 5,  "PID3": 10 },
              "PID2": { "PID1": 5,  "PID2": 1,  "PID3": 15 },
              "PID3": { "PID1": 20, "PID2": 15  }
            }
          }


   Similar to the network map case, array-based encoding for "map" was
   considered, but the current encoding was chosen for clarity.

11.3.  Map-Filtering Service

   The Map-Filtering Service allows ALTO clients to specify filtering
   criteria to return a subset of a full map available in the Map
   Service.

11.3.1.  Filtered Network Map

   A filtered ALTO network map is an ALTO network map information
   resource (Section 11.2.1) for which an ALTO client may supply a list
   of PIDs to be included.  A filtered ALTO network map MAY be provided
   by an ALTO server.







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11.3.1.1.  Media Type

   Since a filtered ALTO network map is still an ALTO network map, it
   uses the media type defined for ALTO network maps at
   Section 11.2.1.1.

11.3.1.2.  HTTP Method

   A filtered ALTO network map is requested using the HTTP POST method.

11.3.1.3.  Accept Input Parameters

   An ALTO client supplies filtering parameters by specifying media type
   "application/alto-networkmapfilter+json" with HTTP POST body
   containing a JSON object of type ReqFilteredNetworkMap, where:

       object {
         PIDName pids<0..*>;
         [AddressType address-types<0..*>;]
       } ReqFilteredNetworkMap;

   with fields:

   pids:  Specifies list of PIDs to be included in the returned filtered
      network map.  If the list of PIDs is empty, the ALTO server MUST
      interpret the list as if it contained a list of all currently
      defined PIDs.  The ALTO server MUST interpret entries appearing
      multiple times as if they appeared only once.

   address-types:  Specifies a list of address types to be included in
      the returned filtered network map.  If the "address-types" field
      is not specified, or the list of address types is empty, the ALTO
      server MUST interpret the list as if it contained a list of all
      address types known to the ALTO server.  The ALTO server MUST
      interpret entries appearing multiple times as if they appeared
      only once.

11.3.1.4.  Capabilities

   None.

11.3.1.5.  Uses

   The resource ID of the network map based on which the filtering is
   performed.






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11.3.1.6.  Response

   The format is the same as unfiltered network maps.  See
   Section 11.2.1.6 for the format.

   The ALTO server MUST only include PIDs in the response that were
   specified (implicitly or explicitly) in the request.  If the input
   parameters contain a PID name that is not currently defined by the
   ALTO server, the ALTO server MUST behave as if the PID did not appear
   in the input parameters.  Similarly, the ALTO server MUST only
   enumerate addresses within each PID that have types specified
   (implicitly or explicitly) in the request.  If the input parameters
   contain an address type that is not currently known to the ALTO
   server, the ALTO server MUST behave as if the address type did not
   appear in the input parameters.

   The version tag included in the "vtag" field of the response MUST
   correspond to the full (unfiltered) network map information resource
   from which the filtered information is provided.  This ensures that a
   single, canonical version tag is used independent of any filtering
   that is requested by an ALTO client.






























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11.3.1.7.  Example

    POST /networkmap/filtered HTTP/1.1
    Host: custom.alto.example.com
    Content-Length: 33
    Content-Type: application/alto-networkmapfilter+json
    Accept: application/alto-networkmap+json,application/alto-error+json

    {
      "pids": [ "PID1", "PID2" ]
    }


    HTTP/1.1 200 OK
    Content-Length: 342
    Content-Type: application/alto-networkmap+json

    {
      "meta" : {
        "vtag" : {
           "resource-id": "my-default-network-map",
           "tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
        }
      },
      "network-map" : {
        "PID1" : {
          "ipv4" : [
            "192.0.2.0/24",
            "198.51.100.0/24"
          ]
        },
        "PID2" : {
          "ipv4": [
            "198.51.100.128/24"
          ]
        }
      }
    }


11.3.2.  Filtered Cost Map

   A filtered ALTO cost map is a cost map information resource
   (Section 11.2.3) for which an ALTO client may supply additional
   parameters limiting the scope of the resulting cost map.  A filtered
   ALTO cost map MAY be provided by an ALTO server.





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11.3.2.1.  Media Type

   Since a filtered ALTO cost map is still an ALTO cost map, it uses the
   media type defined for ALTO cost maps at Section 11.2.3.1.

11.3.2.2.  HTTP Method

   A filtered ALTO cost map is requested using the HTTP POST method.

11.3.2.3.  Accept Input Parameters

   The input parameters for a filtered cost map are supplied in the
   entity body of the POST request.  This document specifies the input
   parameters with a data format indicated by the media type
   "application/alto-costmapfilter+json", which is a JSON object of type
   ReqFilteredCostMap, where:

       object {
         CostType   cost-type;
         [JSONString constraints<0..*>;]
         [PIDFilter  pids;]
       } ReqFilteredCostMap;

       object {
         PIDName srcs<0..*>;
         PIDName dsts<0..*>;
       } PIDFilter;

   with fields:

   cost-type:  The CostType (Section 10.7) for the returned costs.  The
      "cost-metric" and "cost-mode" fields MUST match one of the
      supported cost types indicated in this resource's "capabilities"
      field (Section 11.3.2.4).  The ALTO client SHOULD omit the
      "description" field, and if present, the ALTO server MUST ignore
      the "description" field.

   constraints:  Defines a list of additional constraints on which
      elements of the cost map are returned.  This parameter MUST NOT be
      specified if this resource's "capabilities" field
      (Section 11.3.2.4) indicate that constraint support is not
      available.  A constraint contains two entities separated by
      whitespace: (1) an operator, "gt" for greater than, "lt" for less
      than, "ge" for greater than or equal to, "le" for less than or
      equal to, or "eq" for equal to and (2) a target cost value.  The
      cost value is a number that MUST be defined in the same units as





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      the cost metric indicated by the "cost-metric" parameter.  ALTO
      servers SHOULD use at least IEEE 754 double-precision floating
      point [IEEE.754.2008] to store the cost value, and SHOULD perform
      internal computations using double-precision floating-point
      arithmetic.  If multiple "constraint" parameters are specified,
      they are interpreted as being related to each other with a logical
      AND.

   pids:  A list of source PIDs and a list of destination PIDs for which
      path costs are to be returned.  If a list is empty, the ALTO
      server MUST interpret it as the full set of currently defined
      PIDs.  The ALTO server MUST interpret entries appearing in a list
      multiple times as if they appeared only once.  If the "pids" field
      is not present, both lists MUST be interpreted by the ALTO server
      as containing the full set of currently defined PIDs.

11.3.2.4.  Capabilities

   The URI providing this resource supports all capabilities documented
   in Section 11.2.3.4 (with identical semantics), plus additional
   capabilities.  In particular, the capabilities are defined by a JSON
   object of type FilteredCostMapCapabilities:

       object {
         JSONString cost-type-names<1..*>;
         JSONBool cost-constraints;
       } FilteredCostMapCapabilities;

   with fields:

   cost-type-names:  See Section 11.2.3.4 and note that the array can
      have one to many cost types.

   cost-constraints:  If true, then the ALTO server allows cost
      constraints to be included in requests to the corresponding URI.
      If not present, this field MUST be interpreted as if it specified
      false.  ALTO clients should be aware that constraints may not have
      the intended effect for cost maps with the ordinal cost mode since
      ordinal costs are not restricted to being sequential integers.

11.3.2.5.  Uses

   The resource ID of the network map based on which the cost map will
   be filtered.







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11.3.2.6.  Response

   The format is the same as an unfiltered ALTO cost map.  See
   Section 11.2.3.6  for the format.

   The "dependent-vtags" field in the "meta" field provides an array
   consisting of a single element, which is the version tag of the
   network map used in filtering.  ALTO clients should verify that the
   version tag included in the response is equal to the version tag of
   the network map used to generate the request (if applicable).  If it
   is not, the ALTO client may wish to request an updated network map,
   identify changes, and consider requesting a new filtered cost map.

   The returned cost map MUST contain only source/destination pairs that
   have been indicated (implicitly or explicitly) in the input
   parameters.  If the input parameters contain a PID name that is not
   currently defined by the ALTO server, the ALTO server MUST behave as
   if the PID did not appear in the input parameters.

   If any constraints are specified, source/destination pairs for which
   the path costs do not meet the constraints MUST NOT be included in
   the returned cost map.  If no constraints were specified, then all
   path costs are assumed to meet the constraints.




























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11.3.2.7.  Example

       POST /costmap/filtered HTTP/1.1
       Host: custom.alto.example.com
       Content-Type: application/alto-costmapfilter+json
       Content-Length: 181
       Accept: application/alto-costmap+json,application/alto-error+json

       {
         "cost-type" : {"cost-mode": "numerical",
                        "cost-metric": "routingcost"
         },
         "pids" : {
           "srcs" : [ "PID1" ],
           "dsts" : [ "PID1", "PID2", "PID3" ]
         }
       }


       HTTP/1.1 200 OK
       Content-Length: 341
       Content-Type: application/alto-costmap+json

       {
         "meta" : {
           "dependent-vtags" : [
             {"resource-id": "my-default-network-map",
              "tag": "75ed013b3cb58f896e839582504f622838ce670f"
             }
           ],
           "cost-type": {"cost-mode" : "numerical",
                         "cost-metric" : "routingcost"
           }
         },
         "cost-map" : {
              "PID1": { "PID1": 0,  "PID2": 1,  "PID3": 2 }
         }
       }


11.4.  Endpoint Property Service

   The Endpoint Property Service provides information about endpoint
   properties to ALTO clients.







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11.4.1.  Endpoint Property

   An endpoint property resource provides information about properties
   for individual endpoints.  In addition to the required "pid" endpoint
   property (see Sections 7.1.1 and 11.4.1.4), further endpoint
   properties MAY be provided by an ALTO server.

11.4.1.1.  Media Type

   The media type of an endpoint property resource is "application/
   alto-endpointprop+json".

11.4.1.2.  HTTP Method

   The endpoint property resource is requested using the HTTP POST
   method.

11.4.1.3.  Accept Input Parameters

   The input parameters for an endpoint property request are supplied in
   the entity body of the POST request.  This document specifies the
   input parameters with a data format indicated by the media type
   "application/alto-endpointpropparams+json", which is a JSON object of
   type ReqEndpointProp:

       object {
         EndpointPropertyType  properties<1..*>;
         TypedEndpointAddr     endpoints<1..*>;
       } ReqEndpointProp;

   with fields:

   properties:  List of endpoint properties to be returned for each
      endpoint.  Each specified property MUST be included in the list of
      supported properties indicated by this resource's "capabilities"
      field (Section 11.4.1.4).  The ALTO server MUST interpret entries
      appearing multiple times as if they appeared only once.

   endpoints:  List of endpoint addresses for which the specified
      properties are to be returned.  The ALTO server MUST interpret
      entries appearing multiple times as if they appeared only once.

11.4.1.4.  Capabilities

   The capabilities of an ALTO server URI providing endpoint properties
   are defined by a JSON object of type EndpointPropertyCapabilities:





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       object {
         EndpointPropertyType prop-types<1..*>;
       } EndpointPropertyCapabilities;

   with field:

   prop-types:  The endpoint properties (see Section 10.8) supported by
      the corresponding URI.

   In particular, the information resource closure MUST provide the
   lookup of pid for every ALTO network map defined.

11.4.1.5.  Uses

   None.

11.4.1.6.  Response

   The "dependent-vtags" field in the "meta" field of the response MUST
   be an array that includes the version tags of all ALTO network maps
   whose "pid" is queried.

   The data component of an endpoint properties response is named
   "endpoint-properties", which is a JSON object of type
   EndpointPropertyMapData, where:

       object {
         EndpointPropertyMapData endpoint-properties;
       } InfoResourceEndpointProperties : ResponseEntityBase;

       object-map {
         TypedEndpointAddr -> EndpointProps;
       } EndpointPropertyMapData;

       object {
         EndpointPropertyType -> JSONValue;
       } EndpointProps;

   Specifically, an EndpointPropertyMapData object has one member for
   each endpoint indicated in the input parameters (with the name being
   the endpoint encoded as a TypedEndpointAddr).  The requested
   properties for each endpoint are encoded in a corresponding
   EndpointProps object, which encodes one name/value pair for each
   requested property, where the property names are encoded as strings
   of type EndpointPropertyType.  An implementation of the protocol in






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   this document SHOULD assume that the property value is a JSONString
   and fail to parse if it is not, unless the implementation is using an
   extension to this document that indicates when and how property
   values of other data types are signaled.

   The ALTO server returns the value for each of the requested endpoint
   properties for each of the endpoints listed in the input parameters.

   If the ALTO server does not define a requested property's value for a
   particular endpoint, then it MUST omit that property from the
   response for only that endpoint.

11.4.1.7.  Example


  POST /endpointprop/lookup HTTP/1.1
  Host: alto.example.com
  Content-Length: 181
  Content-Type: application/alto-endpointpropparams+json
  Accept: application/alto-endpointprop+json,application/alto-error+json

  {
    "properties" : [ "my-default-networkmap.pid",
                     "priv:ietf-example-prop" ],
    "endpoints"  : [ "ipv4:192.0.2.34",
                     "ipv4:203.0.113.129" ]
  }


  HTTP/1.1 200 OK
  Content-Length: 396
  Content-Type: application/alto-endpointprop+json

  {
    "meta" : {
      "dependent-vtags" : [
        {"resource-id": "my-default-network-map",
         "tag": "7915dc0290c2705481c491a2b4ffbec482b3cf62"
        }
      ]
    },
    "endpoint-properties": {
      "ipv4:192.0.2.34"    : { "my-default-network-map.pid": "PID1",
                               "priv:ietf-example-prop": "1" },
      "ipv4:203.0.113.129" : { "my-default-network-map.pid": "PID3" }
    }
  }




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11.5.  Endpoint Cost Service

   The Endpoint Cost Service provides information about costs between
   individual endpoints.

   In particular, this service allows lists of endpoint prefixes (and
   addresses, as a special case) to be ranked (ordered) by an ALTO
   server.

11.5.1.  Endpoint Cost

   An endpoint cost resource provides information about costs between
   individual endpoints.  It MAY be provided by an ALTO server.

   How an ALTO server provides the endpoint cost resource is
   implementation dependent.  An ALTO server may use either fine-grained
   costs among individual endpoints or coarse-grained costs based on the
   costs between the PIDs corresponding to the endpoints.  See
   Section 15.3 for additional details.

11.5.1.1.  Media Type

   The media type of the endpoint cost resource is "application/alto-
   endpointcost+json".

11.5.1.2.  HTTP Method

   The endpoint cost resource is requested using the HTTP POST method.

11.5.1.3.  Accept Input Parameters

   An ALTO client supplies the endpoint cost parameters through a media
   type "application/alto-endpointcostparams+json", with an HTTP POST
   entity body of a JSON object of type ReqEndpointCostMap:

       object {
         CostType          cost-type;
         [JSONString       constraints<0..*>;]
         EndpointFilter    endpoints;
       } ReqEndpointCostMap;

       object {
         [TypedEndpointAddr srcs<0..*>;]
         [TypedEndpointAddr dsts<0..*>;]
       } EndpointFilter;






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   with fields:

   cost-type:  The cost type (Section 10.7) to use for returned costs.
      The "cost-metric" and "cost-mode" fields MUST match one of the
      supported cost types indicated in this resource's "capabilities"
      fields (Section 11.5.1.4).  The ALTO client SHOULD omit the
      "description" field, and if present, the ALTO server MUST ignore
      the "description" field.

   constraints:  Defined equivalently to the "constraints" input
      parameter of a filtered cost map (see Section 11.3.2).

   endpoints:  A list of source endpoints and destination endpoints for
      which path costs are to be returned.  If the list of source or
      destination endpoints is empty (or not included), the ALTO server
      MUST interpret it as if it contained the endpoint address
      corresponding to the client IP address from the incoming
      connection (see Section 13.3 for discussion and considerations
      regarding this mode).  The source and destination endpoint lists
      MUST NOT be both empty.  The ALTO server MUST interpret entries
      appearing multiple times in a list as if they appeared only once.

11.5.1.4.  Capabilities

   This document defines EndpointCostCapabilities as the same as
   FilteredCostMapCapabilities.  See Section 11.3.2.4.

11.5.1.5.  Uses

   It is important to note that although this resource allows an ALTO
   server to reveal costs between individual endpoints, the ALTO server
   is not required to do so.  A simple implementation of ECS may compute
   the cost between two endpoints as the cost between the PIDs
   corresponding to the endpoints, using one of the exposed network and
   cost maps defined by the server.  ECS MUST NOT specify the "use"
   field to indicate a network or cost map.  Hence, the ECS cost is the
   cost from the source endpoint to the destination endpoint.  A future
   extension may allow ECS to state that it "uses" a network map.  The
   extension then will need to define the semantics.

11.5.1.6.  Response

   The "meta" field of an endpoint cost response MUST include the "cost-
   type" field, to indicate the cost type used.

   The data component of an endpoint cost response is named
   "endpoint-cost-map", which is a JSON object of type
   EndpointCostMapData:



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       object {
         EndpointCostMapData endpoint-cost-map;
       } InfoResourceEndpointCostMap : ResponseEntityBase;

       object-map {
         TypedEndpointAddr -> EndpointDstCosts;
       } EndpointCostMapData;

       object-map {
         TypedEndpointAddr -> JSONValue;
       } EndpointDstCosts;


   Specifically, an EndpointCostMapData object is a dictionary map with
   each key representing a TypedEndpointAddr string identifying the
   source endpoint specified in the input parameters.  For each source
   endpoint, an EndpointDstCosts dictionary map object denotes the
   associated cost to each destination endpoint specified in input
   parameters.  An implementation of the protocol in this document
   SHOULD assume that the cost value is a JSONNumber and fail to parse
   if it is not, unless the implementation is using an extension to this
   document that indicates when and how costs of other data types are
   signaled.  If the ALTO server does not define a cost value from a
   source endpoint to a particular destination endpoint, it MAY be
   omitted from the response.


























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11.5.1.7.  Example

  POST /endpointcost/lookup HTTP/1.1
  Host: alto.example.com
  Content-Length: 248
  Content-Type: application/alto-endpointcostparams+json
  Accept: application/alto-endpointcost+json,application/alto-error+json

  {
    "cost-type": {"cost-mode" : "ordinal",
                  "cost-metric" : "routingcost"},
    "endpoints" : {
      "srcs": [ "ipv4:192.0.2.2" ],
      "dsts": [
        "ipv4:192.0.2.89",
        "ipv4:198.51.100.34",
        "ipv4:203.0.113.45"
      ]
    }
  }


  HTTP/1.1 200 OK
  Content-Length: 274
  Content-Type: application/alto-endpointcost+json

  {
    "meta" : {
      "cost-type": {"cost-mode" : "ordinal",
                    "cost-metric" : "routingcost"
      }
    },
    "endpoint-cost-map" : {
      "ipv4:192.0.2.2": {
        "ipv4:192.0.2.89"    : 1,
        "ipv4:198.51.100.34" : 2,
        "ipv4:203.0.113.45"  : 3
      }
    }
  }


12.  Use Cases

   The sections below depict typical use cases.  While these use cases
   focus on peer-to-peer applications, ALTO can be applied to other
   environments such as Content Distribution Networks (CDNs)
   [ALTO-USE-CASES].



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12.1.  ALTO Client Embedded in P2P Tracker

   Many deployed P2P systems use a tracker to manage swarms and perform
   peer selection.  Such a P2P tracker can already use a variety of
   information to perform peer selection to meet application-specific
   goals.  By acting as an ALTO client, the P2P tracker can use ALTO
   information as an additional information source to enable more
   network-efficient traffic patterns and improve application
   performance.

   A particular requirement of many P2P trackers is that they must
   handle a large number of P2P clients.  A P2P tracker can obtain and
   locally store ALTO information (e.g., ALTO network maps and cost
   maps) from the ISPs containing the P2P clients, and benefit from the
   same aggregation of network locations done by ALTO servers.

       .---------.   (1) Get Network Map    .---------------.
       |         | <----------------------> |               |
       |  ALTO   |                          |  P2P Tracker  |
       | Server  |   (2) Get Cost Map       | (ALTO client) |
       |         | <----------------------> |               |
       `---------'                          `---------------'
                                               ^     |
                                 (3) Get Peers |     | (4) Selected Peer
                                               |     v     List
                 .---------.                 .-----------.
                 | Peer 1  | <-------------- |   P2P     |
                 `---------'                 |  Client   |
                     .      (5) Connect to   `-----------'
                     .        Selected Peers     /
                 .---------.                    /
                 | Peer 50 | <------------------
                 `---------'

               Figure 4: ALTO Client Embedded in P2P Tracker

   Figure 4 shows an example use case where a P2P tracker is an ALTO
   client and applies ALTO information when selecting peers for its P2P
   clients.  The example proceeds as follows:

   1.  The P2P tracker requests from the ALTO server a network map, so
       that it locally map P2P clients into PIDs.

   2.  The P2P tracker requests from the ALTO server the cost map
       amongst all PIDs identified in the preceding step.

   3.  A P2P client joins the swarm, and requests a peer list from the
       P2P tracker.



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   4.  The P2P tracker returns a peer list to the P2P client.  The
       returned peer list is computed based on the network map and the
       cost map returned by the ALTO server, and possibly other
       information sources.  Note that it is possible that a tracker may
       use only the network map to implement hierarchical peer selection
       by preferring peers within the same PID and ISP.

   5.  The P2P client connects to the selected peers.

   Note that the P2P tracker may provide peer lists to P2P clients
   distributed across multiple ISPs.  In such a case, the P2P tracker
   may communicate with multiple ALTO servers.

12.2.  ALTO Client Embedded in P2P Client: Numerical Costs

   P2P clients may also utilize ALTO information themselves when
   selecting from available peers.  It is important to note that not all
   P2P systems use a P2P tracker for peer discovery and selection.
   Furthermore, even when a P2P tracker is used, the P2P clients may
   rely on other sources, such as peer exchange and DHTs, to discover
   peers.

   When a P2P client uses ALTO information, it typically queries only
   the ALTO server servicing its own ISP.  The "my-Internet view"
   provided by its ISP's ALTO server can include preferences to all
   potential peers.

   .---------.   (1) Get Network Map    .---------------.
   |         | <----------------------> |               |
   |  ALTO   |                          |  P2P Client   |
   | Server  |   (2) Get Cost Map       | (ALTO client) |
   |         | <----------------------> |               |    .---------.
   `---------'                          `---------------' <- |  P2P    |
             .---------.                 /  |      ^    ^    | Tracker |
             | Peer 1  | <--------------    |      |     \   `---------'
             `---------'                    |    (3) Gather Peers
                 .      (4) Select Peers    |      |       \
                 .        and Connect      /   .--------.  .--------.
             .---------.                  /    |  P2P   |  |  DHT   |
             | Peer 50 | <----------------     | Client |  `--------'
             `---------'                       | (PEX)  |
                                               `--------'

               Figure 5: ALTO Client Embedded in P2P Client

   Figure 5 shows an example use case where a P2P client locally applies
   ALTO information to select peers.  The use case proceeds as follows:




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   1.  The P2P client requests the network map covering all PIDs from
       the ALTO server servicing its own ISP.

   2.  The P2P client requests the cost map providing path costs amongst
       all PIDs from the ALTO server.  The cost map by default specifies
       numerical costs.

   3.  The P2P client discovers peers from sources such as peer exchange
       (PEX) from other P2P clients, distributed hash tables (DHT), and
       P2P trackers.

   4.  The P2P client uses ALTO information as part of the algorithm for
       selecting new peers and connects to the selected peers.

12.3.  ALTO Client Embedded in P2P Client: Ranking

   It is also possible for a P2P client to offload the selection and
   ranking process to an ALTO server.  In this use case, the ALTO client
   embedded in the P2P client gathers a list of known peers in the
   swarm, and asks the ALTO server to rank them.  This document limits
   the use case to when the P2P client and the ALTO server are deployed
   by the same entity; hence, the P2P client uses the ranking provided
   by the ALTO server directly.

   As in the use case using numerical costs, the P2P client typically
   only queries the ALTO server servicing its own ISP.

   .---------.                          .---------------.
   |         |                          |               |
   |  ALTO   | (2) Get Endpoint Ranking |  P2P Client   |
   | Server  | <----------------------> | (ALTO client) |
   |         |                          |               |    .---------.
   `---------'                          `---------------' <- |  P2P    |
             .---------.                 /  |      ^    ^    | Tracker |
             | Peer 1  | <--------------    |      |     \   `---------'
             `---------'                    |    (1) Gather Peers
                 .      (3) Connect to      |      |       \
                 .        Selected Peers   /   .--------.  .--------.
             .---------.                  /    |  P2P   |  |  DHT   |
             | Peer 50 | <----------------     | Client |  `--------'
             `---------'                       | (PEX)  |
                                               `--------'

           Figure 6: ALTO Client Embedded in P2P Client: Ranking







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   Figure 6 shows an example of this scenario.  The use case proceeds as
   follows:

   1.  The P2P client discovers peers from sources such as Peer Exchange
       (PEX) from other P2P clients, Distributed Hash Tables (DHT), and
       P2P trackers.

   2.  The P2P client queries the ALTO server's ranking service (i.e.,
       the ECS Service), by including the discovered peers as the set of
       destination endpoints, and indicating the "ordinal" cost mode.
       The response indicates the ranking of the candidate peers.

   3.  The P2P client connects to the peers in the order specified in
       the ranking.

13.  Discussions

13.1.  Discovery

   The discovery mechanism by which an ALTO client locates an
   appropriate ALTO server is out of scope for this document.  This
   document assumes that an ALTO client can discover an appropriate ALTO
   server.  Once it has done so, the ALTO client may use the information
   resource directory (see Section 9.2) to locate an information
   resource with the desired ALTO information.

13.2.  Hosts with Multiple Endpoint Addresses

   In practical deployments, a particular host can be reachable using
   multiple addresses (e.g., a wireless IPv4 connection, a wireline IPv4
   connection, and a wireline IPv6 connection).  In general, the
   particular network path followed when sending packets to the host
   will depend on the address that is used.  Network providers may
   prefer one path over another.  An additional consideration may be how
   to handle private address spaces (e.g., behind carrier-grade NATs).

   To support such behavior, this document allows multiple endpoint
   addresses and address types.  With this support, the ALTO Protocol
   allows an ALTO service provider the flexibility to indicate
   preferences for paths from an endpoint address of one type to an
   endpoint address of a different type.










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13.3.  Network Address Translation Considerations

   In this day and age of NAT v4<->v4, v4<->v6 [RFC6144], and possibly
   v6<->v6 [RFC6296], a protocol should strive to be NAT friendly and
   minimize carrying IP addresses in the payload or provide a mode of
   operation where the source IP address provides the information
   necessary to the server.

   The protocol specified in this document provides a mode of operation
   where the source network location is computed by the ALTO server
   (i.e., the Endpoint Cost Service) from the source IP address found in
   the ALTO client query packets.  This is similar to how some P2P
   trackers (e.g., BitTorrent trackers -- see "Tracker HTTP/HTTPS
   Protocol" in [BitTorrent]) operate.

   There may be cases in which an ALTO client needs to determine its own
   IP address, such as when specifying a source endpoint address in the
   Endpoint Cost Service.  It is possible that an ALTO client has
   multiple network interface addresses, and that some or all of them
   may require NAT for connectivity to the public Internet.

   If a public IP address is required for a network interface, the ALTO
   client SHOULD use the Session Traversal Utilities for NAT (STUN)
   [RFC5389].  If using this method, the host MUST use the "Binding
   Request" message and the resulting "XOR-MAPPED-ADDRESS" parameter
   that is returned in the response.  Using STUN requires cooperation
   from a publicly accessible STUN server.  Thus, the ALTO client also
   requires configuration information that identifies the STUN server,
   or a domain name that can be used for STUN server discovery.  To be
   selected for this purpose, the STUN server needs to provide the
   public reflexive transport address of the host.

   ALTO clients should be cognizant that the network path between
   endpoints can depend on multiple factors, e.g., source address and
   destination address used for communication.  An ALTO server provides
   information based on endpoint addresses (more generally, network
   locations), but the mechanisms used for determining existence of
   connectivity or usage of NAT between endpoints are out of scope of
   this document.

13.4.  Endpoint and Path Properties

   An ALTO server could make available many properties about endpoints
   beyond their network location or grouping.  For example, connection
   type, geographical location, and others may be useful to
   applications.  This specification focuses on network location and
   grouping, but the protocol may be extended to handle other endpoint
   properties.



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14.  IANA Considerations

   This document defines registries for application/alto-* media types,
   ALTO cost metrics, ALTO endpoint property types, ALTO address types,
   and ALTO error codes.  Initial values for the registries and the
   process of future assignments are given below.

14.1.  application/alto-* Media Types

   This document registers multiple media types, listed in Table 2.

    +-------------+------------------------------+-------------------+
    | Type        | Subtype                      | Specification     |
    +-------------+------------------------------+-------------------+
    | application | alto-directory+json          | Section 9.2.1     |
    | application | alto-networkmap+json         | Section 11.2.1.1  |
    | application | alto-networkmapfilter+json   | Section 11.3.1.1  |
    | application | alto-costmap+json            | Section 11.2.3.1  |
    | application | alto-costmapfilter+json      | Section 11.3.2.1  |
    | application | alto-endpointprop+json       | Section 11.4.1.1  |
    | application | alto-endpointpropparams+json | Section 11.4.1.1  |
    | application | alto-endpointcost+json       | Section 11.5.1.1  |
    | application | alto-endpointcostparams+json | Section 11.5.1.1  |
    | application | alto-error+json              | Section 8.5.1     |
    +-------------+------------------------------+-------------------+

                    Table 2: ALTO Protocol Media Types

   Type name:  application

   Subtype name:  This documents registers multiple subtypes, as listed
      in Table 2.

   Required parameters:  n/a

   Optional parameters:  n/a

   Encoding considerations:  Encoding considerations are identical to
      those specified for the "application/json" media type.  See
      [RFC7159].

   Security considerations:  Security considerations relating to the
      generation and consumption of ALTO Protocol messages are discussed
      in Section 15.

   Interoperability considerations:  This document specifies format of
      conforming messages and the interpretation thereof.




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   Published specification:  This document is the specification for
      these media types; see Table 2 for the section documenting each
      media type.

   Applications that use this media type:  ALTO servers and ALTO clients
      either stand alone or are embedded within other applications.

   Additional information:

      Magic number(s):  n/a

      File extension(s):  This document uses the mime type to refer to
         protocol messages and thus does not require a file extension.

      Macintosh file type code(s):  n/a

   Person & email address to contact for further information:  See
      Authors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  n/a

   Author:  See Authors' Addresses section.

   Change controller:  Internet Engineering Task Force
      (mailto:iesg@ietf.org).

14.2.  ALTO Cost Metric Registry

   IANA has created and now maintains the "ALTO Cost Metric Registry",
   listed in Table 3.

                   +-------------+---------------------+
                   | Identifier  | Intended Semantics  |
                   +-------------+---------------------+
                   | routingcost | See Section 6.1.1.1 |
                   | priv:       | Private use         |
                   +-------------+---------------------+

                        Table 3: ALTO Cost Metrics

   This registry serves two purposes.  First, it ensures uniqueness of
   identifiers referring to ALTO cost metrics.  Second, it provides
   references to particular semantics of allocated cost metrics to be
   applied by both ALTO servers and applications utilizing ALTO clients.





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   New ALTO cost metrics are assigned after IETF Review [RFC5226] to
   ensure that proper documentation regarding ALTO cost metric semantics
   and security considerations has been provided.  The RFCs documenting
   the new metrics should be detailed enough to provide guidance to both
   ALTO service providers and applications utilizing ALTO clients as to
   how values of the registered ALTO cost metric should be interpreted.
   Updates and deletions of ALTO cost metrics follow the same procedure.

   Registered ALTO cost metric identifiers MUST conform to the
   syntactical requirements specified in Section 10.6.  Identifiers are
   to be recorded and displayed as strings.

   As specified in Section 10.6, identifiers prefixed with "priv:" are
   reserved for Private Use.

   Requests to add a new value to the registry MUST include the
   following information:

   o  Identifier: The name of the desired ALTO cost metric.

   o  Intended Semantics: ALTO costs carry with them semantics to guide
      their usage by ALTO clients.  For example, if a value refers to a
      measurement, the measurement units must be documented.  For proper
      implementation of the ordinal cost mode (e.g., by a third-party
      service), it should be documented whether higher or lower values
      of the cost are more preferred.

   o  Security Considerations: ALTO costs expose information to ALTO
      clients.  As such, proper usage of a particular cost metric may
      require certain information to be exposed by an ALTO service
      provider.  Since network information is frequently regarded as
      proprietary or confidential, ALTO service providers should be made
      aware of the security ramifications related to usage of a cost
      metric.

   This specification requests registration of the identifier
   "routingcost".  Semantics for the this cost metric are documented in
   Section 6.1.1.1, and security considerations are documented in
   Section 15.3.












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14.3.  ALTO Endpoint Property Type Registry

   IANA has created and now maintains the "ALTO Endpoint Property Type
   Registry", listed in Table 4.

                    +------------+--------------------+
                    | Identifier | Intended Semantics |
                    +------------+--------------------+
                    | pid        | See Section 7.1.1  |
                    | priv:      | Private use        |
                    +------------+--------------------+

                   Table 4: ALTO Endpoint Property Types

   The maintenance of this registry is similar to that of the preceding
   ALTO cost metrics.  That is, the registry is maintained by IANA,
   subject to the description in Section 10.8.2.

   New endpoint property types are assigned after IETF Review [RFC5226]
   to ensure that proper documentation regarding ALTO endpoint property
   type semantics and security considerations has been provided.
   Updates and deletions of ALTO endpoint property types follow the same
   procedure.

   Registered ALTO endpoint property type identifiers MUST conform to
   the syntactical requirements specified in Section 10.8.1.
   Identifiers are to be recorded and displayed as strings.

   As specified in Section 10.8.1, identifiers prefixed with "priv:" are
   reserved for Private Use.

   Requests to add a new value to the registry MUST include the
   following information:

   o  Identifier: The name of the desired ALTO endpoint property type.

   o  Intended Semantics: ALTO endpoint properties carry with them
      semantics to guide their usage by ALTO clients.  Hence, a document
      defining a new type should provide guidance to both ALTO service
      providers and applications utilizing ALTO clients as to how values
      of the registered ALTO endpoint property should be interpreted.
      For example, if a value refers to a measurement, the measurement
      units must be documented.

   o  Security Considerations: ALTO endpoint properties expose
      information to ALTO clients.  ALTO service providers should be
      made aware of the security ramifications related to the exposure
      of an endpoint property.



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   In particular, the request should discuss the sensitivity of the
   information, and why such sensitive information is required for ALTO-
   based operations.  It may recommend that ISP provide mechanisms for
   users to grant or deny consent to such information sharing.
   Limitation to a trust domain being a type of consent bounding.

   A request defining new endpoint properties should focus on exposing
   attributes of endpoints that are related to the goals of ALTO --
   optimization of application-layer traffic -- as opposed to more
   general properties of endpoints.  Maintaining this focus on
   technical, network-layer data will also help extension developers
   avoid the privacy concerns associated with publishing information
   about endpoints.  For example:

   o  An extension to indicate the capacity of a server would likely be
      appropriate, since server capacities can be used by a client to
      choose between multiple equivalent servers.  In addition, these
      properties are unlikely to be viewed as private information.

   o  An extension to indicate the geolocation of endpoints might be
      appropriate.  In some cases, a certain level of geolocation (e.g.,
      to the country level) can be useful for selecting content sources.
      More precise geolocation, however, is not relevant to content
      delivery, and is typically considered private.

   o  An extension indicating demographic attributes of the owner of an
      endpoint (e.g., age, sex, income) would not be appropriate,
      because these attributes are not related to delivery optimization,
      and because they are clearly private data.

   This specification requests registration of the identifier "pid".
   Semantics for this property are documented in Section 7.1.1, and
   security considerations are documented in Section 15.4.


















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14.4.  ALTO Address Type Registry

   IANA has created and now maintains the "ALTO Address Type Registry",
   listed in Table 5.

   +------------+-----------------+-----------------+------------------+
   | Identifier | Address         | Prefix Encoding | Mapping to/from  |
   |            | Encoding        |                 | IPv4/v6          |
   +------------+-----------------+-----------------+------------------+
   | ipv4       | See Section     | See Section     | Direct mapping   |
   |            | 10.4.3          | 10.4.4          | to IPv4          |
   | ipv6       | See Section     | See Section     | Direct mapping   |
   |            | 10.4.3          | 10.4.4          | to IPv6          |
   +------------+-----------------+-----------------+------------------+

                        Table 5: ALTO Address Types

   This registry serves two purposes.  First, it ensures uniqueness of
   identifiers referring to ALTO address types.  Second, it states the
   requirements for allocated address type identifiers.

   New ALTO address types are assigned after IETF Review [RFC5226] to
   ensure that proper documentation regarding the new ALTO address types
   and their security considerations has been provided.  RFCs defining
   new address types should indicate how an address of a registered type
   is encoded as an EndpointAddr and, if possible, a compact method
   (e.g., IPv4 and IPv6 prefixes) for encoding a set of addresses as an
   EndpointPrefix.  Updates and deletions of ALTO address types follow
   the same procedure.

   Registered ALTO address type identifiers MUST conform to the
   syntactical requirements specified in Section 10.4.2.  Identifiers
   are to be recorded and displayed as strings.

   Requests to add a new value to the registry MUST include the
   following information:

   o  Identifier: The name of the desired ALTO address type.

   o  Endpoint Address Encoding: The procedure for encoding an address
      of the registered type as an EndpointAddr (see Section 10.4.3).

   o  Endpoint Prefix Encoding: The procedure for encoding a set of
      addresses of the registered type as an EndpointPrefix (see
      Section 10.4.4).  If no such compact encoding is available, the
      same encoding used for a singular address may be used.  In such a
      case, it must be documented that sets of addresses of this type
      always have exactly one element.



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   o  Mapping to/from IPv4/IPv6 Addresses: If possible, a mechanism to
      map addresses of the registered type to and from IPv4 or IPv6
      addresses should be specified.

   o  Security Considerations: In some usage scenarios, endpoint
      addresses carried in ALTO Protocol messages may reveal information
      about an ALTO client or an ALTO service provider.  Applications
      and ALTO service providers using addresses of the registered type
      should be made aware of how (or if) the addressing scheme relates
      to private information and network proximity.

   This specification requests registration of the identifiers "ipv4"
   and "ipv6", as shown in Table 5.

14.5.  ALTO Error Code Registry

   IANA has created and now maintains the "ALTO Error Code Registry".
   Initial values are listed in Table 1, and recommended usage of the
   error codes is specified in Section 8.5.2.

   Although the error codes defined in Table 1 are already quite
   complete, future extensions may define new error codes.  The "ALTO
   Error Code Registry" ensures the uniqueness of error codes when new
   error codes are added.

   New ALTO error codes are assigned after IETF Review [RFC5226] to
   ensure that proper documentation regarding the new ALTO error codes
   and their usage has been provided.

   A request to add a new ALTO error code to the registry MUST include
   the following information:

   o  Error Code: A string starting with E_ to indicate the error.

   o  Intended Usage: ALTO error codes carry with them semantics to
      guide their usage by ALTO servers and clients.  In particular, if
      a new error code indicates conditions that overlap with those of
      an existing ALTO error code, recommended usage of the new error
      code should be specified.

15.  Security Considerations

   Some environments and use cases of ALTO require consideration of
   security attacks on ALTO servers and clients.  In order to support
   those environments interoperably, the ALTO requirements document
   [RFC6708] outlines minimum-to-implement authentication and other
   security requirements.  This document considers the following threats
   and protection strategies.



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15.1.  Authenticity and Integrity of ALTO Information

15.1.1.  Risk Scenarios

   An attacker may want to provide false or modified ALTO information
   resources or an information resource directory to ALTO clients to
   achieve certain malicious goals.  As an example, an attacker may
   provide false endpoint properties.  For example, suppose that a
   network supports an endpoint property named "hasQuota", which reports
   whether an endpoint has usage quota.  An attacker may want to
   generate a false reply to lead to unexpected charges to the endpoint.
   An attack may also want to provide a false cost map.  For example, by
   faking a cost map that highly prefers a small address range or a
   single address, the attacker may be able to turn a distributed
   application into a Distributed-Denial-of-Service (DDoS) tool.

   Depending on the network scenario, an attacker can attack
   authenticity and integrity of ALTO information resources using
   various techniques, including, but not limited to, sending forged
   DHCP replies in an Ethernet, DNS poisoning, and installing a
   transparent HTTP proxy that does some modifications.

15.1.2.  Protection Strategies

   ALTO protects the authenticity and integrity of ALTO information
   (both information directory and individual information resources) by
   leveraging the authenticity and integrity mechanisms in TLS (see
   Section 8.3.5).

   ALTO service providers who request server certificates and
   certification authorities who issue ALTO-specific certificates SHOULD
   consider the recommendations and guidelines defined in [RFC6125].

   Software engineers developing and service providers deploying ALTO
   should make themselves familiar with possibly updated standards
   documents as well as up-to-date Best Current Practices on configuring
   HTTP over TLS.

15.1.3.  Limitations

   The protection of HTTP over TLS for ALTO depends on that the domain
   name in the URI for the information resources is not comprised.  This
   will depend on the protection implemented by service discovery.

   A deployment scenario may require redistribution of ALTO information
   to improve scalability.  When authenticity and integrity of ALTO
   information are still required, then ALTO clients obtaining ALTO
   information through redistribution must be able to validate the



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   received ALTO information.  Support for this validation is not
   provided in this document, but it may be provided by extension
   documents.

15.2.  Potential Undesirable Guidance from Authenticated ALTO
       Information

15.2.1.  Risk Scenarios

   The ALTO services make it possible for an ALTO service provider to
   influence the behavior of network applications.  An ALTO service
   provider may be hostile to some applications and, hence, try to use
   ALTO information resources to achieve certain goals [RFC5693]:

      ...redirecting applications to corrupted mediators providing
      malicious content, or applying policies in computing cost maps
      based on criteria other than network efficiency.

   See [ALTO-DEPLOYMENT] for additional discussions on faked ALTO
   guidance.

   A related scenario is that an ALTO server could unintentionally give
   "bad" guidance.  For example, if many ALTO clients follow the cost
   map or the Endpoint Cost Service guidance without doing additional
   sanity checks or adaptation, more preferable hosts and/or links could
   get overloaded while less preferable ones remain idle; see AR-14 of
   [RFC6708] for related application considerations.

15.2.2.  Protection Strategies

   To protect applications from undesirable ALTO information resources,
   it is important to note that there is no protocol mechanism to
   require conforming behaviors on how applications use ALTO information
   resources.  An application using ALTO may consider including a
   mechanism to detect misleading or undesirable results from using ALTO
   information resources.  For example, if throughput measurements do
   not show "better-than-random" results when using an ALTO cost map to
   select resource providers, the application may want to disable ALTO
   usage or switch to an external ALTO server provided by an
   "independent organization" (see AR-20 and AR-21 in [RFC6708]).  If
   the first ALTO server is provided by the access network service
   provider and the access network service provider tries to redirect
   access to the external ALTO server back to the provider's ALTO server
   or try to tamper with the responses, the preceding authentication and
   integrity protection can detect such a behavior.






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15.3.  Confidentiality of ALTO Information

15.3.1.  Risk Scenarios

   In many cases, although ALTO information resources may be regarded as
   non-confidential information, there are deployment cases in which
   ALTO information resources can be sensitive information that can pose
   risks if exposed to unauthorized parties.  This document discusses
   the risks and protection strategies for such deployment scenarios.

   For example, an attacker may infer details regarding the topology,
   status, and operational policies of a network through its ALTO
   network and cost maps.  As a result, a sophisticated attacker may be
   able to infer more fine-grained topology information than an ISP
   hosting an ALTO server intends to disclose.  The attacker can
   leverage the information to mount effective attacks such as focusing
   on high-cost links.

   Revealing some endpoint properties may also reveal additional
   information than the provider intended.  For example, when adding the
   line bitrate as one endpoint property, such information may be
   potentially linked to the income of the habitants at the network
   location of an endpoint.

   In Section 5.2.1 of [RFC6708], three types of risks associated with
   the confidentiality of ALTO information resources are identified:
   risk type (1) Excess disclosure of the ALTO service provider's data
   to an authorized ALTO client; risk type (2) Disclosure of the ALTO
   service provider's data (e.g., network topology information or
   endpoint addresses) to an unauthorized third party; and risk type (3)
   Excess retrieval of the ALTO service provider's data by collaborating
   ALTO clients.  [ALTO-DEPLOYMENT] also discusses information leakage
   from ALTO.

15.3.2.  Protection Strategies

   To address risk types (1) and (3), the provider of an ALTO server
   must be cognizant that the network topology and provisioning
   information provided through ALTO may lead to attacks.  ALTO does not
   require any particular level of details of information disclosure;
   hence, the provider should evaluate how much information is revealed
   and the associated risks.

   To address risk type (2), the ALTO Protocol needs confidentiality.
   Since ALTO requires that HTTP over TLS must be supported, the
   confidentiality mechanism is provided by HTTP over TLS.





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   For deployment scenarios where client authentication is desired to
   address risk type (2), ALTO requires that HTTP Digestion
   Authentication is supported to achieve ALTO client authentication to
   limit the number of parties with whom ALTO information is directly
   shared.  TLS client authentication may also be supported.  Depending
   on the use case and scenario, an ALTO server may apply other access
   control techniques to restrict access to its services.  Access
   control can also help to prevent Denial-of-Service attacks by
   arbitrary hosts from the Internet.  See [ALTO-DEPLOYMENT] for a more
   detailed discussion on this issue.

   See Section 14.3 on guidelines when registering endpoint properties
   to protect endpoint privacy.

15.3.3.  Limitations

   ALTO information providers should be cognizant that encryption only
   protects ALTO information until it is decrypted by the intended ALTO
   client.  Digital Rights Management (DRM) techniques and legal
   agreements protecting ALTO information are outside of the scope of
   this document.

15.4.  Privacy for ALTO Users

15.4.1.  Risk Scenarios

   The ALTO Protocol provides mechanisms in which the ALTO client
   serving a user can send messages containing network location
   identifiers (IP addresses or fine-grained PIDs) to the ALTO server.
   This is particularly true for the Endpoint Property, the Endpoint
   Cost, and the fine-grained Filtered Map services.  The ALTO server or
   a third party who is able to intercept such messages can store and
   process obtained information in order to analyze user behaviors and
   communication patterns.  The analysis may correlate information
   collected from multiple clients to deduce additional application/
   content information.  Such analysis can lead to privacy risks.  For a
   more comprehensive classification of related risk scenarios, see
   cases 4, 5, and 6 in [RFC6708], Section 5.2.

15.4.2.  Protection Strategies

   To protect user privacy, an ALTO client should be cognizant about
   potential ALTO server tracking through client queries, e.g., by using
   HTTP cookies.  The ALTO Protocol as defined by this document does not
   rely on HTTP cookies.  ALTO clients MAY decide not to return cookies
   received from the server, in order to make tracking more difficult.
   However, this might break protocol extensions that are beyond the
   scope of this document.



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   An ALTO client may consider the possibility of relying only on ALTO
   network maps for PIDs and cost maps amongst PIDs to avoid passing IP
   addresses of other endpoints (e.g., peers) to the ALTO server.  When
   specific IP addresses are needed (e.g., when using the Endpoint Cost
   Service), an ALTO client SHOULD minimize the amount of information
   sent in IP addresses.  For example, the ALTO client may consider
   obfuscation techniques such as specifying a broader address range
   (i.e., a shorter prefix length) or by zeroing out or randomizing the
   last few bits of IP addresses.  Note that obfuscation may yield less
   accurate results.

15.5.  Availability of ALTO Services

15.5.1.  Risk Scenarios

   An attacker may want to disable the ALTO services of a network as a
   way to disable network guidance to large scale applications.  In
   particular, queries that can be generated with low effort but result
   in expensive workloads at the ALTO server could be exploited for
   Denial-of-Service attacks.  For instance, a simple ALTO query with n
   source network locations and m destination network locations can be
   generated fairly easily but results in the computation of n*m path
   costs between pairs by the ALTO server (see Section 5.2).

15.5.2.  Protection Strategies

   The ALTO service provider should be cognizant of the workload at the
   ALTO server generated by certain ALTO Queries, such as certain
   queries to the Map Service, the Map-Filtering Service and the
   Endpoint Cost (Ranking) Service.  One way to limit Denial-of-Service
   attacks is to employ access control to the ALTO server.  The ALTO
   server can also indicate overload and reject repeated requests that
   can cause availability problems.  More advanced protection schemes
   such as computational puzzles [SIP] may be considered in an extension
   document.

   An ALTO service provider should also leverage the fact that the Map
   Service allows ALTO servers to pre-generate maps that can be
   distributed to many ALTO clients.

16.  Manageability Considerations

   This section details operations and management considerations based
   on existing deployments and discussions during protocol development.
   It also indicates where extension documents are expected to provide
   appropriate functionality discussed in [RFC5706] as additional
   deployment experience becomes available.




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16.1.  Operations

16.1.1.  Installation and Initial Setup

   The ALTO Protocol is based on HTTP.  Thus, configuring an ALTO server
   may require configuring the underlying HTTP server implementation to
   define appropriate security policies, caching policies, performance
   settings, etc.

   Additionally, an ALTO service provider will need to configure the
   ALTO information to be provided by the ALTO server.  The granularity
   of the topological map and the cost maps is left to the specific
   policies of the ALTO service provider.  However, a reasonable default
   may include two PIDs, one to hold the endpoints in the provider's
   network and the second PID to represent full IPv4 and IPv6
   reachability (see Section 11.2.2), with the cost between each source/
   destination PID set to 1.  Another operational issue that the ALTO
   service provider needs to consider is that the filtering service can
   degenerate into a full map service when the filtering input is empty.
   Although this choice as the degeneration behavior provides
   continuity, the computational and network load of serving full maps
   to a large number of ALTO clients should be considered.

   Implementers employing an ALTO client should attempt to automatically
   discover an appropriate ALTO server.  Manual configuration of the
   ALTO server location may be used where automatic discovery is not
   appropriate.  Methods for automatic discovery and manual
   configuration are discussed in [ALTO-SERVER-DISC].

   Specifications for underlying protocols (e.g., TCP, HTTP, TLS) should
   be consulted for their available settings and proposed default
   configurations.

16.1.2.  Migration Path

   This document does not detail a migration path for ALTO servers since
   there is no previous standard protocol providing the similar
   functionality.

   There are existing applications making use of network information
   discovered from other entities such as whois, geo-location databases,
   or round-trip time measurements, etc.  Such applications should
   consider using ALTO as an additional source of information; ALTO need
   not be the sole source of network information.







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16.1.3.  Dependencies on Other Protocols and Functional Components

   The ALTO Protocol assumes that HTTP client and server implementations
   exist.  It also assumes that JSON encoder and decoder implementations
   exist.

   An ALTO server assumes that it can gather sufficient information to
   populate Network and Cost maps.  "Sufficient information" is
   dependent on the information being exposed, but likely includes
   information gathered from protocols such as IGP and EGP Routing
   Information Bases (see Figure 1).  Specific mechanisms have been
   proposed (e.g., [ALTO-SVR-APIS]) and are expected to be provided in
   extension documents.

16.1.4.  Impact and Observation on Network Operation

   ALTO presents a new opportunity for managing network traffic by
   providing additional information to clients.  In particular, the
   deployment of an ALTO server may shift network traffic patterns, and
   the potential impact to network operation can be large.  An ALTO
   service provider should ensure that appropriate information is being
   exposed.  Privacy implications for ISPs are discussed in
   Section 15.3.

   An ALTO service provider should consider how to measure impacts on
   (or integration with) traffic engineering, in addition to monitoring
   correctness and responsiveness of ALTO servers.  The measurement of
   impacts can be challenging because ALTO-enabled applications may not
   provide related information back to the ALTO service provider.
   Furthermore, the measurement of an ALTO service provider may show
   that ALTO clients are not bound to ALTO server guidance as ALTO is
   only one source of information.

   While it can be challenging to measure the impact of ALTO guidance,
   there exist some possible techniques.  In certain trusted deployment
   environments, it may be possible to collect information directly from
   ALTO clients.  It may also be possible to vary or selectively disable
   ALTO guidance for a portion of ALTO clients either by time,
   geographical region, or some other criteria to compare the network
   traffic characteristics with and without ALTO.

   Both ALTO service providers and those using ALTO clients should be
   aware of the impact of incorrect or faked guidance (see
   [ALTO-DEPLOYMENT]).







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16.2.  Management

16.2.1.  Management Interoperability

   A common management API would be desirable given that ALTO servers
   may typically be configured with dynamic data from various sources,
   and ALTO servers are intended to scale horizontally for fault-
   tolerance and reliability.  A specific API or protocol is outside the
   scope of this document, but may be provided by an extension document.

   Logging is an important functionality for ALTO servers and, depending
   on the deployment, ALTO clients.  Logging should be done via syslog
   [RFC5424].

16.2.2.  Management Information

   A Management Information Model (see Section 3.2 of [RFC5706]) is not
   provided by this document, but should be included or referenced by
   any extension documenting an ALTO-related management API or protocol.

16.2.3.  Fault Management

   An ALTO service provider should monitor whether any ALTO servers have
   failed.  See Section 16.2.5 for related metrics that may indicate
   server failures.

16.2.4.  Configuration Management

   Standardized approaches and protocols to configuration management for
   ALTO are outside the scope of this document, but this document does
   outline high-level principles suggested for future standardization
   efforts.

   An ALTO server requires at least the following logical inputs:

   o  Data sources from which ALTO information resources is derived.
      This can be either raw network information (e.g., from routing
      elements) or pre-processed ALTO-level information in the forms of
      network maps, cost maps, etc.

   o  Algorithms for computing the ALTO information returned to clients.
      These could return either information from a database or
      information customized for each client.

   o  Security policies mapping potential clients to the information
      that they have privilege to access.





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   Multiple ALTO servers can be deployed for scalability.  A centralized
   configuration database may be used to ensure they are providing the
   desired ALTO information with appropriate security controls.  The
   ALTO information (e.g., network maps and cost maps) being served by
   each ALTO server, as well as security policies (HTTP authentication,
   TLS client and server authentication, TLS encryption parameters)
   intended to serve the same information should be monitored for
   consistency.

16.2.5.  Performance Management

   An exhaustive list of desirable performance information from ALTO
   servers and ALTO clients are outside of the scope of this document.
   The following is a list of suggested ALTO-specific metrics to be
   monitored based on the existing deployment and protocol development
   experience:

   o  Requests and responses for each service listed in an information
      directory (total counts and size in bytes);

   o  CPU and memory utilization;

   o  ALTO map updates;

   o  Number of PIDs;

   o  ALTO map sizes (in-memory size, encoded size, number of entries).

16.2.6.  Security Management

   Section 15 documents ALTO-specific security considerations.
   Operators should configure security policies with those in mind.
   Readers should refer to HTTP [RFC7230] and TLS [RFC5246] and related
   documents for mechanisms available for configuring security policies.
   Other appropriate security mechanisms (e.g., physical security,
   firewalls, etc.) should also be considered.

17.  References

17.1.  Normative References

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers", RFC
              1812, June 1995.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.




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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, August 2006.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

17.2.  Informative References

   [ALTO-DEPLOYMENT]
              Stiemerling, M., Ed., Kiesel, S., Ed., Previdi, S., and M.
              Scharf, "ALTO Deployment Considerations", Work in
              Progress, February 2014.

   [ALTO-INFOEXPORT]
              Shalunov, S., Penno, R., and R. Woundy, "ALTO Information
              Export Service", Work in Progress, October 2008.




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   [ALTO-MULTI-PS]
              Das, S., Narayanan, V., and L. Dondeti, "ALTO: A Multi
              Dimensional Peer Selection Problem", Work in Progress,
              October 2008.

   [ALTO-QUERYRESPONSE]
              Das, S. and V. Narayanan, "A Client to Service Query
              Response Protocol for ALTO", Work in Progress, March 2009.

   [ALTO-SERVER-DISC]
              Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
              H. Song, "ALTO Server Discovery", Work in Progress,
              September 2013.

   [ALTO-SVR-APIS]
              Medved, J., Ward, D., Peterson, J., Woundy, R., and D.
              McDysan, "ALTO Network-Server and Server-Server APIs",
              Work in Progress, March 2011.

   [ALTO-USE-CASES]
              Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
              S. Previdi, "Use Cases for ALTO within CDNs", Work in
              Progress, June 2012.

   [BitTorrent]
              "Bittorrent Protocol Specification v1.0",
              <http://wiki.theory.org/BitTorrentSpecification>.

   [Fielding-Thesis]
              Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", University of
              California, Irvine, Dissertation 2000, 2000.

   [IEEE.754.2008]
              Institute of Electrical and Electronics Engineers,
              "Standard for Binary Floating-Point Arithmetic", IEEE
              Standard 754, August 2008.

   [P4P-FRAMEWORK]
              Alimi, R., Pasko, D., Popkin, L., Wang, Y., and Y. Yang,
              "P4P: Provider Portal for P2P Applications", Work in
              Progress, November 2008.

   [P4P-SIGCOMM08]
              Xie, H., Yang, Y., Krishnamurthy, A., Liu, Y., and A.
              Silberschatz, "P4P: Provider Portal for (P2P)
              Applications", SIGCOMM 2008, August 2008.




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   [P4P-SPEC] Wang, Y., Alimi, R., Pasko, D., Popkin, L., and Y. Yang,
              "P4P Protocol Specification", Work in Progress, March
              2009.

   [PROXIDOR] Akonjang, O., Feldmann, A., Previdi, S., Davie, B., and D.
              Saucez, "The PROXIDOR Service", Work in Progress, March
              2009.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693, October
              2009.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions", RFC
              5706, November 2009.

   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, April 2011.

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, June 2011.

   [RFC6708]  Kiesel, S., Previdi, S., Stiemerling, M., Woundy, R., and
              Y. Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", RFC 6708, September 2012.

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

   [SIP]      Jennings, C., "Computational Puzzles for SPAM Reduction in
              SIP", Work in Progress, July 2007.















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Appendix A.  Acknowledgments

   Thank you to Jan Seedorf (NEC) for substantial contributions to the
   Security Considerations section.  Ben Niven-Jenkins (Velocix),
   Michael Scharf, and Sabine Randriamasy (Alcatel-Lucent) gave
   substantial feedback and suggestions on the protocol design.

   We would like to thank the following people whose input and
   involvement was indispensable in achieving this merged proposal:

      Obi Akonjang (DT Labs/TU Berlin),

      Saumitra M.  Das (Qualcomm Inc.),

      Syon Ding (China Telecom),

      Doug Pasko (Verizon),

      Laird Popkin (Pando Networks),

      Satish Raghunath (Juniper Networks),

      Albert Tian (Ericsson/Redback),

      Yu-Shun Wang (Microsoft),

      David Zhang (PPLive),

      Yunfei Zhang (China Mobile).

   We would also like to thank the following additional people who were
   involved in the projects that contributed to this merged document:
   Alex Gerber (ATT), Chris Griffiths (Comcast), Ramit Hora (Pando
   Networks), Arvind Krishnamurthy (University of Washington), Marty
   Lafferty (DCIA), Erran Li (Bell Labs), Jin Li (Microsoft), Y.  Grace
   Liu (IBM Watson), Jason Livingood (Comcast), Michael Merritt (ATT),
   Ingmar Poese (DT Labs/TU Berlin), James Royalty (Pando Networks),
   Damien Saucez (UCL), Thomas Scholl (ATT), Emilio Sepulveda
   (Telefonica), Avi Silberschatz (Yale University), Hassan Sipra (Bell
   Canada), Georgios Smaragdakis (DT Labs/TU Berlin), Haibin Song
   (Huawei), Oliver Spatscheck (ATT), See-Mong Tang (Microsoft), Jia
   Wang (ATT), Hao Wang (Yale University), Ye Wang (Yale University),
   Haiyong Xie (Yale University).

   Stanislav Shalunov would like to thank BitTorrent, where he worked
   while contributing to ALTO development.





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Appendix B.  Design History and Merged Proposals

   The ALTO Protocol specified in this document consists of
   contributions from

   o  P4P [P4P-FRAMEWORK], [P4P-SIGCOMM08], [P4P-SPEC];

   o  ALTO Info-Export [ALTO-INFOEXPORT];

   o  Query/Response [ALTO-QUERYRESPONSE], [ALTO-MULTI-PS]; and

   o  Proxidor [PROXIDOR].

Authors' Addresses

   Richard Alimi (editor)
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

   EMail: ralimi@google.com


   Reinaldo Penno (editor)
   Cisco Systems, Inc.
   170 West Tasman Dr
   San Jose, CA  95134
   USA

   EMail: repenno@cisco.com


   Y. Richard Yang (editor)
   Yale University
   51 Prospect St
   New Haven, CT  06511
   USA

   EMail: yry@cs.yale.edu











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   Sebastian Kiesel
   University of Stuttgart Information Center
   Networks and Communication Systems Department
   Allmandring 30
   Stuttgart  70550
   Germany

   EMail: ietf-alto@skiesel.de


   Stefano Previdi
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   EMail: sprevidi@cisco.com


   Wendy Roome
   Alcatel-Lucent
   600 Mountain Ave.
   Murray Hill, NJ  07974
   USA

   EMail: w.roome@alcatel-lucent.com


   Stanislav Shalunov
   Open Garden
   751 13th St
   San Francisco, CA  94130
   USA

   EMail: shalunov@shlang.com


   Richard Woundy
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103
   USA

   EMail: Richard_Woundy@cable.comcast.com






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