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ALTO WG                                                    R. Alimi, Ed.
Internet-Draft                                                    Google
Intended status: Standards Track                           R. Penno, Ed.
Expires: August 29, 2013                                   Cisco Systems
                                                            Y. Yang, Ed.
                                                         Yale University
                                                            Feb 25, 2013


                             ALTO Protocol
                    draft-ietf-alto-protocol-14.txt

Abstract

   Applications using the Internet already have access to 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 application
   clients.  What is missing is knowledge of the underlying network
   topologies from the point of view of ISPs (henceforth referred as
   Providers).  In other words, what a Provider prefers in terms of
   traffic optimization -- and a way to distribute it.

   The Application-Layer Traffic Optimization (ALTO) Service provides
   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 Service.
   Although the ALTO Service would primarily be provided by the network
   operator (e.g., an ISP), content service providers and third parties
   could also operate this service.  Applications that could use this
   service 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.

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].

Status of this Memo



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   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 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.






















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Background and Problem Statement . . . . . . . . . . . . .  6
     1.2.  Design History and Merged Proposals  . . . . . . . . . . .  6
     1.3.  Solution Benefits  . . . . . . . . . . . . . . . . . . . .  7
       1.3.1.  Service Providers  . . . . . . . . . . . . . . . . . .  7
       1.3.2.  Applications . . . . . . . . . . . . . . . . . . . . .  7
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.1.  Endpoint . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.2.  Endpoint Address . . . . . . . . . . . . . . . . . . .  8
       2.1.3.  ASN  . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.4.  Network Location . . . . . . . . . . . . . . . . . . .  8
       2.1.5.  ALTO Information . . . . . . . . . . . . . . . . . . .  8
       2.1.6.  ALTO Information Base  . . . . . . . . . . . . . . . .  9
     2.2.  ALTO Service and Protocol Scope  . . . . . . . . . . . . .  9
     2.3.  ALTO Information Reuse and Redistribution  . . . . . . . . 10
   3.  ALTO Information Service Framework . . . . . . . . . . . . . . 11
     3.1.  ALTO Information Services  . . . . . . . . . . . . . . . . 12
       3.1.1.  Map Service  . . . . . . . . . . . . . . . . . . . . . 12
       3.1.2.  Map Filtering Service  . . . . . . . . . . . . . . . . 12
       3.1.3.  Endpoint Property Service  . . . . . . . . . . . . . . 12
       3.1.4.  Endpoint Cost Service  . . . . . . . . . . . . . . . . 12
   4.  Network Map  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Provider-defined Identifier (PID)  . . . . . . . . . . . . 13
     4.2.  Endpoint Addresses . . . . . . . . . . . . . . . . . . . . 13
       4.2.1.  IP Addresses . . . . . . . . . . . . . . . . . . . . . 14
     4.3.  Example Network Map  . . . . . . . . . . . . . . . . . . . 14
   5.  Cost Map . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  Cost Attributes  . . . . . . . . . . . . . . . . . . . . . 15
       5.1.1.  Cost Type  . . . . . . . . . . . . . . . . . . . . . . 15
       5.1.2.  Cost Mode  . . . . . . . . . . . . . . . . . . . . . . 16
     5.2.  Cost Map Structure . . . . . . . . . . . . . . . . . . . . 17
     5.3.  Network Map and Cost Map Dependency  . . . . . . . . . . . 17
   6.  Endpoint Properties  . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Endpoint Property Type . . . . . . . . . . . . . . . . . . 18
       6.1.1.  Endpoint Property Type: pid  . . . . . . . . . . . . . 18
   7.  Protocol Specification: General Processing . . . . . . . . . . 18
     7.1.  Overall Design . . . . . . . . . . . . . . . . . . . . . . 18
     7.2.  Notation . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.3.  Basic Operation  . . . . . . . . . . . . . . . . . . . . . 19
       7.3.1.  Discovering Information Resources  . . . . . . . . . . 19
       7.3.2.  Requesting Information Resources . . . . . . . . . . . 19
       7.3.3.  Response . . . . . . . . . . . . . . . . . . . . . . . 20
       7.3.4.  Client Behavior  . . . . . . . . . . . . . . . . . . . 21
       7.3.5.  Authentication and Encryption  . . . . . . . . . . . . 21
       7.3.6.  HTTP Cookies . . . . . . . . . . . . . . . . . . . . . 21



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       7.3.7.  Parsing  . . . . . . . . . . . . . . . . . . . . . . . 21
     7.4.  Information Resource Attributes  . . . . . . . . . . . . . 21
       7.4.1.  Capability Advertisement . . . . . . . . . . . . . . . 22
       7.4.2.  Accept Input Parameters  . . . . . . . . . . . . . . . 22
       7.4.3.  Media Type . . . . . . . . . . . . . . . . . . . . . . 22
     7.5.  Information Resource Media Type Encoding . . . . . . . . . 22
       7.5.1.  Meta Information . . . . . . . . . . . . . . . . . . . 22
       7.5.2.  ALTO Information . . . . . . . . . . . . . . . . . . . 23
       7.5.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 23
     7.6.  Information Resource Directory . . . . . . . . . . . . . . 23
       7.6.1.  Media Type . . . . . . . . . . . . . . . . . . . . . . 24
       7.6.2.  Encoding . . . . . . . . . . . . . . . . . . . . . . . 24
       7.6.3.  Example  . . . . . . . . . . . . . . . . . . . . . . . 25
       7.6.4.  Usage Considerations . . . . . . . . . . . . . . . . . 28
     7.7.  Protocol Errors  . . . . . . . . . . . . . . . . . . . . . 29
       7.7.1.  Media Type . . . . . . . . . . . . . . . . . . . . . . 29
       7.7.2.  Resource Format  . . . . . . . . . . . . . . . . . . . 30
       7.7.3.  Error Codes  . . . . . . . . . . . . . . . . . . . . . 30
       7.7.4.  Overload Conditions and Server Unavailability  . . . . 31
   8.  Protocol Specification: Basic ALTO Data Types  . . . . . . . . 31
     8.1.  PID Name . . . . . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  Version Tag  . . . . . . . . . . . . . . . . . . . . . . . 31
     8.3.  Endpoints  . . . . . . . . . . . . . . . . . . . . . . . . 32
       8.3.1.  Address Type . . . . . . . . . . . . . . . . . . . . . 32
       8.3.2.  Endpoint Address . . . . . . . . . . . . . . . . . . . 32
       8.3.3.  Endpoint Prefixes  . . . . . . . . . . . . . . . . . . 33
       8.3.4.  Endpoint Address Group . . . . . . . . . . . . . . . . 33
     8.4.  Cost Mode  . . . . . . . . . . . . . . . . . . . . . . . . 34
     8.5.  Cost Type  . . . . . . . . . . . . . . . . . . . . . . . . 34
     8.6.  Endpoint Property  . . . . . . . . . . . . . . . . . . . . 35
   9.  Protocol Specification: Service Information Resources  . . . . 35
     9.1.  Map Service  . . . . . . . . . . . . . . . . . . . . . . . 35
       9.1.1.  Network Map  . . . . . . . . . . . . . . . . . . . . . 35
       9.1.2.  Cost Map . . . . . . . . . . . . . . . . . . . . . . . 37
     9.2.  Map Filtering Service  . . . . . . . . . . . . . . . . . . 40
       9.2.1.  Filtered Network Map . . . . . . . . . . . . . . . . . 40
       9.2.2.  Filtered Cost Map  . . . . . . . . . . . . . . . . . . 42
     9.3.  Endpoint Property Service  . . . . . . . . . . . . . . . . 46
       9.3.1.  Endpoint Property  . . . . . . . . . . . . . . . . . . 46
     9.4.  Endpoint Cost Service  . . . . . . . . . . . . . . . . . . 49
       9.4.1.  Endpoint Cost  . . . . . . . . . . . . . . . . . . . . 49
   10. Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     10.1. ALTO Client Embedded in P2P Tracker  . . . . . . . . . . . 54
     10.2. ALTO Client Embedded in P2P Client: Numerical Costs  . . . 55
     10.3. ALTO Client Embedded in P2P Client: Ranking  . . . . . . . 56
   11. Discussions  . . . . . . . . . . . . . . . . . . . . . . . . . 57
     11.1. Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 57
     11.2. Hosts with Multiple Endpoint Addresses . . . . . . . . . . 58



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     11.3. Network Address Translation Considerations . . . . . . . . 58
     11.4. Endpoint and Path Properties . . . . . . . . . . . . . . . 59
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 59
     12.1. application/alto-* Media Types . . . . . . . . . . . . . . 59
     12.2. ALTO Cost Type Registry  . . . . . . . . . . . . . . . . . 60
     12.3. ALTO Endpoint Property Type Registry . . . . . . . . . . . 62
     12.4. ALTO Address Type Registry . . . . . . . . . . . . . . . . 62
     12.5. ALTO Error Code Registry . . . . . . . . . . . . . . . . . 63
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 64
     13.1. Privacy Considerations for ISPs  . . . . . . . . . . . . . 64
     13.2. ALTO Clients . . . . . . . . . . . . . . . . . . . . . . . 64
     13.3. Authentication, Integrity Protection, and Encryption . . . 65
     13.4. ALTO Information Redistribution  . . . . . . . . . . . . . 65
     13.5. Denial of Service  . . . . . . . . . . . . . . . . . . . . 66
     13.6. ALTO Server Access Control . . . . . . . . . . . . . . . . 66
   14. Manageability Considerations . . . . . . . . . . . . . . . . . 66
     14.1. Operations . . . . . . . . . . . . . . . . . . . . . . . . 67
       14.1.1. Installation and Initial Setup . . . . . . . . . . . . 67
       14.1.2. Migration Path . . . . . . . . . . . . . . . . . . . . 67
       14.1.3. Requirements on Other Protocols and Functional
               Components . . . . . . . . . . . . . . . . . . . . . . 67
       14.1.4. Impact and Observation on Network Operation  . . . . . 68
     14.2. Management . . . . . . . . . . . . . . . . . . . . . . . . 68
       14.2.1. Management Interoperability  . . . . . . . . . . . . . 68
       14.2.2. Management Information . . . . . . . . . . . . . . . . 69
       14.2.3. Fault Management . . . . . . . . . . . . . . . . . . . 69
       14.2.4. Configuration Management . . . . . . . . . . . . . . . 69
       14.2.5. Performance Management . . . . . . . . . . . . . . . . 69
       14.2.6. Security Management  . . . . . . . . . . . . . . . . . 70
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 70
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 70
     15.2. Informative References . . . . . . . . . . . . . . . . . . 71
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 73
   Appendix B.  Authors . . . . . . . . . . . . . . . . . . . . . . . 74
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 75
















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1.  Introduction

1.1.  Background and Problem Statement

   Today, network information available to applications is mostly from
   the view of endhosts.  There is no clear mechanism for a network to
   convey to network applications its point of view on its network
   topological structures and path preferences, forcing applications to
   make approximations using data sources such as BGP Looking Glass
   and/or applications' own measurements, which can be misleading or
   inaccurate.  On the other hand, modern network applications can be
   adaptive, with the potential to become more network-efficient (e.g.,
   reduce network resource consumption) and achieve better application
   performance (e.g., accelerated download rate), by leveraging better
   network-provided information.

   This document defines the ALTO protocol to implement the ALTO
   Service, which provides a simple mechanism to convey useful network
   topological and path preference information to applications from the
   underlying network's Provider's point of view.  The ALTO protocol
   meets the ALTO requirements [I-D.ietf-alto-reqs], and unifies
   multiple protocols previously designed with similar intentions (see
   Section 1.2).

   The ALTO protocol uses a REST-ful design [Fielding-Thesis], and
   encodes its requests and responses using JSON [RFC4627].  These
   designs are chosen because of their flexibility and extensibility.
   In addition, these designs make it possible for ALTO to leverage the
   existing HTTP [RFC2616] implementations and infrastructures for
   better, scalable deployment.

1.2.  Design History and Merged Proposals

   The ALTO Protocol specified in this document consists of
   contributions from

   o  P4P [I-D.p4p-framework], [P4P-SIGCOMM08],
      [I-D.wang-alto-p4p-specification];

   o  ALTO Info-Export [I-D.shalunov-alto-infoexport];

   o  Query/Response [I-D.saumitra-alto-queryresponse],
      [I-D.saumitra-alto-multi-ps];

   o  ATTP [ATTP]; and

   o  Proxidor [I-D.akonjang-alto-proxidor].




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   See Appendix A for a list of people who have made significant
   contributions to this effort as well as the aforementioned projects
   and proposals.

1.3.  Solution Benefits

   At a high level, the ALTO Service allows a Service Provider (e.g., an
   ISP) to publish information about network locations and costs between
   them at configurable granularities.

   A mechanism to publish such information can benefit both Service
   Providers (providers of the information) and Applications (consumers
   of the information).  We enumerate some benefits below.

1.3.1.  Service Providers

   Service Providers that use the ALTO Service can benefit in achieving
   better traffic management.  For example, by using ALTO as a tool to
   interact with applications, a Service Provider gives network
   information to applications to manage traffic on more expensive or
   difficult to provision links such as long distance, transit or backup
   links.  This improves the efficiency of provisioning the networking
   infrastructure of the Service Provider.

1.3.2.  Applications

   Applications that use the ALTO Service can benefit in achieving
   better network cooperation and reducing overhead.  Specifically,
   applications taking advantage of the ISP's knowledge can both avoid
   network bottlenecks and boost application performance.  By using ALTO
   information, applications can reduce the reliance on obtaining
   network information through third-party databases.  Applications
   relying on measuring path performance metrics themselves can reduce
   the measurement overhead by conducting only fine-tuning or fault-
   tolerant measurements on top of ALTO information.  A specific example
   application that can use ALTO information is peer-to-peer overlay
   applications who can use ALTO information in peer selection.


2.  Architecture

   We start by introducing the terminology.  Then we define the ALTO
   architecture and the ALTO Protocol's place in the overall
   architecture.







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2.1.  Terminology

   We use the following terms defined in [RFC5693]: Application, Overlay
   Network, Peer, Resource, Resource Identifier, Resource Provider,
   Resource Consumer, Resource Directory, Transport Address, Host
   Location Attribute, ALTO Service, ALTO Server, ALTO Client, ALTO
   Query, ALTO Reply, ALTO Transaction, Local Traffic, Peering Traffic,
   Transit Traffic.

   We also use the following additional terms: Endpoint Address,
   Autonomous System Number (ASN), Network Location, ALTO Information,
   and ALTO Information Base.

2.1.1.  Endpoint

   An Endpoint is an entity that is capable of communicating (sending
   and/or receiving messages) on a network.

   An Endpoint is typically either a Resource Provider or Resource
   Consumer.

2.1.2.  Endpoint Address

   An Endpoint Address represents the communication address of an
   endpoint.  An Endpoint Address can be network-attachment based (IP
   address) or network-attachment agnostic.  Common forms of Endpoint
   Addresses include IP address, MAC address, overlay ID, and phone
   number.

   Each Endpoint Address has an associated Address Type, which indicates
   both its syntax and semantics.

2.1.3.  ASN

   An Autonomous System Number.

2.1.4.  Network Location

   Network Location is a generic term denoting 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.

2.1.5.  ALTO Information

   ALTO Information is a generic term referring to the network
   information sent by an ALTO Server.





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2.1.6.  ALTO Information Base

   Internal representation of the ALTO Information maintained by the
   ALTO Server.  Note that the structure of this internal representation
   is not defined by this document.

2.2.  ALTO Service and Protocol Scope

   Each network region in the global Internet can provide its ALTO
   Service, which conveys 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.

   Specifically, the ALTO Service of a network region defines 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.  Hence, we say that the network
   information provided by the ALTO Service of a network region
   represents the "my-Internet View" of the network region.

   To better understand the ALTO Service and the role of the ALTO
   Protocol, we show in Figure 1 the overall ALTO system architecture.
   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 can be seen as a policy
   which is updated at a larger time-scale.

   Figure 1 illustrates that the ALTO Information provided by an ALTO
   Server may be influenced (at the operator's discretion) by other
   systems.  In particular, the ALTO Server can aggregate information
   from multiple systems to provide an abstract, unified, useful network
   view 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 ALTO may convey dynamic network information, it is not
   intended to replace near-real-time congestion control protocols.

   It may also be possible for an ALTO Server to exchange network
   information with other ALTO Servers (either within the same



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   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.

   +-------------------------------------------------------------------+
   |                         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.

2.3.  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.

   Beyond integration with existing HTTP caching infrastructure, ALTO
   information may also be cached or redistributed using application-
   dependent mechanisms, such as P2P DHTs or P2P file-sharing.  This
   document does not define particular mechanisms for such



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   redistribution.

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

   If caching or redistribution is used, the response message may be
   returned from another (possibly third-party) entity.


3.  ALTO Information Service Framework

   The ALTO Protocol conveys network information through services, where
   each service defines a set of related functionalities.  An ALTO
   Client can query 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.

   In this document, we focus on achieving the goal of conveying Network
   Locations, which denote the locations of Endpoints at a network, and
   provider-defined costs for paths between pairs of Network Locations.
   We achieve the goal by defining the Map Service, which provides the
   core ALTO information to clients, and three additional services: the
   Map Filtering Service, Endpoint Property Service, and Endpoint Cost
   Service.  Additional services can be defined in companion documents.
   Below we give an overview of the services.  Details of the services
   will be presented in the following sections.


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

                     Figure 2: ALTO Service Framework.




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3.1.  ALTO Information Services

3.1.1.  Map Service

   The Map Service provides batch information to ALTO Clients in the
   form of Network Map and Cost Map. The Network Map (See Section 4)
   provides the full set of Network Location groupings defined by the
   ALTO Server and the Endpoints contained with each grouping.  The Cost
   Map (see Section 5) provides costs between the defined groupings.

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

3.1.2.  Map Filtering Service

   Resource constrained ALTO Clients may benefit from query results
   being filtered at the ALTO Server.  This avoids an ALTO Client
   spending network bandwidth or CPU collecting results and performing
   client-side filtering.  The Map Filtering Service allows ALTO Clients
   to query for the ALTO Server Network Map and Cost Map based on
   additional parameters.

3.1.3.  Endpoint Property Service

   This service allows ALTO Clients to look up properties for individual
   Endpoints.  An example endpoint property is its Network Location
   (i.e., its grouping defined by the ALTO Server).  Another endpoint
   property is its connectivity type such as ADSL (Asymmetric Digital
   Subscriber Line), Cable, or FTTH (Fiber To The Home).

3.1.4.  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 either numerical costs or ordinal costs
   (rankings) directly amongst Endpoints.


4.  Network Map

   An ALTO Network Map defines a grouping of network endpoints.  In this
   document, we use Network Map to refer 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.

   The definition of Network Map is based on the observation that in
   reality, many endpoints are close by to one another in terms of



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   network connectivity.  By treating a group of close-by 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 Map).

4.1.  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 US-ASCII string of type
   PIDName (see Section 8.1) and its associated set of Endpoint
   Addresses.  As we 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.

   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
   ISP may prefer that endpoints associated with the same PoP (Point-of-
   Presence) in a P2P application communicate locally instead of
   communicating with endpoints in other PoPs.  The ISP may aggregate
   endhosts within a PoP into a single PID in the 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 5 provides further details on using
   PIDs to represent costs in an ALTO Cost Map.

4.2.  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.  The current specification only
   considers IP addresses.







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4.2.1.  IP Addresses

   When either an ALTO Client or ALTO Server needs to determine which
   PID in a Network Map contains a particular IP address, longest-prefix
   matching MUST be used.

   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.  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.

   Each endpoint MUST map into exactly one PID.  Since longest-prefix
   matching is used to map an endpoint to a PID, this can be
   accomplished by ensuring that no two PIDs contain an identical IP
   prefix.

4.3.  Example Network Map

   Figure 3 illustrates an example Network Map. PIDs are used to
   identify network-agnostic aggregations.

   .-----------------------------------------------------------.
   | ALTO Network Map                                          |
   |                                                           |
   |  .-----------------------------------.  .---------------. |
   |  | NetLoc: PID-1                     |  | NetLoc: PID-2 | |
   |  |  .------------------------------. |  |    ...        | |
   |  |  | 192.0.2.0/24                 | |  `---------------` |
   |  |  | .--------------------------. | |                    |
   |  |  | | Endpoint: 192.0.2.34     | | |  .---------------. |
   |  |  | `--------------------------` | |  | NetLoc: PID-3 | |
   |  |  `------------------------------` |  |    ...        | |
   |  |  .------------------------------. |  `---------------` |
   |  |  | 198.51.100.0/25              | |                    |
   |  |  | .--------------------------. | |  .---------------. |
   |  |  | | Endpoint: 198.51.100.100 | | |  | NetLoc: PID-4 | |
   |  |  | `--------------------------` | |  |    ...        | |
   |  |  `------------------------------` |  `---------------` |
   |  `-----------------------------------`                    |
   |                                                           |
   `-----------------------------------------------------------`

                      Figure 3: Example Network Map.





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5.  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 needs.

   An ALTO Cost Map defines Path Costs pairwise amongst sets of source
   and destination Network Locations defined by PIDs.  Each Path Cost is
   the end-to-end cost from the source to the destination.

   As cost directional from the source to the destination, an
   application, when using ALTO, may independently determine how the
   Resource Consumer and Resource Provider are designated as the source
   or destination, and hence how to utilize the Path Cost provided by
   ALTO.  For example, if the cost is expected to be correlated with
   throughput, a typical application concerned with bulk data retrieval
   may use the Resource Provider as the source, and Resource Consumer as
   the destination.

   One advantage of separating ALTO information into a Network Map and a
   Cost Map is that the two components 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 dynamic network conditions.

   As used in this document, the 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.

5.1.  Cost Attributes

   Path Costs have attributes:

   o  Type: identifies what the costs represent;

   o  Mode: identifies how the costs should be interpreted.

   Certain queries for Cost Maps allow the ALTO Client to indicate the
   desired Type and Mode.

5.1.1.  Cost Type

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

   Cost types are indicated in protocol messages as strings.




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5.1.1.1.  Cost Type: routingcost

   An ALTO Server MUST define the 'routingcost' Cost Type.

   This Cost Type conveys a generic measure for the cost of routing
   traffic from a source to a destination.  Lower values indicate a
   higher preference for traffic to be sent from a source to a
   destination.

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

5.1.2.  Cost Mode

   The Mode attribute indicates how costs should be interpreted.
   Specifically, the 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 'numerical' and 'ordinal'
   costs.  An ALTO Client SHOULD be cognizant of operations when a
   desired cost mode is not supported.  For example, an ALTO Client
   desiring numerical costs may adjust behavior if only the ordinal Cost
   Mode is available.  Alternatively, an ALTO Client desiring ordinal
   costs may construct ordinal costs given numerical values if only the
   numerical Cost Mode is available.

5.1.2.1.  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.

5.1.2.2.  Cost Mode: ordinal

   This Cost Mode is indicated by the string 'ordinal'.  This mode
   indicates that the costs values in a Cost Map are a ranking (relative



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   to all other values in the Cost Map), with lower values indicating a
   higher preference.  The values are non-negative integers.  Ordinal
   cost values in a Cost Map need not be unique nor contiguous.  In
   particular, it is possible that two entries in a 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.

   It is important to note that the values in the Cost Map provided with
   the ordinal Cost Mode are not necessarily the actual cost known to
   the ALTO Server.

5.2.  Cost Map Structure

   A query for a Cost Map either explicitly or implicitly includes 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 query has a list of multiple Source
   Network Locations, say [Src_1, Src_2, ..., Src_m], and a list of
   multiple 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 a Path Cost for a particular pair, it may be omitted.  We
   refer 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.

5.3.  Network Map and Cost Map Dependency

   If a Cost Map contains PIDs in the list of Source Network Locations
   or the list of Destination Network Locations, the Path Costs are
   generated based on a particular Network Map (which defines the PIDs).
   Version Tags are introduced to ensure that ALTO Clients are able to
   use consistent information even though the information is provided in
   two maps.

   A Version Tag is an opaque string associated with a Network Map
   maintained by the ALTO Server.  Two Version Tags match only if their
   strings are the same.  Whenever the content of the Network Map
   maintained by the ALTO Server changes, the Version Tag MUST also be
   changed.  Possibilities for generating a Version Tag include the



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   last-modified timestamp for the Network Map, or a hash of its
   contents, where the collision probability is considered zero in
   practical deployment scenarios.

   A Network Map distributed by the ALTO Server includes its Version
   Tag. A Cost Map referring to PIDs also includes the Version Tag of
   the Network Map on which it is based.


6.  Endpoint Properties

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

6.1.  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.

6.1.1.  Endpoint Property Type: pid

   An ALTO Server MUST define the 'pid' Endpoint Property Type, which
   provides the PID of an endpoint.  Since the PID of an endpoint
   depends on the Network Map, Version Tag of the Network Map used to
   return the pid property MUST be included.


7.  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.

7.1.  Overall Design

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

   o  Information Resources: Each service provides network information
      as a set of resources, which are distinguished by their media
      types [RFC2046].  An ALTO Client may construct an HTTP request for
      a particular resource (including any parameters, if necessary),
      and an ALTO Server returns the requested resource in an HTTP
      response.





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   o  Information Resource Directory: An ALTO Server provides to ALTO
      Clients a list of available resources and the URI at which each is
      provided.  This document refers to this list as the Information
      Resource Directory.  This directory is the single entry point to
      an ALTO Service.  ALTO Clients consult the directory to determine
      the services provided by an ALTO Server.

7.2.  Notation

   This document uses an adaptation of the C-style struct notation to
   define the required and optional members of JSON objects.  Unless
   explicitly noted, each member of a struct is REQUIRED.

   The types 'JSONString', 'JSONNumber', 'JSONBool' indicate the JSON
   string, number, and boolean types, respectively.  'JSONValue'
   indicates a JSON value, as specified in Section 2.1 of [RFC4627].

   Note that no standard, machine-readable interface definition or
   schema is provided.  Extension documents may document these as
   necessary.

7.3.  Basic Operation

   The ALTO Protocol employs standard HTTP [RFC2616].  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 MUST be compliant with
   [RFC2616].

7.3.1.  Discovering Information Resources

   To discover available resources, an ALTO Client requests the
   Information Resource Directory, which an ALTO Server provides at the
   URI found by the ALTO Discovery protocol.

   Informally, an Information Resource Directory enumerates URIs at
   which an ALTO Server offers Information Resources.  Each entry in the
   directory indicates a URI at which an ALTO Server accepts requests,
   and returns either the requested Information Resource or an
   Information Resource Directory that references additional Information
   Resources.  See Section 7.6 for a detailed specification.

7.3.2.  Requesting Information Resources

   Through the retrieved Information Resource Directories, 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



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   available.

   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.

   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.

   It is possible for an ALTO Server to leverage caching HTTP
   intermediaries for responses to both GET and POST requests by
   including explicit freshness information (see Section 14 of
   [RFC2616]).  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.

7.3.3.  Response

   Upon receiving a request, an ALTO server either returns the requested
   resource, provides the ALTO Client an Information Resource Directory
   indicating how to reach the desired resource, or returns an error.

   The type of response MUST be indicated by the media type attached to
   the response (the Content-Type HTTP header).  If an ALTO Client
   receives an Information Resource Directory, it can consult the
   received directory to determine if any of the offered URIs contain
   the desired Information Resource.

   The generic encoding for an Information Resource is specified in
   Section 7.4.

   Errors are indicated via either ALTO-level error codes, or via HTTP
   status codes; see Section 7.7.







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7.3.4.  Client Behavior

7.3.4.1.  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.

7.3.4.2.  Error Conditions

   If an ALTO Client does not successfully receive a desired Information
   Resource from a particular ALTO Server, it 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).  An ALTO Client may also retry
   the request at a later time.

7.3.5.  Authentication and Encryption

   An ALTO Server MUST support SSL/TLS [RFC5246] to implement server
   and/or client authentication, encryption, and/or integrity
   protection.  See [RFC6125] for considerations regarding verification
   of server identity.

7.3.6.  HTTP Cookies

   If cookies are included in an HTTP request received by an ALTO
   Server, they MUST be ignored.

7.3.7.  Parsing

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

7.4.  Information Resource Attributes

   An Information Resource encodes the ALTO Information desired by an
   ALTO Client.  This document specifies multiple Information Resources
   that can be provided by an ALTO Server.  Each Information Resource
   has certain attributes associated with it, including its
   capabilities, the accepted input parameters, and output data format.






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7.4.1.  Capability Advertisement

   An ALTO Server may advertise to an ALTO Client that it supports
   certain capabilities in requests for an Information Resource.  For
   example, if an ALTO Server allows requests for a Cost Map to include
   constraints, it may advertise that it supports this capability.

7.4.2.  Accept Input Parameters

   An ALTO Server may allow an ALTO Client to supply input parameters
   when requesting certain Information Resources.  The format of the
   input parameters (i.e., as contained in the entity body of the HTTP
   POST request) is indicated by the media type [RFC2046].

7.4.3.  Media Type

   An ALTO Server uses Media Type [RFC2046] to uniquely indicate the
   data format of the Information Resource that it returns in the HTTP
   entity body.

7.5.  Information Resource Media Type Encoding

   Though each Information Resource may have a distinct syntax, they are
   designed to have a common structure containing generic ALTO-layer
   metadata about the resource, as well as data itself.

   Specifically, each Information Resource has a single top-level JSON
   object of type InfoResourceEntity:


   object {
     InfoResourceMetaData   meta;    [OPTIONAL]
     [InfoResourceDataType] data;
   } InfoResourceEntity;


   with members:

   meta  meta-information pertaining to the Information Resource

   data  the data contained in the Information Resource

7.5.1.  Meta Information

   Meta information is encoded as a JSON object.  This document does not
   specify any members, but it is defined here as a standard container
   for extensibility.  Specifically, InfoResourceMetaData is defined as:




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   object {
   } InfoResourceMetaData;


7.5.2.  ALTO Information

   The "data" member of the InfoResourceEntity encodes the resource-
   specific data; the structure of this member is detailed later for
   each particular Information Resource.

7.5.3.  Example

   The following is an example of the encoding for an Information
   Resource:


   HTTP/1.1 200 OK
   Content-Length: 40
   Content-Type: application/alto-costmap+json

   {
     "meta" : {},
     "data" : {
       ...
     }
   }


7.6.  Information Resource Directory

   An Information Resource Directory indicates to ALTO Clients which
   Information Resources are made available by an ALTO Server.

   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 an 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 could be
   accomplished by manual configuration or by returning the URI of an
   Information Resource Directory from the ALTO Discovery Protocol
   [I-D.ietf-alto-server-discovery].  For recommendations on how the URI
   may look like, see [I-D.ietf-alto-server-discovery].



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7.6.1.  Media Type

   The media type is "application/alto-directory+json".

7.6.2.  Encoding

   An Information Resource Directory is a JSON object of type
   InfoResourceDirectory:


   object {
     ...
   } Capabilities;

   object {
     JSONString   uri;
     JSONString   media-types<1..*>;
     JSONString   accepts<0..*>;        [OPTIONAL]
     Capabilities capabilities;         [OPTIONAL]
   } ResourceEntry;

   object {
     ResourceEntry resources<0..*>;
   } InfoResourceDirectory;


   where the "resources" array indicates a list of Information Resources
   provided by an ALTO Server.  Note that the list of available
   resources is enclosed in a JSON object for extensibility; future
   protocol versions may specify additional members in the
   InfoResourceDirectory object.

   Any URI endpoint indicated in an Information Resource Directory MAY
   provide a response to an OPTIONS request that is in the format of an
   Information Resource Directory response.  This provides ALTO Clients
   a means to discover resources and capabilities offered by that URI
   endpoint.  ALTO Servers that reply with an HTTP 300 status code
   ("Multiple Choices") SHOULD use the Information Resource Directory
   format in the reply.

   Each entry in the directory 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 and MUST
      be resolved according to section 5 of [RFC3986].





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   media-types  The list of all media types of Information Resources
      (see Section 7.4.3) available via GET or POST requests to the
      corresponding URI or URIs discoverable via the URI.

   accepts  The list of all media types of input parameters (see
      Section 7.4.2) accepted by POST requests to the corresponding URI
      or URIs discoverable via the URI.  If this member is not present,
      it MUST be assumed to be an empty array.

   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
      member 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.

   If an entry has an empty list for "accepts", then the corresponding
   URI MUST support GET requests.  If an entry has a non-empty list for
   "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.

7.6.3.  Example

   The following is an example Information Resource Directory returned
   by an ALTO Server.  In this example, the ALTO Server provides
   additional Network and Cost Maps via a separate subdomain,
   "custom.alto.example.com".  The maps available via this subdomain are
   Filtered Network and Cost Maps as well as pre-generated maps for the
   "hopcount" and "routingcost" Cost Types in the "ordinal" Cost Mode.

   An ALTO Client can discover the maps available by
   "custom.alto.example.com" by successfully performing an OPTIONS
   request to "http://custom.alto.example.com/maps".

   In this example, the ALTO server provides the Endpoint Cost Service
   for Cost Types 'routingcost' and 'hopcount', each available for both
   'numerical' and 'ordinal' mode".


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




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   HTTP/1.1 200 OK
   Content-Length: 1472
   Content-Type: application/alto-directory+json

   {
     "resources" : [
       {
         "uri" : "http://alto.example.com/networkmap",
         "media-types" : [ "application/alto-networkmap+json" ]
       }, {
         "uri" : "http://alto.example.com/costmap/num/routingcost",
         "media-types" : [ "application/alto-costmap+json" ],
         "capabilities" : {
           "cost-modes" : [ "numerical" ],
           "cost-types" : [ "routingcost" ]
         }
       }, {
         "uri" : "http://alto.example.com/costmap/num/hopcount",
         "media-types" : [ "application/alto-costmap+json" ],
         "capabilities" : {
           "cost-modes" : [ "numerical" ],
           "cost-types" : [ "hopcount" ]
         }
       }, {
         "uri" : "http://custom.alto.example.com/maps",
         "media-types" : [
           "application/alto-networkmap+json",
           "application/alto-costmap+json"
         ],
         "accepts" : [
           "application/alto-networkmapfilter+json",
           "application/alto-costmapfilter+json"
         ]
       }, {
         "uri" : "http://alto.example.com/endpointprop/lookup",
         "media-types" : [ "application/alto-endpointprop+json" ],
         "accepts" : [ "application/alto-endpointpropparams+json" ],
         "capabilities" : {
           "prop-types" : [ "pid" ]
         }
       }, {
         "uri" : "http://alto.example.com/endpointcost/lookup",
         "media-types" : [ "application/alto-endpointcost+json" ],
         "accepts" : [ "application/alto-endpointcostparams+json" ],
         "capabilities" : {
           "cost-constraints" : true,
           "cost-modes" : [ "ordinal", "numerical" ],
           "cost-types" : [ "routingcost", "hopcount" ]



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         }
       }
     ]
   }




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








































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   HTTP/1.1 200 OK
   Content-Length: 1001
   Content-Type: application/alto-directory+json

   {
     "resources" : [
       {
         "uri" : "http://custom.alto.example.com/networkmap/filtered",
         "media-types" : [ "application/alto-networkmap+json" ],
         "accepts" : [ "application/alto-networkmapfilter+json" ]
       }, {
         "uri" : "http://custom.alto.example.com/costmap/filtered",
         "media-types" : [ "application/alto-costmap+json" ],
         "accepts" : [ "application/alto-costmapfilter+json" ],
         "capabilities" : {
           "cost-constraints" : true,
           "cost-modes" : [ "ordinal", "numerical" ],
           "cost-types" : [ "routingcost", "hopcount" ]
         }
       }, {
         "uri" : "http://custom.alto.example.com/ord/routingcost",
         "media-types" : [ "application/alto-costmap+json" ],
         "capabilities" : {
           "cost-modes" : [ "ordinal" ],
           "cost-types" : [ "routingcost" ]
         }
       }, {
         "uri" : "http://custom.alto.example.com/ord/hopcount",
         "media-types" : [ "application/alto-costmap+json" ],
         "capabilities" : {
           "cost-modes" : [ "ordinal" ],
           "cost-types" : [ "hopcount" ]
         }
       }
     ]
   }


7.6.4.  Usage Considerations

7.6.4.1.  ALTO Client

   This document specifies no requirements or constraints on ALTO
   Clients with regards to how they process an Information Resource
   Directory to identify the URI corresponding to a desired Information
   Resource.  However, some advice is provided for implementors.

   It is possible that multiple entries in the directory match a desired



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   Information Resource.  For instance, in the example in Section 7.6.3,
   a full Cost Map with "numerical" Cost Mode and "routingcost" Cost
   Type 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.

7.6.4.2.  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".

7.7.  Protocol Errors

   If there is an error processing a request, an 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's entity
   body.

   If no ALTO-layer information is available, an ALTO Server may omit an
   ALTO Error resource from the response.  An appropriate HTTP status
   code MUST be set.

   It is important to note that the HTTP Status Code and ALTO Error Code
   have distinct roles.  An ALTO Error Code provides detailed
   information about why a particular request for an ALTO Resource was
   not successful.  The HTTP status code indicates to HTTP processing
   elements (e.g., intermediaries and clients) how the response should
   be treated.

   An ALTO Client MUST interpret both HTTP Status Code and ALTO Error
   Code.  If the ALTO Error Code indicates an error, the ALTO Client
   should consider that the request has failed.

7.7.1.  Media Type

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







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7.7.2.  Resource Format

   An ALTO Error Resource has the format:


   object {
     JSONString code;
   } ErrorResourceEntity;


   where:

   code  An ALTO Error Code defined in Table 1

7.7.3.  Error Codes

   This document defines ALTO Error Codes to support the error
   conditions needed for purposes of this document.  Additional status
   codes may be defined in companion or extension documents.

   The HTTP status codes corresponding to each ALTO Error Code are
   defined to provide correct behavior with HTTP intermediaries and
   clients.  When an ALTO Server returns a particular ALTO Error Code,
   it MUST indicate one of the corresponding HTTP status codes in
   Table 1 in the HTTP response.

   +-------------------------+-------------+---------------------------+
   | ALTO Error Code         | HTTP Status | Description               |
   |                         | Code(s)     |                           |
   +-------------------------+-------------+---------------------------+
   | E_SYNTAX                | 400         | Parsing error in request  |
   |                         |             | (including identifiers)   |
   | E_JSON_FIELD_MISSING    | 400         | Required field missing    |
   | E_JSON_VALUE_TYPE       | 400         | JSON Value of unexpected  |
   |                         |             | type                      |
   | E_INVALID_COST_MODE     | 400         | Invalid cost mode         |
   | E_INVALID_COST_TYPE     | 400         | Invalid cost type         |
   | E_INVALID_PROPERTY_TYPE | 400         | Invalid property type     |
   +-------------------------+-------------+---------------------------+

                     Table 1: Defined ALTO Error Codes

   If multiple errors are present in a single request (e.g., a request
   uses a JSONString when a JSONInteger 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 technique.



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7.7.4.  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 [RFC2616], a 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.

   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.


8.  Protocol Specification: Basic ALTO Data Types

   This section details the format for particular data values used in
   the ALTO Protocol.

8.1.  PID Name

   A PID Name is encoded as a US-ASCII string.  The string MUST be no
   more than 64 characters, and MUST NOT contain any ASCII character
   below 0x21 or above 0x7E or the '.' separator (0x2E).  The '.'
   separator is reserved for future use and MUST NOT be used unless
   specifically indicated by a companion or extension document.

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

8.2.  Version Tag

   A Version Tag is encoded as a US-ASCII string.  The string MUST be no
   more than 64 characters, and MUST NOT contain any ASCII character
   below 0x21 or above 0x7E.

   The type 'VersionTag' is used in this document to indicate a string
   of this type.






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8.3.  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.

8.3.1.  Address Type

   Address Types are encoded as US-ASCII strings consisting of only
   alphanumeric characters (code points 0x30-0x39, 0x41-0x5A, and 0x61-
   0x7A).  This document defines the address type 'ipv4' to refer to
   IPv4 addresses, and 'ipv6' to refer to IPv6 addresses.  All Address
   Type identifiers appearing in an HTTP request or response with an
   'application/alto-*' media type MUST be registered in the ALTO
   Address Type registry Section 12.4.

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

8.3.2.  Endpoint Address

   Endpoint Addresses are encoded as US-ASCII strings.  The exact
   characters and format depend on the type of endpoint address.

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

8.3.2.1.  IPv4

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

8.3.2.2.  IPv6

   IPv6 Endpoint Addresses are encoded as specified in Section 4 of
   [RFC5952].

8.3.2.3.  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 type.




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   Typed Endpoint Addresses are encoded as US-ASCII strings of the
   format 'AddressType:EndpointAddr' (with the ':' character as a
   separator).  The type 'TypedEndpointAddr' is used to indicate a
   string of this format.

8.3.3.  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 US-ASCII 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.

8.3.3.1.  IPv4

   IPv4 Endpoint Prefixes are encoded as specified in Section 3.1 of
   [RFC4632].

8.3.3.2.  IPv6

   IPv6 Endpoint Prefixes are encoded as specified in Section 7 of
   [RFC5952].

8.3.4.  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 {
     EndpointPrefix [AddressType]<0..*>;
     ...
   } EndpointAddrGroup;


   In particular, an Endpoint Address Group is a JSON object with the
   name of each member being the string corresponding to the address
   type, and the member's corresponding value being a list of prefixes
   of addresses of that type.




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   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"
     ]
   }


8.4.  Cost Mode

   A Cost Mode is encoded as a US-ASCII string.  The string MUST either
   have the value 'numerical' or 'ordinal'.

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

8.5.  Cost Type

   A Cost Type is encoded as a US-ASCII string.  The string MUST be no
   more than 32 characters, and MUST NOT contain characters other than
   alphanumeric characters (code points 0x30-0x39, 0x41-0x5A, and 0x61-
   0x7A), the hyphen ('-', code point 0x2D), or the colon (':', code
   point 0x3A).

   Identifiers prefixed with 'priv:' are reserved for Private Use
   [RFC5226].  Identifiers prefixed with 'exp:' are reserved for
   Experimental use.  For an identifier with the 'priv:' or 'exp:'
   prefix, an additional string (e.g., company identifier or random
   string) MUST follow to reduce potential collisions.  For example, a
   short string after 'exp:' to indicate the starting time of a specific
   experiment is recommended.  All other identifiers appearing in an
   HTTP request or response with an 'application/alto-*' media type MUST
   be registered in the ALTO Cost Types registry Section 12.2.

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







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8.6.  Endpoint Property

   An Endpoint Property is encoded as a US-ASCII string.  The string
   MUST be no more than 32 characters, and MUST NOT contain characters
   other than alphanumeric characters (code points 0x30-0x39, 0x41-0x5A,
   and 0x61-0x7A), the hyphen ('-', code point 0x2D), or the colon (':',
   code point 0x3A).

   Identifiers prefixed with 'priv:' are reserved for Private Use
   [RFC5226].  Identifiers prefixed with 'exp:' are reserved for
   Experimental use.  All other identifiers appearing in an HTTP request
   or response with an 'application/alto-*' media type MUST be
   registered in the ALTO Endpoint Property registry Section 12.3.

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


9.  Protocol Specification: Service Information Resources

   This section documents the individual Information Resources defined
   to provide the services define in this document.

9.1.  Map Service

   The Map Service provides batch information to ALTO Clients in the
   form of two types of maps: a Network Map and Cost Map.

9.1.1.  Network Map

   The Network Map Information Resource lists for each PID, the network
   locations (endpoints) within the PID.  It MUST be provided by an ALTO
   Server.

9.1.1.1.  Media Type

   The media type is "application/alto-networkmap+json".

9.1.1.2.  HTTP Method

   This resource is requested using the HTTP GET method.

9.1.1.3.  Accept Input Parameters

   None.






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9.1.1.4.  Capabilities

   None.

9.1.1.5.  Response

   The returned InfoResourceEntity object "data" member of type
   InfoResourceNetworkMap:


   object {
     EndpointAddrGroup [pidname]<0..*>;
     ...
   } NetworkMapData;

   object {
     VersionTag     map-vtag;
     NetworkMapData map;
   } InfoResourceNetworkMap;


   with members:

   map-vtag  The Version Tag (Section 5.3) of the Network Map.

   map  The Network Map data itself.

   NetworkMapData is a JSON object with each member representing a
   single PID and its associated set of endpoint addresses.  A member's
   name is a string of type PIDName.

   The returned Network Map MUST include all PIDs known to the ALTO
   Server.

9.1.1.6.  Example


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











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   HTTP/1.1 200 OK
   Content-Length: 370
   Content-Type: application/alto-networkmap+json

   {
     "meta" : {},
     "data" : {
       "map-vtag" : "1266506139",
       "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"
           ]
         }
       }
     }
   }


9.1.2.  Cost Map

   The Cost Map resource lists the Path Cost for each pair of source/
   destination PID defined by the ALTO Server for a given Cost Type and
   Cost Mode.  This resource MUST be provided for at least the
   'routingcost' Cost Type and 'numerical' Cost Mode.

   Note that since this resource, an unfiltered Cost Map requested by an
   HTTP GET, does not indicate the desired Cost Mode or Cost Type as
   input parameters, an ALTO Server MUST indicate in an Information
   Resource Directory a unfiltered Cost Map Information Resource by
   specifying the capabilities (Section 9.1.2.4) with "cost-types" and
   "cost-modes" members each having a single element.  This technique
   will allow an ALTO Client to determine a URI for an unfiltered Cost
   Map of the desired Cost Mode and Cost Type.



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9.1.2.1.  Media Type

   The media type is "application/alto-costmap+json".

9.1.2.2.  HTTP Method

   This resource is requested using the HTTP GET method.

9.1.2.3.  Accept Input Parameters

   None.

9.1.2.4.  Capabilities

   This resource may be defined for across multiple Cost Types and Cost
   Modes.  The capabilities of an ALTO Server URI providing this
   resource are defined by a JSON Object of type CostMapCapability:


   object {
     CostMode cost-modes<0..*>;
     CostType cost-types<0..*>;
   } CostMapCapability;


   with members:

   cost-modes  The Cost Modes ( Section 5.1.2) supported by the
      corresponding URI.  If not present, this member MUST be
      interpreted as an empty array.

   cost-types  The Cost Types ( Section 5.1.1) supported by the
      corresponding URI.  If not present, this member MUST be
      interpreted as an empty array.

   An ALTO Server MUST support all of the Cost Types listed here for
   each of the listed Cost Modes.  Note that an ALTO Server may provide
   multiple Cost Map Information Resources, each with different
   capabilities.

9.1.2.5.  Response

   The returned InfoResourceEntity object has "data" member of type
   InfoResourceCostMap:







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   object DstCosts {
     JSONValue [PIDName];
     ...
   };

   object {
     DstCosts [PIDName]<0..*>;
     ...
   } CostMapData;

   object {
     CostMode    cost-mode;
     CostType    cost-type;
     VersionTag  map-vtag;
     CostMapData map;
   } InfoResourceCostMap;


   with members:

   cost-mode  Cost Mode (Section 5.1.2) used in the Cost Map.

   cost-type  Cost Type (Section 5.1.1) used in the Cost Map.

   map-vtag  The Version Tag (Section 5.3) of the Network Map used to
      generate the Cost Map.

   map  The Cost Map data itself.

   CostMapData is a JSON object with each member representing a single
   Source PID; the name for a member is the PIDName string identifying
   the corresponding Source PID.  For each Source PID, a DstCosts object
   denotes the associated cost to a set of destination PIDs (
   Section 5.2); the name for each member in the object is the PIDName
   string identifying the corresponding Destination PID.  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 a Path Cost is not defined (e.g.,
   both the Source and Destination PIDs contain addresses outside of the
   Network Provider's administrative domain).






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9.1.2.6.  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: 262
   Content-Type: application/alto-costmap+json

   {
     "meta" : {},
     "data" : {
       "cost-mode" : "numerical",
       "cost-type" : "routingcost",
       "map-vtag"  : "1266506139",
       "map" : {
         "PID1": { "PID1": 1,  "PID2": 5,  "PID3": 10 },
         "PID2": { "PID1": 5,  "PID2": 1,  "PID3": 15 },
         "PID3": { "PID1": 20, "PID2": 15  }
       }
     }
   }


9.2.  Map Filtering Service

   The Map Filtering Service allows ALTO Clients to specify filtering
   criteria to return a subset of the full maps available in the Map
   Service.

9.2.1.  Filtered Network Map

   A Filtered Network Map is a Network Map Information Resource
   (Section 9.1.1) for which an ALTO Client may supply a list of PIDs to
   be included.  A Filtered Network Map MAY be provided by an ALTO
   Server.

9.2.1.1.  Media Type

   See Section 9.1.1.1.






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9.2.1.2.  HTTP Method

   This resource is requested using the HTTP POST method.

9.2.1.3.  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 members:

   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 list of address types to be included in the
      returned Filtered Network Map. If 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.

9.2.1.4.  Capabilities

   None.

9.2.1.5.  Response

   See Section 9.1.1.5 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 which were
   specified (implicitly or explicitly) in the request.  If the input
   parameters contain an address type that is not currently known to the



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   ALTO Server, the ALTO Server MUST behave as if the address type did
   not appear in the input parameters.

9.2.1.6.  Example


   POST /networkmap/filtered HTTP/1.1
   Host: custom.alto.example.com
   Content-Length: 27
   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: 255
   Content-Type: application/alto-networkmap+json

   {
     "meta" : {},
     "data" : {
       "map-vtag" : "1266506139",
       "map" : {
         "PID1" : {
           "ipv4" : [
             "192.0.2.0/24",
             "198.51.100.0/24"
           ]
         },
         "PID2" : {
           "ipv4": [
             "198.51.100.128/24"
           ]
         }
       }
     }
   }


9.2.2.  Filtered Cost Map

   A Filtered Cost Map is a Cost Map Information Resource
   (Section 9.1.2) for which an ALTO Client may supply additional
   parameters limiting the scope of the resulting Cost Map. A Filtered
   Cost Map MAY be provided by an ALTO Server.



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9.2.2.1.  Media Type

   See Section 9.1.2.1.

9.2.2.2.  HTTP Method

   This resource is requested using the HTTP POST method.

9.2.2.3.  Accept Input Parameters

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

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


   with members:

   cost-type  The Cost Type ( Section 5.1.1) for the returned costs.
      This MUST be one of the supported Cost Types indicated in this
      resource's capabilities ( Section 9.2.2.4).

   cost-mode  The Cost Mode ( Section 5.1.2) for the returned costs.
      This MUST be one of the supported Cost Modes indicated in this
      resource's capabilities ( Section 9.2.2.4).

   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 ( Section 9.2.2.4)
      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;
      (2) a target cost value.  The cost value is a number that MUST be
      defined in the same units as the Cost Type indicated by the cost-



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      type 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"
      member is not present, both lists MUST be interpreted by the ALTO
      Server as containing the full set of currently-defined PIDs.

9.2.2.4.  Capabilities

   The URI providing this resource supports all capabilities documented
   in Section 9.1.2.4 (with identical semantics), plus additional
   capabilities.  In particular, the capabilities are defined by a JSON
   object of type FilteredCostMapCapability:


   object {
     CostMode cost-modes<0..*>;
     CostType cost-types<0..*>;
     JSONBool cost-constraints;
   } FilteredCostMapCapability;


   with members:

   cost-modes  See Section 9.1.2.4.

   cost-types  See Section 9.1.2.4.

   cost-constraints  If true, then the ALTO Server allows cost
      constraints to be included in requests to the corresponding URI.
      If not present, this member 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.








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9.2.2.5.  Response

   See Section 9.1.2.5  for the format.

   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.

   Note that ALTO Clients should verify that the Version Tag included in
   the response is consistent with 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.































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9.2.2.6.  Example


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

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


   HTTP/1.1 200 OK
   Content-Length: 177
   Content-Type: application/alto-costmap+json

   {
     "meta" : {},
     "data" : {
       "cost-mode" : "numerical",
       "cost-type" : "routingcost",
       "map-vtag" : "1266506139",
       "map" : {
         "PID1": { "PID1": 0,  "PID2": 1,  "PID3": 2 }
       }
     }
   }


9.3.  Endpoint Property Service

   The Endpoint Property Service provides information about Endpoint
   properties to ALTO Clients.

9.3.1.  Endpoint Property

   The Endpoint Property resource provides information about properties
   for individual endpoints.  It MAY be provided by an ALTO Server.  If
   an ALTO Server provides one or more Endpoint Property resources, then
   at least one MUST provide the 'pid' property.





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9.3.1.1.  Media Type

   The media type is "application/alto-endpointprop+json".

9.3.1.2.  HTTP Method

   This resource is requested using the HTTP POST method.

9.3.1.3.  Accept Input Parameters

   An ALTO Client supplies the endpoint properties to be queried through
   a media type "application/alto-endpointpropparams+json", and
   specifies in the HTTP POST entity body a JSON Object of type
   ReqEndpointProp:


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


   with members:

   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
      (Section 9.3.1.4).  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.

9.3.1.4.  Capabilities

   This resource may be defined across multiple types of endpoint
   properties.  The capabilities of an ALTO Server URI providing
   Endpoint Properties are defined by a JSON Object of type
   EndpointPropertyCapability:


   object {
     EndpointPropertyType prop-types<0..*>;
   } EndpointPropertyCapability;


   with members:



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   prop-types  The Endpoint Properties (see Section 8.6) supported by
      the corresponding URI.  If not present, this member MUST be
      interpreted as an empty array.

9.3.1.5.  Response

   The returned InfoResourceEntity object has "data" member of type
   InfoResourceEndpointProperty, where:


   object {
     JSONValue [EndpointPropertyType];
     ...
   } EndpointProps;

   object {
     EndpointProps  [TypedEndpointAddr]<0..*>;
     ...
   } EndpointPropertyMapData;

   object {
     VersionTag              map-vtag;
     EndpointPropertyMapData map;
   } InfoResourceEndpointProperty;


   EndpointPropertyMapData 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 EndpointProperty.  An implementation
   of the protocol in 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.

   The ALTO Server MAY include the Version Tag (Section 5.3) of the
   Network Map used to generate the response (if desired and applicable)
   as the 'map-vtag' member in the response.  If the 'pid' property is
   returned for any endpoints in the response, the 'map-vtag' member is



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   REQUIRED.  Otherwise, it is OPTIONAL.

9.3.1.6.  Example


  POST /endpointprop/lookup HTTP/1.1
  Host: alto.example.com
  Content-Length: 96
  Content-Type: application/alto-endpointpropparams+json
  Accept: application/alto-endpointprop+json,application/alto-error+json

  {
    "properties" : [ "pid", "example-prop" ],
    "endpoints" : [ "ipv4:192.0.2.34", "ipv4:203.0.113.129" ]
  }


  HTTP/1.1 200 OK
  Content-Length: 149
  Content-Type: application/alto-endpointprop+json

  {
    "meta" : {},
    "data": {
      "map-vtag" : "1266506139",
      "map" : {
        "ipv4:192.0.2.34"    : { "pid": "PID1", "example-prop": "1" },
        "ipv4:203.0.113.129" : { "pid": "PID3" }
      }
    }
  }


9.4.  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.

9.4.1.  Endpoint Cost

   The Endpoint Cost resource provides information about costs between
   individual endpoints.  It MAY be provided by an ALTO Server.

   It is important to note that although this resource allows an ALTO



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   Server to reveal costs between individual endpoints, an ALTO Server
   is not required to do so.  A simple alternative would be to compute
   the cost between two endpoints as the cost between the PIDs
   corresponding to the endpoints.  See Section 13.1 for additional
   details.

9.4.1.1.  Media Type

   The media type is "application/alto-endpointcost+json".

9.4.1.2.  HTTP Method

   This resource is requested using the HTTP POST method.

9.4.1.3.  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 {
     TypedEndpointAddr srcs<0..*>;          [OPTIONAL]
     TypedEndpointAddr dsts<1..*>;
   } EndpointFilter;

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


   with members:

   cost-mode  The Cost Mode ( Section 5.1.2) to use for returned costs.
      This MUST be one of the Cost Modes indicated in this resource's
      capabilities ( Section 9.4.1.4).

   cost-type  The Cost Type ( Section 5.1.1) to use for returned costs.
      This MUST be one of the Cost Types indicated in this resource's
      capabilities ( Section 9.4.1.4).

   constraints  Defined equivalently to the "constraints" input
      parameter of a Filtered Cost Map (see Section 9.2.2).





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   endpoints  A list of Source Endpoints and Destination Endpoints for
      which Path Costs are to be returned.  If the list of Source
      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 11.3 for discussion and considerations regarding this
      mode).  The list of destination Endpoints MUST NOT be empty.  The
      ALTO Server MUST interpret entries appearing multiple times in a
      list as if they appeared only once.

9.4.1.4.  Capabilities

   See Section 9.2.2.4.

9.4.1.5.  Response

   The returned InfoResourceEntity object has "data" member equal to
   InfoResourceEndpointCostMap, where:


   object EndpointDstCosts {
     JSONValue [TypedEndpointAddr];
     ...
   };

   object {
     EndpointDstCosts [TypedEndpointAddr]<0..*>;
     ...
   } EndpointCostMapData;

   object {
     CostMode            cost-mode;
     CostType            cost-type;
     EndpointCostMapData map;
   } InfoResourceEndpointCostMap;


   InfoResourceEndpointCostMap has members:

   cost-mode  The Cost Mode used in the returned Cost Map.

   cost-type  The Cost Type used in the returned Cost Map.

   map  The Endpoint Cost Map data itself.

   EndpointCostMapData is a JSON object with each member representing a
   single Source Endpoint specified in the input parameters; the name
   for a member is the TypedEndpointAddr string identifying the



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   corresponding Source Endpoint.  For each Source Endpoint, a
   EndpointDstCosts object denotes the associated cost to each
   Destination Endpoint specified in the input parameters; the name for
   each member in the object is the TypedEndpointAddr string identifying
   the corresponding Destination Endpoint.  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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9.4.1.6.  Example


  POST /endpointcost/lookup HTTP/1.1
  Host: alto.example.com
  Content-Length: 195
  Content-Type: application/alto-endpointcostparams+json
  Accept: application/alto-endpointcost+json,application/alto-error+json

  {
    "cost-mode" : "ordinal",
    "cost-type" : "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: 231
  Content-Type: application/alto-endpointcost+json

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



10.  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



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   environments such as CDNs [I-D.jenkins-alto-cdn-use-cases].

10.1.  ALTO Client Embedded in P2P Tracker

   Many currently-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 (the Network Map and Cost Map) 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 the Network Map covering all PIDs from
       the ALTO Server using the Network Map query.  The Network Map
       includes the IP prefixes contained in each PID, allowing the P2P
       tracker to locally map P2P clients into a PIDs.





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   2.  The P2P Tracker requests the Cost Map amongst all PIDs from the
       ALTO Server.

   3.  A P2P Client joins the swarm, and requests a peer list from the
       P2P Tracker.

   4.  The P2P Tracker returns a peer list to the P2P client.  The
       returned peer list is computed based on the Network Map and 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.

10.2.  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 an 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.



















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   .---------.   (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:

   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 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.

10.3.  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
   gathers a list of known peers in the swarm, and asks the ALTO Server
   to rank them.

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








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   .---------.                          .---------------.
   |         |                          |               |
   |  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

   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,
       including discovered peers as the set of Destination Endpoints,
       and indicates 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.


11.  Discussions

11.1.  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 7.6) to locate an Information
   Resource with the desired ALTO Information.







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11.2.  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.

11.3.  Network Address Translation Considerations

   At this day and age of NAT v4<->v4, v4<->v6 [RFC6144], and possibly
   v6<->v6[I-D.mrw-nat66], 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 provide 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 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 where 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.




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   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.

11.4.  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.


12.  IANA Considerations

12.1.  application/alto-* Media Types

   This document requests the registration of multiple media types,
   listed in Table 2.

      +-------------+------------------------------+---------------+
      | Type        | Subtype                      | Specification |
      +-------------+------------------------------+---------------+
      | application | alto-directory+json          | Section 7.6   |
      | application | alto-networkmap+json         | Section 9.1.1 |
      | application | alto-networkmapfilter+json   | Section 9.2.1 |
      | application | alto-costmap+json            | Section 9.1.2 |
      | application | alto-costmapfilter+json      | Section 9.2.2 |
      | application | alto-endpointprop+json       | Section 9.3.1 |
      | application | alto-endpointpropparams+json | Section 9.3.1 |
      | application | alto-endpointcost+json       | Section 9.4.1 |
      | application | alto-endpointcostparams+json | Section 9.4.1 |
      | application | alto-error+json              | Section 7.7   |
      +-------------+------------------------------+---------------+

                    Table 2: ALTO Protocol Media Types

   Type name:  application

   Subtype name:  This documents requests the registration of multiple
      subtypes, as listed in Table 2.





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   Required parameters:  n/a

   Optional parameters:  n/a

   Encoding considerations:  Encoding considerations are identical to
      those specified for the 'application/json' media type.  See
      [RFC4627].

   Security considerations:  Security considerations relating to the
      generation and consumption of ALTO protocol messages are discussed
      in Section 13.

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

   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 standalone or 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).

12.2.  ALTO Cost Type Registry

   This document requests the creation of an ALTO Cost Type registry,
   listed in Table 3, to be maintained by IANA.




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                   +-------------+---------------------+
                   | Identifier  | Intended Semantics  |
                   +-------------+---------------------+
                   | routingcost | See Section 5.1.1.1 |
                   | priv:       | Private use         |
                   | exp:        | Experimental use    |
                   +-------------+---------------------+

                         Table 3: ALTO Cost Types.

   This registry serves two purposes.  First, it ensures uniqueness of
   identifiers referring to ALTO Cost Types.  Second, it provides
   references to particular semantics of allocated Cost Types to be
   applied by both ALTO Servers and applications utilizing ALTO Clients.

   New ALTO Cost Types are assigned after Expert Review [RFC5226].  The
   Expert Reviewer will generally consult the ALTO Working Group or its
   successor.  Expert Review is used to ensure that proper documentation
   regarding ALTO Cost Type semantics and security considerations has
   been provided.  The provided documentation 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
   Type should be interpreted.  Updates and deletions of ALTO Cost Types
   follow the same procedure.

   Registered ALTO Cost Type identifiers MUST conform to the syntactical
   requirements specified in Section 8.5.  Identifiers are to be
   recorded and displayed as ASCII strings.

   Identifiers prefixed with 'priv:' are reserved for Private Use.
   Identifiers prefixed with 'exp:' are reserved for Experimental use.

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

   o  Identifier: The name of the desired ALTO Cost Type.

   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 Type may
      require certain information to be exposed by an ALTO Service
      Provider.  Since network information is frequently regarded as



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      proprietary or confidential, ALTO Service Providers should be made
      aware of the security ramifications related to usage of a Cost
      Type.

   This specification requests registration of the identifier
   'routingcost'.  Semantics for the this Cost Type are documented in
   Section 5.1.1.1, and security considerations are documented in
   Section 13.1.

12.3.  ALTO Endpoint Property Type Registry

   This document requests the creation of an ALTO Endpoint Property
   Types registry, listed in Table 4, to be maintained by IANA.

                    +------------+--------------------+
                    | Identifier | Intended Semantics |
                    +------------+--------------------+
                    | pid        | See Section 6.1.1  |
                    | priv:      | Private use        |
                    | exp:       | Experimental use   |
                    +------------+--------------------+

                  Table 4: ALTO Endpoint Property Types.

   The maintenance of this registry is similar to that of the preceding
   ALTO Cost Types.

12.4.  ALTO Address Type Registry

   This document requests the creation of an ALTO Address Type registry,
   listed in Table 5, to be maintained by IANA.

   +------------+----------------+----------------+--------------------+
   | Identifier | Address        | Prefix         | Mapping to/from    |
   |            | Encoding       | Encoding       | IPv4/v6            |
   +------------+----------------+----------------+--------------------+
   | ipv4       | See            | See            | Direct mapping to  |
   |            | Section 8.3.2  | Section 8.3.3  | IPv4               |
   | ipv6       | See            | See            | Direct mapping to  |
   |            | Section 8.3.2  | Section 8.3.3  | 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.




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   New ALTO Address Types are assigned after Expert Review [RFC5226].
   The Expert Reviewer will generally consult the ALTO Working Group or
   its successor.  Expert Review is used to ensure that proper
   documentation regarding the new ALTO Address Types and their security
   considerations has been provided.  The provided documentation 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 8.3.1.  Identifiers are
   to be recorded and displayed as ASCII 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 8.3.2).

   o  Endpoint Prefix Encoding: The procedure for encoding a set of
      addresses of the registered type as an EndpointPrefix (see
      Section 8.3.3).  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.

   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.

12.5.  ALTO Error Code Registry

   This document requests the creation of an ALTO Error Code registry,
   listed in Table 1, to be maintained by IANA.



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13.  Security Considerations

13.1.  Privacy Considerations for ISPs

   ISPs must be cognizant of the network topology and provisioning
   information provided through ALTO Interfaces.  ISPs should evaluate
   how much information is revealed and the associated risks.  On the
   one hand, providing overly fine-grained information may make it
   easier for attackers to infer network topology.  In particular,
   attackers may try to infer details regarding ISPs' operational
   policies or inter-ISP business relationships by intentionally posting
   a multitude of selective queries to an ALTO server and analyzing the
   responses.  Such sophisticated attacks may reveal more information
   than an ISP hosting an ALTO server intends to disclose.  On the other
   hand, revealing overly coarse-grained information may not provide
   benefits to network efficiency or performance improvements to ALTO
   Clients.

   It is possible that one or multiple ALTO Clients issue queries in an
   effort to reverse-engineer specific details (e.g., network topology)
   that was used to produce ALTO information.  Operators should have
   security policies in place such that confidential information or
   information that could be reverse-engineered to reveal confidential
   information is not sent to unauthorized ALTO Clients.

   ISPs must also be cognizant that ALTO may reveal additional
   information about IP addresses and associated information about it.
   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.

13.2.  ALTO Clients

   Applications using the information must be cognizant of the
   possibility that the information is malformed or incorrect.  Even if
   an ALTO Server has been properly authenticated by the ALTO Client,
   the information provided may be malicious because the ALTO Server and
   its credentials have been compromised (e.g., through malware).  Other
   considerations (e.g., relating to application performance) can be
   found in Section 6 of [RFC5693].

   ALTO Clients should also be cognizant of revealing Network Location
   Identifiers (IP addresses or fine-grained PIDs) to the ALTO Server,
   as doing so may allow the ALTO Server to infer communication
   patterns.  As an ALTO Server may collect information from multiple
   client queries, the server may deduce additional application/content
   information through correlation.  One possibility is for the ALTO
   Client to only rely on Network Map for PIDs and Cost Map amongst PIDs



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   to avoid passing IP addresses of other endpoints (e.g., peers) to the
   ALTO Server.

   In addition, ALTO clients should be cautious not to unintentionally
   or indirectly disclose the resource identifier (of which they try to
   improve the retrieval through ALTO-guidance), e.g., the name/
   identifier of a certain video stream in P2P live streaming, to the
   ALTO server.  Note that the ALTO Protocol specified in this document
   does not explicitly reveal any resource identifier to the ALTO
   Server.  However, for instance, depending on the popularity or other
   specifics (such as language) of the resource, an ALTO server could
   potentially deduce information about the desired resource from
   information such as the Network Locations the client sends as part of
   its request to the server.

13.3.  Authentication, Integrity Protection, and Encryption

   SSL/TLS [RFC5246] can provide encryption and integrity protection of
   transmitted messages as well as authentication of the ALTO Client and
   Server.  HTTP Basic or Digest authentication can provide
   authentication of the client (combined with SSL/TLS, it can
   additionally provide encryption, integrity protection and server
   authentication).

   Issues resulting from an attacker controlling the data received by an
   ALTO Client are discussed in Section 13.2.

   An ALTO Server may optionally use authentication (and potentially
   encryption) to limit the parties with whom ALTO information is
   directly shared.  There may be special use cases where encryption of
   ALTO information is desirable.  In many cases, however, information
   sent out by an ALTO Server may be regarded as non-confidential
   information.

   ISPs 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.

13.4.  ALTO Information Redistribution

   It is possible for applications to redistribute ALTO information to
   improve scalability.  Even with such a distribution scheme, ALTO
   Clients obtaining ALTO information must be able to validate the
   received ALTO information to ensure that it was generated by an
   appropriate ALTO Server.  Support for this validation is not provided
   in this document, but may be provided by extension documents.



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13.5.  Denial of Service

   ISPs should be cognizant of the workload at the ALTO Server generated
   by certain ALTO Queries, such as certain queries to the Map Filtering
   Service and Ranking Service.  In particular, queries which 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).  One way to limit Denial-of-Service
   attacks is to employ access control to the ALTO server.  The ALTO
   server can also indicate overload.  Yet another possible mechanism
   for an ALTO Server to protect itself against a multitude of
   computationally expensive bogus requests is to demand that each ALTO
   Client to solve a computational puzzle first before allocating
   resources for answering a request (see, e.g.,
   [I-D.jennings-sip-hashcash]).  The current specification does not use
   such computational puzzles, and discussion regarding tradeoffs of
   such an approach would be needed before including such a technique in
   the ALTO Protocol.

   ISPs should also leverage the fact that the the Map Service allows
   ALTO Servers to pre-generate maps that can be useful to many ALTO
   Clients.

13.6.  ALTO Server Access Control

   In order to limit access to an ALTO server (e.g., for an ISP to only
   allow its users to access its ALTO server, or to prevent Denial-of-
   Service attacks by arbitrary hosts from the Internet), an ALTO server
   may employ access control policies.  Depending on the use-case and
   scenario, an ALTO server may restrict access to its services more
   strictly or rather openly (see [I-D.ietf-alto-deployments] for a more
   detailed discussion on this issue).


14.  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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14.1.  Operations

14.1.1.  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 operator of an ALTO Server will need to configure
   the ALTO information to be provided by the ALTO Server.  The
   granularity of the topological map and the cost map is left to the
   specific policies of the operator of the ALTO Server.  However, a
   reasonable default may include two PIDs, one to hold the endpoints in
   the operator's network and the second PID to represent full IPv4 and
   IPv4 reachability (see Section 4.2.1), with the cost between each
   source/destination PID set to 1.  Another operational issue that the
   operator of an ALTO Server 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 operational impact 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 [I-D.ietf-alto-server-discovery].

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

14.1.2.  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.

14.1.3.  Requirements 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



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   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., [I-D.medved-alto-svr-apis]) and are expected to be
   provided in extension documents.

14.1.4.  Impact and Observation on Network Operation

   ALTO presents a new opportunity for managing network traffic by
   providing additional information to clients.  The potential impact to
   network operation is large.

   Deployment of an ALTO Server may shift network traffic patterns.
   Thus, operators should consider impacts on (or integration with)
   traffic engineering and the deployment of a monitoring service to
   observe the effects of ALTO operations.  Note that ALTO-specific
   monitoring and metrics are discussed in 6.3 of
   [I-D.ietf-alto-deployments] and future versions of that document.  In
   particular, operators may observe that ALTO Clients are not bound to
   ALTO Server guidance as ALTO is only one source of information.

   Operators providing an ALTO Server should ensure that appropriate
   information is being exposed.  Privacy implications for ISPs are
   discussed in Section 13.1.  Both operators and ALTO Servers and those
   using ALTO Clients should be aware of the impact of incorrect or
   faked guidance (see Section 10.3 of [I-D.ietf-alto-deployments] and
   future versions of that document).

14.2.  Management

14.2.1.  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].






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14.2.2.  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.

14.2.3.  Fault Management

   Monitoring ALTO Servers and Clients is described in Section 6.3 of
   [I-D.ietf-alto-deployments] and future versions of that document.

14.2.4.  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 is derived.  This can
      either be raw network information (e.g., from routing elements) or
      pre-processed ALTO-level information in the form of a Network Map,
      Cost Map, etc.

   o  Algorithms for computing the ALTO information returned to clients.
      These could either return 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.

   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,
   SSL/TLS client and server authentication, SSL/TLS encryption
   parameters) intended to serve the same information should be
   monitored for consistency.

14.2.5.  Performance Management

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




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   o  Requests and responses for each service listed in a 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)

14.2.6.  Security Management

   Section 13 documents ALTO-specific security considerations.
   Operators should configure security policies with those in mind.
   Readers should refer to HTTP [RFC2616] and SSL/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.


15.  References

15.1.  Normative References

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

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

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.




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   [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.

15.2.  Informative References

   [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.

   [I-D.akonjang-alto-proxidor]
              Akonjang, O., Feldmann, A., Previdi, S., Davie, B., and D.
              Saucez, "The PROXIDOR Service",
              draft-akonjang-alto-proxidor-00 (work in progress),
              March 2009.

   [I-D.gu-alto-redistribution]
              Yingjie, G., Alimi, R., and R. Even, "ALTO Information
              Redistribution", draft-gu-alto-redistribution-03 (work in
              progress), July 2010.




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   [I-D.ietf-alto-deployments]
              Stiemerling, M., Kiesel, S., and S. Previdi, "ALTO
              Deployment Considerations", draft-ietf-alto-deployments-06
              (work in progress), February 2013.

   [I-D.ietf-alto-reqs]
              Previdi, S., Stiemerling, M., Woundy, R., and Y. Yang,
              "Application-Layer Traffic Optimization (ALTO)
              Requirements", draft-ietf-alto-reqs-08 (work in progress),
              March 2011.

   [I-D.ietf-alto-server-discovery]
              Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M., and
              S. Yongchao, "ALTO Server Discovery",
              draft-ietf-alto-server-discovery-07 (work in progress),
              January 2013.

   [I-D.jenkins-alto-cdn-use-cases]
              Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
              S. Previdi, "Use Cases for ALTO within CDNs",
              draft-jenkins-alto-cdn-use-cases-03 (work in progress),
              June 2012.

   [I-D.jennings-sip-hashcash]
              Jennings, C., "Computational Puzzles for SPAM Reduction in
              SIP", draft-jennings-sip-hashcash-06 (work in progress),
              July 2007.

   [I-D.medved-alto-svr-apis]
              Medved, J., Ward, D., Peterson, J., Woundy, R., and D.
              McDysan, "ALTO Network-Server and Server-Server APIs",
              draft-medved-alto-svr-apis-00 (work in progress),
              March 2011.

   [I-D.mrw-nat66]
              Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", draft-mrw-nat66-16 (work in progress),
              April 2011.

   [I-D.p4p-framework]
              Alimi, R., Pasko, D., Popkin, L., Wang, Y., and Y. Yang,
              "P4P: Provider Portal for P2P Applications",
              draft-p4p-framework-00 (work in progress), November 2008.

   [I-D.saumitra-alto-multi-ps]
              Das, S., Narayanan, V., and L. Dondeti, "ALTO: A Multi
              Dimensional Peer Selection Problem",
              draft-saumitra-alto-multi-ps-00 (work in progress),



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              October 2008.

   [I-D.saumitra-alto-queryresponse]
              Das, S. and V. Narayanan, "A Client to Service Query
              Response Protocol for ALTO",
              draft-saumitra-alto-queryresponse-00 (work in progress),
              March 2009.

   [I-D.shalunov-alto-infoexport]
              Shalunov, S., Penno, R., and R. Woundy, "ALTO Information
              Export Service", draft-shalunov-alto-infoexport-00 (work
              in progress), October 2008.

   [I-D.wang-alto-p4p-specification]
              Wang, Y., Alimi, R., Pasko, D., Popkin, L., and Y. Yang,
              "P4P Protocol Specification",
              draft-wang-alto-p4p-specification-00 (work in progress),
              March 2009.

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

   [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.


Appendix A.  Acknowledgments

   Thank you to Jan Seedorf for contributions to the Security
   Considerations section.

   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.),




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      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 (AT&T), 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 (AT&T),
   Ingmar Poese (DT Labs/TU Berlin), James Royalty (Pando Networks),
   Damien Saucez (UCL) Thomas Scholl (AT&T), Emilio Sepulveda
   (Telefonica), Avi Silberschatz (Yale University), Hassan Sipra (Bell
   Canada), Georgios Smaragdakis (DT Labs/TU Berlin), Haibin Song
   (Huawei), Oliver Spatscheck (AT&T), See-Mong Tang (Microsoft), Jia
   Wang (AT&T), Hao Wang (Yale University), Ye Wang (Yale University),
   Haiyong Xie (Yale University).


Appendix B.  Authors

   [[CmtAuthors: RFC Editor: Please move information in this section to
   the Authors' Addresses section at publication time.]]

   Stefano Previdi
   Cisco

   Email: sprevidi@cisco.com


   Stanislav Shalunov
   BitTorrent

   Email: shalunov@bittorrent.com





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   Richard Woundy
   Comcast

   Richard_Woundy@cable.comcast.com


Authors' Addresses

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

   Email: ralimi@google.com


   Reinaldo Penno (editor)
   Cisco Systems
   170 West Tasman Dr
   San Jose  CA
   USA

   Email: repenno@cisco.com


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

   Email: yry@cs.yale.edu


















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