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ALTO WG                                                    R. Alimi, Ed.
Internet-Draft                                           Yale University
Intended status: Standards Track                           R. Penno, Ed.
Expires: January 13, 2011                               Juniper Networks
                                                            Y. Yang, Ed.
                                                         Yale University
                                                           July 12, 2010


                             ALTO Protocol
                    draft-ietf-alto-protocol-05.txt

Abstract

   Networking applications today already have access to a great amount
   of Inter-Provider network topology information.  For example, views
   of the Internet routing table are easily available at looking glass
   servers and entirely practical to be downloaded by clients.  What is
   missing is knowledge of the underlying network topology from the ISP
   or Content Provider (henceforth referred as Provider) point of view.
   In other words, what a Provider prefers in terms of traffic
   optimization -- and a way to distribute it.

   The ALTO Service provides information such as preferences of network
   resources with the goal of modifying network resource consumption
   patterns while maintaining or improving application performance.
   This document describes a protocol implementing the ALTO Service.
   While such service would primarily be provided by the network (i.e.,
   the ISP), content providers and third parties could also operate this
   service.  Applications that could use this service are those that
   have a choice in connection endpoints.  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 [1].

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.



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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Background and Problem Statement . . . . . . . . . . . . .  5
     1.2.  Design History and Merged Proposals  . . . . . . . . . . .  5
     1.3.  Solution Benefits  . . . . . . . . . . . . . . . . . . . .  5
       1.3.1.  Service Providers  . . . . . . . . . . . . . . . . . .  5
       1.3.2.  Applications . . . . . . . . . . . . . . . . . . . . .  6
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.1.  Endpoint Address . . . . . . . . . . . . . . . . . . .  6
       2.1.2.  ASN  . . . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.3.  Network Location . . . . . . . . . . . . . . . . . . .  7
       2.1.4.  ALTO Information . . . . . . . . . . . . . . . . . . .  7
       2.1.5.  ALTO Information Base  . . . . . . . . . . . . . . . .  7
     2.2.  ALTO Service and Protocol Scope  . . . . . . . . . . . . .  7
   3.  Protocol Structure . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Server Information Service . . . . . . . . . . . . . . . .  9
     3.2.  ALTO Information Services  . . . . . . . . . . . . . . . . 10
       3.2.1.  Map Service  . . . . . . . . . . . . . . . . . . . . . 10
       3.2.2.  Map Filtering Service  . . . . . . . . . . . . . . . . 10
       3.2.3.  Endpoint Property Service  . . . . . . . . . . . . . . 10
       3.2.4.  Endpoint Cost Service  . . . . . . . . . . . . . . . . 10
   4.  Network Map  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  PID  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  Endpoint Addresses . . . . . . . . . . . . . . . . . . . . 12
       4.2.1.  IP Addresses . . . . . . . . . . . . . . . . . . . . . 12
     4.3.  Example Network Map  . . . . . . . . . . . . . . . . . . . 12
   5.  Cost Map . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Cost Attributes  . . . . . . . . . . . . . . . . . . . . . 13
       5.1.1.  Cost Type  . . . . . . . . . . . . . . . . . . . . . . 13
       5.1.2.  Cost Mode  . . . . . . . . . . . . . . . . . . . . . . 13
     5.2.  Cost Map Structure . . . . . . . . . . . . . . . . . . . . 14
     5.3.  Network Map and Cost Map Dependency  . . . . . . . . . . . 14
   6.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Design Approach  . . . . . . . . . . . . . . . . . . . . . 15
       6.1.1.  Use of Existing Infrastructure . . . . . . . . . . . . 15
       6.1.2.  ALTO Information Reuse and Redistribution  . . . . . . 16
   7.  Protocol Messaging . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Notation . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.2.  Message Format . . . . . . . . . . . . . . . . . . . . . . 17
       7.2.1.  Protocol Versioning Approach . . . . . . . . . . . . . 17
       7.2.2.  Request Message  . . . . . . . . . . . . . . . . . . . 17
       7.2.3.  Response Message . . . . . . . . . . . . . . . . . . . 18
     7.3.  General Processing . . . . . . . . . . . . . . . . . . . . 21
     7.4.  ALTO Status Codes  . . . . . . . . . . . . . . . . . . . . 21
     7.5.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . 22
       7.5.1.  Successful Response  . . . . . . . . . . . . . . . . . 22



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       7.5.2.  Error Conditions . . . . . . . . . . . . . . . . . . . 22
     7.6.  HTTP Usage . . . . . . . . . . . . . . . . . . . . . . . . 22
       7.6.1.  Authentication and Encryption  . . . . . . . . . . . . 23
       7.6.2.  Cookies  . . . . . . . . . . . . . . . . . . . . . . . 23
       7.6.3.  Caching Parameters . . . . . . . . . . . . . . . . . . 23
     7.7.  ALTO Requests  . . . . . . . . . . . . . . . . . . . . . . 23
       7.7.1.  Server Information Service . . . . . . . . . . . . . . 24
       7.7.2.  Map Service  . . . . . . . . . . . . . . . . . . . . . 27
       7.7.3.  Map Filtering Service  . . . . . . . . . . . . . . . . 31
       7.7.4.  Endpoint Property Service  . . . . . . . . . . . . . . 35
       7.7.5.  Endpoint Cost Service  . . . . . . . . . . . . . . . . 38
     7.8.  Redistributable Responses  . . . . . . . . . . . . . . . . 40
       7.8.1.  Server Capability Requirements . . . . . . . . . . . . 40
       7.8.2.  Service ID . . . . . . . . . . . . . . . . . . . . . . 40
       7.8.3.  Server and Request Parameters  . . . . . . . . . . . . 41
       7.8.4.  Expiration Time  . . . . . . . . . . . . . . . . . . . 41
       7.8.5.  Signature  . . . . . . . . . . . . . . . . . . . . . . 42
   8.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     8.1.  ALTO Client Embedded in P2P Tracker  . . . . . . . . . . . 43
     8.2.  ALTO Client Embedded in P2P Client: Numerical Costs  . . . 45
     8.3.  ALTO Client Embedded in P2P Client: Ranking  . . . . . . . 46
   9.  Discussions  . . . . . . . . . . . . . . . . . . . . . . . . . 46
     9.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 47
     9.2.  Hosts with Multiple Endpoint Addresses . . . . . . . . . . 47
     9.3.  Network Address Translation Considerations . . . . . . . . 47
     9.4.  Mapping IPs to ASNs  . . . . . . . . . . . . . . . . . . . 48
     9.5.  Endpoint and Path Properties . . . . . . . . . . . . . . . 48
     9.6.  P2P Peer Selection . . . . . . . . . . . . . . . . . . . . 49
       9.6.1.  Client-based Peer Selection  . . . . . . . . . . . . . 49
       9.6.2.  Server-based Peer Selection  . . . . . . . . . . . . . 49
       9.6.3.  Protocol Extension: 'Location-Only' Peer Selection . . 50
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 51
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 51
     11.1. Privacy Considerations for ISPs  . . . . . . . . . . . . . 51
     11.2. ALTO Clients . . . . . . . . . . . . . . . . . . . . . . . 51
     11.3. Authentication, Integrity Protection, and Encryption . . . 52
     11.4. ALTO Information Redistribution  . . . . . . . . . . . . . 52
     11.5. Denial of Service  . . . . . . . . . . . . . . . . . . . . 53
     11.6. ALTO Server Access Control . . . . . . . . . . . . . . . . 53
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 54
     12.2. Informative References . . . . . . . . . . . . . . . . . . 54
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 56
   Appendix B.  Authors . . . . . . . . . . . . . . . . . . . . . . . 57
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57






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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 to convey
   information about the network's preferences to applications.  By
   leveraging better network-provided information, applications have the
   potential to become more network-efficient (e.g., reduce network
   resource consumption) and achieve better application performance
   (e.g., accelerated download rate).  The ALTO Service intends to
   provide a simple way to convey network information to applications.

   The goal of this document is to specify a simple and unified protocol
   that meets the ALTO requirements [8] while providing a migration path
   for Internet Service Providers (ISP), Content Providers, and clients
   that have deployed protocols with similar intentions (see below).
   This document is a work in progress and will be updated with further
   developments.

1.2.  Design History and Merged Proposals

   The protocol specified here consists of contributions from

   o  P4P [9], [10];

   o  ALTO Info-Export [11];

   o  Query/Response [12], [13];

   o  ATTP [ATTP];

   o  Proxidor [14].

   See Appendix A for a list of people that have contributed
   significantly to this effort and the projects and proposals listed
   above.

1.3.  Solution Benefits

   The ALTO Service offers many benefits to both end-users (consumers of
   the service) and Internet Service Providers (providers of the
   service).

1.3.1.  Service Providers

   The ALTO Service enables ISPs to influence the peer selection process
   in distributed applications in order to increase locality of traffic,



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   improve user-experience, amongst others.  It also helps ISPs to
   efficiently engineer traffic that traverses more expensive links such
   as transit and backup links, thus allowing a better provisioning of
   the networking infrastructure.

1.3.2.  Applications

   Applications that use the ALTO Service can benefit in multiple ways.
   For example, they may no longer need to infer topology information,
   and some applications can reduce reliance on measuring path
   performance metrics themselves.  They can take advantage of the ISP's
   knowledge to avoid bottlenecks and boost performance.

   An example type of application is a Peer-to-Peer overlay where peer
   selection can be improved by including ALTO information in the
   selection process.


2.  Architecture

   Two key design objectives of the ALTO Protocol are simplicity and
   extensibility.  At the same time, it introduces additional techniques
   to address potential scalability and privacy issues.  After an
   introduction to the terminology, the ALTO architecture and the ALTO
   Protocol's place in the overall architecture are defined.

2.1.  Terminology

   We use the following terms defined in [15]: 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, ASN,
   and Network Location.

2.1.1.  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.






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2.1.2.  ASN

   An Autonomous System Number.

2.1.3.  Network Location

   Network Location is a generic term denoting a single endpoint or
   group of endpoints.

2.1.4.  ALTO Information

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

2.1.5.  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

   An ALTO Server conveys the network information from the perspective
   of a network region; the ALTO Server presents its "my-Internet View"
   [16] of the network region.  A network region in this context can be
   an Autonomous System, an ISP, or perhaps a smaller region or set of
   ISPs; the details depend on the deployment scenario and discovery
   mechanism.

   To better understand the ALTO Service and the role of the ALTO
   Protocol, we show in Figure 1 the overall 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.

   More specifically, the ALTO Information provided by an ALTO Server
   may be influenced (at the operator's discretion) by other systems.
   Examples 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 outside
   the scope of this specification.




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

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

                     Figure 1: Basic ALTO Architecture


3.  Protocol Structure

   The ALTO Protocol uses a simple extensible framework to convey
   network information.  In the general framework, the ALTO protocol
   will convey properties on both Network Locations and the paths
   between Network Locations.




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   In this document, we focus on a particular Endpoint property to
   denote the location of an endpoint, and provider-defined costs for
   paths between pairs of Network Locations.

   The ALTO Protocol is built on a common transport protocol, messaging
   structure and encoding, and transaction model.  The protocol is
   subdivided into services of related functionality.  ALTO-Core
   provides the Server Information Service and the Map Service to
   provide ALTO Information.  Other ALTO Information services can
   provide additional functionality.  There are three such services
   defined in this document: the Map Filtering Service, Endpoint
   Property Service, and Endpoint Cost Service.  Additional services may
   be defined in in companion documents.  Note that functionality
   offered in different services are not totally non-overlapping (e.g.,
   the Map Service and Map Filtering Service).


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

                     Figure 2: ALTO Protocol Structure

3.1.  Server Information Service

   The Server Capability Service lists the details on the information
   that can be provided by an ALTO Server and perhaps other ALTO Servers
   maintained by the network provider.  The configuration includes, for
   example, details about the operations and cost metrics supported by
   the ALTO Server and other related ALTO Servers that may be usable by
   an ALTO Client.  The capability document can be downloaded by ALTO
   Clients.  The capability information could also be provisioned to
   devices, but care must be taken to update it appropriately.



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3.2.  ALTO Information Services

   Multiple, distinct services are defined to allow ALTO Clients to
   query ALTO Information from an ALTO Server.  The ALTO Server
   internally maintains an ALTO Information Base that encodes the
   network provider's preferences.  The ALTO Information Base encodes
   the Network Locations defined by the ALTO Server (and their
   corresponding properties), as well as the provider-defined costs
   between pairs of Network Locations.

3.2.1.  Map Service

   The Map Service provides batch information to ALTO Clients in the
   form of a 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.

3.2.2.  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.

3.2.3.  Endpoint Property Service

   This service allows ALTO Clients to look up properties for individual
   endpoints.  An example endpoint property is its Network Location (its
   grouping defined by the ALTO Server) or connectivity type (e.g.,
   ADSL, Cable, or FioS).

3.2.4.  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

   In reality, many endpoints are very close to one another in terms of



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   network connectivity, for example, endpoints on the same site of an
   enterprise.  By treating a group of endpoints together as a single
   entity in ALTO, we can achieve much greater scalability without
   losing critical information.

   The Network Location endpoint property allows an ALTO Server to group
   endpoints together to indicate their proximity.  The resulting set of
   groupings is called the ALTO Network Map.

   The definition of 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 PoP may be
   considered close.

   As used in this document, the Network 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.

4.1.  PID

   Each group of Endpoints is identified by a provider-defined Network
   Location identifier called a PID.  There can be many different ways
   of grouping the endpoints and assigning PIDs.

   A PID is an identifier that provides an indirect and network-agnostic
   way to specify a network aggregation.  For example, a PID may be
   defined by the ALTO service provider to denote a subnet, a set of
   subnets, a metropolitan area, a PoP, an autonomous system, or a set
   of autonomous systems.  Aggregation of endpoints into PIDs can
   indicate proximity and can improve scalability.  In particular,
   network preferences (costs) may be specified between PIDs, allowing
   cost information to be more compact and updated at a smaller time
   scale than the network aggregations themselves.

   Using PIDs, the Network Map may also be used to communicate simple
   preferences with only minimal information from the Cost Map. 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 Map may be encoded to indicate that peering within the same
   PID is preferred; for example, cost(PID_i, PID_i) == c* and
   cost(PID_i, PID_j) > c* for i != j.  Section 5 provides further
   details about Cost Map structure.





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4.2.  Endpoint Addresses

   Communicating endpoints may have many types of addresses, such as IP
   addresses, MAC addresses, or overlay IDs.  The current specification
   only considers IP addresses.

4.2.1.  IP Addresses

   The endpoints aggregated into a PID are denoted by a list of IP
   prefixes.  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.  A RECOMMENDED way to satisfy this property is to
   define a PID containing the 0.0.0.0/0 prefix for IPv4 or ::/0 (for
   IPv6).

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.

   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 (numerical or
      ordinal).

   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 alphanumeric
   strings.  An ALTO Server MUST at least define the routing cost type
   denoted by the string 'routingcost'.

   Note that an ISP may internally compute routing cost using any method
   it chooses (including air-miles or hop-count).

5.1.2.  Cost Mode

   The Mode attribute indicates how costs should be interpreted.  For
   example, an ALTO Server could return costs that should be interpreted



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   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, such as summation, 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 alphanumeric
   strings.  An ALTO Server MUST at least define the modes 'numerical'
   and 'ordinal'.

   If an ALTO Client requests a Cost Mode that is not supported, the
   ALTO Server MUST reply with costs having Mode either 'numerical' or
   'ordinal'.  Thus, an ALTO Server must implement at least one of
   'numerical' or 'ordinal' Costs, but it may choose which to support.
   ALTO Clients may choose how to handle such situations.  Two
   particular possibilities are to use the returned costs as-is (e.g.,
   treat numerical costs as ordinal rankings) or ignore the ALTO
   information altogether.

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 communicating pair
   (i.e., Src_1 -> Dst_1, ..., Src_1 -> Dst_n, ..., Src_m -> Dst_1, ...,
   Src_m -> Dst_n).  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



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   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.  When the Network Map changes, the
   Version Tag SHOULD also be changed.  (Thus, the Version Tag is
   defined similarly to HTTP's ETag.)  Possibilities for generating a
   Version Tag included the last-modified timestamp for the Network Map,
   or a hash of its contents.

   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.  Protocol Overview

6.1.  Design Approach

   The ALTO Protocol design uses a REST-like interface with the goal of
   leveraging current HTTP [2] [3] implementations and infrastructure,
   as well as familiarity with existing REST-like services in popular
   use.  ALTO messages are denoted with HTTP Content-Type "application/
   alto" and use JSON [4] to encode message bodies.

   This document currently specifies both services and and the message
   encoding in a descriptive fashion.  Care is taken to make
   descriptions precise and unambiguous, but it still lacks benefits of
   automatic tooling that exists for certain encoding formats.

   Standards such as WSDL 2.0 and WADL are capable of describing
   available interfaces.  JSON Schema [17] allows message encodings to
   be specified precisely and messages may be verified against the
   schema.  It is not yet clear whether such an approach should be taken
   in this document.

   Other benefits enabled by these design choices include easier
   understanding and debugging, flexible ALTO Server implementation
   strategies, and more importantly, simple caching and redistribution
   of ALTO information to increase scalability.

6.1.1.  Use of Existing Infrastructure

   HTTP is a natural choice for integration with existing applications
   and infrastructure.  In particular, the ALTO Protocol design
   leverages:





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   o  the huge installed base of infrastructure, including HTTP caches,

   o  mature software implementations,

   o  the fact that many P2P clients already have an embedded HTTP
      client, and

   o  authentication and encryption mechanisms in HTTP and SSL/TLS.

6.1.2.  ALTO Information Reuse and Redistribution

   ALTO information may be useful to a large number of applications and
   users.  For example, an identical Network Map may be used by all ALTO
   Clients querying a particular ALTO Server.  At the same time,
   distributing ALTO information must be efficient and not become a
   bottleneck.  Therefore, the ALTO Protocol specified in this document
   integrates with existing HTTP caching infrastructure to allow reuse
   of ALTO information by ALTO Clients and reduce load on ALTO servers.

   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 redistribution, but it does define the primitives (e.g., digital
   signatures) needed to support such a mechanism.  See [18] for further
   discussion.

   Note that if caching or redistribution is used, the Response message
   may be returned from another (possibly third-party) entity.  Reuse
   and Redistribution is further discussed in Section 11.4.  Protocol
   support for redistribution is specified in Section 7.8.


7.  Protocol Messaging

   This section specifies client and server processing, as well as
   messages in the ALTO Protocol.  Details common to ALTO Server
   processing of all messages is first discussed, followed by details of
   the individual messages.

7.1.  Notation

   This document uses 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.




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   This document only includes object members used by this
   specification.  It is possible that protocol extensions include
   additional members to JSON objects defined in this document; such
   additional members will be silently ignored by ALTO Servers and
   Clients only implementing the base protocol defined in this document.

7.2.  Message Format

   Request and Response follow the standard format for HTTP Request and
   Response messages [2] [3].

   The following subsections provide an overview of how ALTO Requests
   and Responses are encoded in HTTP, and discusses rationale for
   certain design decisions.

7.2.1.  Protocol Versioning Approach

   The ALTO Protocol uses a simple and clean approach to versioning that
   permits evolution between versions even if ALTO information is being
   served as static, pre-generated files.

   In particular, it is assumed that a single host responding to ALTO
   Requests implements a single protocol version.  Note that virtual
   hosting can be used if multiple protocol versions need to be
   supported by a single physical server.

   A common query (Server List, detailed in Section 7.7.1.1) to be
   present in all ALTO protocol versions allows an ALTO Client to
   discover additional ALTO Servers and the ALTO Protocol version number
   of each.

   This approach keeps the ALTO Server implementation free from parsing
   and directing each request based on version number.  Although ALTO
   Requests are free from protocol version numbers, the protocol version
   number is echoed in each ALTO Response to keep responses self-
   contained to, for example, ease reading persisted or redistributed
   ALTO responses.

   This document specifies ALTO Protocol version 1.

7.2.2.  Request Message

   An ALTO Request is a standard HTTP Request generated by an ALTO
   Client, with certain components defined by the ALTO Protocol.

   The basic syntax of an ALTO Request is:

       <Method> /<Resource> HTTP/1.1



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       Host: <Host>

   For example:

       GET /info/capability HTTP/1.1
       Host: alto.example.com:6671

7.2.2.1.  Standard HTTP Headers

   The Host header MUST follow the standard rules for the HTTP 1.1 Host
   Header.

   The Content-Length header MUST follow the standard rules defined in
   HTTP 1.1.

   The Content-Type HTTP Header MUST have value "application/alto" if
   the Body is non-empty.

7.2.2.2.  Method and Resource

   Next, both the HTTP Method and URI-Path (denoted as Resource)
   indicate the operation requested by the ALTO Client.  In this
   example, the ALTO Client is requesting basic capability information
   from the ALTO Server.

7.2.2.3.  Input Parameters

   Certain operations defined by the ALTO Protocol (e.g., in the Map
   Filtering Service) allow the ALTO Client to supply additional input
   parameters.  Such input parameters are encoded in a URI-Query-String
   where possible and appropriate.  However, due to practical
   limitations (e.g. underlying HTTP implementations may have
   limitations on the total length of a URI and the Query-String is
   better-suited for simple unstructured parameters and lists), some
   operations in the ALTO Protocol use input parameters encoded in the
   HTTP Request Body.

7.2.3.  Response Message

   A Response message is a standard HTTP Response generated by an ALTO
   Server with certain components defined by the ALTO Protocol.

   The basic syntax of an ALTO Response is:

       HTTP/1.1 <StatusCode> <StatusMsg>
       Content-Length: <ContentLength>
       Content-Type: <ContentType>




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       <ALTOResponse>

   where the HTTP Response Body is an ALTOResponse JSON Object (defined
   in Section 7.2.3.3).  For example:

       HTTP/1.1 200 OK
       Content-Length: 1000
       Content-Type: application/alto

       {
           "meta" : {
               "version": 1,
               "status" : {
                   "code" : 1,
                   "reason" : "Success"
               },
               ...
           },
           "type" : "capability",
           "data" : {
               ...
           }
       }

7.2.3.1.  Standard HTTP Headers

   The Content-Length header MUST follow the standard rules defined in
   HTTP 1.1.

   The Content-Type HTTP Header MUST have value "application/alto" if
   the Body is non-empty.

7.2.3.2.  Status Code and Message

   Two sets of status codes are used in the ALTO Protocol.  First, an
   ALTO Status Code provides detailed information about the success or
   failure of a particular operation.  Second, an HTTP Status Code
   indicates to HTTP processing elements (e.g., intermediaries and
   clients) how the response should be treated.

7.2.3.3.  HTTP Body

   The Response body MUST encode a single top-level JSON object of type
   ALTOResponse:







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       struct {
           RspMetaData   meta;
           JSONString    type;
           [RspDataType] data;
       } ALTOResponse;

   The ALTOResponse object has distinct sections for:

   o  meta information encoded in an extensible way,

   o  the type of ALTO Information to follow, and

   o  the requested ALTO Information.

7.2.3.3.1.  Meta Information

   Meta information is encoded as a JSON object with type RspMetaData:

       struct {
           JSONNumber     code;
           JSONString     reason;           [OPTIONAL]
       } RspStatus;

       struct {
           JSONNumber     version;
           RspStatus      status;
           RspRedistInfo  redistribution;   [OPTIONAL]
       } RspMetaData;

   with members:

   o  version: the ALTO Protocol version

   o  status: the ALTO Status Code indicating a more detailed reason for
      the success or failure of a request than HTTP Status Codes permit.
      See Section 7.4 for a list of ALTO Status Codes defined in this
      document.  The corresponding 'reason' is a free-form string
      providing a human-readable indication of the particular status
      code.

   o  redistribution: additional meta information used by ALTO
      information redistribution (see Section 7.8)

7.2.3.3.2.  ALTO Information

   If the Response is successful (see Section 7.4), then the "type" and
   "data" members of the ALTOResponse object are REQUIRED. "type"
   encodes a Response-specific string which indicates to the ALTO Client



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   the type of data encoded in the message.  The "data" member encodes
   the actual Response-specific data.  The structure of this member is
   detailed later in this section for each particular ALTO Response.

7.2.3.4.  Signature

   An ALTO Server MAY additionally supply a signature asserting that it
   generated a particular response.  In order to allow the signature to
   be computed over the entire response message, the signature itself is
   specified in an HTTP Header or Trailer (see Section 7.8.5).

7.3.  General Processing

   The protocol is structured in such a way that, independent of the
   query type, there are a set of general processing steps.  The ALTO
   Client selects a specific ALTO Server with which to communicate,
   establishes a TCP connection, and constructs and sends ALTO Request
   messages which MUST conform to Section 7.7.  In response to Request
   messages, an ALTO Server constructs and sends ALTO Response messages
   which also MUST conform to Section 7.7.

7.4.  ALTO Status Codes

   This document defines ALTO Status Codes to support the operations
   defined in this document.  Additional status codes may be defined in
   companion or extension documents.

   An ALTO Server MUST return the 'Success' code in Table 1 if and only
   if the Request message is successfully processed and the requested
   ALTO information is returned by the ALTO Server.

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

   If multiple errors are present in a single ALTO 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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   +----------------------+------------------+-------------------------+
   | ALTO Status Code     | HTTP Status      | Description             |
   |                      | Code(s)          |                         |
   +----------------------+------------------+-------------------------+
   | SUCCESS              | 2xx              | Success                 |
   | E_JSON_SYNTAX        | 400              | JSON parsing error in   |
   |                      |                  | request                 |
   | E_JSON_FIELD_MISSING | 400              | Required field missing  |
   | E_JSON_VALUE_TYPE    | 400              | JSON Value of           |
   |                      |                  | unexpected type         |
   | E_INVALID_OPERATION  | 501              | Invalid operation       |
   |                      |                  | requested               |
   | E_INVALID_COST_TYPE  | 501              | Invalid cost type       |
   +----------------------+------------------+-------------------------+

                    Table 1: Defined ALTO Status Codes

   Status codes described in Table 1 are a work in progress.  The set of
   status codes will be modified or expanded as implementation
   experience is gained; feedback is welcomed.

   In addition, feedback from implementers of ALTO Clients is welcomed
   to identify if there is a need to communicate multiple status codes
   in a single response.

7.5.  Client Behavior

7.5.1.  Successful Response

   This specification does not indicate any required actions taken by
   ALTO Clients upon receiving a successful response from an ALTO
   Server.  Although ALTO Clients are suggested to interpret the
   received ALTO Information and adapt application behavior, ALTO
   Clients may also choose to ignore the received information.

7.5.2.  Error Conditions

   If an ALTO Client does not receive a successful response from the
   ALTO Server, it can either choose another server or fall back to a
   default behavior (e.g., perform peer selection without the use of
   ALTO information).  An ALTO Client may also retry the request at a
   later time.

7.6.  HTTP Usage







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7.6.1.  Authentication and Encryption

   An ALTO Server MAY support SSL/TLS to implement server and/or client
   authentication, as well as encryption.

   An ALTO Server MAY support HTTP Digest authentication.

7.6.2.  Cookies

   Cookies MUST NOT be used.

7.6.3.  Caching Parameters

   If the Response generated by the ALTO Server is cachable, the ALTO
   Server MAY include 'Cache-Control' and 'Expires' HTTP headers.

   If a Response generated by the ALTO Server is not cachable, the ALTO
   Server MUST specify the "Cache-Control: no-cache" HTTP Header.

7.7.  ALTO Requests

   This section documents the individual operations supported in the
   ALTO Protocol.  See Section 7.2.2 and Section 7.2.3 for
   specifications of HTTP Request/Response components common to all
   operations in the ALTO Protocol.

   Table 2 provides an summary of the HTTP Method and URI-Paths used for
   ALTO Requests:

      +----------------+--------------+----------------------------+
      | Service        | Operation    | HTTP Method and URI-Path   |
      +----------------+--------------+----------------------------+
      | Server Info    | List Servers | GET  /info/servers         |
      | Server Info    | Capability   | GET  /info/capability      |
      |                |              |                            |
      | Map            | Network Map  | GET  /map/core/pid/net     |
      | Map            | Cost Map     | GET  /map/core/pid/cost    |
      |                |              |                            |
      | Map Filtering  | Network Map  | POST /map/filter/pid/net   |
      | Map Filtering  | Cost Map     | POST /map/filter/pid/cost  |
      |                |              |                            |
      | Endpoint Prop. | Lookup       | GET  /endpoint/prop/<name> |
      |                |              | POST /endpoint/prop/lookup |
      |                |              |                            |
      | Endpoint Cost  | Lookup       | POST /endpoint/cost/lookup |
      +----------------+--------------+----------------------------+

                    Table 2: Overview of ALTO Requests



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7.7.1.  Server Information Service

   The Server Information Service provides information about available
   ALTO Servers and their capabilities (e.g., supported services).

   An ALTO Server MUST support the Server Information Service and MUST
   implement all operations defined in this section.

7.7.1.1.  Server List

   The Server List request allows an ALTO Client to discover other ALTO
   Servers provided by the ALTO Service Provider.  Upon discovering an
   additional ALTO Server, the ALTO Client may then query the server
   capabilities (see Section 7.7.1.2) to test if it supports desired
   functionality.

   The Server List request is intended to help an ALTO Client find an
   ALTO Server supporting the desired ALTO Protocol version and
   capabilities.  It is not intended to serve as a substitute for the
   ALTO Server Discovery which helps an ALTO Client locate an initial
   ALTO Server.

   This operation MUST be supported by the ALTO Server.

7.7.1.1.1.  Request Syntax

       GET /info/servers HTTP/1.1
       Host: <Host>

7.7.1.1.2.  Response Syntax

       HTTP/1.1 200 <StatusMsg>
       Content-Length: <BodyLength>
       Content-Type: application/alto

       <ALTOResponse>

   where the ALTOResponse object has "type" member equal to the string
   "server-list" and "data" member of type RspServerList:

       struct {
           JSONString   uri;
           JSONNumber   version;
       } ServerItem;

       struct {
           ServerItem   servers<0..*>;
       } RspServerList;



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   RspServerList has members:

   o  servers: Array of available ALTO Servers, detailing the URI of the
      ALTO Server and the ALTO Protocol version that it implements.  The
      array must at least contain an entry corresponding to the ALTO
      Server at the URI from which it is retrieving the server list.

7.7.1.1.3.  Example

       GET /info/servers HTTP/1.1
       Host: alto.example.com:6671


       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "server-list",
           "data" : {
               "servers" : [
                   {
                       "uri": "http://alto.example.com:6671",
                       "version" : 1
                   }
               ]
           }
       }

7.7.1.2.  Server Capability

   The Server Capability request allows an ALTO Client to determine the
   functionality supported by the queried ALTO Server.

   This operation MUST be supported by the ALTO Server.

7.7.1.2.1.  Request Syntax

       GET /info/capability HTTP/1.1
       Host: <Host>





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7.7.1.2.2.  Response Syntax

       HTTP/1.1 200 <StatusMsg>
       Content-Length: <BodyLength>
       Content-Type: application/alto

       <ALTOResponse>

   where the ALTOResponse object has "type" member equal to the string
   "capability" and "data" member of type RspCapability:

       enum {
           map,
           map_filtering,
           endpoint_property,
           endpoint_cost
       } ServiceType;          [Note: encoded as JSONString's]

       struct {
           ServiceType  services<0..*>;
           JSONString   cost_types<0..*>;      [OPTIONAL]
           JSONBool     cost_constraints;      [OPTIONAL]
           JSONString   service_id;            [OPTIONAL+]
           JSONString   certificate;           [OPTIONAL+]
       } RspCapability;

   See Section 7.8.1 for additional notes concerning the 'service_id'
   and 'certificate' fields for ALTO Servers enabling response
   redistribution.

   RspCapability has members:

   o  services: Lists the services supported by the ALTO Server.  The
      service names defined in this document are are "map",
      "map_filtering", "endpoint_property", and "endpoint_cost".

   o  cost_types: Array of cost type information for additional
      supported ALTO Cost types, detailing the name for each supported
      additional type.  [[Comment.1: Need to discuss IANA implications
      or alternate approaches.  Note that current definition assumes the
      unit for a cost type is fixed.]]

   o  cost_constraints: Indicates if the ALTO Server supports cost
      constraints.  The value 'false' is implied if this member is not
      present.

   o  service_id: UUID [5] indicating an set of ALTO Servers (possibly
      just a single ALTO Server) serving equivalent ALTO Information



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      (see Section 7.8).

   o  certificate: PEM-encoded X.509 certificate used by the ALTO Server
      to sign distributed information (see Section 7.8).

7.7.1.2.3.  Example

       GET /info/capability HTTP/1.1
       Host: alto.example.com:6671


       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "capability",
           "data" : {
               "services" : [ "map", "map-filtering" ],
               "cost_types": [
                   "routingcost",
                   "hopcount"
               ],
               "cost_constraints": false
           }
       }

7.7.2.  Map Service

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

   An ALTO Server MUST support the Map Service and MUST implement all
   operations defined in this section.

7.7.2.1.  Network Map

   The full Network Map lists for each PID, the network locations
   (endpoints) within the PID.






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7.7.2.1.1.  Request Syntax

       GET /map/core/pid/net HTTP/1.1
       Host: <Host>

7.7.2.1.2.  Response Syntax

       HTTP/1.1 200 <StatusMsg>
       Content-Length: <BodyLength>
       Content-Type: application/alto

       <ALTOResponse>

   where the ALTOResponse object has "type" member equal to the string
   "network_map" and "data" member of type RspNetworkMap:

       struct {
           CIDRString [pidname]<0..*>;
           ...
       } NetworkMapData;

       struct {
           JSONString     map_vtag;
           NetworkMapData map;
       } RspNetworkMap;

   RspNetworkMap has members:

   o  map_vtag: The Version Tag of the Network Map (Section 5.3)

   o  map: The network map data itself.

   NetworkMapData is a JSON object with each member representing a
   single PID and its associated set of IP Prefixes (encoded as a string
   in CIDR notation).  A member's name is a JSONString denoting the
   PID's name.

7.7.2.1.3.  Example

       GET /map/core/pid/net HTTP/1.1
       Host: alto.example.com:6671










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       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "network_map",
           "data" : {
               "map_vtag" : "1266506139",
               "map" : {
                   "PID1" : [
                       "192.0.2.0/24",
                       "198.51.100.0/25"
                   ],
                   "PID2" : [
                       "198.51.100.128/25"
                   ],
                   "PID3" : [
                       "0.0.0.0/0"
                   ]
               }
           }
       }

7.7.2.2.  Cost Map

   The Map Service Cost Map query is a batch operation in which the ALTO
   Server returns the Path Cost for each pair of source/destination PID
   defined by the ALTO Server.

   The ALTO Server provides costs using the default Cost Type
   ('routingcost') and default Cost Mode ('numerical').

7.7.2.2.1.  Request Syntax

       GET /map/core/pid/cost HTTP/1.1
       Host: <Host>









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7.7.2.2.2.  Response Syntax

       HTTP/1.1 200 <StatusMsg>
       Content-Length: <BodyLength>
       Content-Type: application/alto

       <ALTOResponse>

   where the ALTOResponse object has "type" member equal to the string
   "cost_map" and "data" member of type RspCostMap:

       struct DstCosts {
           JSONNumber [dstname];
           ...
       };

       struct {
           DstCosts [srcname]<0..*>;
           ...
       } CostMapData;

       struct {
           JSONString  map_vtag;
           JSONString  cost_type;
           JSONString  cost_mode;
           CostMapData map;
       } RspCostMap;

   RspCostMap has members:

   o  map_vtag: The Version Tag of the Network Map used to generate the
      Cost Map (Section 5.3).

   o  cost_type: Cost Type used in the map (Section 5.1.1)

   o  cost_mode: Cost Mode used in the map (Section 5.1.2)

   o  map: The cost map data itself.

   CostMapData is a JSON object with each member representing a single
   Source PID.  For each Source PID, a DstCosts structure denotes the
   associated cost to a set of destination PIDs (Section 5.2).  DstCosts
   has a single member for each destination PID in the map.

7.7.2.2.3.  Example

       GET /map/core/pid/cost HTTP/1.1
       Host: alto.example.com:6671



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       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "cost_map",
           "data" : {
               "map_vtag"  : "1266506139",
               "cost_type" : "routingcost",
               "cost_mode" : "numerical",
               "map" : {
                   "PID1": { "PID1": 1,  "PID2": 5,  "PID3": 10 },
                   "PID2": { "PID1": 5,  "PID2": 1,  "PID3": 15 },
                   "PID3": { "PID1": 20, "PID2": 15, "PID3": 1  }
               }
           }
       }

7.7.3.  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.

   An ALTO Server MAY support the Map Filtering Service.  If an ALTO
   Server supports the Map Filtering Service, all operations defined in
   this section MUST be implemented.

7.7.3.1.  Network Map

   ALTO Clients can query for a subset of the full network map (see
   Section 7.7.2.1).

7.7.3.1.1.  Request Syntax

       POST /map/filter/pid/net HTTP/1.1
       Host: <Host>
       Content-Length: <BodyLength>

       <ReqNetworkMap>

   where:



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       struct {
           JSONString pids<0..*>;
       } ReqNetworkMap;

   The Body of the request encodes an array of PIDs to be included in
   the resulting 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.

7.7.3.1.2.  Response Syntax

   The Response syntax is identical to that of the Map Service's Network
   Map Response (Section 7.7.2.1.2).

   The ALTO Server MUST only include PIDs in the Response that were
   specified (implicitly or explicitly) in the Request.  If the Request
   contains 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
   request.
































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7.7.3.1.3.  Example

       POST /map/filter/pid/net HTTP/1.1
       Host: alto.example.com:6671
       Content-Length: <BodyLength>

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


       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "network_map",
           "data" : {
               "map_vtag" : "1266506139",
               "map" : {
                   "PID1" : [
                       "192.0.2.0/24",
                       "198.51.100.0/24",
                   ],
                   "PID2" : [
                       "198.51.100.128/24",
                   ]
               }
           }
       }

7.7.3.2.  Cost Map

   ALTO Clients can query for the Cost Map (see Section 7.7.2.2) based
   on additional parameters.

7.7.3.2.1.  Request Syntax

       POST /map/filter/pid/cost?<URI-Query-String> HTTP/1.1
       Host: <Host>

       <ReqCostMap>



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   where:

       struct {
           JSONString srcs<0..*>;
           JSONString dsts<0..*>;
       } ReqCostMap;

   The Query String may contain the following parameters:

   o  type: The requested Cost Type (Section 5.1.1).  If not specified,
      the default value is "routingcost".  This parameter MUST NOT be
      specified multiple times.

   o  mode: The requested Cost mode (Section 5.1.2).  If not specified,
      the default value is "numerical".  This parameter MUST NOT be
      specified multiple times.

   o  constraint: Defines a constraint on which elements of the Cost Map
      are returned.  This parameter MUST NOT be used if the Server
      Capability Response (Section 7.7.1.2) indicates that constraint
      support is not available.  A constraint contains two entities
      separated by whitespace (before URL encoding): (1) an operator
      either 'gt' for greater than , 'lt' for less than or 'eq' for
      equal to with 10 percent on either side, (2) a target numerical
      cost.  The numerical cost is a number that MUST be defined in the
      units specified in the Server Capability Response.  If multiple
      'constraint' parameters are specified, the ALTO Server assumes
      they are related to each other with a logical AND.  If no
      'constraint' parameters are specified, then the ALTO Server
      returns the full Cost Map.

   The Request body MAY specify a list of Source PIDs, and a list of
   Destination PIDs.  If a list is empty, it is interpreted by the ALTO
   Server as the full set of currently-defined PIDs.  The ALTO Server
   returns costs between each pair of source/destination PID.  If the
   Request body is empty, both lists are interpreted to be empty.

7.7.3.2.2.  Response Syntax

   The Response syntax is identical to that of the Map Service's Cost
   Map Response (Section 7.7.2.2.2).

   The Response MUST NOT contain any source/destination pair that was
   not indicated (implicitly or explicitly) in the Request.  If the
   Request contains 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 request.




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7.7.3.2.3.  Example

       POST /map/filter/pid/cost?type=hopcount HTTP/1.1
       Host: alto.example.com:6671

       {
           "srcs" : [ "PID1" ],
           "dsts" : [ "PID1", "PID2", "PID3" ]
       }


       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "cost_map",
           "data" : {
               "map_vtag"  : "1266506139",
               "cost_type" : "hopcount",
               "cost_mode" : "numerical",
               "map" : {
                   "PID1": { "PID1": 0,  "PID2": 1,  "PID3": 2 }
               }
           }
       }

7.7.4.  Endpoint Property Service

   The Endpoint Property Lookup query allows an ALTO Client to lookup
   properties of Endpoints known to the ALTO Server.  If the ALTO Server
   provides the Endpoint Property Service, the ALTO Server MUST define
   at least the 'pid' property for Endpoints.

   An ALTO Server MAY support the Endpoint Property Service.  If an ALTO
   Server supports the Endpoint Property Service, all operations defined
   in this section MUST be implemented.

7.7.4.1.  Endpoint Property Lookup






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7.7.4.1.1.  Request Syntax

       POST /endpoint/prop/lookup?<URI-Query-String> HTTP/1.1
       Host: <Host>
       Content-Length: <BodyLength>

       <ReqEndpointProp>

   where:

       struct {
           JSONString endpoints<0..*>;
       } ReqEndpointProp;

   The Query String may contain the following parameters:

   o  prop: The requested property type.  This parameter MUST be
      specified at least once, and MAY be specified multiple times
      (e.g., to query for multiple different properties at once).

   The body encodes a list of endpoints (IP addresses) as strings.

   An alternate syntax is supported for the case when properties are
   requested for a single endpoint:

       GET /endpoint/prop/<Endpoint>?<URI-Query-String> HTTP/1.1
       Host: <Host>

   where the Query String is the same as in the first form.

7.7.4.1.2.  Response Syntax

       HTTP/1.1 200 <StatusMsg>
       Content-Length: <BodyLength>
       Content-Type: application/alto

       <ALTOResponse>

   where the ALTOResponse object has "type" member equal to the string
   "endpoint_property" and "data" member of type RspEndpointProperty:











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       struct {
           JSONString [propertyname];
           ...
       } EndpointProps;

       struct {
           EndpointProps  [endpointname]<0..*>;
           ...
       } RspEndpointProperty;

   RspEndpointProperty has one member for each endpoint indicated in the
   Request.  The requested properties for each endpoint are encoded in a
   corresponding EndpointProps object, which encodes one name/value pair
   for each requested property.  Note that property values are JSON
   Strings.  If the ALTO Server does not define a requested property for
   a particular endpoint, then it MUST omit it from the Response for
   only that endpoint.

7.7.4.1.3.  Example

       POST /endpoint/prop/lookup?prop=pid HTTP/1.1
       Host: alto.example.com:6671
       Content-Length: [TODO]

       {
           "endpoints" : [ "192.0.2.34", "203.0.113.129" ]
       }


       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

       {
           "meta" : {
               "version" : 1,
               "status" : {
                   "code" : 1
               }
           },
           "type" : "endpoint_property",
           "data": {
               "192.0.2.34"    : { "pid": "PID1" },
               "203.0.113.129" : { "pid": "PID3" }
           }
       }





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7.7.5.  Endpoint Cost Service

   The Endpoint Cost Service allows ALTO Clients to directly supply
   endpoints to an ALTO Server.  The ALTO Server replies with costs
   (numerical or ordinal) amongst the endpoints.

   In particular, this service allows lists of Endpoint addresses to be
   ranked (ordered) by an ALTO Server.

   An ALTO Server MAY support the Endpoint Cost Service.  If an ALTO
   Server supports the Endpoint Cost Service, all operations defined in
   this section MUST be implemented.

7.7.5.1.  Endpoint Cost Lookup

7.7.5.1.1.  Request Syntax

       POST /endpoint/cost/lookup?<URI-Query-String> HTTP/1.1
       Host: <Host>
       Content-Length: <BodyLength>

       <ReqCostMap>

   The request body includes a list of source and destination endpoints
   that should be assigned a cost by the ALTO Server.  The allowed Query
   String parameters are defined identically to Section 7.7.3.2.

   The request body MUST specify a list of source Endpoints, and a list
   of destination Endpoints, using an structure identical to
   Section 7.7.3.2 with the exception that identifiers are endpoints
   instead of PIDs.  If the list of source Endpoints is empty (or it is
   not included), the ALTO Server MUST treat it as if it contained the
   Endpoint address of the requesting client.  The list of destination
   Endpoints MUST NOT be empty.  The ALTO Server returns costs between
   each pair of source/destination Endpoint.

7.7.5.1.2.  Response Syntax

       HTTP/1.1 200 <StatusMsg>
       Content-Length: <BodyLength>
       Content-Type: application/alto

       <ALTOResponse>

   where ALTOResponse is encoded identically to Section 7.7.2.2.2 with
   the following exceptions:





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   o  ALTO Response's "type" member must be equal to
      "endpoint_cost_map",

   o  The "map_vtag" member of RspCostMap MUST be omitted, and

   o  Identifiers refer to endpoints instead of PIDs.

7.7.5.1.3.  Example

       POST /endpoint/cost/lookup?mode=ordinal HTTP/1.1
       Host: alto.example.com:6671
       Content-Length: [TODO]

       {
         "src": [ "192.0.2.2" ],
         "dst": [ "192.0.2.89", "198.51.100.34", "203.0.113.45" ]
       }


       HTTP/1.1 200 OK
       Content-Length: [TODO]
       Content-Type: application/alto

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








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7.8.  Redistributable Responses

   An ALTO Server MAY indicate that a response is suitable for
   redistribution by including the "redistribution" member in the
   RspMetaData JSON object of an ALTO Response message.  This additional
   member has type RspRedistInfo:

       struct {
           JSONString service_id;
           JSONString request_uri;
           JSONValue  request_body;
           JSONString expires;
       } RspRedistInfo;

   If an ALTO Server indicates the that the response is redistributable,
   the Response message MUST satisfy all requirements in this section.

   The ALTO Server generating the response indicates its own unique
   identifier (Service ID) and any input parameters used to generate the
   response.  This allows ALTO Clients to which the information is
   distributed to understand the context of the query and interpret the
   results.  This information is encoded in the RspRedistInfo JSON
   Object.

7.8.1.  Server Capability Requirements

   The 'service_id' and 'certificate' fields of the Server Capability
   response are REQUIRED if an ALTO Server generates redistributable
   responses.

7.8.2.  Service ID

   For scalability and fault tolerance, multiple ALTO Servers may be
   deployed to serve equivalent ALTO Information.  In such a scenario,
   ALTO Responses from any such redundant server should be seen as
   equivalent for the purposes of redistribution.  For example, if two
   ALTO Servers A and B are deployed by the service provider to
   distribute equivalent ALTO Information, then clients contacting
   Server A should be able to redistribute ALTO Responses to Server B.

   One technique for doing this would be to rely on the server's DNS
   name.  However, such an approach would mandate that all ALTO Servers
   resolved by a particular DNS name would need to provide equivalent
   ALTO information, which may be unneccessarily restrictive.  Another
   technique would be to rely on the server's IP adddress.  However,
   this suffers similar problems as the DNS name in deployment scenarios
   using IP Anycast.




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   To avoid such restrictions, the ALTO Protocol allows an ALTO Service
   Provider to explicitly denote ALTO Servers that provide equivalent
   ALTO Information by giving them identical Service IDs.

   If an ALTO Server generates redistributable responses, the Server
   Capability response's 'service_id' field MUST be set to the ALTO
   Server's Service ID.

   To help prevent ALTO Servers from mistakenly claiming to distribute
   equivalent ALTO Information, ALTO Server Implementations SHOULD by
   default generate a new UUID at installation time.  The default,
   generated UUID may be overridden by the service provider.

7.8.3.  Server and Request Parameters

   This section defines the members of the RspRedistInfo JSON object.

   The 'service_id' member is REQUIRED and MUST have a value equal to
   the ALTO Server's Service ID.

   The 'request_uri' member is REQUIRED and MUST specify the HTTP
   Request-URI that was passed in the HTTP Request.

   If the HTTP Request body was non-empty, the 'request_body' member
   MUST specify full JSON value passed in the HTTP Request (note that
   whitespace may differ, as long as the JSON Value is identical).  If
   the HTTP Request was empty, then the 'request_body' MUST NOT be
   included.

   Note that information about ALTO Client performing the Request and
   any HTTP Headers passed in the request are not included.  If any such
   information or headers influence the response generated by the ALTO
   Server, the response SHOULD NOT be indicated as redistributable.

7.8.4.  Expiration Time

   ALTO Responses marked as redistributable SHOULD indicate a time after
   which the information is considered stale and should be refreshed
   from the ALTO Server (or possibly another ALTO Client).

   The 'expires' element is RECOMMENDED and, if present, MUST specify a
   time in UTC formatted according to [6].

   If an expiration time is present, the ALTO Server SHOULD ensure that
   it is reasonably consistent with the expiration time that would be
   computed by HTTP header fields.  If the expiration time in the
   'expires' element is earlier, some ALTO Clients may refresh data from
   the ALTO Server earlier than expected.  If the expiration time



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   included in the response body is later, some ALTO Clients may refresh
   the data later than expected.

7.8.5.  Signature

   ALTO Responses marked as redistributable MUST include a signature
   used to assert that the ALTO Server Provider generated the ALTO
   Information.

   Verification of the signature requires the ALTO Client to retrieve
   the ALTO Server's public key.  There are multiple possibilities to
   retrieve it:

   o  SSL/TLS connection with the ALTO Server: The public key algorithm
      and public key may be retrieved from the ALTO Server's X.509
      Certificate used on an HTTPS connection between the ALTO Server
      and ALTO Client.

   o  Included in ALTO Server's Server Capability Response: If the ALTO
      Client requests from the ALTO Server over a non SSL/TLS
      connection, an X.509 certificate (including the public key and
      public key algorithm) can be included in the Server Capability
      Response.

   To reduce requirements on the underlying transport (i.e., requiring
   SSL/TLS), the ALTO Protocol uses the latter option.  This
   specification makes the following requirements of the X.509
   certificates:

   o  The certificate's corresponding private key MUST be used to sign
      redistributable responses.

   o  The certificate for each ALTO Server with an identical Service ID
      MUST be identical.

   ALTO Clients SHOULD verify that the certificate satisfies any local
   policies (e.g., Issuer, expiration date, etc).

   The ALTO Server may include the Hash Algorithm, Signature Algorithm,
   and Signature in either HTTP Headers or Trailers.  Headers may be
   useful if Responses are pre-generated, while Trailers may be useful
   if Responses are dynamically generated (e.g., to avoid buffering
   large responses in memory while the hash value is computed).

   The following HTTP Headers (the ALTO Server MAY specify them as HTTP
   Trailers) MUST be used to encode the Signature parameters for
   redistributable ALTO Responses:




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       ALTO-HashAlgorithm: <HashAlgorithm>
       ALTO-SignatureAlgorithm: <SignatureAlgorithm>
       ALTO-SignatureDigest: <Signature>

   where <HashAlgorithm> and <SignatureAlgorithm> are an integer values
   from the IANA TLS HashAlgorithm and SignatureAlgorithm registries,
   and <Signature> is the corresponding PEM-encoded signature.

   ALTO Clients SHOULD verify the signature on any ALTO information
   received via redistribution before adjusting application behavior
   based on it.

   If an ALTO Client consumes ALTO Information from multiple ALTO
   Servers, it can locally maintain a map of the corresponding
   certificate for each ALTO Server.  Upon receiving redistributed
   information, it may lookup the appropriate certificate to use for
   signature verification based on the Service ID contained in the
   redistributed ALTO Response.  Note that verifying the signature also
   protects against hijacking of a Service ID as long as the initial
   certificate retrieval (via the Server Capability Response) is secure
   from hijacking.

   ALTO Clients SHOULD pass the ALTO Server Certificate, Signature, and
   Signature Algorithm along with the body of the ALTO Response.  The
   mechanism for redistributing such information is not specified by the
   ALTO Protocol, but one possibility is to add additional messages or
   fields to the application's native protocol.


8.  Use Cases

   The sections below depict typical use cases.

8.1.  ALTO Client Embedded in P2P Tracker

   Many P2P currently-deployed P2P systems use a Tracker to manage
   swarms and perform peer selection.  P2P trackers may currently use a
   variety of information to perform peer selection to meet application-
   specific goals.  By acting as an ALTO Client, an 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.



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   .---------.   (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.

   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.




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8.2.  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.

   .---------.   (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.




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8.3.  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.

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


9.  Discussions






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9.1.  Discovery

   The particular mechanism by which an ALTO Client discovers its ALTO
   Server is an important component to the ALTO architecture and
   numerous techniques have been discussed [19].  However, the discovery
   mechanism is out of scope for this document.  This document assumes
   that an ALTO Client can discover an appropriate ALTO Server.  Note
   that the Server List query in the Server Information Service is
   intended to aid an ALTO Client in selecting an available ALTO Server
   with the capabilities necessary for its application.

   Some ISPs have proposed the possibility of delegation, in which an
   ISP provides information for customer networks which do not wish to
   run ALTO Servers themselves.  A consideration for delegation is that
   customer networks may wish to explicitly configure such delegation.

9.2.  Hosts with Multiple Endpoint Addresses

   In practical deployments, especially during the transition from IPv4
   to IPv6, a particular host may be reachable using multiple addresses.
   Furthermore, the particular network path followed when sending
   packets to the host may differ based on the address that is used.
   Network providers may perfer one path over another (e.g., one path my
   have a NAT64 middlebox).  An additional consideration may be how to
   handle private address spaces (e.g., behind carrier-grade NATs).

   Note that to support such behavior, Endpoints must be associated with
   a particular address type (e.g., IPv4 or IPv6).  One simple
   possibility may be to prefix each endpoint address with its type
   (e.g., "ipv4:198.51.100.128/25").  However, we may want to discuss if
   a more efficient/compact encoding is possible in some cases (e.g.,
   all addresses in the same PID are IPv6).

   There are limitations as to what information ALTO can provide in this
   regard.  In particular, a particular ALTO Service provider may not be
   able to determine if connectivity with a particular endhost will
   succeed over IPv4 or IPv6, as this may depend upon information
   unknown to the ISP such as particular application implementations.

   Exploration of these issues is being considered in a separate
   Internet Draft [20].  Once a suitable solution emerges, it will be
   included in this document.

9.3.  Network Address Translation Considerations

   At this day and age of NAT v4<->v4, v4<->v6 [21], and possibly
   v6<->v6[22], a protocol should strive to be NAT friendly and minimize
   carrying IP addresses in the payload, or provide a mode of operation



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   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 (via
   the Endpoint Property Lookup interface) 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 [23]) operate.

   The ALTO client SHOULD use the Session Traversal Utilities for NAT
   (STUN) [7] to determine a public IP address to use as a source
   Endpoint address.  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.

9.4.  Mapping IPs to ASNs

   It may be desired for the ALTO Protocol to provide ALTO information
   including ASNs.  Thus, ALTO Clients may need to identify the ASN for
   a Resource Provider to determine the cost to that Resource Provider.

   Applications can already map IPs to ASNs using information from a BGP
   Looking Glass.  To do so, they must download a file of about 1.5MB
   when compressed (as of October 2008, with all information not needed
   for IP to ASN mapping removed) and periodically (perhaps monthly)
   refresh it.

   Alternatively, the Network Map query in the Map Filtering Service
   defined in this document could be extended to map ASNs into a set of
   IP prefixes.  The mappings provided by the ISP would be both smaller
   and more authoritative.

   For simplicity of implementation, it's highly desirable that clients
   only have to implement exactly one mechanism of mapping IPs to ASNs.

9.5.  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.  The current draft focuses on network location and
   grouping, but the protocol may be extended to handle other Endpoint



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   properties.

9.6.  P2P Peer Selection

   This section discusses possible approaches to peer selection using
   ALTO information (Network Location Identifiers and associated Costs)
   from an ALTO Server.  Specifically, the application must select which
   peers to use based on this and other sources of information.  With
   this in mind, the usage of ALTO Costs is intentionally flexible,
   because:

      Different applications may use the information differently.  For
      example, an application that connects to just one address may have
      a different algorithm for selecting it than an application that
      connects to many.

      Though initial experiments have been conducted [24], more
      investigation is needed to identify other methods.

   In addition, the application might account for robustness, perhaps
   using randomized exploration to determine if it performs better
   without ALTO information.

9.6.1.  Client-based Peer Selection

   One possibility is for peer selection using ALTO costs to be done
   entirely by a P2P client.  The following are some techniques have
   been proposed and/or used:

   o  Prefer network locations with lower ordinal rankings (i.e., higher
      priority) [14] [11].

   o  Optimistically unchoking low-cost peers with higher probability
      [11].

9.6.2.  Server-based Peer Selection

   Another possibility is for ALTO costs to be used by an Application
   Tracker (e.g., BitTorrent Tracker) when returning peer lists.  The
   following are techniques that have been proposed and/or used:

   o  Using bandwidth matching (e.g., at an Application Tracker) and
      choosing solution (within bound of optimal) with minimal network
      cost [24].







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9.6.3.  Protocol Extension: 'Location-Only' Peer Selection

   This section discusses a promising peer selection algorithm that was
   recently used in experiments with a P2P live streaming application.
   However, to support it, a small protocol extension would be required,
   but the protocol extension is general and may be applicable in other
   contexts as well.  Feedback from the WG is greatly encouraged.

   Experiments in the context of live streaming have shown significant
   benefits of a simple "location-only" algorithm that primarily makes
   use of the Network Map. A benefit of this algorithm is that it can
   provide a simple integration path for applications wishing to utilize
   ALTO.

   In particular, the algorithm proceeds as follows to select an ordered
   list of peers for a particular incoming (or existing peer):

   1.  Insert into the result list a number (up to a threshold) of peers
       from the same PID as the incoming peer.

   2.  Insert into the result list a number (up to a threshold) of peers
       from the same ISP as the incoming peer.

   3.  Insert into the result list a number (up to a threshold) of peers
       from different ISPs than the incoming peer.

   In the experiments, this algorithm was implemented at a tracker and
   executed for peer selection when peers initially join and when
   requesting new peers.

   This algorithm makes two assumptions about the preferences
   communicated by the Network Map:

   o  The ISP prefers peers within the same PID to peer with each other
      (see Section 4); and

   o  The ALTO Server can indicate which PIDs describe network locations
      within the same ISP.

   The second assumption is currently not satisfied by the ALTO
   protocol, but could be accomplished by including a PID attribute.
   For example, a boolean attribute name "Intra-Region" with value
   'true' could be added to PIDs within the ALTO Server's Network
   Region.







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10.  IANA Considerations

   This document request the registration of a new media type:
   "application/alto"


11.  Security Considerations

11.1.  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, inter-ISP business relationships, etc. by intentionally
   posting a multitude of selective queries to an ALTO server (and
   carefully 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.

11.2.  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 [15].

   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.  One possibility is for the ALTO Client to only rely on
   Network Map for PIDs and Cost Map amongst PIDs to avoid passing IP
   addresses of their 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



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   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.

11.3.  Authentication, Integrity Protection, and Encryption

   SSL/TLS can provide encryption 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 and
   authentication of the server).

   An ALTO Server may optionally use authentication (and potentially
   encryption) to protect ALTO information it provides.  This can be
   achieved by digitally signing a hash of the ALTO information itself
   and attaching the signature to the ALTO information.  There may be
   special use cases where encryption of ALTO information is desirable.
   In most cases, however, information sent out by an ALTO Server is
   most likely to 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.

11.4.  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 actually generated by
   the correct ALTO Server.  Further, to prevent the ALTO Server from
   being a target of attack, the verification scheme must not require
   ALTO Clients to contact the ALTO Server to validate every set of
   information.  Note that in any case, contacting the originating ALTO
   server for information validation would undermine the intended effect
   of redistribution and is therefore not desirable.

   Note that the redistribution scheme must additionally handle details
   such as ensuring ALTO Clients retrieve ALTO information from the
   correct ALTO Server.  See [18] for further discussion.  Details of a
   particular redistribution scheme are outside the scope of this
   document.

   To fulfill these requirements, ALTO Information meant to be
   redistributable contains a digital signature which includes a hash of



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   the ALTO information signed by the ALTO Server with its private key.
   The corresponding public key should either be part of the ALTO
   information itself, or it could be included in the server capability
   response.  The public key SHOULD include the hostname of the ALTO
   Server and it SHOULD be signed by a trusted authority (i.e., in a
   certificate).  This enables an ALTO client retrieving redistributed
   ALTO information to verify the correctness of the ALTO Server's
   signature, given that it trusts the authority which signed the ALTO
   Server's certificate.  Note that in some cases this requires that the
   retrieving ALTO Client must be able to derive a transitive
   certificate chain (including a Root-CA) to the trusted authority
   which signed the ALTO Server's certificate.  This requirement may not
   be possible to fulfill between every ALTO Client / ALTO Server
   combination on the Internet due to the lack of a world-wide public
   key infrastructure.

11.5.  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.  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., [25]).  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.

11.6.  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 [26] for a more detailed discussion on



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   this issue).


12.  References

12.1.  Normative References

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

   [2]   Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
         Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [3]   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.

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

   [5]   Leach, P., Mealling, M., and R. Salz, "A Universally Unique
         IDentifier (UUID) URN Namespace", RFC 4122, July 2005.

   [6]   Klyne, G., Ed. and C. Newman, "Date and Time on the Internet:
         Timestamps", RFC 3339, July 2002.

   [7]   Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session
         Traversal Utilities for (NAT) (STUN)",
         draft-ietf-behave-rfc3489bis-18 (work in progress), July 2008.

12.2.  Informative References

   [8]   Kiesel, S., Popkin, L., Previdi, S., Woundy, R., and Y. Yang,
         "Application-Layer Traffic Optimization (ALTO) Requirements",
         draft-kiesel-alto-reqs-01 (work in progress), November 2008.

   [9]   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.

   [10]  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.

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




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

   [13]  Das, S., Narayanan, V., and L. Dondeti, "ALTO: A Multi
         Dimensional Peer Selection Problem",
         draft-saumitra-alto-multi-ps-00 (work in progress),
         October 2008.

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

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

   [16]  Yang, Y., Popkin, L., Penno, R., and S. Shalunov, "An
         Architecture of ALTO for P2P Applications",
         draft-yang-alto-architecture-00 (work in progress), March 2009.

   [17]  Zyp, K., "A JSON Media Type for Describing the Structure and
         Meaning of JSON Documents", draft-zyp-json-schema-02 (work in
         progress), March 2010.

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

   [19]  Song, H., Even, R., Pascual, V., and Y. Zhang, "Application-
         Layer Traffic Optimization (ALTO): Discover ALTO Servers",
         draft-song-alto-server-discovery-00 (work in progress),
         March 2009.

   [20]  Penno, R. and J. Medved, "ALTO and IPv4/IPv6 Co-existence and
         Transition", draft-penno-alto-ipv4v6-00 (work in progress),
         June 2010.

   [21]  Baker, F., Li, X., and C. Bao, "Framework for IPv4/IPv6
         Translation", draft-baker-behave-v4v6-framework-02 (work in
         progress), February 2009.

   [22]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Address
         Translation (NAT66)", draft-mrw-behave-nat66-02 (work in
         progress), March 2009.

   [23]  "Bittorrent Protocol Specification v1.0",
         http://wiki.theory.org/BitTorrentSpecification, 2009.




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

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

   [26]  Stiemerling, M. and S. Kiesel, "ALTO Deployment
         Considerations", draft-stiemerling-alto-deployments-04 (work in
         progress), July 2010.


Appendix A.  Acknowledgments

   Thank you to Jan Seedorf for contributions to the Security
   Considerations section.  We would like to thank Yingjie Gu and Roni
   Even for helpful input and design concerning ALTO Information
   redistribution.

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

      Obi Akonjang (DT Labs/TU Berlin),

      Saumitra M. Das (Qualcomm Inc.),

      Syon Ding (China Telecom),

      Doug Pasko (Verizon),

      Laird Popkin (Pando Networks),

      Satish Raghunath (Juniper Networks),

      Albert Tian (Ericsson/Redback),

      Yu-Shun Wang (Microsoft),

      David Zhang (PPLive),

      Yunfei Zhang (China Mobile).

   We would also like to thank the following additional people who were
   involved in the projects that contributed to this merged document:
   Alex Gerber (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



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   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

   [[Comment.2: 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


   Richard Woundy
   Comcast

   Richard_Woundy@cable.comcast.com


Authors' Addresses

   Richard Alimi (editor)
   Yale University

   Email: richard.alimi@yale.edu












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   Reinaldo Penno (editor)
   Juniper Networks
   1194 N Mathilda Avenue
   Sunnyvale,   CA
   USA

   Email: rpenno@juniper.net


   Y. Richard Yang (editor)
   Yale University

   Email: yry@cs.yale.edu






































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