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ALTO WG                                                    R. Alimi, Ed.
Internet-Draft                                           Yale University
Intended status: Standards Track                           R. Penno, Ed.
Expires: June 19, 2010                                  Juniper Networks
                                                            Y. Yang, Ed.
                                                         Yale University
                                                       December 16, 2009


                             ALTO Protocol
                    draft-ietf-alto-protocol-01.txt

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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 an 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].

























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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
     2.2.  ALTO Service and Protocol Scope  . . . . . . . . . . . . .  7
   3.  Protocol Structure . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Server Capability  . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Services . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Map Service  . . . . . . . . . . . . . . . . . . . . .  9
       3.2.2.  Map Filtering Service  . . . . . . . . . . . . . . . . 10
       3.2.3.  Endpoint Property Service  . . . . . . . . . . . . . . 10
       3.2.4.  Ranking Service  . . . . . . . . . . . . . . . . . . . 10
   4.  Network Map  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  PID  . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  Example Network Map  . . . . . . . . . . . . . . . . . . . 11
   5.  Cost Map . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Cost Attributes  . . . . . . . . . . . . . . . . . . . . . 12
       5.1.1.  Cost Type  . . . . . . . . . . . . . . . . . . . . . . 12
       5.1.2.  Cost Mode  . . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Cost Map Structure . . . . . . . . . . . . . . . . . . . . 13
     5.3.  Network Map and Cost Map Dependency  . . . . . . . . . . . 13
   6.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Design Approach  . . . . . . . . . . . . . . . . . . . . . 14
       6.1.1.  Use of Existing Infrastructure . . . . . . . . . . . . 14
       6.1.2.  ALTO Information Reuse and Redistribution  . . . . . . 14
     6.2.  Message Format . . . . . . . . . . . . . . . . . . . . . . 15
       6.2.1.  Query Message  . . . . . . . . . . . . . . . . . . . . 15
       6.2.2.  Response Message . . . . . . . . . . . . . . . . . . . 15
       6.2.3.  Query and Response Body Encoding . . . . . . . . . . . 16
   7.  Protocol Messaging . . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  Client Processing  . . . . . . . . . . . . . . . . . . . . 16
       7.1.1.  General Processing . . . . . . . . . . . . . . . . . . 16
       7.1.2.  General Error Conditions . . . . . . . . . . . . . . . 17
     7.2.  Server Processing  . . . . . . . . . . . . . . . . . . . . 17
       7.2.1.  Successful Responses . . . . . . . . . . . . . . . . . 17
       7.2.2.  General Error Conditions . . . . . . . . . . . . . . . 17
       7.2.3.  Caching Parameters . . . . . . . . . . . . . . . . . . 17
     7.3.  ALTO Queries . . . . . . . . . . . . . . . . . . . . . . . 18
       7.3.1.  Server Capability  . . . . . . . . . . . . . . . . . . 18



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       7.3.2.  Map Service  . . . . . . . . . . . . . . . . . . . . . 18
       7.3.3.  Map Filtering Service  . . . . . . . . . . . . . . . . 19
       7.3.4.  Endpoint Property Service  . . . . . . . . . . . . . . 21
       7.3.5.  Ranking Service  . . . . . . . . . . . . . . . . . . . 22
   8.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  ALTO Client Embedded in P2P Tracker  . . . . . . . . . . . 23
     8.2.  ALTO Client Embedded in P2P Client: Numerical Costs  . . . 25
     8.3.  ALTO Client Embedded in P2P Client: Ranking  . . . . . . . 26
   9.  Discussions  . . . . . . . . . . . . . . . . . . . . . . . . . 27
     9.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 27
     9.2.  Network Address Translation Considerations . . . . . . . . 27
     9.3.  Mapping IPs to ASNs  . . . . . . . . . . . . . . . . . . . 28
     9.4.  Endpoint and Path Properties . . . . . . . . . . . . . . . 28
     9.5.  P2P Peer Selection . . . . . . . . . . . . . . . . . . . . 28
       9.5.1.  Client-based Peer Selection  . . . . . . . . . . . . . 29
       9.5.2.  Server-based Peer Selection  . . . . . . . . . . . . . 29
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 29
     11.1. ISPs . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     11.2. ALTO Clients . . . . . . . . . . . . . . . . . . . . . . . 29
     11.3. ALTO Information . . . . . . . . . . . . . . . . . . . . . 30
     11.4. ALTO Information Redistribution  . . . . . . . . . . . . . 30
     11.5. Denial of Service  . . . . . . . . . . . . . . . . . . . . 31
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
     12.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 33
   Appendix B.  Authors . . . . . . . . . . . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34






















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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
   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 [5] 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 [6], [7];

   o  ALTO Info-Export [8];

   o  Query/Response [9], [10];

   o  ATTP [ATTP].

   o  Proxidor [19].

   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 backbone and transit 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.  Below we start
   with an introduction to the terminology.  Then we define the overall
   architecture and how the ALTO Protocol fits into the architecture.

2.1.  Terminology

   We use the following terms defined in [11]: 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 end
   point.  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 concept denoting a single endpoint or
   group of endpoints.  Whenever we say Network Location, we refer to
   either a single endpoint or a group of endpoints.

2.2.  ALTO Service and Protocol Scope

   An ALTO Server conveys the network information from the perspective
   of a network region.  We say that the ALTO Server presents its "my-
   Internet View" [12] of the network region.  A network region in this
   context can be an Autonomous System, an ISP, perhaps a smaller
   region, or perhaps a 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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   +-------------------------------------------------------------------+
   |                               ISP                                 |
   |                                                                   |
   |                    +-----------+                                  |
   |                    | Routing   |                                  |
   |  +--------------+  | Protocols |                                  |
   |  | Provisioning |  +-----------+                                  |
   |  | Policy       |        |                                        |
   |  +--------------+\       |                                        |
   |                   \      |                                        |
   |                    \     |                                        |
   |  +-----------+      \+---------+                      +--------+  |
   |  |Dynamic    |       | ALTO    | ALTO Protocol        | ALTO   |  |
   |  |Network    |.......| Server  | -------------------- | Client |  |
   |  |Information|       +---------+                      +--------+  |
   |  +-----------+      /                                /            |
   |                    /         ALTO SD Query/Response /             |
   |                   /                                /              |
   |          +----------+                  +--------------+           |
   |          | External |                  | ALTO Service |           |
   |          | Interface|                  | Discovery    |           |
   |          +----------+                  +--------------+           |
   |               |                                                   |
   |               |           Figure 1: Basic ALTO Architecture.      |
   |               |                                                   |
   +-------------------------------------------------------------------+
                   |
         +------------------+
         | Third Parties    |
         |                  |
         | Content Providers|
         +------------------+

                             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 Endpoints and paths between network
   locations.

   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



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   structure and encoding, and transaction model.  The protocol is
   subdivided into services of related functionality.  ALTO-Core
   provides the Map Service.  Other services can provide additional
   functionality.  There are three such services defined in this
   document: the Map Filtering Service, Endpoint Property Service, and
   Ranking Service.  Additional services may be defined in the future in
   companion documents.


   .-------------------------------------------------------------------.
   |                                                                   |
   | .----------. .---------------. .---------------. .-------------.  |
   | |          | | Map Filtering | | Endpont Prop. | |   Ranking   |  |
   | |          | |    Service    | |    Service    | |   Service   |  |
   | |          | `---------------' `---------------' `-------------'  |
   | | Server   | .-------------------------------------------------.  |
   | |Capability| |  Map Service                                    |  |
   | |          | |         .-------------.  .--------------.       |  |
   | |          | |         | Network Map |  |  Cost Map    |       |  |
   | |          | |         `-------------'  `--------------'       |  |
   | `----------' `-------------------------------------------------'  |
   |                                                                   |
   `-------------------------------------------------------------------'

                     Figure 1: ALTO Protocol Structure

3.1.  Server Capability

   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.  The capability document can be downloaded by ALTO
   Clients or provisioned into devices.

3.2.  Services

3.2.1.  Map Service

   The Map Service provides batch information to ALTO Clients.  Two maps
   are provided in this document.  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.




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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 ALTO Server maps 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.  Ranking Service

   Some ALTO Clients may also benefit from querying for rankings and
   costs based on endpoints.  The Ranking Service allows an ALTO Server
   to return either numerical costs or ordinal costs (rankings) for
   additional network locations types such as Endpoints.


4.  Network Map

   In reality, many endpoints are very close to one another in terms of
   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
   loosing 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 Network Map may also be used to communicate simple preferences.
   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.

   Note that 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.






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4.1.  PID

   Each group can be 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 a identifier providing 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.

4.2.  Example Network Map

   Figure 2 illustrates an example Network Map. PIDs are used to
   identify network-agnostic aggregations.

   .--------------------------------------------------------.
   | ALTO Network Map                                       |
   |                                                        |
   |  .--------------------------------.  .---------------. |
   |  | NetLoc: PID-1                  |  | NetLoc: PID-2 | |
   |  |  .---------------------------. |  |    ...        | |
   |  |  | 128.36.0.0/16             | |  `---------------` |
   |  |  | .-----------------------. | |                    |
   |  |  | | Endpoint: 128.36.9.8  | | |  .---------------. |
   |  |  | `-----------------------` | |  | NetLoc: PID-3 | |
   |  |  `---------------------------` |  |    ...        | |
   |  |  .---------------------------. |  `---------------` |
   |  |  | 130.132.0.0/16            | |                    |
   |  |  | .-----------------------. | |  .---------------. |
   |  |  | | Endpoint: 130.132.1.2 | | |  | NetLoc: PID-4 | |
   |  |  | `-----------------------` | |  |    ...        | |
   |  |  `---------------------------` |  `---------------` |
   |  `--------------------------------`                    |
   |                                                        |
   `--------------------------------------------------------`

                       Figure 2: 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.

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).

   If an ALTO Client requests a Cost Type that is not available, the
   ALTO Server responds with an error as specified in Section 7.2.2.2.

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
   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



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   make sense for numerical values, do not make sense for ordinal
   rankings.  ALTO Clients may want to 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.  (Note 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 Path Cost query contains any PID in the list of Source Network
   Locations or the list of Destination Network Locations, we say that
   the Path Costs are generated based on a particular Network Map (which
   defines that PID).  Version Tags are introduced to ensure that ALTO
   Clients are able to use consistent information even though the
   information is provided in two maps.  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.




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6.  Protocol Overview

6.1.  Design Approach

   The ALTO Protocol design uses a RESTful interface with the goal of
   leveraging current HTTP [2] [3] implementations and infrastructure.
   ALTO messages are denoted with HTTP Content-Type "application/alto".
   Message encodings use a structured, text-based format.  The exact
   encoding will be documented in a future revision, as the current
   focus is overall protocol architecture and operations.

   These design decisions make the protocol easier to understand and
   debug, and allows for flexible ALTO Server implementation strategies.
   More importantly, however, this enables use of existing
   implementations and infrastructure, and allows for simple caching and
   redistribution of ALTO information to increase scalability.

6.1.1.  Use of Existing Infrastructure

   An important design consideration for the ALTO Protocol is easy
   integration with existing applications and infrastructure.  As
   outlined above, HTTP is a natural choice.  In particular, this ALTO
   Protocol design leverages:

   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.

6.1.2.  ALTO Information Reuse and Redistribution

   ALTO information may be useful to a large number of applications and
   users.  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.  For example, a full Network Map may be reused by all ALTO
   Clients using the ALTO Server.







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6.2.  Message Format

   The ALTO Protocol uses a RESTful design operating over HTTP.  Both
   Query and Response follow the standard format for HTTP Request and
   Response messages [2] [3].  This section provides an overview of the
   components of a Query message sent from an ALTO Client to an ALTO
   Server, as well as the components of a Response message returned by
   an ALTO Server.  Note that if caching or redistribution is used, the
   Response message may be returned from another (possibly third-party)
   entity.  Reuse and Redistrubution is further discussed in
   Section 11.4.

6.2.1.  Query Message

   A Query message is generated by an ALTO Client and sent to an ALTO
   Server.  The ALTO Protocol employs the following components of the
   HTTP request message:

   Method:  Indicates operation requested by the ALTO Client (along with
      URI Path).

   URI Path:  Indicates the operation requested by the ALTO Client
      (along with Method).

   URI Query String Parameters:  Indicates parameters for the requested
      operation.  Note that in the messaging specification in Section 7,
      we abbreviate these as 'URI QS Params'.  Order of query string
      parameters is not specified.  Some parameters are restricted in
      how many times they appear.  We use the notation 'min..max' to
      denote the the minimum and maximum times they may appear, where
      'max' may be '*' to denote unbounded.  If no parameters are
      defined for a particular ALTO request message, the query string
      MUST be empty (and there MUST be no '?' included in the URI Path).

   Headers:  Indicates encoding of the Body.  If the HTTP body of the
      request is non-empty, the Content-Type header MUST be set to
      "application/alto".

   Body:  Indicates additional request parameters that are not concisely
      representable as Query String parameters.  If no Body is defined
      for a particular ALTO request message, the HTTP request body MUST
      be empty.

6.2.2.  Response Message

   A Response message is generated by an ALTO Server, which corresponds
   to a particular Query message.  The ALTO Protocol employs the
   following components of the HTTP Response message:



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   Status Code:  Indicates either success or an error condition.

   Headers:  Indicates encoding of the Body and caching directives.  If
      the HTTP body of the response is non-empty, the Content-Type
      header MUST be set to "application/alto"

   Body:  Contains data requested by the ALTO Client.

6.2.3.  Query and Response Body Encoding

   When the Body of a Query or Response message is not empty, it MUST
   contain a properly-encoded message.  This section will be updated to
   include common requirements when an encoding format is decided.


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.

   Note that the primary focus of the current draft is the architecture
   and protocol operations, and only the structure of messages is
   specified (without actual message encoding).  Additionally, the
   following details have been omitted for clarity:

   o  protocol version number

   o  transaction ID

   o  map version tags

   o  HTTP URL encoding

   This section will be updated as revisions are made to protocol
   details and encodings.

7.1.  Client Processing

7.1.1.  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 query messages
   which MUST conform to Section 7.3.  In response to query messages, an
   ALTO Server constructs and sends response messages which also MUST



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   conform to Section 7.3.

   An ALTO Server MAY support SSL/TLS to implement server and/or client
   authentication.  An ALTO Server also MAY support HTTP Digest
   authentication.  Cookies MUST NOT be used.  ALTO Clients and ALTO
   Servers MUST additionally follow any TCP transport and HTTP
   processing and encoding considerations.

7.1.2.  General Error Conditions

   In the case that the client does not receive an appropriate response
   from the server it can choose another server to communicate or fall
   back to a default behavior (e.g., perform peer selection without the
   use of ALTO information).

7.2.  Server Processing

7.2.1.  Successful Responses

   If a Query message is successfully processed and an ALTO response is
   generated, the HTTP status code in the response MUST be set to 200.

7.2.2.  General Error Conditions

   This section specifies ALTO Server behavior when it recevies a Query
   message that cannot be processed due to a problem with processing the
   Query message itself.

7.2.2.1.  Invalid Query Format

   If any component of the Query message is formatted incorrectly (i.e.,
   it does not follow the formats in Section 7.3), the ALTO Server MUST
   return HTTP Status Code 400.

7.2.2.2.  Unsupported Query

   If an ALTO Server does not support the operation indicated in the
   Query message, the ALTO Server MUST return HTTP Status Code 501.

7.2.3.  Caching Parameters

   If the response generated by the ALTO Server is cachable, the ALTO
   Server MAY include 'Cache-Control' and 'Expires' HTTP headers,
   enabling it to be cached by intermediary HTTP Caches or the ALTO
   Client itself.






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7.3.  ALTO Queries

7.3.1.  Server Capability

   The Server Capability query allows an ALTO Client to determine the
   available operations of a particular ALTO Server

   This query MUST be supported.

7.3.1.1.  Query

       Method        : 'GET'
       URI Path      : '/capability'

7.3.1.2.  Example Response Structure

       {
         "instance-name": "alto.example.com",
         "uri": "http:\/\/alto.example.com:6671",
         "cost": [
           {"type":"latency",  "units":"ms"},
           {"type":"pDistance","units":"scalar"},
           {"type":"bandwidth","units":"kbps"}
           ],
         "constraint-support": false
       }

7.3.2.  Map Service

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

7.3.2.1.  Network Map

   The full Network Map is an instantiation of the Reverse Property
   Lookup where the ALTO Server lists for each PID, the network
   locations (endpoints) within the PID.

7.3.2.1.1.  Query

       Method        : 'GET'
       URI Path      : '/prop/pid/map'








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7.3.2.1.2.  Example Response Structure

       {
         "PID1" : [
             "128.36.1.0/24",
             "132.130.1.0/24",
             "132.130.2.0/24"
           ],
         "PID2" : [ "130.132.3.0/24" ],
         "PID3" : [ "0.0.0.0/0" ]
       }

7.3.2.2.  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').

7.3.2.2.1.  Query

       Method        : 'GET'
       URI Path      : '/cost/pid/map'

7.3.2.2.2.  Example Response Structure

       {
         "Type": "routingcost",
         "Mode": "numerical",
         "Map" : {
           "PID1": { "PID1": 1,  "PID2": 5 , "PID3": 10 },
           "PID2": { "PID1": 5 , "PID2": 1 , "PID3": 15 },
           "PID3": { "PID1": 20, "PID2": 15, "PID3": 1  }
         }
       }

7.3.3.  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.

   The services defined in this section are OPTIONAL.






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7.3.3.1.  Network Map

   ALTO Clients can query for a subset of the full network map (see
   Section 7.3.2.1).

7.3.3.1.1.  Query

       Method        : 'POST'
       URI Path      : '/prop/pid/filter'

7.3.3.1.2.  Example Request Structure

       POST /prop/pid/filter ...

       [ "PID1", "PID2" ]

7.3.3.1.3.  Example Response Structure

       {
         "PID1" : [
             "128.36.1.0/24",
             "132.130.1.0/24",
             "132.130.2.0/24"
           ],
         "PID2" : [ "130.132.3.0/24" ],
         "PID3" : [ "0.0.0.0/0" ]
       }

7.3.3.2.  Cost Map

   ALTO Clients can query for the Cost Map (see Section 7.3.2.2) based
   on additional parameters.

7.3.3.2.1.  Query

       Method        : 'POST'
       URI Path      : '/cost/pid/filter'
       URI QS Params : 'type=[costtype]'          (multiplicity: 0..1)
                       'mode=[costmode]'          (multiplicity: 0..1)
                       'constraint=[constraint]'  (multiplicity: 0..*)

   The 'constraint' parameter is optional and is to be used only if the
   ALTO service supports it.  It allows a client to specify a target
   numerical cost.  The constraint contains two entities: (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 ALTO service configuration document obtained



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   from ALTO service discovery.  If multiple 'constraint' parameters are
   specified, the ALTO Server assumes they are related to each other
   with a logical AND.

   If the query does not specify the 'type' and 'mode' query string
   parameters, then the server assumes the type to be 'routingcost' and
   the mode to be 'numerical'.  A Query MUST contain no more than one
   'type' parameter, and no more than one 'mode' parameter.

   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 PIDs.  The ALTO Server returns costs
   between each pair of source/destination PID.

7.3.3.2.2.  Example Request Structure

       POST /cost/pid/filter

       {
         "src": [ "PID1" ],
         "dst": [ "PID1", "PID2", "PID3" ]
       }

7.3.3.2.3.  Example Response Structure

       {
         "Type": "routingcost",
         "Mode": "numerical",
         "Map" : {
           "PID1": { "PID1" : 1, "PID2": 5 , "PID3": 10 },
         }
       }

7.3.4.  Endpoint Property Service

   The Endpoint Property Lookup query allows an ALTO Client to query for
   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.  Additional supported
   properties can be defined in the Server Capability response.

   The services defined in this section are OPTIONAL.

7.3.4.1.  Query

       Method        : 'POST'
       URI Path      : '/endpoint/m'
       URI QS Params : 'prop=[propertyname]'   (multiplicity: 1..*)



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   The request body includes a list of Endpoints for which the property
   value should be returned.  If the request body is empty, the ALTO
   Server implicitly assumes that the request contains a single-element
   list with the Endpoint address of the requesting client.

   Also note that the 'prop' parameter may be specified multiple times
   to query for multiple properties simultaneously.  For example, the
   query string could be 'prop=pid&prop=bandwidth'.

7.3.4.2.  Example Request Structure

       POST /endpoint/m?prop=pid ...

       [ "ipv4:128.36.1.34" ]

7.3.4.3.  Example Response Structure

       {
         "ipv4:128.36.1.34" : { "pid": "PID1" }
       }

7.3.5.  Ranking Service

   The Ranking 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.

   The services defined in this section are OPTIONAL.

7.3.5.1.  Ranking Query

7.3.5.1.1.  Query

       Method        : 'POST'
       URI Path      : '/cost/endpoint/ranking'
       URI QS Params : 'type=[costtype]'          (multiplicity: 0..1)
                       'mode=[costmode]'          (multiplicity: 0..1)
                       'constraint=[constraint]'  (multiplicity: 0..*)

   The request body includes a list of source and destination endpoints
   that should be assigned a cost by the ALTO Server.  The 'type',
   'mode', and 'constraint' parameters behave as specified in
   Section 7.3.3.2.

   The request body MUST specify a list of source Endpoints, and a list



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   of destination Endpoints.  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.

7.3.5.1.2.  Example Request Structure

       POST /cost/endpoint/ranking?mode=ordinal ...

       {
         "src": "ipv4:128.30.24.2"
         "dst": [
             "ipv4:128.30.24.89",
             "ipv4:12.32.67.3",
             "ipv4:130.132.33.4"
           ]
       }

7.3.5.1.3.  Example Response Structure

       {
         "Type": "routingcost",
         "Mode": "ordinal",
         "Ranking" : {
           "ipv4:128.30.24.2": {
               "ipv4:128.30.24.89" : 1,
               "ipv4:130.132.33.4" : 2,
               "ipv4:12.32.67.3"   : 3
             }
         }
       }


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.




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

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

               Figure 3: ALTO Client Embedded in P2P Tracker

   Figure 3 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.




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   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.

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 4: ALTO Client Embedded in P2P Client

   Figure 4 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.





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   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.

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 5: ALTO Client Embedded in P2P Client: Ranking

   Figure 5 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.





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


9.  Discussions

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 [13] [14].  However, the
   discovery mechanism is out of scope for this document.

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

9.2.  Network Address Translation Considerations

   At this day and age of NAT v4<->v4, v4<->v6 [15], and possibly
   v6<->v6[16], a protocol should strive to be NAT friendly and minimize
   carrying IP addresses in the payload, or provide a mode of operation
   where the source IP address provide the information necessary to the
   server.

   The protocol specified in this document provides a mode of operation
   where the source network location is computed by the ALTO Server (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 [17]) operate.

   The ALTO client SHOULD use the Session Traversal Utilities for NAT
   (STUN) [4] to determine a public IP address to use as a source NL-ID.
   If using this method, the host MUST use the "Binding Request" message
   and the resulting "XOR-MAPPED-ADDRESS" parameter that is returned in
   the response.  Using STUN requires cooperation from a publicly
   accessible STUN server.  Thus, the ALTO client also requires
   configuration information that identifies the STUN server, or a
   domain name that can be used for STUN server discovery.  To be
   selected for this purpose, the STUN server needs to provide the
   public reflexive transport address of the host.







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9.3.  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.

9.4.  Endpoint and Path Properties

   An ALTO Server could make available many properties about Endpoints
   beyond their network location or grouping.  For example, connection
   type, geographical location, and others may be useful to
   applications.  The current draft focuses on network location and
   grouping, but the protocol may be extended to handle other Endpoint
   properties.

9.5.  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 [18], 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



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   without ALTO information.

9.5.1.  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) [19] [8].

   o  Optimistically unchoking low-cost peers with higher probability
      [8].

9.5.2.  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 [18].


10.  IANA Considerations

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


11.  Security Considerations

11.1.  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.  In
   particular, providing overly fine-grained information may make it
   easier for attackers to infer network topology.  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.  Other
   considerations (e.g., relating to application performance) can be



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   found in Section 6 of [11].

   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.

11.3.  ALTO Information

   An ALTO Server may optionally use authentication and encryption to
   protect ALTO information.  SSL/TLS can provide encryption as well as
   authentication of the 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).

   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 the redistribution scheme must additionally handle details
   such as ensuring ALTO Clients retrieve ALTO information from the
   correct ALTO Server.  See [20] and [21] 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
   the ALTO information signed by the ALTO Server's 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.



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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.  Therefore ISPs should employ security
   measures such as firewalls to prevent queries originating from
   unauthorized networks.

   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.


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]   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

   [5]   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.

   [6]   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.

   [7]   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.

   [8]   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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   [9]   Das, S. and V. Narayanan, "A Client to Service Query Response
         Protocol for ALTO", draft-saumitra-alto-queryresponse-00 (work
         in progress), March 2009.

   [10]  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.

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

   [12]  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.

   [13]  Garcia, G., Tomsu, M., and Y. Wang, "ALTO Discovery Protocols",
         draft-wang-alto-discovery-00 (work in progress), March 2009.

   [14]  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.

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

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

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

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

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

   [20]  Yingjie, G., Alimi, R., and R. Even, "ALTO Information
         Redistribution", draft-gu-alto-redistribution-01 (work in
         progress), October 2009.

   [21]  Stiemerling, M., "ALTO Information Redistribution Considered



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         Harmful", draft-stiemerling-alto-info-redist-00 (work in
         progress), August 2009.


Appendix A.  Acknowledgments

   First, 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
   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.1: RFC Editor: Please move information in this section to
   the Authors' Addresses section at publication time.]]



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   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


   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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