Network Working Group                                         J. Seedorf
Internet-Draft                                                       NEC
Intended status: Informational                                 E. Burger
Expires: October 23, 2009                            This Space for Sale
                                                          April 21, 2009

    Application-Layer Traffic Optimization (ALTO) Problem Statement

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   Peer-to-peer applications, such as file sharing, real-time
   communication, and live media streaming, use a significant amount of

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   Internet resources.  Such applications often transfer large amounts
   of data in direct peer-to-peer connections.  However, they usually
   have little knowledge of the underlying network topology.  As a
   result, they may choose their peers based on measurements and
   statistics that, in many situations, may lead to suboptimal choices.
   This document describes problems related to optimizing traffic
   generated by peer-to-peer applications and associated issues such
   optimizations raise in the use of network-layer information.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  State-of-the-Art . . . . . . . . . . . . . . . . . . . . .  4
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The Problem  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  File sharing . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2.  Cache/Mirror Selection . . . . . . . . . . . . . . . . . .  8
     4.3.  Live Media Streaming . . . . . . . . . . . . . . . . . . .  9
     4.4.  Realtime Communications  . . . . . . . . . . . . . . . . .  9
     4.5.  Distributed Hash Tables  . . . . . . . . . . . . . . . . .  9
   5.  Aspects of the Problem . . . . . . . . . . . . . . . . . . . .  9
     5.1.  ALTO Service Providers . . . . . . . . . . . . . . . . . . 10
     5.2.  ALTO Service Implementation  . . . . . . . . . . . . . . . 10
     5.3.  User Privacy . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Topology Hiding  . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Coexistence with Caching . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   10. Informative References . . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

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1.  Introduction

1.1.  Overview

   Peer-to-peer (P2P) applications, such as file sharing, real-time
   communication, and live media streaming, use a significant amount of
   Internet resources[WWW.wired.fuel].  Different from the client/server
   architecture, P2P applications access resources such as files or
   media relays distributed across the Internet and exchange large
   amounts of data in connections that they establish directly with
   nodes sharing such resources.

   One advantage of P2P systems results from the fact that the resources
   such systems offer are often available through multiple replicas.
   However, applications generally do not have reliable information of
   the underlying network and thus have to select among available
   instances based on information they deduce from empirical
   measurements that, in some situations, lead to suboptimal choices.
   For example, one popular metric is an estimation of round-trip time.
   This choice occurs before actual data transmission begins and thus
   before the peer can deduce actual throughput.  This is one reason why
   a peer selection algorithm that simply uses round-trip time often
   results in a sub-optimal choice of peers.

   Many of today's P2P systems use an overlay network consisting of
   direct peer connections.  Such connections often do not account for
   the underlying network topology.  In addition to having suboptimal
   performance, such networks can lead to congestion and cause serious
   inefficiencies.  As shown in [ACM.fear], traffic generated by popular
   P2P applications often cross network boundaries multiple times,
   overloading links which are frequently subject to congestion
   [ACM.bottleneck].  Moreover, such transits, besides resulting in a
   poor experience for the user, can be quite costly to the network

   Recent studies [ACM.ispp2p] [WWW.p4p.overview] [ACM.ono] show a
   possible solution to this problem.  Internet Service Providers (ISP),
   network operators or third parties can collect reliable network
   information.  This information includes relevant information such as
   topology or instantaneous bandwidth available.  Normally, such
   information is rather "static", i.e., information which can change
   over time but on a much longer time scale than information used for
   congestion control on the transport layer.  By providing this
   information to P2P applications, it would be possible to greatly
   increase application performance, reduce congestion and optimize the
   overall traffic across different networks.  Presumably, both the
   application and the network operator can benefit from such
   information.  Thus, network operators have an incentive to provide,

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   either directly themselves or indirectly through a third party, such
   information and applications have an incentive to use such
   information.  This document gives the problem statement of optimizing
   traffic generated by P2P applications using information provided by a
   separate party.

   Section 3 introduces the problem.  Section 4 describes some use cases
   where both P2P applications and network operators benefit from a
   solution to such a problem.  Section 5 describes the main issues to
   consider when designing such a solution.  Note a companion document
   to this document, the ALTO Requirements [I-D.ietf-alto-reqs], goes
   into the details of these issues.

1.2.  State-of-the-Art

   The papers [I-D.bonaventure-informed-path-selection] and [ACM.ispp2p]
   [WWW.p4p.overview] are examples of contemporary solution proposals
   that address the problem described in this document.  Moreover, these
   proposals have encouraging simulation and field test results.  These
   and similar, independent, solutions all consist of two essential
   o  a discovery mechanism which a P2P application uses to find a
      reliable information source and
   o  a protocol P2P applications use to query such sources in order to
      retrieve the information needed to perform better-than-random
      selection of the endpoints providing a desired resource.

   It is not easy to foresee how such solutions will perform in the
   Internet.  A more accurate evaluation requires representative data
   collected from real systems by a critical mass of users.  However,
   wide adoption is unlikely without an agreement on a common solution
   based upon an open standard.

2.  Definitions

   The following terms have special meaning in the definition of the
   Application-Layer Traffic Optimization (ALTO) problem.

   Application:  A distributed communication system (e.g., file sharing)
      that uses the ALTO service to improve its performance or quality
      of experience while optimizing resource consumption in the
      underlying network infrastructure.  Applications may use the P2P
      model to organize themselves, use the client-server model, or use
      a hybrid of both.

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   Peer:  A specific participant in an application.  Colloquially, a
      peer refers to a participant in a P2P network or system, and this
      definition does not violate that assumption.  If the basis of the
      application is the client-server or hybrid model, then the usage
      of the terms "client" and "server" disambiguates the peer's role.
   P2P:  Peer-to-Peer.
   Resource:  Content, such as a file or a chunk of a file or a server
      process, for example to relay a media stream or perform a
      computation, which applications can access.  In the ALTO context,
      a resource is often available in several equivalent replicas.  In
      addition, different peers share these resources, often
   Resource Identifier:  An application layer identifier used to
      identify a resource, no matter how many replicas exist.
   Resource Provider:  For P2P applications, a resource provider is a
      specific peer that provides some resources.  For client-server or
      hybrid applications, a provider is a server that hosts a resource.
   Resource Consumer:  For P2P applications, a resource consumer is a
      specific peer that needs to access resources.  For client-server
      or hybrid applications, a consumer is a client that needs to
      access resources.
   Transport Address:  All address information that a resource consumer
      needs to access the desired resource at a specific resource
      provider.  This information usually consists of the resource
      provider's IP address and possibly other information, such as a
      transport protocol identifier or port numbers.
   Overlay Network:  A virtual network consisting of direct connections
      on top of another network, established by a group of peers.
   Resource Directory:  An entity that is logically separate from the
      resource consumer that assists a resource consumer to identify a
      set of resource providers.  Some P2P applications refer to the
      resource directory as a P2P tracker.
   Host Location Attribute:  Information about the location of a host in
      the network topology.  The ALTO service gives recommendations
      based on this information.  A host location attribute may consist
      of, for example, an IP address, an address prefix or address range
      that contains the host, an autonomous system (AS) number, or any
      other localization attribute.  These different options may provide
      different levels of detail.  Depending on the system architecture,
      this may have implications on the quality of the recommendations
      ALTO is able to provide, on whether recommendations can be
      aggregated, and on how much privacy-sensitive information about
      users might be disclosed to additional parties.
   ALTO Service:  Several resource providers may be able to provide the
      same resource.  The ALTO service gives guidance to a resource
      consumer or resource directory about which resource provider(s) to
      select in order to optimize the client's performance or quality of
      experience while optimizing resource consumption in the underlying

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      network infrastructure.
   ALTO Server:  A logical entity that provides interfaces to query the
      ALTO service.
   ALTO Client:  The logical entity that sends ALTO queries.  Depending
      on the architecture of the application one may embed it in the
      resource consumer or in the resource directory.
   ALTO Query:  A message sent from an ALTO client to an ALTO server,
      which requests guidance from the ALTO Service.
   ALTO Response:  A message sent from an ALTO server to an ALTO client,
      which contains guiding information from the ALTO service.
   ALTO Transaction:  An ALTO transaction consists of an ALTO query and
      the corresponding ALTO response.
   Local Traffic:  Traffic that stays within the network infrastructure
      of one Internet Service Provider (ISP).  This type of traffic
      usually results in the least cost for the ISP.
   Peering Traffic:  Internet traffic exchanged by two Internet Service
      Providers whose networks connect directly.  Apart from
      infrastructure and operational costs, peering traffic is often
      free to the ISPs, within the contract of a peering agreement.
   Transit Traffic:  Internet traffic exchanged on the basis of economic
      agreements amongst Internet Service Providers (ISP).  An ISP
      generally pays a transit provider for the delivery of traffic
      flowing between its network and remote networks to which the ISP
      does not have a direct connection.
   Application Protocol:  A protocol used by the application for
      establishing an overlay network between the peers and exchanging
      data on it, as well as for data exchange between peers and
      resource directories if applicable.  These protocols play an
      important role in the overall ALTO architecture.  However,
      defining them is out of the scope of the ALTO WG.
   ALTO Client Protocol:  The protocol used for sending ALTO queries and
      ALTO replies between an ALTO client and ALTO Server.
   Provisioning Protocol:  A protocol used for populating the ALTO
      server with topology-related information.
   Inter-ALTO Server Protocol:  The protocol used for synchronization,
      query forwarding, or referral between ALTO servers that have been
      provisioned with only partial knowledge of the topology-related
      information (e.g., on a per-domain basis).

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                                           +-----+  |  Peers
           +-----+       +------+    +=====|     |--+
           |     |.......|      |====+     +--*--+
           +-----+       +------+    |        *
         Source of        ALTO       |        *
         topological      service    |     +--*--+
         information                 +=====|     | Super-peer
                                           +-----+ (Tracker, proxy)
         === ALTO client protocol
         *** Application protocol (out of scope)
         ... Provisioning or initialization (out of scope)

     Figure 1 - Overview of protocol interaction between ALTO elements

   Figure 1 shows the scope of the ALTO client protocol: Peers or super-
   peers can use such a protocol to query an ALTO-service.  The mapping
   of topological information onto an ALTO service as well as the
   application protocol interaction between peers and super-peers are
   out of scope for the ALTO client protocol.

3.  The Problem

   Network engineers have been facing the problem of traffic
   optimization for a long time and have designed mechanisms like MPLS
   [RFC3031] and DiffServ [RFC3260] to deal with it.  The problem these
   protocols address consists in finding (or setting) optimal routes for
   packets traveling between specific source and destination addresses
   and based on requirements such as low latency, high reliability, and
   priority.  Such solutions are usually implemented at the link and
   network layers, and tend to be almost transparent.  At best,
   applications can only "mark" the traffic they generate with the
   corresponding properties.

   However, P2P applications that are today posing serious challenges to
   Internet infrastructures do not benefit much from the above route-
   based techniques.  Cooperating with external services aware of the
   network topology could greatly optimize the traffic the P2P
   application generates.  In fact, when a P2P application needs to
   establish a connection, the logical target is not a host, but rather
   a resource (e.g., a file or a media relay) that is often available in
   multiple instances on different peers.  Selection of the closest one
   -- or, in general, the best from an overlay topological proximity --
   has much more impact on the overall traffic than the route followed

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   by its packets to reach the endpoint.

   Optimization of peer selection is particularly important in the
   initial phase of the process.  Consider a P2P protocol such as
   BitTorrent, where a querying peer receives a list of candidate
   destinations where a resource resides.  From this list, the peer will
   derive a smaller set of candidates to connect to and exchange
   information with.  In another example, a streaming video client may
   be provided with a list of destinations from which it can stream
   content.  In both cases, the use of topology information in an early
   stage will allow applications to improve their performance and will
   help ISPs make a better use of their network resources.  In
   particular, an economic goal for ISPs is to reduce the transit
   traffic on interdomain links.

   Addressing the Application-Layer Traffic Optimization (ALTO) problem
   means, on the one hand, deploying an ALTO service to provide
   applications with information regarding the underlying network and,
   on the other hand, enhancing applications in order to use such
   information to perform better-than-random selection of the endpoints
   they establish connections with.

4.  Use Cases

4.1.  File sharing

   File sharing applications allow users to search for content shared by
   other users and download it.  Typically, search results consist of
   many instances of the same file (or chunk of a file) available from
   multiple sources.  The goal of an ALTO solution is to help peers find
   the best ones according to the underlying networks.

   On the application side, integration of ALTO functionalities may
   happen at different levels.  For example, in the completely
   decentralized Gnutella network, selection of the best sources is
   totally up to the user.  In systems like BitTorrent and eDonkey,
   central elements such as trackers or servers act as mediators.
   Therefore, in the former case, optimization would require
   modification in the applications, while in the latter it could just
   be implemented in some central elements.

4.2.  Cache/Mirror Selection

   Providers of popular content like media and software repositories
   usually resort to geographically distributed caches and mirrors for
   load balancing.  Selection of the proper mirror/cache for a given
   user is today based on inaccurate geolocation data, on proprietary

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   network location systems or often delegated to the user himself.  An
   ALTO solution could be easily adopted to ease such a selection in an
   automated way.

4.3.  Live Media Streaming

   P2P applications for live streaming allow users to receive multimedia
   content produced by one source and targeted to multiple destinations,
   in a real-time or near-real-time way.  This is particularly important
   for users or networks that do not support multicast.  Peers often
   participate in the distribution of the content, acting as both
   receivers and senders.  The goal of an ALTO solution is to help peers
   to find the best sources and the best destinations for media flows
   they receive and relay.

4.4.  Realtime Communications

   P2P real-time communications allow users to establish direct media
   flows for real-time audio, video, and real-time text calls or to have
   text chats.  In the basic case, media flows directly between the two
   endpoints.  However, unfortunately a significant portion of users
   have limited access to the Internet due to NATs, firewalls or
   proxies.  Thus, other elements need to relay the media.  Such media
   relays are distributed over the Internet with a public addresses.  An
   ALTO solution needs to help peers to find the best relays.

4.5.  Distributed Hash Tables

   Distributed hash tables (DHT) are a class of overlay algorithms used
   to implement lookup functionalities in popular P2P systems, without
   using centralized elements.  In such systems, peers maintain
   addresses of other peers participating in the same DHT in a routing
   table, sorted according to specific criteria.  An ALTO solution will
   provide valuable information for DHT algorithms.

5.  Aspects of the Problem

   This section introduces some aspects of the problem that some people
   may not be aware of when they first start studying the problem space.

   The goal of an ALTO service is to provide applications with
   information they can use to perform better-than-random peer

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5.1.  ALTO Service Providers

   At least three different kinds of entities can provide ALTO services:
   1.  Network operators.  Network operators usually have full knowledge
       of the network they administer and are aware of their network
       topology and transit and peering traffic policies.
   2.  Third parties.  Third parties are entities separate from network
       operators, but which may have either collected network
       information or have arrangements with network operators to learn
       the network information.  Examples of such entities are content
       delivery networks like Akamai, which control wide and highly
       distributed infrastructures, or companies providing an ALTO
       service on behalf of ISPs.
   3.  User communities.  User communities run distributed algorithms,
       for example for estimating the topology of the Internet.

5.2.  ALTO Service Implementation

   It is important for the reader to understand there are significant
   user communities that expect an ALTO Server to be a centralized
   service.  Likewise, there are other user communities that expect a
   service that services P2P applications be itself a distributed,
   possibly even a P2P, service.

   A result of this is one can reasonably expect there to be some sort
   of service discovery mechanism to go along with the ALTO protocol

5.3.  User Privacy

   On the one hand, there are data elements an ALTO client could provide
   in its query to an ALTO server that could help increase the level of
   accuracy in the replies.  For example, if the querying client
   indicates what kind of application it is using (e.g. real-time
   communications or bulk data transfer), the server will be able to
   indicate priorities in its replies accommodating the requirements of
   the traffic the application will generate.  On the other hand,
   applications might consider such information private.  In addition,
   some applications may not know a priori what kind of request they
   will be making.

5.4.  Topology Hiding

   Operators, with their intimate knowledge of their network topology,
   can play an important role in addressing the ALTO problem.  However,
   operators often consider such network information to be confidential.

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5.5.  Coexistence with Caching

   Caching is a common approach to optimizing traffic generated by
   applications that require large data transfers.  In some cases, such
   techniques have proven to be extremely effective in both enhancing
   user experience and saving network resources.

   A cache, either explicitly or transparently, replaces the content
   source.  Thus, the cache must use the same protocol as the querying
   peer.  That is, if a cache stores web content, it must present an
   HTTP interface to the web client.  Any cache solution for a given
   protocol needs to present that same protocol to the client.  Said
   differently, each caching solution for a different protocol needs to
   implement that specific protocol.  For this reason, one can only
   reasonably expect caching solutions for the most popular protocols,
   such as HTTP and BitTorrent.

   It is extremely important to realize that caching and ALTO are
   entirely orthogonal.  ALTO, especially if it is aware of caches, can
   in fact direct clients to nearby caches where the user could get a
   much better quality of experience.

6.  Security Considerations

   This document is neither a requirements document nor a protocol
   specification.  However, we believe it is important for the reader to
   understand areas of security and privacy that will be important for
   the design and implementation of an ALTO solution.  Moreover, issues
   such as digital rights management are out of scope for ALTO, as they
   are not technically enforceable at this level.

   The approach proposed in this document asks P2P applications to
   delegate a portion of their routing capability to third parties.
   This gives the third party a significant role in P2P systems.

   In the case where the network operator deploys an ALTO solution, it
   the P2P community might consider it hostile because the operator
   could, for example:
   o  use ALTO to prevent content distribution and enforce copyrights;
   o  redirect applications to corrupted mediators providing malicious
   o  track connections to perform content inspection or logging; and
   o  apply policies based on criteria other than network efficiency.
      For example, the service provider may suggest routes sub-optimal
      from the user's perspective to avoid peering points regulated by
      inconvenient economic agreements.

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   It is important to note there is no protocol mechanism to require
   ALTO for P2P applications.  If, for some reason, ALTO fails to
   improve the performance of P2P applications, ALTO will not gain
   popularity and the P2P community will not use it.

   At the time of this writing, the privacy issues described in the
   previous section are relevant for an ALTO solution.  Users may be
   reluctant to disclose sensitive information to an ALTO server.
   Operators, on the other hand, may not wish to disclose information
   that would expose details of their interior topology.  When exploring
   the solution space in detail, one needs to consider these issues so
   that an ALTO protocol does not presume mandatory information
   disclosure, by either clients or servers.

7.  IANA Considerations


8.  Contributors

   The basis of this document is draft-marocco-alto-problem-statement,
   written by Enrico Marocco and Vijay Gurbani.  They continue to
   provide significant edits and inputs to the current document editors.

9.  Acknowledgments

   Vinay Aggarwal and the P4P working group conducted the research work
   done outside the IETF.  Emil Ivov, Rohan Mahy, Anthony Bryan,
   Stanislav Shalunov, Laird Popkin, Stefano Previdi, Reinaldo Penno,
   Dimitri Papadimitriou, Sebastian Kiesel, Greg DePriest and many
   others provided insightful discussions, specific comments and much
   needed corrections.

   Jan Seedorf and Sebastian Kiesel are partially supported by the NAPA-
   WINE project (Network-Aware P2P-TV Application over Wise Networks,, a research project supported by the
   European Commission under its 7th Framework Program (contract no.
   214412).  The views and conclusions contained herein are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either expressed or implied, of
   the NAPA-WINE project or the European Commission.

   Thanks in particular to Richard Yang for several reviews.

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10.  Informative References

              Akella, A., Seshan, S., and A. Shaikh, "An Empirical
              Evaluation of WideArea Internet Bottlenecks",  Proceedings
              of ACM SIGCOMM, October 2003.

              Karagiannis, T., Rodriguez, P., and K. Papagiannaki,
              "Should ISPs fear Peer-Assisted Content Distribution?",
               In ACM USENIX IMC, Berkeley 2005.

              Aggarwal, V., Feldmann, A., and C. Scheideler, "Can ISPs
              and P2P systems co-operate for improved performance?",  In
              ACM SIGCOMM Computer Communications Review
              (CCR), 37:3, pp. 29-40.

   [ACM.ono]  Choffnes, D. and F. Bustamante, "Taming the Torrent: A
              practical approach to reducing cross-ISP traffic in P2P
              systems",  Proceedings of ACM SIGCOMM, August 2008.

              Saucez, D. and B. Donnet, "The case for an informed path
              selection service",
              draft-bonaventure-informed-path-selection-00 (work in
              progress), February 2008.

              Kiesel, S., Popkin, L., Previdi, S., Woundy, R., and Y.
              Yang, "Application-Layer Traffic Optimization (ALTO)
              Requirements", draft-ietf-alto-reqs-00 (work in progress),
              April 2009.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, April 2002.

              Gummadi, K., Gummadi, R., Ratnasamy, S., Gribble, S.,
              Shenker, S., and I. Stoica, "The impact of DHT routing
              geometry on resilience and proximity",  Proceedings of ACM
              SIGCOMM, August 2003.

              Xie, H., Krishnamurthy, A., Silberschatz, A., and R. Yang,

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              "P4P: Explicit Communications for Cooperative Control
              Between P2P and Network Providers",

              Glasner, J., "P2P fuels global bandwidth binge",

Authors' Addresses

   Jan Seedorf
   NEC Laboratories Europe, NEC Europe Ltd.
   Kurfuersten-Anlage 36
   Heidelberg  69115

   Phone: +49 (0) 6221 4342 221

   Eric W. Burger
   This Space for Sale
   New Hampshire

   Fax:   +1 530 267 7447

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