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Versions: 00 01 RFC 5594

Network Working Group                                        J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                               A. Cooper
Expires: August 27, 2009               Center for Democracy & Technology
                                                       February 23, 2009


   Report from the IETF workshop on P2P Infrastructure, May 28, 2008
                  draft-p2pi-cooper-workshop-report-01

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Abstract

   This document reports the outcome of a workshop organized by the



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   Real-time Applications and Infrastructure Area Directors of the IETF
   to discuss network delay and congestion issues resulting from
   increased P2P traffic volumes.  The workshop was held on May 28, 2008
   at MIT in Cambridge, MA, USA.  The goals of the workshop were
   twofold: to understand the technical problems ISPs and end users are
   experiencing as a result of high volumes of P2P traffic, and to begin
   to understand how the IETF may be helpful in addressing these
   problems.  Gaining an understanding of where in the IETF this work
   might be pursued and how to extract out feasible work items were
   highlighted as important tasks in pursuit of the latter goal.  The
   workshop was very well attended and produced several work items that
   have since been taken up by members of the IETF community.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4

   2.  Scoping of the Problem and Solution Spaces . . . . . . . . . .  5

   3.  Service Provider Perspective . . . . . . . . . . . . . . . . .  5
     3.1.  DOCSIS Architecture and Upstream Contention  . . . . . . .  5
     3.2.  TCP Flow Fairness and Service Flows  . . . . . . . . . . .  6
     3.3.  Service Provider Responses . . . . . . . . . . . . . . . .  7

   4.  Application Provider Perspective . . . . . . . . . . . . . . .  8

   5.  Potential Solution Areas . . . . . . . . . . . . . . . . . . .  8
     5.1.  Improving Peer Selection: Information Sharing,
           Localization, and Caches . . . . . . . . . . . . . . . . .  9
       5.1.1.  Leveraging AS Numbers  . . . . . . . . . . . . . . . . 10
       5.1.2.  P4P: Provider Portal for P2P Applications  . . . . . . 10
       5.1.3.  Multi-Layer Tracker-Based Architecture . . . . . . . . 11
       5.1.4.  ISP-Aided Neighbor Selection . . . . . . . . . . . . . 12
       5.1.5.  Caches . . . . . . . . . . . . . . . . . . . . . . . . 13
       5.1.6.  Potential IETF Work  . . . . . . . . . . . . . . . . . 13
     5.2.  New Approaches to Congestion Control . . . . . . . . . . . 15
       5.2.1.  End-to-End Congestion Control  . . . . . . . . . . . . 15
       5.2.2.  Weighted Congestion Control  . . . . . . . . . . . . . 16
     5.3.  Quality of Service . . . . . . . . . . . . . . . . . . . . 17

   6.  Applications Opening Multiple TCP Connections  . . . . . . . . 18

   7.  Costs and Congestion . . . . . . . . . . . . . . . . . . . . . 18

   8.  Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Transport Issues . . . . . . . . . . . . . . . . . . . . . 19
     8.2.  Improved Peer Selection  . . . . . . . . . . . . . . . . . 20



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   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20

   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20

   11. Informative References . . . . . . . . . . . . . . . . . . . . 20

   Appendix A.  Program Committee . . . . . . . . . . . . . . . . . . 21

   Appendix B.  Workshop Participants . . . . . . . . . . . . . . . . 21

   Appendix C.  Workshop Agenda . . . . . . . . . . . . . . . . . . . 23

   Appendix D.  Slides and Position Papers  . . . . . . . . . . . . . 23

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23




































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1.  Introduction

   Increasingly, large ISPs are encountering issues with P2P traffic.
   The transfer of static, delay-tolerant data between nodes on the
   Internet is a well-understood problem, but traditional management of
   fairness at the transport level is under strain from applications
   designed to achieve the best end-user transfer rates.  At peak times
   this results in networks running near absolute capacity, causing all
   traffic to incur delays; the applications that bear the brunt of this
   additional latency are real-time applications like VoIP and Internet
   gaming.  To explore how IETF standards work could be useful in
   addressing these issues, the Real-time Applications and
   Infrastructure Area Directors organized a "P2P Infrastructure"
   workshop and invited contributions from subject matter experts in the
   problem and solution spaces.

   The goals of the workshop were twofold: to understand the technical
   problems ISPs and end users are experiencing as a result of high
   volumes of P2P traffic, and to begin to understand how the IETF may
   be helpful in addressing these problems.  Gaining an understanding of
   where in the IETF this work might be pursued and how to extract out
   feasible work items were highlighted as important tasks in pursuit of
   the latter goal.  The workshop's focus was on engineering solutions
   that promise some imminent benefit to the Internet as a whole, as
   opposed to longer-term research or closed properietary solutions.
   While public policy must inform work in this space, crafting or
   debating public policy was outside the scope of the workshop.

   Position papers were solicited in the weeks prior to the workshop,
   and a limited number of speakers were invited to present their views
   at the workshop based on these submissions.  This report is a summary
   of all participants' contributions.  The program committee and
   participant list are attached in Appendix A and Appendix B,
   respectively.  The agenda of the workshop can be found in Appendix C.
   A link to the presentations given at the workshop and the position
   papers submitted prior to the workshop is in Appendix D.

   The workshop showcased the IETF community's recognition of the impact
   of P2P and other high-volume applications on the Internet as a whole.
   Participants welcomed the opportunity to discuss potential
   standardization work that network operators, applications providers,
   and end users would all find mutually beneficial.  Two transport-
   related work items gained significant traction: designing a protocol
   for very deferential end-to-end congestion control for delay-tolerant
   applications, and producing an informational document about the
   reasoning behind and effects of applications opening multiple
   transport connections at once.  A seperate area of interest that
   emerged at the workshop focused on improving peer selection by having



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   networks make more information available to applications.  Finally,
   presenters also covered traditional approaches to multiple service-
   tier queuing such a diffserv.


2.  Scoping of the Problem and Solution Spaces

   The genesis for the P2PI workshop grew in large part out of specific
   pain points that ISPs are experiencing as a result of high volumes of
   P2P traffic.  However, several workshop participants felt that the
   IETF should approach a more general space of problems, of which P2P-
   related congestion may be merely one instance.

   For example, high-volume applications besides P2P, whether they
   already exist or have yet to be developed, could cause congestion
   issues similar to those caused by P2P. And while much attention has
   been paid to congestion on access links, increased traffic volumes
   could impact other parts of the network.  Although the workshop
   focused primarily on the specific causes and effects of current P2P
   traffic volumes, it may be useful in the future for the IETF to
   consider how to pursue solutions to these larger problems.

   Obtaining more data about Internet congestion may also be a helpful
   step before the IETF pursues solutions.  This data collection could
   focus on where in the network congestion is occurring, its duration
   and frequency, its effects, and its root causes.  Although individual
   service providers expressed interest in sharing congestion data,
   strategies for reliably and regularly obtaining and disseminating
   such data on a broad scale remain elusive.


3.  Service Provider Perspective

   To help participants gain a fuller understanding of one specific
   network operator view of P2P-induced congestion, Jason Livingood and
   Rich Woundy provided an overview of Comcast's network and approach to
   management of P2P traffic.

3.1.  DOCSIS Architecture and Upstream Contention

   In the Data Over Cable Service Interface Specification (DOCSIS)
   architecture [DOCSIS] used for many cable systems, there may be a
   single Cable Modem Termination System (CMTS) serving hundreds or
   thousands of residential cable customers.  Each CMTS has multiple
   DOCSIS domains, each of which typically has a single downstream link
   and a number of upstream links.  Each CMTS is connected through a
   hybrid fiber-coaxial (HFC) network to subscribers' cable modems.




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   The limiting resource in this architecture is usually bandwidth, so
   bandwidth is typically the measure used for capacity planning.  As
   with all networks, congestion manifests itself when instantaneous
   load exceeds available capacity.

   In the upstream direction, any cable modem connected to a CMTS can
   make a request to the CMTS to transmit, and requests are randomized
   to minimize collisions.  With many cable modems issuing requests at
   once, the requests may collide, resulting in delays.  DOCSIS does not
   specify a size for cable modem buffers, but buffer delays of one to
   four seconds have been observed with various cable modems from
   different vendors.

   Once the CMTS has granted a cable modem the ability to transmit its
   data PDU, the modem can piggyback its next request on top of that
   data PDU.  In situations with a lot of upstream traffic, piggybacking
   happens more often, which sends heavy upstream users to the front of
   the CMTS queue, ahead of interactive but less-upstream-intensive
   applications.  For example, if the CMTS is granting requests
   approximately every one to three milliseconds, then a cable modem
   transmitting data for a service like VoIP with a packetization delay
   of 20-30 milliseconds may get into contention with another modem on
   the same CMTS that is constantly transmitting upstream and
   piggybacking each new request.  This may explain how heavy upstream
   users ultimately dominate the pipe over more interactive
   applications.  Consequentially, it is imperative that assessments of
   the problem space, and potential solutions, are mindful of the
   influence that specific layer-2 networks may exert on the behavior of
   Internet traffic, especially when considering the alleviation of
   congestion in an access network.

3.2.  TCP Flow Fairness and Service Flows

   How TCP flow fairness applies to upstream requests to the CMTS is an
   open question.  A CMTS sees many service flows, each of which could
   be a single TCP flow or many TCP flows (or UDP).  The CMTS is not
   aware of the source or destination IP address of a packet until it
   has already been transmitted upstream, so those cannot be used to
   impose flow fairness.

   A particular cable modem can have multiple service flows defined.
   For example, a modem that is also a VoIP endpoint can provision a
   service flow for VoIP that would allow VoIP traffic to avoid the
   upstream request process to the CMTS (and thereby avoid contention
   with other modems).  The service flow would have upstream capacity
   provisioned for it.  The modem would have a separate service flow for
   best efforts traffic.  Some ISPs provision such a flow for their own
   VoIP offerings; others allow subscribers to pay extra to have



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   particular traffic assigned to a provisioned service flow.

   It may also be possible for an ISP to provision such a flow on the
   fly when it recognizes the need for it.  DiffServ [RFC2475] bits set
   by the customer premises equipment could be used to classify flows,
   for example.

3.3.  Service Provider Responses

   Starting in 2005, ISP customers were increasingly complaining about
   the performance of delay-sensitive traffic (VoIP and gaming), due in
   part to the issues arising out of the DOCSIS architecture as
   described above.  At the same time, ISPs were seeing heavy growth in
   P2P traffic, and an increasing correlation between high levels of P2P
   activity and packet loss.

   In responding to this situation, cable ISPs have several avenues to
   pursue.  The newest generation of the DOCSIS specification, DOCSIS
   3.0, enables faster transfer rates than most cable systems currently
   support.  While the rollout of DOCSIS 3.0 will provide additional
   capacity, it will likely not obviate the need for congestion
   management in an environment where client software is designed to
   maximize bandwidth consumption regardless of available capacity.

   Congestion management can take many forms; Jason and Rich explained
   the new protocol-agnostic approach that Comcast is currently
   trialing.  Prior to these trials, all traffic was marked as "best
   efforts."  During the trials, all traffic is re-classified as
   "priority."  When a CMTS is approaching peak congestion on a
   particular upstream or downstream port (the "Near Congestion State"),
   some subscribers will have traffic re-classified as "best efforts."
   The threshold for determining when a CMTS port is in Near Congestion
   State and the number of minutes it remains in this state are both
   parameters being explored during the trials.  To re-classify upstream
   traffic, a new default DOCSIS service flow is used that has the same
   provisioned bandwidth as the "priority" stream, but is treated with
   lower priority.

   The subscribers whose traffic is re-marked will be selected by
   determining whether they have temporarily entered a "Long Duration
   Bulk Consumption State."  This state is achieved by consuming a
   certain amount of bandwidth over a certain period of minutes (both
   tweakable parameters being explored during the trials).  These
   thresholds will depend on the subscriber's service tier --
   subscribers who pay for more bandwidth will have higher thresholds.
   The re-marking will not distinguish between multiple users of the
   same subscriber connection, so one family member's P2P usage could
   cause another family member's Web browsing traffic to be lowered in



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   priority.  There is no current mechanism for users to determine that
   their traffic has been re-marked.

   By temporarily reducing the traffic priority of subscribers who have
   been consuming bandwidth in bulk for lengthy periods, this congestion
   management technique aims to preserve a good user experience for
   subscribers with burstier traffic patterns, including those using
   real-time applications.  As compared to an approach that reduces
   particular subscribers' bandwidth during periods of congestion, this
   technique eliminates the ability for applications to set their own
   priority levels, but it also avoids the negative connotations that
   some users may associate with bandwidth reductions.

   This approach involves many tweakable parameters.  A large part of
   the trial process is aimed at determining the best settings for these
   parameters, but there may also be opportunities to work with the
   research community to identify the best way to adjust the thresholds
   necessary to optimize the performance of the management technique.


4.  Application Provider Perspective

   Stanislav Shalunov provided an overview of BitTorrent's view of the
   impact of increased P2P traffic volumes and potential mitigations.
   The impact is described here; his proposed solutions (comprising the
   bulk of his talk) are addressed in the appropriate subsections of
   Section 5.

   As uptake in P2P usage has grown, so has end-user latency.  For
   example, a user whose uplink capacity is 250-500 Kbps and whose modem
   buffer has a capacity of 32-64 Kbps may easily fill the buffer
   (unless the modem uses AQM, which is uncommon).  This can result in
   delay on the order of seconds, with disastrous effects on application
   performance.  On a cable system with shared capacity between
   neighbors, one neighbor could saturate the buffer and affect the
   latency of another neighbor's traffic.


5.  Potential Solution Areas

   The submissions received in advance of the workshop covered a broad
   array of work addressing specific aspects of P2P traffic volume and
   other related issues.  Solution suggestions generally fell into one
   or more of three topic areas: improving peer selection, new
   approaches to congestion control, and quality of service mechanisms.
   The workshop discussions and outcomes in each area are described
   below.




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5.1.  Improving Peer Selection: Information Sharing, Localization, and
      Caches

   Peer selection is an integral factor in determining the efficiency of
   P2P networks from both the ISP and the P2P client point of view.  How
   peers are selected will determine both network load and client
   performance.

   The way that P2P clients select peers today varies from protocol to
   protocol and client to client, but as a general matter peers are
   largely oblivious to routing-level and network topology information.
   This results in P2P topologies that are agnostic of underlay
   topologies and constraints.

   Approaches to closing this gap generally involve an entity that has
   knowledge of network topology, costs, or constraints (e.g., an ISP)
   making some of this information available to P2P clients or trackers.
   This information may be used to localize traffic based on some metric
   of locality, or otherwise alter peer selection decisions based on the
   provided network information (hereafter referred to simply as
   "localization").  One special case of this kind of approach would
   help peers find caches containing the content they seek.

   Any alteration to current peer selection algorithms will have
   engineering trade-offs.  BitTorrent, for example, used randomized
   peer selection by design.  Choosing peers randomly out of a large
   selection helps to average out problems among peers, and it allows
   for connections to good peers that may be far away.  Randomized peer
   selection also supports "rarest first" piece selection, which allows
   swarms to continue even when the original seed disappears and
   distributes pieces so that more peers are likely to have pieces of
   interest to other peers.  Any move away from randomized selection
   would have to take these factors into account.

   Although localization has the potential to improve peer selection,
   the incentives for both parties to the information exchange are
   complex.  ISPs may want to move traffic off of their own networks,
   which could motivate them to provide information to peers that has
   the opposite effect of what the peers would expect.  Likewise, peers
   will want the use of the information they receive to result in
   performance improvements; otherwise, they have no incentive to
   consult with the network before selecting peers.  Even when both
   parties find the information sharing to be beneficial, user
   experiences will not necessarily be uniform depending on the scope of
   the information provided and the peer's location.  Localization
   information could form one component of a peer selection decision,
   but it will likely need to be balanced against other factors.




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   Workshop participants discussed both current research efforts in this
   area and how IETF standards work may be useful in furthering the
   general concept of improved peer selection.  Those discussions are
   summarized below.

5.1.1.  Leveraging AS Numbers

   One simple way to potentially make peer selection more efficient
   would be for a peer to prefer peers within its own AS.  Transfers
   between peers within the same AS may be faster on some networks,
   although more data is needed to determine the extent of the potential
   improvement.  On mobile networks, for example, the utility of AS
   numbers is limited since they do not correlate to geographic
   location.  Peers may also see improvements by connecting to other
   peers within a specific set of ASes or IP prefixes provided by their
   ISPs.  Some ISPs may have an incentive to expose this granularity of
   information because it will potentially reduce their transit costs.

   A case study was conducted with the four most popular BitTorrent
   torrents to determine what the effect of localizing to an AS might
   be.  The swarm sizes for the torrents were 9984, 3944, 2561, and
   2023, with the size distributions appearing to be polynomial.  With
   more than 20 peers in a single AS, peers within an AS could trade
   only with each other, avoiding interdomain traffic.  More than half
   (57%) of peers in the four swarms were in ASes like this.  Thus, in
   these cases connecting to peers within an AS could reduce transit
   traffic by at least 57%.  If the ASes have asymmetric upload and
   download links, however, the resulting user experience may
   deteriorate since each peer's download speed would be limited by
   slower upload speeds.

   With the largest swarm size at 9984, the probability of two peers
   being in the same neighborhood is too low to make localization to the
   neighborhood level worthwhile.  Attempting a simple localization
   scheme, such as the AS localization described above, and determining
   its effectiveness likely makes more sense as a first step.

5.1.2.  P4P: Provider Portal for P2P Applications

   The P4P project [P4P] aims to design a framework to enable
   cooperation between "providers" and applications (including P2P),
   where providers may be ISPs, content distribution networks, or
   caching services.  In this architecture, each provider can
   communicate information to P2P clients through a portal known as an
   iTracker.  An iTracker could be identified through a DNS SRV record
   (perhaps with its own new record type), a whois look-up, or through a
   trusted third party.




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   An iTracker has different interfaces for different types of
   information that the provider may want to share.  The core interface
   allows the provider to express the "virtual cost" of its intradomain
   or interdomain links.  Virtual cost may reflect any kind of provider
   preferences, and may be based on the provider's choice of metrics,
   including utilization, transit costs, or geography.  It is up to the
   provider to decide how dynamic it wants to be in updating its virtual
   cost determinations.

   In tests of this framework, two parallel swarms were created with
   approximately the same number of clients and similar geographical and
   network distributions, both sharing the same file.  One of the swarms
   used the P4P framework, with the ISP's network topology map as input
   to its iTracker, and the other swarm used traditional peer selection.
   The swarm without P4P saw 98% of traffic to and from peers external
   to the ISP, whereas with P4P that number was 50%.  Download
   completion times for the P4P-enabled swarm improved approximately 20%
   on average.

5.1.3.  Multi-Layer Tracker-Based Architecture

   The multi-layer tracker-based P2P scheme described at the workshop is
   a generic example of an architecture that demonstrates how
   localization may be useful in principle.

   In a traditional tracker-based P2P system, trackers provide clients
   with information about seeds and peers where clients can find the
   content they seek.  A multi-layered tracker architecture incorporates
   additional "local" trackers that provide the same information, but
   only for content located within their own local network scope.
   Client queries are re-directed from the global tracker to the
   appropriate local trackers.  Local trackers may also exist on
   multiple levels, in which case queries would be further re-directed.
   This sort of architecture could also serve hybrid P2P/content
   delivery networks, where the global tracker functions as both a
   tracker and a content server, and local trackers track locally
   provisioned caches in addition to seeds and peers.

   One challenge in this architecture is determining what "local" means
   for trackers, seeds and peers.  Locality could be dependent on
   traffic conditions, load balancing, static topology, policy or some
   other metric.  These same considerations would also be crucial for
   determining appropriate cache placement in a hybrid network.

   This architecture presents in the abstract the problem of re-
   directing from a global entity to a local entity.  Client queries
   need to find their way to the appropriate local tracker.  This can be
   accomplished through an off-path, explicit mechanism where local



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   trackers register with the global tracker in advance, or through an
   on-path approach where the network proxies P2P requests.  The off-
   path tracker format approach is preferable for performance and
   reliability reasons.

   Inasmuch as the multi-layer scheme might require ISPs to aid peers in
   finding the optimal paths to unauthorized copies of copyrighted
   content, ISPs may be concerned about the legal liability of
   participating.

5.1.4.  ISP-Aided Neighbor Selection

   ISPs have a lot of knowledge about their networks: everything from
   the bandwidth, geography, and service class of particular nodes to
   overarching routing policies, OSPF and BGP metrics, and distances to
   peering points.  The ISP-aided neighbor selection service described
   below seeks to leverage this knowledge without requiring ISPs to
   reveal any information that could not already be discerned through
   reverse-engineering by client applications.

   The service consists of an "oracle" hosted by an ISP.  The oracle
   receives a list of IP addresses from a network node, sorts the list
   according to its own confidential criteria, and returns the sorted
   list to the node.  The peer ranking provided by the oracle could be
   viewed as a special case of the virtual cost interface described in
   the previous section.

   This service could be used by P2P clients or trackers, or any other
   application that would benefit from learning its ISP's connection
   preferences.  The oracle could be run as a web server or UDP service
   at a known location (perhaps similar to BIND).

   For interdomain ranking, an ISP could rank its own peers first, or it
   could base its ranking on the AS number of the IPs in the provided
   list.  Another option would be for multiple ISPs to work together to
   have their oracles exchange lists with each other.

   The main challenge in implementing the oracle service is scalability.
   If peers need to communicate to the oracle the IP address of every
   peer they know, the size of oracle requests may be inordinately
   large.  Additionally, today's largest swarms approach 10000 peers,
   and with every peer requesting a sorted list, oracle request volume
   will swell.  With the growth of business models dependent upon P2P
   for distribution of content, swarms in the future may be far larger,
   further exacerbating the problem.  Potential mitigations include
   having trackers instead of peers issue oracle requests, and using
   other peers' sorted lists as input rather than always using an
   unsorted list.



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   On the other hand, this approach is advantageous from a legal
   liability perspective, because it does not require ISPs to have any
   knowledge of where particular content might be located, or any role
   in directing peers to particular content.

5.1.5.  Caches

   Deploying caches as peers in P2P networks was suggested as a
   component of multiple different proposals put forth at the workshop.
   Caches may help to ease network load by reducing the need for peers
   to upload to each other and by localizing traffic.

   The two main concerns about P2P caches relate to network capacity and
   legal liability.  For caches to be useful, they will likely need to
   be large (one suggestion was that a 1 TB cache could service 30% of
   requests within a single AS, and a 100 TB cache could service 80% of
   requests).  Large caches will require sizable bandwidth in order to
   avoid contention among peers.  Caches would not be usefully placed
   within an HFC network on a cable system, for example.

   The legal liability attached to hosting a P2P cache likely reduces
   the incentives to do so.  Even under legal regimes where liability
   for caching may be unclear, ISPs and others may view hosting a cache
   as too great of a legal risk to be worthwhile.

5.1.6.  Potential IETF Work

   Much of the localization work discussed at the workshop is still in
   its initial stages, and many questions remain about the value that
   localization provides for varying network and overlay architectures.
   More data is needed to evaluate the effects on both traffic load and
   client performance.  Understanding swarm distributions is important;
   swarms with long tails may not particularly benefit from
   localization.

   Against this backdrop, the key task for the IETF as identified at the
   workshop is to pinpoint incrementally beneficial work items in the
   the spaces discussed above.  In the future it may be possible to
   standardize entire P2P mechanisms, but as a starting point it makes
   more sense to single out core manageable components for
   standardization.  The focus should be on items that are not so
   specific to one ISP or P2P network that standardization is rendered
   useless.  Ideally, any mechanisms resulting from this work might
   apply to future applications that exhibit the same bandwidth-
   intensive properties as today's P2P file-sharing.

   In considering any of these items, it will be necessary to ensure
   that the information exchanged by networks and applications does not



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   harm any of the parties involved.  Not every piece of information
   exchanged with be beneficial or verifiable, and this fact must be
   recognized and accounted for.  Solutions that leave applications or
   networks worse off than they already are today will not gain any
   traction.

   It should also not be assumed that a particular party will be best
   suited to provide a particular kind of information.  For example, an
   ISP may not know what the connection costs are in other ISPs'
   networks, whereas an overlay network that receives cost information
   from several ISPs may have a better handle on this kind of data.
   Standardization of information sharing should not assume the identity
   of particular parties doing the sharing.

   The list of potential work items discussed at the workshop is
   provided below.  Workshop participants showed particular interest in
   pursuing the first three items further.

5.1.6.1.  AS Numbers

   P2P clients are currently reliant on IP-to-AS mapping tables when
   they want to determine AS numbers.  Providing a standard, easier way
   for clients to obtain this information may help to make peer
   selection more efficient on certain networks.

5.1.6.2.  Querying for Preferred Peers

   In situations where a peer or tracker can make requests in real time
   to a service that expresses its ISP's peering preferences,
   standardizing a format for requests and responses may be useful.  The
   focus would be on the communication of the information, not on the
   criteria used to decide preferences.  The information provided to
   peers would have to be crafted to ensure that it protects the privacy
   of other peers and safeguards proprietary network information.

5.1.6.3.  Local Tracker, iTracker, Oracle, or Cache Discovery

   With the deployment of trackers, iTrackers, oracles or other
   mechanisms that provide some information specific to a node's
   locality, nodes will need a way to find these resources.  One task
   for the IETF could be to explore a way to do discovery, potentially
   by leveraging an existing discovery protocol (DNS, DHCP, anycast,
   etc.).  Depending on the resource to be discovered, discovery may
   require only a simple look-up, or it may require a more complex
   determination of which resource is "closest" to the node issuing the
   request.





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5.1.6.4.  ISP Account Usage Information

   Where ISP subscribers are bound by network usage policies or volume-
   based quotas, it may be useful to have a standard way of
   communicating the subscriber's current usage status.  This would be
   similar to information about how many minutes of cell phone airtime
   are left in a subscriber's billing cycle.  Applications could use
   this information to make decisions about when and how to transfer
   data.  One challenge in implementing such a standard would be support
   for potentially limitless different ISP business models.  The level
   of granularity that an ISP is able to provide may also be constrained
   depending on the pricing model and how dynamic the information is
   expected to be.

5.1.6.5.  Tracker Formats

   A multi-layered tracker approach could potentially be aided by a
   standard tracker format for re-directing from a global tracker to a
   local tracker.  While the extent to which existing trackers will be
   willing to consult with other trackers is unclear, the re-direction
   format may have an analog in another context -- many HTTP servers
   build their own indexes of mirror information for a similar purpose,
   though these are not standardized.  If the two problem spaces prove
   to be similar enough, there may be room to standardize a format
   across both.

5.2.  New Approaches to Congestion Control

   One recent informal survey presented at the workshop found that ISPs
   perceive traffic volumes from heavy users to be a problem, but no
   single congestion management tool has been put to wide use.  Within
   developer and research communities, congestion issues raised by
   increased P2P traffic volumes have spurred new thinking about
   congestion control mechanisms at both the transport layer and the
   application layer.  The subsections below explore some of these new
   ideas and highlight areas where IETF work may be appropriate.

5.2.1.  End-to-End Congestion Control

   As noted previously, uptake in P2P usage can result in perceptible
   end-user latency on the order of seconds for interactive
   applications.  One approach to resolving this "RTT in seconds"
   problem would be for P2P clients to implement better congestion
   control that keeps the bottleneck full while yielding to keep the
   delay of competing traffic low.  Such an algorithm has been
   implemented in BitTorrent's client by continuously sampling one-way
   delay (separating propagation from queuing delay) and targeting a
   small queuing delay value.  This essentially approximates a scavenger



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   service class in an end-to-end congestion control mechanism by by
   forcing bulk, elastic traffic to yield to competitors under
   congestion.

   In a similar vein, the P4P framework supports a component that allows
   applications to mark traffic as "bulk data" (not time sensitive).
   Applications adjust their behavior according to the feedback they
   receive from such markings.

   Experimenting with the standardization of these kinds of techniques
   or any congestion control framework with design goals that differ
   from those of TCP may be helpful work for the IETF to pursue.

5.2.2.  Weighted Congestion Control

   Congestion control has typically been implemented at a protocol
   level, as a optional, cooperative effort between endpoints
   experiencing congestion, but in looking for a long-term approach to
   congestion control, we may need a more rigorous way for available
   bandwidth to be allocated by and between the hosts using a network.
   The idea behind weighted congestion control is to allow hosts to give
   more weight to interactive applications during times of congestion.

   Comparing such an approach with DiffServ showcases its strengths and
   weaknesses.  Unlike DiffServ, weighted congestion control could be
   implemented on hosts with a simple extension to socket APIs (although
   consensus among OSes would be necessary for portability).  Control
   resides with the host, whereas even when DiffServ APIs are available,
   it is difficult for a host to know that the network is complying with
   its classifications.  With weighted congestion control, hosts need
   some disincentive to setting their weights at maximum levels, whereas
   DiffServ was not designed for individual users to employ.  Both
   approaches must rely on traffic senders to set policies, meaning that
   the congestion issues stemming from P2P use on the receiver side are
   not aided by either mechanism.  With DiffServ, a light user may waste
   his or her priority connecting to a heavy user on another network,
   which is not a problem with host-controlled weighting.

   Weighted congestion control is just one example of a generalized set
   of features that characterize useful approaches to congestion
   control.  These characteristics include full user control of
   priorities within a user's own scope, and no possibility of
   interpreting ISP behavior as discriminatory.  The former means that
   ISPs should not override user decisions arbitrarily (though this does
   not preclude an ISP from offering prioritization as an option).  The
   latter means that the metric for decision-making needs to obviate
   suspicion of ISP motivations.




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   One metric that meets these criteria is a harm (cost) metric, where
   cost is equal to the amount of data that was not served to its
   destination.  Using this metric, cost is greatest when traffic peaks
   are greatest.  It allows for a policy of not sending too much data
   during times of congestion, without specifying exactly how much is
   too much.  The cost metric could be used either for a DiffServ
   approach or for weighted congestion control.

   One important limitation on ISPs from a congestion control
   perspective is that they do not have a window into congestion on
   other ISPs' networks.  Solving this problem requires a separate
   mechanism to express congestion across domains.

   One potential avenue for the IETF or IRTF to pursue would be to
   establish a long-term design team to assess congestion problems in
   general and the long-term effects of any proposed quick fixes.  These
   issues are not necessarily confined to P2P and should be viewed in
   the broader context of massive increases in bandwidth use.

5.3.  Quality of Service

   Although ISPs have implemented a wide variety of short-term
   approaches to dealing with congestion, several of these may not be
   viable in the long term.  For example, some ISPs have found that
   using deep packet inspection to change the delivery characteristics
   of certain traffic at times of congestion is more cost effective than
   adding additional bandwidth.  Over time, this approach could lead to
   a cat-and-mouse game where applications providers continually adapt
   to avoid being correctly classified by DPI equipment.  Similarly,
   ISPs implementing traffic analysis to identify P2P traffic may find
   that in the long run the overhead required of an effective
   classification scheme will be excessive.

   Quality of service (QoS) arrangements may be more suitable in the
   long term.  One approach that distinguishes certain classes of
   traffic during times of congestion was described in Section 3.3.  A
   standardized mechanism that may be useful for implementing QoS is
   DiffServ Code Points (DSCP) [RFC2474].

   With DSCP, devices at the edge of the network mark packets with the
   service level they should receive.  Nodes within the network do not
   need to remember anything about service flows, and applications do
   not need to request a particular service level.  Users effectively
   avoid self-interference through service classification.

   Although DSCP may have many uses, perhaps the most relevant to the
   P2P congestion issue is its ability to facilitate usage-based
   charging.  User pricing agreements that charge a premium for real-



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   time traffic and best effort traffic could potentially shape user
   behavior, resulting in reduced congestion (although ISPs would need a
   mechanism to mitigate the risk of charging subscribers for things
   like unintentional malware downloads or DoS attacks).  DSCP could
   also be used to limit a user's supply of high-priority bandwidth,
   resulting in a similar effect.

   Equipment to support DSCP is already available.  Although there has
   been some concern about a perceived lack of DSCP deployment, it is
   widely used by enterprise customers, and growth has been strong due
   to uptake in VoIP at the enterprise level.

   However, DSCP still faces deployment hurdles on many networks.
   Perhaps the largest barrier of all to wide deployment is the lack of
   uniform code points to be used across networks.  For example, the
   latest Windows Vista API marks voice traffic as CS7, above the
   priority reserve for router traffic.  To properly take advantage of
   this change, every switch will need to re-mark all traffic.  In
   addition, disparate ISPs are currently without a means of verifying
   each others' markings, and thus may be unwilling to trust the
   markings they receive.


6.  Applications Opening Multiple TCP Connections

   The workshop discussions about P2P congestion spurred a related
   discussion about applications (P2P or otherwise) that open multiple
   TCP connections.  With multiple users sharing one link, TCP flow
   fairness gives users with multiple open connections a larger
   proportion of bandwidth.  Since some P2P protocols use multiple open
   connections for a single file transfer, and users often pursue
   multiple transfers at once, this can cause a P2P user to have many
   more open connections at once than other users on the same link.  The
   same is true for users of other applications that open multiple
   connections.  A single user with multiple open connections is not
   necessarily a problem on its face, but the fact that fairness is
   determined per flow rather than per user leaves that impression.
   Workshop participants thought it may be useful for the IETF to
   provide some information about such situations.


7.  Costs and Congestion

   Workshop participants expressed diverging opinions about how much the
   cost of transferring data -- as experienced by ISPs, and by
   extension, by their subscribers -- should factor into IETF thinking
   on P2P traffic issues.




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   On one hand, bandwidth costs may be significant, even when viewed in
   isolation from congestion issues.  Some estimates put the total cost
   of shipping 1 GB between $0.10 and $2.  The cost of transit bandwidth
   in markets where subscribers are charged flat rates appears to have
   leveled off and may no longer be getting cheaper.  Thus, it may be
   reasonable to expect more service providers to move to volume-based
   pricing (where they have not already done so) as a means to control
   congestion and increase revenues.  This is only feasible if bandwidth
   consumption is visible to end users, which argues for some mechanism
   of exposing quotas and usage to applications.  However, expressing
   cost information may be outside of the technical purview of the IETF.

   On the other hand, congestion can be viewed merely as a manifestation
   of cost.  An ISP that invests in capacity could be considered to be
   paying to relieve congestion.  Or, if subscribers are charged for
   congesting the network, then cost and congestion could be viewed as
   one and the same.  The distinction between them may thus be
   artificial.

   Workshop participants felt that the issues highlighted here may be
   useful fodder for IRTF work.


8.  Next Steps

   The IETF community recognizes the significance of both growing P2P
   traffic volumes and increased network load at large.  The importance
   of addressing the impact of high-volume, delay-tolerant data transfer
   on end user experiences was highly apparent at the workshop.

   At the conclusion of the workshop and in the days following, it
   became clear that the largest areas of interest fell into two
   categories: transport-related issues and improved peer selection.

8.1.  Transport Issues

   Two main transport-related work items evolved out of the workshop.
   The first was the creation of a standardized delay-based end-to-end
   congestion control mechanism that applications such as P2P clients
   could use to reduce their own impact on interactive applications in
   use on shared links (as described in Section 5.2.1).  The second was
   an informational document that describes the current practice of P2P
   applications opening multiple transport connections and makes
   recommendations about the best practices for doing so (as discussed
   in Section 6).






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8.2.  Improved Peer Selection

   Participants expressed strong interest in further pursuing the range
   of concepts described in Section 5.1 that support mechanisms for
   information sharing between networks and applications to help improve
   peer selection.  Adding to the appeal of this topic is its potential
   utility for applications other than P2P that may also benefit from
   information about the network.  Because the scope of potential
   solutions discussed at the workshop was broad, extracting out the
   most feasible pieces to pursue is the necessary first step.


9.  Security Considerations

   The workshop discussions covered a range of potential engineering
   activities, each with its own security considerations.  For example,
   if networks are to provide preference or topology information to
   applications, the applications may desire some means of verifying the
   authenticity of the information.  As the IETF community begins to
   pursue specific avenues arising out of this workshop, addressing
   relevant security requirements will be crucial.


10.  Acknowledgements

   The IETF would like to thank MIT, which hosted the workshop, and all
   those people at MIT and elsewhere who assisted with the organization
   and logistics of the workshop.

   The IETF is grateful to the program committee (listed in Appendix A)
   for their time and energy in organizing the workshop, reviewing the
   position papers, and crafting an event of value for all participants.
   The IETF would also like to thank the scribes, Spencer Dawkins and
   Alissa Cooper, who diligently recorded the proceedings during the
   workshop.

   A special thanks to all the participants in the workshop (listed in
   Appendix B), who took the time, came to the workshop to participate
   in the discussions, and who put in the effort to make this workshop a
   success.  The IETF especially appreciates the effort of those that
   prepared and made presentations at the workshop.


11.  Informative References

   [DOCSIS]   CableLabs, "DOCSIS Specifications - DOCSIS 2.0 Interface",
              2008, <http://www.cablemodem.com/specifications/
              specifications20.html>.



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   [P4P]      Xie, H., Yang, Y., Krishnamurthy, A., and A. Silberschatz,
              "P4P: Provider Portal for Applications", August 2008, <htt
              p://uwnews.org/relatedcontent/2008/August/
              rc_parentID43281_thisID43282.pdf>.

   [RFC2475]  Carlson, M., Weiss, W., Blake, S., Wang, Z., Black, D.,
              and E. Davies, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2475,
              December 1998.


Appendix A.  Program Committee

      Dave Clark, MIT
      Lars Eggert, TSV AD
      Cullen Jennings, RAI AD
      John Morris, Center for Democracy and Technology
      Jon Peterson, RAI AD
      Danny Weitzner, MIT


Appendix B.  Workshop Participants

      Vinay Aggarwal, Deutsche Telekom Labs, TU Berlin
      Marvin Ammori, Free Press
      Loa Andersson, Acreo AB
      Jari Arkko, Ericsson
      Alan Arolovitch, PeerApp
      Timothy Balcer
      Mary Barnes, Nortel
      Colby Barth, Cisco Systems
      John Barlett, NetForecast
      Salman Baset, Columbia University
      Chris Bastian, Comcast
      Matthew Bell, Charter Communications
      Donald Bowman, Sandvine Inc.
      Scott Bradner, Harvard University
      Bob Briscoe, British Telecom
      David Bryan, SIPeerior Technologies
      Rex Bullinger, National Cable & Telecommunications Association







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      Gonzalo Camarillo, Ericsson
      Mary-Luc Champel, Thomson
      William Check, NCTA
      Alissa Cooper, Center for Democracy and Technology
      Patrick Crowley, Washington University
      Leslie Daigle, Internet Society
      Spencer Dawkins
      John Dickinson, Bright House Networks
      Lisa Dusseault, CommerceNet
      Lars Eggert, Nokia Research Center
      Joe Godas, Cablevision
      Vernon Groves, Microsoft
      Daniel Grunberg, Immedia Semiconductor
      Carmen Guerrero, University Carlos III Madrid
      Vijay Gurbani, Bell Laboratories/Alcatel-Lucent
      William Hawkins III, ITT
      Volker Hilt, Bell Labs, Alcatel-Lucent
      Russell Housley, Vigil Security, LLC
      Robert Jackson, Camiant
      Cullen Jennings, Cisco Systems
      Paul Jessop, RIAA
      XingFeng Jiang, Huawei
      Michael Kelsen, Time Warner Cable
      Tom Klieber, Comcast
      Eric Klinker, BitTorrent Inc.
      Umesh Krishnaswamy
      Gregory Lebovitz, Juniper
      Erran Li, Bell-Labs
      Jason Livingood, Comcast
      Andrew Malis, Verizon
      Enrico Marocco, Telecom Italia Lab
      Marcin Matuszewski, Nokia
      Danny McPherson, Arbor Networks, Inc.
      Michael Merritt, AT&T
      Lyle Moore, Bell Canada
      John Morris, Center for Democracy and Technology
      Jean-Francois Mule, Cablelabs
      David Oran, Cisco Systems
      Reinaldo Penno, Juniper Networks
      Jon Peterson, NeuStar
      Howard Pfeffer, Time Warner Cable
      Laird Popkin, Pando Networks
      Stefano Previdi, Cisco systems
      Satish Putta







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      Eric Pescorla
      Benny Rodrig, Avaya
      Damien Saucez, UCLouvain (UCL)
      Henning Schulzrinne, Columbia University
      Michael Sheehan, Juniper Networks
      Don Shulzrinne, Immedia Semiconductor
      David Sohn, Center for Democracy and Technology
      Martin Stiemerling, NEC
      Clint Summers, Cox Communications
      Robert Topolski
      Mark Townsley, Cisco Systems
      Yushun Wang, Microsoft
      Hao Wang, Yale University
      Ye Wang, Yale University
      David Ward, Cisco
      Nicholas Weaver, ICSI
      Daniel Weitzner, MIT
      Magnus Westerlund, Ericsson
      Thomas Woo, Bell Labs
      Steve Worona, EDUCAUSE
      Richard Woundy, Comcast
      Haiyong Xie
      Richard Yang, Yale University


Appendix C.  Workshop Agenda

   1.  Welcome/Note Well/Intro Slides
   2.  Service Provider Perspective (Comcast)
   3.  Application Designer Perspective (BitTorrent)
   4.  Lightning Talks & General Discussion
   5.  Localization and Caches
   6.  New Approaches to Congestion
   7.  Quality of Service
   8.  Conclusions & Wrap-Up


Appendix D.  Slides and Position Papers

   Slides and position papers are available at: http://
   trac.tools.ietf.org/area/rai/trac/wiki/PeerToPeerInfrastructure










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Authors' Addresses

   Jon Peterson
   NeuStar
   USA

   Email: jon.peterson@neustar.biz


   Alissa Cooper
   Center for Democracy & Technology
   1634 Eye St. NW, Suite 1100
   Washington, DC  20006
   USA

   Email: acooper@cdt.org



































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