Skip to main content

Comcast's ISP Experiences in a Proactive Network Provider Participation for P2P (P4P) Technical Trial
RFC 5632

Document Type RFC - Informational (September 2009) Errata
Was draft-livingood-woundy-p4p-experiences (individual in app area)
Authors Y. Richard Yang , Laird Popkin , Richard Woundy , Chris Griffiths , Jason Livingood
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
IESG Responsible AD Lisa M. Dusseault
Send notices to (None)
RFC 5632
Network Working Group                                       C. Griffiths
Request for Comments: 5632                                  J. Livingood
Category: Informational                                          Comcast
                                                               L. Popkin
                                                               R. Woundy
                                                                 Y. Yang
                                                          September 2009

Comcast's ISP Experiences in a Proactive Network Provider Participation
                     for P2P (P4P) Technical Trial


   This document describes the experiences of Comcast, a large cable
   broadband Internet Service Provider (ISP) in the U.S., in a Proactive
   Network Provider Participation for P2P (P4P) technical trial in July
   2008.  This trial used P4P iTracker technology, which is being
   considered by the IETF as part of the Application Layer Transport
   Optimization (ALTO) working group.

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Griffiths, et al.            Informational                      [Page 1]
RFC 5632                Comcast P4P Experiences           September 2009

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  High-Level Details . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Differences between the P4P iTrackers Used . . . . . . . . . .  4
     3.1.  P4P Fine Grain . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  P4P Coarse Grain . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  P4P Generic Weighted . . . . . . . . . . . . . . . . . . .  5
   4.  High-Level Trial Results . . . . . . . . . . . . . . . . . . .  5
     4.1.  Swarm Size . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Impact on Download Speed . . . . . . . . . . . . . . . . .  7
     4.3.  General Impacts on Upstream and Downstream Traffic and
           Other Interesting Data . . . . . . . . . . . . . . . . . .  7
   5.  Important Notes on Data Collected  . . . . . . . . . . . . . .  8
   6.  Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   Comcast is a large broadband Internet Service Provider (ISP), based
   in the U.S., serving the majority of its customers via cable modem
   technology.  A trial was conducted in July 2008 with Pando Networks,
   Yale, and several ISP members of the P4P working group, which is part
   of the Distributed Computing Industry Association (DCIA).  Comcast is
   a member of the DCIA's P4P Working Group, whose mission is to work
   with Internet Service Providers (ISPs), peer-to-peer (P2P) companies,
   and technology researchers to develop "P4P" mechanisms, such as so-
   called "iTrackers" (hereafter P4P iTrackers), that accelerate
   distribution of content and optimize utilization of ISP network
   resources.  P4P iTrackers theoretically allow P2P networks to
   optimize traffic within each ISP, reducing the volume of data
   traversing the ISP's infrastructure and creating a more manageable
   flow of data.  P4P iTrackers can also accelerate P2P downloads for
   end users.

   P4P's iTracker technology [SIGCOMM] was conceptually discussed with
   the IETF at the Peer-to-Peer Infrastructure (P2Pi) Workshop held on
   May 28, 2008, at the Massachusetts Institute of Technology (MIT), as
   documented in [RFC5594].  This work was discussed in greater detail
   at the 72nd meeting of the IETF, in Dublin, Ireland, in the ALTO BoF
   (Birds of a Feather meeting) on July 29, 2008.  Due to interest from
   the community, Comcast shared P4P iTracker trial data at the 73rd
   meeting of the IETF, in Minneapolis, Minnesota, in the ALTO BoF on
   November 18, 2008.  Since that time, discussion of P4P iTrackers and
   alternative technologies has continued among participants of the ALTO
   working group.

Griffiths, et al.            Informational                      [Page 2]
RFC 5632                Comcast P4P Experiences           September 2009

   The P4P iTracker trial was conducted, in cooperation with Pando,
   Yale, and three other P4P member ISPs, from July 2 to July 17, 2008.
   This was the first P4P iTracker trial over a cable broadband network.
   The trial used a Pando P2P client, and Pando distributed a special
   21-MB licensed video file in order to measure the effectiveness of
   P4P iTrackers.  A primary objective of the trial was to measure the
   effects that increasing the localization of P2P swarms would have on
   P2P uploads, P2P downloads, and ISP networks, in comparison to normal
   P2P activity.

2.  High-Level Details

   As noted in Section 1 of [DynamicSwarmMgmt], a swarm is defined in
   the following way:

      The content and the set of peers distributing it [a file] is
      usually called a torrent.  A peer that only uploads content is
      called a seed, while a peer that uploads and downloads at the same
      time is called a leecher.  The connected set of peers
      participating in the piece exchanges of a torrent is referred to
      as a swarm.

   There were five different swarms for the content used in the trial.
   The second, third, and fourth used different P4P iTrackers: Generic,
   Coarse Grained, and Fine Grained, all of which are described in
   Section 3.  The fifth was a proprietary Pando mechanism.  (The
   results of the fifth swarm, while satisfactory, are not included here
   since our focus is on open standards and a mechanism that may be
   leveraged for the benefit of the entire community of P2P clients.)
   Comcast deployed a P4P iTracker server in its production network to
   support this trial, and configured multiple iTracker files to provide
   varying levels of localization to clients.

   In the trial itself, a P2P client begins a P2P session by querying a
   pTracker, which runs and manages the P2P network.  The pTracker
   occasionally queries the P4P iTracker, which in this case was
   maintained by Comcast, the ISP.  Other ISPs either managed their own
   P4P iTracker or used Pando or Yale to host their P4P iTracker files.
   The P4P iTracker returns network topology information to the
   pTracker, which then communicates with P2P clients, in order to
   enable P2P clients to make network-aware decisions regarding peers.

   The Pando client was enabled to capture extended logging, when the
   version of the client included support for it.  The extended logging
   included the source and destination IP address of all P2P transfers,
   the number of bytes transferred, and the start and end timestamps.
   This information gives a precise measurement of the data flow in the
   network, allowing computation of data transfer volumes as well as

Griffiths, et al.            Informational                      [Page 3]
RFC 5632                Comcast P4P Experiences           September 2009

   data flow rates at each point in time.  With standard logging, Pando
   captured the start and completion times of every download, as well as
   the average transfer rate observed by the client for the download.

   Pando served the data from an origin server external to Comcast's
   network.  This server served about 10 copies of the file, after which
   all transfers (about 1 million downloads across all ISPs) were
   performed purely via P2P.

   The P2P clients in the trial start with tracker-provided peers, then
   use peer exchange to discover additional peers.  Thus, the initial
   peers were provided according to P4P iTracker guidance (90% guidance
   based on P4P iTracker topology and 10% random guidance), then later
   peers discover the entire swarm via either additional announces or
   peer exchange.

3.  Differences between the P4P iTrackers Used

   Given the size of the Comcast network, it was felt that in order to
   truly evaluate the P4P iTracker application we would need to test
   various network topologies that reflected its network and would help
   gauge the level of effort and design requirements necessary to get
   correct statistical data out of the trial.  In all cases, P4P
   iTrackers were configured with automation in mind, so that any
   successful P4P iTracker configuration would be automatically
   updating, rather than manually configured on an ongoing basis.  All
   P4P iTrackers were hosted on the same small server, and it appeared
   to be relatively easy and inexpensive to scale up a P4P iTracker
   infrastructure should P4P iTracker-like mechanisms become
   standardized and widely adopted.

3.1.  P4P Fine Grain

   The Fine Grain topology was the first and most complex P4P iTracker
   that we built for this trial.  It was a detailed mapping of Comcast
   backbone-connected network Autonomous System Numbers (ASNs) to IP
   Aggregates, which were weighted based on priority and distance from
   each other.  Included in this design was a prioritization of all Peer
   and Internet transit connected ASNs to the Comcast backbone to ensure
   that P4P traffic would prefer settlement-free and lower-cost networks
   first, and then more expensive transit links.  This attempted to
   optimize and lower transit costs associated with this traffic.  We
   then took the additional step of detailing each ASN and IP Aggregate
   into IP subnets down to Optical Transport Nodes (OTNs) where all
   Cable Modem Termination Systems (CMTS, as briefly defined in Section
   2.6 of [RFC3083]) reside .  This design gave a highly localized and
   detailed description of the Comcast network for the iTracker to
   disseminate.  This design defined 1,182 P4P iTracker node

Griffiths, et al.            Informational                      [Page 4]
RFC 5632                Comcast P4P Experiences           September 2009

   identifiers, and resulted in a 107,357-line configuration file.

   This P4P iTracker was obviously the most time-consuming to create and
   the most complex to maintain.  Trial results indicated that this
   level of localization was too high, and was less effective compared
   to lower levels of localization.

3.2.  P4P Coarse Grain

   Given the level of detail in the Fine Grain design, it was important
   that we also enable a high-level design, which still used priority
   and weighting mechanisms for the Comcast backbone and transit links.
   The Coarse Grain design was a limited or summarized version of the
   Fine Grain design, which used the ASN to IP Aggregate and weighted
   data for transit links, but removed all additional localization data.
   This ensured we would get similar data sets from the Fine Grain
   design, but without the more detailed localization of each of the
   networks attached to the Comcast backbone.  This design defined 22
   P4P iTracker node identifiers, and resulted in a 998-line
   configuration file.

   From an overall cost, complexity, risk, and effectiveness standpoint,
   this was judged to be the optimal P4P iTracker for Comcast.
   Importantly, this did not require revealing the complex, internal
   network topology that the Fine Grain did.  Updates to this iTracker
   were also far simpler to automate, which will better ensure that it
   is accurate over time, and keeps administrative overhead relatively
   low.  However, the differences, costs, and benefits of Coarse Grain
   and Generic Weighted (see below) likely merit further study.

3.3.  P4P Generic Weighted

   The Generic Weighted design was a copy of the Coarse Grained design,
   but instead of using ISP-designated priority and weights, all weights
   were defaulted to pre-determined parameters that the Yale team had
   designed.  All other data was replicated from the Coarse Grain
   design.  Gathering and providing the information necessary to support
   the Generic Weighted iTracker was roughly the same level of effort as
   for Coarse Grain.

4.  High-Level Trial Results

   Trial data was collected by Pando Networks and Yale University, and
   raw trial results were shared with Comcast and all of the other ISPs
   involved in the trial.  Analysis of the raw results was performed by
   Pando and Yale, and these organizations delivered an analysis of the
   P4P iTracker trial.  Using the raw data, Comcast also analyzed the
   trial results.  Furthermore, the raw trial results for Comcast were

Griffiths, et al.            Informational                      [Page 5]
RFC 5632                Comcast P4P Experiences           September 2009

   shared with Net Forecast, Inc., which performed an independent
   analysis of the trial for Comcast.

4.1.  Swarm Size

   During the trial, downloads peaked at 24,728 per day, per swarm, or
   nearly 124,000 per day for all five swarms.  The swarm size peaked at
   11,703 peers per swarm, or nearly 57,000 peers for all five swarms.
   We observed a comparable number of downloads in each of the five

   For each swarm, Table 1 below gives the number of downloads per swarm
   from Comcast that finished downloading, and the number of downloads
   from Comcast that canceled downloading before finishing.

                  Characteristics of P4P iTracker Swarms:

   |   Swarm   | Completed | Cancellations |    Total   | Cancellation |
   |           | Downloads |               |  Attempts  |     Rate     |
   |   Random  |   2,719   |       89      |    2,808   |     3.17%    |
   | (Control) |           |               |            |              |
   | --------- | --------- |  -----------  | ---------- |  ----------- |
   |  P4P Fine |   2,846   |       64      |    2,910   |     2.20%    |
   |  Grained  |           |               |            |              |
   | --------- | --------- |  -----------  | ---------- |  ----------- |
   |    P4P    |   2,775   |       63      |    2,838   |     2.22%    |
   |  Generic  |           |               |            |              |
   |   Weight  |           |               |            |              |
   | --------- | --------- |  -----------  | ---------- |  ----------- |
   |    P4P    |   2,886   |       52      |    2,938   |     1.77%    |
   |   Coarse  |           |               |            |              |
   |  Grained  |           |               |            |              |

              Table 1: Per-Swarm Size and Cancellation Rates

Griffiths, et al.            Informational                      [Page 6]
RFC 5632                Comcast P4P Experiences           September 2009

4.2.  Impact on Download Speed

   The results of the trial indicated that P4P iTrackers can improve the
   speed of downloads to P2P clients.  In addition, P4P iTrackers were
   effective in localizing P2P traffic within the Comcast network.

                   Impact of P4P iTrackers on Downloads:

   |     Swarm    | Global Avg |   Change   | Comcast Avg |   Change   |
   |              |     bps    |            |     bps     |            |
   |    Random    |   144,045  |     n/a    | 254,671 bps |     n/a    |
   |   (Control)  |     bps    |            |             |            |
   |  ----------  | ---------- | ---------- |  ---------- | ---------- |
   |   P4P Fine   |   162,344  |    +13%    | 402,043 bps |    +57%    |
   |    Grained   |     bps    |            |             |            |
   |  ----------  | ---------- | ---------- |  ---------- | ---------- |
   |  P4P Generic |   163,205  |    +13%    | 463,782 bps |    +82%    |
   |    Weight    |     bps    |            |             |            |
   |  ----------  | ---------- | ---------- |  ---------- | ---------- |
   |  P4P Coarse  |   166,273  |    +15%    | 471,218 bps |    +85%    |
   |    Grained   |     bps    |            |             |            |

           Table 2: Per-Swarm Global and Comcast Download Speeds

4.3.  General Impacts on Upstream and Downstream Traffic and Other
      Interesting Data

   An analysis of the effects of P4P iTracker use on upstream
   utilization and Internet transit was also interesting.  It did not
   appear that P4P iTrackers significantly increased upstream
   utilization in the Comcast access network; in essence, uploading was
   already occurring no matter what and a P4P iTracker in and of itself
   did not appear to materially increase uploading for this specific,
   licensed content.  (A P4P iTracker is not intended as a solution for
   the potential of network congestion to occur.)  Random was 143,236 MB
   and P4P Generic Weight was 143,143 MB, while P4P Coarse Grained was
   139,669 MB.  We also observed that using a P4P iTracker reduced
   outgoing Internet traffic by an average of 34% at peering points.
   Random was 134,219 MB and P4P Generic Weight was 91,979 MB, while P4P
   Coarse Grained was 86,652 MB.

   In terms of downstream utilization, we observed that the use of a P4P
   iTracker reduced incoming Internet traffic by an average of 80% at
   peering points.  Random was 47,013 MB, P4P Generic Weight was 8,610
   MB, and P4P Coarse Grained was 7,764 MB.  However, we did notice that

Griffiths, et al.            Informational                      [Page 7]
RFC 5632                Comcast P4P Experiences           September 2009

   download activity in the Comcast access network increased somewhat,
   from 56,030 MB for Random, to 59,765 MB for P4P Generic Weight, and
   60,781 MB for P4P Coarse Grained.  Note that for each swarm, the
   number of downloaded bytes according to logging reports is very close
   to the number of downloads multiplied by file size.  But they do not
   exactly match due to log report errors and duplicated chunks.  One
   factor contributing to the differences in access network download
   activity is that different swarms have different numbers of
   downloaders, due to random variations during uniform random
   assignment of downloaders to swarms (see Table 1).  One interesting
   observation is that Random has higher cancellation rate (3.17%) than
   that of the guided swarms (1.77%-2.22%).  Whether guided swarms
   achieve lower cancellation rate is an interesting issue for future

5.  Important Notes on Data Collected

   Raw data is presented in this document.  We did not normalize traffic
   volume data (e.g., upload and download) by the number of downloads in
   order to preserve this underlying raw data.

   We also recommend that readers not focus too much on the absolute
   numbers, such as bytes downloaded from internal sources and bytes
   downloaded from external sources.  Instead, we recommend readers
   focus on ratios such as the percentage of bytes downloaded that came
   from internal sources in each swarm.  As a result, the small random
   variation between number of downloads of each swarm does not distract
   readers from important metrics like shifting traffic from external to
   internal sources, among other things.

   We also wish to note that the data was collected from a sample of the
   total swarm.  Specifically, there were some peers running older
   versions of the Pando client that did not implement the extended
   transfer logging.  For those nodes, which participated in the swarms
   but did not report their data transfers, we have download counts.
   The result of this is that, for example, the download counts
   generated from the standard logging are a bit higher than the
   download counts generated by the extended logging.  That being said,
   over 90% of downloads were by peers running the newer software, which
   we believe shows that the transfer records are highly representative
   of the total data flow.

   In terms of which analysis was performed from the standard logging
   compared to extended logging, all of the data flow analysis was
   performed using the extended logging.  Pando's download counts and
   performance numbers were generated via standard logging (i.e., all
   peers report download complete/cancel, data volumes, and measured
   download speed on the client).  Yale's download counts and

Griffiths, et al.            Informational                      [Page 8]
RFC 5632                Comcast P4P Experiences           September 2009

   performance numbers were derived via extended logging (e.g., by
   summing the transfer records, counting IP addresses reported, etc.).

   One benefit of having two data sources is that we can compare the
   two.  In this case, the two approaches both reported comparable

6.  Next Steps

   One objective of this document is to share with the IETF community
   the results of one P4P iTracker trial in a large broadband network,
   given skepticism regarding the benefits to P2P users as well as to
   ISPs.  From the perspective of P2P users, P4P iTrackers potentially
   deliver faster P2P downloads.  At the same time, ISPs can increase
   the localization of swarms, enabling them to reduce bytes flowing
   over transit points, while also delivering an optimized P2P
   experience to customers.  However, an internal analysis of varying
   levels of P4P iTracker adoption by ISPs leads us to believe that,
   while P4P iTracker-type mechanisms are valuable on a single ISP
   basis, the value of P4P iTrackers increases dramatically as many ISPs
   choose to deploy it.

   We believe these results can inform the technical discussion in the
   IETF over how to use P4P iTracker mechanisms.  Should such a
   mechanism be standardized, the use of ISP-provided P4P iTrackers
   should probably be an opt-in feature for P2P users, or at least a
   feature of which they are explicitly aware of and which has been
   enabled by default in a particular P2P client.  In this way, P2P
   users could choose to opt-in either explicitly or by their choice of
   P2P client in order to choose to use the P4P iTracker to improve
   performance, which benefits both the user and the ISP at the same
   time.  Importantly in terms of privacy, the P4P iTracker makes
   available only network topology information, and would not in its
   current form enable an ISP, via the P4P iTracker, to determine which
   P2P clients were downloading any specific content, whether to
   determine, for example, if content was a song or a movie or even the

   It is also possible that a P4P iTracker type of mechanism, in
   combination with a P2P cache, could further improve P2P download
   performance, which merits further study.  In addition, this was a
   limited trial that, while very promising, indicates a need for
   additional technical investigation and trial work.  Such a follow-up
   study should explore the effects of P4P iTrackers when more P2P
   client software variants are involved, with larger swarms, and with
   additional and more technically diverse content (file size, file
   type, duration of content, etc.).

Griffiths, et al.            Informational                      [Page 9]
RFC 5632                Comcast P4P Experiences           September 2009

7.  Security Considerations

   This document does not propose any kind of protocol, practice or

   The experiment did show that an ISP can improve performance without
   exposing fine-grained details about network structure, which might
   otherwise be a security concern (see Section 3.1 (P4P Fine Grain) and
   Section 3.2 (P4P Coarse Grain).  Section 6 (Next Steps) mentions that
   the opt-in architecture allows P2P users to maintain privacy.

   Other security aspects were not considered in the experiment, which
   focused on performance measurements.

8.  Acknowledgements

   The authors wish to acknowledge the hard work of all of the P4P
   working group members, and specifically the focused efforts of the
   teams at both Pando and Yale for the trial itself.  Finally, the
   authors recognize and appreciate Peter Sevcik and John Bartlett of
   NetForecast, Inc., for their valued independent analysis of the trial

9.  Informative References

              Carlsson, N. and G. Dan, "Dynamic Swarm Management for
              Improved BitTorrent Performance", USENIX 8th International
              Workshop on Peer-to-Peer Systems, March 2009,

   [RFC3083]  Woundy, R., "Baseline Privacy Interface Management
              Information Base for DOCSIS Compliant Cable Modems and
              Cable Modem Termination Systems", RFC 3083, March 2001.

   [RFC5594]  Peterson, J. and A. Cooper, "Report from the IETF Workshop
              on Peer-to-Peer (P2P) Infrastructure, May 28, 2008",
              RFC 5594, July 2009.

   [SIGCOMM]  Xie, H., Yang, Y., Krishnamurthy, A., Liu, Y., and A.
              Silberschatz, "ACM SIGCOMM 2008 - P4P: Provider Portal for
              Applications", Association for Computing Machinery SIGCOMM
              2008 Proceedings, August 2008,

Griffiths, et al.            Informational                     [Page 10]
RFC 5632                Comcast P4P Experiences           September 2009

Authors' Addresses

   Chris Griffiths
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103


   Jason Livingood
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103


   Laird Popkin
   Pando Networks
   520 Broadway Street
   10th Floor
   New York, NY  10012


   Richard Woundy
   Comcast Cable Communications
   27 Industrial Avenue
   Chelmsford, MA  01824


Griffiths, et al.            Informational                     [Page 11]
RFC 5632                Comcast P4P Experiences           September 2009

   Richard Yang
   Yale University
   51 Prospect Street
   New Haven, CT  06520


Griffiths, et al.            Informational                     [Page 12]