Application Layer Traffic                                      N. Weaver
Optimization (ALTO) Working Group         International Computer Science
Internet-Draft                                                 Institute
Intended status: Informational                             March 4, 2009
Expires: September 5, 2009

           Peer to Peer Localization Services and Edge Caches

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   Without caches in the infrastructure, peer to peer content delivery's
   primary effect is cost shifting rather than cost savings.  Even with

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   perfect localization, depending on the relative cost of last-mile
   uplink bandwidth verses transport bandwidth, P2P may substantially
   increase aggregate cost.  Yet the addition of edge caches, caches
   located in the ISPs near the customers, radically change the
   economics of P2P content delivery.  Edge caches interact very
   strongly with localization services for P2P content delivery, and any
   localization service must be tightly integrated into edge-cache

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  The Design of Edge Caches . . . . . . . . . . . . . . . . . . . 3
     2.1.  Safe Incentives for Edge Caches . . . . . . . . . . . . . . 4
   3.  An Economic Model for Delivery Costs  . . . . . . . . . . . . . 5
     3.1.  The Limits of Localization  . . . . . . . . . . . . . . . . 6
   4.  Edge-Cache Interactions with Localization . . . . . . . . . . . 6
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   When compared with conventional content delivery, peer to peer
   content delivery of bulk data is significant at shifting costs from
   the content provider to the ISPs, but can often significantly magnify
   the aggregate cost of delivery.  Depending on the particular costs to
   an ISP, even perfect localization (restriction of P2P activity to
   within the ISP's network) may still result in significantly higher
   aggregate costs over conventional content delivery, although
   localization does reduce transit costs.

   However, if edge-caches are introduced into the architecture, the
   economics can change radically.  Rather than increasing transport
   costs, P2P with ISP-provided edge caches reduce transport costs for
   all parties, achiving costs reductions for the ISP analogous to those
   seen with edge-based HTTP servers such as Akamai [akamai].  Yet
   unlike edge-based web servers, edge-caches for P2P are failure-
   transparent: when they fail, or do not have the right data, the
   failure does not impact correct operation of the P2P system.

   It is critical that ALTO or other localization services for bulk-data
   P2P be both edge-cache aware and assist edge-caches in their
   operation, for localization without edge-caches may not produce
   significant cost savings to the ISPs or performance benefits to the
   customers, but edge-caches need localization services both to ease
   client discovery and to provide necessary topological information for
   edge-cache operation.

   This document begins with a brief discussion of edge caches for P2P
   (Section 2), then outlines a simple cost model of content delivery
   (Section 3), which argues why both localization and edge-caches are
   necessary for cost-effective content delivery.  It then discusses how
   localization and edge-caches should interact (Section 4), before a
   brief conclusions section (Section 5)

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  The Design of Edge Caches

   An edge-cache is simply a special P2P node which lives in the ISP's
   network close to, but not at, the final recipients.  Thus it incurs
   no transit cost in communicating with ISP-local peers, and is close
   in latency and has a high-bandwidth connection into the ISP's
   internal network.

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   The role of an edge cache is to coordinate transfers between local
   peers and the rest of the Internet, as well as to cache data for
   subsequent use, within the existing or modified P2P protocol.  For
   example, a BitTorrent edge cache can participate in a swarm, offering
   up data only to ISP-local peers once it has a complete file, and
   refusing to seed or leech (but only tit-for-tat) with peers outside
   the ISP before it has obtained the entire file.

   One feature of an edge-cache is that it can be unreliable.  Since,
   from the point of view of the other peers, it is simply another P2P
   participant, if the edge-cache fails to include a block, a file, or
   fails altogether, the P2P system will still work properly.  This is
   in sharp contrast to edge-based HTTP caches or CDNs, where a failure
   in the node may result in failures to the user.

   A side consequence of unreliability is that an edge-cache can
   therefore be inexpensive.  For example, a 1U server (based on a Mini-
   ITX motherboard) capable of holding 4 SATA disks might cost less than
   $800.  With a price of $130 for a 1.5TB drive, an edge cache costing
   less than $1400 could cache over 5 TB of data.  Such a low-cost
   system might suffer significantly higher transient failure rates than
   a higher-quality server, necessitating a reboot, reimage, and
   disabling of bad disks, but as failures are low-consequence, such
   caches can be cheap to deploy.

   Finally, a P2P edge-cache doesn't require changing existing P2P
   protocols.  As long as local peers will find the edge cache, or the
   edge-cache can find the local peers, edge-caches can be introduced
   into existing protocols without change.  In particular, BitTorrent is
   highly amenable to edge-caches without requiring client changes.

2.1.  Safe Incentives for Edge Caches

   The biggest impediment to building edge-caches is not technical but
   legal.  Given a P2P swarm, a single edge cache or collection of
   caches should be able to monitor the swarm and find participants.
   But an edge cache needs to be notified both about a particular P2P
   swarm and that it is acceptable to cache the swarm.

   It is outside the scope of this document for a detailed discussion,
   but there exist many possibilities, such as P2P content providers
   (such as Linux ISO images) registering their content, users of the
   ISP asserting that a swarm is legitimate (and consenting to be
   identified if a copyright holder objects), and agreements with third
   party data providers (such as Amazon S3) which support BitTorrent and
   other P2P content distribution.

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3.  An Economic Model for Delivery Costs

   For purposes of this discussion, we assume that different portions of
   the network have different costs to transmit or receive one unit of
   data.  Although costs really vary by time of day and network
   conditions (for example, the cost to an ISP of traffic on an
   uncongested uplink on the last mile is effectively 0, but can be huge
   if there is congestion, or peering arrangements may make the cost of
   uplink transit negative), for simplicity we will ignore these effects
   for now.

   CP: This is the cost for the content provider to send one unit of

   CDN: This is the cost for the content provider to send one unit of
   data through a third party, edge-based CDN

   CT: This is the cost for the ISP to receive one unit of data from the
   general internet

   CTU: This is the cost for the ISP to send one unit of data to the
   general internet

   CL: This is the cost for the ISP to send one unit of data to the end
   customer across the last mile

   CLU: This is the cost for the ISP to receive one unit of data from an
   end customer across the last mile.

   With such a basic cost model, it becomes possible to estimate the
   costs for for different content delivery mechanisms.

   Central (conventional) HTTP traffic: For such traffic, the content
   provider pays N*CP, while the ISP pays N*(CT+CL).  The costs
   increases linearly with the number of requests.

   Edge-located HTTP content delivery networks (such as Akamai): For
   such traffic, the content provider pays N*CDN, while the ISP pays
   N*CL.  This is obviously the best case for the ISP, but the cost of
   the CDN may not be favorable to the content provider.

   Conventional P2P without localization: If we assume the P2P system is
   highly efficient, the content provider pays only CP regardless of the
   number of users.  The ISP will need to pay N*(CL + CLU) for all users
   on the last mile, and some value less than N*(CT + CTU) for transit.

   Conventional P2P with perfect localization: If the P2P system is
   perfect, including localizing the traffic completely within the ISP,

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   the content provider pays only CP, while the ISP will need to pay
   N*(CL + CLU) but only (CT + CTU) for transit.

   Conventional P2P with perfect localization and perfect edge caches:
   Adding in edge-caches changes the situation.  Now the content
   provider pays only CP, while the ISP pays N*CL + CT + CTU.

3.1.  The Limits of Localization

   Such a simple cost model illustrates the major limitation of
   localization.  If CLU, the cost of the last mile uplink, is more than
   CT, the cost of the transit downlink, P2P can significantly increase
   the costs to the ISP over conventional HTTP delivery, even with
   perfect localization and perfect operation.  For some networks, such
   as DOCSIS cable modems, this is often the case, as increasing network
   capacity on the shared last mile may require new infrastructure or
   repurposing bandwidth otherwise used for higher-value services such
   as television channels.

   Yet it shows that if edge-caches are added into the system, everybody
   sees a cost savings: both the content provider and the ISP benefit
   from lower cost, but without the reliability concerns present in
   edge-based HTTP CDNs.  Thus edge-caches represent the best of both
   worlds: for a content provider, edge-caches in the P2P system have
   the same low cost as a conventional P2P system, but for the ISP, the
   edge-caches have the same low cost as an edge-located CDN.

4.  Edge-Cache Interactions with Localization

   Since edge-caches are critical to realize the true potential of P2P
   to create an aggregate cost savings, they need to be considered when
   developing other portions of a common P2P infrastructure.  In
   particular, edge-caches both interact with and benefit from
   localization services, and thus it is critical that both localization
   and edge-caching be codesigned to interoperate.  Thus some edge-cache
   concerns which directly relate to localization.

   Edge-cache discovery: Any localization service which supports the
   discovery of "preferable" nodes should give preference to any
   relevant edge-caches in the system.  Thus the localization service
   will drive traffic towards the relevant edge caches, resulting in
   greater performance and lower cost-of-delivery.

   Edge-cache content notification: Any localization service should also
   act as content notification, notifying the edge-cache about a user's
   desire to fetch a particular piece of content.  The edge-cache may
   use this information, along with other constraints and heuristics, to

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   determine whether it should participate in this distribution system.
   For example, a particular ISP's edge-cache for BitTorrent could be
   configured to cache torrents requested from Amazon S3 or other
   sources based on a contractual relationship, but reject torrents
   hosted elsewhere.

   Peer-access control: The edge-cache, when contacted by a peer, needs
   to know whether the peer is local to its network.  Thus the
   localization service should support queries from the edge cache as to
   whether a peer would be considered local to the ISP.

   Support for file descriptors: In order for both the localization
   service and the edge-cache to track files as they are requested, ALTO
   requests from peers should include both a per-file unique ID and a
   variable length field containing the protocol's representation of the
   file requested (eg, for BitTorrent, the .torrent file).  This has
   some minor privacy implications, but greatly enhances both the
   ability of localization to know which peers are involved in a
   particular transfer and the ability of edge-caches to determine which
   data to fetch.

5.  Conclusions

   Edge-caches are critical if P2P is to achieve the promised aggregate
   cost savings.  Without an edge-cache, localization's benefits are
   limited, as even perfect localization is unable to reduce the
   transfers over the last-mile uplink.  Yet edge-caches also need to
   rely on localization, both to drive traffic to the edge cache, to
   discover new content, and to determine which peers are allowed to
   access the edge-cache.  Thus localization protocols should include
   edge-caches in their focus, and edge-caches will need to use
   localization protocols.

6.  Acknowledgements

   Grant info here.  All opinions are those of the author, not the
   funding institution.

   Feedback on the general concept and economic models for P2P edge
   caches from Richard Woundy, Jason Livingood, Vern Paxson, Christian
   Kreibich, and others.

7.  IANA Considerations


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8.  Security Considerations

   The privacy concerns of edge-caches and localization are only mild to
   moderate.  It is already possible for P2P nodes to observe what other
   nodes are downloading or making available, and an edge-cache simply
   represents another such node in the system.  Any P2P system which
   wishes to avoid this problem will not want to use localization
   (because of the impacts on traffic analysis), and ISPs will not want
   to cache such data (because most of the data will represent illegal

   This is also why localization services such as ALTO should have a
   query interface that doesn't just give a list of IP addressees to
   rank, but also has query modes which present ALTO with a UUID and a
   content identifier, so a localization system can keep track of other
   systems which have already requested the same content.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

   [akamai]   Akamai Inc, "The Akamai CDN", 2008,

Author's Address

   Nicholas Weaver
   International Computer Science Institute
   1947 Center Street suite 600
   Berkeley, CA  94704

   Phone: +1 510 666 2903

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