Network Working Group E. Marocco
Internet-Draft Telecom Italia
Intended status: Informational I. Rimac
Expires: August 30, 2009 V. Gurbani
Bell Labs, Alcatel-Lucent
February 26, 2009
Mythbustering Peer-to-peer Traffic Localization
draft-marocco-p2prg-mythbustering-00
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Abstract
Peer-to-peer traffic optimization techniques that aim at improving
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locality in the peer selection process have attracted great interest
in the research community and have been subject of much discussion.
Some of this discussion has produced controversial myths, some rooted
in reality while others remain unfounded. This document evaluates
the most prominent myths attributed to P2P optimization techniques by
referencing the most relevant study (or studies) that have addressed
facts pertaining to the myth. Using these studies, we hope to either
confirm or refute each specific myth.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Seeder . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Leecher . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Swarm . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Tit-for-tat . . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Surplus Mode . . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Transit . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.7. Peering . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Myth: Reduced Cross-domain Traffic . . . . . . . . . . . . . . 6
3.1. Facts . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 6
4. Myth: Increased Application Performance . . . . . . . . . . . 6
4.1. Facts . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 7
5. Myth: Increased Uplink Bandwidth Usage . . . . . . . . . . . . 7
5.1. Facts . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 8
6. Myth: Impacts on Peering Agreements . . . . . . . . . . . . . 8
6.1. Facts . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 8
6.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 9
7. Myth: Impacts on Transit . . . . . . . . . . . . . . . . . . . 9
7.1. Facts . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 10
7.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 10
8. Myth: Swarm Weakening . . . . . . . . . . . . . . . . . . . . 10
8.1. Facts . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 10
8.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
11. Informative References . . . . . . . . . . . . . . . . . . . . 11
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Peer-to-peer (P2P) applications used for file-sharing, streaming and
realtime communications exchange large amounts of data in connections
established among the peers themselves and are responsible for an
important part of the Internet traffic. Since applications have
generally no knowledge of the underlying network topology, the
traffic they generate is frequent cause of congestions in inter-
domain links and significantly contributes to the raising of transit
costs paid by network operators and Internet Service Providers (ISP).
One approach to reduce congestions and transit costs caused by P2P
applications consists of enhancing the peer selection process with
the introduction of proximity information. This allows the peers to
identify the topologically closer resource among all the instances of
the resources they are looking for. Several solutions following such
an approach have recently been proposed [Choffnes] [Aggarwal] [Xie],
some of which are now being considered for standardization in the
IETF [ALTO].
Despite extensive research based on simulations and field trials, it
is hard to predict how proposed solutions would perform in a real-
world systems made of millions of peers. For this reason, possible
effects and side-effects of optimization techniques based on P2P
traffic localization have been a matter of frequent debate. This
document describes some of the most interesting effects, referencing
relevant studies which have addressed them and trying to determine
whether and in what measure they are likely to happen.
Each possible effect -- or Myth -- is examined in three phases:
o Facts: in which a list of relevant data is presented, usually
collected from simulations or field trials;
o Discussion: in which the reasons for and against the myth are
discussed based on the facts previously listed;
o Conclusions: in which the authors try to epress a reasonable
measure of the plausibility of the myth.
This document at the current stage is little more than a strawman.
With the help of the IRTF community, the authors would like to
improve it, in the number of the Facts, in the quality of the
Discussion and, particularly, in the trustworthiness of the
Conclusions.
2. Definitions
Terminology defined in [I-D.marocco-alto-problem-statement] is reused
here; other definitions should be consistent with it.
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2.1. Seeder
A peer which has a complete copy of the content it is sharing, and
still offers it for upload. The term "seeder" is adopted from
BitTorrent terminology and is used in this document to indicate
upload-only peers also in other kinds of P2P applications.
2.2. Leecher
A peer which has not yet completed the download of a specific content
(but usually has already started offering for upload the part it is
in possession of). The term "leecher" is adopted from BitTorrent
terminology and is used in this document to indicate peers which are
both uploading and downloading, also in other kinds of P2P
applications.
2.3. Swarm
The group of peers that are uploading and/or downloading pieces of
the same content. The term "swarm" is commonly used in BitTorrent,
to indicate all seeders and leechers exchanging chuncks of a
particular file; however, in this document it is used more generally,
for example, in the case of P2P streaming applications, to refer to
all peers receiving and/or transmitting the same media stream.
2.4. Tit-for-tat
A content exchange strategy where the amount of data sent by a
leecher to another leecher is roughly equal to the amount of data
received from it. P2P applications, most notably BitTorrent, adopt
such an approach to maximize resources shared by the users.
2.5. Surplus Mode
The status of a swarm where the number of seeders is significantly
greater than the number of leechers (i.e. the total download demand
is abundantly satisfied by the upload capacity).
2.6. Transit
The service through which a network can exchange IP packets with all
other networks it is not directly connected to. The transit service
is always regulated by a contract, according to which the custumer
(i.e. a network operator or an ISP) pays the transit provider per
amount of data exchanged.
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2.7. Peering
The direct interconnection between two separate networks for the
purpose of exchanging traffic without recurring to a transit
provider. Peering is usually regulated by agreements taking in
account the amount of traffic generated by each party in each
direction.
3. Myth: Reduced Cross-domain Traffic
The reduction in cross-domain traffic (and thus in transit costs due
to it) is one of the positive effects P2P traffic localization
techniques are expected to cause, and also the main reason way ISPs
look at them with interest. Simulations and field tests have shown a
reduction varying from 20% to 80%.
3.1. Facts
1. Various simulations and initial field trials of the P4P solution
[Xie] on average show a 70% reduction of cross-domain traffic.
2. Data observed in Comcast's P4P trial
[I-D.livingood-woundy-p4p-experiences] show a 34% reduction of
the outgoing P2P traffic and an 80% reduction of the incoming.
3. Simulations of the "oracle-based" approach [Aggarwal] proposed by
researchers at TU Berlin show an increase in local exchanges from
10% in the unbiased case to 60%-80% in the localized case.
3.2. Discussion
Tautologically, P2P traffic localization techniques tend to localize
content exchanges, and thus reduce cross-domain traffic.
3.3. Conclusions
Confirmed.
4. Myth: Increased Application Performance
Ostensibly, the increase in application performance is the main
reason for the consideration of P2P traffic localization techniques
in academia and industry. The expected increase depends on the
specific application: file sharing applications witness an increase
in the download rate, realtime communication applications observe
lower delay and jitter, and streaming applications can benefit by a
high constant bitrate.
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4.1. Facts
1. Various simulations and initial field trials of the P4P solution
[Xie] show an average reduction of download completion times
between 10% and 23%.
2. Data observed in Comcast's P4P trial
[I-D.livingood-woundy-p4p-experiences] show and increase in
download rates between 13% and 85%. Interestingly, the collected
data also indicate that fine-grained localization is less
effective in improving download performance compared to lower
levels of localization.
3. Data collected in the Ono experiment [Choffnes] show a 31%
average download rate improvement.
* In networks where the ISP provides higher bandwidth for in-
network traffic (e.g. as in the case of RDSNET, described in
[Choffnes]), the increase is significantly higher.
* In networks with relatively low uplink bandwidth (as the case
of Easynet, described in [Choffnes]), traffic localization
slightly degrades application performance.
4. Simulations of the "oracle-based" approach [Aggarwal] proposed by
researchers at TU Berlin show a reduction in download times
between 16% and 34%.
5. Simulations by Bell Labs [Seetharaman] indicate that localization
is not as effective in all scenarios and that the user experience
can suffer in certain locality-aware swarms based on the actual
implementation of locality.
4.2. Discussion
It seems that traffic localization techniques often cause an
improvement in application performance. However, it must be noted
that such beneficial effects heavily depend on the network
infrastructures. In some cases, for example in networks with
relatively low uplink bandwidth, localization seems to be useless if
not harmful. Also, beneficial effects depend on the swarm size;
imposing locality when only a small set of local peers are available
may even decrease download performance for local peers.
4.3. Conclusions
Very likely, especially for large swarms and in networks with high
capacity.
5. Myth: Increased Uplink Bandwidth Usage
The increase in uplink bandwidth usage would be a negative effect,
especially in environments where the access network is based on
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technologies providing asymmetric upstream/downstream bandwidth (e.g.
DSL or DOCSIS).
5.1. Facts
1. Data observed in Comcast's P4P trial
[I-D.livingood-woundy-p4p-experiences] show no increase in the
uplink traffic.
5.2. Discussion
Mathematically, average uplink traffic remains the same as long as
the swarm is not in surplus mode. However, in some particular cases
where surplus capacity is available, localization may lead to local
low-bandwiwth leechers connecting to each other instead of trying the
external seeders. Even if such a phenomenon has not been observed in
simulations and field trials, it could occur to applications that use
localization as the only means for optimization when some content
becomes popular in different areas at different times (as is the case
of prime time TV shows distributed on BitTorrent networks minutes
after getting aired in North America).
5.3. Conclusions
Unlikely.
6. Myth: Impacts on Peering Agreements
Peering agreements are usually established on a reciprocity basis,
assuming that the amount of data sent and received by each party is
roughly the same (or, in case of asymmetric traffic volumes, a
compensation fee is paid by the party which would otherwise obtain
the most gain). P2P traffic localization techniques aim at reducing
cross-domain traffic and thus might also impact peering agreements.
6.1. Facts
No significant publications, simulations or trials have tried to
understand how traffic localization techniques can influence factors
that rule how peering agreements are established and maintained.
6.2. Discussion
This is a key topic for network operators and ISPs, and certainly
deserves to be analyzed more accurately. Some random thoughts
follow.
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It seems reasonable to expect different effects depending on the
kinds of agreements. For example:
o ISPs with different customer bases: the ISP whose customers
generate more P2P traffic can achieve a greater reduction of
cross-domain traffic and thus could probably be in a position to
re-negotiate the contract ruling the peering agreement;
o ISPs with similar customer bases:
* ISPs with different access technologies: customers of the ISP
which provides higher bandwidth -- and, in particular, higher
uplink bandwidth -- will have more incentives for keeping their
P2P traffic local. Consequently, the ISP with a better
infrastructure will be able to achieve a greater reduction in
cross-domain traffic and will be probably in a position to re-
negotiate the peering agreement;
* ISPs with similar access technologies: both ISPs would achieve
roughly the same reduction in cross-domain traffic and thus the
conditions under which the peering agreement had been
established would not change much.
As a consequence of the reasoning above, it seems reasonable to
expect that the simple fact that one ISP starts localizing its P2P
traffic will be a strong incentive for the ISPs it peers with to do
that as well.
6.3. Conclusions
Likely.
7. Myth: Impacts on Transit
One of the main goals of P2P traffic localization techniques is to
allow ISPs to keep local a part of the traffic generated by their
customers and thus save on transit costs. However, similar
techniques based on de-localization rather than localization may be
used by those ISP which are also transit providers to artificially
increase the amount of data exchanged with networks they provide
transit to (i.e. pushing the peers run by their custormers to
establish connections with peers in the networks that pay them for
transit).
7.1. Facts
No significant publications, simulations or trials have tried to
study effects of traffic localization techniques on the dynamics of
transit provision economics.
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7.2. Discussion
It is actually very hard to predict how the economics of transit
provision would be affected by the tricks some transit providers
could play on their customers making use of P2P traffic localization
-- or, in this particular case, de-localization -- techniques. This
is also a key topic for ISPs, definitely worth an accurate
investigation.
Probably, the only lesson contentions concerning transit and peering
agreement have teached so far [CogentVsAOL] [SprintVsCogent] is that,
at the end of the day, no economic factor, no matter how much
relevant it is, is able to isolate different networks from each
other.
7.3. Conclusions
Likely.
8. Myth: Swarm Weakening
Peer selection techniques based on locality information are certainly
beneficial in areas where the density of peers is high enough, but
may cause damages otherwise. Some studies have tried to understand
to what extent locality can be pushed without damaging peers in
isolated parts of the network.
8.1. Facts
1. Simulations run by researchers at INRIA [Le Blond] have shown
that, in BitTorrent, even when peer selection is heavily based on
locality, swarms do not get damaged.
2. Simulations by Bell Labs [Seetharaman] indicate that the user
experience can suffer in certain locality-aware swarms based on
the actual implementation of locality.
8.2. Discussion
It seems reasonable to expect that excessive traffic localization
will cause some degree of deterioration in P2P swarms based on the
tit-for-tat approach, and the damages of such deterioration will
likely affect most users in networks with lower density of peers.
However, as shown in [Le Blond], the right balance of randomness and
locality depends on the P2P algorithm.
On the other hand, P2P systems not adopting the tit-for-tat approach
(e.g. the eDonkey network) should not be damaged by locality-based
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optimizations.
8.3. Conclusions
Plausible, in some circustancies.
9. Security Considerations
No considerations at this time.
10. Acknowledgments
This documents tries to summarize discussions happened in live
meetings and on several mailing lists: all those who are reading this
have probably contributed more ideas and more material than the
authors themselves.
11. Informative References
[ALTO] "Application-Layer Traffic Optimization (ALTO) Working
Group", <http://ietf.org/html.charters/alto-charter.html>.
[Aggarwal]
Aggarwal, V., Akonjang, O., and A. Feldmann, "Improving
User and ISP Experience through ISP-aided P2P
Localityraffic in P2P systems".
[Choffnes]
Choffnes, D. and F. Bustamante, "Taming the Torrent: A
practical approach to reducing cross-ISP traffic in P2P
systems".
[CogentVsAOL]
Washington Post, "Peering Dispute With AOL Slows Cogent
Customer Access", <http://legalminds.lp.findlaw.com/list/
cyberia-l/msg42080.html>.
[I-D.livingood-woundy-p4p-experiences]
Griffiths, C., Livingood, J., and R. Woundy, "Comcast's
ISP Experiences In a Recent P4P Technical Trial",
draft-livingood-woundy-p4p-experiences-02 (work in
progress), October 2008.
[I-D.marocco-alto-problem-statement]
Seedorf, J. and E. Burger, "Application-Layer Traffic
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Optimization (ALTO) Problem Statement",
draft-marocco-alto-problem-statement-04 (work in
progress), February 2009.
[Le Blond]
Le Blond, S., Legout, A., and W. Dabbous, "Pushing
BitTorrent Locality to the Limit".
[Seetharaman]
Seetharaman, S., Hilt, V., Rimac, I., and M. Ammar,
"Applicability and Limitations of Locality-Awareness in
BitTorrent File-Sharing".
[SprintVsCogent]
PC World, "Sprint-Cogent Dispute Puts Small Rip in Fabric
of Internet", <http://www.pcworld.com/businesscenter/
article/153123/
sprintcogent_dispute_puts_small_rip_in_fabric_of_internet.
html>.
[Xie] Xie, H., Yang, Y., Krishnamurthy, A., Liu, Y., and A.
Silberschatz, "P4P: Provider Portal for Applications".
Authors' Addresses
Enrico Marocco
Telecom Italia
Email: enrico.marocco@telecomitalia.it
Ivica Rimac
Bell Labs, Alcatel-Lucent
Email: rimac@bell-labs.com
Vijay K. Gurbani
Bell Labs, Alcatel-Lucent
Email: vkg@alcatel-lucent.com
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