Internet DRAFT - draft-crowley-alto-importance
draft-crowley-alto-importance
Internet Engineering Task Force P. Crowley
Internet Draft Washington University
Intended status: Informational July 7, 2008
Expires: January 1, 2009
On the Relative Importance of P2P Peer Selection
draft-crowley-alto-importance-01.txt
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Abstract
This Internet-draft discusses the relative importance of path
selection in peer-to-peer (P2P) applications in light of the recent
discussions highlighting the conflict between the use of P2P
applications and the costs borne by network infrastructure operators.
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Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
Table of Contents
1. Introduction...................................................2
2. Context & Importance...........................................2
2.1. Technical Strengths.......................................3
3. Problems with P2P Applications.................................4
4. Proposed Solutions.............................................5
5. Problems Solved via Standardization............................6
6. What is Needed.................................................6
7. Security Considerations........................................7
8. IANA Considerations............................................7
9. Conclusions....................................................7
10. Acknowledgments...............................................7
11. Informative References........................................7
Author's Addresses................................................8
Intellectual Property Statement...................................8
Disclaimer of Validity............................................8
1. Introduction
The purpose of this document is to juxtapose peer-to-peer (P2P)
application context, importance, and problems with solutions offered
by standardization. In particular, the goal is to explore whether the
current challenges created by P2P applications can be effectively
solved, or at least ameliorated, through standardization efforts.
2. Context & Importance
P2P file-sharing is a significant consumer Internet traffic source. A
recent report from Cisco Systems [1,2] indicates that in 2007 P2P
applications accounted for 51% of global consumer Internet traffic.
While there are non-file-sharing P2P applications, file sharing is
the dominant source of bandwidth consumption. P2P-based streaming
video is significant in some parts of the world, but that is not
counted among the P2P numbers in the Cisco report. We can conclude,
therefore, that P2P file-sharing is an important application for
users and infrastructure owners.
However, it is also reasonable to assume that the majority of the
bandwidth consumed by P2P applications is devoted to the distribution
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of video files. While some may assume that the popularity of P2P
applications is content-independent, it is reasonable to suppose that
the popularity of P2P applications, the least when measured by
bandwidth consumed, is due entirely to the popularity of sharing
video files. Anecdotally, it seems that this file-sharing is mostly
illegal; whether this is blatant theft or a form of civil
disobedience is a topic left for another time. More to the point,
the Cisco study also reports that non-P2P video traffic has
experienced, and will continue to experience, an even greater rate of
growth than P2P applications. Indeed, despite accounting for 51% of
global Internet traffic, P2P traffic shrank year-on-year as a total
percentage while non-P2P video grew.
Before discussing the technical strengths and problems with P2P
applications, we note that this is an unwieldy conversation because
many topics are customarily grouped together, while specific problems
and solutions typically apply only to a single topic. Topics that are
usually aggregated included:
o BitTorrent and the characteristics that distinguish it from other
P2P clients and protocols.
o File Sharing as opposed to other applications on peer-to-peer
platforms, such as video-on-demand, voice and instant messaging.
o P2P versus client-server service architectures.
In this Internet-draft, we will focus the discussion on the use of
P2P applications for file-sharing, because this usage dominates both
the importance and the popularity of P2P systems in the world today.
2.1. Technical Strengths
P2P systems, and in particular BitTorrent, are in most ways
remarkably efficient technologies.
BitTorrent effectively eliminates costs associated with distributing
large digital files. Individuals with consumer DSL connections can be
global-scale publishers of multi-gigabyte files. It is for this
reason that people around the world can rip and share extremely large
video files at very little cost.
P2P systems deploy and evolve at the rate of change of end-user
habits. There is very low friction resisting development and
deployment. There are no encumbrances from service or infrastructure
providers to slow things down.
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Perhaps most remarkably, file distribution with BitTorrent scales
automatically with available resources, because it does not require
manual intervention to utilize capacity increases.
3. Problems with P2P Applications
The problems caused by P2P applications can be categorized according
to the party experiencing the problem.
Home users, those who install and use P2P clients on their own
computers, experience at least two kinds of problems. First, P2P
file-sharing applications interfere with latency sensitive
applications, such as voice over IP (VOIP), because they tend to fill
all available buffers between a computer and the end of its access
link. As recently pointed out by S. Shalunov at the IETF P2P
infrastructure workshop in May 2008, mass-market consumer access
points and switches feature small amounts of buffer space that
BitTorrent tends to fill, increasing ping times and jitter for
latency sensitive applications. Second, P2P applications can easily
exhaust bandwidth caps associated with broadband connections.
For ISPs and network infrastructure operators, P2P applications
present at least three problems: large bandwidth usage, usage that
scales with available capacity, and steady, long-lived connections
rather than bursty ones.
Since files shared on P2P networks tend to be large, the aggregate
bandwidth used is also large.
As mentioned above, a P2P application such as BitTorrent
automatically scales to take advantage of the available network
resources. This creates a tremendous problem for increasing
capacity: if rising P2P traffic has inspired an accelerated capacity
increase and that P2P traffic can automatically scale to take
advantage of any added capacity, then it is difficult to make
rational, cost-effective capacity investment plans.
Long-lived, non-bursty traffic is troublesome because it runs afoul
of the assumptions that govern the oversubscription rates used to
price broadband connections.
It can be seen that the problem is in fact due to the conjunction of
these three. For example, FTP is also steady, large, and long-lasting
but very few use it so it causes no large problem.
It is worth asking whether these are technical problems or business
model problems. Certainly, a technical decision could be taken to
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remove buffering from access points and switches, although this would
not apply to the installed base of tens of millions of devices.
However, the other problems are at least equally dependent on
business model or social factors, rather than on solely technical
questions.
4. Proposed Solutions
A high-level notion of ISP cost is the problem most discussed in
popular media and research literature. Many proposals for P2P peer-
localization, content caching, and hybrid server-peer architectures
have been proposed. Additionally, commercial offerings include
devices that can measure P2P traffic and apply traffic shaping
policies.
ALTO is particularly concerned with the mechanisms used to select
peers, which in turn can achieve localization or effective caching.
Examples of these mechanisms are nicely surveyed in the ALTO abstract
presented at the IETF P2P infrastructure workshop in May 2008, where
two major approaches are summarized: end-to-end measurement and
application-to-network layer cooperation.
In general, however, these peer selection ideas are only useful in
contexts where peers are abundant and many options of equivalent
quality exist. Whenever peers must be chosen based on the rare
content they can provide, peer selection mechanisms will not apply.
Of course, it is natural to assume that a disproportionately large
amount of P2P file sharing bandwidth can be accounted for by a
relatively small number of distinct files. That is, a subset of
popular videos will likely account for the majority of traffic at any
given time.
However, as other legal online video options proliferate, it is
possible that P2P file-sharing will cease to be the primary video
distribution platform. In recent years, the number of ad-supported
and subscription-based online video services has flourished.
Continued developments along this path-along with the possibility
that the public-at-large may eventually lose its appetite for illegal
file sharing-may significantly alter P2P dynamics.
And more to the point, as video continues to dominate global Internet
traffic, the Internet will experience dramatic increases in the
number of high-bandwidth, non-bursty, long-lived connections. P2P
applications are singled out as the problematic ones today, but in
the near future there will be many more video-oriented applications
that will cause similar problems.
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5. Problems Solved via Standardization
What Problems do Standards Solve?
In general, standards have two effects. They avoid the problems
associated with a single solution provider, such as monopoly pricing
power and technical frailty, by encouraging commoditization of the
object of standardization. They also avoid the problems associated
with the reinvention and fragmentation of common solutions, such as:
safety (i.e., standards can reduce the likelihood of errors and
mistakes), interoperability, and quality.
6. What is Needed
There is no shortage of feasible technical solutions to the problems
raised by P2P applications, but the effectiveness of a given solution
will be determined by business models and end-user willingness to
adopt them.
Examples of connectivity business models include the following.
o Fixed prices with bandwidth caps.
o Pay-per-bit.
o Traffic categories, with per-category pricing models.
The current user vocabulary defines as categories phone, video, and
data. This is obvious and clear to consumer customers of telephone
and cable companies. P2P traffic characteristics cause a problem for
existing data pricing models. Some have suggested that data pricing
models require further elaboration, with finer subdivisions. For
example, one can imagine web/e-mail/data, P2P, gaming, voice, and
video all be as coming subcategories within a data plan, each with
their own pricing model. Might users understand and accept such a
pricing model? Would such a model make sense for broadband
providers?
This line of thinking leads one to ask: Is the Internet a utility, an
infrastructure, or a service platform?
If it is a utility, then we can consider the pricing model for
electricity, in which users are charged for each kilowatt-hour used,
without regard for how the units are used (but perhaps with regard to
when they are used).
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If it is an infrastructure, then we can consider the pricing model
for interstate highways. Such a model is very different from
today's, since this type of infrastructure does not produce profit
directly, but enables profitable uses.
If the Internet is a service platform, such as mobile phones and
mobile phone networks, then one can imagine pricing models that
separate out distinct uses, such as with mobile voice, data, and SMS
pricing models. In such a model, not all bits carry the same price.
7. Security Considerations
Security risks tend to increase with complexity. Any proposal that
increases the complexity of infrastructure or its interfaces, such as
those that aim to provide access to direct or indirect topology
information, invites the risk of complicating security. More
specifically, if a service provider publishes information that is
meant to both improve end-user performance and reduce service
provider cost, there will always be the opportunity for subversion,
or perhaps the more insidious suspicion that the service provider is
paying greater attention to its costs than to the performance
provided. An effective antidote for this situation is independent
verification.
8. IANA Considerations
9. Conclusions
In conclusion, because standardization seems ill-positioned to
address the most pressing problems with P2P systems, it is likely
premature for the IETF to pursue any strong activities in
standardization. There may be other reasons to pursue
standardization, but there does not appear to be a strong, objective
case for doing so for the sake of improving P2P file-sharing.
10. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
11. Informative References
[1] Cisco Systems, "Cisco Visual Networking Index-Forecast and
Methodology, 2007-2012", White Paper, http://www.cisco.com,
June 2008.
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[2] Cisco Systems, "Approaching the Zettabyte Era", White Paper,
http://www.cisco.com, June 2008.
Author's Addresses
Patrick Crowley
Washington University
Dept. of CSE/Campus Box 1045
One Brookings Drive
St. Louis, MO 63130
USA
Email: pcrowley@wustl.edu
URL: http://www.arl.wustl.edu/~pcrowley
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