Network Working Group E. Marocco
Internet-Draft Telecom Italia
Intended status: Informational V. Gurbani
Expires: January 11, 2009 Bell Laboratories, Alcatel-Lucent
July 10, 2008
Application-Layer Traffic Optimization (ALTO) Problem Statement
draft-marocco-alto-problem-statement-02
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Abstract
A significant part of the Internet traffic today is generated by
peer-to-peer applications used for file sharing, realtime
communications and live media streaming. Such applications often
deal with large amounts of data in direct peer-to-peer connections,
but they usually have little knowledge of the underlying topology,
both at the overlay layer and the network layer. As a result, they
may choose their peers based on measurements and statistics which, in
some specific situations, often lead to suboptimal choices. This
document describes problems related to optimizing traffic generated
by peer-to-peer applications through the use of link and network
layer information.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Research or Engineering? . . . . . . . . . . . . . . . . . 4
2. The Problem . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Caching . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. Information Distribution . . . . . . . . . . . . . . . 5
2.1.3. Topology Hiding . . . . . . . . . . . . . . . . . . . 6
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. File sharing . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Cache/Mirror Selection . . . . . . . . . . . . . . . . . . 6
3.3. Live media streaming . . . . . . . . . . . . . . . . . . . 6
3.4. Realtime communications . . . . . . . . . . . . . . . . . 7
3.5. Distributed Hash Tables . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
6. Informative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
A significant part of the Internet traffic is today generated by
peer-to-peer (P2P) applications used for file sharing, realtime
communications and live media streaming [WWW.cachelogic.picture]
[WWW.wired.fuel]. Contrary to client/server architectures, P2P
applications access resources (e.g. files or media relays)
distributed across the Internet and exchange large amounts of data in
connections that they establish directly with nodes hosting such
resources.
One of the advantages of P2P systems comes from the fact that the
resources they offer are often made available through multiple
instances. Yet, applications generally ignore the topology of the
latent overlay network and have to select among available instances
based on information they deduce from empirical measurements which,
in some particular situations, could lead to suboptimal choices.
For example, popular metrics based on round trip time estimation
sometimes used for initial sources selection (i.e. before actual
data transmissions begin, when goodput values are unknown) perform
quite badly for file sharing applications, as they tend to ignore
bandwidth and reliability of underlying links, which have much
more influence than delay on file transfers.
Many of the existing overlay networks are built on top of connections
between peers that are established regardless of the underlying
network topology. In addition to simply achieving suboptimal
performance, such networks can lead to congestions and cause serious
inefficiencies. As shown in [ACM.fear], traffic generated by popular
P2P applications often cross network boundaries multiple times,
overloading links which are frequently subject to congestion
[ACM.bottleneck].
Recent studies [ACM.ispp2p] [WWW.p4p.overview] [ACM.ono] have shown
that if Internet Service Providers (ISP), network operators or third
parties in general provide reliable topology and/or bandwidth
information to P2P applications, it would be possible to greatly
increase application performance, reduce congestions and optimize the
overall traffic across different networks.
This document describes the problem of optimizing traffic generated
by P2P applications using information provided by third parties.
Section 2 introduces the problem and the main issues to keep in mind
when designing a solution, while Section 3 describes some use cases
where both P2P users and network operators would benefit from such a
solution.
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1.1. Research or Engineering?
At the time of writing, several solutions have been proposed to
address the problem described in this document, both inside and
outside the IETF [I-D.bonaventure-informed-path-selection]
[ACM.ispp2p] [WWW.p4p.overview], all accompanied by encouraging
simulation and field test results. Such solutions have been proposed
independently, but all consists of two essential parts:
o a discovery mechanism which can be used by a P2P application to
find a reliable information source;
o a protocol used by P2P applications to query such sources in order
to retrieve the information needed to choose the best endpoint
among those which host a desired resource.
It is not easy to foresee how such solutions would perform in the
Internet, but a more accurate evaluation would require representative
data collected from real systems by a critical mass of users.
However, wide adoption will probably never happen without an
agreement on a common solution based on open standards; whether such
a solution should be still studied as a research problem, published
as a "Proposed Standard" or an "Experimental" RFC [RFC2026] is an
open issue.
2. The Problem
Network engineers have been facing the problem of traffic
optimization for a long time now and have already designed mechanisms
like MPLS [RFC3031] and DiffServ [RFC3260] to deal with it. The
problem they address consists in finding (or setting) an optimal
route for packets traveling between specific source and destination
addresses and based on requirements such as low latency, high
reliability, and priority. Such solutions are usually implemented at
the link and network layers, and tend to be almost transparent. At
best, applications can only "mark" the traffic they generate with the
corresponding properties.
However, P2P applications which are today posing serious challenges
to Internet infrastructures, do not benefit much from the above
techniques and "cooperating" with external services aware of the
network topology could greatly optimize the traffic they generate.
In fact, when a P2P application needs to establish a connection, the
logical target is not a host, but rather a resource (e.g. a file or a
media relay) generally available in multiple instances on different
hosts; selection of the closest one -- or, in general, the best from
an overlay topological proximity -- has much more impact on the
overall traffic than the route followed by its packets to reach the
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endpoint.
Addressing the Application-Layer Traffic Optimization (ALTO) problem
means, on the one hand, providing topology information regarding the
underlying network and, on the other hand, enhancing P2P applications
in order to use such information to select the best endpoints among
those that are available for the connections they are going to
establish.
2.1. Issues
2.1.1. Caching
A common approach to optimize traffic generated by applications which
require large data transfers is based on caching techniques. In some
cases, such techniques have proven to be extremely effective in both
enhancing user experience and saving network resources; however, they
have two main limits in respect to the solutions based on provision
of topology information:
1. Application specificity: since a cache is meant to replace the
source of the content being accessed -- either explicitly or
transparently -- it must be able to speak the same protocol with
the querying peer. For this reason, caching solutions can be
reasonably adopted only for most popular applications (e.g. HTTP
and BitTorrent).
2. Content awareness: since caches need to actually store the
content being delivered, they are subject to legal threats
whenever the user has not the right to access such content. This
limitation makes caching approaches unusable in today's popular
file-sharing systems.
In general, solutions based on provision of topology information do
not interfere with caching; to the contrary, if the peer selection
service used by applications is aware of the presence of chaches, it
can give them higher priorities in its replies and thus achieve
greater optimization.
2.1.2. Information Distribution
As a direct consequence of the total distribution of the Internet, it
seems almost impossible to centralize all information P2P
applications may need to optimize traffic they generate. It is quite
likely that such information would be highly distributed, for
example, at an ISP or domain level. It is also reasonable to expect
that, in some cases, the same network administrators will control
provision of such information.
However, as applications usually have no knowledge of the
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administrative entities running the network they are using, any
solution will need to define a discovery mechanism (e.g. based on or
similar to reverse DNS [RFC2317]) and perhaps an infrastructure to
certify information sources.
2.1.3. Topology Hiding
Operators can play an important role in addressing the ALTO problem,
but they generally consider topology of the networks they control to
be confidential information; therefore, in order to succeed and
achieve wide adoption, any solution should provide a method to help
P2P applications in peer selection without explicitly disclosing
topology of the underlying network.
3. Use Cases
3.1. File sharing
File sharing applications allow users to search for content shared by
other users and download it. Typically, search results consist of
many instances of the same file available from multiple sources; the
goal of an ALTO solution would be to help peers find the best ones
according to the underlying networks.
On the application side, integration of ALTO functionalities may
happen at different levels. For example, while in the completely
decentralized Gnutella network selection of the best sources is
totally up to the user, in systems like BitTorrent and eDonkey,
central elements (i.e. trackers or servers) act as mediators.
Therefore, in the former case, optimization would require
modification in the applications, while in the latter it could just
be implemented in some central elements.
3.2. Cache/Mirror Selection
Providers of popular content like media and software repositories
usually resort to geographically distributed caches and mirrors for
load balancing. Selection of the proper mirror/cache for a given
user is today based on inaccurate geolocation data, on proprietary
network location systems or often delegated to the user himself; an
ALTO solution could be easily adopted to ease such a selection in an
automated way.
3.3. Live media streaming
P2P applications for live streaming allow users to receive multimedia
content produced by one source and targeted to multiple destinations,
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in a realtime or near-realtime way without recurring to multicast.
Such applications typically participate in the distribution of the
content, acting as both receivers and senders; the goal of an ALTO
solution would be to help peers to find the best sources and the best
destinations for media flows they receive and relay.
3.4. Realtime communications
P2P realtime communications allow users to establish direct media
flows, usually to place audio and video calls, or to have text chats.
In the basic case, media would flow directly between the two
endpoints; however, in the general case, a significant portion of
communications between users with limited access to the Internet
(e.g. users behind NATs, firewalls or HTTP proxies) need to be
relayed by other elements. Such media relays are distributed over
the Internet -- in some cases co-located with applications with a
public address; the goal of an ALTO solution would be to help peers
to find the best relays.
3.5. Distributed Hash Tables
Distributed hash tables (DHT) are a class of overlay algorithms used
to implement lookup functionalities in popular P2P systems, without
recurring to centralized elements. In such systems, peers maintain
addresses of other peers participating in the same DHT in a routing
table, sorted according to specific criteria. An ALTO solution would
provide valuable information for DHT algorithms which, in order to
reduce path latency of distributed queries, include round trip time
estimations among such criteria [SIGCOMM.resprox].
4. Security Considerations
The approach proposed in this document requires P2P applications to
delegate a portion of their routing capability to third parties,
giving them a significant role in systems where that would be
otherwise excluded.
In the case where an ALTO solution is deployed by the network
operator, it is conceivable that the P2P community would consider it
hostile because the operator could, for example:
o redirect applications to corrupted mediators providing malicious
content;
o track connections to perform content inspection;
o apply policies based on criteria other than network efficiency
(for example, to avoid peering points regulated by inconvenient
economic agreements).
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However, ALTO is completely optional for P2P applications and its
purpose is to help improve performance of such applications. If, for
some reason, it fails to achieve this purpose, it would simply fail
to gain popularity and would not be used.
Even in cases where the ALTO service provider would decide to
maliciously alter results returned by queries only after the solution
has gained popularity (i.e. it behaves for a while to become popular
and then starts misbehaving), it would be fairly easy for P2P
application maintainers and users to revert to solutions that are not
using it. After all, it would all come down to change some
application settings in cases where the protocol is implemented
inside the client and upgrading centralized elements for
architectures like BitTorrent and eDonkey.
5. Acknowledgments
We have to acknowledge many people. For the record: Vinay Aggarwal
and the P4P working group for the research work done outside the
IETF. Emil Ivov, Rohan Mahy, Anthony Bryan, Stanislav Shalunov,
Laird Popkin, Stefano Previdi, Reinaldo Penno, Dimitri Papadimitriou
and many others for interesting discussions, comments and
corrections.
6. Informative References
[ACM.bottleneck]
Akella, A., Seshan, S., and A. Shaikh, "An Empirical
Evaluation of WideArea Internet Bottlenecks", Proceedings
of ACM SIGCOMM, October 2003.
[ACM.fear]
Karagiannis, T., Rodriguez, P., and K. Papagiannaki,
"Should ISPs fear Peer-Assisted Content Distribution?",
In ACM USENIX IMC, Berkeley 2005.
[ACM.ispp2p]
Aggarwal, V., Feldmann, A., and C. Scheideler, "Can ISPs
and P2P systems co-operate for improved performance?", In
ACM SIGCOMM Computer Communications Review (CCR), 37:3,
pp. 29-40.
[ACM.ono] Choffnes, D. and F. Bustamante, "Taming the Torrent: A
practical approach to reducing cross-ISP traffic in P2P
systems", Proceedings of ACM SIGCOMM, August 2008.
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[I-D.bonaventure-informed-path-selection]
Saucez, D. and B. Donnet, "The case for an informed path
selection service",
draft-bonaventure-informed-path-selection-00 (work in
progress), February 2008.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[SIGCOMM.resprox]
Gummadi, K., Gummadi, R., Ratnasamy, S., Gribble, S.,
Shenker, S., and I. Stoica, "The impact of DHT routing
geometry on resilience and proximity", Proceedings of ACM
SIGCOMM, August 2003.
[WWW.cachelogic.picture]
Parker, A., "The true picture of peer-to-peer
filesharing", <http://www.cachelogic.com>.
[WWW.p4p.overview]
Xie, H., Krishnamurthy, A., Silberschatz, A., and R. Yang,
"P4P: Explicit Communications for Cooperative Control
Between P2P and Network Providers",
<http://www.dcia.info/documents/P4P_Overview.pdf>.
[WWW.wired.fuel]
Glasner, J., "P2P fuels global bandwidth binge",
<http://www.wired.com/techbiz/media/news/2005/04/67202>.
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Authors' Addresses
Enrico Marocco
Telecom Italia
Via G. Reiss Romoli, 274
Turin 10148
Italy
Email: enrico.marocco@telecomitalia.it
Vijay K. Gurbani
Bell Laboratories, Alcatel-Lucent
2701 Lucent Lane
Lisle, IL 60532
USA
Email: vkg@alcatel-lucent.com
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