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12_draft-wang-p2pi-localization-00.txt
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P2PI Workshop Y. Wang
S. Tan
R. Grove
Internet Draft Microsoft
Intended status: Informational May 9, 2008
Expires: November 2008
Traffic Localization with Multi-Layer, Tracker-Based Peer-to-Peer
Content Distribution Architecture
draft-wang-p2pi-localization-00.txt
Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
The goal of this document is to provide a high level overview of the
challenges to the infrastructure created by peer-to-peer
applications coupled with the emergence of media-centric content. To
illustrate the need for solution areas IETF could address, this
document sketches out a straw man peer-to-peer content delivery
framework based on a multi-layer tracker scheme to localize peer
traffic. The mechanisms and protocols required to achieve traffic
localization and to reduce impact on the infrastructure are
summarized at the end.
1. Introduction
Peer-to-peer (P2P) applications and the emergence of media-centric
content present a host of challenges to the current Internet
infrastructure. The delivery model for P2P systems differs from
traditional distributed systems in the follow manner:
1. Symmetric delivery (peer distribution) vs. asymmetric download
(client/server)
2. Multiple, concurrent, dynamic connections vs. single, static
connection per content unit
3. Routing at multiple layers vs. traditional end-to-end connection
at the Network layer
These new characteristics coupled with the growing size for rich-
media content stretch current network service provider
infrastructure to the limit in handling the traffic. Capacity
provisioning is falling behind demand. Recent, anecdotal report [1]
Wang, et al. Expires November 9, 2008 [Page 1]
P2PI Workshop P2PI Challenges May 2008
shows the 80/20 phenomenon in that a small portion of high volume
users is consuming the majority of the bandwidth. Most of the
regular user experience degrades as a result of the capacity
exhaustion, and also because of the lack of an isolation mechanism
for per-user or per-application traffic.
There are several approaches to mitigate the problems, such as
increasing the infrastructure bandwidth provisioning, reducing the
traffic though codec optimization, selectively dropping traffic with
layered encoding, or signaling and admission control to achieve
fairness. For an infrastructure-centric approach, this document
explores traffic localization to lessen the impact. The next section
presents an example framework for P2P content delivery with traffic
localization to illustrate some mechanisms and protocols required to
achieve the desired results.
2. Straw Man P2P Content Delivery Architecture with Multi-Layer
Trackers and Traffic Localization
The framework described in this section is not a formal proposal for
a solution, but rather a generic, albeit simplified example to
demonstrate the need for several mechanisms to achieve traffic
localization. It is built on a multi-layer, tracker-based P2P
content delivery scheme [2], although the localization principle is
applicable to other P2P streaming content types.
In a normal tracker-based P2P system, a client searches an index to
locate a tracker for the desired content. The tracker typically
contains the content information, including seeds and active peers
to retrieve various parts of the content. The retrieval can happen
concurrently with multiple peer nodes and seeds based on a
predefined scheduling algorithm.
The multi-layered tracker scheme introduces additional trackers
(local trackers) for the same content located in participating
provider networks, each with seeds and peers for the content located
within its network scope respectively. The key is how the initial
query to the global tracker results in the right local tracker based
on the network location of the client. There are two elements to
this approach:
1. The network locality algorithm to determine the "nearest" tracker,
and also for selecting the seeds and peers within the local
tracker
2. The mechanisms to redirect the query from the global tracker to
the selected local tracker
Wang, et al Expires November 9, 2008 [Page 2]
P2PI Workshop P2PI Challenges May 2008
Tracker redirection can be accomplished with the following options:
1. Explicit delegation where each local tracker registers with the
global tracker in advance, and the client will try to locate the
nearest tracker for its provider network
2. Implicit redirection where the infrastructure intercepts and
redirects the original query to its local trackers
3. Proxy-based approach where the service network provides a proxy
for P2P services
Among the redirection options listed, the preference is to adopt an
explicit standard mechanism that is not in-path for both reliability
and performance consideration.
Once a tracker is located, the client may still need to determine
the "closest" seeds or peers to further localize the traffic. The
locality information can either be recorded in the tracker by the
network operators (the oracle approach), or can be determined
dynamically through network synthetic coordinate protocols similar
to Vivaldi [3], but focus on network locality instead of latency.
Network operators can further optimize the traffic by providing one
or more local caches within their networks as content seeds or
regular peers in the local trackers. The cache can be provisioned
beforehand with content providers or on demand upon the first
request.
The same approach for multi-layered trackers and traffic
localization also applies to streaming content types. Although in
the streaming distribution, a cache or proxy becomes necessary as an
aggregation point for a provider network that both aggregates the
streaming data from the source (or the global P2P delivery network)
and serves all the local participating peers.
This simple example highlights the need for network synthetic
coordinates, either through operator provisioning or dynamic
measurements, and the need for a generic tracker redirection or
delegation mechanism. Note though there are two additional aspects
not covered in this example: a federation-based authentication
and/or authorization scheme if necessary, and cross-layer topology
matching to improve traffic efficiency, especially for ad hoc,
wireless networks.
3. Summary - Potential Solution Areas
This section summarizes the potential work areas for IETF. It is not
the intent of this document to provide details for each target
areas, but merely the high level requirements and rationales.
Wang, et al Expires November 9, 2008 [Page 3]
P2PI Workshop P2PI Challenges May 2008
3.1. Network Synthetic Coordinate Protocol
This is the key element for localizing the traffic. It is used to
select both the nearest local tracker for a client and the nearest
seeds and peers within from the tracker to limit the scope of the
traffic. Network operators can also utilize this protocol to
construct a pseudo network map (i.e., the oracle). A solution should
take into account both the relative coordinates between node
addresses in question and the condition of the network traffic
whenever possible. From an infrastructure perspective, the notion of
"scope" and the capability to administratively inject or define
scopes in the coordinate systems are also necessary.
3.2. Multi-Layer Tracker Redirection
It is useful to standardize on the redirection mechanism for similar
type of multi-layer tracker architecture. The redirection will work
in conjunction with the network coordinate measurement to direct a
client query to the nearest local tracker. Off-path, explicit
mechanisms are preferred for reliability reasons because it will be
easier to diagnose, and can avoid introducing new bottlenecks to the
packet flow.
3.3. Cross-Layer Topology Matching
Topology matching between the overlay and the underlying Link layer
affects the efficiency especially in ad hoc wireless networks.
Topology mismatch may cause a single hop between two peer nodes to
traverse multiple hops in the Link layer topology unnecessarily,
often results in redundant packets over the same physical link.
3.4. QoS-Related Signaling and Admission Control
QoS signaling and admission control are not considered for this
solution. The notion of end-to-end path reservation based on QoS
policy [4] is not scalable in the Internet scale. Any packet marking
scheme [5] suffers from the tragedy of the commons problem. Note
though these may work in a closed environment or within a single
provider network.
4. Security Considerations
Security considerations are not addressed in this document.
Wang, et al Expires November 9, 2008 [Page 4]
P2PI Workshop P2PI Challenges May 2008
5. Acknowledgments
This document has benefited from discussion with Scott Briggs, Nick
Holt, Paul Harris, and Tyler Barton.
References
[1] http://gigaom.com/2008/04/22/shocking-new-facts-about-p2p-and-
broadband-usage/
[2] Cohen, B. "The BitTorrent Protocol Specification," BEP-003,
http://www.bittorrent.org/beps/bep_0003.html, January 2008.
[3] Dabek, F., Cox, R., Kaashoek, F., and R. Morris, "Vivaldi: A
Decentralized Network Coordinate System," SIGCOMM 2004, August
2004.
[4] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[5] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition
of the Differentiated Services Field (DS Field) in the IPv4
and IPv6 Headers", RFC 2474, December 1998.
Authors' Addresses
Yu-Shun Wang
See-Mong Tan
Rich Groves
Microsoft Corp
One Microsoft Way
Redmond, WA 98052
US
Phone: +1 425 722 6980
Email: [email protected]
Wang, et al Expires November 9, 2008 [Page 5]