PPSP Y. Zhang
Internet Draft China Mobile
Intended status: Informational N.Zong
HuaweiTech
G.Camarillo
Ericsson
J.seng
PPlive
R.Yang
Yale University
Expires: July 16, 2011 January 16, 2011
Problem Statement of P2P Streaming Protocol (PPSP)
draft-ietf-ppsp-problem-statement-01
Abstract
P2P streaming systems show more and more popularity in current
Internet with proprietary protocols. This document identifies
problems of the proprietary protocols, proposes standard signaling
protocols called PPSP and discusses the scope and use case of PPSP.
Status of this Memo
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This Internet-Draft will expire on July 16, 2011.
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Table of Contents
1. Introduction ................................................. 4
2. Terminology and concepts ..................................... 6
3. Problem statement ............................................ 7
3.1. ISP's difficulties in deploying P2P caches .............. 7
3.2. Difficulties in building open streaming delivery
infrastructure ............................................... 7
3.3. Difficulties in mobile and wireless environment...........8
3.4. Terminal physical resource starvation ................... 9
4. PPSP:Standard peer to peer streaming protocols .............. 10
5. Use cases of PPSP ........................................... 13
5.1. Worldwide provision of open P2P live streaming services. 13
5.2. CDN supporting P2P streaming ........................... 13
5.3. PPSP supporting cross-screen streaming in heterogeneous
environment ................................................. 14
5.4. Supporting P2P streaming in cellular mobile network..... 15
5.5. Cache service supporting P2P streaming ................. 16
6. Security Considerations ..................................... 17
6.1. Tracker Protocol ....................................... 17
6.2. Peer Protocol .......................................... 17
7. Acknowledgements ............................................ 18
8. References .................................................. 19
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1. Introduction
Streaming traffic is among the fastest growing traffic on the
Internet. As Cisco Visual Network Traffic index measured, video
streaming already generates the largest volume of Internet traffic in
the year of 2010, and the percentage is expected to rise to as high
as 91% of the total Internet traffic by 2014[Cisco].
There are two basic architectures for delivering streaming traffic on
the global Internet: the client-server paradigm and the peer to peer
(P2P) paradigm [Survey].The basic advantage of the P2P paradigm is
its scalability and fault tolerance against failures of centralized
infrastructures. As an example, PPLive [PPLive], one of the largest
P2P streaming vendors, is able to distribute large-scale, live
streaming programs such as the CCTV Spring Festival Gala to more than
3 million users with only a handful of servers. It can also deliver
VoD streaming to a scale of some hundred of thousands simultaneous
users using the same structure and similar protocols[VoD]. The effect
of P2P technologies is also well demonstrated in delivering real and
VoD streaming effectively in current practice like CNN [CNN] PPstream
[PPstream],UUSee [UUSee]and CNTV[CNTV]. The latest release of Adobe
Flash, a major platform of streaming distribution in the Internet,
has also introduced Cirrus [Cirrus], a peer assisted data exchange
mode. Almost all these systems use their proprietary signaling
protocols and standard data delivery protocols (like UDP and TCP).
What's more, along with the new players like CDN vendors (e.g.,Akamai
[Akamai], ChinaCache[ChinaCache]) and ISPs(e.g., ComCast
[ComCast])joining in the P2P streaming delivery game, the P2P
streaming ecosystem is becoming more complex with participants from
the source, infrastructure side, edge delivery even to the terminals.
Given the above increasing integration of P2P streaming into the
global content delivery infrastructure, the lacking of an open,
standard P2P streaming signaling protocol suite becomes a major
missing component in the Internet protocol stack. Multiple, similar
but proprietary signaling protocols result in repetitious development
efforts for new systems, and the lock-in effects lead to substantial
difficulties in the integration. For example, in the enhancement of
existing caches and CDN systems to support P2P streaming, the open
protocols will dynamically reduce the complexity of the interaction
with different P2P streaming applications.
In this document we propose an open P2P streaming protocol named PPSP,
to standardize signaling operations on two important components, peer
and tracker for information exchange. The problems of proprietary
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signaling protocols and benefit of PPSP are explained further in
section 3.
PPSP will serve as an enabling technology, building on the
development experiences of existing P2P streaming systems. Its design
will allow it to integrate with IETF protocols on distributed
resource location, traffic localization, and streaming control and
data transfer mechanisms for building a complete streaming system or
a streaming delivery infrastructure.
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2. Terminology and concepts
Chunk: A chunk is a basic unit of partitioned streaming. Peers may
use a chunk as a unit of storage, advertisement and exchange among
peers [VoD]. Note that a streaming system may use different units for
advertisement and data exchange, using chunks during data exchange,
and a larger unit such as a set of chunks during advertisement.
Content Distribution Network (CDN): A CDN node refers to a network
entity that is deployed in the network (e.g., at the network edge or
data centers) to store content provided by the original servers, and
serves content to the clients located nearby topologically.
Live streaming: It refers to a scenario where all clients receive
streaming content for the same ongoing event. It is desired that the
lags between the play points of the clients and that of the streaming
source be small.
P2P cache: A P2P cache refers to a network entity that caches P2P
traffic in the network, and either transparently or explicitly as a
peer distributes content to other peers.
Peer: A peer refers to a participant in a P2P streaming system that
not only receives streaming content, but also stores and uploads
streaming content to other participants.
PPSP: The abbreviation of P2P streaming protocols. PPSP refer to the
key signaling protocols among various P2P streaming system components,
including the tracker and the peer.
Swarm: A swarm refers to a group of clients (i.e., peers) who exchang
data to distribute the same content (e.g. video/audio program,
digital file, etc) at a given time.
Tracker: A tracker refers to a directory server which maintains a
list of peers storing chunks for a specific channel or streaming file,
and answers queries from peers for peer lists. The tracker is a logic
component which can be centralized or distributed.
Video-on-demand (VoD): It refers to a scenario where different
clients may watch different parts of the same recorded media content
during a past event.
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3. Problem statement
Let's take a look what problems proprietary signaling brings out for
P2P streaming applications.
3.1. ISP's difficulties in deploying P2P caches
Facing with many P2P streaming applications, ISPs are witnessing a
big traffic tension on their backbone and inter-networking ports.P2P
cache is used to reduce the traffic by dynamically storing the
frequently accessed streaming content (maybe in chunk or in file
granularity). It's often deployed in the edge or inter-networking
places of the network. However, the cache nodes need to execute DPI
(deep packet inspection) for identifying different P2P streaming
systems. Multiple ever changing proprietary P2P streaming protocols
require the P2P cache updating its matching library constantly which
increases the operator's cost dramatically.
With PPSP, P2P cache can detect P2P streaming applications much
easier. This reduces the ISP workload to a large extent.
3.2. Difficulties in building open streaming delivery infrastructure
The future Internet is content-centric [CCN][DONA]because the vast
majority of current Internet usage (a "high 90% level of
traffic"[Van]) consists of data. Most of the content-centric works
are seeking for building an open global content delivery
infrastructure. In content-centric networks there is no doubt that
P2P streaming data is going to account for a large portion. If
current multiple proprietary protocols continue to work, there will
exist lots of specific and independent systems to deliver vast
streaming content. This brings more burdens for identifying and
sharing the same contents, increases the storage, forwarding and
maintenance cost in the intermediate nodes for repeated content and
for the same content, many on-the-way data spread over the Internet.
All these will definitely increase the cost of streaming distribution
and causes possible congestion in the network.
In an open P2P streaming delivery infrastructure, It must involve
different participants in the distribution chain ranging from the
source (or current P2P streaming vendors) , infrastructure (e.g, CDN)
and delivery side(caches or terminal peers).
Let's first consider a simplest open P2P streaming environment where
different source vendors spread in different regions cooperatively
deliver a broadcasting event. Suppose PPLive cooperatively broadcasts
live Chinese spring festival gala for American Chinese with Pando
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networks. At a first sight, this is reasonable because PPlive has
relatively few users in USA and Utilizing Pando's peer resources can
help to realize efficient streaming delivery. However, different
messages and interaction semantics between the two systems is a big
challenge for the cooperative delivery.
Consider a more complex case where source vendors cooperate with CDN
providers. Such integration is already practiced by UUSee[UUSee],
RayV[RayV] and Forcetech[Forcetech]. The effect of the involvement of
infrastructure devices such as CDN nodes has been verified to improve
the total performance of P2P streaming (e.g., with lower latency) by
providing more stable "super peers" and reduce traffic in ISP network
[CDN+P2P] [RFC 5693].However, there are substantial obstacles in
deploying infrastructure nodes supporting proprietary P2P streaming
protocols [HTPT]. Unlike the Web who has already equipped with the
standard HTTP protocol and all kinds of the infrastructure devices
support the protocol, in order to support P2P streaming, the
infrastructure devices need to understand and keep updated with
various protocols to take part in the delivery. Similar to the cache
case, this introduces complexity and deployment cost for the
infrastructure devices.
With PPSP, CDN nodes can be designed to inter-operate with only the
standard protocols, reducing the case by case negotiation between the
source providers and multiple CDN providers.
3.3. Difficulties in mobile and wireless environment
Mobility and wireless are becoming increasingly important features to
support in future Internet deployments [GENI], [FIND]. Currently
there are more and more mobile and wireless Internet users. It is
predicted that by the end of 2012, the number of mobile Internet
users will surpass that of fixed Internet users in China [Statistics].
Along with this trend, mobile streaming is becoming a key offering
[MobileTV]. In Korea the number of mobile TV subscriber has reached
seventeen millions, accounting for one third of the mobile
subscribers. During the 2008 Beijing Olympic Games, more than one
million users enjoyed mobile TV service.
Although mobile handsets are more eligible to be peers with much
better bandwidth and higher CPU frequency, larger storage and memory
than several years ago, mobile and wireless peers may face bigger
challenges for supporting P2P streaming with unsteady and relatively
lower (esp. in uplink) network connections, less steady power than
PCs. And what are worse, current protocols are designed mainly for
the fixed Internet and do not address these challenges.
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In what kinds of network dynamical conditions, what kinds of mobile
nodes can act as peers should be investigated to reflect in the
standard protocols from the beginning of the design.
3.4. Terminal physical resource starvation
Private protocols may require a terminal to install multiple
different software at the same time. For example a user installs CBox
for CCTV programs [CNTV], and PPLive for Japanese and Korean movies
[PPLive]. However it may be difficult to install multiple clients in
one resource constraint peer like mobile handsets or Pads. The
limited CPU, storage and memory often limit the total number of
installed applications as well as concurrent threads and processes.
Note that for many client software, even it's not used by the users
right now, the daemon may be invoked to facilitate other peers for
free data delivery assistance.
Standard protocols reduce the complexity of coexisting multiple
closed systems and create possibilities to use one client software
accommodating different systems.
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4. PPSP:Standard peer to peer streaming protocols
The objective of this working group is to design PPSP, unified peer
to peer streaming protocols to address the problems discussed in the
preceding section.
There are basically two kinds of P2P streaming systems, pull-based
and push-based. In pull-based P2P streaming systems, a centralized
tracker or distributed trackers maintains information about which
peers are in which swarms and answers the peers' query on such
information with a peer-list. After receiving the message, the peer
can connect with the candidates in a swarm, exchange its content
availability in its memory or storage (depending on it's real-time or
VoD streaming) with other peers and then retrieve for wanted
streaming data. The swarm is a mesh topology. Most of the current
practices are belonging to this genre. The advantages of pull-based
mode are its robustness to the peer churn and acceptable latency for
a smooth play. On the other hand, in push-based P2P streaming
systems, there is a head node maintaining the topology e.g., a tree.
The peers in this topology share the same interest on content. The
signaling and data distribution are both based on this topology. For
one program or video file, the peer queries the head node for its
location to join and the head node replies with a peer-list(maybe
with recommended connection order). After receiving this peer-list,
the peer can connect with the candidates for being a node in certain
place of the topology and receive the data along this topology
without the need of exchanging content availability with its siblings.
In this sense the head node is acting as the tracker. The push mode
has the advantages of lower latency but the topology is fragile to
the peer churn. Few practical systems use this mode. A more practical
mode is a hybrid pull-push mode where the peers exchange content
availability with its siblings for retrieving unfounded data.
As discussed above, in essence, there are two important entities in
P2P streaming, i.e., trackers and peers. PPSP is targeted to
standardize the signaling protocols in tracker-based architectures
for supporting both live and VoD streaming.
In live streaming, all peers are interested in the media coming from
an ongoing event, which means that all peers share nearly the same
streaming content at a given point of time. In live streaming, some
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peers may store the live media for further distribution, which is
known as TSTV (time-shift TV), where the stored media are separated
into chunks and distributed in a VoD-like manner.
In VoD, different peers watch different parts of the recorded media
content during a past event. In this case, each peer keeps asking
other peers which media chunks are stored in which peers, and then
gets the required media from certain/selected peers.
In detail, PPSP designs a protocol for signaling between trackers and
peers (the PPSP "tracker protocol") and a signaling protocol for
communication among the peers (the PPSP "peer protocol") as shown in
Figure 1. The two protocols enable peers to receive streaming data
within the time constraints required by specific content items. The
tracker protocol handles the initial and periodic exchange of meta
information between trackers and peers, such as peer-list and content
information. The peer protocol controls the advertising and exchange
of media data between the peers.
Note that in the pull mode and hybrid pull-push mode, both tracker
protocol and peer protocol can be used; while in the push mode, only
tracker protocol is used.
What's more, existing protocols should be investigated and evaluated
for being reused or extended as the protocols among peers, e.g., HTTP.
Considering that there can be a large number of peers, the protocol
should consider some lightweight (possibly binary) encoding.
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+------------------------------------------------+
| |
| +--------------------------------+ |
| | Tracker(Head Node) | |
| +--------------------------------+ |
| | ^ ^ |
|Tracker | | Tracker |Tracker |
|Protocol| | Procotol |Protocol |
| | | | |
| V | | |
| +---------+ Peer +---------+ |
| | Peer |<----------->| Peer | |
| +---------+ Protocol +---------+ |
| | ^ |
| | |Peer |
| | |Protocol |
| V | |
| +---------------+ |
| | Peer | |
| +---------------+ |
| |
| |
+------------------------------------------------+
Figure 1 PPSP System Architecture
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5. Use cases of PPSP
5.1. Worldwide provision of open P2P live streaming services
The cooperative vendors can easily expand the broadcasting scale with
PPSP. In figure 2 the interactions between vendor A's tracker and vendor
B and vendor C's super-nodes (SN in short) can be normalized using
tracker protocol; and peer protocol can be used among SNs/peers spread
in different vendors.
+-------------------------------------------------------------------+
| |
| +------------------+ |
| +------------>| A's Tracker |<----------+ |
| | +------------------+ | |
| Tracker| ^ ^ | |
| Protocol| Tracker| |Tracker |Tracker |
| | Protocol| |Protocol |Protocol |
| | | | | |
| | | | | |
| v v v v |
| +------+ Peer +------+ +------+ +------+ |
| | B's |<------->| B's | | C's | | C's | |
| | SN1 |Protocol | SN2 | | SN1 | | SN2 | |
| +------+ +------+ +------+ +------+ |
| ^ ^ ^ ^ |
| | | | | |
| | | Peer Protocol Peer Protocol| | |
| Peer | +-------------+ +--------------+ |Peer |
| Procotol| | | |protocol|
| | | | | |
| | | | | |
| | | | | |
| v v v v |
| +------+ Peer +------+ +---------+ Peer +---------+ |
| | A's |<------> | B's | |A's |<------> |C's | |
| | User1|Protocol | User2| | User1 |Protocol | User2 | |
| +------+ +------+ +---------+ +---------+ |
| |
+-------------------------------------------------------------------+
Figure 2 Cooperative Vendors Interactions
5.2. CDN supporting P2P streaming
This scenario is similar to use case 1 but it involves more kinds of
participants: besides P2P streaming vendors, non-P2P streaming
vendors and CDN providers are also involved in the delivery.
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The CDN surrogates can act as the super-nodes of different P2P
streaming vendors with PPSP. The interactions among these network
entities are the same as shown in Figure 2. The P2P streaming vendors
can rent CDN resources to provide better QoS assured services for VIP
users than services provides by only ordinary peers.
Another case is that the CDN providers offer P2P streaming
distribution with PPSP. This often occurs for an operator or CDN
vendor to build a new CDN system supporting streaming applications
with low cost. This service is very useful for a small streaming
provider who has no much money to distribute its stream worldwide. It
can also be used by an operator to launch self-operating streaming
service from the beginning.
5.3. PPSP supporting cross-screen streaming in heterogeneous environment
In this scenario PC, Setbox/TV and mobile terminals from both fixed
network and mobile network work together sharing the content they
store/cache and finishing the streaming delivery.
Using PPSP, peers can identify the types of networks, peer abilities
and and get to know what content other peers have (maybe with the
conversion of the content availability expression in different
networks) even in different network conditions as shown in Figure 3.
This will play an important role on the sharing among heterogeneous
peers.
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+-------------------------------------------------------------------+
| |
| Tracker Protocol +---------+ Tracker Protocol |
| +-------------> | Tracker |<------------------+ |
| | +---------+ | |
| | ^ | |
| | | | |
| | | | |
| V | V |
| +------+ | +------------+ |
| | STB | Tracker Protocol |Mobile Phone| |
| +------+ | +------------+ |
| ^ | ^ |
| | | | |
| | | | |
| | V | |
| |Peer Protocol +---------+ Peer Protocol | |
| +-------------> | PC |<------------------+ |
| +---------+ |
| |
+-------------------------------------------------------------------+
Figure 3 Heterogeneous P2P Streaming Interactions with PPSP
5.4. Supporting P2P streaming in cellular mobile network
In a cellular mobile environment like 3G or 4G, with the increase in
bandwidth and smart mobile terminal capabilities, P2P streaming is
easier to be realized than before. Note that the mobile terminals are
not compulsorily to be peers. Network peers who are deployed by the
ISPs or operators and mobile peers with WiFi connections are more
often selected. For example, in 3GPP, there is a P2P CDS work item on
the requirement of mobile operators to prefer use deployed network-
side equipments (e.g., serving gateways or GGSNs, one access point
from cellular mobile network to the Internet) to act as super-peers
when there are no enough eligible peers to realize P2P streaming[P2P
CDS]. Because they are deployed by the operators, the stability and
storage size are better guaranteed than ordinary peers.
In this case the PPSP tracker protocol will identify the network
types and dynamics as well as the terminal types and return super-
peers in the peer-list to these super-peers and normal mobile peers.
If mobile terminals are not eligible to be peers, they can simply
receive data from these super-peers without contributing any data to
others.
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5.5. Cache service supporting P2P streaming
As discussed in the section3, deploying cache nodes in the network
edges can greatly decrease the inter-network traffic and increase
user experience in streaming service.
With PPSP, the cache nodes can identify the P2P streaming genre even
it may include different applications, cache the frequent visited
content (or part of) and report what they cache to the provider's
tracker like a normal peer and serve other requesting peers in
sharing data as shown in Figure 4. The cache nodes needn't update
their library when new applications are introduced, which enable the
cache nods spend less cost to support more applications.
+-------------------------------------------------------------------+
| |
| Tracker Protocol +---------+ Tracker Protocol |
| +-------------> | Tracker |<--------------------+ |
| | +---------+ | |
| | | |
| | | |
| | | |
| V V |
| +-----------+ Peer Protocol +------------+ |
| | Cache |<------------------------------->| Peer | |
| +-----------+ +------------+ |
+-------------------------------------------------------------------+
Figure 4 Cache Service Supporting Streaming with PPSP
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6. Security Considerations
PPSP will not attempt to provide a solution on security and copyright
issues like malicious content distribution, content pollution and DRM
for a general P2P streaming system. Instead PPSP security
considerations involve the security problems related to PPSP
protocols.
6.1. Tracker Protocol
Malicious peers may issue denial of service attack to the trackers by
sending large amount of requests with tracker protocol. Distributed
trackers deployment may alleviate the problem.
On the other hand, malicious peers may report fake information (e.g.,
cheating trackers and other peers by claiming itself owning some
unexisting data).
So it may be optional in some cases to realize authentication to the
peers before accepting the request for the tracker. But this may add
up the tracker's workload on authentication.
In the above discussion tracker is trustful. Things are worse when
the malicious peer acts as part of the distributed trackers, who is
untrustful with much possibility. The malicious acting tracker may
reply the peers with fake peer-list. Peers may find they cannot find
desired data with the fake peer-list.
6.2. Peer Protocol
Similar to the behavior in the tracker-peer interaction, malicious
peers may also create fake information on chunk availability and
exchange it with other peers. Some techniques to check the data
integrity (e.g., using checksum) may be useful for detecting the data.
But this part is out of scope of PPSP.
The protocol documents will contain a complete description on the
security/privacy issues relevant to any usage of PPSP.
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7. Acknowledgements
We would like to acknowledge the following people who provided review,
feedback and suggestions to this document: M. Stiemerling;D. Bryan E.
Marocco; V. Gurbani; R. Even; H. Zhang; C. Schmidt;L. Xiao; C.
Williams; V. Pasual; D. Zhang; J. Lei.
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8. References
[Cisco] Cisco Visual Networking Index: Forecast and Methodology,
2009-2014,
http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns7
05/ns827/white_paper_c11-
481360_ns827_Networking_Solutions_White_Paper.html
[PPLive] www.pplive.com
[VoD]Challenges, Design and Analysis of a Large-scale P2P-VoD
System,Yan Huang et al, Sigcomm08.
[CNN] www.cnn.com
[PPStream] www.ppstream.com
[UUSee] www.uusee.com
[CNTV] www.cntv.com
[Cirrus] labs.adobe.com/technologies/cirrus/
[Akamai] Understanding hybrid CDN-P2P: why limelight needs its own
Red Swoosh, C. Huang et al,Proceedings of the 18th International
Workshop on Network and Operating Systems Support for Digital Audio
and Video, May 28-30, 2008, Braunschweig, Germany.
[ChinaCache]www.chinacache.com
[ComCast]http://www.afterdawn.com/news/article.cfm/2008/05/20/comcast
_invests_in_p2p_streaming_startup
[CCN] Content-Centric Networking,V. Jacobson,
https://wiki.tools.isoc.org/@api/deki/files/2634//=1.vj.isoc.mar10.pd
f
[DONA]A Data-Oriented (and Beyond) Network Architecture, T. Koponen
et al, Sigcomm 2007.
[Van] A New Way to Look at Networking,Van Jacobson,
http://video.google.com/videoplay?docid=-6972678839686672840
[RayV]www.rayv.com
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[Forcetech]http://www.forcetech.net/english/solutions
[CDN+P2P]Efficient Large-scale Content Distribution with Combination
of CDN and P2P Networks, H. Jiang et al, International Journal of
Hybrid Information Technology, Vol.2, No.2, April, 2009.
[RFC 5693], Application-Layer Traffic Optimization (ALTO) Problem
Statement, E. Marocco et al, http://datatracker.ietf.org/doc/rfc5693/
[HPTP] HPTP: Relieving the Tension between ISPs and P2P, Guobin Shen
et al, IPTPS 2007.
[GENI] www.geni.net
[FIND]www.nets-find.net
[Statistics] http://labs.chinamobile.com/news/48283
[P2P CDS] 3GPP TR 22.906, Study on IMS based peer-to-peer content
distribution services,http://www.3gpp.org/ftp/Specs/html-
info/22906.htm
Author's Addresses
Yunfei Zhang
China Mobile Communication Corporation
zhangyunfei@chinamobile.com
Ning Zong
Huawei Technologies Co., Ltd.
zongning@huawei.com
Gonzalo Camarillo
Ericsson
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Gonzalo.Camarillo@ericsson.com
James Seng
PPLive
james.seng@pplive.com
Richard Yang
Yale University
yry@cs.yale.edu
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