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PPSP                                                               Y. Gu
Internet-Draft                                                   N. Zong
Intended status: Standards Track                                  Huawei
Expires: April 25, 2010                                       Hui. Zhang
                                                       NEC Labs America.
                                                           Yunfei. Zhang
                                                            China Mobile
                                                      Gonzalo. Camarillo
                                                                Ericsson
                                                               Yong. Liu
                                                  Polytechnic University
                                                        October 22, 2009


                  Survey of P2P Streaming Applications
                        draft-gu-ppsp-survey-01

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document presents a survey of popular Peer-to-Peer streaming
   applications on the Internet.  We focus on the Architecture and Peer
   Protocol/Tracker Signaling Protocol description in the presentation,
   and study a selection of well-known P2P streaming systems, including
   Joost, PPlive, and more.  Through the survey, we summarize a common
   P2P streaming process model and the correspondent signaling process
   for P2P Streaming Protocol standardization.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminologies and concepts . . . . . . . . . . . . . . . . . .  3
   3.  Survey of P2P streaming system . . . . . . . . . . . . . . . .  4
     3.1.  Joost  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Octoshape  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  PeerCast . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Conviva  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  PPLive . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.6.  PPStream . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.7.  SopCast  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.8.  TVants . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  A common P2P Streaming Process Model . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16



















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1.  Introduction

   Toward standardizing the signaling protocols used in today's Peer-to-
   Peer (P2P) streaming applications, we surveyed several popular P2P
   streaming systems on their architectures and signaling protocols
   between peers, as well as, between peers and trackers.  The studied
   P2P streaming systems, running worldwide or domestically, include
   PPLive, Joost, Cybersky-TV, Octoshape, and more.  This document does
   not intend to cover all design options of P2P streaming applications.
   Instead, we choose a representative set of applications and focus on
   the respective signaling characteristics of each kind.  Through the
   survey, we generalize a common streaming process model from those P2P
   streaming systems, and summarize the companion signaling process as
   the base of P2P Streaming Protocol standardization.


2.  Terminologies and concepts

   Chunk: A chunk is a basic unit of partitioned streaming media, which
   is used by a peer for the purpose of storage, advertisement and
   exchange among peers [Sigcomm:P2P streaming].

   Content Distribution Network (CDN) node: A CDN node refers to a
   network entity that usually is deployed at the network edge to store
   content provided by the original servers, and serves content to the
   clients located nearby topologically.

   Live streaming: The scenario where all clients receive streaming
   content for the same ongoing event.  The lags between the play points
   of the clients and that of the streaming source are small..

   P2P cache: A P2P cache refers to a network entity that caches P2P
   traffic in the network, and either transparently or explicitly
   distributes content to other peers.

   P2P streaming protocols: P2P streaming protocols refer to multiple
   protocols such as streaming control, resource discovery, streaming
   data transport, etc. which are needed to build a P2P streaming
   system.

   Peer/PPSP peer: A peer/PPSP peer refers to a participant in a P2P
   streaming system.  The participant not only receives streaming
   content, but also stores and uploads streaming content to other
   participants.

   PPSP protocols: PPSP protocols refer to the key signaling protocols
   among various P2P streaming system components, including the tracker
   and peers.  PPSP protocols are a part of P2P streaming protocols.



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   Swarm: A swarm refers to a group of clients (i.e. peers) sharing the
   same content (e.g. video/audio program, digital file, etc) at a given
   time.

   Tracker/PPSP tracker: A tracker/PPSP tracker refers to a directory
   service which maintains the lists of peers/PPSP peers storing chunks
   for a specific channel or streaming file, and answers queries from
   peers/PPSP peers.

   Video-on-demand (VoD): The scenario where different clients watch
   different parts of the media recorded and stored during past events.


3.  Survey of P2P streaming system

3.1.  Joost

   Joost announced to give up P2P technology last year.  However, as a
   once very popular P2P streaming application, it's worthwhile to
   understand how Joost works.

   The key components of Joost include servers, super nodes and peers.
   There are 5 types of servers: Tracker server, Version server, Backend
   server, Content server and Graphics server.  The architecture of
   Joost system is shown in Figure 1.

   First, we introduce the functionalities of Joost's key components
   through three basic phases.  Then we will discuss the Peer protocol
   and Tracker protocol of Joost.

   Installation: Backend server is involved in the installation phase.
   Backend server provides peer with an initial channel list in a SQLite
   file.  No other parameters, such as local cache, node ID, or
   listening port, are configured in this file.

   Bootstrapping: In case of a newcomer, Tracker server provides several
   super node addresses and possibly some content server addresses.
   Then the peer connects Version server for the latest software
   version.  Finally, the peer starts to connect some super nodes to
   obtain the list of other available peers and begin streaming video
   contents.  Different from Skype, super nodes in Joost only deal with
   control traffic.  They do not relay/forward any media data.

   Channel switching: Super nodes are responsible for redirecting
   clients to content server or peers.

   Peers communicate with servers over HTTP/HTTPs and with super nodes/
   other peers over UDP.



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   Tracker Protocol: Because there are super nodes responsible for
   providing the peerlist/content servers, protocol spoken between
   tracker server and peers is rather simple.  Peers get the addresses
   of super nodes and content servers from Tracker server over HTTP.
   After that, Tracker sever will not appear in any stage, e.g. channel
   switching, VoD interaction.  In fact, the protocol spoken between
   peers and super nodes is more like what we normally called "Tracker
   Protocol".  It enables super nodes to check peer status, maintain
   peer lists for several, if not all, channels.  It provides peer list/
   content servers to peers.  So in the rest of this section, when we
   mention Tracker Protocol, we mean the one spoken between peers and
   super nodes.

   Peers will communicate with super nodes in some scenarios using
   Tracker Protocol.

   1 When a peer starts Joost software, after the installation and
   bootstrapping, the peer will communicate with one or several super
   nodes to get a list of available peers/content servers.

   2 For on-demand video functions, super nodes periodically exchange
   small UDP packets for peer management.

   3 When switching between channels, peers contact super nodes and the
   latter help the peers find available peers to fetch the requested
   media data.

   Peer Protocol: Until now, we have not gotten enough materials to show
   what is negotiation process between peers.  However, we try to
   reverse-engineer what is negotiated based on the data in [Joost-
   experiment].  We omitted the analysis process and directly show our
   conclusion.  Media data in Joost is split into chunks and then
   encrypted.  A chunk is about 5-10 seconds of video data.  After
   receiving peer list from super nodes, a peer negotiates with some or
   all of the peers in the list to find out what chunks they have.  Then
   the peer makes decision about from which peers to get the chunks.  No
   peer capability information is exchanged in the Peer Protocol.














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                   +---------------+       +-------------------+
                   | Version Server|       |   Tracker Server  |
                   +---------------+       +-------------------+
                             \                       |
                              \                      |
                               \                     | +---------------+
                                \                    | |Graphics Server|
                                 \                   | +---------------+
                                  \                  |     |
   +--------------+        +-------------+        +--------------+
   |Content Server|--------|    Peer1    |--------|Backend Server|
   +--------------+        +-------------+        +--------------+
                                     |
                                     |
                                     |
                                     |
                              +------------+       +---------+
                              | Super Node |-------|  Peer2  |
                              +------------+       +---------+

   Figure 1, Architecture of Joost system

3.2.  Octoshape

   CNN has been working with a P2P Plug-in, from a Denmark-based company
   Octoshape, to broadcast its living streaming.  Octoshape helps CNN
   serve a peak of more than a million simultaneous viewers.  Figure 2
   depicts the architecture of the Octoshape system.

   Octoshape maintains a mesh overlay topology.  Its overlay topology
   maintenance scheme is similar to that of P2P file-sharing
   applications, such as BitTorrent.  There is no Tracker server in
   Octoshape, thus no Tracker Protocol.  Peers obtain live streaming
   from content servers and peers over Octoshape Protocol.  Several data
   streams are constructed from live stream.  No data streams are
   identical and any number K of data streams can reconstruct the
   original live stream.  The number K is based on the original media
   playback rate and the playback rate of each data stream.  For
   example, a 400Kbit/s media is split into four 100Kbit/s data streams,
   and then k = 4.  Data streams are constructed in peers, instead of
   Broadcast server, which release server from large burden.  The number
   of data streams constructed in a particular peer equals the number of
   peers downloading data from the particular peer, which is constrained
   by the upload capacity of the particular peer.  To get the best
   performance, the upload capacity of a peer should be larger than the
   playback rate of the live stream.  If not, an artificial peer may be
   added to deliver extra bandwidth.




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   Each single peer has an address book of other peers who is watching
   the channel.  A Standby list is set up based on the address book.
   The peer periodically probes/asks the peers in the standby list to be
   sure that they are ready to take over if one of the current senders
   stops or gets congested.  [Octoshape]

   Peer Protocol: The live stream is firstly sent to a few peers in the
   network and then be spread to the rest.  When a peer joins a channel,
   it notifies all the other peers about its presence over Peer
   Protocol, which will drive the others to add it into their address
   books.  Although [Octoshape] declares that each peer records all the
   peers joining the channel, we suspect that not all the peers are
   recorded, considering the notification traffic will be large and
   peers will be busy with recording when a popular program starts in a
   channel and lots of peers switch to this channel.  Maybe some
   geographic or topological neighbors are notified and the peer gets
   its address book from these neighbors.

   The peer sends requests to some selected peers for the live stream
   and the receivers answers OK or not according to their upload
   capacity.  The peer continues sending requests to peers until it
   finds enough peers to provide the needed data streams to redisplay
   the original live stream.  The details of Octoshape are (not?)
   disclosed yet, we hope someone else can provide much specific
   information.
            +------------+   +--------+
            |   Peer 1   |---| Peer 2 |
            +------------+   +--------+
                 |    \    /      |
                 |     \  /       |
                 |      \         |
                 |     / \        |
                 |    /   \       |
                 |  /      \      |
      +--------------+    +-------------+
      |     Peer 4   |----|    Peer3    |
      +--------------+    +-------------+

      *****************************************
                         |
                         |
                 +---------------+
                 | Content Server|
                 +---------------+

      Figure 2, Architecture of Octoshape system





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3.3.  PeerCast

   PeerCast adopts a Tree structure.  The architecture of PeerCast is
   shown in Figure 3.

   Peers in one channel construct the Broadcast Tree and the Broadcast
   server is the root of the Tree.  A Tracker can be implemented
   independently or merged in the Broadcast server.  Tracker in Tree
   based P2P streaming application selects the parent nodes for those
   new peers who join in the Tree.  A Transfer node in the Tree receives
   and transfers data simultaneously.

   Peer Protocol: The peer joins a channel and gets the broadcast server
   address.  First of all, the peer sends a request to the server, and
   the server answers OK or not according to its idle capability.  If
   the broadcast server has enough idle capability, it will include the
   peer in its child-list.  Otherwise, the broadcast server will choose
   at most eight nodes of its children and answer the peer.  The peer
   records the nodes and contacts one of them, until it finds a node
   that can server it.

   In stead of requesting the channel by the peer, a Transfer node
   pushes live stream to its children, which can be a transfer node or a
   receiver.  A node in the tree will notify its status to its parent
   periodically, and the latter will update its child-list according to
   the received notifications.

























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               ------------------------------
               |            +---------+      |
               |            | Tracker |      |
               |            +---------+      |
               |                  |          |
               |                  |          |
               |   +---------------------+   |
               |   |   Broadcast server  |   |
               |   +---------------------+   |
               |------------------------------
                     /                     \
                    /                       \
                   /                         \
                  /                           \
            +---------+                  +---------+
            |Transfer1|                  |Transfer2|
            +---------+                  +---------+
             /      \                       /      \
            /        \                     /        \
           /          \                   /          \
      +---------+  +---------+     +---------+  +---------+
      |Receiver1|  |Receiver2|     |Receiver3|  |Receiver4|
      +---------+  +---------+     +---------+  +---------+

      Figure 3, Architecture of PeerCast system

3.4.  Conviva

   Conviva[TM][conviva] is a real-time media control platform for
   Internet multimedia broadcasting.  For its early prototype, End
   System Multicast (ESM) [ESM04] is the underlying networking
   technology on organizing and maintaining an overlay broadcasting
   topology.  Next we present the overview of ESM.  ESM adopts a Tree
   structure.  The architecture of ESM is shown in Figure 4.

   ESM has two versions of protocols: one for smaller scale conferencing
   apps with multiple sources, and the other for larger scale
   broadcasting apps with Single source.  We focus on the latter version
   in this survey.

   ESM maintains a single tree for its overlay topology.  Its basic
   functional components include two parts: a bootstrap protocol, a
   parent selection algorithm, and a light-weight probing protocol for
   tree topology construction and maintenance; a separate control
   structure decoupled from tree, where a gossip-like algorithm is used
   for each member to know a small random subset of group members;
   members also maintain pathes from source.




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   Upon joining, a node gets a subset of group membership from the
   source (the root node); it then finds parent using a parent selection
   algorithm.  The node uses light-weight probing heuristics to a subset
   of members it knows, and evaluates remote nodes and chooses a
   candidate parent.  It also uses the parent selection algorithm to
   deal with performance degradation due to node and network churns.

   ESM Supports for NATs.  It allows NATs to be parents of public hosts,
   and public hosts can be parents of all hosts including NATs as
   children.
               ------------------------------
               |            +---------+      |
               |            | Tracker |      |
               |            +---------+      |
               |                  |          |
               |                  |          |
               |   +---------------------+   |
               |   |    Broadcast server |   |
               |   +---------------------+   |
               |------------------------------
                     /                     \
                    /                       \
                   /                         \
                  /                           \
            +---------+                   +---------+
            |  Peer1   |                  |  Peer2  |
            +---------+                   +---------+
             /      \                       /      \
            /        \                     /        \
           /          \                   /          \
      +---------+  +---------+     +---------+  +---------+
      |  Peer3  |  |  Peer4  |     |  Peer5  |  |  Peer6  |
      +---------+  +---------+     +---------+  +---------+

      Figure 4, Architecture of ESM system

3.5.  PPLive

   PPLive is one of the most popular P2P streaming software in China.
   It has two major communication protocols.  One is Registration and
   peer discovery protocol, i.e.  Tracker Protocol, and the other is P2P
   chunk distribution protocol, i.e.  Peer Protocol.  Figure 5 shows the
   architecture of PPLive.

   Tracker Protocol: First, a peer gets the channel list from the
   Channel server, in a way similar to that of Joost.  Then the peer
   chooses a channel and asks the Tracker server for the peerlist of
   this channel.



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   Peer Protocol: The peer contacts the peers in its peerlist to get
   additional peerlists, which are aggregated with its existing list.

   For the video-on-demand (VoD) operation, because different peers
   watch different parts of the channel, a peer buffers up to a few
   minutes worth of chunks within a sliding window to share with each
   others.  Some of these chunks may be chunks that have been recently
   played; the remaining chunks are chunks scheduled to be played in the
   next few minutes.  Peers upload chunks to each other.  To this end,
   peers send to each other "buffer-map" messages; a buffer-map message
   indicates which chunks a peer currently has buffered and can share.
   The buffer-map message includes the offset (the ID of the first
   chunk), the length of the buffer map, and a string of zeroes and ones
   indicating which chunks are available (starting with the chunk
   designated by the offset).  PPlive transfer Data over UDP.

   Video Download Policy of PPLive

      1 Top ten peers contribute to a major part of the download
      traffic.  Meanwhile, the top peer session is quite short compared
      with the video session duration.  This would suggest that PPLive
      gets video from only a few peers at any given time, and switches
      periodically from one peer to another;

      2 PPLive can send multiple chunk requests for different chunks to
      one peer at one time;

   PPLive maintains a constant peer list with relatively small number of
   peers.  [P2PIPTV-measuring]
            +------------+    +--------+
            |   Peer 2   |----| Peer 3 |
            +------------+    +--------+
                     |          |
                     |          |
                    +--------------+
                    |    Peer 1    |
                    +--------------+
                            |
                            |
                            |
                    +---------------+
                    | Tracker Server|
                    +---------------+

      Figure 5, Architecture of PPlive system






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3.6.  PPStream

   The system architecture and working flows of PPStream is similar to
   PPLive.  PPStream transfers data using mostly TCP, only occasionally
   UDP.

   Video Download Policy of PPStream

      1 Top ten peers do not contribute to a large part of the download
      traffic.  This would suggest that PPStream gets the video from
      many peers simultaneously, and its peers have long session
      duration;

      2 PPStream does not send multiple chunk requests for different
      chunks to one peer at one time;

   PPStream maintains a constant peer list with relatively large number
   of peers.  [P2PIPTV-measuring]

3.7.  SopCast

   The system architecture and working flows of SopCast is similar to
   PPLive.  SOPCast transfer data mainly using UDP, occasionally TCP;

   Top ten peers contribute to about half of the total download traffic.
   SOPCast's download policy is similar to PPLive's policy in that it
   switches periodically between provider peers.  However, SOPCast seems
   to always need more than one peer to get the video, while in PPLive a
   single peer could be the only video provider;

   SOPCast's peer list can be as large as PPStream's peer list.  But
   SOPCast's peer list varies over time.  [P2PIPTV-measuring]

3.8.  TVants

   The system architecture and working flows of TVants is similar to
   PPLive.  TVAnts is more balanced between TCP and UDP in data
   transmission;

   The system architecture and working flows of TVants is similar to
   PPLive.  TVAnts is more balanced between TCP and UDP in data
   transmission;

   TVAnts' peer list is also large and varies over time.  [P2PIPTV-
   measuring]

   We extract the common Main components and steps of PPLive, PPStream,
   SopCast and TVants, which is shown in Figure 6.



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                        +------------+
                        |   Tracker  |
                       /+------------+
                      /
                     /    +------+
                1,2/     /|Peer 1|
                  /     / +------+
                 /     /3,4,6
           +---------+/              +------+
           |New Peer |---------------|Peer 2|
           +---------+\     4,6      +------+
           |5  |       \
           |---|        \ +------+
                   3,4,6 \|Peer 3|
                          +------+

   Figure 6, Main components and steps of PPLive, PPStream, SopCast and Tvants

   The main steps are:

      (1) A new peer registers with tracker / distributed hash table
      (DHT) to join the peer group which shares a same channel / media
      content;

      (2) Tracker / DHT returns an initial peer list to the new peer;

      (3) The new peer harvests peer lists by gossiping (i.e. exchange
      peer list) with the peers in the initial peer list to aggregate
      more peers sharing the channel / media content;

      (4) The new peer randomly (or with some guide) selects some peers
      from its peer list to connect and exchange peer information (e.g.
      buffer map, peer status, etc) with connected peers to know where
      to get what data;

      (5) The new peer decides what data should be requested in which
      order / priority using some scheduling algorithm and the peer
      information obtained in Step (4);

      (6) The new peer requests the data from some connected peers.


4.  A common P2P Streaming Process Model

   As shown in Figure 7, a common P2P streaming process can be
   summarized from Section 3:





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      1) When a peer wants to receive streaming content:

         1.1) Peer acquires a list of peers/parent nodes from the
         tracker.

         1.2) Peer exchanges its content availability with the peers on
         the obtained peer list, or requests to be adopted by the parent
         nodes.

         1.3) Peer identifies the peers with desired content, or the
         available parent node.

         1.4) Peer requests for the content from the identified peers,
         or receives the content from its parent node.

      2) When a peer wants to share streaming content with others:

         2.1) Peer sends information to the tracker about the swarms it
         belongs to, plus streaming status and/or content availability.

                  +---------------------------------------------------------+
                  |   +--------------------------------+                    |
                  |   |              Tracker           |                    |
                  |   +--------------------------------+                    |
                  |        ^  |                    ^                        |
                  |        |  |                    |                        |
                  |  query |  | peer list/         |streaming Status/       |
                  |        |  | Parent nodes       |Content availability/   |
                  |        |  |                    |node capability         |
                  |        |  |                    |                        |
                  |        |  V                    |                        |
                  |   +-------------+         +------------+                |
                  |   |    Peer1    |<------->|  Peer 2    |                |
                  |   +-------------+ content/+------------+                |
                  |                   join requests                         |
                  +---------------------------------------------------------+
   Figure 7, A common P2P streaming process model

   The functionality of Tracker and data transfer in Mesh-based
   application and Tree-based is a little different.  In the Mesh-based
   applications, such as Joost and PPLive, Tracker maintains the lists
   of peers storing chunks for a specific channel or streaming file.  It
   provides peer list for peers to download from, as well as upload to,
   each other.  In the Tree-based applications, such as PeerCast and
   Canviva, Tracker directs new peers to find parent nodes and the data
   flows from parent to child only.





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5.  Security Considerations

   This document does not consider security issues.  It follows the
   security consideration in [draft-zhang-ppsp-problem-statement].


6.  Acknowledgments

   We would like to acknowledge Jiang xingfeng for providing good ideas
   for this document.


7.  Informative References

   [PPLive]   "www.pplive.com".

   [PPStream]
              "www.ppstream.com".

   [CNN]      "www.cnn.com".

   [OctoshapeWeb]
              "www.octoshape.com".

   [Joost-Experiment]
              Lei, Jun, et al., "An Experimental Analysis of Joost Peer-
              to-Peer VoD Service".

   [Sigcomm_P2P_Streaming]
              Huang, Yan, et al., "Challenges, Design and Analysis of a
              Large-scale P2P-VoD System", 2008.

   [Octoshape]
              Alstrup, Stephen, et al., "Introducing Octoshape-a new
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   [Conviva]  "http://www.rinera.com/".

   [ESM04]    Zhang, Hui., "End System Multicast,
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   [draft-zhang-alto-traceroute-00]
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   [P2PStreamingSurvey]
              Zong, Ning, et al., "Survey of P2P Streaming", Nov. 2008.

   [P2PIPTV_measuring]
              Silverston, Thomas, et al., "Measuring P2P IPTV Systems".

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              Li, Bo, et al., "Peer-to-Peer Live Video Streaming on the
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Authors' Addresses

   Gu Yingjie
   Huawei
   Baixia Road No. 91
   Nanjing, Jiangsu Province  210001
   P.R.China

   Phone: +86-25-84565868
   Fax:   +86-25-84565888
   Email: guyingjie@huawei.com


   Zong Ning
   Huawei
   Baixia Road No. 91
   Nanjing, Jiangsu Province  210001
   P.R.China

   Phone: +86-25-84565866
   Fax:   +86-25-84565888
   Email: zongning@huawei.com


   Hui Zhang
   NEC Labs America.

   Email: huizhang@nec-labs.com


   Zhang Yunfei
   China Mobile

   Email: zhangyunfei@chinamobile.com





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   Gonzalo Camarillo
   Ericsson

   Email: Gonzalo.Camarillo@ericsson.com


   Liu Yong
   Polytechnic University

   Email: yongliu@poly.edu









































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