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PPSP                                                               Y. Gu
Internet-Draft                                              N. Zong, Ed.
Intended status: Standards Track                                  Huawei
Expires: August 29, 2013                                        Y. Zhang
                                                            China Mobile
                                                              F. Piccolo
                                                                   Cisco
                                                                 S. Duan
                                                                    CATR
                                                       February 25, 2013


                  Survey of P2P Streaming Applications
                       draft-ietf-ppsp-survey-04

Abstract

   This document presents a survey of some of the most popular Peer-to-
   Peer (P2P) streaming applications on the Internet.  Main selection
   criteria were popularity and availability of information on operation
   details at writing time.  In doing this, selected applications will
   not be reviewed as a whole, but we will focus exclusively on the
   signaling and control protocol used to establish and maintain overlay
   connections among peers and to advertise and download streaming
   content.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminologies and concepts . . . . . . . . . . . . . . . . . .  4
   3.  Classification of P2P Streaming Applications Based on
       Overlay Topology . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Mesh-based P2P Streaming Applications  . . . . . . . . . .  5
       3.1.1.  Octoshape  . . . . . . . . . . . . . . . . . . . . . .  6
       3.1.2.  PPLive . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.1.3.  Zattoo . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.4.  PPStream . . . . . . . . . . . . . . . . . . . . . . . 11
       3.1.5.  SopCast  . . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.6.  Tribler  . . . . . . . . . . . . . . . . . . . . . . . 13
       3.1.7.  QQLive . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.2.  Tree-based P2P streaming applications  . . . . . . . . . . 16
       3.2.1.  End System Multicast (ESM) . . . . . . . . . . . . . . 17
     3.3.  Hybrid P2P streaming applications  . . . . . . . . . . . . 18
       3.3.1.  New Coolstreaming  . . . . . . . . . . . . . . . . . . 19
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   5.  Author List  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

















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1.  Introduction

   An ever increasing number of multimedia streaming systems have been
   adopting Peer-to-Peer (P2P) paradigm to stream multimedia audio and
   video contents from a source to a large number of end users.  This is
   the reference scenario of this document, which presents a survey of
   some of the most popular P2P streaming applications available on the
   nowadays Internet.  The presented survey does not aim at being
   exhaustive.  Reviewed applications have indeed been selected mainly
   based on their popularity and on the information publicly available
   on P2P operation details at writing time.

   In addition, selected applications are not reviewed as a whole, but
   with exclusive focus on signaling and control protocols used to
   construct and maintain the overlay connections among peers and to
   advertise and download multimedia content.  More precisely, we assume
   throughout the document the high level system model reported in
   Figure 1.
                          +--------------------------------+
                          |              Tracker           |
                          |   Information on multimedia    |
                          |   content and peer set         |
                          +--------------------------------+
                             ^  |                    ^  |
                             |  |                    |  |
                      Trcker |  |            Tracker |  |
                    Protocol |  |           Protocol |  |
                             |  |                    |  |
                             |  |                    |  |
                             |  V                    |  V
                       +-------------+          +------------+
                       |    Peer1    |<-------->|  Peer 2    |
                       +-------------+   Peer   +------------+
                                       Protocol

             Figure 1, High level model of P2P streaming systems assumed
                         as reference througout the document
   As Figure 1 shows, it is possible to identify in every P2P streaming
   system two main types of entity: peers and trackers.  Peers represent
   end users, which join dynamically the system to send and receive
   streamed media content, whereas trackers represent well-known nodes,
   which are stably connected to the system and provide peers with
   metadata information about the streamed content and the set of active
   peers.  According to this model, it is possible to distinguish among
   two different control and signaling protocols:

      the protocol that regulates the interaction between trackers and
      peers and will be denoted as "tracker protocol" in the document;



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      the protocol that regulates the interaction between peers and will
      be denoted as "peer protocol" in the document.

   Hence, whenever possible, we will always try to identify tracker and
   peer protocols and we will provide the corresponding details.

   This document is organized as follows.  Section 2 introduces
   terminology and concepts used throughout the current survey.  Since
   overlay topology built on connections among peers impacts some
   aspects of tracker and peer protocols, Section 2 classifies P2P
   streaming application according to the main overlay topologies: mesh-
   based, tree-based and hybrid.  Then, Section 3 presents some of the
   most popular mesh-based P2P streaming applications: Octoshape,
   PPLive, Zattoo, PPStream, SopCast, Tribler, QQLive.  Likewise,
   Section 4 presents End System Multicast as example of tree-based P2P
   streaming applications.  Finally Section 5 presents New Coolstreaming
   as example of hybrid-topology P2P streaming application.


2.  Terminologies and concepts

   Chunk: A chunk is a basic unit of data organized in P2P streaming for
   storage, scheduling, advertisement and exchange among peers.

   Live streaming: It refers to a scenario where all the audiences
   receive streaming content for the same ongoing event.  It is desired
   that the lags between the play points of the audiences and streaming
   source be small.

   Peer: A peer refers to a participant in a P2P streaming system that
   not only receives streaming content, but also caches and streams
   streaming content to other participants.

   Peer protocol: Control and signaling protocol that regulates
   interaction among peers.

   Pull: Transmission of multimedia content only if requested by
   receiving peer.

   Push: Transmission of multimedia content without any request from
   receiving peer.

   Swarm: A swarm refers to a group of peers who exchange data to
   distribute chunks of the same content at a given time.

   Tracker: A tracker refers to a directory service that maintains a
   list of peers participating in a specific audio/video channel or in
   the distribution of a streaming file.



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   Tracker protocol: Control and signaling protocol that regulates
   interaction among peers and trackers.

   Video-on-demand (VoD): It refers to a scenario where different
   audiences may watch different parts of the same recorded streaming
   with downloaded content.


3.  Classification of P2P Streaming Applications Based on Overlay
    Topology

   Depending on the topology that can be associated with overlay
   connections among peers, it is possible to distinguish among the
   following general types of P2P streaming applications:

      - tree-based: peers are organized to form a tree-shape overlay
      network rooted at the streaming source, and multimedia content
      delivery is push-based.  Peers that forward data are called parent
      nodes, and peers that receive it are called children nodes.  Due
      to their structured nature, tree-based P2P streaming applications
      present a very low cost of topology maintenance and are able to
      guarantee good performance in terms of scalability and delay.  On
      the other side, they are not very resilient to peer churn, that
      may be very high in a P2P environment;

      - mesh-based: peers are organized in a randomly connected overlay
      network, and multimedia content delivery is pull-based.  This is
      the reason why these systems are also referred to as "data-
      driven".  Due to their unstructured nature, mesh-based P2P
      streaming application are very resilient with respect to peer
      churn and are able to guarantee network resource utilization
      higher than for tree-based applications.  On the other side, the
      cost to maintain overlay topology may limit performance in terms
      of scalability and delay, and pull-based data delivery calls for
      large size buffer where to store chunks;

      - hybrid: this category includes all the P2P application that
      cannot be classified as simply mesh-based or tree-based and
      present characteristics of both mesh-based and tree-based
      categories.

3.1.  Mesh-based P2P Streaming Applications

   In mesh-based P2P streaming application peers self-organize in a
   randomly connected overlay graph where peers interact with a limited
   subset of peers (neighbors) and explicitly request chunks they need
   (pull-based or data-driven delivery).  This type of content delivery
   may be associated with high overhead, not only because peers



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   formulate requests to in order to download chunks they need, but also
   because in some applications peers exchange information about chunks
   they own (in form of so called buffer-maps, a sort of bit maps with a
   bit "1" in correspondence of chunks stored in the local buffer).  The
   main advantage of this kind of applications lies in that a peer does
   not rely on a single peer for retrieving multimedia content.  Hence,
   these applications are very resilient to peer churn.  On the other
   side, overlay connections are not persistent and highly dynamic
   (being driven by content availability), and this makes content
   distribution efficiency unpredictable.  In fact, different chunks may
   be retrieved via different network paths, and this may turn at end
   users into playback quality degradation ranging from low bit rates,
   to long startup delays, to frequent playback freezes.  Moreover,
   peers have to maintain large buffers to increase the probability of
   satisfying chunk requests received by neighbors.

3.1.1.  Octoshape

   Octoshape [Octoshape] is popular for the realization of the P2P
   plug-in CNN [CNN] that has been using Octoshape to broadcast its
   living streaming.  Octoshape helps CNN serve a peak of more than a
   million simultaneous viewers.  But Octoshape has also provided
   several innovative delivery technologies such as loss resilient
   transport, adaptive bit rate, adaptive path optimization and adaptive
   proximity delivery.

   Figure 2 depicts the architecture of the Octoshape system.
            +------------+   +--------+
            |   Peer 1   |---| Peer 2 |
            +------------+   +--------+
                 |    \    /      |
                 |     \  /       |
                 |      \         |
                 |     / \        |
                 |    /   \       |
                 |  /      \      |
      +--------------+    +-------------+
      |     Peer 4   |----|    Peer3    |
      +--------------+    +-------------+
      *****************************************
                         |
                         |
                 +---------------+
                 | Content Server|
                 +---------------+

      Figure 2, Architecture of Octoshape system




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   As it can be seen from the picture, there are no trackers and
   consequently no tracker protocol is necessary.

   As regards the peer protocol, as soon as a peer joins a channel, it
   notifies all the other peers about its presence, in such a way that
   each peer maintains a sort of address book with the information
   necessary to contact other peers who are watching the same channel.
   Although Octoshape inventors claim in [Octoshape] that each peer
   records all peers joining a channel, we suspect that it is very
   unlikely that all peers are recorded.  In fact, the corresponding
   overhead traffic would be large, especially when a popular program
   starts in a channel and lots of peers switch to this channel.  Maybe
   only some geographic or topological neighbors are notified and the
   joining peer gets the address book from these nearby neighbors.

   Regarding data distribution strategy, in the Octoshape solution the
   original stream is split into a number K of smaller equal-sized data
   streams, but a number N > K of unique data streams are actually
   constructed, in such a way that a peer receiving any K of the N
   available data streams is able to play the original stream.  For
   instance, if the original live stream is a 400 kbit/sec signal, for
   K=4 and N=12, 12 unique data streams are constructed, and a peer that
   downloads any 4 of the 12 data streams is able to play the live
   stream.  In this way, each peer sends requests of data streams to
   some selected peers, and it receives positive/negative answers
   depending on availability of upload capacity at requested peers.  In
   case of negative answers, a continues sending requests it finds K
   peers willing to upload the minimum number if data streams needed to
   redisplay the original live stream.  Since the number of peers served
   by a given peer is limited by its upload capacity, the upload
   capacity at each peer should be larger than the playback rate of the
   live stream.  Otherwise, artificial peers may be added to offer extra
   bandwidth.

   In order to mitigate the impact of peer loss, the address book is
   also used at each peer to derive the so called Standby List, which
   Octoshape peers use to probe other peers and be sure that they are
   ready to take over if one of the current senders leaves or gets
   congested.

   Finally, in order to optimize bandwidth utilization, Octoshape
   leverages peers within a network to minimize external bandwidth usage
   and to select the most reliable and "closest" source to each viewer.
   It also chooses the best matching available codecs and players, and
   it scales bit rate up and down according to available internet
   connection.





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3.1.2.  PPLive

   PPLive [PPLive] is one of the most popular P2P streaming software in
   China.  The PPLive system includes six parts.

   (1) Video streaming server: providing the source of video content and
   coding the content for adapting the network transmission rate and the
   client playing.

   (2) Peer: also called node or client.  The peers compose the self-
   organizing network logically and each peer can join or leave
   whenever.  When the client downloads the content, it also provides
   its own content to the other client at the same time.

   (3) Directory server: server which the PPLive client, when launched
   or shut down by user, automatically registers user information to and
   cancels user information from.

   (4) Tracker server: server that records the information of all users
   watching the same content.  In more detail, when the PPLive client
   requests some content, this server will check if there are other
   peers owning the content and send the information to the client.

   (5) Web server: providing PPLive software updating and downloading.

   (6) Channel list server: server that stores the information of all
   the programs which can be watched by end users, including VoD
   programs and live broadcasting programs.

   PPLive uses two major communication protocols.  The first one is the
   Registration and Peer Discovery protocol, the equivalent of tracker
   protocol, and the second one is the P2P Chunk Distribution protocol,
   the equivalent of peer protocol.  Figure 3 shows the architecture of
   PPLive system.

















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            +------------+    +--------+
            |   Peer 2   |----| Peer 3 |
            +------------+    +--------+
                     |          |
                     |          |
                    +--------------+
                    |    Peer 1    |
                    +--------------+
                            |
                            |
                            |
                    +---------------+
                    | Tracker Server|
                    +---------------+

      Figure 3, Architecture of PPlive system

   As regards the tracker protocol, firstly a peer gets the channel list
   from the Channel list server; secondly it chooses a channel and asks
   the Tracker server for a peer-list associated with the selected
   channel.

   As regards the peer protocol, a peer contacts the peers in its peer-
   list to get additional peer-lists, to be merged with the original one
   received by Tracker server with the goal of constructing and
   maintaining an overlay mesh for peer management and data delivery.
   According to [P2PIPTVMEA], PPLive peers maintain a constant peer-list
   when the number of peers is relatively small.

   For the video-on-demand (VoD) operation, because different peers
   watch different parts of the channel, a peer buffers chunks up to a
   few minutes of content within a sliding window.  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.  In order
   to upload chunks to each other, peers exchange "buffer-map" messages.
   A buffer-map message indicates which chunks a peer currently has
   buffered and can share, and it 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.

   The download policy of PPLive may be summarized with the following
   three points:

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



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      periodically from one peer to another;

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

      PPLive is observed to have the download scheduling policy of
      giving higher priority to rare chunks and to chunks closer to play
      out deadline.

3.1.3.  Zattoo

   Zattoo [Zattoo] is P2P live streaming system which serves over 3
   million registered users over European countries.The system delivers
   live streaming using a receiver-based, peer-division multiplexing
   scheme.  Zattoo reliably streams media among peers using the mesh
   structure.

   Figure 4 depicts a typical procedure of single TV channel carried
   over Zattoo network.  First, Zattoo system broadcasts a live TV
   channel, captured from satellites, onto the Internet.  Each TV
   channel is delivered through a separate P2P network.
      -------------------------------
      |   ------------------        |         --------
      |   |  Broadcast     |        |---------|Peer1 |-----------
      |   |  Servers       |        |         --------          |
      |   Administrative Servers    |                      -------------
      |   ------------------------  |                      | Super Node|
      |   | Authentication Server | |                      -------------
      |   | Rendezvous Server     | |                           |
      |   | Feedback Server       | |         --------          |
      |   | Other Servers         | |---------|Peer2 |----------|
      |   ------------------------| |         --------
      ------------------------------|

      Figure 4, Basic architecture of Zattoo system

   In order to receive a TV channel, users are required to be
   authenticated through Zattoo Authentication Server.  Upon
   authentication, users obtain a ticket identifying the interest TV
   channel with a specific lifetime.  Then, users contact the Rendezvous
   Server, which plays the role of tracker and based on the received
   ticket sends back a list joined of peers carrying the channel.

   As regards the peer protocol, a peer establishes overlay connections
   with other peers randomly selected in the peer-list received by the
   Rendezvous Server.

   For reliable data delivery, each live stream is partitioned into



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   video segments.  Each video segment is coded for forward error
   correction with Reed-Solomon error correcting code into n sub-stream
   packets such that having obtained k correct packets of a segment is
   sufficient to reconstruct the remaining n-k packets of the same video
   segment.  To receive a video segment, each peer then specifies the
   sub-stream(s) of the video segment it would like to receive from the
   neighboring peers.

   Peers decide how to multiplex a stream among its neighboring peers
   based on the availability of upload bandwidth.  With reference to
   such aspect, Zattoo peers rely on Bandwdith Estimation Server to
   initially estimate the amount of available uplink bandwidth at a
   peer.  Once a peer starts to forward substream to other peers, it
   receives QoS feedback from its receivers if the quality of sub-stream
   drops below a threshold.

   Zattoo uses Adaptive Peer-Division Multiplexing (PDM) scheme to
   handle longer term bandwidth fluctuations.  According to this scheme,
   each peer determines how many sub-streams to transmit and when to
   switch partners.  Specifically, each peer continuously estimates the
   amount of available uplink bandwidth based initially on probe packets
   sent to Zattoo Bandwidth Estimation Server and subsequently on peer
   QoS feedbacks, by using different algorithms depending on the
   underlying transport protocol.  A peer increases its estimated
   available uplink bandwidth, if the current estimate is below some
   threshold and if there has been no bad quality feedback from
   neighboring peers for a period of time, according to some algorithm
   similar to how TCP maintains its congestion window size.  Each peer
   then admits neighbors based on the currently estimated available
   uplink bandwidth.  In case a new estimate indicates insufficient
   bandwidth to support the existing number of peer connections, one
   connection at a time, preferably starting with the one requiring the
   least bandwidth, is closed.  On the other hand, if loss rate of
   packets from a peer's neighbor reaches a certain threshold, the peer
   will attempt to shift the degraded neighboring peer load to other
   existing peers, while looking for a replacement peer.  When one is
   found, the load is shifted to it and the degraded neighbor is
   dropped.  As expected if a peer's neighbor is lost due to departure,
   the peer initiates the process to replace the lost peer.  To optimize
   the PDM configuration, a peer may occasionally initiate switching
   existing partnering peers to topologically closer peers.

3.1.4.  PPStream

   The system architecture of PPStream [PPStream] is similar to the one
   of PPLive.

   To ensure data availability, PPStream uses some form of chunk



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   retransmission request mechanism and shares buffer map at high rate.
   Each data chunk, identified by the play time offset encoded by the
   program source, is divided into 128 sub-chunks of 8KB size each.  The
   chunk id is used to ensure sequential ordering of received data
   chunk.  The buffer map consists of one or more 128-bit flags denoting
   the availability of sub-chunks, and it includes information on time
   offset.  Usually, a buffer map contains only one data chunk at a
   time, and it also contains sending peer's playback status, because as
   soon as a data chunk is played back, the chunk is deleted or replaced
   by the next data chunk.

   At the initiating stage a peer can use up to four data chunks,
   whereas on a stabilized stage a peer uses usually one data chunk.
   However, in transient stage, a peer uses variable number of chunks.
   Sub-chunks within each data chunks are fetched nearly in random
   without using rarest or greedy policy.  The same fetching pattern for
   one data chunk seems to repeat itself in the subsequent data chunks.
   Moreover, higher bandwidth PPStream peers tend to receive chunks
   earlier and thus to contribute more than lower bandwidth peers.

   Based on the experimental results reported in [P2PIPTVMEA], download
   policy of PPStream may be summarized with the following two points:

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

      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.

3.1.5.  SopCast

   The system architecture of SopCast [SopCast] is similar to the one of
   PPLive.

   SopCast allows for software updates via HTTP through a centralized
   web server, and it makes list of channels available via HTTP through
   another centralized server.

   SopCast traffic is encoded and SopCast TV content is divided into
   video chunks or blocks with equal sizes of 10KB.  Sixty percent of
   its traffic is signaling packets and 40% is actual video data
   packets.  SopCast produces more signaling traffic compared to PPLive,
   PPStream, with PPLive producing the minimum of signaling traffic.  It
   has been observed in [P2PIPTVMEA] that SopCast traffic has long-range
   dependency, which also means that eventual QoS mitigation mechanisms



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   may be ineffective.  Moreover, according to [P2PIPTVMEA], SopCast
   communication mechanism starts with UDP for the exchange of control
   messages among its peers by using a gossip-like protocol and then
   moves to TCP for the transfer of video segments.  It also seems that
   top-ten peers contribute to about half of the total download traffic.
   Finally, SopCast peer-list can be as large as PPStream peer-list, but
   differently from PPStream SopCast peer-list varies over time.

3.1.6.  Tribler

   Tribler [tribler] is a BitTorrent client that is able to go very much
   beyond BitTorrent model also thanks to the support for video
   streaming.  Initially developed by a team of researchers at Delft
   University of Technology, Tribler was able to attract attention from
   other universities and media companies and to receive European Union
   research funding (P2P-Next and QLectives projects).

   Differently from BitTorrent, where a tracker server centrally
   coordinates uploads/downloads of chunks among peers and peers
   directly interact with each other only when they actually upload/
   download chunks to/from each other, there is no tracker server in
   Tribler and, as a consequence, there is no need of tracker protocol.

   Peer protocol is instead used to organize peers in an overlay mesh.
   In more detail, Tribler bootstrap process consists in preloading well
   known super-peer addresses into peer local cache, in such a way that
   a joining peer randomly selects a super-peer to retrieve a random
   list of already active peers to establish overlay connections with.
   A gossip-like mechanism called BuddyCast allows Tribler peers to
   exchange their preference lists, that is their downloaded file, and
   to build the so called Preference Cache.  This cache is used to
   calculate similarity levels among peers and to identify the so called
   "taste buddies" as the peers with highest similarity.  Thanks to this
   mechanism each peer maintains two lists of peers: i) a list of its
   top-N taste buddies along with their current preference lists, and
   ii) a list of random peers.  So a peer alternatively selects a peer
   from one of the lists and sends it its preference list, taste-buddy
   list and a selection of random peers.  The goal behind the
   propagation of this kind of information is the support for the remote
   search function, a completely decentralized search service that
   consists in querying Preference Cache of taste buddies in order to
   find the torrent file associated with an interest file.  If no
   torrent is found in this way, Tribler users may alternatively resort
   to web-based torrent collector servers available for BitTorrent
   clients.

   As already said, Tribler supports video streaming in two different
   forms: video on demand and live streaming.



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   As regards video on demand, a peer first of all keeps informed its
   neighbors about the chunks it has.  Then, on the one side it applies
   suitable chunk-picking policy in order to establish the order
   according to which to request the chunks he wants to download.  This
   policy aims to assure that chunks come to the media player in order
   and in the same time that overall chunk availability is maximized.
   To this end, the chunk-picking policy differentiates among high, mid
   and low priority chunks depending on their closeness with the
   playback position.  High priority chunks are requested first and in
   strict order.  When there are no more high priority chunks to
   request, mid priority chunks are requested according to a rarest-
   first policy.  Finally, when there are no more mid priority chunks to
   request, low priority chunks are requested according to a rarest-
   first policy as well.  On the other side, Tribler peers follow the
   give-to-get policy in order to establish which peer neighbors are
   allowed to request chunks (according to BitTorrent jargon to be
   unchoked).  In more detail, time is subdivided in periods and after
   each period Tribler peers first sort their neighbors according to the
   decreasing numbers of chunks they have forwarded to other peers,
   counting only the chunks they originally received from them.  In case
   if tie, Tribler sorts their neighbors according to the decreasing
   total number of chunks they have forwarded to other peers.  Since
   children could lie regarding the number of chunks forwarded to
   others, Tribler peers do directly not ask their children, but their
   grandchildren.  In this way, Tribler peer unchokes the three highest-
   ranked neighbours and, in order to saturate upload bandwidth and in
   the same time not decrease the performance of individual connections,
   it further unchokes a limited number of neighbors.  Moreover, in
   order to search for better neighbors, Tribler peers randomly select a
   new peer in the rest of the neighbours and optimistically unchoke it
   every two periods.

   As regards live streaming, differently from video on demand scenario,
   the number of chunks cannot be known in advance.  As a consequence a
   sliding window of fixed width is used to identify chunks of interest:
   every chunk that falls out the sliding window is considered out-
   dated, is locally deleted and is considered as deleted by peer
   neighbors as well.  In this way, when a peer joins the network, it
   learns about chunks its neighbors possess and identifies the most
   recent one.  This is assumed as beginning of the sliding window at
   the joining peer, which starts downloading and uploading chunks
   according to the description provided for video on demand scenario.
   Finally, differently from what happens for video on demand scenario,
   where torrent files includes a hash for each chunk in order to
   prevent malicious attackers from corrupting data, torrent files in
   live streaming scenario include the public key of the stream source.
   Each chunk is then assigned with absolute sequence number and
   timestamp and signed by source public key.  Such a mechanism allows



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   Tribler peers to use the public key included in torrent file and
   verity the integrity of each chunk.

3.1.7.  QQLive

   QQLive [QQLive] is large-scale video broadcast software including
   streaming media encoding, distribution and broadcasting.  Its client
   can apply for web, desktop program or other environments and provides
   abundant interactive function in order to meet the watching
   requirements of different kinds of users.

   Due to the lack of technical details from QQLive vendor, we got some
   knowledge about QQLive from paper [QQLivePaper], whose authors did
   some measurements and based on this identify the main components and
   working flow of QQLive.

   Main components of QQLive include:

      login server, storing user login information and channel
      information;

      authentication server, processing user login authentication;

      channel server, storing all information about channels including
      channel connection nodes watching a channel;

      program server, storing audio and video data information;

      log server, recording the beginning and ending information of
      channels;

      peer node, watching programs and transporting streaming media.

   Main working flow of QQLive includes startup stage and play stage.

   Startup stage includes only interactions between peers and
   centralized QQLive servers, so it may be regarded as associated with
   tracker protocol.  This stage begins when a peer launches QQLive
   client.  Peer provides authentication information in an
   authentication message, which it sends to the authentication server.
   Authentication server verifies QQLive provided credentials and if
   these are valid, QQLive client starts communicating with login server
   through SSL.  QQLive client sends a message including QQLive account
   and nickname, and login serve returns a message including information
   such as membership point, total view time, upgrading time and so on.
   At this point, QQLive client requests channel server for updating
   channel list.  QQLive client firstly loads an old channel list stored
   locally and then it overwrites the old list with the new channel list



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   received from channel server.  The full channel list is not obtained
   via a single request.  QQLive client firstly requests for channel
   classification and then requests the channel list within a specific
   channel category selected by the user.  This approach will give
   higher real-time performance to QQLive.

   Play stage includes interactions between peers and centralized QQLive
   servers and between QQLive peers, so it may be regarded as associated
   to both tracker protocol and peer protocol.  IN more detail, play
   stage is structured in the following phases:

      Open channel.  QQLive client sends a message to dogin server with
      the ID of chosen channel through UDP, whereas login server replies
      with a message including channel ID, channel name and program
      name.  Afterwards, QQLive client communicates with program server
      through SSL to access program information.  Finally QQLive client
      communicates with channel server through UDP to obtain initial
      peer information.

      View channel.  QQLive client establishes connections with peers
      and sends packets with fixed length of 118 bytes, which contains
      channel ID.  QQLive client maintains communication with channel
      server by reporting its own information and obtaining updated
      information.  Peer nodes transport stream packet data through UDP
      with fixed-port between 13000 and14000.

      Stop channel.  QQLive client continuously sends five identical UDP
      packets to channel server with each data packet fixed length of 93
      bytes.

      Close client.  QQLive client sends a UDP message to notify log
      server and an SSL message to login server, then it continuously
      sends five identical UDP packets to channel server with each data
      packet fixed length of 45 bytes.

3.2.  Tree-based P2P streaming applications

   In tree-based P2P streaming applications peers self-organize in a
   tree-shape overlay network, where peers do not ask for a specific
   content chunk, but simply receive it from their so called "parent"
   node.  Such content delivery model is denoted as push-based.
   Receiving peers are denoted as children, whereas sending nodes are
   denoted as parents.  Overhead to maintain overlay topology is usually
   lower for tree-based streaming applications than for mesh-based
   streaming applications, whereas performance in terms of scalability
   and delay are usually higher.  On the other side, the greatest
   drawback of this type of application lies in that each node depends
   on one single node, its father in overlay tree, to receive streamed



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   content.  Thus, tree-based streaming applications suffer from peer
   churn phenomenon more than mesh-based ones.

3.2.1.  End System Multicast (ESM)

   Even though End System Multicast (ESM) project is ended by now and
   ESM infrastructure is not being currently implemented anywhere, we
   decided to include it in this survey for a twofold reason.  First of
   all, it was probably the first and most significant research work
   proposing the possibility of implementing multicast functionality at
   end hosts in a P2P way.  Secondly, ESM research group at Carnegie
   Mellon University developed the world's first P2P live streaming
   system, and some members founded later Conviva [conviva] live
   platform.

   The main property of ESM is that it constructs the multicast tree in
   a two-step process.  The first step aims at the construction of a
   mesh among participating peers, whereas the second step aims at the
   construction of data delivery trees rooted at the stream source.
   Therefore a peer participates in two types of topology management
   structures: a control structure that guarantees peers are always
   connected in a mesh, and a data delivery structure that guarantees
   data gets delivered in an overlay multicast tree.

   There exist two versions of ESM.

   The first version of ESM architecture [ESM1] was conceived for small
   scale multi-source conferencing applications.  Regarding the mesh
   construction phase, when a new member wants to join the group, an
   out-of-bandwidth bootstrap mechanism provides the new member with a
   list of some group member.  The new member randomly selects a few
   group members as peer neighbors.  The number of selected neighbors
   does not exceed a given bound, which reflects the bandwidth of the
   peer's connection to the Internet.  Each peer periodically emits a
   refresh message with monotonically increasing sequence number, which
   is propagated across the mesh in such a way that each peer can
   maintain a list of all the other peers in the system.  When a peer
   leaves, either it notifies its neighbors and the information is
   propagated across the mesh to all participating peers, or peer
   neighbors detect the condition of abrupt departure and propagate it
   through the mesh.  To improve mesh/tree quality, on the one side
   peers constantly and randomly probe each other to add new links; on
   the other side, peers continually monitor existing links to drop the
   ones that are not perceived as good-quality-links.  This is done
   thanks to the evaluation of a utility function and a cost function,
   which are conceived to guarantee that the shortest overlay delay
   between any pair of peers is comparable to the unicast delay among
   them.  Regarding multicast tree construction phase, peers run a



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   distance-vector protocol on top of the tree and use latency as
   routing metric.  In this way, data delivery trees may be constructed
   from the reverse shortest path between source and recipients.

   The second and subsequent version of ESM architecture [ESM2] was
   conceived for an operational large scale single-source Internet
   broadcast system.  As regards the mesh construction phase, a node
   joins the system by contacting the source and retrieving a random
   list of already connected nodes.  Information on active participating
   peers is maintained thanks to a gossip protocol: each peer
   periodically advertises to a randomly selected neighbor a subset of
   nodes he knows and the last timestamps it has heard for each known
   node.

   The main difference with the first version is that the second version
   constructs and maintains the data delivery tree in a completely
   distributed manner according to the following criteria: i) each node
   maintains a degree bound on the maximum number of children it can
   accept depending on its uplink bandwidth, ii) tree is optimized
   mainly for bandwidth and secondarily for delay.  To this end, a
   parent selection algorithm allows identifying among the neighbors the
   one that guarantees the best performance in terms of throughput and
   delay.  The same algorithm is also applied either if a parent leaves
   the system or if a node is experiencing poor performance (in terms of
   both bandwidth and packet loss).  As loop prevention mechanism, each
   node keeps also the information about the hosts in the path between
   the source and its parent node.It then constructs a (reverse)
   shortest path spanning trees of the mesh with the root being the
   source.

   This second ESM prototype is also able to cope with receiver
   heterogeneity and presence of NAT/firewalls.  In more detail, audio
   stream is kept separated from video stream and multiple bit-rate
   video streams are encoded at source and broadcast in parallel though
   the overlay tree.  Audio is always prioritized over video streams,
   and lower quality video is always prioritized over high quality
   videos.  In this way, system can dynamically select the most suitable
   video stream according to receiver bandwidth and network congestion
   level.  Moreover, in order to take presence of hosts behind NAT/
   firewalls, tree is structured in such a way that public host use
   hosts behind NAT/firewalls as parents.

3.3.  Hybrid P2P streaming applications

   This type of applications aims at integrating the main advantages of
   mesh-based and tree-based approaches.  To this end, overlay topology
   is mixed mesh-tree, and content delivery model is push-pull.




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3.3.1.  New Coolstreaming

   Coolstreaming, first released in summer 2004 with a mesh-based
   structure, arguably represented the first successful large-scale P2P
   live streaming.  Nevertheless, it suffers poor delay performance and
   high overhead associated each video block transmission.  In the
   attempt of overcoming such a limitation, New Coolstreaming
   [NEWCOOLStreaming] adopts a hybrid mesh-tree overlay structure and a
   hybrid pull-push content delivery mechanism.

   Figure 5 illustrates New Coolstreaming architecture.
                   ------------------------------
                  |            +---------+      |
                  |            | Tracker |      |
                  |            +---------+      |
                  |                  |          |
                  |                  |          |
                  |   +---------------------+   |
                  |   |    Content server   |   |
                  |   +---------------------+   |
                  |------------------------------
                        /                     \
                       /                       \
                      /                         \
                     /                           \
               +---------+                   +---------+
               |  Peer1  |                   |  Peer2  |
               +---------+                   +---------+
                /      \                       /      \
               /        \                     /        \
              /          \                   /          \
         +---------+  +---------+     +---------+  +---------+
         |  Peer2  |  |  Peer3  |     |  Peer1  |  |  Peer3  |
         +---------+  +---------+     +---------+  +---------+

                Figure 5, New Coolstreaming Architecture

   The video stream is divided into equal-size blocks or chunks, which
   are assigned with a sequence number to implicitly define the playback
   order in the stream.  Video stream is subdivided into multiple sub-
   streams without any coding, so that each node can retrieve any sub-
   stream independently from different parent nodes.  This consequently
   reduces the impact on content delivery due to a parent departure or
   failure.  The details of hybrid push-pull content delivery scheme are
   as follows:

      a node first subscribes to a sub-stream by connecting to one of
      its partners via a single request (pull) in buffer map, the



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      requested partner, i.e., the parent node.  The node can subscribe
      more sub-streams to its partners in this way to obtain higher play
      quality;

      the selected parent node will continue pushing all blocks of the
      sub-stream to the requesting node.

   This not only reduces the overhead associated with each video block
   transfer, but more importantly it significantly reduces the delay in
   retrieving video content.

   Video content is processed for ease of delivery, retrieval, storage
   and play out.  To manage content delivery, a video stream is divided
   into blocks with equal size, each of which is assigned a sequence
   number to represent its playback order in the stream.  Each block is
   further divided into K sub-blocks and the set of i-th sub-blocks of
   all blocks constitutes the i-th sub-stream of the video stream, where
   i is a value bigger than 0 and less than K+1.  To retrieve video
   content, a node receives at most K distinct sub-streams from its
   parent nodes.  To store retrieved sub-streams, a node uses a double
   buffering scheme having a synchronization buffer and a cache buffer.
   The synchronization buffer stores the received sub-blocks of each
   sub-stream according to the associated block sequence number of the
   video stream.  The cache buffer then picks up the sub-blocks
   according to the associated sub-stream index of each ordered block.
   To advertise the availability of the latest block of different sub-
   streams in its buffer, a node uses a Buffer Map which is represented
   by two vectors of K elements each.  Each entry of the first vector
   indicates the block sequence number of the latest received sub-
   stream, and each bit entry of the second vector if set indicates the
   block sequence index of the sub-stream that is being requested.

   For data delivery, a node uses a hybrid push and pull scheme with
   randomly selected partners.  A node having requested one or more
   distinct sub-streams from a partner as indicated in its first Buffer
   Map will continue to receive the sub-streams of all subsequent blocks
   from the same partner until future conditions cause the partner to do
   otherwise.  Moreover, users retrieve video indirectly from the source
   through a number of strategically located servers.

   To keep the parent-children relationship above a certain level of
   quality, each node constantly monitors the status of the on-going
   sub-stream reception and re-selects parents according to sub-stream
   availability patterns.  Specifically, if a node observes that the
   block sequence number of the sub-stream of a parent is much smaller
   than any of its other partners by a predetermined amount, the node
   then concludes that the parent is lagging sufficiently behind and
   needs to be replaced.  Furthermore, a node also evaluates the maximum



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   and minimum of the block sequence numbers in its synchronization
   buffer to determine if any parent is lagging behind the rest of its
   parents and thus needs also to be replaced.


4.  Security Considerations

   This document does not raise security issues.


5.  Author List

   The authors of this document are listed as below.

      Hui Zhang, NEC Labs America.

      Jun Lei, University of Goettingen.

      Gonzalo Camarillo, Ericsson.

      Yong Liu, Polytechnic University.

      Delfin Montuno, Huawei.

      Lei Xie, Huawei.


6.  Acknowledgments

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


7.  Informative References

   [Octoshape] Alstrup, Stephen, et al., "Introducing Octoshape-a new
   technology for large-scale streaming over the Internet".

   [CNN] CNN web site, www.cnn.com

   [PPLive] PPLive web site, www.pplive.com

   [P2PIPTVMEA] Silverston, Thomas, et al., "Measuring P2P IPTV
   Systems", June 2007.

   [Zattoo] Zattoo web site, http: //zattoo.com/

   [PPStream] PPStream web site, www.ppstream.com



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   [SopCast] SopCast web site, http://www.sopcast.com/

   [tribler] Tribler Protocol Specification, January 2009, on line
   available at http://svn.tribler.org/bt2-design/proto-spec-unified/
   trunk/proto-spec-current.pdf

   [QQLive] QQLive web site, http://v.qq.com

   [QQLivePaper] Liju Feng, et al., "Research on active monitoring based
   QQLive real-time information Acquisition System", 2009.

   [conviva] Conviva web site, http://www.conviva.com

   [ESM1] Chu, Yang-hua, et al., "A Case for End System Multicast", June
   2000. (http://esm.cs.cmu.edu/technology/papers/
   Sigmetrics.CaseForESM.2000.pdf)

   [ESM2] Chu, Yang-hua, et al., "Early Experience with an Internet
   Broadcast System Based on Overlay Multicast", June 2004. (http://
   static.usenix.org/events/usenix04/tech/general/full_papers/chu/
   chu.pdf)

   [NEWCOOLStreaming] Li, Bo, et al., "Inside the New Coolstreaming:
   Principles,Measurements and Performance Implications", April 2008.


Authors' Addresses

   Gu Yingjie
   Huawei
   No.101 Software Avenue
   Nanjing  210012
   P.R.China

   Phone: +86-25-56624760
   Fax:   +86-25-56624702
   Email: guyingjie@huawei.com














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   Zong Ning (editor)
   Huawei
   No.101 Software Avenue
   Nanjing  210012
   P.R.China

   Phone: +86-25-56624760
   Fax:   +86-25-56624702
   Email: zongning@huawei.com


   Zhang Yunfei
   China Mobile

   Email: zhangyunfei@chinamobile.com


   Francesca Lo Piccolo
   Cisco

   Email: flopicco@cisco.com


   Duan Shihui
   CATR
   No.52 HuaYuan BeiLu
   Beijing  100191
   P.R.China

   Phone: +86-10-62300068
   Email: duanshihui@catr.cn




















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