PPSP Y. Gu
Internet-Draft N. Zong
Intended status: Standards Track Huawei
Expires: September 12, 2011 Hui. Zhang
NEC Labs America.
Yunfei. Zhang
China Mobile
J. Lei
University of Goettingen
Gonzalo. Camarillo
Ericsson
Yong. Liu
Polytechnic University
Delfin. Montuno
Lei. Xie
Huawei
March 11, 2011
Survey of P2P Streaming Applications
draft-ietf-ppsp-survey-01
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, andother popular existing systems. Through the
survey, we summarize a common P2P streaming process model and the
correspondent signaling process for P2P Streaming Protocol
standardization.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on September 12, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminologies and concepts . . . . . . . . . . . . . . . . . . 4
3. Survey of P2P streaming system . . . . . . . . . . . . . . . . 5
3.1. Mesh-based P2P streaming systems . . . . . . . . . . . . . 5
3.1.1. Joost . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2. Octoshape . . . . . . . . . . . . . . . . . . . . . . 9
3.1.3. PPLive . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.4. Zattoo . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.5. PPStream . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.6. SopCast . . . . . . . . . . . . . . . . . . . . . . . 18
3.1.7. TVants . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2. Tree-based P2P streaming systems . . . . . . . . . . . . . 21
3.2.1. PeerCast . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.2. Conviva . . . . . . . . . . . . . . . . . . . . . . . 23
3.3. Hybrid P2P streaming system . . . . . . . . . . . . . . . 26
3.3.1. New Coolstreaming . . . . . . . . . . . . . . . . . . 26
4. A common P2P Streaming Process Model . . . . . . . . . . . . . 29
5. Security Considerations . . . . . . . . . . . . . . . . . . . 30
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
7. Informative References . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
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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 regarding their architectures and signaling
protocols between peers, as well as, between peers and trackers. The
studied P2P streaming systems, running worldwide or domestically,
include such as PPLive, Joost, Cybersky-TV, and Octoshape. 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 for P2P Streaming Protocol
(PPSP) 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
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and peers.
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): A kind of application that allows users to
select and watch video content on demand
3. Survey of P2P streaming system
In this section, we summarize some existing P2P streaming systems.
The construction techniques used in these systems can be largely
classified into two categories: tree-based and mesh-based structures.
Tree-based structure: Group members self-organize into a tree
structure, based on which group management and data delivery is
performed. Such structure and push-based content delivery have small
maintenance cost and good scalability and low delay in retrieving the
content(associated with startup delay) and can be easily implemented.
However, it may result in low bandwidth usage and less reliability.
Mesh-based structure: In contrast to tree-based structure, a mesh
uses multiple links between any two nodes. Thus, the reliability of
data transmission is relatively high. Besides, multiple links
results in high bandwidth usage. Nevertheless, the cost of
maintaining such mesh is much larger than that of a tree, and pull-
based content delivery lead to high overhead associated each video
block transmission, in particular the delay in retrieving the
content.
Hybrid structure: Combine tree-based and mesh-based structure,
combine pull-based and push-based content delivery to utilize the
advantages of two structures. It has high reliability as much as
mesh-based structure, lower delay than mesh-based structure, lower
overhead associated each video block transmission and high topology
maintenance cost as much as mesh-based structure.
3.1. Mesh-based P2P streaming systems
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3.1.1. Joost
Joost announced to give up P2P technology on its desktop version last
year, though it introduced a flash version for browsers and iPhone
application. The key reason why Joost shut down its desktop version
is probably the legal issues of provided media content. However, as
one of the most popular P2P VoD application in the past years, it's
worthwhile to understand how Joost works. The peer management and
data transmission in Joost mainly relies on mesh-based structure.
The three key components of Joost are servers, super nodes and peers.
There are five 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. Later, the peer starts to connect some super nodes to
obtain the list of other available peers and begins streaming video
contents. Different from Skype [skype], super nodes in Joost only
deal with control and peer management 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.
Tracker Protocol: Because super nodes here are responsible for
providing the peerlist/content servers to peers, protocol used
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
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peer list/content servers to peers. Thus, in the rest of this
section, when we mention Tracker Protocol, we mean the one used
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 purpose.
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: The following investigations are mainly motivated from
[Joost- experiment ], in which a data-driven reverse-engineer
experiments are performed. We omitted the analysis process and
directly show the conclusion. Media data in Joost is split into
chunks and then encrypted. Each chunk is packetized with about 5-10
seconds of video data. After receiving peer list from super nodes, a
peer negotiates with some or, if necessary, 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
The following sections describe Joost QoS related features, extracted
mostly from [Joost- experiment], [JO2-Moreira] and [JO7-Joost Network
Architecture].
For peer selection, Host Cache of a peer, which is refreshed
periodically, stores a list of Joost super nodes IP addresses and
ports. The selection strategy is influenced by the number of peers
accessing the same content. Specifically, the number of candidate
peers made available is proportional to the number of active peers.
If there are a few of them, then Joost content server is made
available to assist in the data delivery. Although there is no
explicit consideration for peer heterogeneities in peer selection,
low capacity peers tend to partner with low capacity peer. Peers
under the same NAT also tend to serve each other preferentially [JO2-
Moreira]. It may consider geographical locality but not have AS-
level awareness or exploit topological locality and thus may have
impact on the efficiency of video distribution.
To maintain the overlay networks, super nodes probe clients, clients
probe clients and super nodes, and super nodes communicate with super
nodes and servers. To make up for inadequate bandwidth and to be
scalable, Joost forms groups of Joost Server Islands, each island
consisting of one streaming control server controlling ten streaming
servers. Moreover, STUN protocol enables a client to discover
whether it is behind a NAT or firewall and the type of the NAT or
firewall.
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For data delivery, audio and video traffic are streamed separately to
allow for multi-lingual programming. Content comes mostly from peers
and occasionally from content server for !olong-tail!+/- content. As
peers are assumed to contribute in a best-effort manner,
infrastructures are needed to make up for insufficient bandwidth,
including in the asymmetric scenario. However, super nodes are not
part of the bandwidth supplying infrastructures as they only relay
control traffic but not data traffic to clients. To support the P2P
media distribution services, Joost uses an agent based peer-to-peer
system called Anthill. Joost also employs Local Video Cache for
later viewing and to avoid reloading but will still require
authorization from Joost server when accessing the video file at a
later time.
Joost provides large buffering and thus causes longer start-up delay
for VoD traffic than for live media streaming traffic. It affords
more FEC for VoD traffic but gives higher priority in delivery to
live media streaming traffic.
For Joost, load-balancing and fault-tolerance is shifted directly
into the client and all is done natively in the p2p code.
To enhance user viewing experience, Joost provides chat capability
between viewers and user program rating mechanisms.
3.1.2. 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. It 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.
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 is required. 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
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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.
Each single peer has an address book of other peers who is watching
the same 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 spread to the rest of the network. When a peer
joins a channel, it notifies all the other peers about its presence
using 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 nearby 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.
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+------------+ +--------+
| Peer 1 |---| Peer 2 |
+------------+ +--------+
| \ / |
| \ / |
| \ |
| / \ |
| / \ |
| / \ |
+--------------+ +-------------+
| Peer 4 |----| Peer3 |
+--------------+ +-------------+
*****************************************
|
|
+---------------+
| Content Server|
+---------------+
Figure 2, Architecture of Octoshape system
The following sections describe Octoshape QoS related features,
extracted mostly from [OctoshapeWeb], [OC2-Alstrup] and [OC3-
Alstrup]. As it is a closed system, the details of how the features
are implemented are not available.
To spread the burden of data distribution across several peers and
thus limiting the impact of peer loss, Octoshape splits a live stream
into a number of smaller equal-sized sub-streams. For example, a
400kbit/s live stream is split and coded into 12 distinct 100kbit/s
sub-streams. Only a subset of these sub-streams needs to reach a
user for it to reconstruct the !(R)original!_ live stream. The
number of distinct sub-streams could be as many as the number of
active peers.
Therefore, even if the upload capacity of a peer is smaller than its
download capacity, it would now be easier to contribute a sub-stream
than a whole live stream. An Octoshape peer can then receive from
each neighboring peer at least a distinct sub-stream. To make up for
the bandwidth asymmetry, artificial end users are used to deliver
additional bandwidth. Multi OctoServers are also available to
guarantee no single point of failure [OC3-Alstrup].
Octoshape keeps peer!_s availability information in an address book.
Each peer keeps a periodically updated stand-by list and passes it
along with its transmitted sub-stream. With constant monitoring of
the quality and consistency of each content source, the peer can
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switch partners in case of bottleneck and congestion to a better
source.
Octoshape provides operator to control who should and should not
receive certain video signal due to copyright restriction, to control
access based in part on IP numbers, and to obtain real time
statistics during any live events.
To optimize bandwidth utilization, Octoshape leverages computers
within a network to minimize external bandwidth usage and to select
the most reliable and !oclosest!+/- source to each viewer. It also
chooses the best matching available codecs and players and scales bit
rate up and down according to available internet connection.
Octoshape [OctoshapeWeb] claims to have patented resiliency and
throughput technologies to deliver quality streams to the mobile and
wireless edge networks. This throughput optimization technology also
cleans up latent and lossy network connections between the encoder
and the distribution point, providing a stable, high quality, stream
for distribution. Octoshape also claims to be able to deliver true
HD, 1280x720 30fps (720p) video over the Internet and to have
advanced DVR functionalities such as allowing users to move
seamlessly forward and back through the streams with almost no
waiting time.
3.1.3. 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 3 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.
Peer Protocol: The peer contacts the peers in its peerlist to get
additional peerlists, which are aggregated with its existing list.
Through this list, peers can maintain a mesh for peer management and
data delivery.
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
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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 3, Architecture of PPlive system
The following sections describe PPLive QoS related features,
extracted mostly from [PL3-Hei], [PL5-Vu], [PL6-Liu], and [PL7-Liu].
After obtaining an initial peer list from the member server, a peer
periodically updates its peer list by querying both member server and
partner peers. New peers are aggressively contacted at a fixed rate.
In selecting peers as partners, a peer considers their upload-
bandwidth and in part, their location information [PL6-Horvath] in
selecting on a FCFS basis those that have responded [PL7-Liu].
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For data distribution, PPLive, a data-driven or mesh-pull scheme
[PL3-Hei], divides the media content into small portions called
chunks uses and TCP for video streaming. Neighbor peers use a
gossip-like protocol to exchange their buffer map that indicates
chunks available for sharing. Peers obtain one or more their missing
chunks from one or more peers having them. Available chunks may also
be downloaded from the original channel server.
PPLive uses a double buffering mechanism consisting of TV Engine and
Media Player for its stream reassembly and display [PL3-Hei]. The TV
Engine is responsible for downloading video chunks from the PPLive
network and streaming the downloaded video to the Media Player, which
in turns displays the content to the user, after each buffer is
filled up to its respective predetermined threshold.
PPLive is observed to have the download scheduling policy of giving
higher priority to rare chunks and to chunks closer to play out
deadline and to be using a sliding window mechanism to regulate the
buffering of chunks.
To utilize available peer resources, peers in one subscribed overlay
may also be harnessed to support peers in other subscribed overlays
[PL5-Vu].
3.1.4. Zattoo
Zattoo is P2P live streaming system which serves over 3 million
registered users over European countries [Zattoo].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 live TV,
captured from satellites, onto the Internet. Each TV channel is
delivered through a separate P2P network.
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-------------------------------
| ------------------ | --------
| | Broadcast | |---------|Peer1 |-----------
| | Servers | | -------- |
| Administrative Servers | -------------
| ------------------------ | | Super Node|
| | Authentication Server | | -------------
| | Rendezvous Server | | |
| | Feedback Server | | -------- |
| | Other Servers | |---------|Peer2 |----------|
| ------------------------| | --------
------------------------------|
Figure 4, Basic architecture of Zattoo system
Tracker(Rendezvous Server) Protocol: In order to receive the signal
the requested channel, registered users are required to be
authenticated through Zattoo Authentication Server. Upon
authentication, users obtain a ticket with specific lifetime. Then,
users contact Rendezvous Server with the ticket and identify of
interested TV channel. In return, the Rendezvous Server sends back a
list joined peers carrying the channel.
Peer Protocol: Similar to aforementioned procedures in Joost, PPLive,
a new Zattoo peer requests to join an existing peer among the peer
list. Upon the availability of bandwidth, requested peer decides how
to multiplex a stream onto its set of neighboring peers. When
packets arrive at the peer, sub-streams are stored for reassembly
constructing the full stream.
Note Zattoo relies on Bandwidth 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 other receivers if the quality of sub-stream drops
below a threshold.
The following sections describe Zattoo QoS related features,
extracted mostly from [ZT1-Chang].
For reliable data delivery, each live stream is partitioned into
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.
Zattoo uses Peer-Division Multiplexing (PDM) scheme for its data
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delivery topology setup. In this scheme, each new peer independently
executes the Search and Join phases. In the Search Phase, a peer
queries the members of the peer list for sub-streams availability; in
response, receives additional prospective peers, sub-streams
availability, quality indications, and sub-stream sequence numbers;
and then selects, among the responses, partnering peers or quits
after failing two search attempts.
In the Join Phase, a joining peer, having selected the candidate
peers, requests to partner with some of them, spreading the load
among them and preferring topologically close-by peers, if these
peers have less capacity or carry lower quality sub-streams. Barring
departure or performance degradation of neighboring peers, the
established connections stay and the specified sub-stream packet of
every segment continues to be forwarded without further per-packet
handshaking between peers.
To manage stream efficiently for incoming and outgoing destinations,
each peer has a packet buffer, called IOB (Input-Output Buffer). The
IOB is referenced by an input pointer, a repair pointer, and one or
more output pointers, one for each forwarding destination such as
player, file, and other peer. The input pointer points to the slot
in the IOB where the next incoming packet with sequence number higher
than the highest sequence number received so far will be stored, and
the repair pointer always points to one slot beyond the last packet
received in order and is used to regulate packet retransmission and
adaptive PDM (to be described later). A packet map and forwarding
discipline is associated with each output pointer to accommodate the
different forwarding rates and regimes required by the destinations.
Note that retransmission requests are sent to random peers and not to
partnering peers and they are honoured only if the requested packets
are still in IOB and there is sufficient left-over capacity to
transmit all the requested packets. To avoid buffer overrun, a set
of two buffers is used in the IOB instead of a circular buffer.
Zattoo uses Adaptive Peer-Division Multiplexing scheme to handle
longer term bandwidth fluctuations. In this scheme, each peer
determines how many sub-streams to transmit and when to switch
partners. Specifically, each peer continually estimates the amount
of available uplink bandwidth based initially on probe packets to the
Zattoo Bandwidth Estimation Server and later, based on peer QoS
feedbacks, 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
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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 replacement peer. When a replacement 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.
3.1.5. 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]
The following sections describe PPStream QoS related features,
extracted mostly from [PS3-Li], [PS4-Jia] and [PS5-Wei].
PPStream is mainly mesh-based but to some extent it is layered in its
data distribution topology. It uses geographic clustering to some
extent based on geographic longitude and latitude of the IP addresses
[PS4-Jia].
To ensure data availability, some form of chunk retransmission
request mechanism is used and the buffer map is shared at high rate,
although concurrent requests for the same data chunk is rare. 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
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availability of sub-chunks and having a corresponding time offset.
Usually a buffer map contains only one data chunk at a time and is
thus smaller than that of PPLive. It also contains sending peer!_s
playback status to the other peers because as soon as a data chunk is
played back, the chunk is deleted or replaced by the next data chunk
[PS5-Wei].
At the initiating stage, a peer can use up to 4 data chunks and on a
stabilized stage, a peer uses usually one data chunk. However, in
transient stage, a peer uses variable number of chunks. Although,
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 in the following data chunks [PS3-Li].
Moreover, high bandwidth PPStream peers tend to receive chunks
earlier and contributes more than lower bandwidth peers.
3.1.6. 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]
The following sections describe SopCast QoS related features,
extracted mostly from [SC1-Ali], [SC2-Ciullo], [SC4-Fallica], [SC5-
Sentinelli], [SC6-Silverston], and [SC7-Tang].
SopCast allows for software update through (HTTP) a centralized web
server and makes available channel list through (HTTP) another
centralized server.
SopCast traffic is encoded and SopCast TV content is divided into
video chunks or blocks with equal sizes of 10KB [SC7-Tang]. Sixty
percent of its traffic is signaling packets and 40% is actual video
data packets [SC4-Fallica]. SopCast produces more signaling traffic
compared to PPLive, PPStream, and TVAnts, whereas PPLive produces the
least [SC6-Silverston]. Its traffic is also noted to have long-range
dependency [SC6-Silverston], indicating that mitigating it with QoS
mechanisms may be difficult. [SC1-Ali] reported that SopCast
communication mechanism starts with UDP for the exchange of control
messages among its peers using a gossip-like protocol and then moves
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to TCP for the transfer of video segments. This use of TCP for data
transfer seems to contradict others findings [SC4-Fallica, SC6-
Silverston].
To discover candidate peers, a peer requests peer list from Tracker,
or from neighboring peer using a gossip-like protocol. To retrieve
content [SC4-Fallica], a new peer contacts peers selected randomly
from the peer list it obtained from having queried the root servers
(trackers). The process of contacting peers slows down after the
initial bootstrap phase [SC3-Horvath, SC2-Ciullo]. The number of
peers a node typically connects to for download is about 2 to 5 [SC5-
Sentinelli] and there is no observed preference for peers with
shorter paths [SC2-Ciullo]. Partner peers periodically advertise
content availability and exchange sought content. In forming
multiple parent and children relationships, a peer does not exploit
peer location information [SC3-Horvath]. In general, parents are
chosen solely based on performance; however, lower capacity nodes
seem to be choosing parents that are closer to improve performance
and to compensate for its bandwidth constraints [SC1-Ali]. When
needed, a peer can download video streams directly from the Source
Provide, a node that broadcasts the entire video [SC7-Tang]. In the
process of data exchange, there is no enforcement of tit-for-tat like
mechanisms [SC2-Ciullo].
Similar to PPLive, SopCast uses a double-buffering mechanism. The
SopCast buffer downloads video chunks from the network, storing them,
and upon exceeding a predetermined number of stored chunks, launches
the Media player. The Media player buffer then downloads video
content from the local web server listening port and upon receiving
sufficient amount of content, starts video playback.
3.1.7. 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 5.
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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 5, 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.
The following sections describe TVAnts QoS related features,
extracted mostly from [TV1-Alessandria], [TV2-Ciullo], and [TV3-
Horvath].
TVAnts peer discovery mechanism is very greedy during the first part
of a peer life and stabilizes afterwards [TV2-Ciullo].
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For data delivery, peers exhibit mild preference to exchange data
among themselves in the same Autonomous System and also among peers
in the same subnet. TVAnts peer also exhibits some preference to
download from closer peers. According to [TV3-Horvath], TVAnts peer
exploits location information and download mostly from high-bandwidth
peers. However, it does not seem to enforce any tit-for-tat
mechanisms in the data delivery.
TVAnts [TV1-Alessandria] seems to be sensitive to network impairments
such as changes in network capacity, packet loss, and delay. For
capacity loss, a peer will always seek for more peers to download.
In the process of trying to avoid bad paths and selecting good peers
to continue downloading data, aggressive and potentially harmful
behavior for both application and the network results when bottleneck
is affecting all potential peers.
When limited access capacity is experienced, a peer reacts by
increasing redundancy (with FEC or ARQ mechanism) as if reacting to
loss and thus causes higher download rate. To recover from packet
losses, some kind of ARQ mechanism is also used. Although network
conditions do impact video stream distribution such as the network
delay impacting the start-up phase, they seem to have little impact
on the network topology discovery and maintenance process.
3.2. Tree-based P2P streaming systems
3.2.1. PeerCast
PeerCast adopts a Tree structure. The architecture of PeerCast is
shown in Figure 6.
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
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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.
------------------------------
| +---------+ |
| | Tracker | |
| +---------+ |
| | |
| | |
| +---------------------+ |
| | Broadcast server | |
| +---------------------+ |
|------------------------------
/ \
/ \
/ \
/ \
+---------+ +---------+
|Transfer1| |Transfer2|
+---------+ +---------+
/ \ / \
/ \ / \
/ \ / \
+---------+ +---------+ +---------+ +---------+
|Receiver1| |Receiver2| |Receiver3| |Receiver4|
+---------+ +---------+ +---------+ +---------+
Figure 6, Architecture of PeerCast system
The following sections describe PeerCast QoS related features,
extracted mostly from [CVV1-Zhang], [CVV4-Chu], [CVV5-Chu], and
[CVV6-Chu].
Each PeerCast node has a peering layer which is a layer between the
application layer and the transport layer. The peering layer of each
node coordinates among similar nodes to establish and maintain a
multicast tree. Moreover, the peering layer also supports simple,
lightweight redirect primitive. This primitive allows a peer p to
direct another peer c which is either opening a data-transfer session
with p, or has a session already established with p to a target peer
t to try to establish a data-transfer session. Peer discovery starts
at the root (source) or some selected sub-tree root and goes
recursively down the tree structure. When a peer leaves normally, it
informs its parent who then releases the peer, and it also redirects
all its immediate children to find new parents starting at some
target t.
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The peering layer allows for different policies of topology
maintenance. In choosing a parent from among the children of a given
peer, a child can be chosen randomly, one at a time in some fixed
order, or based on least access latency with respect to the choosing
peer. There are also many choices of peers to start and limit the
search. The different combinations are all the descendants of a
leaving peer have to start searching from the root [root-All (RTA)];
only the children of a leaving peer have to start searching from the
root [Root (RT)]; all the descendants of a leaving peer have to start
searching from the parent of the leaving peer [Grandfather-All
(GFA)]; and only the children of the leaving peer have to start
searching from the parent of the leaving peer [Grandfather (GF)].
A heart-beat mechanism at the peer is available to handle failed
peer. With this mechanism, a peer sends keep-alive messages to its
parent and children. If a parent peer detects that a child has
skipped a specified number of heart-beats, it deems the child as lost
and tidies up. Similarly, a child peer starts its search for new
parent once its current parent is deemed to have left.
PeerCast also proposes but has not evaluated a number of algorithms
that use some cost function to optimize the overlay. Some of them
are described next. If a parent is already saturated, a newly
arrived peer replaces one of the costlier children than the newly
arrived peer and the replaced peer tries to reconnect somewhere else
[Knock-Down]. Newly arrived peer replaces the target peer and the
target peer becomes its child [Join-Flip]. Unstable peers are pushed
down to the bottom of the tree [Leaf-Sink]. Existing child and
parent relationship is flipped [Maintain-Flip].
3.2.2. 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 7.
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
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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.
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 7, Architecture of ESM system
The following sections describe ESM QoS related features, extracted
mostly from [CVV1-Zhang], [CVV4-Chu], [CVV5-Chu], and [CVV6-Chu], and
the details of Conviva are not publicly available.
ESM constructs the multicast tree in a two-step process. It
constructs first a mesh of the participating peers; the mesh having
the following properties:
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o The shortest path delay between any pair of peers in the mesh is
at most K times the unicast delay between them, where K is a small
constant.
o Each peer has a limited number of neighbors in the mesh which does
not exceed a given (per-member) bound chosen to reflect the
bandwidth of the peer!_s connection to the Internet.
It then constructs a (reverse) shortest path spanning trees of the
mesh with the root being the source.
Therefore a peer participates in two types of topology management: a
control structure in which peers make sure they are always connected
in a mesh and a data delivery structure in which peers make sure data
gets delivered to them in a tree structure.
To keep connected, each peer maintains communication with a small
number of random neighbors and a complete list of members through a
gossip-like algorithm. When a new node joins, it gets a list of
group members from the source. To look for a parent, it sends probe
request to a subset of the group members it obtained; evaluates them
with respect to delay to the source, application throughput and link
bandwidth; and then chooses from among them a candidate parent that
is not a descendant and is not saturated. In addition to using RTT-
probes, consisting of 1-Kbyte transfers to detect bottleneck
bandwidth, performance history of previously chosen parent is also
considered. The peer also avoids probing hosts with low bottleneck
bandwidth.
When a peer leaves normally, it notifies its neighboring peers and
the neighboring peers propagate the departing peer info. At the same
time, the departing peer continues to forward packets for some time
to minimize transient packet loss. When a peer leaves due to
failure, active peers detect the departure of the peer through its
non-responsiveness to their probe messages. Active peers that
detected the loss then propagate the departed peer info. A departed
peer list that is flushed after a sufficient amount of time has
passed keeps track of leaving and failed peers. The list enables
refreshes from an active peer and a leaving/failed peer to be
distinguished.
Departing peers and failing peers could in some instance partition a
mesh into two or more components. Mesh repair algorithm detects such
occurrences by noticing split in the membership list and tries to
repair by virtually linking between active members to one of the non-
active members, trying one non-active member at a time.
To improve mesh/tree structural and operating quality, each peer
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randomly probes one another to add new links that have perceived gain
in utility; and each peer continually monitors existing links to drop
those links that have perceived drop in utility. Switching parent
occurs if a peer leaves or fails; if there is a persistent congestion
or low bandwidth condition; or if there is a better clustering
configuration. To allow for more public hosts to be available for
becoming parents of NATs, public hosts preferentially choose NATs as
parents.
The data delivery structure, obtained from running a distance vector
protocol on top of the mesh using latency between neighbors as the
routing metric, is maintained using various mechanisms. Each peer
maintains and keeps up to date the routing cost to every other
member, together with the path that leads to such cost. To ensure
routing table stability, data continues to be forwarded along the old
routes for sufficient time until the routing tables converge. The
time is set to be larger than the cost of any path with a valid
route, but smaller than infinite cost. To make better use of the
path bandwidth, streams of different bit-rates are forwarded
according to the following priority scheme: audio being higher than
video streams and lower quality video being higher than quality
video. Moreover, bit-rates of stream are adapted to the peer
performance capability.
3.3. Hybrid P2P streaming system
3.3.1. New Coolstreaming
The Coolstreaming, first released in summer 2004 with a mesh-based
structure, arguably represented the first successful large-scale P2P
live streaming. As the above analysis, it has poor delay performance
and high overhead associated each video block transmission. After
that, New coolstreaming[New CoolStreaming] adopts a hybrid mesh and
tree structure with hybrid pull and push mechanism. All the peers
are organized into mesh-based topology in the similar way like pplive
to ensure high reliability.
Besides, content delivery mechanism is the most important part of New
Coolstreaming. Fig.8 is the content delivery architecture. The
video stream is divided into blocks with equal size, in which each
block is assigned a sequence number to represent its playback order
in the stream. We divide each video stream into multiple sub-streams
without any coding, in which each node can retrieve any sub-stream
independently from different parent nodes. This subsequently reduces
the impact to content delivery due to a parent departure or failure.
The details of hybrid push and pull content delivery scheme are shown
in the following:
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(1) A node first subscribes to a sub-stream by connecting to one of
its partners via a single request (pull) in BM, the 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.)
(2) The selected parent node will continue pushing all blocks in need
of the sub-stream to the requested node.
This not only reduces the overhead associated with each video block
transfer, but more importantly, significantly reduces the timing
involved in retrieving video content.
------------------------------
| +---------+ |
| | Tracker | |
| +---------+ |
| | |
| | |
| +---------------------+ |
| | Content server | |
| +---------------------+ |
|------------------------------
/ \
/ \
/ \
/ \
+---------+ +---------+
| Peer1 | | Peer2 |
+---------+ +---------+
/ \ / \
/ \ / \
/ \ / \
+---------+ +---------+ +---------+ +---------+
| Peer2 | | Peer3 | | Peer1 | | Peer3 |
+---------+ +---------+ +---------+ +---------+
Figure 8 Content Delivery Architecture
The following sections describe Coolstreaming QoS related features,
extracted mostly from [CS1-Bo] and [CS2-Xie].
The basic components of Coolstreaming consist of the source,
bootstrap node, web server, log server, media servers, and peers.
Three basic modules in a peer help it maintain a partial view of the
overlay (Membership Manager); establish and maintain partnership with
other peers with which Buffer Maps indicating available video
content, are exchanged (Partnership Manager),; and manage data
delivery, retrieval, and play out (Stream Manager).
In building the overlay topology, a newly arrived peer contacts the
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bootstrap node for a list of nodes and stores it in its own mCache.
From the stored list, it selects nodes randomly to forms partnership
and then parent-children relationship, where a partnership between
two nodes exists when only block availability information is
exchanged between them, and a parent-children relationship exists
when, in addition to being partner, video content is also exchanged.
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 ith sub-blocks of
all blocks constitutes the ith sub-stream of the video stream, where
i is the 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
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, then the
node concludes that the parent is lagging sufficiently behind and
needs to be replaced. Furthermore, a node also evaluates the maximum
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.
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4. A common P2P Streaming Process Model
As shown in Figure 8, a common P2P streaming process can be
summarized based on Section 3:
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 8, 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,
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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.
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
technology for large-scale streaming over the Internet".
[Zattoo] "http: //zattoo.com/".
[Conviva] "http://www.rinera.com/".
[ESM04] Zhang, Hui., "End System Multicast,
http://www.cs.cmu.edu/~hzhang/Talks/ESMPrinceton.pdf",
May .
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[draft-zhang-alto-traceroute-00]
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Authors' Addresses
Gu Yingjie
Huawei
Baixia Road No. 91
Nanjing, Jiangsu Province 210001
P.R.China
Phone: +86-25-56624760
Fax: +86-25-56624702
Email: guyingjie@huawei.com
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Zong Ning
Huawei
Baixia Road No. 91
Nanjing, Jiangsu Province 210001
P.R.China
Phone: +86-25-56624760
Fax: +86-25-56624702
Email: zongning@huawei.com
Hui Zhang
NEC Labs America.
Email: huizhang@nec-labs.com
Zhang Yunfei
China Mobile
Email: zhangyunfei@chinamobile.com
Lei Jun
University of Goettingen
Phone: +49 (551) 39172032
Email: lei@cs.uni-goettingen.de
Gonzalo Camarillo
Ericsson
Email: Gonzalo.Camarillo@ericsson.com
Liu Yong
Polytechnic University
Email: yongliu@poly.edu
Delfin Montuno
Huawei
Email: delfin.montuno@huawei.com
Gu, et al. Expires September 12, 2011 [Page 34]
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Xie Lei
Huawei
Email: xielei57471@huawei.com
Gu, et al. Expires September 12, 2011 [Page 35]
