PPSP Y. Gu
Internet-Draft Unaffiliated
Intended status: Standards Track N. Zong, Ed.
Expires: January 13, 2014 Huawei
Y. Zhang
Coolpad
F. Lo Piccolo
Cisco
S. Duan
CATR
July 12, 2013
Survey of P2P Streaming Applications
draft-ietf-ppsp-survey-05
Abstract
This document presents a survey of some of the most popular Peer-to-
Peer (P2P) streaming applications on the Internet. Main selection
criteria have been popularity and availability of information on
operation details at writing time. In doing this, selected
applications are not reviewed as a whole, but they are reviewed with
main focus 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 January 13, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
Gu, et al. Expires January 13, 2014 [Page 1]
Internet-Draft Survey of P2P Streaming Applications July 2013
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.3. Zattoo . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.4. PPStream . . . . . . . . . . . . . . . . . . . . . . 11
3.1.5. Tribler . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.6. QQLive . . . . . . . . . . . . . . . . . . . . . . . 14
3.2. Tree-based P2P streaming applications . . . . . . . . . . 15
3.2.1. End System Multicast (ESM) . . . . . . . . . . . . . 16
3.3. Hybrid P2P streaming applications . . . . . . . . . . . . 18
3.3.1. New Coolstreaming . . . . . . . . . . . . . . . . . . 18
4. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5. Author List . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
7. Informative References . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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.
Gu, et al. Expires January 13, 2014 [Page 2]
Internet-Draft Survey of P2P Streaming Applications July 2013
In addition, the selected applications are not reviewed as a whole,
but they are reviewed with main 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 |
+--------------------------------+
^ | ^ |
| | | |
| | Traker | | Traker
| | Protocol | | Protocol
| | | |
| V | V
+-------------+ +------------+
| Peer 1 |<--------| Peer 2 |
| |-------->| |
+-------------+ +------------+
Peer Protocol
Figure 1, High level architecture of P2P streaming systems assumed as
reference model 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/signaling protocols:
1) the "tracker protocol" that regulates the interaction between
trackers and peer;
2) the "peer protocol" that regulates the interaction between
peers.
Hence, whenever possible, we always try to identity tracker and peer
protocols and we provide the corresponding details.
This document is organized as follows. Section 2 introduces
terminology and concepts used throughout the current survey. Since
Gu, et al. Expires January 13, 2014 [Page 3]
Internet-Draft Survey of P2P Streaming Applications July 2013
overlay topology built on connections among peers impacts some
aspects of tracker and peer protocols, Section 2 classifies P2P
streaming applications according to the overlay topology: mesh-based,
tree-based and hybrid. Then, Section 3 presents some of the most
popular mesh-based P2P streaming applications: Octoshape, PPLive,
Zattoo, PPStream, 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
Channel: TV channel from which live streaming content is transmitted
in a P2P streaming application.
Chunk: Basic unit that a streaming media is partitioned into for the
purposes of storage, scheduling, advertisement and exchange among
peers.
Live streaming: Application that allows users to receive almost in
real-time multimedia content related to on ongoing event and streamed
from a source. The lag between the play points at the receivers and
the ones at the streaming source has to be small.
Peer: P2P node that dynamically participates in a P2P streaming
system not only to receive streaming content but also to store and
upload 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 peers.
Push: Transmission of multimedia content without any request from
receiving peers.
Swarm: A group of peers sharing the same streaming content at a given
time.
Tracker: P2P node that stably participates in a P2P streaming system
to provide a directory service by maintaining information both on the
peer set and on the chunks each peer stores.
Tracker protocol: Control and signaling protocol that regulates
interaction among peers and trackers.
Gu, et al. Expires January 13, 2014 [Page 4]
Internet-Draft Survey of P2P Streaming Applications July 2013
Video-on-demand (VoD): Application that allows users to select and
watch video content on demand.
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:
1) 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
guarantee both topology maintenance at very low cost and 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;
2) 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 guarantee higher network resource utilization than the
one associated with tree-based applications. On the other side,
the cost to maintain overlay topology may limit performance in
terms of delay, and pull-based data delivery calls for large size
buffers where to store chunks;
3) hybrid: this category includes all the P2P applications 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 each peer interacts with a
limited subset of other peers (neighbors) and explicitly requests
chunks it needs (pull-based or data-driven delivery). This type of
content delivery may be associated with high overhead, not only
because peers formulate requests 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). On the one side, the main advantage of this
Gu, et al. Expires January 13, 2014 [Page 5]
Internet-Draft Survey of P2P Streaming Applications July 2013
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 highly dynamic and not persistent (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 a P2P plug-in that has been realized by the
homonym Danish company and has become popular for being used by CNN
[CNN] to broadcast living streaming content. Octoshape helps indeed
CNN serve a peak of more than a million simultaneous viewers thanks
not only to the P2P content distribution paradigm, but also to
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
Gu, et al. Expires January 13, 2014 [Page 6]
Internet-Draft Survey of P2P Streaming Applications July 2013
As it can be seen from the picture, there are no trackers and
consequently no tracker protocol is necessary.
As regards the peer protocol, information on peers that already
joined the channel is transmitted in form of metadata when streaming
the live content. In such a way each peer maintains a sort of
Address Book with the information necessary to contact other peers
who are watching the same channel.
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 peer continues sending requests until it
finds K peers willing to upload the minimum number of data streams
needed to display the original live stream. This allows a flexible
use of bandwidth at end users. In fact, since the original stream is
split into smaller data streams, a peer that does not have enough
upload capacity to transmit the original whole stream can transmit a
number of smaller data streams that fits its actual upload capacity.
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 the available Internet
connection.
3.1.2. PPLive
Gu, et al. Expires January 13, 2014 [Page 7]
Internet-Draft Survey of P2P Streaming Applications July 2013
PPLive [PPLive] was first developed in Huazhong University of Science
and Technology in 2004, and it is one of the earliest and most
popular P2P streaming software in China. To give an idea, PPLive
website reached 50 millions of visitors for the opening ceremony of
Beijing 2008 Olympics, and the dedicated Olympics channel attracted
221 millions of views in two weeks.
Even though PPLive was renamed to PPTV in 2010, we continue using the
old name PPLive throughout this document.
PPLive system includes the following main components:
1) video streaming server, that plays the role of source of video
content and copes with content coding issues;
2) peer, also called node or client, that is PPLive entity
downloading video content from other peers and uploading video
content to other peers;
3) channel server, that provides the list of available channels
(live TV or VoD content) to a PPLive, as soon as the peer joins
the system;
4) tracker server, that provides a PPLive peer with the list of
online peers that are watching the same channel as the one the
joining peer is interested in.
Figure 3 illustrates the high level diagram of PPLive system.
+------------+ +------------+
| Peer 2 |----| Peer 3 |
+------------+ +------------+
| | | |
| +--------------+ |
| | Peer 1 | |
| +--------------+ |
| | |
+------------------------------+
| |
| +----------------------+ |
| |Video Streaming Server| |
| +----------------------+ |
| | Channel Server | |
| +----------------------+ |
| | Tracker Server | |
| +----------------------+ |
| |
+------------------------------+
Gu, et al. Expires January 13, 2014 [Page 8]
Internet-Draft Survey of P2P Streaming Applications July 2013
Figure 3, High level overview of PPLive system architecture
As regards the tracker protocol, as soon as a PPLive peer joins the
systems and selects the channel to watch, it retrieves from the
tracker server a list of peers that are watching the same channel.
As regards the peer protocol, it controls both peer discovery and
chunk distribution process. More specifically, peer discovery is
regulated by a kind of gossip-like mechanism. After retrieving the
list of active peers watching a specific channel from tracker server,
a PPLive sends out probes to establish active peer connections, and
some of those peers may return also their own list of active peers to
help the new peer discover more peers in the initial phase. Chunk
distribution process is mainly based on buffer map exchange to
advertise the availability of cached chunks. In more detail, PPLive
software client exploits two local buffers to cache chunks: the
PPLive TV engine buffer and media player buffer. The main reason
behind the double buffer structure is to address the download rate
variations when downloading chunks from PPLive network. In fact,
received chunks are first buffered and reassembled into the PPLive TV
engine buffer; as soon as the number of consecutive chunks in PPLive
TV engine buffer overcomes a predefined threshold, the media player
buffer downloads chunks from the PPLive TV engine buffer; finally,
when the media player buffer fills up to the required level, the
actual video playback starts.
Being the nature of PPLive protocols and algorithm proprietary, most
of known details have been derived from measurement studies.
Specifically, it seems that:
1) number of peers from which a PPLive node downloads live TV
chunks from is constant and relatively low, and the top-ten peers
contribute to a major part of the download traffic, as shown in
[P2PIPTVMEA];
2) PPLive can provide satisfactory performance for popular live TV
and VoD channels. For unpopular live TV channels, performance may
severely degrade, whereas for unpopular VoD channels this problem
rarely happens, as it shown in [CNSR]. Authors of [CNSR] also
demonstrate that the workload in most VoD channels is well
balanced, whereas for live TV channels the workload distribution
is unbalanced, and a small number of peers provide most video
data.
3.1.3. Zattoo
Gu, et al. Expires January 13, 2014 [Page 9]
Internet-Draft Survey of P2P Streaming Applications July 2013
Zattoo [Zattoo] is P2P live streaming system that was launched in
Switzerland in 2006 in coincidence with the EUFA European Football
Championship and in few years was able to attract almost 10 million
registered users in several European countries.
Figure 4 depicts the high level architecture of Zattoo system. The
main reference for the information provided in this document is
[IMC09].
+-------------------------------------+
| ------------------------------- | +------+
| | Broadcast Server | |---|Peer 1|---|
| ------------------------------- | +------+ |
| | Authentication Server | | +--------------+
| ------------------------------- | | Repeater node|
| | Rendezvous Server | | +--------------+
| ------------------------------- | +------+ |
| | Bandwidth Estimation Server | |---|Peer 2|---|
| ------------------------------- | +------+
| | Other Servers | |
| ------------------------------- |
+-------------------------------------+
Figure 4, High level overview of Zattoo system architecture
Broadcast server is in charge of capturing, encoding, encrypting and
sending the TV channel to the Zattoo network. A number N of logical
sub-streams is derived from the original stream, and packets of the
same order in the sub-streams are grouped together into the so-called
segments. Each segment is then coded via a Reed-Salomon error
correcting code in such a way that any number k < N of received
packets in the segment is enough to reconstruct the whole segment.
Authentication server is the first point of contact for a peer the
joins the system. It authenticates Zattoo users and assigns them
with a limited lifetime ticket. Then, a user contacts the Rendezvous
server and specifies the TV channel of interest. It also presents
the tickets received by the authentication server. Provided that the
presented ticket is valid, the rendezvous server returns a list of
Zattoo peers that have already joined the requested channel and a
signed channel ticket. Hence, rendezvous server plays the role of
tracker. At this point the direct interaction between peers starts
and it is regulated by the peer protocol.
A new Zattoo user contacts the peers returned by the rendezvous
server in order to identify a set of neighboring peers covering the
full set of sub-streams in the TV channel. This process is denoted
Gu, et al. Expires January 13, 2014 [Page 10]
Internet-Draft Survey of P2P Streaming Applications July 2013
in Zattoo jargon as Peer Division Multiplexing (PDM). To ease the
identification of neighboring peers, each contacted peer provides
also the list of its own known peers, in such a way that a new Zattoo
user, if needed, can contact new more peers besides the ones
indicated by the rendezvous server. In selecting which peers to
establish connections with, a peer adopts the criterion of
topological closeness. The topological location of a peer is defined
in Zattoo as (in order of preference) its subset number, its
autonomous system number and its country code, and its provided to
each peer by the authentication server.
Zattoo peer protocol provides also a mechanism to make PDM process
adaptive with respect to bandwidth fluctuations. First of all, a
peer controls the admission of new connections based on the available
uplink bandwidth. This is estimated i) at beginning with each peer
sending probe messages to the Bandwidth Estimation server, and ii)
while forwarding sub-streams to other peers based on the quality-of-
service feedback received by those peers. A quality-of-service
feedback is sent from the receiver to the sender only when the
quality of the received sub-stream is below a given threshold. So if
a quality-of-service feedback is received, a Zattoo peer decrements
the estimation of available uplink bandwidth, and if this drops below
the amount needed to supports the current connections, a proper
number of connections is closed. On the other side, if no quality-
of-service feedback is received for a given time interval, a Zattoo
peer increments the estimation of available uplink bandwidth
according to a mechanism very similar to the one of TCP congestion
window (double increase or linear increase depending on whether the
estimate is below or a given threshold).
As it can be seen also in Figure 4, there exist two classes of Zattoo
nodes: simple peers, whose behavior has already been presented, and
Repeater nodes, that serve as bandwidth multiplier, are able to
forward any sub-stream and implement the same peer protocol as simple
peers.
3.1.4. PPStream
PPStream [PPStream] is a very populare P2P streaming software in
China and in many other countries of East Asia.
The system architecture of PPStream is very similar to the one of
PPLive. When a PPStream peer joins the system, it retrieves the list
of channels from the channel list server. After selecting the
channel to watch, a PPStream peer retrieves from the peer list server
the identifiers of peers that are watching the selected channel, and
it establishes connections that are used first of all to exchange
buffer-maps. In more detail, a PPStream chunk is identified by the
Gu, et al. Expires January 13, 2014 [Page 11]
Internet-Draft Survey of P2P Streaming Applications July 2013
play time offset which is encoded by the streaming source and it is
subdivided into sub-chunks. So buffer-maps in PPStream carry the
play time offset information and are strings of bits that indicate
the availability of sub-chunks. After receiving the buffer-maps from
the connected peers, a PPStream peer selects peers to download sub-
chunks from according to a rate-based algorithm, which maximizes the
utility of uplink and downlink bandwidth.
3.1.5. Tribler
Tribler [tribler] is a BitTorrent client that was 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 both i) attract
attention from other universities and media companies and ii) receive
European Union research funding (P2P-Next and QLectives projects).
Differently from BitTorrent, where a tracker server centrally
coordinates peers in uploads/downloads of chunks 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.
This is illustrated also in Figure 5, which depicts the high level
architecture of Tribler.
+------------+
| Superpeer |
+------------+
/ \
/ \
+------------+ +------------+
| Peer 2 |----| Peer 3 |
+------------+ +------------+
/ | \
/ | \
/ +--------------+ \
/ | Peer 1 | \
/ +--------------+ \
/ / \ \
+------------+ / +--------------+
| Peer 4 | / | Peer 5 |
+------------+ / +--------------+
\ / /
\ / /
\ / +------------+
+------------+ | Superpeer |
| Superpeer | +------------+
Gu, et al. Expires January 13, 2014 [Page 12]
Internet-Draft Survey of P2P Streaming Applications July 2013
+------------+
Figure 5, High level overview of Tribler system architecture
Regarding peer protocol and the organization of overlay mesh, Tribler
bootstrap process consists in preloading well known superpeer
addresses into peer local cache, in such a way that a joining peer
randomly selects a superpeer 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 list, that is their downloaded files, 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.
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
Gu, et al. Expires January 13, 2014 [Page 13]
Internet-Draft Survey of P2P Streaming Applications July 2013
decreasing numbers of chunks they have forwarded to other peers,
counting only the chunks they originally received from them. In case
of 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 outdated,
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 identify 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 include 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 Tribler
peers to use the public key included in torrent file and verity the
integrity of each chunk.
3.1.6. 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.
QQLive adopts CDN and P2P architecture for video distribution and is
different from other popular P2P streaming applications. QQLive
provides video source by source servers and CDN and the video content
can be push to every region by CDN throughout China. In each region,
QQLive adopts P2P technology for video content distribution.
Gu, et al. Expires January 13, 2014 [Page 14]
Internet-Draft Survey of P2P Streaming Applications July 2013
One of the main aims for QQLive is to use the simplest architecture
to provide the best user experience. So QQLive take some servers to
implement P2P file distribution. There are two servers in QQLive:
Stun Server and Tracker Server. Stun Server is responsible for NAT
traversing. Tracker Server is responsible for providing content
address information. There are a group of these two Servers for
providing services. There is no Super Peer in QQLive.
Working flow of QQLive includes startup stage and play stage.
1) Startup stage includes only interactions between peers and
Tracker servers. There is a built-in URL in QQLive client
software. When the client startups and connects to the network,
the client gets the Tracker's address through DNS and tells the
Tracker the information of its owned video contents.
2) play stage includes interactions between peers and peers or
peers and CDN. Generally, the client will download the video
content from CDN during the first 30 seconds and then gets
contents from other peers. If unfortunately there is no peer
which owns the content, the client will get the content from CDN
again.
As the client watches the video, the client will store the video to
the hard disk. The default storage space is one Gbyte. If the
storage space is full, the client will delete the oldest content.
When the client do VCR operation, if the video content is stored in
hard disk, the client will not do interactions with other peers or
CDN.
There are two main protocols in QQLive: tracker protocol and peer
protocol. These two protocols are all full private and encrypt the
whole message. The tracker protocol uses UDP and the port for the
tracker server is fixed. For the video streaming, if the client gets
the streaming from CDN, the client use the HTTP with port 80 and no
encryption; if the client gets the streaming from other peers, the
client use UDP to transfer the encrypted media streaming and not RTP/
RTCP.
If there are messages or video content missing, the client will take
retransmission and the retransmission interval is decided by the
network condition. The QQLive doesn't care the strategy of
transmission and chunk selection which is simple and not similar with
BT because of the CDN support.
3.2. Tree-based P2P streaming applications
Gu, et al. Expires January 13, 2014 [Page 15]
Internet-Draft Survey of P2P Streaming Applications July 2013
In tree-based P2P streaming applications peers self-organize in a
tree-shape overlay network, where peers do not ask for a specific
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 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
parent in overlay tree, to receive streamed 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 first P2P live streaming system of
the world, 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 members. The new member randomly selects a few
group members as peer neighbors. The number of selected neighbors
never exceeds 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
Gu, et al. Expires January 13, 2014 [Page 16]
Internet-Draft Survey of P2P Streaming Applications July 2013
leaves, either it notifies its neighbors and the information is
propagated across the mesh to all the 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
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.
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
video. 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 hosts use
hosts behind NAT/firewalls as parents.
Gu, et al. Expires January 13, 2014 [Page 17]
Internet-Draft Survey of P2P Streaming Applications July 2013
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.
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 with 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.
Like in the old Coolstreaming, a newly joined node contacts a special
bootstrap node and retrieves a partial list of active nodes in the
system.
The interaction with bootstrap node is the only one related to the
tracker protocol. The rest of New Coolstreaming interactions are
related to peer protocol.
The newly joined node then establishes a partnership with few active
nodes by periodically exchanging information on content availability.
Streaming content is divided in New Coolstreaming in equal-size
blocks or chunks, which are unambiguously associated with sequence
numbers that represent the playback order. Chunks are then grouped
to form multiple sub-streams.
Like in most of P2P streaming applications information on content
availability is exchanged in form of buffer-maps. However, New
Coolstreaming buffer-maps differ from the usual format of strings of
bits where each bit represents the availability of a chunk. Two
vectors represent indeed buffer-maps in New Coolstreaming. The first
vector reports the sequence numbers of the last chunk received for a
given sub-stream. The second vector is used to explicitly request
chunks from partner peers. In more details, the second vector has as
many bits as sub-streams, and a peer receiving a bit "1" in
correspondence of a given sub-stream is being requested from the
sending peer to upload chunks belonging to that sub-streams. Since
chunks are explicitly requested, data delivery may be regarded as
pull-based. However, data delivery is push-based as well, since
every time a node is requested to upload chunks, it uploads all
chunks for that sub-stream starting from the one indicated in the
first vector of received buffer-map.
Gu, et al. Expires January 13, 2014 [Page 18]
Internet-Draft Survey of P2P Streaming Applications July 2013
In order to improve quality of mesh-tree overlay, each node
continuously monitors the quality of active connections in terms of
mutual delay between sub-streams. If such quality drops below a
predefined threshold, a New Coolstreaming node selects a new partner
among its partners. Parent re-selection is also applied in case of
leaving of the previous parent.
4. Security Considerations
This document does not raise security issues.
5. Author List
Other 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, http://www.cnn.com
[PPLive] PPLive web site, http://www.pplive.com
[P2PIPTVMEA] Silverston, Thomas, et al., "Measuring P2P IPTV
Systems", June 2007.
[CNSR] Li, Ruixuan, et al., "Measurement Study on PPLive Based on
Channel Popularity", May 2011.
[Zattoo] Zattoo web site, http://www.zattoo.com
Gu, et al. Expires January 13, 2014 [Page 19]
Internet-Draft Survey of P2P Streaming Applications July 2013
[IMC09] Chang, Hyunseok, et al., "Live streaming performance of the
Zattoo network", November 2009.
[PPStream] PPStream web site, http:// www.ppstream.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
[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.
8. References
Authors' Addresses
Gu Yingjie
Unaffiliated
Email: guyingjie@gmail.com
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
Gu, et al. Expires January 13, 2014 [Page 20]
Internet-Draft Survey of P2P Streaming Applications July 2013
Zhang Yunfei
Coolpad
Email: hishigh@gmail.com
Francesca Lo Piccolo
Cisco
Via del Serafico 200
Rome 00142
Italy
Phone: +39-06-51645136
Email: flopicco@cisco.com
Duan Shihui
CATR
No.52 HuaYuan BeiLu
Beijing 100191
P.R.China
Phone: +86-10-62300068
Email: duanshihui@catr.cn
Gu, et al. Expires January 13, 2014 [Page 21]
