PPSP Yunfei. Zhang
Internet Draft China Mobile
Intended status: Standards Track February 23, 2009
Expires: August 19, 2009
Problem Statement of P2P Streaming Protocol (PPSP)
draft-zhang-ppsp-problem-statement-00.txt
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Abstract
The document outlines the problem statement of peer to peer streaming
applications and the definition and scope of peer to peer streaming
protocol.
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Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1].
Table of Contents
1. Introduction................................................2
2. Problem Statement of P2P streaming Applications..............3
3. Peer to Peer Streaming Protocol Definition (PPSP) and Scope...4
4. Comparison with related protocols............................6
4.1. P2PSIP.................................................6
4.2. RTSP and related protocols..............................8
5. Scenarios of Inter-working of PPSP...........................8
5.1. Non CDN assistant inter-worked PPSP.....................8
5.2. CDN assistant inter-worked PPSP.........................9
6. Security Considerations......................................9
7. Acknowledgments............................................10
8. References.................................................11
Author's Addresses............................................11
1. Introduction
Nowadays Peer to Peer computing has been successfully used in many
fields, from one to one communication like VoIP, IM to one to many
communication like streaming, file sharing and gaming. In streaming
area, with the popularity of P2P technology, PPlive[1], PPstream[2],
UUSee[3] ,Pando[4] etc. show the prosperity of P2P real time and VOD
streaming applications. Take pplive for example, it has over 5
million online users at the same time for real-time streaming. Also
some web2.0 streaming applications such as youtube[5], tudou [6]are
reported to use or prepare to use P2P engine to accelerate its
downloading rate and cut down the transmission cost esp in the winter
of finance crisis which is being widespread all over the world.
Basically there are two kinds of streaming solutions: client-server
streaming and P2P streaming. Some client-server streaming control
protocols have been developed both within and out of IETF, including
RTSP[7], MMS[8],PNA[9] and HTTP. As for p2p streaming applications,
there already exist a lot of real time and VOD applications like
PPlive, PPStream, UUSee in China and Pando in the USA, each of which
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uses its proprietary protocol. P2P streaming applications account for
more and more Internet traffic. According to statistics in a main
china ISP, PPlive accounts 10% of the total Internet backbone traffic.
In contrast, Bittorrent's traffic share is about 8% in the ISP's
backbone.
Therefore there is no doubt that P2P streaming is more and more
important in the Internet. It's time to draw up an open P2P streaming
protocol in IETF to make P2P streaming wider adoption and regulate
its behavior from the whole Internet point of view.
2. Problem Statement of P2P streaming Applications
Although P2P streaming applications are popular in the Internet,
there are still some unsolved problems surrounding them:
First, the startup delay(20~30s), the latency between the
broadcasting time and the audience view time(120s), the re-buffering
time after a dragging or forward/backward in VoD(6~8s) and the
channel switch time are still long. How to reduce these delays in the
Internet is an open question for researchers;
Second, the video quality is still very low for p2p streaming
applications, most of which has a playback rate of some hundreds kbps.
These may be attributed to several factors, e.g., current low access
bandwidth and asymmetrical upload and download bandwidth like ADSL
and cable modem, How to improve the video quality under such network
environment is an open problem. Even suppose when there is a higher
access bandwidth like LTE which has over 100Mbps bandwidth, it is
still a question if current P2P streaming mechanisms work to support
high quality streaming because of different wireless network
environment.
Third, the dragging and backward/forward performance in VoD is
unacceptable in current p2p streaming applications. The lack of
upload bandwidth where ADSL and cable modem dominating the access
network indicates a peer-assistant method like CDN can make the
dragging performance better. How to change current CDN to accommodate
p2p streaming environment and integrate them together is an open
question.
Fourth, traffic localization and transport protocol optimization are
very important problems in p2p streaming environment being discussed
in ALTO and TANA in IETF. So PPSP can reuse the fruits of ALTO and
TANA.
Last but not least, from the protocol perspective, private protocol
has the following problems: for end users, he needs multiple client
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software to view different programs; for operators, it's difficult to
identify these different applications and for service provider, it
has to pay much attention to both programs source and P2P delivery
technology. So an open peer to peer streaming protocol is required to
meet the requirements mentioned above.
3. Peer to Peer Streaming Protocol Definition (PPSP) and Scope
The basic mission of PPSP is to create a distributed real-time data
retrieval protocol in one to many communication (or data-driven
communication).One to many communication is different from one to one
communication where there is known destination to visit. In one to
many communication, the destinations are unknown and the concrete
data are stored piece by piece in different peers and the key is to
find those data and reassemble them.
Therefore, PPSP focuses on how to negotiate with un-preassigned peers
for needed chunks along with some application requirements parameters
and transmit the retrieved content accordingly. PPSP involves a
bundle of interactions, including interaction between peers, between
peers and trackers, between peer and CDN. Note that CDN can be viewed
as a special peer who has a complete copy of the programs in VoD and
a super-stable peer with higher upload and download bandwidth in
real-time streaming. From the protocol type perspective, PPSP
includes streaming control (Step 1-4 and 7) and transmission protocol
(Step5) which will be discussed in the following part The protocol
stack of PPSP is shown in Fig1.
+------------------------+
| PPSP Application |
+------------------------+
| PPSP Signaling Protocol|
+------------------------+
| PPSP Trans Protocol |
+------------------------+
| Transport Layer |
+------------------------+
Figure 1 PPSP Position in Protocol Stack.
The process of PPSP applications is shown in Fig2. We explain it
as follows:
1. Peer sending PPSP signaling request with parameters(e.g., QoS,
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location, historical records such as online duration)
2. Tracker returning Peer list according to the parameters through
PPSP signaling protocol.
3. Peer Gossiping communication among peer candidates to exchange
chunk bitmap and find a chunk through PPSP signaling protocol.
4. Peer Scheduling where to get the chunk and do cache replacement
e.g., BT like, rarest first .This action is done by peer itself
and doesn't include interaction with other peers or network. So
it's beyond the scope of PPSP.
5. Chunk transmission among peers (including CDN transmission) through
PPSP transmission protocol: There are two levels of work in this
step. One level is to deploy TCP/UDP/RTP as the basic transmission
protocol. Generally UDP is used in practice to reduce the
transmission overhead. An open question exists that if RTP can be
used here. The other level is what kind of rules the peers take in
transmission. These rules are as important as the basic
transmission protocols in one to many communication, because the
basic task there is to get the data from different source as soon
as possible or distribute its data with the lowest cost in a scale.
It makes p2p streaming transmission different from pure TCP/UDP/RTP
level work. The rules vary much according to different requirements.
For instance, a peer can transmit a chunk by maximizing download
rate or minimizing transmission overhead according the network
conditions. These rule leads to different peer's actions, e.g., a
peer can send a request for the same content to multiple neighbors
simultaneously, to ensure it gets the content in time; or request
for different content from multiple neighbors simultaneously; when
a request times out, it is redirected to a different neighbor; or
work with one neighbor at a time; only when that neighbor times out,
try to connect to a different neighbor. Obviously it creates many
critical parameters in transmission, e.g., response time, the
number of simultaneous neighbors to send requests. To tune these
parameters network monitoring is required to regulate in the
protocols.
6. Peer Re-assembling the chunk in its cache to finish playback of the
programs.
7. Peer Reporting to Tracker what chunks it has periodically. This is
publishing process where PPSP signaling protocol can be used.
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+---------+
| Tracker |
+---------+
^ |
| | +----------+
1,7| | 2 | Peer 4 |
| | +----------+
| V
+---------+ +----------+
4,6| Peer 1 |<----3---->| Peer 2 |
+---------+<----5---- +----------+
^ ^
5| |3
| |
| V
+---------+
| Peer 3 |
+---------+
Figure 2 PPSP Process
4. Comparison with related protocols
4.1. P2PSIP
P2PSIP deals with resource location in one to one commutation. The
iterative and recursive routing process inP2PSIP is shown in Fig3,
which is different from PPSP. That is, the data stored in P2P SIP is
user profile data and user knows exactly what the data is (e.g., the
location of Alice@chinamobile.com) using RELOAD to locates the data.
While in PPSP scenarios, there are many peers storing data pieces of
"Mr. and Ms. Smith" and the user doesn't know and needn't know the
belongings of the peers and he just know the metadata of the movie.
He must use a gossip protocol to communicate with other peers to get
the real data quickly.
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+------------------+
| Peer |
+------------------+
^ | ^ |
| | | |
1,2 | | 1' 3,4| |3'
| | | |
V V V V
+-----------+ +-----------+
| Peer |--2'->| Peer |
+-----------+ +-----------+
Fig3 P2PSIP process
The difference between P2PSIP and PPSP are as follows:
1) One to one communication VS One to many communication (End to End
communication VS data centric communication):
a) Because there are lot of peer candidates in PPSP, NAT
transversal is not as important as that in P2PSIP and public peers
can be found with higher probability;
2) PPSP includes transmission Protocol and P2PSIP doesn't involve that.
3) Different Search efficiency requirement:
a) PPSP requires retrieval real-time/para real-time data, iterative
and recursive routing is not suitable for low efficiency.
b) Node organizations are quite different. DHT doesn't fit for
peers in PPSP.
4) Different transmission quality requirements: P2PSIP doesn't require
Voice quality and PPSP need to ensure streaming quality. Therefore
in PPSP the following factors in peers must be considered:
a) Heterogeneous nodes;
b) Node Churn and Data Churn(the data update quicker than P2PSIP)
c) Topology-aware
5) Different applicable services: P2PSIP is suitable for VoIP and
PPSP is suitable for streaming, gaming and file sharing. It's too
comprehensive for P2PSIP (based on text) for non-session applications
like streaming. There are too many signaling interactions with much
overhead compared with P2PSIP which has only one interaction.
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Although P2PSIP doesn't fit for peer organization, it can be deployed
in PPSP environment to some extent. DHT can be used to organize
multiple channel servers in real-time streaming or multiple file
trackers in VoD. Because the search time of which channel or which
file the tracker stores accounts little in the whole searching
procedure, DHT can be used to query for peer list in case of thousand
of channels or million of files which are hard to use one tracker.
But it doesn't fit for quick search for real data among peers yet.
4.2. RTSP and related protocols
At first sight, the function of PPSP control protocol is similar to
traditional C/S style streaming control protocols RTSP, MMS or PNA.
But in fact RTSP MMS or PNA don't involve the problems PPSP has
because the end user requests the streaming from one assigned source
without needing real-time resource discoery, merge and
synchronization, which simplifies the problem. However it also
inherits the shortcomings of all client-server paradigms including
low scalability, high cost both for investment and maintenance as
well as the traffic pressure for the Internet equipments and single
point of failure.
5. Scenarios of Inter-working of PPSP
PPSP can be used not only within a single p2p application, but also
in the inter-working of different p2p applications.
5.1. Non CDN assistant inter-worked PPSP
In this case PPSP can be used by different P2P streaming applications
which can share resources and improve performance with better peers
from allied P2P streaming applications. P2P streaming applications
don't deploy CDN for streaming delivery. The interaction between
different PPSP applications is shown in Fig4. It involves steps
beyond basic PPSP process, e.g., trackers from different vendors
exchange their peer information.
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+----------------------------+
| PPSP APP 1 | PPSP App 2 |
+----------------------------+
| ^ ^ |
| | | |
| +---------------+ |
| PPSP Signaling Protocol |
+----------------------------+
| PPSP Trans Protocol |
+----------------------------+
| Transport Layer |
+----------------------------+
Fig4 Interaction between different P2P applications
5.2. CDN assistant inter-worked PPSP
In this case there is usually a CDN provider that may be run by an
ISP. It provides P2P streaming distribution services for different
PPSP applications. The storage and transmission bandwidth can be
saved in case of the same content transmission for different PPSP
applications. The cooperation between different PPSP providers can be
run both in PPSP transport level and PPSP signaling level as shown in
Fig5.
+----------------------------+
| PPSP APP 1 | PPSP App 2 |
+----------------------------+
| ^ ^ |
| | | |
| +---------------+ |
| PPSP| Signaling Prot|ocol |
+----------------------------+
| +---------------+ |
| PPSP Trans Protocol |
+----------------------------+
| Transport Layer |
+----------------------------+
Fig5 Interaction between different P2P applications
6. Security Considerations
PPSP doesn't relate to security mechanisms currently, but we don't
exclude security mechanisms in PPSP.
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7. Acknowledgments
We have to acknowledge many people. For the record: N.Zong, X.F.Jiang,
H.B.Song. Pick.Li from Huawei.
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References
[1] www.pplive.com
[2] www.ppstream.com
[3] www.uusee.com
[4] www.pando.com
[5] www.youtube.com
[6] www.tudou.com
[7] www.ietf.org/rfc/rfc2326.tx
[8] en.wikipedia.org/wiki/Microsoft_Media_Services
[9] all-streaming-media.com/streaming-media-faq/faq-pnm-
protocol.htm
Author's Addresses
Yunfei Zhang
China Mobile
Phone: 86 13601032119
Email: zhangyunfei@chinamobile.com
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