PPSP Y. Zhang
Internet Draft China Mobile
N.Zong
HuaweiTech
G.Camarillo
Ericsson
R.Yang
Yale University
V. Pascual
Acme Packet
Intended status: Informational February 27, 2012
Expires: August 2012
Problem Statement and Requirements of Peer-to-Peer Streaming
Protocol (PPSP)
draft-ietf-ppsp-problem-statement-08.txt
Abstract
Peer-to-Peer (P2P for short) streaming systems show more and more
popularity in current Internet with proprietary protocols. This
document identifies problems of the proprietary protocols, proposes a
Peer to Peer Streaming Protocol (PPSP) including tracker and peer
signaling components, and discusses the scope, uses cases and
requirements of PPSP.
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Status of this Memo
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Table of Contents
1. Introduction ................................................ 4
2. Terminology and concepts .................................... 5
3. Problem statement ........................................... 7
3.1. Traffic issue and difficulties for ISPs in deploying P2P
caches ...................................................... 7
3.2. Efficiency issue and difficulties in building open streaming
delivery infrastructure ..................................... 7
3.3. Extended applicability issue and difficulties in mobile and
wireless environment......................................... 8
4. PPSP: Standard peer to peer streaming protocols ............. 10
5. Use cases of PPSP ........................................... 12
5.1. Worldwide provision of live/VoD streaming .............. 12
5.2. PPSP supporting cross-screen streaming in heterogeneous
environment ................................................. 14
5.3. Cache service supporting P2P streaming ................. 15
6. Security Considerations ..................................... 17
7. Requirements of PPSP ........................................ 18
7.1. Basic Requirements ..................................... 18
7.2. PPSP Tracker Protocol Requirements ..................... 19
7.3. PPSP Peer Protocol Requirements ........................ 21
7.4. Security Requirements .................................. 22
8. IANA Considerations ......................................... 24
9. Acknowledgments ............................................. 25
10. Informative References ..................................... 26
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1. Introduction
Streaming traffic is among the largest and fastest growing traffic on
the Internet [Cisco], where peer-to-peer (P2P) streaming contribute a
lot. With the advantage of high scalability and fault tolerance
against single point of failure, P2P streaming applications are able
to distribute large-scale, live and VoD streaming programs to
millions of audience with only a handful of servers.
What's more, along with the new players like CDN providers joining in
the effort of using P2P technologies in distributing their serving
streaming content, there are more and more various players in P2P
streaming ecosystem.
Given the increasing integration of P2P streaming into the global
content delivery infrastructure, the lack of an open, standard P2P
streaming signaling protocol suite becomes a major missing component
in the protocol stack. Almost all of existing systems use their
proprietary protocols. Multiple, similar but proprietary protocols
result in repetitious development efforts for new systems, and the
lock-in effects lead to substantial difficulties in their integration
with other players like CDN. For example, in the enhancement of
existing caches and CDN systems to support P2P streaming, proprietary
protocols may increase the complexity of the interaction with
different P2P streaming applications.
In this document we propose an open P2P Streaming Protocol, which is
defined as PPSP, to standardize signaling operations on two important
components, peer and tracker in P2P streaming systems for information
exchange. Note that using PPSP would not hurt current P2P streaming
vendors: Firstly, the openness of signaling interaction would make it
easy to integrate them with some infrastructural components like
ISP's caches or CDNs for better user experience, say, smaller delay
of the play. Secondly, different applications could use the same PPSP
for signaling, but implement system specific mechanisms on top of
that. That is to say, different P2P streaming systems compete on "on
top" things, like scheduling algorithms, which is independent of the
proposed protocols.
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2. Terminology and concepts
Chunk: A chunk is a basic unit of data block organized in P2P
streaming for storage, scheduling, advertisement and exchange among
peers [VoD]. A chunk size varies from several KB to several MB in
different systems. In case of MB size chunk scenario, a sub-chunk
structure named piece is often defined to fit in a single transmitted
packet. A streaming system may use different granularities for
different usage, e.g., using chunks during data exchange, and using a
larger unit such as a set of chunks during advertisement.
Content Distribution Network (CDN): A CDN node refers to a network
entity that is deployed in the network (e.g., at the network edge or
data centers) to store content provided by the original servers, and
serves content to the clients located nearby topologically.
Client: A client refers to the service requester in client/server
computing paradigm. In this draft a client refers to a participant in
a P2P streaming system that only receives streaming content. In some
cases the node is not eligible to be a peer without enough computing
and storage capability is acting as a client. It can be viewed as a
specific kind of peer.
Live streaming: It refers to a scenario where all clients receive
streaming content for the same ongoing event. It is desired that the
lags between the play points of the clients and that of the streaming
source be small.
P2P cache: A P2P cache refers to a network entity that caches P2P
traffic in the network, and either transparently or explicitly as a
peer distributes content to other peers.
Peer: A peer refers to a participant in a P2P streaming system that
not only receives streaming content, but also stores and uploads
streaming content to other participants.
PPSP: The abbreviation of Peer-to-Peer Streaming Protocols. PPSP
refer to the key signaling protocols among various P2P streaming
system components, including the tracker and the peer.
Swarm: A swarm refers to a group of peers who exchange data to
distribute chunks of the same content (e.g. video/audio program,
digital file, etc) at a given time.
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Tracker: A tracker refers to a directory server which maintains a
list of peers which participate in a specific video channel or in the
distribution of a streaming file, and answers queries from peers for
peer lists. The tracker is a logical component which can be
centralized or distributed.
Video-on-demand (VoD): It refers to a scenario where different
clients may watch different parts of the same recorded media with
downloaded content.
Peer list: A list of peers which are in a same swarm maintained by
the tracker. A peer can fetch the peer list of a swarm from either
tracker or other peers to know which peers have the required
streaming content.
Peer ID: An identifier of a peer such that other peers or tracker can
refer the ID for the peer.
Swarm ID: An identifier of a swarm containing a group of peers
sharing a same streaming content.
Chunk ID: An identifier of a chunk in a streaming content.
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3. Problem statement
The problems imposed by proprietary protocols for P2P streaming
applications are listed as follows.
3.1. Traffic issue and difficulties for ISPs in deploying P2P caches
Facing with many P2P streaming applications, ISPs are witnessing a
big traffic tension on their backbone and inter-networking points.P2P
caches are used for ISPs to reduce the traffic by dynamically storing
the frequently accessed streaming content (in chunk or in file
granularity).
However, unlike the Web where all kinds of the infrastructure devices
have been already equipped with standard HTTP protocol, cache systems
have to build a matching library to identify different P2P streaming
protocols firstly. Multiple ever changing proprietary protocols
require the cache system updating its matching library constantly.
This increases the operator's cost dramatically.
With PPSP, P2P caches needn't learn new proprietary protocols any
longer, which would reduce the ISP workload much.
3.2. Efficiency issue and difficulties in building open streaming
delivery infrastructure
Another problem for P2P streaming applications is the efficiency
issue. P2P streaming is often criticized by longer delays (e.g.,
startup delay, seek delay and channel switch delay). Hybrid CDN/P2P
is a good means to solve this problem for operators [Hybrid CDN P2P].
In such design, CDN takes two roles: one is for media streaming
server and the other is for P2P tracker. Consider a CDN vendor
serving for various P2P streaming, similar to the P2P cache issue in
Section 3.1, proprietary P2P streaming protocols introduce
interaction complexity between the peer and tracker, and increase
deployment cost of CDN nodes.
With PPSP, CDN nodes acting as the P2P tracker can be designed to
inter-operate with other devices by only standard protocols, reducing
the case by case negotiation. On the other side, the interface
between edged CDN nodes and user peers could be either via something
like traditional HTTP, or via PPSP peer protocol.
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3.3. Extended applicability issue and difficulties in mobile and
wireless environment
Mobility and wireless are becoming increasingly important in today's
Internet, where streaming service is a major usage. In Korea the
number of mobile TV subscriber has reached seventeen million,
accounting for one third of the mobile subscribers. There are
multiple prior studies exploring P2P streaming in mobile and wireless
networks [Mobile Streaming1] [Mobile Streaming2].
However it's difficult to copy current P2P streaming protocols (even
we suppose we can re-use the proprietary ones) in mobile and wireless
networks. Current protocols are designed mainly for fixed Internet.
Although smart handsets are more eligible to be peers with much
higher bandwidth and CPU frequency, larger storage and memory than
before, peer selection becomes more challenging which needs more
information to exchange during the tracker/peer and peer/peer
communications:
First, the connections are unsteady, lower rate, and costly in terms
of energy consumption and transmission (esp. in uplink). The trackers
and peers may need more information like packet loss rate, peer
battery status and processing capability for peer selection.
Second, current practices often use a "bitmap" message to exchange
chunk availability among peers/trackers. The message is often of some
kilobytes size and exchanged relatively frequently, say, some seconds.
In a scarce bandwidth resource environment, a reasonable optimization
is to reduce the message size, which may require alternative methods
for expressing and distributing bitmap information.
Third, when a peer is moving and if the IP address changes, the on-
going connection and transmission between peers may be affected. Such
information should be reported in time.
Fourth, for a resource constraint peer like mobile handsets or set-
top boxes (STB), the limited resource and the requirements for
installing various applications form big conflicts. On one side, the
limited CPU, storage and memory often limit the total number of
concurrent threads and processes. One the other side, the proprietary
protocols require the user to install many different applications for
different usage, for instance, some applications have rich resources
on TV series or movies while others may offer rich broadcasting
sports program. What's worse, for many P2P applications, even they
are not used by the users right now, the background program may be
invoked to facilitate other peers for free data delivery assistance.
That is to say, there will be multiple background programs running at
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the same time. But it may be difficult to invoke multiple programs in
such a resource constraint peer. PPSP should investigate these
factors and help to reduce the resource consumption in a converged
network.
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4. PPSP: Standard peer to peer streaming protocols
The objective of the PPSP working group is to design a unified peer-
to-peer streaming protocol (PPSP) to address the problems discussed
in the preceding sections.
There are basically two kinds of P2P streaming systems, pull-based
and push-based.
In pull-based P2P streaming systems, a centralized tracker or
distributed trackers maintains information about which peers are in
which swarms and answers the peers' query on such information with a
peer-list. After receiving the message, the peer can connect with the
candidates in a swarm, exchange its content availability in its
memory or storage (depending on it is real-time or VoD streaming)
with other peers and then retrieve the wanted streaming data. The
swarm is a mesh topology. Most of the current practices belong to
this genre. The advantages of pull-based mode are its robustness to
the peer churn and acceptable latency for a smooth play. Most
commercial systems either live streaming and VoD use this mode.
In push-based P2P streaming systems, there is a head node maintaining
the topology, e.g., a tree. The head node acts similar to a tracker.
The peers in this topology share the same interest on content. The
signaling and data distribution are both based on this topology. For
one program or video file, the peer queries the head node by offline
or pre-set head node address information for its location to join and
the head node replies with a peer-list(potentially in a recommended
order). After receiving this peer-list, the peer can connect with the
candidates for being a node in certain place of the topology and
receive the data along this topology without the need of exchanging
content availability with its siblings, as done in pull-based mode.
In this sense the head node is acting as the tracker in the pull-
based mode. The push mode has the advantages of lower latency but the
topology is fragile to the peer churn and is hard to deal with the
VoD scenario. This makes it less robust in practical running. Few
commercially deployed systems use this mode.
PPSP is targeted to standardize signaling protocols for tracker-based
architectures feasible for both modes above that support either live
or offline streaming.
The PPSP design includes a protocol for signaling between trackers
and peers (the PPSP "tracker protocol") and a signaling protocol for
communication among the peers (the PPSP "peer protocol") as shown in
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Figure 1.The two protocols enable peers to receive streaming data
within the time constraints required by specific content items. The
tracker protocol handles the initial and periodic exchange of meta
information between trackers and peers, such as peer-list and content
information. The peer protocol controls the advertising and exchange
of media data between the peers.
Note that in the pull mode, both tracker protocol and peer protocol
can be used; while in the push mode, only tracker protocol is used.
+------------------------------------------------+
| |
| +--------------------------------+ |
| | Tracker(Head Node) | |
| +--------------------------------+ |
| | ^ ^ |
|Tracker | | Tracker |Tracker |
|Protocol| | Procotol |Protocol |
| | | | |
| V | | |
| +---------+ Peer +---------+ |
| | Peer |<----------->| Peer | |
| +---------+ Protocol +---------+ |
| | ^ |
| | |Peer |
| | |Protocol |
| V | |
| +---------------+ |
| | Peer | |
| +---------------+ |
| |
| |
+------------------------------------------------+
Figure 1 PPSP System Architecture
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5. Use cases of PPSP
5.1. Worldwide provision of live/VoD streaming
The content provider can easily expand the broadcasting/VoD scale to
utilize the cooperative content providers' distribution networks or
third party CDN networks with PPSP.
Figure 2 shows the case that provider A broadcasts the program with
the help of provider B and C for a wider coverage. Without PPSP, when
users in B or C's domain (outside A's main serving zone) requests A's
programs, the returned peer-list may include few local peers, which
may decrease the user viewing experience. With PPSP more local
resources from cooperative vendors may be utilized. The content
providers often deploy in-network peers called super-nodes (SN for
short) with better stability and higher storage and bandwidth for
better QoS. With tracker protocol, vendor A's tracker can returns
user request with vendor B and vendor C's SNs in the peer-list. Users
in B and C's domain can exchange data (availability) with these SNs
using peer protocol for better QoS. In this way vendor B and vendor
C's SNs resources are shared with vendor A and vendor A expands its
serving scale with acceptable QoS.
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+-------------------------------------------------------------------+
| |
| +------------------+ |
| +------------>| A's Tracker |<----------+ |
| | +------------------+ | |
| Tracker| ^ ^ | |
| Protocol| Tracker| |Tracker |Tracker |
| | Protocol| |Protocol |Protocol |
| | | | | |
| | | | | |
| v v v v |
| +------+ Peer +------+ +------+ +------+ |
| | B's |<------->| B's | | C's | | C's | |
| | SN1 |Protocol | SN2 | | SN1 | | SN2 | |
| +------+ +------+ +------+ +------+ |
| ^ ^ ^ ^ |
| | | | | |
| | | Peer Protocol Peer Protocol| | |
| Peer | +-------------+ +--------------+ |Peer |
| Procotol| | | |protocol|
| | | | | |
| | | | | |
| | | | | |
| v v v v |
| +------+ Peer +------+ +---------+ Peer +---------+ |
| | A's |<------> | B's | |A's |<------> |C's | |
| | User1|Protocol | User2| | User1 |Protocol | User2 | |
| +------+ +------+ +---------+ +---------+ |
| |
+-------------------------------------------------------------------+
Figure 2 Cooperative Vendors Interaction
Figure 3 is similar to Figure 2 except that the intermediate SNs are
replaced by 3rd party CDN surrogates with PPSP. The P2P streaming
vendors A and B can rent CDN surrogates to provide higher QoS for VIP
users. The CDN nodes talk with the different vendors (including the
peers inside) with the uniform protocols. For users who use browser
equipped with HTTP, scalable streaming is also achieved. The internal
interaction of CDN nodes can be executed by either existing protocol
or peer protocol. The latter is used when building a new CDN system
supporting streaming applications with lower cost.
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+-------------------------------------------------------------------+
| |
| +-------------+ +--------------+ |
| +----->| A's Tracker | | B's Tracker |<---+ |
| | +-------------+ +--------------+ | |
| Tracker| ^ ^ ^ ^ | |
| Protocol| Tracker| |Tracker | |Tracker |Tracker |
| | Protocol| |Protocol| |Protocol |Protocol|
| | | | | | | |
| | | | | | | |
| v v | | v v |
| +------+ Peer +------+| | +------+Internal+------+ |
| | CDN |<------>| CDN || | | CDN |<-----> | CDN | |
| | Node1|Protocol| Node2|| | | Node3|Protocol| Node4| |
| +------+ +------+| | +------+ +------+ |
| ^ ^ | | ^ ^ |
| | | | | | | |
| | | Peer Protocol | | HTTP | | |
| Peer | +-------------+ | | +------+ | Peer |
| Procotol| | | | | Protocol |protocol|
| | | +-+ | | | |
| | | | | | | |
| | | | | | | |
| v v v v v v |
| +------+ Peer +------+ +---------+ Peer +---------+ |
| | A's |<------> | A's | |B's |<------> |B's | |
| | User1|Protocol | User2| | User3 |Protocol | User4 | |
| +------+ +------+ +---------+ +---------+ |
| |
+-------------------------------------------------------------------+
Figure 3 CDN Supporting P2P Streaming
5.2. PPSP supporting cross-screen streaming in heterogeneous environment
In this scenario PC, STB/TV and mobile terminals from both fixed
network and mobile/wireless network share the streaming content. With
PPSP, peers can identify the types of access networks, average load,
peer abilities and get to know what content other peers have
(potentially with the conversion of the content availability
expression in different networks) even in different network
conditions as shown in Figure 4.
These information will play an important role on selecting suitable
peers, e.g., a PC or STB is more likely to be selected to provide
stable content for mobile nodes; a mobile peer within a high-load
base station is unlikely to be selected, which may lead to higher
load on the base station.
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+-------------------------------------------------------------------+
| |
| Tracker Protocol +---------+ Tracker Protocol |
| +-------------> | Tracker |<------------------+ |
| | +---------+ | |
| | ^ | |
| | | | |
| | | | |
| V | V |
| +------+ | +------------+ |
| | STB | Tracker Protocol |Mobile Phone| |
| +------+ | +------------+ |
| ^ | ^ |
| | | | |
| | | | |
| | V | |
| |Peer Protocol +---------+ Peer Protocol | |
| +-------------> | PC |<------------------+ |
| +---------+ |
| |
+-------------------------------------------------------------------+
Figure 4 Heterogeneous P2P Streaming Interaction with PPSP
5.3. Cache service supporting P2P streaming
In Figure 5, when peers request the P2P streaming data, the cache
nodes intercept the requests and ask for the frequently visited
content (or part of) on behalf of the user peers. To do this, it
requests peer-list to the tracker and the tracker replies with
(outward) peers in the peer-list. After the cache nodes exchange data
with these peers, it can report what it cache to the tracker like a
normal peer and serve other requesting peers inside. This operation
greatly decreases the inter-network traffic and increase user
experience in P2P streaming services for an ISP.
The cache nodes needn't update their library when new applications
supporting PPSP are introduced, which enable the cache nodes spend
less cost to support more applications.
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+----------------------------------------------------------------+
| |
| 0:Tracker Protocol +---------+ |
| +----------------> | Tracker | |
| | +---------+ |
| | ^ |
| | | |
| | 2: | Tracker Protocol |
| | | |
| | | |
| | +---------|-------------------------------------|
| | | V |
| | | +---------+ |
| | +----------|---> | Cache |<-------------------+ |
| | | | +---------+ 1,4: Tracker/Peer| |
| | |3: Peer | Protocol | |
| | | Protocol | | |
| | | | | |
| | | | | |
| V V | V |
| +-----------+ | ISP Domain +------------+ |
| | Outward | | | Inside | |
| | Peer | | | Peer | |
| +-----------+ | +------------+ |
+----------------------------------------------------------------+
Figure 5 Cache Service Supporting Streaming with PPSP
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6. Security Considerations
This document discusses the problem statement around Peer-to-Peer
streaming protocols without specifying the protocols. The protocol
specification is deferred to other documents under development in the
PPSP working group. However we believe it is important for the reader
to understand areas of security caused by the P2P nature of the
proposed solution. The main issue is the usage of un-trusted entities
(peers) for service provisioning.
Malicious peers may, for example:
- Issue denial of service (DOS) attacks to the trackers by sending
large amount of requests with the tracker protocol;
- Issue fake information on behalf of other peers;
- Issue fake information about available content;
- Issue fake information about chunk availability;
Malicious peers/trackers may, for example:
- Issue reply instead of the regular tracker (man in the middle
attack).
The PPSP protocol specifications, e.g., the tracker protocol and the
peer protocol, will document the expected threats and how they will
be mitigated for each protocol, but also considerations on threats
and mitigations when combining both protocols in an application. This
will include privacy of the users, protection of the content
distribution, but not protection of the content by Digital Rights
Management (DRM).
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7. Requirements of PPSP
This section enumerates the requirements for the PPSP, which should
be considered when designing PPSP.
7.1. Basic Requirements
PPSP.REQ-1: The tracker and the peer protocols SHOULD be as similar
as possible, in terms of design, message formats and flows.
It is desirable that the peer protocol would be an extension to the
tracker protocol by adding a few message types, or vice versa.
PPSP.REQ-2: The tracker protocol and the peer protocol SHOULD enable
peers to receive streaming content within the required time
constraints, i.e., fulfill streaming feature.
PPSP.REQ-3: Each peer MUST have a unique ID (i.e. peer ID) in a swarm.
It's a basic requirement for a peer to be uniquely identified in a
swarm that other peers or tracker can refer to the peer by ID.
PPSP.REQ-4: The streaming content MUST be uniquely identified by a
swarm ID.
A swarm refers to a group of peers sharing the same streaming content.
A swarm ID uniquely identifies a swarm. The swarm ID can be used in
two cases: 1) a peer requests the tracker for the peer list indexed
by a swarm ID; 2) a peer tells the tracker about the swarms it
belongs to.
PPSP.REQ-5: The streaming content MUST allow to be partitioned into
chunks.
A key characteristic of P2P streaming system is allowing the data
fetching from different peers concurrently. Therefore, the whole
streaming content must allow to be partitioned into small pieces or
chunks for transmission between peers.
PPSP.REQ-6: Each chunk MUST have a unique ID (i.e. chunk ID) in the
swarm.
Each chunk must have a unique ID in the swarm such as the peer can
understand which chunks are stored in which peers and which chunks
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are requested by other peers. An example for generating the chunk ID
is the buffer map approach [I-D.ietf-ppsp-survey].
PPSP.REQ-7: The tracker protocol and peer protocol are recommended to
be carried over TCP (or UDP, when delivery requirements cannot be met
by TCP).
PPSP.REQ-8: The tracker and peer protocol together MUST facilitate
acceptable QoS (e.g. low startup delay, low channel/content switching
time and minimal end-to-end delay) for both on-demand and live
streaming, even for very popular content. The tracker and peer
protocol do not include the algorithm required for scalable
streaming. However, the tracker and peer protocol SHALL NOT restrict
or place limits on any such algorithm.
There are basic QoS requirements for streaming system. Setup time to
receive a new streaming channel or to switch between channels should
be reasonable small. End to end delay (time between content
generation, e.g. camera and content consumption, e.g. user side
monitor) will become critical in case of live streaming. Especially
in provisioning of sports events, end to end delay of 1 minute and
more are not acceptable.
For instance, the tracker and peer protocols can support carrying QoS
related parameters (e.g. video quality, delay requirements) together
with the priorities of these parameters, and QoS situation (e.g.,
performance, available uplink bandwidth) of content providing peers.
There are also some other possible mechanisms, e.g. addition of super
peers, in-network storage, request of alternative peer addresses, and
the usage of QoS information for an advanced peer selection.
7.2. PPSP Tracker Protocol Requirements
The tracker protocol defines how the peers report and request
information to/from the tracker and how the tracker replies to the
requests. The tracker discovery and the possible communication
between trackers are out of the scope of tracker protocol.
PPSP.TP.REQ-1: The tracker MUST implement the tracker protocol for
receiving queries and periodical peer status reports/updates from the
peers and for sending the corresponding replies.
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PPSP.TP.REQ-2: The peer MUST implement the tracker protocol for
sending queries and periodical peer status reports/updates to the
tracker and receiving the corresponding replies.
PPSP.TP.REQ-3: The tracker request message MUST allow the requesting
peer to solicit the peer list from the tracker with respect to a
specific swarm ID.
The tracker request message may also include the requesting peer's
preference parameter, e.g. preferred number of peers in the peer
list, or preferred downloading bandwidth. The track will then be
able to select an appropriate set of peers for the requesting peer
according to the preference.
PPSP.TP.REQ-4: The tracker reply message MUST allow the tracker to
offer the peer list to the requesting peer with respect of a specific
swarm ID.
PPSP.TP.REQ-5: The tracker SHOULD support generating the peer list
with the help of traffic optimization services, e.g. ALTO [I-D.ietf-
alto-protocol].
PPSP.TP.REQ-6: The peer status report/update MUST have the ability to
inform the tracker about the peer's activity in the swarm.
PPSP.TP.REQ-7: The chunk availability information of the peer SHOULD
be reported to tracker when tracker needs such information to steer
peer selection. The chunk information MUST at least contain the
chunk ID.
PPSP.TP.REQ-8: The chunk availability information between peer and
tracker MUST be as expressed as compactly as possible.
The peers may report CHUNK AVAILABILTY DIGEST information (i.e.,
compact expression of chunk availability) to the tracker when
possible to decrease the bandwidth consumption for messages in
bandwidth constraint environment like mobile network. For example,
if a peer has a bitmap like 111111...1(100 continuous 1)xxx..., the
100 continuous "1" can be expressed by one byte with seven bits
representing 100 and one bit representing "1".In this example, 100-
8=92 bits are saved. Considering the frequency of exchange of CHUNK
AVAILBILITY and the fact that many bitmaps have quite a long length
of continuous "1" or "0", such compression makes sense.
PPSP.TP.REQ-9: The status of the peer SHOULD be reported to the
tracker when tracker needs such information to steer peer selection.
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For example, peer status can be online time, physical link status
including DSL/WIFI/etc, battery status, processing capability, and
other capabilities of the peer. Therefore, the tracker is able to
select better candidate peers for streaming.
7.3. PPSP Peer Protocol Requirements
The peer protocol defines how the peers advertise streaming content
availability and exchange status with each other. The peer protocol
also defines the requests and responses of the chunks among the
peers. The first task for this WG will be to decide which signaling
and media transfer protocols will be used. The WG will consider
existing protocols and, if needed, identify potential extensions to
these protocols.
PPSP.PP.REQ-1: The streaming content availability request message
MUST allow the peer to solicit the chunk information from other peers
in the peer list. The chunk information MUST at least contain the
chunk ID. This chunk availability information MUST NOT be passed on
to other peer, unless validated (e.g. prevent hearsay and DoS).
PPSP.PP.REQ-2: The streaming content availability reply message MUST
allow the peer to offer the information of the chunks in its content
buffer. The chunk information MUST at least contain the chunk ID.
PPSP.PP.REQ-3: The streaming content availability request message
SHOULD allow the peer to solicit an additional list of peers to that
received from the tracker - with the same swarm ID. The reply
message MUST contain swarm-membership information of the peers that
have explicitly indicated they are part of the swarm, verifiable by
the receiver. This additional list of peers MUST only contain peers
which have been checked to be valid and online recently (e.g. prevent
hearsay and DoS).
It is possible that a peer may need additional peers for certain
streaming content. Therefore, it is allowed that the peer
communicates with the peers in the current peer list to obtain an
additional list of peers in the same swarm.
PPSP.PP.REQ-4: Streaming content availability update message among
the peers MUST be supported by peer protocol. In the push based
model, where peers advocate their own chunk availability proactively,
the content availability request message described in PP.REQ-1 is not
needed. The peer protocol MUST implement either pull-based, push-
based or both.
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Due to the dynamic change of the buffered streaming content in each
peer and the frequent join/leave of peers in the swarm, the streaming
content availability among a peer's neighbors (i.e. the peers known
to a peer by getting the peer lists from either tracker or peers)
always changes and thus requires being updated on time. This update
should be done at least on demand. For example, when a peer requires
finding more peers with certain chunks, it sends a message to some
other peers in the swarm for streaming content availability update.
Alternatively, each peer in the swarm can advertise its streaming
content availability to some other peers periodically. However, the
detailed mechanisms for this update such as how far to spread such
update message, how often to send this update message, etc should
leave to peer algorithms, rather than protocol concerns.
PPSP.PP.REQ-5: The chunk availability information between peers MUST
be as expressed as compactly as possible.
In PP.REQ-1/2/4, the peers may exchange CHUNK AVAILABILTY DIGEST
information (i.e. compact expression of chunk availability) to with
other peers when possible to decrease the bandwidth consumption for
messages in bandwidth constraint environment like mobile network.
PPSP.PP.REQ-6: The peer status report/update SHOULD be advertised
among the peers to reflect the status of the peer.
Peer status information should be advertised among the peers via the
peer status report/update message. For example, peer status can be
online time, physical link status including DSL/WIFI/etc, battery
status, processing capability, and other capabilities of the peer.
With this information, a peer can select more appropriate peers for
streaming.
PPSP.PP.REQ-7: The peers MUST implement the peer protocol for chunk
data (not availability information) requests and responses among the
peers before the streaming content is transmitted.
7.4. Security Requirements
PPSP.SEC.REQ-1: PPSP MUST support closed swarms, where the peers are
authenticated.
This ensures that only the authenticated users can access the
original media in the P2P streaming system. This can be achieved by
security mechanisms such as user authentication and/or key management
scheme.
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PPSP.SEC.REQ-2: Confidentiality of the streaming content in PPSP
SHOULD be supported and the corresponding key management scheme
SHOULD scale well in P2P streaming system.
PPSP.SEC.REQ-3: PPSP MUST provide an option to encrypt the data
exchange among the PPSP entities.
PPSP.SEC.REQ-4: PPSP MUST have mechanisms to limit potential damage
caused by malfunctioning and badly behaving peers in the P2P
streaming system.
Such an attack will degrade the quality of the rendered media at the
receiver. For example, in a P2P live video streaming system a
polluter can introduce corrupted chunks. Each receiver integrates
into its playback stream the polluted chunks it receives from its
other neighbors. Since the peers forwards chunks to other peers, the
polluted content can potentially spread through much of the P2P
streaming network.
PPSP.SEC.REQ-5: PPSP SHOULD support identifying badly behaving peers,
and exclude or reject them from the P2P streaming system.
PPSP.SEC.REQ-6: PPSP MUST prevent peers from DoS attacks which will
exhaust the P2P streaming system's available resource.
Given the prevalence of DoS attacks in the Internet, it is important
to realize that a similar threat could exist in a large-scale
streaming system where attackers are capable of consuming a lot of
resources with just a small amount of effort.
PPSP.SEC.REQ-7: PPSP SHOULD be robust, i.e., when centralized tracker
fails the P2P streaming system SHOULD still work by supporting
distributed trackers.
PPSP.SEC.REQ-8: Existing P2P security mechanisms SHOULD be re-used as
much as possible in PPSP, to avoid developing new security
mechanisms.
PPSP.SEC.REQ-9: Integrity of the streaming content in PPSP MUST be
supported to provide a peer with the possibility to identify
inauthentic media content (undesirable modified by other entities
rather than its genuine source). The corresponding checksum
distribution and verification scheme SHOULD scale well in P2P
streaming system and be robust against distrustful trackers/peers.
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8. IANA Considerations
This document has no actions for IANA.
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9. Acknowledgments
Thank you to J.Seng for contribution to many sections of this draft.
Thank you to C. Williams and L. Xiao for contributions to PPSP
requirements section.
We would like to acknowledge the following people who provided review,
feedback and suggestions to this document: M. Stiemerling; C. Schmidt;
D. Bryan; E. Marocco; V. Gurbani; R. Even; H. Zhang; V. Pasual; D.
Zhang; J. Lei; Y.Gu; H.Song; X.Jiang; J.Seedorf; D.Saumitra; A.Rahman;
L.Deng; J.Pouwelse; A.Bakker and W.Eddy.
This document was prepared using 2-Word-v2.0.template.dot.
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10. Informative References
[Cisco] Cisco Visual Networking Index: Forecast and Methodology,
2009-2014,
http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/
ns827/white_paper_c11-481360_ns827_Networking_Solutions_White_Paper.html
[VoD] Yan Huang et al, Challenges,"Design and Analysis of a Large-
scale P2P-VoD System", Sigcomm08.
[Mobile Streaming1] Streaming to Mobile Users in a Peer-to-Peer
Network, Jeonghun Noh et al, MOBIMEDIA '09.
[Mobile Streaming2] J.Peltotaloet al.,"A real-time Peer-to-Peer
streaming system for mobile networking environment",in
Proceedings of the INFOCOM and Workshop on Mobile Video
Delivery (MoVID '09), April 2009.
[I-D.ietf-alto-protocol]Alimi, R., Penno, R., and Y. Yang, "ALTO
Protocol", draft-ietf-alto-protocol-10 (work in progress),
October 2011.
[Hybrid CDN P2P]D. Xu, S. Kulkarni, C. Rosenberg, and H. Chai,
"Analysis of a CDN-P2P hybrid architecture for cost-
effective streaming media distribution," Springer
Multimedia Systems, vol.11, no.4, pp.383-399, 2006.
[I-D.ietf-ppsp-survey]Gu, Y., Zong, N., Zhang, H., Zhang, Y., Lei, J.,
Camarillo, G., Liu, Y., Montuno, D., and X. Lei, "Survey
of P2P Streaming Applications", draft-ietf-ppsp-survey-02
(work in progress), July 2011.
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Authors' Addresses
Yunfei Zhang
China Mobile Communication Corporation
zhangyunfei@chinamobile.com
NingZong
Huawei Technologies Co., Ltd.
zongning@huawei.com
Gonzalo Camarillo
Ericsson
Gonzalo.Camarillo@ericsson.com
Richard Yang
Yale University
yry@cs.yale.edu
Victor Pascual
Acme packet
Anabel Segura 10, Madrid 28108, Spain
Vpascual@acmepacket.com
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