Problem Statement and Requirements of Peer-to-Peer Streaming Protocol (PPSP)
draft-ietf-ppsp-problem-statement-11
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 6972.
|
|
|---|---|---|---|
| Authors | Yunfei Zhang , Ning Zong | ||
| Last updated | 2012-12-13 (Latest revision 2012-10-19) | ||
| Replaces | draft-zhang-ppsp-problem-statement | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 6972 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date |
(None)
Needs a YES. |
||
| Responsible AD | Martin Stiemerling | ||
| IESG note | |||
| Send notices to | ppsp-chairs@tools.ietf.org, draft-ietf-ppsp-problem-statement@tools.ietf.org |
draft-ietf-ppsp-problem-statement-11
PPSP Y. Zhang
Internet Draft China Mobile
N.Zong
Huawei Tech
Intended status: Informational October 20, 2012
Expires: April 2013
Problem Statement and Requirements of Peer-to-Peer Streaming
Protocol (PPSP)
draft-ietf-ppsp-problem-statement-11.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
the development of Peer to Peer Streaming Protocol (PPSP) including
the tracker and peer protocol, and discusses the scope, requirements
and use cases of PPSP.
zhang Expires April 20, 2013 [Page 1]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
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), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on April 20, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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.
zhang Expires April 20, 2013 [Page 2]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
Table of Contents
1. Introduction ................................................ 4
1.1. Background ............................................. 4
1.2. Requirements Language ................................... 4
2. Terminology and concepts ..................................... 5
3. Problem statement ........................................... 7
3.1. Heterogeneous P2P traffic and P2P cache deployment....... 7
3.2. QoS issue and CDN deployment ............................ 7
3.3. Extended applicability in mobile and wireless environment 7
4. Tasks of PPSP: Standard peer to peer streaming protocols...... 9
4.1. Tasks and design issues of Tracker protocol ............ 10
4.2. Tasks and design issues of Peer protocol ............... 11
5. Use cases of PPSP .......................................... 12
5.1. Worldwide provision of live/VoD streaming .............. 12
5.2. Enabling CDN for P2P VoD streaming ..................... 12
5.3. Cross-screen streaming ................................. 14
5.4. Cache service supporting P2P streaming ................. 15
5.5. Proxy service supporting P2P streaming ................. 16
5.5.1. Home Networking Scenario .......................... 16
5.5.2. Browser-based HTTP Streaming ...................... 17
6. Requirements of PPSP ........................................ 18
6.1. Basic Requirements ..................................... 18
6.2. PPSP Tracker Protocol Requirements ..................... 19
6.3. PPSP Peer Protocol Requirements ........................ 20
7. Security Considerations ..................................... 21
8. IANA Considerations ........................................ 23
9. Acknowledgments ............................................ 23
10. References ................................................ 24
10.1. Normative References .................................. 24
10.2. Informative References ................................ 24
zhang Expires April 20, 2013 [Page 3]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
1. Introduction
1.1. Background
Streaming traffic is among the largest and fastest growing traffic on
the Internet [Cisco], where peer-to-peer (P2P) streaming contributes
substantially. 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 video on demand (VoD)
streaming programs to a large audience with only a handful of servers.
What's more, along with the 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.
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 the development of an open P2P Streaming
Protocol, which is abbreviated as PPSP, to standardize signaling
operations in P2P streaming systems to solve the above problems.
1.2. Requirements Language
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 [RFC2119] and
indicate requirement levels for compliant implementations.
zhang Expires April 20, 2013 [Page 4]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
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 KBs to several MBs in
different systems. In case of MBs 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.
Chunk ID: The identifier of a chunk in a content stream.
Client: A client in general refers to the service requester in
client/server computing paradigm. In this draft a client also refers
to a participant in a P2P streaming system that only receives
streaming content. In some cases, a node not having enough computing
and storage capabilities will act as a client. Such node can be
viewed as a specific type of peer.
Content Distribution Network (CDN): A CDN is a collection of nodes
that are deployed, in general, at the network edge like Points of
Presence (POP) or Data Centers (DC) and that store content provided
by the original content servers. Typically, CDN nodes serve content
to the clients located nearby topologically.
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 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,
streams content to other peers.
Peer: A peer refers to a participant in a P2P streaming system that
not only receives streaming content, but also caches and streams
streaming content to other participants.
Peer 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 the
tracker or from other peers in order to know which peers have the
required streaming content.
zhang Expires April 20, 2013 [Page 5]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
Peer ID: The identifier of a peer such that other peers, or the
tracker, can refer to the peer by using its ID.
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.
Tracker: A tracker refers to a directory service that maintains a
list of peers participating in a specific audio/video channel or in
the distribution of a streaming file. Also, the tracker answers peer
list queries received from peers. 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 streaming with
downloaded content.
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.
Swarm ID: The identifier of a swarm containing a group of peers
sharing a common streaming content.
Super-node: A super-node is a special kind of peer deployed by ISPs.
This kind of peer is more stable with higher computing, storage and
bandwidth capabilities than normal peers.
zhang Expires April 20, 2013 [Page 6]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
3. Problem statement
The problems caused by proprietary protocols for P2P streaming
applications are listed as follows.
3.1. Heterogeneous P2P traffic and P2P cache deployment
ISPs are faced with different P2P streaming application introducing
substantial traffic into their infrastructure, including their
backbone and their exchange/interconnection points. P2P caches are
used by ISPs in order to locally store content and hence reduce the
P2P traffic. P2P caches usually operate at the chunk or file
granularity.
However, unlike web traffic that is represented by HTTP requests and
responses and therefore allows any caching device to be served (as
long as it supports HTTP), P2P traffic is originated by multiple P2P
applications which require the ISPs to deploy different type of
caches for the different types of P2P streams.
This increases both engineering and operational costs dramatically.
3.2. QoS issue and CDN deployment
P2P streaming is often criticized due to its worse QoS performance
compared to client/server streaming (e.g., longer startup delay,
longer seek delay and channel switch delay). Hybrid CDN/P2P is a good
approach in order to address this problem [Hybrid CDN P2P].
In order to form the hybrid P2P+CDN architecture, the CDN must be
aware of the specific P2P streaming protocol in the collaboration.
Similarly to what is described in section 3.1, proprietary P2P
protocols introduce complexity and deployment cost of CDN.
3.3. Extended applicability in mobile and wireless environment
Mobility and wireless are becoming increasingly important in today's
Internet, where streaming service is a major usage. It's reported
that the average volume of video traffic on mobile networks has risen
up to 50% in the early of 2012 [ByteMobile]. There are multiple prior
studies exploring P2P streaming in mobile and wireless networks
[Mobile Streaming1] [Mobile Streaming2].
zhang Expires April 20, 2013 [Page 7]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
However it's difficult to directly apply current P2P streaming
protocols (even assuming we can re-use some of the proprietary ones)
in mobile and wireless networks.
Following are some illustrative problems:
First, P2P streaming assumes a stable Internet connection in downlink
and uplink direction, with decent capacity and peers that can run for
hours. This isn't the typical setting for mobile terminals. Usually
the connections are unstable and expensive in terms of energy
consumption and transmission (especially in uplink direction). To
enable mobile/wireless P2P streaming feasible, trackers may need more
information on peers like packet loss rate, peer battery status and
processing capability during peer selection compared to fixed peers.
Unfortunately current protocols don't convey this kind of information.
Second, current practices often use a "bitmap" message in order to
exchange chunk availability. The message is of kilobytes in size and
exchanged frequently, e.g., an interval of several seconds or less.
In a mobile environment with scarce bandwidth, the message size may
need to be shortened or it may require more efficient methods for
expressing and distributing chunk availability information, which is
different from wire-line P2P streaming.
Third, for a resource constraint peer like mobile handsets or set-top
boxes (STB), there are severe contentions on limited resource when
using proprietary protocols. The terminal has to install different
streaming client software for different usages, e.g., some for movies
and others for sports. Each of these applications will compete for
the same set of resources even when it is sometimes running in
background mode. PPSP can alleviate this problem with the basic idea
that the "one common client software with PPSP and different
scheduling plug-ins" is better than "different client software
running at the same time" in memory and disk consumption.
zhang Expires April 20, 2013 [Page 8]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
4. Tasks of PPSP: Standard peer to peer streaming protocols
PPSP is targeted to standardize signaling protocols to solve the
above problems that support either live or VoD streaming.
PPSP targets tracker-based architectures, as well as tracker-less
architectures. In tracker-less architecture, the tracker
functionality is distributed in decentralized peers. In the following
part of this section, the tracker is a logic conception, which can be
implemented in a dedicated tracker server or in peers.
The PPSP design includes a signaling protocol between trackers and
peers (the PPSP "tracker protocol") and a signaling protocol among
the peers (the PPSP "peer protocol") as shown in Figure 1. The two
protocols enable peers to receive streaming content within the time
constraints.
PPSP design in general needs to solve the following challenges, e.g.,:
1) When joining a swarm, how does a peer know which peers it should
contact for content?
2) After knowing a set of peers, how does a peer contact with these
peers? in which manner?
3) How to choose peers with better service capabilities, and how to
collect such information from peers?
4) How to improve the efficiency of the communication, e.g. compact
on-the-wire message format and suitable underlying transport
mechanism (UDP or TCP)?
5) How to improve the robustness of the system using PPSP, e.g. when
the tracker fails? How to make the tracker protocol and the peer
protocol loose coupled?
zhang Expires April 20, 2013 [Page 9]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
+------------------------------------------------+
| |
| +--------------------------------+ |
| | Tracker | |
| +--------------------------------+ |
| | ^ ^ |
|Tracker | | Tracker |Tracker |
|Protocol| | Protocol |Protocol |
| | | | |
| V | | |
| +---------+ Peer +---------+ |
| | Peer |<----------->| Peer | |
| +---------+ Protocol +---------+ |
| | ^ |
| | |Peer |
| | |Protocol |
| V | |
| +---------------+ |
| | Peer | |
| +---------------+ |
| |
| |
+------------------------------------------------+
Figure 1 PPSP System Architecture
4.1. Tasks and design issues of Tracker protocol
The tracker protocol handles the initial and periodic exchange of
meta-information between trackers and peers, such as peer list and
content information.
Therefore tracker protocol is best modeled as a request/response
protocol between peers and trackers, and will carry information
needed for the selection of peers suitable for real-time/VoD
streaming.
Special tasks for the design of the tracker protocol are listed as
follows. This is a high-level task-list. The detailed requirements on
the design of the tracker protocol are explicated in section 6.
1) How should a peer be globally identified? This is related to the
peer ID definition, but irrelevant to how the peer ID is generated.
2) How to identify different peers, e.g. peers with public or private
IP address, peers behind or not behind NAT, peers with IPV4 or IPV6
addresses, peers with different property?
zhang Expires April 20, 2013 [Page 10]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
3) How to enable the tracker protocol light-weight, since a tracker
may need to server large amount of peers? This is related to the
encoding issue (e.g., Binary based or Text based) and keep-alive
mechanism.
4) How can the tracker be able to report optimized peer list to serve
a particular content. This is related to status statistic, with which
the tracker can be aware of peer status and content status.
PPSP tracker protocol will consider all these issues in the design
according to the requirements from both peer and tracker perspective
and also taking into consideration deployment and operation
perspectives.
4.2. Tasks and design issues of Peer protocol
The peer protocol controls the advertising and exchange of content
between the peers.
Therefore peer protocol is modeled as a gossip-like protocol with
periodic exchanges of neighbor and chunk availability information.
Special tasks for the design of the peer protocol are listed as
follows. This is a high-level task-list. The detailed requirements on
the design of the peer protocol are explicated in section 6.
1) How does the certain content be globally identified and verified?
Since the content can be retrieved from everywhere, how to ensure the
exchanged content between the peers authentic?
2) How to identify the chunk availability in the certain content?
This is related to the chunk addressing and chunk state maintenance.
Considering the large amount of chunks in the certain content, light-
weight expression is necessary.
3) How to ensure the peer protocol efficient? As we mentioned in
section 3, the chunk availability information exchange is quite
frequent. How to balance the information exchange size and amount is
a big challenge. What kind of encoding and underlying transport
mechanism (UDP or TCP) is used in the messages?
PPSP peer protocol will consider all the above issues in the design
according to the requirements from the peer perspective.
zhang Expires April 20, 2013 [Page 11]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
5. Use cases of PPSP
This section is not the to-do list for the WG, but for the
explanatory effect to show how PPSP could be used in practice.
5.1. Worldwide provision of live/VoD streaming
The content provider can increase live streaming coverage by
introducing PPSP in between different providers.
We suppose a scenario that there is only provider A (e.g,, in China)
providing the live streaming service in provider B(e.g., in USA) and
C(e.g., in Europe)'s coverage. Without PPSP, when a user (e.g., a
Chinese american) in USA requests the program to the tracker (which
is located in A's coverage), the tracker may generally return to the
user with a peer list including most of peers in China, because
generally most users are in China and there are only few users in USA.
This may affect the user experience.
But if we can use the PPSP tracker protocol to involve B and C in the
cooperative provision, as shown in Figure 2, even when the streaming
is not hot to attract many users in USA and Europe to view, the
tracker in A can optimally return the user with a peer list including
B's Super-nodes (SN for short) and C's SN to provide a better user
performance.
Furthermore User@B and User@C can exchange data (availability) with
these local SNs using the peer protocol.
5.2. Enabling CDN for P2P VoD streaming
Figure 3 shows the case of enabling CDN to support P2P VoD streaming
from different content providers by introducing PPSP inside CDN
overlays. It is similar to Figure 2 except that the intermediate SNs
are replaced by 3rd party CDN surrogates. The CDN nodes talk with the
different streaming systems (including trackers and peers) with the
same PPSP protocols.
Furthermore the interaction between the CDN nodes can be executed by
either existing (maybe proprietary) protocols or the PPSP peer
protocol. The peer protocol is useful for building new CDN systems
(e.g., operator CDN) supporting streaming in a low cost.
Note that for compatibility reason both HTTP streaming and P2P
streaming can be supported by CDN from the users' perspective.
zhang Expires April 20, 2013 [Page 12]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
+-------------------------------------------------------------------+
| |
| +------------------+ |
| +------------>| 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 |
| Protocol| | | |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
zhang Expires April 20, 2013 [Page 13]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
+-------------------------------------------------------------------+
| |
| +-------------+ +--------------+ |
| +----->| 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
5.3. Cross-screen streaming
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 even in
different networks (potentially with the conversion of the content
availability expression in different networks) as shown in Figure 4.
Such information will play an important role on selecting suitable
peers, e.g., a PC or STB is more likely to provide stable content and
a mobile peer within a high-load cell is unlikely to be selected,
which may otherwise lead to higher load on the base station.
zhang Expires April 20, 2013 [Page 14]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
+-------------------------------------------------------------------+
| |
| Tracker Protocol +---------+ Tracker Protocol |
| +-------------> | Tracker |<------------------+ |
| | +---------+ | |
| | ^ | |
| | | | |
| | | | |
| V | V |
| +------+ | +------------+ |
| | STB | Tracker Protocol |Mobile Phone| |
| +------+ | +------------+ |
| ^ | ^ |
| | | | |
| | | | |
| | V | |
| |Peer Protocol +---------+ Peer Protocol | |
| +-------------> | PC |<------------------+ |
| +---------+ |
| |
+-------------------------------------------------------------------+
Figure 4 Heterogeneous P2P Streaming with PPSP
5.4. 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 peers. To do this, it asks the
tracker for the peer list and the tracker replies with external peers
in the peer list. After the cache nodes exchange data with these
peers, it can also act as a peer and report what it caches to the
tracker and serve inside requesting peers afterward. This operation
greatly decreases the inter-network traffic in many conditions and
increases user experience.
zhang Expires April 20, 2013 [Page 15]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
The cache nodes do not need to update their library when new
applications supporting PPSP are introduced, which reduces the cost.
+----------------------------------------------------------------+
| |
| Tracker Protocol +---------+ |
| +----------------> | Tracker | |
| | +---------+ |
| | ^ |
| | | |
| | | Tracker Protocol |
| | | |
| | | |
| | +---------|-------------------------------------|
| | | V |
| | | +---------+ |
| | +----------|---> | Cache |<-------------------+ |
| | | | +---------+ Tracker/Peer| |
| | | Peer | Protocol | |
| | | Protocol | | |
| | | | | |
| | | | | |
| V V | V |
| +-----------+ | ISP Domain +------------+ |
| | External | | | Inside | |
| | Peer | | | Peer | |
| +-----------+ | +------------+ |
+----------------------------------------------------------------+
Figure 5 Cache Service Supporting Streaming with PPSP
5.5. Proxy service supporting P2P streaming
5.5.1. Home Networking Scenario
For applications where the peer is not co-located with the Media
Player in the same device (e.g. the peer is located in a Home Media
Gateway), we can use a PPSP Proxy, as shown in figure 6.
As shown in figure 6, the PPSP Proxy terminates both the tracker and
peer protocol allowing the legacy presentation devices to access P2P
streaming content. In figure 6 the DLNA protocol [DLNA] is used in
order to communicate with the presentation devices thanks to its wide
deployment. Obviously, other protocols can also be used.
zhang Expires April 20, 2013 [Page 16]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
+----------------------------------------------------------------+
| |
| Tracker Protocol +---------+ |
| +----------------> | Tracker | |
| | +---------+ |
| | ^ |
| | | |
| | | Tracker Protocol |
| | | |
| | +---------|-------------------------------------|
| | | V |
| | | +---------+ |
| | +----------|---> | PPSP |<-------------------+ |
| | | | | Proxy | DLNA | |
| | | Peer | +---------+ Protocol | |
| | | Protocol | | |
| | | | | |
| V V | V |
| +-----------+ | Home Domain +------------+ |
| | External | | | DLNA Pres.| |
| | Peer | | | Devices | |
| +-----------+ | +------------+ |
+----------------------------------------------------------------+
Figure 6 Proxy service Supporting P2P Streaming
5.5.2. Browser-based HTTP Streaming
P2P Plug-ins are often used in browser-based environment in order to
stream content. With P2P plug-ins, HTTP streaming can be turned into
a de facto P2P streaming. From the browser (and hence the user)
perspective, it's just HTTP based streaming but the PPSP capable
plug-in can actually accelerate the process by leveraging streams
from multiple sources/peers [P2PYoutube]. In this case the plug-ins
behave just like the proxy.
zhang Expires April 20, 2013 [Page 17]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
6. Requirements of PPSP
This section enumerates the requirements that should be considered
when designing PPSP.
6.1. Basic Requirements
PPSP.REQ-1: 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-2: 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-3: The streaming content MUST allow to be partitioned into
chunks.
PPSP.REQ-4: Each chunk MUST have a unique ID (i.e. chunk ID) in the
swarm.
Each chunk must have a unique ID in the swarm so that the peer can
understand which chunks are stored in which peers and which chunks
are requested by other peers.
PPSP.REQ-5: The tracker and peer protocol together MUST facilitate
achieving QoS acceptable to both live and VoD streaming application.
There are basic QoS requirements for streaming systems. Setup time to
receive a new streaming channel or to switch between channels should
be reasonably small. End to end delay, which consists of the time
between content generation (e.g., a camera) and content consumption
(e.g., a monitor), will become critical in case of live streaming
especially in provisioning of sport events where end to end delay of
1 minute and more are not acceptable.
For instance, the tracker and peer protocol can carry QoS related
parameters (e.g. video quality and delay requirements) together with
the priorities of these parameters in addition to the measured QoS
situation (e.g., performance, available uplink bandwidth) of content
providing peers.
zhang Expires April 20, 2013 [Page 18]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
There are also some other possible mechanisms like addition of super
peers, in-network storage, request of alternative peer addresses, and
the usage of QoS information for advanced peer selection mechanisms.
PPSP.REQ-6: The tracker and peer protocol do not include the
algorithm required for scalable streaming. However, the tracker and
peer protocol SHOULD NOT restrict or place limits on any such
algorithm.
PPSP.REQ-7: The tracker SHOULD be robust, i.e., when centralized
tracker fails, the P2P streaming system should still work by
supporting distributed trackers.
6.2. PPSP Tracker Protocol Requirements
PPSP.TP.REQ-1: The tracker protocol MUST allow the peer to solicit
the peer list from the tracker with respect to a specific swarm in a
query and response manner.
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 tracker will then be able to
select an appropriate set of peers for the requesting peer according
to the preference.
PPSP.TP.REQ-2: 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-3: The tracker protocol MUST support the report of the
peer's activity in the swarm to the tracker.
PPSP.TP.REQ-4: The tracker protocol SHOULD support the report of the
peer's chunk availability information to the tracker when tracker
needs such information to steer peer selection.
PPSP.TP.REQ-5: The chunk availability information between peer and
tracker MUST be expressed in a compactable method.
The peers may report chunk availability digest information (i.e.,
compact expression of chunk availability) to the tracker when
possible in order to decrease the bandwidth consumption in mobile
networks. For example, if a peer has a bitmap like 111111...1(one
hundred continuous 1)xxx..., the one hundred continuous "1" can be
expressed by one byte with seven bits representing the number of "1",
i.e., "one hundred" and one bit representing the continuous sequence
is "1" or "0". In this example, 100-8=92 bits are saved. Considering
zhang Expires April 20, 2013 [Page 19]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
the frequency of exchange of chunk availability and the fact that
many bitmaps have a quite long length of continuous "1" or "0", such
compression is quite useful.
PPSP.TP.REQ-6: The tracker protocol SHOULD support the report of the
peer's status to the tracker when tracker needs such information to
steer peer selection.
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.
PPSP.TP.REQ-7: The tracker protocol MUST allow the tracker to
authenticate the peer.
This ensures that only the authenticated users can access the
original content in the P2P streaming system.
6.3. PPSP Peer Protocol Requirements
PPSP.PP.REQ-1: The peer protocol MUST allow the peer to solicit the
chunk information from other peers in a query and response manner.
PPSP.PP.REQ-2: The chunk information exchanged between a pair of
peers MUST NOT be passed to other peers, unless validated (e.g.
prevent hearsay and DoS).
PPSP.PP.REQ-3: The peer protocol SHOULD allow the peer to solicit an
additional list of peers to that received from the tracker.
It is possible that a peer may need additional peers for certain
streaming content. Therefore, it is allowed that the peer
communicates with other peers in the current peer list to obtain an
additional list of peers in the same swarm.
PPSP.PP.REQ-4: When used for soliciting additional list of peers, the
peer protocol MUST contain swarm-membership information of the peers
that have explicitly indicated they are part of the swarm, verifiable
by the receiver.
PPSP.PP.REQ-5: The additional list of peers MUST only contain peers
which have been checked to be valid and online recently (e.g. prevent
hearsay and DoS).
PPSP.PP.REQ-6: The peer protocol MUST support the report of the
peer's chunk availability update.
zhang Expires April 20, 2013 [Page 20]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
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 list 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 the
update message, how often to send this update message, etc. should
leave to the algorithms, rather than protocol concerns.
PPSP.PP.REQ-7: The chunk availability information between peers MUST
be expressed in a compactable method.
The peers may exchange chunk availability digest information with
other peers, when possible, in order to decrease the messages
bandwidth consumption.
PPSP.PP.REQ-8: The peer protocol MUST support the exchange of
additional information, e.g. status about the peers.
This information can be, for instance, information about the access
link or information about whether a peer is running on battery or is
connected to a power supply. With such information, a peer can select
more appropriate peers for streaming.
PPSP.PP.REQ-9: The peer protocol is RECOMMENDED to be carried over
UDP.
This would reduce the message overhead and help the peers to retrieve
streaming content in a timely manner.
7. Security Considerations
This document discusses the problem statement and requirements around
P2P streaming protocols without specifying the protocols. However we
believe it is important for the reader to understand areas of
security introduced by the P2P nature of the proposed solution. The
main issue is the usage of un-trusted entities (peers) for service
provisioning. For example, malicious peers may:
- Originate denial of service (DOS) attacks to the trackers by
sending large amount of requests with the tracker protocol;
zhang Expires April 20, 2013 [Page 21]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
- Originate fake information on behalf of other peers;
- Originate fake information about chunk availability;
For example, malicious peers/trackers may:
- Originate reply instead of the regular tracker (man in the middle
attack).
We list some important security requirements for PPSP protocols as
below:
PPSP.SEC.REQ-1: PPSP MUST support closed swarms, where the peers are
authenticated or in a private network.
This ensures that only the trusted peers can access the original
content in the P2P streaming system. This can be achieved by security
mechanisms such as peer authentication and/or key management scheme.
PPSP.SEC.REQ-2: Confidentiality of the streaming content in PPSP
SHOULD be supported.
In order to achieve this, PPSP should provide mechanisms to encrypt
the data exchange among the peers.
PPSP.SEC.REQ-3: PPSP SHOULD support identifying badly behaving peers,
and exclude or reject them from the P2P streaming system.
PPSP.SEC.REQ-4: Integrity of the streaming content in PPSP MUST be
supported to provide a peer with the possibility to identify
unauthentic content (undesirable modified by other entities rather
than its genuine source).
In a P2P live streaming system a polluter can introduce corrupted
chunks. Each receiver integrates into its playback stream the
polluted chunks it receives from its neighbors. Since the peers
forwards chunks to other peers, the polluted content can potentially
spread through the P2P streaming network.
The PPSP protocol specifications will document the expected threats
(and how they will be mitigated by each protocol) and also
considerations on threats and mitigations when combining both
protocols in an application. This will include privacy of the users
and protection of the content distribution.
PPSP.SEC.REQ-5: The security mechanisms in PPSP, such as key
management and checksum distribution SHOULD scale well in P2P
streaming systems.
zhang Expires April 20, 2013 [Page 22]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
PPSP.SEC.REQ-6: Existing P2P security mechanisms SHOULD be re-used as
much as possible in PPSP, to avoid developing new security mechanisms.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgments
Thank you to J.Seng, G. Camarillo, R. Yang, C. Schmidt, R. Cruz, Y.
Gu, A.Bakker and S. Previdi for contribution to many sections of this
draft. Thank you to C. Williams, V. Pascual 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, D. Bryan,
E. Marocco, V. Gurbani, R. Even, H. Zhang, D. Zhang, J. Lei, H.Song,
X.Jiang, J.Seedorf, D.Saumitra, A.Rahman, L.Deng, J.Pouwelse and
W.Eddy.
This document was prepared using 2-Word-v2.0.template.dot.
zhang Expires April 20, 2013 [Page 23]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[Cisco] Cisco Visual Networking Index: Forecast and Methodology,
2009-2014,
http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns7
05/ns827/white_paper_c11-
481360_ns827_Networking_Solutions_White_Paper.html
[VoD] Y. Huang et al, Challenges, "Design and Analysis of a Large-
scale P2P-VoD System", Sigcomm08.
[ByteMobile] http://www.bytemobile.com/news-
events/2012/archive_230212.html
[Mobile Streaming1] Streaming to Mobile Users in a Peer-to-Peer
Network, J. 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).
[Hybrid CDN P2P]D. Xu et al, "Analysis of a CDN-P2P hybrid
architecture for cost-effective streaming media distribution,"
Springer Multimedia Systems, vol.11, no.4, pp.383-399, 2006.
[PPTV] http://www.pptv.com
[PPStream] http://www.ppstream.com
[DLNA] http://www.dlna.org
[P2PYoutube] https://addons.opera.com/en/extensions/details/p2p-
youtube/
zhang Expires April 20, 2013 [Page 24]
Internet-Draft Problem Statement and Requirements of PPSP October 2012
Authors' Addresses
Yunfei Zhang
China Mobile Communication Corporation
zhangyunfei@chinamobile.com
Ning Zong
Huawei Technologies Co., Ltd.
zongning@huawei.com
zhang Expires April 20, 2013 [Page 25]