PPSP Y. Zhang
Internet Draft China Mobile
N.Zong
HuaweiTech
Intended status: Informational September 19, 2012
Expires: March 2013
Problem Statement and Requirements of Peer-to-Peer Streaming
Protocol (PPSP)
draft-ietf-ppsp-problem-statement-10.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, requirements and uses
cases of PPSP.
zhang Expires March 19, 2013 [Page 1]
Internet-Draft Problem Statement and Requirements of PPSP September 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 March 18, 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 March 19, 2013 [Page 2]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
Table of Contents
1. Introduction ................................................ 4
2. Terminology and concepts. .................................... 5
3. Problem statement ........................................... 7
3.1. Heterogeneous P2P traffic and P2P caches deployment ..... 7
3.2. Latency efficiency difficulties ......................... 7
3.3. Extended applicability in mobile and wireless environment 7
4. PPSP: Standard peer to peer streaming protocols .............. 9
4.1. Tracker protocol candidates discussion and design issues .9
4.2. Peer protocol candidates discussion and design issues . 10
5. Use cases of PPSP .......................................... 11
5.1. Worldwide provision of live/VoD streaming .............. 11
5.2. Cross-screen streaming ................................. 13
5.3. Cache service supporting P2P streaming ................. 14
5.4. Proxy service supporting P2P streaming ................. 15
5.4.1. Home Networking Scenario .......................... 15
5.4.2. Browser-based HTTP Streaming ...................... 16
6. Requirements of PPSP ........................................ 17
6.1. Basic Requirements ..................................... 17
6.2. PPSP Tracker Protocol Requirements ..................... 18
6.3. PPSP Peer Protocol Requirements ........................ 19
7. Security Considerations .................................... 21
8. IANA Considerations ........................................ 23
9. Acknowledgments ............................................ 23
10. Informative References. .................................... 24
zhang Expires March 19, 2013 [Page 3]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
1. Introduction
Streaming traffic is among the largest and fastest growing traffic on
the Internet [Cisco], where peer-to-peer (P2P) streaming contribute
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 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 in P2P streaming
systems to solve the above problems.
zhang Expires March 19, 2013 [Page 4]
Internet-Draft Problem Statement and Requirements of PPSP September 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 stores
(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 stores 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 March 19, 2013 [Page 5]
Internet-Draft Problem Statement and Requirements of PPSP September 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 server 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 media 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.
zhang Expires March 19, 2013 [Page 6]
Internet-Draft Problem Statement and Requirements of PPSP September 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 caches deployment
ISPs are faced to 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 packets and
therefore allows any caching device to be deployed (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 present in the network.
This increases both engineering and operational costs dramatically.
3.2. Latency efficiency difficulties
P2P streaming is often criticized due to its longer delays (e.g.,
startup delay, seek delay and channel switch delay) compared to
client/server streaming. Hybrid CDN/P2P is a good approach in order
to address this problem [Hybrid CDN P2P].
In the Hybrid CDN/P2P approach, the CDN takes two roles: media
streaming server and P2P tracker. Similarly to what described in
section 3.1, proprietary P2P protocols introduce complexity between
peers and CDN trackers because the CDN trackers need to identify each
different P2P streaming protocol. This increases the 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
zhang Expires March 19, 2013 [Page 7]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
[Mobile Streaming1] [Mobile Streaming2].
However it's difficult to apply current P2P streaming protocols (even
assuming we can re-use some of the proprietary ones) in mobile and
wireless networks. Although smart handsets are more eligible to
become peers with much higher bandwidth, CPU frequency, larger
storage and memory than before, peer selection will become more
challenging due to the increase and complexity of exchange between
peers and trackers. Current P2P protocols are not well suited for
these new requirements in the context of mobile and wireless
networks.
Following are some illustrative examples:
First, the connections are unstable and expensive in terms of energy
consumption and transmission (especially in uplink direction). Peers
and trackers may need more information like packet loss rate, peer
battery status and processing capability during peer selection.
Unfortunately current protocols don't cover 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, for example, several seconds. In a mobile
environment with scarce bandwidth, the message size need to be
shortened or it may require more efficient methods for expressing and
distributing chunk availability information, which is different from
current practice.
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 peer has to install many different
streaming applications for different usages, e.g., some for movies
and others for sports and each of these applications will compete for
the same set of memories, flashes or hard disks(some may run in the
background even they are not invoked by the users). Open protocols creat
an opportunity to use one client software accommodating different P2P systems.
This may alleviate this problem.
zhang Expires March 19, 2013 [Page 8]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
4. PPSP: Standard peer to peer streaming protocols
PPSP is targeted to standardize signaling protocols for tracker-based
architectures to solve the above problems that support either live or
VoD streaming.
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 data within the time
constraints. 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.
+------------------------------------------------+
| |
| +--------------------------------+ |
| | Tracker | |
| +--------------------------------+ |
| | ^ ^ |
|Tracker | | Tracker |Tracker |
|Protocol| | Protocol |Protocol |
| | | | |
| V | | |
| +---------+ Peer +---------+ |
| | Peer |<----------->| Peer | |
| +---------+ Protocol +---------+ |
| | ^ |
| | |Peer |
| | |Protocol |
| V | |
| +---------------+ |
| | Peer | |
| +---------------+ |
| |
| |
+------------------------------------------------+
Figure 1 PPSP System Architecture
4.1. Tracker protocol candidates discussion and design issues
Tracker protocol: The tracker protocol is best modeled as a
request/response protocol between peers and trackers, and will carry
zhang Expires March 19, 2013 [Page 9]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
information needed for the selection of peers suitable for real-
time/VoD streaming.
One of the main aspects the new protocol has to address is the format
of the protocol messages. Two options have been identified:
Binary based: Binary based tracker protocols are widely used in
practice, e.g., PPLive[PPLive] and PPStream [PPStream]. Binary based
tracker protocol is simple with the smallest set of semantic
definitions and efficient in peer and tracker resource usage,
especially, for mobile and set-top box terminals.
Text based: HTTP can be easily thought to be the candidate, being a
text based request/response protocol. HTTP messages may be reused for
PPSP semantics and if they don't match PPSP requirements, some new
messages may be redefined. Another approach consists of using
HTTP+XML combination, where HTTP is only used as the underlying
transport protocol (in application-level) while tracker information
and messages are defined in XML format.
PPSP tracker protocol will select the best of the above options
according to the requirements from both peer and tracker perspective
and also taking into consideration deployment and operation
perspectives.
4.2. Peer protocol candidates discussion and design issues
Peer Protocol: The peer protocol is modeled as a gossip-like protocol
with periodic exchanges of neighbor and media chunk availability
information. Namely, the peer protocol is a content-centric protocol
built around the abstraction of a cloud of participants disseminating
the same data in ways and orders that are convenient to the
participants [I-D.ietf-ppsp-peer-protocol]. In that respect and in
light of the above requirements, typical HTTP is neither suitable nor
efficient.
We list two peer protocol candidates:
Websockets for bidirectional HTTP: WebSockets is basically a
bidirectional TCP connection derived from a HTTP connection hence
allowing a bidirectional P2P transport over HTTP. On the negative
side, TCP is not ideally suited for multi-party transfers of the same
content (see Rationale section in I-D.ietf-ppsp-peer-protocol) and
therefore it introduces implementation (i.e., code) complexity.
zhang Expires March 19, 2013 [Page 10]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
UDP based: Unlike TCP or HTTP, UDP is a datagram-based protocol
without any sequential data stream abstraction which is, in most the
cases, unnecessary for PPSP. Compared to the use of TCP, it reduces
the per-connection footprint and complexity of TCP especially in
resource constraint mobile cases.
The PPSP peer protocol will discuss the protocol design rationales in
detail.
5. Use cases of PPSP
5.1. Worldwide provision of live/VoD streaming
The content provider can efficiently increase live streaming coverage
by introducing PPSP in between different providers.
Figure 2 shows the case of provider A broadcasting a TV program with
the help of provider B and C for a wider coverage by introducing PPSP.
Without PPSP, when users outside A requests TV program@A, the
returned peer-list may include few local peers. This may affect the
user experience. With PPSP, B and C can involve in the broadcasting.
The providers often deploy in-network peers called super-nodes (SN
for short) who have better stability and higher storage and bandwidth
for better QoS. With the tracker protocol, the tracker@A can return a
peer-list containing, in addition to peers@A addresses, the SNs owned
by B and C. Hence User@B and User@C can exchange data (availability)
with these local SNs using the peer protocol.
Figure 3 shows the case of cooperative VoD provision by introducing
PPSP inside CDN overlays and in between different CDNs. 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 with the same PPSP protocols. Note that for compatibility
reason both HTTP streaming and P2P streaming can be supported by CDN.
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 supporting
streaming in a low cost.
zhang Expires March 19, 2013 [Page 11]
Internet-Draft Problem Statement and Requirements of PPSP September 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 March 19, 2013 [Page 12]
Internet-Draft Problem Statement and Requirements of PPSP September 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
5.2. 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 March 19, 2013 [Page 13]
Internet-Draft Problem Statement and Requirements of PPSP September 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
5.3. 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 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 requesting peers inside afterward. This operation
greatly decreases the inter-network traffic and increases user
experience.
zhang Expires March 19, 2013 [Page 14]
Internet-Draft Problem Statement and Requirements of PPSP September 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
5.4. Proxy service supporting P2P streaming
5.4.1. 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 March 19, 2013 [Page 15]
Internet-Draft Problem Statement and Requirements of PPSP September 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
5.4.2. Browser-based HTTP Streaming
P2P Plug-ins can be 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 March 19, 2013 [Page 16]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
6. Requirements of PPSP
This section enumerates the requirements that should be considered
when designing PPSP.
6.1. Basic Requirements
PPSP.REQ-1: The tracker and the peer protocol SHOULD allow peers to
receive streaming content within the required time constraints.
PPSP.REQ-2: 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-3: 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-4: 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-5: 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. An example for generating the chunk ID
is the bitmap approach [I-D.ietf-ppsp-survey].
PPSP.REQ-6: The tracker protocol and peer protocol are recommended to
be carried over TCP or UDP.
PPSP.REQ-7: 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 live and VoD streaming
even for very popular content. The tracker and peer protocol do not
zhang Expires March 19, 2013 [Page 17]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
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 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.
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.
6.2. 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, sending the corresponding replies and periodical
send peer status reports/updates.
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 tracker will then be able to
select an appropriate set of peers for the requesting peer according
to the preference.
zhang Expires March 19, 2013 [Page 18]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
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 expressed as compact as possible.
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
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-9: The status of the peer SHOULD be reported to the
tracker when tracker needs such information in order 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.
6.3. 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.
zhang Expires March 19, 2013 [Page 19]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
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 other 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 the peer protocol. The peer protocol
MUST implement either pull-based, push-based or both.
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-5: The chunk availability information between peers MUST
be expressed as compactly as possible.
In PP.REQ-1/2/4, the peers may exchange chunk availability digest
information with other peers, when possible, in order to decrease the
zhang Expires March 19, 2013 [Page 20]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
messages bandwidth consumption.
PPSP.PP.REQ-6: The peer status report/update SHOULD be advertised
among the peers in order 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 such 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. 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;
- 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.
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.
zhang Expires March 19, 2013 [Page 21]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
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 systems.
PPSP.SEC.REQ-3: PPSP MUST provide an option in order to encrypt the
data exchange among the PPSP entities.
PPSP.SEC.REQ-4: PPSP MUST have mechanisms in order 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 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.
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 available resources of the P2P streaming system.
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
unauthentic 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.
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
zhang Expires March 19, 2013 [Page 22]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
and protection of the content distribution. Protection of the content
by Digital Rights Management (DRM) is outside the scope of the PPSP.
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 and S.
Previdi for contribution to many sections of this draft. Thank you to
C. Williams, V. Pasual 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,
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.
zhang Expires March 19, 2013 [Page 23]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
10. 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).
[I-D.ietf-alto-protocol]R. Alimi et al, "ALTO Protocol", draft-ietf-
alto-protocol-10 (work in progress), October 2011.
[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.
[I-D.ietf-ppsp-survey] Y. Gu et al, "Survey of P2P Streaming
Applications", draft-ietf-ppsp-survey-02 (work in progress), July
2011.
[PPTV] http://www.pptv.com
[PPStream] http://www.ppstream.com
[I-D.ietf-ppsp-peer-protocol] A. Bakker et al, Peer-to-Peer Streaming
Peer Protocol (PPSPP),draft-ietf-ppsp-peer-protocol-02, (work in
progress), June 2012.
[DLNA] http://www.dlna.org
[P2PYoutube] https://addons.opera.com/en/extensions/details/p2p-
youtube/
zhang Expires March 19, 2013 [Page 24]
Internet-Draft Problem Statement and Requirements of PPSP September 2012
Authors' Addresses
Yunfei Zhang
China Mobile Communication Corporation
zhangyunfei@chinamobile.com
NingZong
Huawei Technologies Co., Ltd.
zongning@huawei.com
zhang Expires March 19, 2013 [Page 25]