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Versions: (draft-zhang-ppsp-problem-statement) 00 01 02 03 04 05 06 07 08 09 10 11 13 14 15 RFC 6972

PPSP                                                            Y. Zhang
Internet Draft                                              China Mobile
                                                                  N.Zong
                                                              HuaweiTech

Intended status: Informational                             June 28, 2012
Expires: December 2012



        Problem Statement and Requirements of Peer-to-Peer Streaming
                              Protocol (PPSP)
                 draft-ietf-ppsp-problem-statement-09.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, and discusses the design scope, requirements and uses
   cases of PPSP.



























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Status of this Memo

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   This Internet-Draft will expire on December 28, 2012.

Copyright Notice

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   This document is subject to BCP 78 and the IETF Trust's Legal
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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......................................... 7
   4. PPSP: Standard peer to peer streaming protocols ............. 9
      4.1. Candidate protocols discussion and design issues........10
   5. Use cases of PPSP .......................................... 11
      5.1. Worldwide provision of live/VoD streaming ..............11
      5.2. PPSP supporting cross-screen streaming in heterogeneous
      environment ................................................ 13
      5.3. Cache service supporting P2P streaming .................14
   6. Security Considerations..................................... 16
   7. Requirements of PPSP........................................ 17
      7.1. Basic Requirements..................................... 17
      7.2. PPSP Tracker Protocol Requirements .....................18
      7.3. PPSP Peer Protocol Requirements ........................20
      7.4. Security Requirements.................................. 21
   8. IANA Considerations ........................................ 23
   9. Acknowledgments ............................................ 24
  10. Informative References  .................................... 25


















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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 in P2P streaming
   systems to solve the above problems.



















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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.

3.2. Efficiency issue and difficulties in building open streaming
   delivery infrastructure

   P2P streaming is often criticized by its inefficiency in terms of
   longer delays (e.g., startup delay, seek delay and channel switch
   delay) than client/server streaming. 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. A tracker is an essential
   component in P2P streaming systems to direct the requesting peers
   for possible serving peers and contents. Section 2 gives detailed
   description on the tracker.Proprietary protocols introduce complexity
   between the peer and CDN tracker interaction. Like the cache node in
   section 3.1, the CDN tracker has to identify different systems with
   different protocols. This increases the deployment cost.

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].



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   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. Although smart handsets are more eligible to become 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, which are not involved in current protocols
   designed mainly for fixed Internet.

   First, the connections are unsteady 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 in peer selection. Unfortunately current
   protocols don't cover these kinds of information.

   Second, current practices often use a "bitmap" message to exchange
   chunk availability among peers and trackers. The message is some
   kilobytes size and exchanged frequently, say, several seconds. In a
   mobile environment with scarce bandwidth, the message size need to be
   shortened or it may require other methods for expressing and
   distributing chunk availability information.

   Third, for a resource constraint peer like mobile handsets or set-top
   boxes (STB), there are severe contentions on limited resource using
   proprietary protocols. The peer has to install many different
   streaming applications for different usages, e.g., some for movies
   and others for sports. Note that for many P2P applications, even the
   users don't use them at this moment, they may be invoked as the
   background programs to facilitate other users for free data delivery
   assistance. This makes the mobile handsets or STBs more short of
   resources on the running applications. Only open protocols can
   alleviate this problem.
















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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
   offline 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 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.

             +------------------------------------------------+
             |                                                |
             |     +--------------------------------+         |
             |     |            Tracker             |         |
             |     +--------------------------------+         |
             |        |     ^                   ^             |
             |Tracker |     | Tracker           |Tracker      |
             |Protocol|     | Procotol          |Protocol     |
             |        |     |                   |             |
             |        V     |                   |             |
             |     +---------+    Peer     +---------+        |
             |     |   Peer  |<----------->|   Peer  |        |
             |     +---------+   Protocol  +---------+        |
             |       | ^                                      |
             |       | |Peer                                  |
             |       | |Protocol                              |
             |       V |                                      |
             |     +---------------+                          |
             |     |      Peer     |                          |
             |     +---------------+                          |
             |                                                |
             |                                                |
             +------------------------------------------------+
                     Figure 1 PPSP System Architecture




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4.1. Candidate protocols discussion and design issues

   Tracker protocol: The 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/offline streaming. Therefore the HTTP is a natural choice as the
   candidate of the PPSP tracker protocol.

   The following question asked is about the on-the-wire format of the
   carried message. There are three options:

   First, binary based: HTTP is only for the underlying transport (in
   application-level) protocol and the real information are delivered in
   binary carried by HTTP messages (e.g., through BASE64 encoding). This
   is uneasy to read and debug.

   Second, text based: reusing HTTP messages to construct PPSP tracker
   message as much as possible and redefine them if they don't match
   PPSP requirements. Many new messages may be redefined.

   Third, HTTP+XML based: This is also a text-based option. In this way,
   the role of HTTP is similar to the first option. The difference is that
   the tracker information are defined in XML format in HTTP messages. This
   is most flexible.

   The work of selecting best appropriate track protocol is in the scope of
   PPSP tracker protocol.

   Peer Protocol: The peer protocol is modeled as a gossip-like protocol
   with periodic exchanges of neighbor and media chunk availability
   information. That is to say, we need a content-centric protocol built
   around the abstraction of a cloud of participants disseminating the
   same data in any way and order that is convenient to them [I-
   D.ietf-ppsp-peer-protocol]. Obviously typical HTTP is neither
   suitable nor efficient in this abstraction.

   We list two candidates on peer protocol:

   First, Websockets for bidirectional HTTP: WebSockets is basically
   just a TCP connection derived from a HTTP connection. This means you
   can use it as a bidirectional transport to run a P2P protocol over.
   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 having to use HTTP first adds some (code)
   complexity.





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   Second, UDP based: Unlike TCP or HTTP, UDP is a datagram-based
   protocol completely dropping the sequential data stream abstraction,
   which is in most cases unnecessary for PPSP. This also reduces the
   higher per-connection footprint, complexity and less reliablility in
   NAT traversal using TCP especially in resource constraint cases such
   as mobile P2P streaming.

   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 easily expand the broadcasting/VoD scale to
   utilize the cooperative content providers' CDN or third party CDN
   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. This
   may hurt the user experience in P2P environment. 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) who have better stability and higher storage and bandwidth for
   better QoS. With tracker protocol, vendor A's tracker can returns
   with vendor B and vendor C's SNs in the peer-list. User@B and User@C
   can exchange data (availability) with these SNs using peer protocol.
   In this way vendor B and vendor C's SNs are shared 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 CDN nodes talk
   with the different vendors (including the peers inside) with the same
   protocols. In this way both HTTP streaming and P2P streaming can be
   supported.

   The internal interaction of CDN nodes can be executed by either
   existing protocol or peer protocol. The latter is useful in building
   new CDN systems supporting streaming because the cost can be reduced
   in a P2P way.







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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 even in
   different network (potentially with the conversion of the content
   availability expression in different networks) 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 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.



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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 request the frequently visited
   content (or part of) on behalf of the user peers. To do this, it asks
   for the 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 also 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.

   The cache nodes needn't update their library when new applications
   supporting PPSP are introduced, which reduce the cost.











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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.
   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 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 aswarm.

   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, G. Camarillo and R. Yang 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; C.
   Schmidt; 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.


































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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.

   [I-D.ietf-ppsp-peer-protocol] A. Bakker, R. Petrocco, Peer-to-Peer
   Streaming Peer Protocol (PPSPP),draft-ietf-ppsp-peer-protocol-02,
   (work in progress), June 2012.











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Authors' Addresses

   Yunfei Zhang
   China Mobile Communication Corporation
   zhangyunfei@chinamobile.com

   Ning Zong
   Huawei Technologies Co., Ltd.
   zongning@huawei.com





































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