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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
                                                             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.



























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Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   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
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   Legal Provisions and are provided without warranty as described in
   the Simplified BSD License.










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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














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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.










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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 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.




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   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.





















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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].




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   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.















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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?












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             +------------------------------------------------+
             |                                                |
             |     +--------------------------------+         |
             |     |            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?



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   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.






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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.







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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    |
   | 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

















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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

  5.3. 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.




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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 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.














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   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.





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      +----------------------------------------------------------------+
      |                                                                |
      |    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.













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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.


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   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


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   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.


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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 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;



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   - 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.



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   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.




























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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/










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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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