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RFC 6972 - Problem Statement and Requirements of the Peer-to-Pee

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Internet Engineering Task Force (IETF)                          Y. Zhang
Request for Comments: 6972                                       Coolpad
Category: Informational                                          N. Zong
ISSN: 2070-1721                                      Huawei Technologies
                                                               July 2013

                 Problem Statement and Requirements of
               the Peer-to-Peer Streaming Protocol (PPSP)


   Peer-to-Peer (P2P) streaming systems becoming more and more popular
   on the Internet, and most of them are using proprietary protocols.
   This document identifies problems associated with proprietary
   protocols; proposes the development of the Peer-to-Peer Streaming
   Protocol (PPSP), which includes the tracker and peer protocols; and
   discusses the scope, requirements, and use cases of PPSP.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Backgrounds  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Concepts . . . . . . . . . . . . . . . . . . .  3
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Heterogeneous P2P Traffic and P2P Cache Deployment . . . .  5
     3.2.  QoS Issue and CDN Deployment . . . . . . . . . . . . . . .  5
     3.3.  Extended Applicability in Mobile and Wireless
           Environments . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Tasks of PPSP: Standard Peer-to-Peer Streaming Protocols . . .  7
     4.1.  Tasks and Design Issues of the Tracker Protocol  . . . . .  8
     4.2.  Tasks and Design Issues of the Peer Protocol . . . . . . .  9
   5.  Use Cases of PPSP  . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Worldwide Provision of Live/VoD Streaming  . . . . . . . .  9
     5.2.  Enabling CDN for P2P VoD Streaming . . . . . . . . . . . . 11
     5.3.  Cross-Screen Streaming . . . . . . . . . . . . . . . . . . 12
     5.4.  Cache Service Supporting P2P Streaming . . . . . . . . . . 13
     5.5.  Proxy Service Supporting P2P Streaming . . . . . . . . . . 14
       5.5.1.  Home Networking Scenario . . . . . . . . . . . . . . . 14
       5.5.2.  Browser-Based HTTP Streaming . . . . . . . . . . . . . 14
   6.  Requirements of PPSP . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Basic Requirements . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Operational and Management Requirements  . . . . . . . . . 15
       6.2.1.  Operational Considerations . . . . . . . . . . . . . . 16
       6.2.2.  Management Considerations  . . . . . . . . . . . . . . 17
     6.3.  PPSP Tracker Protocol Requirements . . . . . . . . . . . . 17
     6.4.  PPSP Peer Protocol Requirements  . . . . . . . . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 21

1.  Introduction

1.1.  Backgrounds

   Streaming traffic is among the largest and fastest growing traffic on
   the Internet [Cisco].  Peer-to-Peer (P2P) streaming contributes
   substantially to this growth.  With the advantage of high scalability
   and fault tolerance against a 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.  More and more providers are joining the P2P
   streaming ecosystem, e.g., Content Distribution Networks (CDN)
   providers started using P2P technologies to distribute their
   streaming content.

   Given the increasing integration of P2P streaming in the global
   content delivery infrastructure, there is a need for an open and
   standard streaming signaling protocol suite.  Almost all existing
   systems use proprietary protocols.  Having multiple proprietary
   protocols that perform similar functions results in repetitious
   development efforts for new systems, and the lock-in effects lead to
   substantial integration difficulties with other players (e.g., CDN).
   For example, in the enhancement of existing caches and CDN systems to
   support P2P streaming, proprietary protocols may increase the
   complexity of interactions 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 the P2P streaming system to solve the above-
   mentioned problems.

1.2.  Requirements Language

   The key words "MUST" and "MUST NOT" in this document are to be
   interpreted as described in RFC 2119 [RFC2119] and indicate
   requirement levels for compliant implementations.

2.  Terminology and Concepts

   CHUNK: A CHUNK is a basic unit of data 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 the case of the 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.

   CHUNK ID: The identifier of a CHUNK in a content stream.

   CLIENT: A CLIENT 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 a 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 store content provided by
   the original content servers.  Typically, CDN nodes serve content to
   the users located nearby topologically.

   LIVE STREAMING: LIVE STREAMING refers to a scenario where all the
   audiences receive streaming content for the same ongoing event.  It
   is desired that the lags between the play points of the audiences 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 that are in the 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.

   PEER ID: The identifier of a PEER such that other PEERs, or the
   TRACKER, can refer to the PEER by using its ID.

   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
   that can be centralized or distributed.

   VIDEO ON DEMAND (VoD): VIDEO ON DEMAND refers to a scenario in which
   different audiences may watch different parts of the same recorded
   streaming with downloaded content.

   SWARM: A SWARM refers to a group of PEERs that 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.

3.  Problem Statement

   The problems caused by proprietary protocols for P2P streaming
   applications are described in this section.

3.1.  Heterogeneous P2P Traffic and P2P Cache Deployment

   ISPs are faced with different P2P streaming applications introducing
   substantial traffic into their infrastructure, including their
   backbone and their exchange/interconnection points.  P2P caches are
   used by ISPs to locally store content and hence reduce the P2P
   traffic.  P2P caches usually operate at the chunk or file

   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 that 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

   When compared to client/server streaming, P2P streaming is often
   criticized due to its poorer QoS performance (e.g., longer startup
   delay, longer seek delay, and channel switch delay).  Hybrid CDN/P2P
   is a good approach to address this problem [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.
   Similar to what is described in Section 3.1, proprietary P2P
   protocols introduce complexity and the deployment cost of CDN.

3.3.  Extended Applicability in Mobile and Wireless Environments

   Mobile and wireless networks, which make considerable use of
   streaming service, are becoming increasingly important in today's
   Internet.  It's reported that the average volume of video traffic on
   mobile networks had risen up to 50% in the early part of 2012
   [ByteMobile].  There are multiple prior studies exploring P2P
   streaming in mobile and wireless networks [Mobile-Streaming1]

   However, it's difficult to directly apply current P2P streaming
   protocols (even assuming we can reuse 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 directions, 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 make 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 as 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 size is in kilobytes and
      is 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 wireline P2P streaming.

      Third, for resource-constrained peers, like mobile handsets or
      set-top boxes (STB), there are multiple systems competing for
      severely limited resources 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 one of the applications is 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.

4.  Tasks of PPSP: Standard Peer-to-Peer Streaming Protocols

   PPSP aims to solve the problems mentioned above by standardizing
   signaling protocols that support either live or VoD streaming.  PPSP
   supports both centralized and distributed trackers.  In distributed
   trackers, the tracker functionality is distributed in decentralized
   peers.  In this section, the tracker is a logic conception that 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

                |                                                |
                |     +--------------------------------+         |
                |     |            Tracker             |         |
                |     +--------------------------------+         |
                |        |     ^                   ^             |
                |Tracker |     | Tracker           |Tracker      |
                |Protocol|     | Protocol          |Protocol     |
                |        |     |                   |             |
                |        V     |                   |             |
                |     +---------+    Peer     +---------+        |
                |     |   Peer  |<----------->|   Peer  |        |
                |     +---------+   Protocol  +---------+        |
                |       | ^                                      |
                |       | |Peer                                  |
                |       | |Protocol                              |
                |       V |                                      |
                |     +---------------+                          |
                |     |      Peer     |                          |
                |     +---------------+                          |
                |                                                |
                |                                                |

                    Figure 1: PPSP System Architecture

   The PPSP design, in general, needs to solve the following challenges:

      1) When joining a swarm, how does a peer know which peers it
      should contact for content?

      2) After determining a set of peers, how does a peer make 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., which
      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 loosely coupled?

4.1.  Tasks and Design Issues of the Tracker Protocol

   The tracker protocol handles the initial and periodic exchange of
   meta-information between trackers and peers, such as a peer list and
   content information.

   Therefore, 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/VoD

   Special tasks for the design of the tracker protocol are listed
   below.  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

      2) How to identify different peers, e.g., peers with public or
      private IP addresses, peers behind or not behind NAT, peers with
      IPV4 or IPV6 addresses, peers with different properties?

      3) The tracker protocol must be light weight, since a tracker may
      need to serve a large number of peers.  This is related to the
      encoding issue (e.g., Binary based or Text based) and keep-alive

      4) How can the tracker report an optimized peer list to serve
      particular content?  This is related to the status statistic, with
      which the tracker can be aware of the peer status and content

   The PPSP tracker protocol will consider all these issues in the
   design according to the requirements from both the peer and tracker
   perspectives and will also take into consideration deployment and
   operation perspectives.

4.2.  Tasks and Design Issues of the Peer Protocol

   The peer protocol controls the advertising and exchange of content
   between the peers.

   Therefore, the peer protocol is modeled as a gossip-like protocol
   with periodic exchanges of neighbor and chunk-availability

   Special tasks for the design of the peer protocol are listed below.
   This is a high-level task-list.  The detailed requirements on the
   design of the peer protocol are explicated in Section 6.

      1) How is certain content globally identified and verified?  Since
      the content can be retrieved from everywhere, how to ensure the
      exchanged content between the peers is authentic?

      2) How to identify the chunk availability in certain content?
      This is related to the chunk-addressing and chunk-state
      maintenance.  Considering the large amount of chunks in certain
      content, light-weight expression is necessary.

      3) How to ensure the peer protocol efficiency?  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.

   The PPSP peer protocol will consider all the above issues in the
   design according to the requirements from the peer perspective.

5.  Use Cases of PPSP

   This section is not a to-do list for the WG; it provides details on
   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 between different providers.  This is quite similar
   to the case described in CDNI [RFC6707] [RFC6770].

   Let us assume a scenario in which there is only provider A (e.g., in
   China) providing live streaming service in provider B's (e.g., in the
   USA) and C's (e.g., in Europe) coverage.  Without PPSP, when a user
   (e.g., a Chinese American) in the USA requests the program to the
   tracker (which is located in A's coverage), the tracker may generally
   return a peer list to the user including most of the peers in China,
   because generally most users are in China and there are only few

   users in the 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
   does no attract many users in the USA and Europe, the tracker in A
   can optimally return a peer list to the user including B's and C's
   Super-Nodes (SN for short) to provide a better user performance.
   Furthermore, B's User2 and C's User3 can exchange data (availability)
   with these local SNs using the peer protocol.

   |                                                                   |
   |                          +------------------+                     |
   |            +------------>| 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 | User3   | |
   |      +------+         +------+    +---------+         +---------+ |
   |                                                                   |

                 Figure 2: Cooperative Vendors Interaction

5.2.  Enabling CDN for P2P VoD Streaming

   Figure 3 shows an example 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) using the same PPSP protocols.

   |                                                                   |
   |                   +-------------+    +--------------+             |
   |            +----->| 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    |
   | Protocol|                |   |        | | 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

   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) that support streaming at a low cost.

   Note that for compatibility reasons, both HTTP and P2P streaming can
   be supported by CDN from the users' perspective.

5.3.  Cross-Screen Streaming

   In this scenario, PC, STB/TV, and mobile terminals from both fixed
   and mobile/wireless networks share the streaming content.  With PPSP,
   peers can identify the types of access networks, average load, and
   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.

    |                                                                  |
    |      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

   Such information will play an important role in 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 lead to a higher load on the base station.

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
   enhances the user experience.

      |                                                                |
      |    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

   The cache nodes do not need to update their library when new
   applications supporting PPSP are introduced, which reduces the cost.

5.5.  Proxy Service Supporting P2P Streaming

5.5.1.  Home Networking Scenario

   For applications where the peer is not colocated 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.

       |                                                               |
       |    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

   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 Digital Living Network Alliance
   (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.

5.5.2.  Browser-Based HTTP Streaming

   P2P Plug-ins are often used in browser-based environments to stream
   content.  With P2P plug-ins, HTTP streaming can be turned into 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

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).

      It's a basic requirement for a peer to be uniquely identified in a
      P2P streaming system so that other peers or trackers can refer to
      the peer by ID.

      Note that a peer can join multiple swarms with a unique ID or
      change swarm without changing its 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 be partitioned into chunks.

   PPSP.REQ-4: Each chunk MUST have a unique ID (i.e., chunk ID) in the

      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.

6.2.  Operational and Management Requirements

   This section lists some operational and management requirements based
   on the checklist presented in Appendix A of [RFC5706].

6.2.1.  Operational Considerations

   PPSP.OAM.REQ-1: PPSP MUST be sufficiently configurable.

      According to basic requirements, when setting up PPSP, a content
      provider should generate chunk IDs and a swarm ID for each stream
      of content.  An original content server and tracker are configured
      and set up.  The content provider should then publish this
      information, typically by creating web links.

      The configuration should allow the proxy-based and end-client

   PPSP.OAM.REQ-2: PPSP MUST implement a set of configuration parameters
   with default values.

   PPSP.OAM.REQ-3: PPSP MUST support diagnostic operations.

      Mechanisms must be supported by PPSP to verify correct operation.
      The PPSP tracker should collect the status of the peers including
      the peer's activity, whether it obtained chunks in time, etc.
      Such information can be used to monitor the streaming behavior of

   PPSP.OAM.REQ-4: PPSP MUST facilitate achieving quality acceptable to
   the streaming application.

      There are basic quality requirements for streaming systems.  The
      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 sporting
      events where an end-to-end delay of 1 minute or more are not

      For instance, the tracker and peer protocol can carry quality
      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.

      PPSP implementations may use techniques such as scalable streaming
      to handle bandwidth shortages without disrupting playback.

6.2.2.  Management Considerations

   PPSP.OAM.REQ-5: When management objectives need to be supported in
   implementations, PPSP MUST support remote management using a standard
   interface, as well as a basic set of management information.

      Due to large-scale peer networks, PPSP tracker service or seeders
      should remotely collect information from peers and expose the
      information via a standard interface for management purposes.
      Peer information can be collected via a PPSP tracker protocol or
      peer protocol.

      The minimum set of management objects should include swarm
      information such as content characteristics and rate limits;
      tracking information such as swarm list and log events; and peer
      information such as peer activity, chunk statistics, and log

   PPSP.OAM.REQ-6: PPSP MUST support fault monitoring including peer and
   server health, as well as the streaming behavior of peers.

      Peer and server health will at least include node activity and
      connectivity, especially for peers behind NAT.  As mentioned in
      PPSP.OAM.REQ-4, streaming behavior of the peer can be learned from
      chunk distribution information.

   PPSP.OAM.REQ-7: PPSP MUST support configuration management to define
   the configuration parameters.

      A set of configurable parameters related to chunk generation in
      the PPSP setup stage can be defined by content providers via a
      management interface to content servers.

   PPSP.OAM.REQ-8: PPSP MUST support performance management with respect
   to streaming performance based on chunk distribution statistics,
   network load, and tracker and peer monitoring.

   PPSP.OAM.REQ-9: PPSP MUST support security management.  See Section 7
   of this document.

6.3.  PPSP Tracker Protocol Requirements

   PPSP.TP.REQ-1: The tracker protocol MUST allow the peer to solicit a
   peer list in a swarm generated and possibly tailored by the tracker
   in a query and response manner.

      The tracker request message may 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.

      The tracker may also generate the peer list with the help of
      traffic optimization services, e.g., Application-Layer Traffic
      Optimization [ALTO].

   PPSP.TP.REQ-2: The tracker protocol MUST report the peer's activity
   in the swarm to the tracker.

   PPSP.TP.REQ-3: The tracker protocol MUST take the frequency of
   message exchange and efficient bandwidth use into consideration when
   communicating chunk availability information.

      For example, the chunk availability information between peer and
      tracker can be presented in a compact method, e.g., to express a
      sequence of continuous "1" more efficiently.

   PPSP.TP.REQ-4: The tracker protocol MUST have a provision for the
   tracker to authenticate the peer.

      This ensures that only the authenticated users can access the
      original content in the P2P streaming system.

6.4.  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 the chunk information
   is validated (e.g., preventing hearsay and DoS attacks).

   PPSP.PP.REQ-3: The peer protocol MUST 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, the peer is allowed to communicate
      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 an 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,
   which is verifiable by the receiver.

   PPSP.PP.REQ-5: The additional list of peers MUST only contain peers
   that have been checked to be valid and online recently (e.g.,
   preventing hearsay and DoS attacks).

   PPSP.PP.REQ-6: The peer protocol MUST report the peer's chunk
   availability update.

      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 the
      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 a
      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 be left to the
      algorithms, rather than protocol concerns.

   PPSP.PP.REQ-7: The peer protocol MUST take the frequency of message
   exchange and efficient bandwidth use into consideration when
   communicating chunk information.

      For example, the chunk availability information between peers can
      be presented in a compact method.

   PPSP.PP.REQ-8: The peer protocol MUST exchange 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.

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 untrusted entities (peers) for service
   provisioning.  For example, malicious peers/trackers may:

   o  Originate DoS attacks to the trackers by sending a large number of
      requests with the tracker protocol;

   o  Originate fake information on behalf of other peers;

   o  Originate fake information about chunk availability;

   o  Originate fake reply messages on behalf of the tracker;

   o  Leak private information about other peers or trackers.

   We list some important security requirements for PPSP protocols

   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 schemes.

      Another aspect is that confidentiality of the streaming content in
      PPSP needs to be supported.  In order to achieve this, PPSP should
      provide mechanisms to encrypt the data exchange among the peers.

   PPSP.SEC.REQ-2: Integrity of the streaming content in PPSP MUST be
   supported to provide a peer with the possibility of identifying
   unauthentic content (undesirable modifications 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
      forward 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-3: The security mechanisms in PPSP, such as key
   management and checksum distribution, MUST scale well in P2P
   streaming systems.

8.  Acknowledgements

   Thanks to J. Seng, G. Camarillo, R. Yang, C. Schmidt, R. Cruz, Y. Gu,
   A. Bakker, and S. Previdi for contributing to many sections of this
   document.  Thank you to C. Williams, V. Pascual, and L. Xiao for
   contributing to the 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,
   J. Pouwelse, W. Eddy, B. Claise, D. Harrington, J. Arkko, and all the
   AD reviewers.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, November 2009.

   [RFC6707]  Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", RFC 6707, September 2012.

   [RFC6770]  Bertrand, G., Stephan, E., Burbridge, T., Eardley, P., Ma,
              K., and G. Watson, "Use Cases for Content Delivery Network
              Interconnection", RFC 6770, November 2012.

9.2.  Informative References

   [ALTO]     Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", Work
              in Progress, December 2009.

              Bytemobile, "Mobile Video Traffic Hits Nearly 70% on
              Certain Networks", February 2012,

   [CDN-P2P]  Xu, D., Kulkarni, S., Rosenberg, C., and H-K. Chai,
              "Analysis of a CDN-P2P Hybrid Architecture for
              Cost-Effective Streaming Media Distribution", Multimedia
              Systems, vol. 11, no. 4, pp. 383-399, 2006.

   [Cisco]    Cisco, "Cisco Visual Networking Index: Forecast and
              Methodology, 2012 - 2017", Visual Networking Index (VNI),
              ns525/ns537/ns705/ns827/ white_paper_c11-481360_

   [DLNA]     "DLNA", <http://www.dlna.org>.

              Noh, J., Makar, M., and B. Girod, "Streaming To Mobile
              Users In A Peer-to-Peer Network", MOBIMEDIA , 2009.

              Peltotalo, J., Harju, J., Saukkoh, M., Vaatamoinen, L.,
              Bouazizi, I., Curcio, I., and J. van Gassel, "A Real-Time
              Peer-to-Peer Streaming System for Mobile Networking
              Environment", Proceedings of the INFOCOM and Workshop on
              Mobile Video Delivery (MoVID '09), 2009.

              "Youtube Extension-Opera Add-Ons", Opera Software,

   [VoD]      Huang, Y., Fu, T., Chiu, D-M.,  Lui, J., and C. Huang,
              "Challenges, Design and Analysis of a Large-Scale P2P-VoD
              System", SIGCOMM , 2008.

Authors' Addresses

   Yunfei Zhang

   EMail: hishigh@gmail.com

   Ning Zong
   Huawei Technologies

   EMail: zongning@huawei.com


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