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rfc:rfc6972

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)

Abstract

 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
 http://www.rfc-editor.org/info/rfc6972.

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.

Zhang & Zong Informational [Page 1] RFC 6972 PPSP Problem Statement July 2013

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

Zhang & Zong Informational [Page 2] RFC 6972 PPSP Problem Statement July 2013

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.

Zhang & Zong Informational [Page 3] RFC 6972 PPSP Problem Statement July 2013

 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.
 PEER-TO-PEER STREAMING PROTOCOL (PPSP): PPSPs refer to the primary
 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.

Zhang & Zong Informational [Page 4] RFC 6972 PPSP Problem Statement July 2013

 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
 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 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.

Zhang & Zong Informational [Page 5] RFC 6972 PPSP Problem Statement July 2013

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]
 [Mobile-Streaming2].
 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.

Zhang & Zong Informational [Page 6] RFC 6972 PPSP Problem Statement July 2013

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
 constraints.
              +------------------------------------------------+
              |                                                |
              |     +--------------------------------+         |
              |     |            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?

Zhang & Zong Informational [Page 7] RFC 6972 PPSP Problem Statement July 2013

    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
 streaming.
 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
    generated.
    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
    mechanism.
    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
    status.
 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.

Zhang & Zong Informational [Page 8] RFC 6972 PPSP Problem Statement July 2013

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
 information.
 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

Zhang & Zong Informational [Page 9] RFC 6972 PPSP Problem Statement July 2013

 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

Zhang & Zong Informational [Page 10] RFC 6972 PPSP Problem Statement July 2013

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.

Zhang & Zong Informational [Page 11] RFC 6972 PPSP Problem Statement July 2013

 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.

Zhang & Zong Informational [Page 12] RFC 6972 PPSP Problem Statement July 2013

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.

Zhang & Zong Informational [Page 13] RFC 6972 PPSP Problem Statement July 2013

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/

Zhang & Zong Informational [Page 14] RFC 6972 PPSP Problem Statement July 2013

 peers [P2PYoutube].  In this case, the plug-ins behave just like the
 proxy.

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
 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.

6.2. Operational and Management Requirements

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

Zhang & Zong Informational [Page 15] RFC 6972 PPSP Problem Statement July 2013

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
    scenarios.
 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.
 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
    acceptable.
    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.

Zhang & Zong Informational [Page 16] RFC 6972 PPSP Problem Statement July 2013

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
    event.
 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

Zhang & Zong Informational [Page 17] RFC 6972 PPSP Problem Statement July 2013

    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.

Zhang & Zong Informational [Page 18] RFC 6972 PPSP Problem Statement July 2013

 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;

Zhang & Zong Informational [Page 19] RFC 6972 PPSP Problem Statement July 2013

 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
 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 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.

Zhang & Zong Informational [Page 20] RFC 6972 PPSP Problem Statement July 2013

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]
            Bytemobile, "Mobile Video Traffic Hits Nearly 70% on
            Certain Networks", February 2012,
            <http://www.bytemobile.com/news-events/2012/
            archive_230212.html>.

Zhang & Zong Informational [Page 21] RFC 6972 PPSP Problem Statement July 2013

 [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),
            <http://www.cisco.com/en/US/solutions/collateral/ns341/
            ns525/ns537/ns705/ns827/ white_paper_c11-481360_
            ns827_Networking_Solutions_White_Paper.html>.
 [DLNA]     "DLNA", <http://www.dlna.org>.
 [Mobile-Streaming1]
            Noh, J., Makar, M., and B. Girod, "Streaming To Mobile
            Users In A Peer-to-Peer Network", MOBIMEDIA , 2009.
 [Mobile-Streaming2]
            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.
 [P2PYoutube]
            "Youtube Extension-Opera Add-Ons", Opera Software,
            <https://addons.opera.com/en/extensions/details/
            p2p-youtube/>.
 [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
 Coolpad
 EMail: hishigh@gmail.com
 Ning Zong
 Huawei Technologies
 EMail: zongning@huawei.com

Zhang & Zong Informational [Page 22]

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