PPSP                                                            Y. Zhang
Internet Draft                                              China Mobile
                                                         Yale University
Intended status: Informational                        September 25, 2011
Expires: March 2012

             Problem Statement of P2P Streaming Protocol (PPSP)


   P2P streaming systems show more and more popularity in current
   Internet with proprietary protocols. This document identifies
   problems of the proprietary protocols, proposes standard signaling
   protocols called PPSP and discusses the scope and use cases of PPSP.

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Table of Contents

   1. Introduction ................................................ 4
   2. Terminology and concepts ..................................... 6
   3. Problem statement ........................................... 8
      3.1. Difficulties for ISPs in deploying P2P caches ........... 8
      3.2. Difficulties in building open streaming delivery
      infrastructure .............................................. 8
      3.3. Difficulties in mobile and wireless environment .......... 9
      3.4. Difficulties for resource-constraint terminals to run
      multiple background programs at the same time ............... 10
   4. PPSP:Standard peer to peer streaming protocols .............. 11
   5. Use cases of PPSP .......................................... 14
      5.1. Worldwide provision of open P2P live streaming services . 14
      5.2. CDN supporting P2P streaming ........................... 15
      5.3. PPSP supporting cross-screen streaming in heterogeneous
      environment ................................................ 16
      5.4. Supporting P2P streaming in cellular mobile network ..... 16
      5.5. Cache service supporting P2P streaming ................. 17
   6. Security Considerations ..................................... 19
   7. IANA Considerations ........................................ 20
   8. Acknowledgments ............................................ 21
   9. References ................................................. 22
      9.1. Normative References ................................... 22
      9.2. Informative References ................................. 22

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1. Introduction

   Streaming traffic is among the fastest growing traffic on the
   Internet. As Cisco Visual Network Traffic index measured, video
   streaming already generates the largest volume of Internet traffic in
   the year of 2010, and the percentage is expected to rise to as high
   as 91% of the total Internet traffic by 2014[Cisco].

   There are two basic architectures for delivering streaming traffic on
   the Internet: the client-server paradigm and the peer to peer (P2P)
   paradigm [Survey]. The basic advantage of the P2P paradigm is its
   scalability and fault tolerance against failures of centralized
   infrastructures. As an example, PPLive [PPLive], one of the largest
   P2P streaming vendors, is able to distribute large-scale, live
   streaming programs such as the CCTV Spring Festival Gala to more than
   3 million users with only a handful of servers. It can also deliver
   VoD streaming to a scale of some hundred of thousands simultaneous
   users using the same structure and similar protocols [VoD]. The
   effect of P2P technologies is also well demonstrated in delivering
   real and VoD streaming effectively in current practice like CNN [CNN],
   PPstream [PPStream],UUSee [UUSee]and CNTV[CNTV]. The latest release
   of Adobe Flash, a major platform of streaming distribution in the
   Internet, has also introduced Cirrus [Cirrus], a peer assisted data
   exchange mode. One point that should also be noted is that P2P
   approach requires more resources and computational power on the
   clients (when compared to client-server architecture), as well as a
   lot of clients to participate in the P2P network for the network to
   be efficient. This is less challenging for highly increasing
   capability on hardware.

   What's more, along with the new players like CDN providers
   (e.g.,Akamai NetSession [Akamai], ChinaCache[ChinaCache]) joining in
   the effort of using P2P streaming delivery in providing their content,
   the P2P streaming ecosystem is becoming more complex with diverse
   players varying from the source, infrastructure side, edge delivery
   side even to the heterogeneous kinds of terminals.

   Given the increasing integration of P2P streaming into the global
   content delivery infrastructure, the lacking of an open, standard P2P
   streaming signaling protocol suite becomes a major missing component
   in the protocol stack. Almost all of these systems use their
   proprietary signaling protocols. Multiple, similar but proprietary
   signaling protocols result in repetitious development efforts for new
   systems, and the lock-in effects lead to substantial difficulties in
   their integration. For example, in the enhancement of existing caches
   and CDN systems to support P2P streaming, open protocols may reduce

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   the complexity of the interaction with different P2P streaming

   In this document we propose an open P2P streaming protocol named PPSP,
   to standardize signaling operations on two important components, peer
   and tracker in P2P streaming systems for information exchange. The
   problems of proprietary signaling protocols and benefit of PPSP are
   explained further in section 3.

   PPSP will serve as an enabling technology, building on the
   development experiences of existing P2P streaming systems. Its design
   will allow it to integrate with IETF protocols on distributed
   resource location, traffic localization, and streaming control and
   data transfer mechanisms for building a complete streaming system or
   updating /integrating existing cache/CDN to support P2P streaming

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2. Terminology and concepts

   Chunk: A chunk is a basic unit of partitioned streaming. Peers may
   use a chunk as a unit of storage, advertisement and exchange among
   peers [VoD]. Note that a streaming system may use different units for
   advertisement and data exchange, using chunks during data exchange,
   and a larger unit such as a set of chunks during advertisement.

   Content Distribution Network (CDN): A CDN node refers to a network
   entity that is deployed in the network (e.g., at the network edge or
   data centers) to store content provided by the original servers, and
   serves content to the clients located nearby topologically.

   Client: A client refers to the service requester in client/server
   computing paradigm. In this draft a client refers to a participant in
   a P2P streaming system that only receives streaming content. In some
   cases the node is not eligible to be a peer without enough computing
   and storage capability is acting as a client. It can be viewed as a
   specific kind of peer.

   Live streaming: It refers to a scenario where all clients receive
   streaming content for the same ongoing event. It is desired that the
   lags between the play points of the clients and that of the streaming
   source be small.

   P2P cache: A P2P cache refers to a network entity that caches P2P
   traffic in the network, and either transparently or explicitly as a
   peer distributes content to other peers.

   Peer: A peer refers to a participant in a P2P streaming system that
   not only receives streaming content, but also stores and uploads
   streaming content to other participants.

   PPSP: The abbreviation of Peer-to-Peer Streaming Protocols. PPSP
   refer to the key signaling protocols among various P2P streaming
   system components, including the tracker and the peer.

   Swarm: A swarm refers to a group of peers who exchange data to
   distribute the same content (e.g. video/audio program, digital file,
   etc) at a given time.

   Tracker: A tracker refers to a directory server which maintains a
   list of peers which participate in a specific video channel or in the
   distribution of a streaming file, and answers queries from peers for

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   peer lists. The tracker is a logical component which can be
   centralized or distributed.

   Video-on-demand (VoD): It refers to a scenario where different
   clients may watch different parts of the same recorded media with
   downloaded content.

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3. Problem statement

   The problems imposed by proprietary signaling for P2P streaming
   applications are listed as follows.

3.1. Difficulties for ISPs in deploying P2P caches

   Facing with many P2P streaming applications, ISPs are witnessing a
   big traffic tension on their backbone and inter-networking points.P2P
   caches are used to reduce the traffic by dynamically storing the
   frequently accessed streaming content (maybe in chunk or in file
   granularity). However, the cache nodes need to execute DPI (deep
   packet inspection) for identifying different P2P streaming systems.
   Multiple ever changing proprietary P2P streaming protocols require
   the P2P cache updating its matching library constantly which
   increases the operator's cost dramatically.

   With PPSP, P2P caches can detect P2P streaming applications much
   easier without needing to update its library, as there is only a
   single protocol to be detected and not a potentially unknown set of
   proprietary P2P protocols. This reduces the ISP workload to a large
   extent. Note that using standard PPSP won't hurt current P2P
   streaming vendors: Firstly, the openness of signaling interaction
   makes it easy to integrate them with ISP's caches for better user
   experience, say, smaller delay of the play. Secondly, -with PPSP,
   different applications use PPSP for signaling, but implement
   something system specific on top of that. That is to say, different
   P2P streaming systems compete on "on top" things, like scheduling
   algorithms, which is independent of how the peers exchange chunk
   availability. In other words, different systems can have quite
   different scheduling algorithms with same tracker/peer protocol,
   which is easier to be open.

3.2. Difficulties in building open streaming delivery infrastructure

   More and more efforts are seeking for building an open global
   streaming delivery infrastructure, where P2P-type is accounting for a
   large portion. However if current multiple proprietary protocols
   continue to work, there will exist lots of specific and independent
   systems to deliver vast of same streaming content. This brings more
   burdens for identifying and sharing the same contents, increases the
   storage, forwarding and maintenance cost in the intermediate nodes
   for repeated content. This will definitely increase the cost of
   streaming distribution and causes possible congestion in the network.

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   Consider a case where source vendors cooperate with 3rd party CDN
   provider. Such integration is already practiced by UUSee[UUSee],
   RayV[RayV] and Forcetech[Forcetech]. The effect has been verified to
   improve the total performance of P2P streaming (e.g., with lower
   latency) by providing more stable "super peers" and reduce traffic
   for ISP [CDN+P2P] [RFC 5693].However, there are substantial obstacles
   for CDN nodes supporting proprietary P2P streaming protocols [HPTP].
   Unlike the Web where all kinds of the infrastructure devices have
   been already equipped with standard HTTP protocol, an open CDN
   supporting various P2P streaming applications need to understand and
   keep updated on various protocols. Similar to the caching case in
   Section 3.1, this introduces complexity and deployment cost.

   With PPSP, CDN nodes can be designed to inter-operate with other
   devices by only standard protocols, reducing the case by case
   negotiation between the P2P streaming providers and CDN providers.On
   the other side, the interface between CDN nodes and user peers can be
   either HTTP or PPSP.

3.3. Difficulties in mobile and wireless environment

   Mobility and wireless are becoming increasingly important features in
   today's Internet. It is predicted that by the end of 2012, the number
   of mobile Internet users will surpass that of fixed Internet users in
   China [Statistics]. Mobile streaming has becoming a key offering. In
   Korea the number of mobile TV subscriber has reached seventeen
   millions, accounting for one third of the mobile subscribers. During
   the 2008 Beijing Olympic Games, more than one million users enjoyed
   mobile TV service. There are more and more studies exploring P2P
   streaming in mobile and wireless networks [Mobile Streaming1] [Mobile

   However it's difficult to copy current P2P streaming protocols in
   mobile and wireless networks. Current protocols are designed mainly
   for fixed Internet. Although smart handsets are more eligible to be
   peers with much better bandwidth and higher CPU frequency, larger
   storage and memory than before, peer selection is more challenging
   which needs more information to exchange during the tracker/peer and
   peer/peer communications: First, in mobile and wireless networks, the
   connections are unsteady, lower and costly(esp. in uplink). The
   trackers and peers may need more information, compared to fixed
   Internet, like packet loss rate, peer battery status and processing
   capability for peer selection. Note that not all mobile nodes are
   eligible to be peers. The new-added information should help the
   tracker/peer to make the decision. Second, current practices often
   use a "bitmap" message to exchange chunk availability among
   peers/trackers. The message is often of some kilobytes size and

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   relatively frequent to exchange. In the mobile networks, the
   bandwidth is scarce and a reasonable optimization is to reduce the
   message size, which maybe requires a new expression on "bitmap".
   Third, mobility issue. When a peer is moving and the IP address
   changes, the on-going connection and transmission between peers may
   be affected. Therefore such information should be reported in time,
   which is not addressed in current practices.

   PPSP should investigate these factors for a practical converged
   network from the beginning of the design.

3.4. Difficulties for resource-constraint terminals to run multiple
   background programs at the same time

   Private protocols may require a terminal to install different
   software for different applications. Note that for many client
   software, even it's not used by the users right now, the background
   program may be invoked to facilitate other peers for free data
   delivery assistance. In other words, there will be multiple
   background programs running at the same time. However it may be
   difficult to invoke multiple programs in a resource constraint peer
   like mobile handsets or set-top box. The limited CPU, storage and
   memory often limit the total number of concurrent threads and
   processes. Taking storage for example, according to
   [PPStream][UUSee][PPLive Design], the buffer of each peer's hard disk
   contributed to the system is at least 1GB. If each mobile peer, like
   iPhone (version 1) runs two such background applications at the same
   time, the storage cannot be shared for different applications and it
   will consume one fourth of all its storage (8 GB), leaving other data
   with fewer storage.

   PPSP can help to reduce the resource consumption on resource
   constraint devices, such as STBs or mobile phones, by reusing a PPSP
   base library and potentially other optimizations.

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4. PPSP:Standard peer to peer streaming protocols

   The objective of the PPSP working group is to design a unified peer-
   to-peer streaming protocol (PPSP) to address the problems discussed
   in the preceding sections.

   There are basically two kinds of P2P streaming systems, pull-based
   and push-based.

   In pull-based P2P streaming systems, a centralized tracker or
   distributed trackers maintains information about which peers are in
   which swarms and answers the peers' query on such information with a
   peer-list. After receiving the message, the peer can connect with the
   candidates in a swarm, exchange its content availability in its
   memory or storage (depending on it's real-time or VoD streaming) with
   other peers and then retrieve the wanted streaming data. The swarm is
   a mesh topology. Most of the current practices are belonging to this
   genre. The advantages of pull-based mode are its robustness to the
   peer churn and acceptable latency for a smooth play.

   In push-based P2P streaming systems, there is a head node maintaining
   the topology, e.g., a tree. The peers in this topology share the same
   interest on content. The signaling and data distribution are both
   based on this topology. For one program or video file, the peer
   queries the head node for its location to join and the head node
   replies with a peer-list(potentially in a recommended order). After
   receiving this peer-list, the peer can connect with the candidates
   for being a node in certain place of the topology and receive the
   data along this topology without the need of exchanging content
   availability with its siblings. In this sense the head node is acting
   as the tracker. The push mode has the advantages of lower latency but
   the topology is fragile to the peer churn. Few commercially deployed
   systems use this mode.

   A more practical mode is a hybrid pull-push mode where the peers
   exchange content availability with its siblings for retrieving
   requsted data.

   In live streaming, all peers are interested in the media coming from
   an ongoing event, which means that all peers share nearly the same
   streaming content at a given point of time. In live streaming, some
   peers may store the live media for further distribution, which is
   known as TSTV (time-shift TV), where the stored media are separated
   into chunks and distributed in a VoD-like manner.

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   In VoD, different peers watch different parts of the recorded media
   content during a past event. In this case, each peer keeps asking
   other peers which media chunks are stored in which peers, and then
   gets the required media from certain/selected peers.

   To sum up, in essence, there are two important entities in P2P
   streaming, i.e., trackers and peers in P2P streaming systems. PPSP is
   targeted to standardize the signaling protocols in this tracker-based
   architectures for supporting both live and VoD streaming.

   In detail, PPSP designs a protocol for signaling between trackers and
   peers (the PPSP "tracker protocol") and a signaling protocol for
   communication among the peers (the PPSP "peer protocol") as shown in
   Figure 1. The two protocols enable peers to receive streaming data
   within the time constraints required by specific content items. The
   tracker protocol handles the initial and periodic exchange of meta
   information between trackers and peers, such as peer-list and content
   information. The peer protocol controls the advertising and exchange
   of media data between the peers.

   Note that in the pull mode and hybrid pull-push mode, both tracker
   protocol and peer protocol can be used; while in the push mode, only
   tracker protocol is used.

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

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5. Use cases of PPSP

5.1. Worldwide provision of open P2P live streaming services

   The cooperative vendors can easily expand the broadcasting scale with
   PPSP. In figure 2 shows the case that vendor A broadcasts the program
   with the help of vendor B and vendor C for a wider coverage. The
   interaction between vendor A's tracker and vendor B and vendor C's
   super-nodes (SN in short) can be normalized using tracker protocol;
   and peer protocol can be used among SNs/peers spread in different

   |                                                                   |
   |                          +------------------+                     |
   |            +------------>| A's      Tracker |<----------+         |
   |            |             +------------------+           |         |
   |     Tracker|                ^              ^            |         |
   |    Protocol|         Tracker|              |Tracker     |Tracker  |
   |            |        Protocol|              |Protocol    |Protocol |
   |            |                |              |            |         |
   |            |                |              |            |         |
   |            v                v              v            v         |
   |      +------+ Peer    +------+            +------+    +------+    |
   |      | B's  |<------->| B's  |            | C's  |    | C's  |    |
   |      | SN1  |Protocol | SN2  |            | SN1  |    | SN2  |    |
   |      +------+         +------+            +------+    +------+    |
   |         ^  ^                                           ^ ^        |
   |         |  |                                           | |        |
   |         |  | Peer Protocol                Peer Protocol| |        |
   | Peer    |  +-------------+              +--------------+ |Peer    |
   | Procotol|                |              |                |protocol|
   |         |                |              |                |        |
   |         |                |              |                |        |
   |         |                |              |                |        |
   |         v                v              v                v        |
   |      +------+ Peer    +------+    +---------+  Peer   +---------+ |
   |      | A's  |<------> | B's  |    |A's      |<------> |C's      | |
   |      | User1|Protocol | User2|    | User1   |Protocol | User2   | |
   |      +------+         +------+    +---------+         +---------+ |
   |                                                                   |
                 Figure 2 Cooperative Vendors Interaction

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5.2. CDN supporting P2P streaming

   This scenario is similar to use case 1 except that the intermediate
   SNs are replaced by 3rd party CDN surrogates with PPSP. The P2P
   streaming vendors A and B can rent CDN surrogates to provide higher
   QoS services for VIP users than services provides by only ordinary
   peers. The interaction among these network entities are shown in
   Figure 3. The CDN nodes talk with the different trackers and peers
   with the uniform Tracker and peer protocols. It can also communicate
   with end users using HTTP for legacy equipments. The internal
   interaction of CDN nodes can be executed by either original internal
   protocol or new peer protocol. The latter is used when building a new
   CDN system supporting streaming applications with low cost deploying
   P2P delivery inside the network.

   |                                                                   |
   |                   +-------------+    +--------------+             |
   |            +----->| A's Tracker |    |  B's Tracker |<---+        |
   |            |      +-------------+    +--------------+    |        |
   |     Tracker|              ^  ^        ^    ^             |        |
   |    Protocol|       Tracker|  |Tracker |    |Tracker      |Tracker |
   |            |      Protocol|  |Protocol|    |Protocol     |Protocol|
   |            |              |  |        |    |             |        |
   |            |              |  |        |    |             |        |
   |            v              v  |        |    v             v        |
   |      +------+ Peer   +------+|        |  +------+Internal+------+ |
   |      | CDN  |<------>| CDN  ||        |  | CDN  |<-----> | CDN  | |
   |      | Node1|Protocol| Node2||        |  | Node3|Protocol| Node4| |
   |      +------+        +------+|        |  +------+        +------+ |
   |         ^  ^                 |        |        ^         ^        |
   |         |  |                 |        |        |         |        |
   |         |  | Peer Protocol   |        |   HTTP |         |        |
   | Peer    |  +-------------+   |        | +------+         | Peer   |
   | Procotol|                |   |        | | Protocol       |protocol|
   |         |                | +-+        | |                |        |
   |         |                | |          | |                |        |
   |         |                | |          | |                |        |
   |         v                v v          v v                v        |
   |      +------+ Peer    +------+    +---------+  Peer   +---------+ |
   |      | A's  |<------> | A's  |    |B's      |<------> |B's      | |
   |      | User1|Protocol | User2|    | User3   |Protocol | User4   | |
   |      +------+         +------+    +---------+         +---------+ |
   |                                                                   |
                   Figure 3 CDN Supporting P2P Streaming

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5.3. PPSP supporting cross-screen streaming in heterogeneous environment

   In this scenario PC, Setbox/TV and mobile terminals from both fixed
   network and mobile network share the content they store/cache. Peers
   from heterogeneous networks

   With PPSP Peers can identify the types of access networks, average
   load, peer abilities and get to know what content other peers have
   (potentially with the conversion of the content availability
   expression in different networks) even in different network
   conditions as shown in Figure 4. These information will play an
   important role on selecting suitable peers, e.g., a PC or STB node is
   more likely to be selected to provide stable content for mobile nodes;
   a mobile peer within a high-load base station is unlikely to be
   selected, which may lead to higher load on the base station.

   |                                                                   |
   |      Tracker Protocol  +---------+   Tracker Protocol             |
   |        +-------------> | Tracker |<------------------+            |
   |        |               +---------+                   |            |
   |        |                    ^                        |            |
   |        |                    |                        |            |
   |        |                    |                        |            |
   |        V                    |                        V            |
   |    +------+                 |                +------------+       |
   |    |  STB |           Tracker Protocol       |Mobile Phone|       |
   |    +------+                 |                +------------+       |
   |        ^                    |                        ^            |
   |        |                    |                        |            |
   |        |                    |                        |            |
   |        |                    V                        |            |
   |        |Peer Protocol  +---------+    Peer Protocol  |            |
   |        +-------------> |    PC   |<------------------+            |
   |                        +---------+                                |
   |                                                                   |
         Figure 4 Heterogeneous P2P Streaming Interaction with PPSP

5.4. Supporting P2P streaming in cellular mobile network

   In a cellular mobile environment like 3G or 4G, with the increase in
   bandwidth and smart mobile terminal capabilities, P2P (including P2P

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   streaming) is easier to be realized than before. In a provincial
   network of China Mobile, P2P has accounted for more than 30
   percentage of the traffic, ranked second.

   Note that the mobile terminals are not compulsorily to be peers. Here
   they act as clients. Network peers who are deployed by the ISPs or
   operators and mobile peers with WiFi connections are more likely to
   be selected. For example, in 3GPP, there is a P2P CDS work item
   working on the requirement of mobile operators to prefer use deployed
   network-side equipments (e.g., serving gateways or GGSNs, one access
   point from cellular mobile network to the Internet) to act as super-
   peers when there are no enough eligible peers to realize P2P
   streaming[P2P CDS]. Because they are deployed by the operators, the
   stability and storage size are better guaranteed than ordinary peers.

   Similar with case 5.3, PPSP tracker protocol will help to identify
   and return the super-peers in the peer-list with preference. If
   mobile terminals are not eligible to be peers, they can simply
   receive data from these super-peers without contributing any data to

5.5. Cache service supporting P2P streaming

   As discussed in the Section 3, deploying cache nodes at the network
   edges can greatly decrease the inter-network traffic and increase
   user experience in streaming service.

   With PPSP, the cache nodes can identify the P2P streaming genre even
   it may include different applications. When a peer requests the
   streaming data, cache detects the request and requests the frequent
   visited content (or part of) to the original tracker as a normal peer.
   The tracker replies with (outward) peers. After the cache connectes
   with the peers, it can report what it cache to the provider's tracker
   like a normal peer and serve other requesting peers inside to reduce
   the cross-ISP traffic as shown in Figure 5. The cache nodes needn't
   update their library when new applications supporting PPSP are
   introduced, which enable the cache nodes spend less cost to support
   more applications.

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Internet-Draft        Problem Statement of PPSP         September 2011

   |                                                                |
   |  0:Tracker Protocol +---------+                                |
   |  +----------------> | Tracker |                                |
   |  |                  +---------+                                |
   |  |                       ^                                     |
   |  |                       |                                     |
   |  |                    2: | Tracker Protocol                    |
   |  |                       |                                     |
   |  |                       |                                     |
   |  |             +---------|-------------------------------------|
   |  |             |         V                                     |
   |  |             |     +---------+                               |
   |  |  +----------|---> | Cache   |<-------------------+          |
   |  |  |          |     +---------+   1,4: Tracker/Peer|          |
   |  |  |3: Peer   |                       Protocol     |          |
   |  |  | Protocol |                                    |          |
   |  |  |          |                                    |          |
   |  |  |          |                                    |          |
   |  V  V          |                                    V          |
   |  +-----------+ |        ISP Domain             +------------+  |
   |  |  Outward  | |                               |   Inside   |  |
   |  |  Peer     | |                               |   Peer     |  |
   |  +-----------+ |                               +------------+  |

           Figure 5 Cache Service Supporting Streaming with PPSP

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6. Security Considerations

   This document discusses the problem statement around peer-to-peer
   streaming protocols without specifying the protocols. The protocol
   specification is deferred to other documents under development in the
   PPSP working group. However we believe it is important for the reader
   to understand areas of security caused by the p2p nature of the
   proposed solution. The main issue is the usage of untrusted entities
   (peers) for service provisioning.

   Malicious peers may, for example:

   - Issue denial of service (DOS) attacks to the trackers by sending
   large amount of requests with the tracker protocol;

   - Issue fake information on behalf of other peers;

   - Issue fake information about available content;

   - Issue fake information about chunk availability;

   Malicious peers/trackers may, for example:

   - Issue reply instead of the regular tracker (man in the middle

   The PPSP protocol specifications, e.g., the tracker protocol and the
   peer protocol, will document the expected threats and how they will
   be mitigated for each protocol, but also considerations on threats
   and mitigations when combining both protocols in an application. This
   will include privacy of the users, protection of the content
   distribution, but not protection of the content by Digital Rights
   Management (DRM).

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7. IANA Considerations

   This document has no actions for IANA.

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8. Acknowledgments

   We would like to acknowledge the following people who provided review,
   feedback and suggestions to this document: M. Stiemerling; C. Schmidt;
   D. Bryan; E. Marocco; V. Gurbani; R. Even; H. Zhang; L. Xiao; C.
   Williams; V. Pasual; D. Zhang; J. Lei.

   This document was prepared using 2-Word-v2.0.template.dot.

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9. References

9.1. Normative References

   [RFC 5693], Application-Layer Traffic Optimization (ALTO) Problem
             Statement, E. Marocco et al,

9.2. Informative References

    [Cisco] Cisco Visual Networking Index: Forecast and Methodology,

   [PPLive] www.pplive.com

   [VoD] Yan Huang et al,Challenges, "Design and Analysis of a Large-
             scale P2P-VoD System", Sigcomm08.

   [CNN] www.cnn.com

   [PPStream] www.ppstream.com

   [UUSee] www.uusee.com

   [CNTV] www.cntv.com

   [Cirrus] labs.adobe.com/technologies/cirrus/

   [Akamai] Peer-to-Peer Systems, Rodrigo Rodrigues et al,
             Communications of the ACM,Vol. 53 No. 10, Pages 72-82.


   [Survey] Yong Liu et al, "A survey on peer-to-peer video streaming
             systems", Peer to Peer Networking and
             Applications (2008) Volume 1, Number 1,:18-28,Springer.

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   [RayV] http://www.rayv.com

   [Forcetech] http://www.forcetech.net/english/solutions

   [CDN+P2P] H. Jiang et al,"Efficient Large-scale Content Distribution
             with Combination of CDN and P2P Networks", International
             Journal of Hybrid Information Technology, Vol.2, No.2,
             April, 2009.

   [HPTP] HPTP: Relieving the Tension between ISPs and P2P, Guobin Shen
             et al, IPTPS 2007.

   [Statistics] http://labs.chinamobile.com/news/48283

   [P2P CDS] 3GPP TR 22.906, Study on IMS based peer-to-peer content
             distribution services,http://www.3gpp.org/ftp/Specs/html-

   [Mobile Streaming1] Streaming To Mobile Users In A Peer-to-Peer
             Network,Jeonghun Noh et al,MOBIMEDIA '09.

   [Mobile Streaming2] J. Peltotalo et al.,"A real-time peer-to-peer
             streaming system for mobile networking environment", in
             Proceedings of the INFOCOM and Workshop on Mobile Video
             Delivery (MoVID '09), April 2009.

   [PPLive Design] Y. Huang, T. Fu, D. Chiu, J. Lui, and C.
             Huang ,"Challenges, design and analysis of a large-scale
             p2p-vod system", ACM SIGCOMM Computer Communication Review,
             38(4):375-388, 2008.

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Authors' Addresses

   Yunfei Zhang
   China Mobile Communication Corporation

   Ning Zong
   Huawei Technologies Co., Ltd.

   Gonzalo Camarillo

   James Seng

   Richard Yang
   Yale University

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