PPSP Y. Zhang Internet Draft China Mobile N.Zong HuaweiTech G.Camarillo Ericsson R.Yang Yale University V. Pascual Acme Packet Intended status: Informational February 27, 2012 Expires: August 2012 Problem Statement and Requirements of Peer-to-Peer Streaming Protocol (PPSP) draft-ietf-ppsp-problem-statement-08.txt Abstract Peer-to-Peer (P2P for short) streaming systems show more and more popularity in current Internet with proprietary protocols. This document identifies problems of the proprietary protocols, proposes a Peer to Peer Streaming Protocol (PPSP) including tracker and peer signaling components, and discusses the scope, uses cases and requirements of PPSP. zhang Expires August 27, 2012 [Page 1] Internet-Draft Problem Statement and Requirments of PPSP February 2012 Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on August 27, 2012. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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 Expires August 27, 2012 [Page 2] Internet-Draft Problem Statement and Requirments of PPSP February 2012 Table of Contents 1. Introduction ................................................ 4 2. Terminology and concepts .................................... 5 3. Problem statement ........................................... 7 3.1. Traffic issue and difficulties for ISPs in deploying P2P caches ...................................................... 7 3.2. Efficiency issue and difficulties in building open streaming delivery infrastructure ..................................... 7 3.3. Extended applicability issue and difficulties in mobile and wireless environment......................................... 8 4. PPSP: Standard peer to peer streaming protocols ............. 10 5. Use cases of PPSP ........................................... 12 5.1. Worldwide provision of live/VoD streaming .............. 12 5.2. PPSP supporting cross-screen streaming in heterogeneous environment ................................................. 14 5.3. Cache service supporting P2P streaming ................. 15 6. Security Considerations ..................................... 17 7. Requirements of PPSP ........................................ 18 7.1. Basic Requirements ..................................... 18 7.2. PPSP Tracker Protocol Requirements ..................... 19 7.3. PPSP Peer Protocol Requirements ........................ 21 7.4. Security Requirements .................................. 22 8. IANA Considerations ......................................... 24 9. Acknowledgments ............................................. 25 10. Informative References ..................................... 26 zhang Expires August 27, 2012 [Page 3] Internet-Draft Problem Statement and Requirments of PPSP February 2012 1. Introduction Streaming traffic is among the largest and fastest growing traffic on the Internet [Cisco], where peer-to-peer (P2P) streaming contribute a lot. With the advantage of high scalability and fault tolerance against single point of failure, P2P streaming applications are able to distribute large-scale, live and VoD streaming programs to millions of audience with only a handful of servers. What's more, along with the new players like CDN providers joining in the effort of using P2P technologies in distributing their serving streaming content, there are more and more various players in P2P streaming ecosystem. Given the increasing integration of P2P streaming into the global content delivery infrastructure, the lack of an open, standard P2P streaming signaling protocol suite becomes a major missing component in the protocol stack. Almost all of existing systems use their proprietary protocols. Multiple, similar but proprietary protocols result in repetitious development efforts for new systems, and the lock-in effects lead to substantial difficulties in their integration with other players like CDN. For example, in the enhancement of existing caches and CDN systems to support P2P streaming, proprietary protocols may increase the complexity of the interaction with different P2P streaming applications. In this document we propose an open P2P Streaming Protocol, which is defined as PPSP, to standardize signaling operations on two important components, peer and tracker in P2P streaming systems for information exchange. Note that using PPSP would not hurt current P2P streaming vendors: Firstly, the openness of signaling interaction would make it easy to integrate them with some infrastructural components like ISP's caches or CDNs for better user experience, say, smaller delay of the play. Secondly, different applications could use the same PPSP for signaling, but implement system specific mechanisms on top of that. That is to say, different P2P streaming systems compete on "on top" things, like scheduling algorithms, which is independent of the proposed protocols. zhang Expires August 27, 2012 [Page 4] Internet-Draft Problem Statement and Requirments of PPSP February 2012 2. Terminology and concepts Chunk: A chunk is a basic unit of data block organized in P2P streaming for storage, scheduling, advertisement and exchange among peers [VoD]. A chunk size varies from several KB to several MB in different systems. In case of MB size chunk scenario, a sub-chunk structure named piece is often defined to fit in a single transmitted packet. A streaming system may use different granularities for different usage, e.g., using chunks during data exchange, and using a larger unit such as a set of chunks during advertisement. Content Distribution Network (CDN): A CDN node refers to a network entity that is deployed in the network (e.g., at the network edge or data centers) to store content provided by the original servers, and serves content to the clients located nearby topologically. Client: A client refers to the service requester in client/server computing paradigm. In this draft a client refers to a participant in a P2P streaming system that only receives streaming content. In some cases the node is not eligible to be a peer without enough computing and storage capability is acting as a client. It can be viewed as a specific kind of peer. Live streaming: It refers to a scenario where all clients receive streaming content for the same ongoing event. It is desired that the lags between the play points of the clients and that of the streaming source be small. P2P cache: A P2P cache refers to a network entity that caches P2P traffic in the network, and either transparently or explicitly as a peer distributes content to other peers. Peer: A peer refers to a participant in a P2P streaming system that not only receives streaming content, but also stores and uploads streaming content to other participants. PPSP: The abbreviation of Peer-to-Peer Streaming Protocols. PPSP refer to the key signaling protocols among various P2P streaming system components, including the tracker and the peer. Swarm: A swarm refers to a group of peers who exchange data to distribute chunks of the same content (e.g. video/audio program, digital file, etc) at a given time. zhang Expires August 27, 2012 [Page 5] Internet-Draft Problem Statement and Requirments of PPSP February 2012 Tracker: A tracker refers to a directory server which maintains a list of peers which participate in a specific video channel or in the distribution of a streaming file, and answers queries from peers for peer lists. The tracker is a logical component which can be centralized or distributed. Video-on-demand (VoD): It refers to a scenario where different clients may watch different parts of the same recorded media with downloaded content. Peer list: A list of peers which are in a same swarm maintained by the tracker. A peer can fetch the peer list of a swarm from either tracker or other peers to know which peers have the required streaming content. Peer ID: An identifier of a peer such that other peers or tracker can refer the ID for the peer. Swarm ID: An identifier of a swarm containing a group of peers sharing a same streaming content. Chunk ID: An identifier of a chunk in a streaming content. zhang Expires August 27, 2012 [Page 6] Internet-Draft Problem Statement and Requirments of PPSP February 2012 3. Problem statement The problems imposed by proprietary protocols for P2P streaming applications are listed as follows. 3.1. Traffic issue and difficulties for ISPs in deploying P2P caches Facing with many P2P streaming applications, ISPs are witnessing a big traffic tension on their backbone and inter-networking points.P2P caches are used for ISPs to reduce the traffic by dynamically storing the frequently accessed streaming content (in chunk or in file granularity). However, unlike the Web where all kinds of the infrastructure devices have been already equipped with standard HTTP protocol, cache systems have to build a matching library to identify different P2P streaming protocols firstly. Multiple ever changing proprietary protocols require the cache system updating its matching library constantly. This increases the operator's cost dramatically. With PPSP, P2P caches needn't learn new proprietary protocols any longer, which would reduce the ISP workload much. 3.2. Efficiency issue and difficulties in building open streaming delivery infrastructure Another problem for P2P streaming applications is the efficiency issue. P2P streaming is often criticized by longer delays (e.g., startup delay, seek delay and channel switch delay). Hybrid CDN/P2P is a good means to solve this problem for operators [Hybrid CDN P2P]. In such design, CDN takes two roles: one is for media streaming server and the other is for P2P tracker. Consider a CDN vendor serving for various P2P streaming, similar to the P2P cache issue in Section 3.1, proprietary P2P streaming protocols introduce interaction complexity between the peer and tracker, and increase deployment cost of CDN nodes. With PPSP, CDN nodes acting as the P2P tracker can be designed to inter-operate with other devices by only standard protocols, reducing the case by case negotiation. On the other side, the interface between edged CDN nodes and user peers could be either via something like traditional HTTP, or via PPSP peer protocol. zhang Expires August 27, 2012 [Page 7] Internet-Draft Problem Statement and Requirments of PPSP February 2012 3.3. Extended applicability issue and difficulties in mobile and wireless environment Mobility and wireless are becoming increasingly important in today's Internet, where streaming service is a major usage. In Korea the number of mobile TV subscriber has reached seventeen million, accounting for one third of the mobile subscribers. There are multiple prior studies exploring P2P streaming in mobile and wireless networks [Mobile Streaming1] [Mobile Streaming2]. However it's difficult to copy current P2P streaming protocols (even we suppose we can re-use the proprietary ones) in mobile and wireless networks. Current protocols are designed mainly for fixed Internet. Although smart handsets are more eligible to be peers with much higher bandwidth and CPU frequency, larger storage and memory than before, peer selection becomes more challenging which needs more information to exchange during the tracker/peer and peer/peer communications: First, the connections are unsteady, lower rate, and costly in terms of energy consumption and transmission (esp. in uplink). The trackers and peers may need more information like packet loss rate, peer battery status and processing capability for peer selection. Second, current practices often use a "bitmap" message to exchange chunk availability among peers/trackers. The message is often of some kilobytes size and exchanged relatively frequently, say, some seconds. In a scarce bandwidth resource environment, a reasonable optimization is to reduce the message size, which may require alternative methods for expressing and distributing bitmap information. Third, when a peer is moving and if the IP address changes, the on- going connection and transmission between peers may be affected. Such information should be reported in time. Fourth, for a resource constraint peer like mobile handsets or set- top boxes (STB), the limited resource and the requirements for installing various applications form big conflicts. On one side, the limited CPU, storage and memory often limit the total number of concurrent threads and processes. One the other side, the proprietary protocols require the user to install many different applications for different usage, for instance, some applications have rich resources on TV series or movies while others may offer rich broadcasting sports program. What's worse, for many P2P applications, even they are not used by the users right now, the background program may be invoked to facilitate other peers for free data delivery assistance. That is to say, there will be multiple background programs running at zhang Expires August 27, 2012 [Page 8] Internet-Draft Problem Statement and Requirments of PPSP February 2012 the same time. But it may be difficult to invoke multiple programs in such a resource constraint peer. PPSP should investigate these factors and help to reduce the resource consumption in a converged network. zhang Expires August 27, 2012 [Page 9] Internet-Draft Problem Statement and Requirments of PPSP February 2012 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 is 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 belong to this genre. The advantages of pull-based mode are its robustness to the peer churn and acceptable latency for a smooth play. Most commercial systems either live streaming and VoD use this mode. In push-based P2P streaming systems, there is a head node maintaining the topology, e.g., a tree. The head node acts similar to a tracker. 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 by offline or pre-set head node address information 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, as done in pull-based mode. In this sense the head node is acting as the tracker in the pull- based mode. The push mode has the advantages of lower latency but the topology is fragile to the peer churn and is hard to deal with the VoD scenario. This makes it less robust in practical running. Few commercially deployed systems use this mode. PPSP is targeted to standardize signaling protocols for tracker-based architectures feasible for both modes above that support either live or offline streaming. The PPSP design includes 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 zhang Expires August 27, 2012 [Page 10] Internet-Draft Problem Statement and Requirments of PPSP February 2012 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, both tracker protocol and peer protocol can be used; while in the push mode, only tracker protocol is used. +------------------------------------------------+ | | | +--------------------------------+ | | | Tracker(Head Node) | | | +--------------------------------+ | | | ^ ^ | |Tracker | | Tracker |Tracker | |Protocol| | Procotol |Protocol | | | | | | | V | | | | +---------+ Peer +---------+ | | | Peer |<----------->| Peer | | | +---------+ Protocol +---------+ | | | ^ | | | |Peer | | | |Protocol | | V | | | +---------------+ | | | Peer | | | +---------------+ | | | | | +------------------------------------------------+ Figure 1 PPSP System Architecture zhang Expires August 27, 2012 [Page 11] Internet-Draft Problem Statement and Requirments of PPSP February 2012 5. Use cases of PPSP 5.1. Worldwide provision of live/VoD streaming The content provider can easily expand the broadcasting/VoD scale to utilize the cooperative content providers' distribution networks or third party CDN networks with PPSP. Figure 2 shows the case that provider A broadcasts the program with the help of provider B and C for a wider coverage. Without PPSP, when users in B or C's domain (outside A's main serving zone) requests A's programs, the returned peer-list may include few local peers, which may decrease the user viewing experience. With PPSP more local resources from cooperative vendors may be utilized. The content providers often deploy in-network peers called super-nodes (SN for short) with better stability and higher storage and bandwidth for better QoS. With tracker protocol, vendor A's tracker can returns user request with vendor B and vendor C's SNs in the peer-list. Users in B and C's domain can exchange data (availability) with these SNs using peer protocol for better QoS. In this way vendor B and vendor C's SNs resources are shared with vendor A and vendor A expands its serving scale with acceptable QoS. zhang Expires August 27, 2012 [Page 12] Internet-Draft Problem Statement and Requirments of PPSP February 2012 +-------------------------------------------------------------------+ | | | +------------------+ | | +------------>| A's Tracker |<----------+ | | | +------------------+ | | | Tracker| ^ ^ | | | Protocol| Tracker| |Tracker |Tracker | | | Protocol| |Protocol |Protocol | | | | | | | | | | | | | | v v v v | | +------+ Peer +------+ +------+ +------+ | | | B's |<------->| B's | | C's | | C's | | | | SN1 |Protocol | SN2 | | SN1 | | SN2 | | | +------+ +------+ +------+ +------+ | | ^ ^ ^ ^ | | | | | | | | | | Peer Protocol Peer Protocol| | | | Peer | +-------------+ +--------------+ |Peer | | Procotol| | | |protocol| | | | | | | | | | | | | | | | | | | | v v v v | | +------+ Peer +------+ +---------+ Peer +---------+ | | | A's |<------> | B's | |A's |<------> |C's | | | | User1|Protocol | User2| | User1 |Protocol | User2 | | | +------+ +------+ +---------+ +---------+ | | | +-------------------------------------------------------------------+ Figure 2 Cooperative Vendors Interaction Figure 3 is similar to Figure 2 except that the intermediate SNs are replaced by 3rd party CDN surrogates with PPSP. The P2P streaming vendors A and B can rent CDN surrogates to provide higher QoS for VIP users. The CDN nodes talk with the different vendors (including the peers inside) with the uniform protocols. For users who use browser equipped with HTTP, scalable streaming is also achieved. The internal interaction of CDN nodes can be executed by either existing protocol or peer protocol. The latter is used when building a new CDN system supporting streaming applications with lower cost. zhang Expires August 27, 2012 [Page 13] Internet-Draft Problem Statement and Requirments of PPSP February 2012 +-------------------------------------------------------------------+ | | | +-------------+ +--------------+ | | +----->| A's Tracker | | B's Tracker |<---+ | | | +-------------+ +--------------+ | | | Tracker| ^ ^ ^ ^ | | | Protocol| Tracker| |Tracker | |Tracker |Tracker | | | Protocol| |Protocol| |Protocol |Protocol| | | | | | | | | | | | | | | | | | v v | | v v | | +------+ Peer +------+| | +------+Internal+------+ | | | CDN |<------>| CDN || | | CDN |<-----> | CDN | | | | Node1|Protocol| Node2|| | | Node3|Protocol| Node4| | | +------+ +------+| | +------+ +------+ | | ^ ^ | | ^ ^ | | | | | | | | | | | | Peer Protocol | | HTTP | | | | Peer | +-------------+ | | +------+ | Peer | | Procotol| | | | | Protocol |protocol| | | | +-+ | | | | | | | | | | | | | | | | | | | | | v v v v v v | | +------+ Peer +------+ +---------+ Peer +---------+ | | | A's |<------> | A's | |B's |<------> |B's | | | | User1|Protocol | User2| | User3 |Protocol | User4 | | | +------+ +------+ +---------+ +---------+ | | | +-------------------------------------------------------------------+ Figure 3 CDN Supporting P2P Streaming 5.2. PPSP supporting cross-screen streaming in heterogeneous environment In this scenario PC, STB/TV and mobile terminals from both fixed network and mobile/wireless network share the streaming content. With PPSP, peers can identify the types of access networks, average load, peer abilities and get to know what content other peers have (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 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. zhang Expires August 27, 2012 [Page 14] Internet-Draft Problem Statement and Requirments of PPSP February 2012 +-------------------------------------------------------------------+ | | | Tracker Protocol +---------+ Tracker Protocol | | +-------------> | Tracker |<------------------+ | | | +---------+ | | | | ^ | | | | | | | | | | | | | V | V | | +------+ | +------------+ | | | STB | Tracker Protocol |Mobile Phone| | | +------+ | +------------+ | | ^ | ^ | | | | | | | | | | | | | V | | | |Peer Protocol +---------+ Peer Protocol | | | +-------------> | PC |<------------------+ | | +---------+ | | | +-------------------------------------------------------------------+ Figure 4 Heterogeneous P2P Streaming Interaction with PPSP 5.3. Cache service supporting P2P streaming In Figure 5, when peers request the P2P streaming data, the cache nodes intercept the requests and ask for the frequently visited content (or part of) on behalf of the user peers. To do this, it requests peer-list to the tracker and the tracker replies with (outward) peers in the peer-list. After the cache nodes exchange data with these peers, it can report what it cache to the tracker like a normal peer and serve other requesting peers inside. This operation greatly decreases the inter-network traffic and increase user experience in P2P streaming services for an ISP. 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. zhang Expires August 27, 2012 [Page 15] Internet-Draft Problem Statement and Requirments of PPSP February 2012 +----------------------------------------------------------------+ | | | 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 zhang Expires August 27, 2012 [Page 16] Internet-Draft Problem Statement and Requirments of PPSP February 2012 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 un-trusted entities (peers) for service provisioning. Malicious peers may, for example: - Issue denial of service (DOS) attacks to the trackers by sending large amount of requests with the tracker protocol; - Issue fake information on behalf of other peers; - Issue fake information about available content; - Issue fake information about chunk availability; Malicious peers/trackers may, for example: - Issue reply instead of the regular tracker (man in the middle attack). The PPSP protocol specifications, 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). zhang Expires August 27, 2012 [Page 17] Internet-Draft Problem Statement and Requirments of PPSP February 2012 7. Requirements of PPSP This section enumerates the requirements for the PPSP, which should be considered when designing PPSP. 7.1. Basic Requirements PPSP.REQ-1: The tracker and the peer protocols SHOULD be as similar as possible, in terms of design, message formats and flows. It is desirable that the peer protocol would be an extension to the tracker protocol by adding a few message types, or vice versa. PPSP.REQ-2: The tracker protocol and the peer protocol SHOULD enable peers to receive streaming content within the required time constraints, i.e., fulfill streaming feature. PPSP.REQ-3: Each peer MUST have a unique ID (i.e. peer ID) in a swarm. It's a basic requirement for a peer to be uniquely identified in a swarm that other peers or tracker can refer to the peer by ID. PPSP.REQ-4: The streaming content MUST be uniquely identified by a swarm ID. A swarm refers to a group of peers sharing the same streaming content. A swarm ID uniquely identifies a swarm. The swarm ID can be used in two cases: 1) a peer requests the tracker for the peer list indexed by a swarm ID; 2) a peer tells the tracker about the swarms it belongs to. PPSP.REQ-5: The streaming content MUST allow to be partitioned into chunks. A key characteristic of P2P streaming system is allowing the data fetching from different peers concurrently. Therefore, the whole streaming content must allow to be partitioned into small pieces or chunks for transmission between peers. PPSP.REQ-6: Each chunk MUST have a unique ID (i.e. chunk ID) in the swarm. Each chunk must have a unique ID in the swarm such as the peer can understand which chunks are stored in which peers and which chunks zhang Expires August 27, 2012 [Page 18] Internet-Draft Problem Statement and Requirments of PPSP February 2012 are requested by other peers. An example for generating the chunk ID is the buffer map approach [I-D.ietf-ppsp-survey]. PPSP.REQ-7: The tracker protocol and peer protocol are recommended to be carried over TCP (or UDP, when delivery requirements cannot be met by TCP). PPSP.REQ-8: The tracker and peer protocol together MUST facilitate acceptable QoS (e.g. low startup delay, low channel/content switching time and minimal end-to-end delay) for both on-demand and live streaming, even for very popular content. The tracker and peer protocol do not include the algorithm required for scalable streaming. However, the tracker and peer protocol SHALL NOT restrict or place limits on any such algorithm. There are basic QoS requirements for streaming system. Setup time to receive a new streaming channel or to switch between channels should be reasonable small. End to end delay (time between content generation, e.g. camera and content consumption, e.g. user side monitor) will become critical in case of live streaming. Especially in provisioning of sports events, end to end delay of 1 minute and more are not acceptable. For instance, the tracker and peer protocols can support carrying QoS related parameters (e.g. video quality, delay requirements) together with the priorities of these parameters, and QoS situation (e.g., performance, available uplink bandwidth) of content providing peers. There are also some other possible mechanisms, e.g. addition of super peers, in-network storage, request of alternative peer addresses, and the usage of QoS information for an advanced peer selection. 7.2. PPSP Tracker Protocol Requirements The tracker protocol defines how the peers report and request information to/from the tracker and how the tracker replies to the requests. The tracker discovery and the possible communication between trackers are out of the scope of tracker protocol. PPSP.TP.REQ-1: The tracker MUST implement the tracker protocol for receiving queries and periodical peer status reports/updates from the peers and for sending the corresponding replies. zhang Expires August 27, 2012 [Page 19] Internet-Draft Problem Statement and Requirments of PPSP February 2012 PPSP.TP.REQ-2: The peer MUST implement the tracker protocol for sending queries and periodical peer status reports/updates to the tracker and receiving the corresponding replies. PPSP.TP.REQ-3: The tracker request message MUST allow the requesting peer to solicit the peer list from the tracker with respect to a specific swarm ID. The tracker request message may also include the requesting peer's preference parameter, e.g. preferred number of peers in the peer list, or preferred downloading bandwidth. The track will then be able to select an appropriate set of peers for the requesting peer according to the preference. PPSP.TP.REQ-4: The tracker reply message MUST allow the tracker to offer the peer list to the requesting peer with respect of a specific swarm ID. PPSP.TP.REQ-5: The tracker SHOULD support generating the peer list with the help of traffic optimization services, e.g. ALTO [I-D.ietf- alto-protocol]. PPSP.TP.REQ-6: The peer status report/update MUST have the ability to inform the tracker about the peer's activity in the swarm. PPSP.TP.REQ-7: The chunk availability information of the peer SHOULD be reported to tracker when tracker needs such information to steer peer selection. The chunk information MUST at least contain the chunk ID. PPSP.TP.REQ-8: The chunk availability information between peer and tracker MUST be as expressed as compactly as possible. The peers may report CHUNK AVAILABILTY DIGEST information (i.e., compact expression of chunk availability) to the tracker when possible to decrease the bandwidth consumption for messages in bandwidth constraint environment like mobile network. For example, if a peer has a bitmap like 111111...1(100 continuous 1)xxx..., the 100 continuous "1" can be expressed by one byte with seven bits representing 100 and one bit representing "1".In this example, 100- 8=92 bits are saved. Considering the frequency of exchange of CHUNK AVAILBILITY and the fact that many bitmaps have quite a long length of continuous "1" or "0", such compression makes sense. PPSP.TP.REQ-9: The status of the peer SHOULD be reported to the tracker when tracker needs such information to steer peer selection. zhang Expires August 27, 2012 [Page 20] Internet-Draft Problem Statement and Requirments of PPSP February 2012 For example, peer status can be online time, physical link status including DSL/WIFI/etc, battery status, processing capability, and other capabilities of the peer. Therefore, the tracker is able to select better candidate peers for streaming. 7.3. PPSP Peer Protocol Requirements The peer protocol defines how the peers advertise streaming content availability and exchange status with each other. The peer protocol also defines the requests and responses of the chunks among the peers. The first task for this WG will be to decide which signaling and media transfer protocols will be used. The WG will consider existing protocols and, if needed, identify potential extensions to these protocols. PPSP.PP.REQ-1: The streaming content availability request message MUST allow the peer to solicit the chunk information from other peers in the peer list. The chunk information MUST at least contain the chunk ID. This chunk availability information MUST NOT be passed on to other peer, unless validated (e.g. prevent hearsay and DoS). PPSP.PP.REQ-2: The streaming content availability reply message MUST allow the peer to offer the information of the chunks in its content buffer. The chunk information MUST at least contain the chunk ID. PPSP.PP.REQ-3: The streaming content availability request message SHOULD allow the peer to solicit an additional list of peers to that received from the tracker - with the same swarm ID. The reply message MUST contain swarm-membership information of the peers that have explicitly indicated they are part of the swarm, verifiable by the receiver. This additional list of peers MUST only contain peers which have been checked to be valid and online recently (e.g. prevent hearsay and DoS). It is possible that a peer may need additional peers for certain streaming content. Therefore, it is allowed that the peer communicates with the peers in the current peer list to obtain an additional list of peers in the same swarm. PPSP.PP.REQ-4: Streaming content availability update message among the peers MUST be supported by peer protocol. In the push based model, where peers advocate their own chunk availability proactively, the content availability request message described in PP.REQ-1 is not needed. The peer protocol MUST implement either pull-based, push- based or both. zhang Expires August 27, 2012 [Page 21] Internet-Draft Problem Statement and Requirments of PPSP February 2012 Due to the dynamic change of the buffered streaming content in each peer and the frequent join/leave of peers in the swarm, the streaming content availability among a peer's neighbors (i.e. the peers known to a peer by getting the peer lists from either tracker or peers) always changes and thus requires being updated on time. This update should be done at least on demand. For example, when a peer requires finding more peers with certain chunks, it sends a message to some other peers in the swarm for streaming content availability update. Alternatively, each peer in the swarm can advertise its streaming content availability to some other peers periodically. However, the detailed mechanisms for this update such as how far to spread such update message, how often to send this update message, etc should leave to peer algorithms, rather than protocol concerns. PPSP.PP.REQ-5: The chunk availability information between peers MUST be as expressed as compactly as possible. In PP.REQ-1/2/4, the peers may exchange CHUNK AVAILABILTY DIGEST information (i.e. compact expression of chunk availability) to with other peers when possible to decrease the bandwidth consumption for messages in bandwidth constraint environment like mobile network. PPSP.PP.REQ-6: The peer status report/update SHOULD be advertised among the peers to reflect the status of the peer. Peer status information should be advertised among the peers via the peer status report/update message. For example, peer status can be online time, physical link status including DSL/WIFI/etc, battery status, processing capability, and other capabilities of the peer. With this information, a peer can select more appropriate peers for streaming. PPSP.PP.REQ-7: The peers MUST implement the peer protocol for chunk data (not availability information) requests and responses among the peers before the streaming content is transmitted. 7.4. Security Requirements PPSP.SEC.REQ-1: PPSP MUST support closed swarms, where the peers are authenticated. This ensures that only the authenticated users can access the original media in the P2P streaming system. This can be achieved by security mechanisms such as user authentication and/or key management scheme. zhang Expires August 27, 2012 [Page 22] Internet-Draft Problem Statement and Requirments of PPSP February 2012 PPSP.SEC.REQ-2: Confidentiality of the streaming content in PPSP SHOULD be supported and the corresponding key management scheme SHOULD scale well in P2P streaming system. PPSP.SEC.REQ-3: PPSP MUST provide an option to encrypt the data exchange among the PPSP entities. PPSP.SEC.REQ-4: PPSP MUST have mechanisms to limit potential damage caused by malfunctioning and badly behaving peers in the P2P streaming system. Such an attack will degrade the quality of the rendered media at the receiver. For example, in a P2P live video streaming system a polluter can introduce corrupted chunks. Each receiver integrates into its playback stream the polluted chunks it receives from its other neighbors. Since the peers forwards chunks to other peers, the polluted content can potentially spread through much of the P2P streaming network. PPSP.SEC.REQ-5: PPSP SHOULD support identifying badly behaving peers, and exclude or reject them from the P2P streaming system. PPSP.SEC.REQ-6: PPSP MUST prevent peers from DoS attacks which will exhaust the P2P streaming system's available resource. Given the prevalence of DoS attacks in the Internet, it is important to realize that a similar threat could exist in a large-scale streaming system where attackers are capable of consuming a lot of resources with just a small amount of effort. PPSP.SEC.REQ-7: PPSP SHOULD be robust, i.e., when centralized tracker fails the P2P streaming system SHOULD still work by supporting distributed trackers. PPSP.SEC.REQ-8: Existing P2P security mechanisms SHOULD be re-used as much as possible in PPSP, to avoid developing new security mechanisms. PPSP.SEC.REQ-9: Integrity of the streaming content in PPSP MUST be supported to provide a peer with the possibility to identify inauthentic media content (undesirable modified by other entities rather than its genuine source). The corresponding checksum distribution and verification scheme SHOULD scale well in P2P streaming system and be robust against distrustful trackers/peers. zhang Expires August 27, 2012 [Page 23] Internet-Draft Problem Statement and Requirments of PPSP February 2012 8. IANA Considerations This document has no actions for IANA. zhang Expires August 27, 2012 [Page 24] Internet-Draft Problem Statement and Requirments of PPSP February 2012 9. Acknowledgments Thank you to J.Seng for contribution to many sections of this draft. Thank you to C. Williams and L. Xiao for contributions to PPSP requirements section. We would like to acknowledge the following people who provided review, feedback and suggestions to this document: M. Stiemerling; C. Schmidt; D. Bryan; E. Marocco; V. Gurbani; R. Even; H. Zhang; V. Pasual; D. Zhang; J. Lei; Y.Gu; H.Song; X.Jiang; J.Seedorf; D.Saumitra; A.Rahman; L.Deng; J.Pouwelse; A.Bakker and W.Eddy. This document was prepared using 2-Word-v2.0.template.dot. zhang Expires August 27, 2012 [Page 25] Internet-Draft Problem Statement and Requirments of PPSP February 2012 10. Informative References [Cisco] Cisco Visual Networking Index: Forecast and Methodology, 2009-2014, http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ ns827/white_paper_c11-481360_ns827_Networking_Solutions_White_Paper.html [VoD] Yan Huang et al, Challenges,"Design and Analysis of a Large- scale P2P-VoD System", Sigcomm08. [Mobile Streaming1] Streaming to Mobile Users in a Peer-to-Peer Network, Jeonghun Noh et al, MOBIMEDIA '09. [Mobile Streaming2] J.Peltotaloet al.,"A real-time Peer-to-Peer streaming system for mobile networking environment",in Proceedings of the INFOCOM and Workshop on Mobile Video Delivery (MoVID '09), April 2009. [I-D.ietf-alto-protocol]Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-ietf-alto-protocol-10 (work in progress), October 2011. [Hybrid CDN P2P]D. Xu, S. Kulkarni, C. Rosenberg, and H. Chai, "Analysis of a CDN-P2P hybrid architecture for cost- effective streaming media distribution," Springer Multimedia Systems, vol.11, no.4, pp.383-399, 2006. [I-D.ietf-ppsp-survey]Gu, Y., Zong, N., Zhang, H., Zhang, Y., Lei, J., Camarillo, G., Liu, Y., Montuno, D., and X. Lei, "Survey of P2P Streaming Applications", draft-ietf-ppsp-survey-02 (work in progress), July 2011. zhang Expires August 27, 2012 [Page 26] Internet-Draft Problem Statement and Requirments of PPSP February 2012 Authors' Addresses Yunfei Zhang China Mobile Communication Corporation zhangyunfei@chinamobile.com NingZong Huawei Technologies Co., Ltd. zongning@huawei.com Gonzalo Camarillo Ericsson Gonzalo.Camarillo@ericsson.com Richard Yang Yale University yry@cs.yale.edu Victor Pascual Acme packet Anabel Segura 10, Madrid 28108, Spain Vpascual@acmepacket.com zhang Expires August 27, 2012 [Page 27]