PPSP Y. Zhang Internet Draft China Mobile N.Zong HuaweiTech Intended status: Informational September 19, 2012 Expires: March 2013 Problem Statement and Requirements of Peer-to-Peer Streaming Protocol (PPSP) draft-ietf-ppsp-problem-statement-10.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, requirements and uses cases of PPSP. zhang Expires March 19, 2013 [Page 1] Internet-Draft Problem Statement and Requirements of PPSP September 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 March 18, 2013. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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 March 19, 2013 [Page 2] Internet-Draft Problem Statement and Requirements of PPSP September 2012 Table of Contents 1. Introduction ................................................ 4 2. Terminology and concepts. .................................... 5 3. Problem statement ........................................... 7 3.1. Heterogeneous P2P traffic and P2P caches deployment ..... 7 3.2. Latency efficiency difficulties ......................... 7 3.3. Extended applicability in mobile and wireless environment 7 4. PPSP: Standard peer to peer streaming protocols .............. 9 4.1. Tracker protocol candidates discussion and design issues .9 4.2. Peer protocol candidates discussion and design issues . 10 5. Use cases of PPSP .......................................... 11 5.1. Worldwide provision of live/VoD streaming .............. 11 5.2. Cross-screen streaming ................................. 13 5.3. Cache service supporting P2P streaming ................. 14 5.4. Proxy service supporting P2P streaming ................. 15 5.4.1. Home Networking Scenario .......................... 15 5.4.2. Browser-based HTTP Streaming ...................... 16 6. Requirements of PPSP ........................................ 17 6.1. Basic Requirements ..................................... 17 6.2. PPSP Tracker Protocol Requirements ..................... 18 6.3. PPSP Peer Protocol Requirements ........................ 19 7. Security Considerations .................................... 21 8. IANA Considerations ........................................ 23 9. Acknowledgments ............................................ 23 10. Informative References. .................................... 24 zhang Expires March 19, 2013 [Page 3] Internet-Draft Problem Statement and Requirements of PPSP September 2012 1. Introduction Streaming traffic is among the largest and fastest growing traffic on the Internet [Cisco], where peer-to-peer (P2P) streaming contribute substantially. With the advantage of high scalability and fault tolerance against single point of failure, P2P streaming applications are able to distribute large-scale, live and video on demand (VoD) streaming programs to 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 in P2P streaming systems to solve the above problems. zhang Expires March 19, 2013 [Page 4] Internet-Draft Problem Statement and Requirements of PPSP September 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 KBs to several MBs in different systems. In case of MBs size chunk scenario, a sub-chunk structure named piece is often defined to fit in a single transmitted packet. A streaming system may use different granularities for different usage, e.g., using chunks during data exchange, and using a larger unit such as a set of chunks during advertisement. Chunk ID: The identifier of a chunk in a content stream. Client: A client in general refers to the service requester in client/server computing paradigm. In this draft a client also refers to a participant in a P2P streaming system that only receives streaming content. In some cases, a node not having enough computing and storage capabilities will act as a client. Such node can be viewed as a specific type of peer. Content Distribution Network (CDN): A CDN is a collection of nodes that are deployed, in general, at the network edge like Points of Presence (POP) or Data Centers (DC) and that store content provided by the original content servers. Typically, CDN nodes serve content to the clients located nearby topologically. Live streaming: It refers to a scenario where all clients receive streaming content for the same ongoing event. It is desired that the lags between the play points of the clients and streaming source be small. P2P cache: A P2P cache refers to a network entity that stores (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 stores and streams streaming content to other participants. Peer list: A list of peers which are in a same swarm maintained by the tracker. A peer can fetch the peer list of a swarm from the tracker or from other peers in order to know which peers have the required streaming content. zhang Expires March 19, 2013 [Page 5] Internet-Draft Problem Statement and Requirements of PPSP September 2012 Peer ID: The identifier of a peer such that other peers, or the tracker, can refer to the peer by using its ID. PPSP: The abbreviation of Peer-to-Peer Streaming Protocols. PPSP refer to the key signaling protocols among various P2P streaming system components, including the tracker and the peer. Tracker: A tracker refers to a directory server that maintains a list of peers participating in a specific audio/video channel or in the distribution of a streaming file. Also, the tracker answers peer list queries received from peers. The tracker is a logical component which can be centralized or distributed. Video-on-demand (VoD): It refers to a scenario where different clients may watch different parts of the same recorded media with downloaded content. Swarm: A swarm refers to a group of peers who exchange data to distribute chunks of the same content (e.g. video/audio program, digital file, etc) at a given time. Swarm ID: The identifier of a swarm containing a group of peers sharing a common streaming content. zhang Expires March 19, 2013 [Page 6] Internet-Draft Problem Statement and Requirements of PPSP September 2012 3. Problem statement The problems caused by proprietary protocols for P2P streaming applications are listed as follows. 3.1. Heterogeneous P2P traffic and P2P caches deployment ISPs are faced to different P2P streaming application introducing substantial traffic into their infrastructure, including their backbone and their exchange/interconnection points. P2P caches are used by ISPs in order to locally store content and hence reduce the P2P traffic. P2P caches usually operate at the chunk or file granularity. However, unlike Web traffic that is represented by HTTP packets and therefore allows any caching device to be deployed (as long as it supports HTTP), P2P traffic is originated by multiple P2P applications which require the ISPs to deploy different type of caches for the different types of P2P streams present in the network. This increases both engineering and operational costs dramatically. 3.2. Latency efficiency difficulties P2P streaming is often criticized due to its longer delays (e.g., startup delay, seek delay and channel switch delay) compared to client/server streaming. Hybrid CDN/P2P is a good approach in order to address this problem [Hybrid CDN P2P]. In the Hybrid CDN/P2P approach, the CDN takes two roles: media streaming server and P2P tracker. Similarly to what described in section 3.1, proprietary P2P protocols introduce complexity between peers and CDN trackers because the CDN trackers need to identify each different P2P streaming protocol. This increases the deployment cost of CDN. 3.3. Extended applicability in mobile and wireless environment Mobility and wireless are becoming increasingly important in today's Internet, where streaming service is a major usage. It's reported that the average volume of video traffic on mobile networks has risen up to 50% in the early of 2012 [ByteMobile]. There are multiple prior studies exploring P2P streaming in mobile and wireless networks zhang Expires March 19, 2013 [Page 7] Internet-Draft Problem Statement and Requirements of PPSP September 2012 [Mobile Streaming1] [Mobile Streaming2]. However it's difficult to apply current P2P streaming protocols (even assuming we can re-use some of the proprietary ones) in mobile and wireless networks. Although smart handsets are more eligible to become peers with much higher bandwidth, CPU frequency, larger storage and memory than before, peer selection will become more challenging due to the increase and complexity of exchange between peers and trackers. Current P2P protocols are not well suited for these new requirements in the context of mobile and wireless networks. Following are some illustrative examples: First, the connections are unstable and expensive in terms of energy consumption and transmission (especially in uplink direction). Peers and trackers may need more information like packet loss rate, peer battery status and processing capability during peer selection. Unfortunately current protocols don't cover this kind of information. Second, current practices often use a "bitmap" message in order to exchange chunk availability. The message is of kilobytes in size and exchanged frequently, for example, several seconds. In a mobile environment with scarce bandwidth, the message size need to be shortened or it may require more efficient methods for expressing and distributing chunk availability information, which is different from current practice. Third, for a resource constraint peer like mobile handsets or set-top boxes (STB), there are severe contentions on limited resource when using proprietary protocols. The peer has to install many different streaming applications for different usages, e.g., some for movies and others for sports and each of these applications will compete for the same set of memories, flashes or hard disks(some may run in the background even they are not invoked by the users). Open protocols creat an opportunity to use one client software accommodating different P2P systems. This may alleviate this problem. zhang Expires March 19, 2013 [Page 8] Internet-Draft Problem Statement and Requirements of PPSP September 2012 4. PPSP: Standard peer to peer streaming protocols PPSP is targeted to standardize signaling protocols for tracker-based architectures to solve the above problems that support either live or VoD streaming. 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 data within the time constraints. 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. +------------------------------------------------+ | | | +--------------------------------+ | | | Tracker | | | +--------------------------------+ | | | ^ ^ | |Tracker | | Tracker |Tracker | |Protocol| | Protocol |Protocol | | | | | | | V | | | | +---------+ Peer +---------+ | | | Peer |<----------->| Peer | | | +---------+ Protocol +---------+ | | | ^ | | | |Peer | | | |Protocol | | V | | | +---------------+ | | | Peer | | | +---------------+ | | | | | +------------------------------------------------+ Figure 1 PPSP System Architecture 4.1. Tracker protocol candidates discussion and design issues Tracker protocol: The tracker protocol is best modeled as a request/response protocol between peers and trackers, and will carry zhang Expires March 19, 2013 [Page 9] Internet-Draft Problem Statement and Requirements of PPSP September 2012 information needed for the selection of peers suitable for real- time/VoD streaming. One of the main aspects the new protocol has to address is the format of the protocol messages. Two options have been identified: Binary based: Binary based tracker protocols are widely used in practice, e.g., PPLive[PPLive] and PPStream [PPStream]. Binary based tracker protocol is simple with the smallest set of semantic definitions and efficient in peer and tracker resource usage, especially, for mobile and set-top box terminals. Text based: HTTP can be easily thought to be the candidate, being a text based request/response protocol. HTTP messages may be reused for PPSP semantics and if they don't match PPSP requirements, some new messages may be redefined. Another approach consists of using HTTP+XML combination, where HTTP is only used as the underlying transport protocol (in application-level) while tracker information and messages are defined in XML format. PPSP tracker protocol will select the best of the above options according to the requirements from both peer and tracker perspective and also taking into consideration deployment and operation perspectives. 4.2. Peer protocol candidates discussion and design issues Peer Protocol: The peer protocol is modeled as a gossip-like protocol with periodic exchanges of neighbor and media chunk availability information. Namely, the peer protocol is a content-centric protocol built around the abstraction of a cloud of participants disseminating the same data in ways and orders that are convenient to the participants [I-D.ietf-ppsp-peer-protocol]. In that respect and in light of the above requirements, typical HTTP is neither suitable nor efficient. We list two peer protocol candidates: Websockets for bidirectional HTTP: WebSockets is basically a bidirectional TCP connection derived from a HTTP connection hence allowing a bidirectional P2P transport over HTTP. On the negative side, TCP is not ideally suited for multi-party transfers of the same content (see Rationale section in I-D.ietf-ppsp-peer-protocol) and therefore it introduces implementation (i.e., code) complexity. zhang Expires March 19, 2013 [Page 10] Internet-Draft Problem Statement and Requirements of PPSP September 2012 UDP based: Unlike TCP or HTTP, UDP is a datagram-based protocol without any sequential data stream abstraction which is, in most the cases, unnecessary for PPSP. Compared to the use of TCP, it reduces the per-connection footprint and complexity of TCP especially in resource constraint mobile cases. The PPSP peer protocol will discuss the protocol design rationales in detail. 5. Use cases of PPSP 5.1. Worldwide provision of live/VoD streaming The content provider can efficiently increase live streaming coverage by introducing PPSP in between different providers. Figure 2 shows the case of provider A broadcasting a TV program with the help of provider B and C for a wider coverage by introducing PPSP. Without PPSP, when users outside A requests TV program@A, the returned peer-list may include few local peers. This may affect the user experience. With PPSP, B and C can involve in the broadcasting. The providers often deploy in-network peers called super-nodes (SN for short) who have better stability and higher storage and bandwidth for better QoS. With the tracker protocol, the tracker@A can return a peer-list containing, in addition to peers@A addresses, the SNs owned by B and C. Hence User@B and User@C can exchange data (availability) with these local SNs using the peer protocol. Figure 3 shows the case of cooperative VoD provision by introducing PPSP inside CDN overlays and in between different CDNs. 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 with the same PPSP protocols. Note that for compatibility reason both HTTP streaming and P2P streaming can be supported by CDN. 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 supporting streaming in a low cost. zhang Expires March 19, 2013 [Page 11] Internet-Draft Problem Statement and Requirements of PPSP September 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 | | Protocol| | | |protocol| | | | | | | | | | | | | | | | | | | | v v v v | | +------+ Peer +------+ +---------+ Peer +---------+ | | | A's |<------> | B's | |A's |<------> |C's | | | | User1|Protocol | User2| | User1 |Protocol | User2 | | | +------+ +------+ +---------+ +---------+ | | | +-------------------------------------------------------------------+ Figure 2 Cooperative Vendors Interaction zhang Expires March 19, 2013 [Page 12] Internet-Draft Problem Statement and Requirements of PPSP September 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. Cross-screen streaming In this scenario PC, STB/TV and mobile terminals from both fixed network and mobile/wireless network share the streaming content. With PPSP, peers can identify the types of access networks, average load, peer abilities and get to know what content other peers have even in different networks (potentially with the conversion of the content availability expression in different networks) as shown in Figure 4. Such information will play an important role on selecting suitable peers, e.g., a PC or STB is more likely to provide stable content and a mobile peer within a high-load cell is unlikely to be selected, which may otherwise lead to higher load on the base station. zhang Expires March 19, 2013 [Page 13] Internet-Draft Problem Statement and Requirements of PPSP September 2012 +-------------------------------------------------------------------+ | | | Tracker Protocol +---------+ Tracker Protocol | | +-------------> | Tracker |<------------------+ | | | +---------+ | | | | ^ | | | | | | | | | | | | | V | V | | +------+ | +------------+ | | | STB | Tracker Protocol |Mobile Phone| | | +------+ | +------------+ | | ^ | ^ | | | | | | | | | | | | | V | | | |Peer Protocol +---------+ Peer Protocol | | | +-------------> | PC |<------------------+ | | +---------+ | | | +-------------------------------------------------------------------+ Figure 4 Heterogeneous P2P Streaming with PPSP 5.3. 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 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 requesting peers inside afterward. This operation greatly decreases the inter-network traffic and increases user experience. zhang Expires March 19, 2013 [Page 14] Internet-Draft Problem Statement and Requirements of PPSP September 2012 The cache nodes do not need to update their library when new applications supporting PPSP are introduced, which reduces the cost. +----------------------------------------------------------------+ | | | Tracker Protocol +---------+ | | +----------------> | Tracker | | | | +---------+ | | | ^ | | | | | | | | Tracker Protocol | | | | | | | | | | | +---------|-------------------------------------| | | | V | | | | +---------+ | | | +----------|---> | Cache |<-------------------+ | | | | | +---------+ Tracker/Peer| | | | | Peer | Protocol | | | | | Protocol | | | | | | | | | | | | | | | | V V | V | | +-----------+ | ISP Domain +------------+ | | | External | | | Inside | | | | Peer | | | Peer | | | +-----------+ | +------------+ | +----------------------------------------------------------------+ Figure 5 Cache Service Supporting Streaming with PPSP 5.4. Proxy service supporting P2P streaming 5.4.1. Home Networking Scenario For applications where the peer is not co-located with the media player in the same device (e.g. the peer is located in a home media gateway), we can use a PPSP proxy, as shown in figure 6. As shown in figure 6, the PPSP proxy terminates both the tracker and peer protocol allowing the legacy presentation devices to access P2P streaming content. In figure 6 the DLNA protocol [DLNA] is used in order to communicate with the presentation devices thanks to its wide deployment. Obviously, other protocols can also be used. zhang Expires March 19, 2013 [Page 15] Internet-Draft Problem Statement and Requirements of PPSP September 2012 +----------------------------------------------------------------+ | | | Tracker Protocol +---------+ | | +----------------> | Tracker | | | | +---------+ | | | ^ | | | | | | | | Tracker Protocol | | | | | | | +---------|-------------------------------------| | | | V | | | | +---------+ | | | +----------|---> | PPSP |<-------------------+ | | | | | | Proxy | DLNA | | | | | Peer | +---------+ Protocol | | | | | Protocol | | | | | | | | | | V V | V | | +-----------+ | Home Domain +------------+ | | | External | | | DLNA Pres.| | | | Peer | | | Devices | | | +-----------+ | +------------+ | +----------------------------------------------------------------+ Figure 6 Proxy service Supporting P2P Streaming 5.4.2. Browser-based HTTP Streaming P2P Plug-ins can be used in browser-based environment in order to stream content. With P2P plug-ins, HTTP streaming can be turned into a de facto P2P streaming. From the browser (and hence the user) perspective, it's just HTTP based streaming but the PPSP capable plug-in can actually accelerate the process by leveraging streams from multiple sources/peers [P2PYoutube]. In this case the plug-ins behave just like the proxy. zhang Expires March 19, 2013 [Page 16] Internet-Draft Problem Statement and Requirements of PPSP September 2012 6. Requirements of PPSP This section enumerates the requirements that should be considered when designing PPSP. 6.1. Basic Requirements PPSP.REQ-1: The tracker and the peer protocol SHOULD allow peers to receive streaming content within the required time constraints. PPSP.REQ-2: 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-3: 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-4: 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-5: 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. An example for generating the chunk ID is the bitmap approach [I-D.ietf-ppsp-survey]. PPSP.REQ-6: The tracker protocol and peer protocol are recommended to be carried over TCP or UDP. PPSP.REQ-7: 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 live and VoD streaming even for very popular content. The tracker and peer protocol do not zhang Expires March 19, 2013 [Page 17] Internet-Draft Problem Statement and Requirements of PPSP September 2012 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 systems. Setup time to receive a new streaming channel or to switch between channels should be reasonably small. End to end delay, which consists of the time between content generation (e.g., a camera) and content consumption (e.g., a monitor), will become critical in case of live streaming especially in provisioning of sport events where end to end delay of 1 minute and more are not acceptable. For instance, the tracker and peer protocol can carry QoS related parameters (e.g. video quality and delay requirements) together with the priorities of these parameters in addition to the measured QoS situation (e.g., performance, available uplink bandwidth) of content providing peers. There are also some other possible mechanisms like addition of super peers, in-network storage, request of alternative peer addresses, and the usage of QoS information for advanced peer selection mechanisms. 6.2. 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, sending the corresponding replies and periodical send peer status reports/updates. 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 tracker will then be able to select an appropriate set of peers for the requesting peer according to the preference. zhang Expires March 19, 2013 [Page 18] Internet-Draft Problem Statement and Requirements of PPSP September 2012 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 expressed as compact as possible. The peers may report chunk availability digest information (i.e., compact expression of chunk availability) to the tracker when possible in order to decrease the bandwidth consumption in mobile networks. For example, if a peer has a bitmap like 111111...1(one hundred continuous 1)xxx..., the one hundred continuous "1" can be expressed by one byte with seven bits representing the number of "1", i.e., "one hundred" and one bit representing the continuous sequence is "1" or "0". In this example, 100-8=92 bits are saved. Considering the frequency of exchange of chunk availability and the fact that many bitmaps have a quite long length of continuous "1" or "0", such compression is quite useful. PPSP.TP.REQ-9: The status of the peer SHOULD be reported to the tracker when tracker needs such information in order to steer peer selection. For example, peer status can be online time, physical link status including DSL/WiFi/etc., battery status, processing capability and other capabilities of the peer. Therefore, the tracker is able to select better candidate peers for streaming. 6.3. 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. zhang Expires March 19, 2013 [Page 19] Internet-Draft Problem Statement and Requirements of PPSP September 2012 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 other 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 the peer protocol. The peer protocol MUST implement either pull-based, push-based or both. Due to the dynamic change of the buffered streaming content in each peer and the frequent join/leave of peers in the swarm, the streaming content availability among a peer's neighbors (i.e. the peers known to a peer by getting the peer list from either tracker or peers) always changes and thus requires being updated on time. This update should be done at least on demand. For example, when a peer requires finding more peers with certain chunks, it sends a message to some other peers in the swarm for streaming content availability update. Alternatively, each peer in the swarm can advertise its streaming content availability to some other peers periodically. However, the detailed mechanisms for this update such as how far to spread the update message, how often to send this update message, etc. should leave to the algorithms, rather than protocol concerns. PPSP.PP.REQ-5: The chunk availability information between peers MUST be expressed as compactly as possible. In PP.REQ-1/2/4, the peers may exchange chunk availability digest information with other peers, when possible, in order to decrease the zhang Expires March 19, 2013 [Page 20] Internet-Draft Problem Statement and Requirements of PPSP September 2012 messages bandwidth consumption. PPSP.PP.REQ-6: The peer status report/update SHOULD be advertised among the peers in order 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 such 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. Security Considerations This document discusses the problem statement and requirements around P2P streaming protocols without specifying the protocols. However we believe it is important for the reader to understand areas of security introduced by the P2P nature of the proposed solution. The main issue is the usage of un-trusted entities (peers) for service provisioning. For example, malicious peers may: - Originate denial of service (DOS) attacks to the trackers by sending large amount of requests with the tracker protocol; - Originate fake information on behalf of other peers; - Originate fake information about chunk availability; For example, malicious peers/trackers may: - Originate reply instead of the regular tracker (man in the middle attack). We list some important security requirements for PPSP protocols as below: PPSP.SEC.REQ-1: PPSP MUST support closed swarms, where the peers are authenticated. 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 March 19, 2013 [Page 21] Internet-Draft Problem Statement and Requirements of PPSP September 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 systems. PPSP.SEC.REQ-3: PPSP MUST provide an option in order to encrypt the data exchange among the PPSP entities. PPSP.SEC.REQ-4: PPSP MUST have mechanisms in order 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 streaming system a polluter can introduce corrupted chunks. Each receiver integrates into its playback stream the polluted chunks it receives from its neighbors. Since the peers forwards chunks to other peers, the polluted content can potentially spread through the P2P streaming network. 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 available resources of the P2P streaming system. 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 unauthentic 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. 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 zhang Expires March 19, 2013 [Page 22] Internet-Draft Problem Statement and Requirements of PPSP September 2012 and protection of the content distribution. Protection of the content by Digital Rights Management (DRM) is outside the scope of the PPSP. 8. IANA Considerations This document has no actions for IANA. 9. Acknowledgments Thank you to J.Seng, G. Camarillo, R. Yang, C. Schmidt, R. Cruz and S. Previdi for contribution to many sections of this draft. Thank you to C. Williams, V. Pasual and L. Xiao for contributions to PPSP requirements section. We would like to acknowledge the following people who provided review, feedback and suggestions to this document: M. Stiemerling, D. Bryan, E. Marocco, V. Gurbani, R. Even, H. Zhang, D. Zhang, J. Lei, 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 March 19, 2013 [Page 23] Internet-Draft Problem Statement and Requirements of PPSP September 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/ns7 05/ns827/white_paper_c11- 481360_ns827_Networking_Solutions_White_Paper.html [VoD] Y. Huang et al, Challenges, "Design and Analysis of a Large- scale P2P-VoD System", Sigcomm08. [ByteMobile] http://www.bytemobile.com/news- events/2012/archive_230212.html [Mobile Streaming1] Streaming to Mobile Users in a Peer-to-Peer Network, J. Noh et al, MOBIMEDIA '09. [Mobile Streaming2] J.Peltotaloet al.,"A real-time Peer-to-Peer streaming system for mobile networking environment", in Proceedings of the INFOCOM and Workshop on Mobile Video Delivery (MoVID '09). [I-D.ietf-alto-protocol]R. Alimi et al, "ALTO Protocol", draft-ietf- alto-protocol-10 (work in progress), October 2011. [Hybrid CDN P2P]D. Xu et al, "Analysis of a CDN-P2P hybrid architecture for cost-effective streaming media distribution," Springer Multimedia Systems, vol.11, no.4, pp.383-399, 2006. [I-D.ietf-ppsp-survey] Y. Gu et al, "Survey of P2P Streaming Applications", draft-ietf-ppsp-survey-02 (work in progress), July 2011. [PPTV] http://www.pptv.com [PPStream] http://www.ppstream.com [I-D.ietf-ppsp-peer-protocol] A. Bakker et al, Peer-to-Peer Streaming Peer Protocol (PPSPP),draft-ietf-ppsp-peer-protocol-02, (work in progress), June 2012. [DLNA] http://www.dlna.org [P2PYoutube] https://addons.opera.com/en/extensions/details/p2p- youtube/ zhang Expires March 19, 2013 [Page 24] Internet-Draft Problem Statement and Requirements of PPSP September 2012 Authors' Addresses Yunfei Zhang China Mobile Communication Corporation zhangyunfei@chinamobile.com NingZong Huawei Technologies Co., Ltd. zongning@huawei.com zhang Expires March 19, 2013 [Page 25]