P2P Live Streaming Yang Gao, Nazanin Magharei, Reza Rejaie, "Mesh or Multiple-Tree: A Comparative Study of Live P2P Streaming Approaches" INFOCOM 07 Y Liu, Y Guo, "A survey on peer-to-peer video streaming systems", Peer-to-peer Networking and Applications, 2008. S Ali, A Mathur, "Measurement of commercial peer-to-peer live video streaming", Recent Advances in Peer-to-Peer Streaming, 2006 . Deepak Kumar Agarwal ( 71404423 ) Ajay Narayan ( 60006864 ) Nishchint Raina ( 67569992 )
Paper 1. Mesh or Multiple-Tree: A Comparative Study of Live P2P Streaming Approaches - Analyze tree based and mesh based overlays as content delivery overlays - Evaluates performance of their content delivery mechanisms over a properly connected overlay - similarities and differences - ability to tolerate churn - mesh based > tree based by all measures !
P2P streaming Using P2P overlay for streaming live media over network Participating end-systems (or peers) actively contribute their resources by forwarding their available content to their connected peers. Push based content delivery over multiple tree shaped overlays. The tree-based P2P streaming approach expands on the idea of end-system multicast by organizing participating peers into multiple diverse trees. Mesh-based approach uses swarming content delivery over a randomly connected mesh.
Terms Churn: a peer can leave or join the p2p system at arbitrary time Deadlock: In the presence of churn, a tree could become saturated and thus unable to accept any new leaf node. Content Bottleneck: When a parent does not have sufficient number of useful packets for a child peer, the bandwidth of its congestion controlled connection to that child peer can not be fully utilized. Bandwidth Utilization: ratio of the number of data packets to the total number of delivered packets. Average Quality: the average number of descriptions ( of Multiple Description Coded (MDC) content ) it receives during a session. Multiple Description Coding (MDC): Encoding streams into multiple sub-streams called description. Each description can be independently decoded. Furthermore, receiving multiple unique descriptions results in a higher quality.
Delivery Trees Mesh – based approach Tree – based approach
Tree Overlay Construction Peer decides number of trees to join based on its access link bandwidth Each peer is placed as an internal node in only one tree and as a leaf node in other trees. Join: peer contacts the bootstrapping node to identify a parent in the desired number of trees Leave: subtree nodes rejoin the tree Balance tree: peer is added as an internal node to the tree that has the minimum number of internal nodes. Short tree: a new internal node is placed as a child for the node with the lowest depth
Mesh Overlay Construction Participating peers form a randomly connected overlay Each peer tries to maintain a certain number of parents (i.e., incoming degree) Each peer serves a specific number of child peers (i.e., outgoing degree). Upon arrival, a peer contacts a bootstrapping node to receive a set of peers that can potentially serve as parents.
...Mesh Overlay Construction • The bootstrapping node maintains the outgoing degree of all participating peers. Then, it selects a random subset of peers that can accommodate new child peers in response to an incoming request for parents. • Individual peers periodically report their newly available packets to their child peers and request specific packets from individual parent peers • A parent peer periodically receives an ordered list of requested packets from each child peer, and delivers the packets in the requested order. The requested packets from individual parents are determined by a packet scheduling algorithm at each child peer.
Packet scheduling algorithm ( PRIME ) Each peer maintains two pieces of information for individual parents: the available packets, and the weighted average bandwidth ( b/w budget ) Each peer monitors the aggregate incoming bandwidth from all parents and slowly adapt the number of requested descriptions (or their target quality) with the aggregate bandwidth. Each peer invokes the algorithm every ∆ seconds to request packets from parent ( with n target quality ) as follows: scheduler identifies the packets with the highest timestamp that have become available among parents since the last request (during last ∆ seconds). the missing packets for each timestamp (up to n descriptions per timestamp) are identified and a random subset of these packets is requested from all parents to fully utilize their bandwidth. to balance the load among parents, when a packet is available at more than one parent, it is requested from the parent that has the lowest fraction of its bandwidth budget utilized.
Similarities Both approaches leverage MDC to accommodate the bandwidth heterogeneity among participating peers. Superimposed view of multiple diverse trees is same as directed random mesh overlays. Content delivery in both enables individual peers to receive different pieces of content. All peers receive data from multiple parents and send it down to different child peers. Both require peers to maintain a loosely synchronized playout time that is sufficiently (τ seconds) behind source’s playout time.
Delivery Tree in Mesh Maximize outgoing bandwidth Diffusion Phase: Once a new packet becomes available at the source, a single peer p in level, i pulls the packet during the next interval ∆ s. Swarming Phase: During the swarming phase, peers on different diffusion subtrees exchange their new packets to contribute their outgoing bandwidth. Delivery tree of a packet consists of two parts: top portion shall be a diffusion subtree bottom portion is a collection of swarming connections hanging from the diffusion subtree.
Effect of Per Connection Bandwidth Tree-based approach has a sweet spot for the ratio of per-connection bandwidth to description bandwidth where high resource utilization and thus high delivered quality is achieved.
Effect of bandwidth heterogeneity Mesh: as the % of high bandwidth peers increases, the aggregate performance improves Tree: increasing the % of high bandwidth peers rapidly drops depth of all trees which in turn improves both utilization and the delivered quality.
Summary Identifies the key differences between mesh-based and tree-based approaches to P2P streaming. This in turn sheds an insightful light on the inherent limitations and potentials of these two approaches Identifies the underlying causes for the observed differences between tree- and mesh-based approaches.
Paper 2 A survey on peer-to-peer video streaming systems Yong Liu; Yang Guo; Chao Liang
Introduction • Classification of Video Streaming : • Live Streaming • Video on Demand • Different models to achieve video streaming over internet: • Client-Server Model • Content Delivery Network • Peer-to-Peer Networking
P2P Live Streaming • Live video content is disseminated to all users in real-time. Video playbacks on all users are synchronized. • Overlay Structures for P2P live streaming : • Tree Based Systems • Single-tree streaming • Multi-tree streaming • Mesh-based Systems
Tree Based System [P2P Live Streaming] • Tree Based Systems • A peer has only one parent in a single streaming tree and downloads all content of the video stream from that parent. • Single Tree Streaming • Users form a tree at the application layer, rooted at the video server. • Considerations while constructing a streaming tree: • Depth of the tree. • Fan out of the tree. • Tree maintenance
Single Tree Construction & Maintenance • Achieved in 2 ways: • Centralized • central server controls the tree construction and recovery • Disadvantage: Performance bottleneck and the single point of failure • Distributed • cannot recovery fast enough to handle frequent peer churn.
Multi – tree Streaming • Server divides the stream into multiple sub-streams • One sub-tree for each sub-stream • Each peer joins all sub-trees to retrieve every sub-stream. • Each peer has a different position in different sub trees.
Mesh-based Systems • Peers establish and terminate peering relationships dynamically • A peer maintains peering relationship with multiple neighboring peers • Extremely robust against peer churn
Mesh formation and Maintenance • A mesh streaming system maintains a tracker. • Keeps track of the active peers in the video session. • Each peer, when joining the network, contacts the tracker: • Peer reports its IP address, port number etc. • Tracker returns a subset of active list of peers in the session.
Mesh Maintenance • Peers identify new node by exchanging peer list with neighbors. • Also request for active peer list from tracker. • Graceful departure of a peer is informed to the tracker. • Unexpected Peer departure: • Peers regularly exchange keep-alive messages
P2P Video on Demand • Video on Demand [VoD] • allows users to watch any point of video at any time • offers more flexibility and convenience to users • key feature to attract consumers to IPTV service • Overlays to support VoD: • Tree based P2P systems • Mesh based P2P systems
Tree Based P2P Systems • Users grouped into sessions based on arrival time. • The server and users in the same session form an application level multicast tree.[Base tree] • Server streams entire video over the base tree. • Users who join the session later, should obtain the ‘patch’ [ Content that is missed]
Tree Based P2P Systems • Users act like peers in a P2P network. Each of them provide the following 2 functions: • Base Stream Forwarding • Users forward the received base stream to child nodes • Patch Serving • Users cache initial part of stream and forward to peers joining newly.
Cache-and-relay P2P VoD • Based on the concept of interval caching. • Server caches a moving window of video content. • Efficiently utilizes memory at the server • Serve clients whose viewing point falls into the caching window. • Serves all clients asynchronously.
Cache-and-relay P2P VoD • Each peer buffers a moving window of video content around the point where they are watching. • Serves other users who are watching around that point by forwarding the stream.
Mesh-based P2P VoD • Achieves fast file downloading by swarming • Server disperses data blocks to different users. • Diversity Requirement: • The data blocks at different users are better-off to be different from each other so that there is always something to exchange. • Fully utilize users upload bandwidth • Achieve highest downloading throughput.
Mesh-based P2P VoD • Challenges face in building a mesh based P2P: • effective rate of video play back is poor as data blocks are retrieved in a fairly random order. • availability of different content blocks is also skewed by users behavior • Requires right balance between the overall system efficiency and the conformation to the sequential playback • Example of Mesh-based P2P VoD: BiToS
BiToS: Mesh-based P2P VoD • BiToS has 3 components : • Received Buffer : Stores all data blocks that have arrived. • High Priority Set: Contanins the list of data blocks that are close to playback but are not downloaded yet. • Remaining Pieces : List of all blocks that are yet to be downloaded.
Mesh-based P2P VoD • Availability of Content in Mesh-based P2P: • If video is downloaded in the order of playback • newly arrived user can make little contribution • Not many have content earlier users are looking for • Earlier arrived peers serve content to the newly arrived users. • The number of peers that serve content to earlier arrived peers constantly reduces, as users might leave the network. • One Solution is to introduce a source server.
Conclusion • Existing Limitations in P2P systems: • Quality of Experience is not comparable to traditional TV. • Long channel start up times and channel delays. • Considerable lag among peers. • Low resolution videos due to limited uploading capacity.
Conclusion • High traffic volumes pose a challenge to ISP’s network capabilities. • Video content distribution load is shifted to the ISPs without any profit to them. • Requires further investigation to identify an effective method to regulate and manage P2P video streaming traffic and maintain stability of the ISP’s network infrastructure.
Agenda • Challenges with analyzing P2P apps • How is measurement done? • Analysis of Control Protocols • Defining Metrics • Analysis of Data Plane • Summary and Conclusion
P2P Systems • Bright side • Ubiquity, Resilience, Scalability • Distributed Applications • Academic interest generated for Video applications • Popular • Not-so-bright side • Little understanding of protocols • Proprietary nature makes it difficult
Challenges with proprietary apps • No specification of protocols • Forced to conduct black-box tests • No documentation or API • Can’t write test scripts • Manual interaction to be done
How is it done? • Collecting packet traces with Ethereal • Separating control traffic from data traffic • Reverse engineering the protocols • By analyzing control traffic • Data plane analysis on some metrics • Applications • PPLive • SOPCast