Peer-to-Peer Based Multimedia Distribution Service

1. Peer-to-Peer Based Multimedia Distribution Service Zhe Xiang, Qian Zhang, Wenwu Zhu, Zhensheng ZhangIEEE Transactions on Multimedia, Vol. 6, No. 2, April 2004 Presented by Ho Tsz Kin14/04/2004

2. Agenda • Introduction • Architecture • Topology-ware Overlay • Replication Strategies • Intergroup Replication • Intragroup Replication • Performance Evaluation • Conclusion

3. Multimedia distribution services • Centralized multimedia distribution • Mirroring, Proxy caching • Bottleneck bandwidth problem • Measurement between University of Washington and a set of 13,656 servers • Over 90% is less than 10 Mbps • Not scalable • Content distribution network (CDN) • Deploys a large number of servers at the edge of the network • Objective is to efficiently redirect user requests to appropriate servers so that request latency is reduced and load among servers are balanced

4. Multimedia distribution services • Capacity of the edge server is not large enough to support multimedia service • Where and when to place those edge servers is a difficult problem • Peer-to-peer network • Some rely on servers to disseminate information • Single point of failure • Overlay network in a P2P system is not aware of the underlying topology • Availability depend on peer’s reliability • Cannot provide good QoS-provision • Propose a novel framework based on P2P network

5. Architecture Client join the P2P network, and contribute resources Determine how many replicas and how they place Determine grouping among peers

6. Topology-aware Overlay • Routing overhead is a key performance metric • If randomly constructed, overlay network may actually be far away in the underlying network • Nearby peers in the underlying network are clustered into groups • A group consists of a set of nodes that are close to each other • Close means if the distance is less than some predefined value • Distance can be network latency, or round trip time

7. Topology-aware Overlay • Two different groups are communicating with each other through the shortest distance • Predefined distance threshold • Given a certain transmission delay requirement

8. Content delivery • When a request to obtain certain content is issued • Found within the same group • Content can be directly distributed to the requesting peer • Peer may decide to replicate according to the replication strategies • Not found, flooding search is carried out • A shortest communication path is setup between two groups • The content in source will first be sent to some host in target group, that host in target group will send the content to requester

9. Replication Strategies • Global level replication decision • relies on complete information about the network such as distances between groups or between peers, storage capacity of each group, and each peer • such global information is difficult to obtain in a distributed environment • Divide the problem into two sub-problems • Intergroup and Intragroup replication

10. Intergroup Replication • Provide low latency and QoS-aware service within group level • Seed • Group-level replica • Number of seeds = number of groups holding this • Seed capacity is the total capacity of a group to store different seed • Minimize • the average distance between requesting group and the group providing content • Subject to • the constraint of each group’s seed capacity

11. Intergroup Replication • Variation of K-center problem • NP-Complete • Ignore seed capacity of each group, and only consider the totally seed capacity • Idea of heuristic Seed of each content ci should be uniformly distributed over the network, let number be L Average distance L 2D Euclidean space

12. Intergroup Replication • Average access distance • Modified problem, with S is total capacity, popularity of content ci is ri Weighted average minimum distance Storage capacity constraints • Applying Lagrange Function

13. Intergroup Replication • Substitute back to find the average distance • Proposed heuristic • If distance between the requestor and the peer who has a replica is larger than , then replicate Estimated using local information

14. Intragroup Replication • Improving the availability of the content • Replica is copies of the content within the group • Replica replication matrix • Availability of content ci N peers Reliability of pj

15. Intragroup Replication • Optimization problem • Variation of the knapsack problem • NP-complete size of content ci storage capacity of peer pj

16. Intragroup Replication • Proposed heuristic • Climb-hill based algorithm • Adding a new replica for content cr will improve its availability • Deleting the stored contents cj also decreases its availability • A(cr): availability of content cr • A’(ci):availability of content ci if we delete this content • If A’(ci) > A(cr) • Deleting ci does not conflict with the objective

17. Performance Evaluation • Network topology • Euclidean space model • Nodes are randomly located • Edge longitudes are fixed as 3000 ms • 200 groups are generated • Latency within group are very small • Packet loss model mainly due to the congestion occurred at routers • Number of hops between two peers increases linearly to the distance between two peers • Largest hop is ten • Bandwidth of link range from 800 Kbps to 1.4 Mbps, and average is about 1.2 Mbps

18. Performance Evaluation • Content distribution • 10,000 MPEG-4 format video clips encoded in 1.28 Mbps • Length follows a normal distribution in range of 3 min to 5 min, correspondingly to 37.8 MB to 48 MB in files sizes • Request distribution • Zipf distribution • Truncated Geometric Distribution (TGD) • Truncated Pareto Distribution (TPD)

19. Performance Evaluation • Peer Storage capacity and reliability • Storage contributed by a peer follows a normal distribution in the range of 300 MB and 2 GB, which approximately supports 8 to 50 video clips • Peer reliability of sustaining service follows normal distribution in the range of 0.1 to 0.9 • Comparison • Freenet • Always makes a replica for each requested content • LRU replacement policy • Random replication system • Contents are uniformly distributed into peer’s storage

20. Performance Evaluation • Performance metrics • Average latency • Average access distance between the requestor peer and the content provider peer • Video quality • Perceived video quality by the client • PSNR • Weighted availability • Represents the service availability provided by contents in a certain area (within distance d) • Defined as:

21. Performance Evaluation • Average latency • Varying number of content from 8000 to 12500 • Varying skew factor with 10000 content

22. Performance Evaluation • Video quality • Varying peer storage • Varying average packet loss ratio of network links with peer storage capacity as 960 MB

23. Performance Evaluation • Availability • Varying distance d

24. Conclusion • Propose and analyze • A topology-aware overlay • Replication strategies • Intergroup replication • Intragroup replication • Comments: • Assume equal sizes in intergroup replication, but different sizes in intragroup replication • Topology-aware techniques can also be applied to clustering in SLVoD • How to formulate and resolve stripping strategies