Peer-to-Peer (P2P) Communication

Transcription

1 eer-to-eer (2) Communication 1

2 References Lv, Cao, Cohen, Li and Shenker, Search and Replication in Unstructured eer-to-eer Networks, In 16 th ACM Intl Conf on Supercomputing (ICS), S. Kang and M. Mutka, Efficient Mobile Access to Internet Data via a Wireless eer-to-eer Network, IEEE Int l Conf on ervasive Computing and Communications, March

3 Different Models of User-level Communication Client-server (request-response) model ush model eer-to-peer model 3

4 Client-server model Request Server Network Client Response 4

5 ush model Server Network Client Data 5

6 2 model Network Server Support Optional User = client/server/both User User User User User User 6

7 Two 2 systems Server cluster S S S S Q R Q D Q: Time to Live, flooding D Q Q Q R Q Napster Gnutella 7

8 2 in General Idea: Share (unused) resources Applications» Disk space, CU cycles, network resources» File distribution» Online games» Instant messaging ros: Aggregation of resources. No single point of failure Cons: Intermittent presence of nodes Difficult to locate and use resources efficiently. 8

9 Environmental Characteristics of 2 network Heterogeneity Nodes: Largely a server or a client ( free-riders ) Bandwidth: Asymmetric (Higher download) Latency: Large variance (The closest 20% of peers are 4 times closer than the furthest 20%.) Availability» I-level: Connected to the Internet (20% nodes with 90+% uptime)» Application-level: Connected to the 2 system Degree of sharing» Gnutella: 10% of peers share more files than the rest combined Large % of free-riders» Zero up-load peers for modems, more for cable modems 9

10 Environmental Characteristics of 2 network (contd.) Topology eers form connections with highly available nodes with high bandwidth very robust Misreporting Users misreport their bandwidth to discourage download requests.» Legality concern for sharing files» Valid concern for users with lower bandwidth 10

11 Architectural Considerations Level of control: centralized vs decentralized Caching and replication Searching 11

12 Centralized (Napster) Level of control A central directory is maintained eers send requests to the directory server rone to failure Decentralized, but structured (Chord) A certain topology is maintained Files are placed at specific locations Decentralized, but unstructured (Gnutella) No central control and no topology is maintained A node knows its neighbors only 12

13 Caching and Replication Better BW utilization and access latency Data is placed near the requester Achieved by caching and/or replicating data at various points in the net. Caching: Store a downloaded file for future use. Replication: A node makes copies of a file and places it at multiple locations.» ro-active replication (random nodes)» ath replication (ut a copy on all the nodes on the path) 13

14 Searching: Knowing a peer that holds a copy of the desired object Request/reply Flooding (TTL value terminates search process) Expanding ring (successive floods with increased TTL) Random walk and multiple random walks TTL and Checking based termination Topology update + flooding based search 14

15 Wireless 2 Networks 3G connectivity almost everywhere Date rate: 2 Mbps Charging: based on data volume Volume based charging Expensive 2 comm Low cost solution Each peer downloads a part of the file eers form an ad hoc network to exchange the rest 15

16 Network Formation Content rovider Internet Associated Server (n) 6. GID 4. Register 5. GID 3. MS info beacon 1. Join 2. Accept Init. Master Server (1) AS -Global topology -Schedule becon 16

17 Gathering neighborhood information eers broadcasts HELLO(GID) packets eers know their neighbors from received HELLOs eers send their neighbor lists to the MS via AS MS constructs global topology and gives it to AS 17

18 eers download a part of the file Ideally, peers download an equal fraction of the file To accommodate dynamic join: A file distribution is incremental in nature File size N = D*t (t is an integer) In each round, a peer downloads D amount of data Let p r be # of peers at the end of round r and n new peers appear for round r+1 (i.e. p r+1 - p r = n) Master computes the MIN value of n that satisfies» D*p r + D*n*(r+1) <= Remaining file size If n = 0, no more new peers are accepted. 18

19 Content distribution Each peer needs to do a net-wide broadcast of its part of the file. Uncontrolled broadcast Broadcast storm problem For each (source) peer, its AS Computes the min set of rebrodcasting peers from the global topology Notifies other AS s corresponding to members of the rebroadcasting set Other AS s direct their peers to rebroadcast packets from the given source The next peer starts broadcasting when its predecessor completes its broadcast using DONE 19