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اسلاید 1 :

شبكه هاي Overlay

 شبكه هاي Overlay چيست؟

 به هر شبكه اي بر روي يك شبكه ديگر ايجاد مي شود  Overlay گفته مي شود. ولي امروزه شبكه هاي Overlay به شبكه هايي اطلاق مي شود كه در لايه كاربرد و برروي شبكه IP تشكيل مي شوند. شبكه هاي اشتراك فايل از قبيل Gnutella, KaZaA  و غيره نمونه هايي از شبكه هاي  Overlay هستند.

كاربردهاي شبكه هاي Overlay :

سيستم هاي اشتراك فايل (Peer-to-Peer File Sharing)

چند  پخشی در اينترنت  (Multicasting in the Internet)

کنفرانس ويدئوِيی

بازی ها ی  گروهی در شبکه

 ....

اسلاید 2 :

سيستم هاي اشتراك فايل Peer-to-Peer

nAlice chooses one of the peers, Bob.

nFile is copied from Bob’s PC to Alice’s notebook: HTTP

nWhile Alice downloads, other users uploading from Alice.

nAlice’s peer is both a Web client and a transient Web server.

All peers are servers = highly scalable!

Example

nAlice runs P2P client application on her notebook computer

nIntermittently connects to Internet; gets new IP address for each connection

nAsks for “Hey Jude”

nApplication displays other peers that have copy of Hey Jude.

 

اسلاید 3 :

P2P: centralized directory

original “Napster” design

1) when peer connects, it informs central server:

qIP address

qcontent

2) Alice queries for “Hey Jude”

3) Alice requests file from Bob

اسلاید 4 :

P2P: problems with centralized directory

nSingle point of failure

nPerformance bottleneck

nCopyright infringement

اسلاید 5 :

Query flooding: Gnutella

overlay network: graph

nedge between peer X and Y if there’s a TCP connection

nall active peers and edges is overlay net

nEdge is not a physical link

nGiven peer will typically be connected with < 10 overlay neighbors

nfully distributed

qno central server

npublic domain protocol

nmany Gnutella clients implementing protocol

 

اسلاید 6 :

Gnutella: protocol

r Query message
sent over existing TCP
connections

r peers forward
Query message

r QueryHit
sent over
reverse
path

اسلاید 7 :

Gnutella: Peer joining

1.Joining peer X must find some other peer in Gnutella network: use list of candidate peers

2.X sequentially attempts to make TCP with peers on list until connection setup with Y

3.X sends Ping message to Y; Y forwards Ping message.

4.All peers receiving Ping message respond with Pong message

5.X receives many Pong messages. It can then setup additional TCP connections

Peer leaving: see homework problem!

اسلاید 8 :

Exploiting heterogeneity: KaZaA

nEach peer is either a group leader or assigned to a group leader.

qTCP connection between peer and its group leader.

qTCP connections between some pairs of group leaders.

nGroup leader tracks the content in  all its children.

 

اسلاید 9 :

KaZaA: Querying

nEach file has a hash and a descriptor

nClient sends keyword query to its group leader

nGroup leader responds with matches:

qFor each match: metadata, hash, IP address

nIf group leader forwards query to other group leaders, they respond with matches

nClient then selects files for downloading

qHTTP requests using hash as identifier sent to peers holding desired file

 

اسلاید 10 :

Kazaa tricks

nLimitations on simultaneous uploads

nRequest queuing

nIncentive priorities

nParallel downloading

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