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Peer to Peer _P2P_ Networks

VIEWS: 39 PAGES: 18

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Peer to Peer (P2P) Networks
           Section 2.6
      Ali Erkan & John Barr
          Ithaca College
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       An Extremely Simplified History of P2P

Napster paved the way for decentralized P2P file-sharing programs
such as Kazaa, Limewire, iMesh, Morpheus, and BearShare
[-W IKIPEDIA]

After Napster, unofficial Napster servers proliferated, aided by a
program known as Napigator, and a second generation of P2P
protocols (including FastTrack and Gnutella) were quickly developed.
[-W IKIPEDIA]

Most P2P networks based on overlay networks:
• Edge between peer X and Y if there’s a TCP connection
• Edge is not a physical link
• Given peer will typically be connected with < 10 overlay neighbors
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                 Centralized Directory Paradigm
                                                                     Bob



                                                          pd   ate
                                                     nd u
                                              rm a
                                       Info
                                   1                             e
                                                     nd updat
                                       Inform a




                                                                     File transfer
                                   1
           Centralized                                                               Peers
        directory server               Inform an
                                   1                   d update

                                   1 Inform
                           2                      and
                                                      upda                       3
                               Que                         te
                                   ry f
                                        or c
                                             onte
                                                  nt



Locating content is centralized                Alice

File transfer is decentralized
Problems: Single point of failure, performance bottleneck
                                   02-068
                                   AW/Kurose and Ross
                                   Computer Networking
                                   KR02.32 ar1
                                   26p2 Wide x16p9 Deep
                                   2/c
                                   04/24/02GM
                                                                           4

          Non-centralized Directory Paradigm
                                                 File transfer


                                                                 Query

                                                            Query hit
                         e   ry                             Qu
                      Qu                    it                   er
                                     r   yh                           y
                              Q   ue

                                  Query

                             Query hit
                             Qu
                               er
                                         y




Locating content is decentralized
                                  AW/Kurose and Ross
File transfer is decentralized    Computer Networking 3/e
                                  KR02.24
Problem: How do you find something? Wide x 17p8 Deep
                                  19p10
                                                          2/c
                                                          03/18/04 rossi
                                                                             5

                  Exploiting Heterogeneity
                                          Key:

                                                 Ordinary peer

                                                 Group-leader peer

                                                 Neighboring relationships
                                                 in overlay network




Each peer is either a group leader or assigned to a group leader.
TCP connection between peer and its group leader
                          02-068
                          AW/Kurose and Ross
                           some Networking
TCP connections betweenComputerar1
                          KR02.33 pairs of group leaders
                          24p4 Wide x
Group leader tracks the content in14p3 Deep children
                          2/c         all its
                          04/24/02GM
                                                                      6

     Exploiting Heterogeneity: The Kazaa Design

Each file has a hash and a descriptor
Client sends keyword query to its group leader
Group leader responds with matches (one for each match):
 metadata, hash, IP address
If group leader forwards query to other group leaders, they respond
with matches
Client then selects files for downloading
Client sends HTTP request to peer holding desired file using hash as
identifier
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                      Sample Problem

Consider an overlay network with N active peers, with each pair of
peers having an active TCP connection. Additionally, suppose that the
TCP connections pass through a total of M routers. How many nodes
and edges are there in the corresponding overlay network?
                                                                     8

                      Sample Problem

What is the least number of connections a P2P network could have?
A

In a sparse network of N nodes, the number of edges “tends” to A .
                                                                    9

                      Sample Problem

What is the most number of edges a graphs could have?
A

In a dense network of N nodes, the number of edges “tends” to A .
                                                                        10

                       Sample Problem

Suppose every participating node in a Gnutella network maintains
TCP connections to at least four distinct peers at all times. Suppose
peer X, which has five TCP connections to other peers, wants to leave:
• If X explicitly closes his application, what actions would each of
  the five formerly connected peers take?
• If X abruptly gets disconnected, what happens?
                                                                         11

                       Sample Problem

If X explicitly closes his application, what actions would each of the
five formerly connected peers take?
A
                                                 12

                      Sample Problem

If X abruptly gets disconnected, what happens?
A
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                      Sample Problem

Suppose Alice issues a Query message which Bob receives.
Ordinarily, the QueryHit message would follow the reverse path of the
Query. What are the advantages and disadvantages of Bob opening a
direct TCP connection to Alice and send the QueryHit that way?
A
                                                                     14

                     Sample Problem

In the Gnutella protocol, when the peer Alice generates a Query
message, it inserts a unique ID in the message’s MessageID field.
When the peer Bob has a match, it generates QueryHit message using
the same MessageID as the Query message. Describe how peers can
use the MessageID field and local routing tables to accomplish
reverse-path routing.
A
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                      Sample Problem

As an alternative approach, peers can augment the query message
with their IP number. How would this work out?
A
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                      Sample Problem

In a KaZaA network, suppose each super-group leader is roughly
responsible for 200 group leaders and each group leader is
responsible for 200 ordinary peers. How many super-group leaders
would be necessary for a network of four million peers?
A
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                     Sample Problem

What information might each group leader store? What information
might each super-group leader store? How might a search be
performed in such a three-tier design?
A
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                       Sample Problem

Suppose that each peer in a P2P network is connected to at most N
neighbors in the overlay network. Also, suppose that the node-count
field is initially set at K. Suppose Alice makes a query. Find an upper
bound on the number of query messages that are sent into the overlay
network.
A

								
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