by shobana padmanabhan sep 12 2007 cse 473 n.
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By Shobana Padmanabhan Sep 12, 2007 CSE 473. Class #4: P2P Section 2.6 of textbook (some pictures here are from the book). Recap. Links Packet switched networks Store-and-forward routing Delays d prop = distance/ propagation speed d tran = L /R = packet size/ capacity d q d proc

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by shobana padmanabhan sep 12 2007 cse 473
By

Shobana Padmanabhan

Sep 12, 2007

CSE 473

Class #4: P2PSection 2.6 of textbook (some pictures here are from the book)

recap
Recap
  • Links
  • Packet switched networks
  • Store-and-forward routing
  • Delays
    • dprop = distance/ propagation speed
    • dtran = L /R = packet size/ capacity
    • dq
    • dproc
  • IPv4 address: 32 bits
    • 172.16.2.15 (roach.int.cec.wustl.edu)
    • 172.16.2.8 (butterfly.int.cec.wustl.edu)
      • 10101100 00010000 00000010 00001000
    • Subnet mask
      • 172.16.2.8/24
        • First 24 bits constitute “prefix”
today s lecture
Today’s lecture
  • Application layer
    • Applications don’t see links
    • Client-server or P2P
      • Today, we focus on P2P
applications
Applications

Gmail

request

response

Internet

request

response

New mail:

“Watch Federer video.

Tell Cathy, Denis, Ed, and Frank also.”

applications1
Applications

Gmail

  • Server sends 5 copies
  • If 50 requests, 50 copies; 5 million requests, 5 million copies!

request

response

Internet

request

response

client server
Client-server
  • Suppose
    • File is F bits
    • N users
    • Upload rate of server’s access link is us
    • Upload rate of useri’s access link is ui
    • Download rate of useri’s access link is di
  • Time to distribute a copy to all users
    • NF/us a
      • A user may have very low download rate
        • Max {NF/us, F/dmin} where dmin= min {d1,d2,…dN }
client server1
Client-server
  • Other problems
    • Significant reliance on always-on servers
    • Multiple copies, along with other traffic, can cause congestion on links
      • Longest-prefix matching is not sensitive to traffic load
      • Queuing delays at routers
        • Potential dropping of packets (i.e.) data loss
  • Peer-to-peer architecture addresses these…
peer to peer p2p
Peer-to-peer (P2P)
  • Pairs of computers, called peers, communicate directly.

Source: wikipedia.org

  • Server sends file once.
  • Peers redistribute file chunks using their upload capacity.
    • Max {F/us, F/dmin, NF/(us+u1+u2+…+uN)}
bittorrent
BitTorrent
  • P2p protocol for file distribution
bittorrent1
BitTorrent
  • When a peer (Alice) joins a torrent, she registers with a tracker.
    • Periodically, informs tracker of its presence.
  • Tracker sends Alice IP addresses of n random participating peers.
  • Alice tries to establish connection with them all.
    • All connected peers are neighboring peers.
      • These peers may leave and others may join over time.
  • Periodically, Alice asks each neighbor for their list of chunks
    • Alice then requests chunks that she wants.
  • Important decisions Alice makes are:
    • Request “rarest first” chunks
    • “Trading algo” to determine neighbors to whom to send
      • Eliminates “free-riding”
bittorrent2
BitTorrent
  • Estimated BitTorrent traffic is 18 - 35%
  • BitTorrent does not offer users anonymity
    • obtaining IP addresses exposes users with insecure systems to attacks
  • Peers may leave selfishly after downloading a file
  • Users with low upload capacity may see slower download speeds until they upload more
    • Some trackers exempt dial-up users

Source: wikipedia.org

indexing
Indexing
  • Map info to locations
    • Centralized index
    • Query flooding
    • Hierarchical overlay
    • DHTs (Distributed Hash Table)
2 query flooding index
2. Query flooding index
  • Limited-scope query flooding
  • Handling of peers joining and leaving overlay
    • Bootstrap, maybe with a tracker
  • The new peer tries to setup TCP connections
  • “Ping”, “pong”
3 hierarchical overlay index
3. Hierarchical overlay index
  • “Super” peers maintain index for files of their children
  • Super peers interconnect themselves
4 dht index
4. DHT index
  • A fully decentralized index
  • Allows users to determine all locations of a file
    • Without generating excessive traffic
skype
Skype
  • P2P Internet telephony
    • P2P techniques for also user location
  • Proprietary protocol; all packet transmissions encrypted
  • Nodes organized into hierarchical overlay net
    • Each peer classified as super or ordinary
  • Index to map username to current IP address
    • Distributed over super peers
longest prefix matching1
Longest-prefix matching

// insert sorted

if (new_mask == curr_mask) {

if (new_destAddr == curr_destAddr) {

//entry already exists

return 0;

}

return (new_destAddr > curr_destAddr) ? 1 : -1;

}

return (new_mask > curr_mask) ? 1 : -1;

// if return value > 0, insert above current entry

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