L6.md

Lecture 6

File distribution: client-server vs P2P

• How much time to distribute file (size F) from one server to N peers
  • peer upload/download capacity is limited resource

File distribution time: client-server

• server transmission: must sequentially send (upload) N file copies:
  • time to send one copy: F/u_s
  • time to send N copies: NF/u_s
• client: each client must download file copy
  • d_min = min client download rate
  • min client download time: F/d_min
• time to distribute F to N clients using client server approach = D_cs-s >= max(NF/U_s, F/d_min)

File distribution time: P2P

• server transmission must upload at least one copy
  • time to send one copy: F/u_s
• client: each client must download file copy
  • min client download time: F/d_min
• clients: as aggregate must download NF bits
  • max upload rate (limiting max download rate) is u_s + sigma u_i
• D_P2P > max(F/u_s, F/d_min, NF/(u_s + sigmau_i))

client server vs P2P: exampls

• P2P is much more scalable

P2P file distribution: BitTorrent

• file divided into 256Kb chinks

• peers in torrent send/receive file chunks

  • tracker tracks peers participating in torrent
    • only tracks who is in the system not who has what chunk
  • torrent: group of peers exchanging chunks of a file
  • if someone arrives, obtains list of peers from tracker and begins exchangiong chunks with the torrent

• peer joining torrent: has no chunks, but will accumulate them over time from other peers

• registers with tracker to get list of peers, connects to subset of peers ("neighbors") not all peers

• while downloading, peer uploads chunks to other peers

• peer may change e=peers with whom it exchanges chunks

• churn: peers may come and go

• once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent

BitTorrent: requesting, sending file chunks

• requesting chunks:
  • at any given time, different peers have different subsets of file chunks
  • periodically, alice asks each peer for list of chunks that they have
  • alice requests missing chunks from peers, rarest first
• sending chunks: tit-for-tat
  • alice sends chunks to those four peers currently sending her chunks at highest rate
    • other peers are choked by alice (do not receive chunks from her)
    • re-evaluate top 4 every 10 seconds
  • every 30 secs: randomly select another peer, starts sending chunks
    • "optiistically unchoke" this peer
    • newly chosen peer may join top 4

BitTorrent: tit-for-tat

• Alice optimisticallly unchokes bob
• alice becomes obe of bob's top 4 providers, bob reciprocates
• bob becomes one of alice's top 4 providers
• higher upload rate: find better trading partners, get file faster

Socket programming

• goal: learn how to build client/server applications that communicate using sockets
• socket: door between application process and end-end-transport protocol
• two socket types for two transport services:
  • UDP unreliable datagram
  • TCP: reliable, byte stream-oriented
• Application example:
  • client reads a line of characters from its keyboard and sends data to server
  • server receives the data and converts characters to uppercase
  • server sends modified data to client
  • client receives modified data and siplays line on its screen

Socket programming with UDP

• UDP: no "connection" between client and server
  • no handshaking before sending data
  • sender explicitly attaches IP destination addresss and port # to each packet
  • receiver extracts sender IP address and port # from received packet
• UDP: transmitted data may be lost or received out of order
• Application viewpoint
  • UDP provides unreliable transfer of groups of bytes between client and server

Client server socket interaction: UDP

• 
• client closes socket after every interaction

Socket programming with TCP

• Client must contact server
  • server process must first be running
  • server must have created socket that welcomes client's contact
• client contacts server by:
  • creating TCP socket, specifying IP address, port number of server process
  • when client creates socket: client TCP establishes connection to server
• when contacted by client, server TCP creates new socket for server process to communicate with that particular client
  • allows server to talk with multiple clients
  • source port number used to distinguish clients

Transport layer

Transport services and protocols

• provide logical communication between application processes running on different hosts
• transport protocols action in end systems:
  • sender breaks application messages into segments, passes to network layer
  • receiver reassembles segments into messages, passes to application layer
• two transport protocols
  • TCP, UDP

Transport vs network layer services and protocols

• network layer, logical communication between hosts
• transport layer, logcial communication between processes
  • relies on enhences, network layer services
• sender
  • passed an application layer message
  • determines segment header field values
  • creates segments
  • passes segment to IP
• reveiver
  • receives segment from IP
  • checks header value
  • extracts application layer messages
  • demultiplexes message up to application via socket

Two principal internet transport protocols

• TCP:
  • transmission control protocol
  • reliable, in order delivery
  • congestion control
  • flow control
  • connection setup
• UDP
  • User datagram protocol
  • unreliable, unordered delivery
  • no-frill
• services not available
  • delay guarantees
  • bandwidth guarantees

Multiplexing/demultiplexing

• multiplexing at sender
  • hande data from multiple sockets, add transport header (later used for demultiplexing)
• demultiplexing at receiver