Computer networks -- 2007-2008 -- info.uvt.ro/Course 8

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Important! These pages are somehow outdated and it is recommended to consult the newer version at Computer networks -- 2008-2009 -- info.uvt.ro (by Ciprian Crăciun).

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Transport layer[edit]

  • where it sits in the layer stack;
  • generically it involves managing dialogues between hosts;
  • functions:
    • connection management;
    • flow control;
    • congestion control;
    • error detection / correction;
    • multiplexing;
    • service addressing;
    • buffering;
  • issues:
    • endpoint / connection addressing;
    • connection establishment / termination;
    • message acknowledgment;
    • message buffering;
  • service primitives:
    • listen / connect;
    • receive / send;
    • disconnect;
  • service access points:
    • TSPA -- Transport Service Access Point;
    • NSPA -- Network Service Access Point;

TCP[edit]

  • TCP -- Transport Control Protocol;
  • characteristics:
    • unicast protocol;
    • connection oriented;
    • ordered;
    • reliable;
    • full-duplex;
  • services:
    • it takes a stream of octets and breaks it into smaller blocks;
    • it presents a virtual circuit to the application layer;
    • it allows multiple streams at the same time;
  • functions:
    • connection setup and shutdown;
    • sequencing;
    • retransmission;
    • error detection;
    • service addressing;
    • flow control:
      • by using a sliding window;
    • congestion control;
  • ports;
  • links:

Segment[edit]

  • structure:
    • IP packet header -- 160 bits -- 20 octets;
    • source port -- 16 bits;
    • destination port -- 16 bits;
    • sequence number -- 32 bits:
      • what is the current offset in the entire stream for sent (outbound) data;
    • acknowledgment number -- 32 bits:
      • what is the expected offset in the entire stream for received (inbound) data;
    • header length -- 4 bits:
      • expressed in 32 bit words;
    • reserved -- 6 bits -- must be all 0;
    • flags -- 6 bits -- used for connection control;
    • window -- 16 bits:
      • the amount of receivable (inbound) data that will be accepted next;
    • checksum -- 16 bits:
      • CRC for both header and data;
    • urgent -- 16 bits:
      • the offset of urgent data;
    • options -- 0 or 32 (or a multiple of 32) bits;
    • data;
  • flags:
    • URG -- indicates that the urgent pointer field is significant;
    • ACK -- indicates that the acknowledgment field is significant;
    • PSH -- push function;
    • RST -- reset the connection;
    • SYN -- synchronize sequence numbers;
    • FIN -- no more data from sender;
  • checksum: the 16 bit one's complement of the one's complement sum of all 16-bit words in the header and text;

Connection life-cycle[edit]

  • the big picture:
    • connection establishment;
    • data exchange;
    • connection termination;
  • connection establishment:
    • three way handshake;
    • passive opening: done by the server;
    • active opening: done by the client;
    • Client: SYN:
      • it also selects a random number for the sequence field -- x;
    • Server: SYN-ACK:
      • it updates the acknowledgment field to x + 1;
      • it selects a random number for the sequence field -- y;
    • Client: ACK:
      • sequence -- x + 1;
      • acknowledgment -- y + 1;
  • data exchange:
    • the sequence and acknowledgment fields are used;
  • connection termination:
    • it is independent for each way of the connection;
    • there is no strict way of doing it:
      • as each end point could start the termination at any time;
      • as the messages could be merged together;
    • 'four way handshake;
    • 'three way handshake;
    • 'two way handshake;
    • in principle:
      • End-1: FIN;
      • End-2: ACK;
  • connection states:

Miscellaneous[edit]

  • MSS -- Maximum Segment Size:
    • how much data is sent into a single packet;
  • cumulative acknowledgment;
  • selective acknowledgment;
  • out of bound data;
  • congestion control:
    • receiver window;
    • congestion window;
    • threshold;
  • TCP and wireless connections;
  • links:

UDP[edit]

  • UDP -- User Datagram Protocol;
  • characteristics:
    • unicast or broadcast protocol;
    • connection-less;
    • unordered;
    • unreliable;
  • services:
    • it takes a block of data from the application layer, and it passes it -- intact -- to the destination;
    • it allows multiple services at the same time;
  • functions:
    • error control;
    • service addressing;
  • ports;

Datagram[edit]

SCTP[edit]

  • SCTP -- Stream Control Transmission Protocol;
  • characteristics:
    • connection-oriented;
    • message-oriented;
    • reliable;
    • ordered (can be turned off);
    • multi-streaming (multiplexing);
    • multi-homing;
  • links:
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