Transport%20Control%20Protocol%20(TCP)

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Transport Control Protocol (TCP)

• Features of TCP, packet loss and retransmission,
  adaptive retransmission, flow control, three way
  handshake, congestion control

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Reliable transport

• IP is unreliable
• Application programmers typically require
  reliability (although not always)
• The Transmission Control Protocol (TCP)
  establishes a reliable end-to-end communication
  service on top of IP

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Seven major features of TCP

• 1. Connection oriented
• 2. Point to point
• 3. Complete reliability - delivered as sent
• 4. Full duplex communication
• 5. Stream interface
• 6. Reliable connection startup
• 7. Graceful connection shutdown

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End to end communication

• TCP has no knowledge of the underlying Internet
  structure

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Fundamental reliability problems

• Unreliable delivery by the communication system
  (IP)
• Unreliable end hosts (e.g., crashes and reboots
• TCP handles these through a variety of techniques

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Packet loss and retransmission

• Sender sets a timer
• Receiver sends a acknowledgement
• Timeout results in retransmission

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Adaptive retransmission

• Sensible timer values vary greatly on an Internet
• TCP monitors the delay on a connection and adapts
  the timer
• notes time taken to receive acknowledgements
• computes weighted average and variance over many
  transmissions and uses these to set the timer

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Two different timeouts
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Flow control

• TCP uses a window mechanism
• Each end of the connection allocates a buffer and
  notifies the other end of its size
• Receiver sends available window size in each
  acknowledgement (window advertisement)
• Receiver sends window advertisement when the
  application consumes some data
• Zero window advertisement tells the sender to
  stop transmitting until further notice

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Three way handshake

• Uses special synchronisation (SYN) and finish
  (FIN) messages to open and close connections
• Also confirms that all data has been received at
  both sides
• Each end of each new connection randomly
  generates a 32 bit connection identifier

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Congestion control

• TCP monitors congestion through message loss
• first lost message, TCP backs right off and sends
  just one small message
• if this is not lost, doubles data size and sends
  two
• continues exponential growth until half
  receivers window size is reached then slows down
  rate of increase

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TCP segment format

• TCP messages are called segments

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• Example exam question
• http//www.cs.nott.ac.uk/mvr/3-ccn.doc