Congestion

As the amount of traffic on a network increases:

  1. Data packets are queued at routers
  2. Acknowledgement packets are also queued at routers
  3. Acknowledgements take longer to reach their destination
  4. Senders timeout and retransmit their data packets
  5. Additional (duplicated packets) are introducted to the network
  6. .....the cycle continues, getting worse with each iteration.....

As TCP was not originally aware of congestion, the 1980s saw a number of congestion collapse events which rendered the Internet unable to transmit data - all network resources were used to forward packets which never reached their destinations and so data throughput seriously reduced.

In order to prevent further congestion collapse events, the behaviour of TCP was extended. This section discusses several of the schemes which were used to improve the behaviour of the protocol when operating on congested networks.


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