TCP transmission policy

• Sender & receiver negotiate on window size
• In the figure below the sender sends 2K data and the initial seq no
• The receiver sends the ack for the next seq no it is expecting and also advertises
the widow size
• Suppose the window is zero then sender waits and then sends a probe and then
sends the next set of data
• The diagram shows how the two sides communicate

• Suppose there is only one byte to be sent the 41 byte packet to be sent instead Nagle algorithm suggest that when the first byte comes it is sent and the other buffered till the ack received and then send the rest and wait for the ack and like this a sufficient no of bytes go in one segment

Silly window syndrome
At the receiver side even if a byte available at its buffer it advertised and the sender sends
the buffer is full the sender waits again and probes to get the window size so this will
continue and a loop formed to avoid this the receiver is forced to wait till good amount of
buffer space availability and then advertises and avoids the loop.
TCP Congestion control
• informally: “too many sources sending too much data too fast for network to
• different from flow control!
• manifestations:
– lost packets (buffer overflow at routers)
– long delays (queueing in router buffers)
Problems when congestion happens