A large file is being sent from a host over a TCP connection

A large file is being sent from a host over a TCP connection that has no loss. Suppose TCP uses AIMD for its congestion control without slow start. Assume that cwnd increases by 1 MSS every time a batch of ACKs is received and also assume that the round-trip time remains constant.

a.) How long does it take for cwnd to increase from 5 MSS to 11 MSS, if there are no loss events?

b.) What is the average throughput (in terms of RTT and MSS) for the described connection up through time = 6 RTT?

Solution

Answer :

         A TCP window that w in size will have w outstandings segments in flight.We cloud say that the \'batch\' of ACKs corresponding to the outstanding segments have arrived,when all w segments have been acknowledged.At each ACK reception,we increment the window by a fraction such that when the full batch comes back the window is incremented by an MSS.

1.1 RTT to increase the congwindow from 5 to 6 MSS

2.2 RTT to increase the congwindow from 6 to 7 MSS

3.3 RTT to increase the congwindow from 7 to 8 MSS

4.4 RTT to increase the congwindow from 8 to 9 MSS

5.5 RTT to increase the congwindow from 9 to 10 MSS

6.6 RTT to increase the congwindow from 10 to 11 MSS

            First RTT,5 MSS were sent,

            Second RTT 6 MSS,

            3rd RTT 7 MSS,

            4th RTT 8 MSS,

            5th RTT 9 MSS,

            6th RTT 10 MSS were sent.

A total of 6 RTT 5+6+7+8+9+10=45 MSS were Sent.

Average throughput =45/6 MSS/RTT.....