Question

Consider sending a large file from a host to another over a TCP connectionthat has no loss.a. Suppose TCP uses AIMD for its congestion control without slow start.Assuming cwnd increases by 1 MSS every time a batch of ACKs is receivedand assuming approximately constant round-trip times, how long does it takefor cwnd increase from 1 MSS to 6 MSS (assuming no loss events)?b. What is the average throughout (in terms of MSS and RTT) for this connectionup through time = 5 RTT?

Answer 1

Given : TCP uses AIMD for its congestion control without slow start.

Also given congestion window(cwnd) increases by 1MSS everytime a batch of ACK is received

a. Time taken for cwnd to increase from 1MSS to 6MSS :

It takes , 1 RTT for cwnd to increase from 1to 2 MSS

2 RTT for cwnd to increase from 2 to 3 MSS

  3 RTT for cwnd to increase from 3 to 4 MSS

4 RTT for cwnd to increase from 4 to 5 MSS

5 RTT for cwnd to increase from 5 to 6 MSS  

therefore it takes 5 RTT's to increase cwnd from 1 to 6 MSS.

b. We know average throughput = total data/total time

So in the 1st RTT 1 MSS was sent

  2nd RTT 2 MSS was sent

3rd RTT 3 MSS was sent

4th RTT 4 MSS was sent

5th RTT 5 RSS was sent

Total data sent = 1+2+3+4+5

= 15MSS

Total time = 5RTT's

therefore average throughput = 15/5

= 3 MSS per RTT