Question

Consider sending a large file from one host to another over a TCP connection that has no loss. a) Suppose TCP uses AIMD for its congestion control without slow start. Assuming cwnd increases by 1MSS every time a batch of ACKs is received and assuming constant roundtrip times,

how long does it take for cwnd to increase from 1 MSS to 6 MSS?

What is the average throughput (in terms of MSS and RTT) for this connection up through time = 5RTT?

Answer 1

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

Congestion window(cwnd) increases by 1 MSS.

After 1 RTT, CWND = 2 MSS

After 2 RTT, CWND = 3 MSS

After 3 RTT, CWND = 4 MSS

After 4 RTT, CWND = 5 MSS

After 5 RTT, CWND = 6 MSS

So, it takes 5 RTT's to increase cwnd from 1 MSS to 6 MSS

Average throughput = (Total data) / (Total time)

Total data = No.of segment during 5 RTT's = 1+2+3+4+5 = 15 MSS

Total time = 5RTT's

Average throughput = (15 MSS)/(5 RTT) = 3 MSS/RTT =3 MSS per RTT