Lectures notes for 2010 - KTH

Congestion Avoidance Algorithm
Slow start keeps increasing cwnd, but at some point we hit a limit due to
intervening routers and packets start to be dropped.
The algorithm assumes that packet loss means congestion 1 . Signs of packet loss:
• timeout occuring
• receipt of duplicate ACKs
Introduce another variable for each connection: ssthresh == slow start threshold
when data is acknowledged we increase cwnd:
• if cwnd < ssthreshold we are doing slow start; increases continue until
we are half way to where we hit congestion before
• else we are doing congestion avoidance; then increase by
1/cwnd + 1/8 of segment size each time an ACK is received
(See Stevens, Vol. 1, figure 21.8, pg. 311 for a plot of this behavior)
1. Note: if your losses come from other causes (such as bit errors on the link) it will still think it is due to congestion!
Maguire TCP header continued 5: 29 of 77
maguire@kth.se 2010.03.21 Internetworking/Internetteknik