Lectures notes for 2010 - KTH

Van Jacobson’s Fast retransmit and Fast Recovery Algorithm
TCP is required to generate an immediate ACK (a duplicate ACK) when an
out-of-order segment is received. This duplicate ACK should not be delayed. The
purpose is to tell the sender that the segment arrived out of order and what segment
number the receiver expects.
Cause:
• segments arriving out of order OR
• lost segment
If more than a small number (3) of duplicate ACKs are detected, assume that a
segment has been lost; then retransmit the missing segment immediately (without
waiting for a retransmission timeout) and perform congestion avoidance - but not
slow start.
Why not slow start? Because the only way you could have gotten duplicate ACKs
is if subsequent segments did arrive - which means that data is getting through.
Maguire TCP header continued 5: 30 of 77
maguire@kth.se 2010.03.21 Internetworking/Internetteknik