Jade Cheng
March 4th, 2009 (late)
ICS 451
Exercise 2


[Question]

In Section 3.5.3 we discussed TCP's estimation of RTT. Why do you think TCP
avoids measuring the SampleRTT for retransmitted segments?

Answer:

The retransmitted segments contain the same data and the same sequence numbers
as the previously sent packets.  Therefore, for the retransmitted segments,
we would expect the same ACK numbers as the previously sent TCP segments.
When we receive these paticular ACKs from the receiver, we don't really know
or care whether they were sent as responses to the retransmitted segments or
to the previously sent TCP segment.

It is possible that after a very short time, we receive the ACK for the
retransmitted data segment.  But that could actually be a delayed ACK for the
original TCP segment.  Either the data transmission or the ACK reply was
probably delayed.  Both of them could result in a late ACK.  If we count
this time in to compute the RTT estimation, we would mistakenly drop the RTT
average by a big percentage.  This RTT would trigger a faster retransmission,
which would worsen the already jammed network traffic.