Subject: question on TCP SACK

From: Tang Oo <[email-masked]>
Date: Wed, 18 Nov 1998 10:38:46 +0800

I am not clear about the behavior of TCP SACK. If an ACK indicates multiple
losses of segments, the congestion window should be cut by half, or cut by
multiple halves? Is the fast retransmit and fast recovery still appliable to
SACK?

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From: Ketan Bajaj <[email-masked]>
Date: Wed, 18 Nov 1998 11:35:10 +0530 (IST)

I think Sack does the following:
The fast retransmit & fast recovery procedures remain the same, and fast
retransmit is triggered based on ack information only(3 dup acks).
The ACK indicates the highest in sequence data received by the receiver,
whereas the SACK provides information about data in receiver's queue
beyond this.

So, the sender uses this information to retransmit selectively, & not
retransmitting what is already reached at receiver end.

TCP with "forward acknowledgements" uses this information to control the
congestion window during the fast recovery period. It makes a better
estimate of the outstanding data in network pipe, and thus pushes in more
data than the Tcp without using this information. It gives some
algorithms for controlling the congestion window, you can see the detail
in "Forward Acknowledgment: Refining TCP Congestion Control" by Mathis &
Mahdavi, Pittsburgh Supercomputing Center.

ketan

Ketan Bajaj
M.Tech Student
Deptt. Of Computer Sci. & Engg.
Indian Institute of Technology
Delhi

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From: Tom Henderson <[email-masked]>
Date: Wed, 18 Nov 1998 17:37:26 -0800 (PST)

The RFCs are not clear on this issue, but the basic congestion avoidance
principle is to only reduce once for a burst of losses within one window.