Strange TCP SACK behaviour in Linux TCP

From: Oumer Teyeb
Date: Tue Jul 18 2006 - 12:18:54 EST

Hello Guys,

I have some questions regarding TCP SACK implementation in Linux .
As I am a subscriber, could you please cc the reply to me? thanks!

I am doing these experiments to find out the impact of reordering. So I have different TCP versions (newReno, SACK, FACk, DSACK, FRTO,....) as implemented in Linux. and I am trying their combination to see how they behave. What struck me was that when I dont use timestamps, introducing SACK increases the download time but decreases the total number of retransmissions.
When timestamps is used, SACK leads to an increase in both the download time and the retransmissions.

So I looked further into the results, and what I found was that when SACK is used, the retransmissions seem to happen earlier .
you can find the pic of cdf of the time when the first TCP retransmission occured for the four combinations of SACK and timestamps after hundrends of downloads of a 100K file for the different conditions under network reordering...

This explains the reason why the download time increases with SACK, because the earlier we go into fast recovery the longer the time we spend on congestion avoidance, and the longer the download time....

...but I couldnt figure out why the retransmissions occur earlier for SACK than no SACK TCP. As far as I know, for both SACK and non SACK cases, we need three (or more according to the setting) duplicate ACKs to enter the fast retransmission /recovery state.... which would have resulted in the same behaviour to the first occurance of a retransmission..... or is there some undocumented enhancment in Linux TCP when using SACK that makes it enter fast retransmit earlier... the ony explanation I could imagine is something like this

non SACK case
1 2 3 4 5 6 7 8 9 10..... were sent and 2 was reorderd....and assume we are using delayed ACKs...and we get a triple duplicate ACK after pkt#8 is received. (i.e 3&4--first duplicate ACK, 5&6..second duplicate ACK and 7&8...third duplicate ACK.....)...

so if SACK behaved like this...

3&4 SACKEd.... 2 packets out of order received
5&6 SACKEd....4 packets out of order received.... start fast reorderd is greater than 3.... (this is true when it comes to marking packets as lost during fast recovery, but is it true als for the first retransmission?)

.. any ideas why this is happening???

Thanks in advance,

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