Strange TCP SACK behaviour in Linux TCP
From: Oumer Teyeb
Date: Tue Jul 18 2006 - 12:18:54 EST
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
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
retransmission....as 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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