Hi Stephen,
Thanks for the quick response.
I have done what you asked and you can find the files at
www.kom.auc.dk/~oumer/sackstuff.tar.gz
I have run the different cases 10 times each,
NT_NSACK[1-10].dat---no timestamp, no SACK
NT_SACK[1-10].dat----no timestamp, SACK
T_NSACK[1-10].dat---timestamp, no SACK
T_SACK[1-10].dat----timestamp. SACK
the files without extension are just two column files that summarize the
ten runs for the four different cases, the first column in the #
retransmission, and second column is the download time, the values are
gathered from tcptrace
the two eps files are just the plot summarizing the above average
download time and average retransmission # for each case...
one more thing in the trace files, you will find 3 tcp connections, the
first one is not modified by my emulator that causes the reordering
(actually, that is the connection through which I reset the destination
catch that stores some metrics from previous runs using some commands
via ssh), the second one is the ftp control channel and the third one is
the ftp data channel....the emulator affects the last two channels
and causes reordering once in a while.....
please dont hesistate to ask me if anything is not clear...
Thanks a lot for taking the time
Regards,
Oumer
Stephen Hemminger wrote:
On Tue, 18 Jul 2006 18:20:47 +0200
Oumer Teyeb <[EMAIL PROTECTED]> wrote:
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 .
at www.kom.auc.dk/~oumer/first_transmission_times.pdf
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,
Oumer
Could you post some short tcpdump snapshot summaries to [EMAIL PROTECTED]
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