Subject: Re: Strange TCP SACK behaviour in Linux TCP

From: Oumer Teyeb <[email-masked]>
Date: Wed, 19 Jul 2006 00:05:10 +0200

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-masked]> 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-masked]?
>  
>