Hi Alexy, Is there anything linux specific about the DSACK
implementation that might lead to increase in the number of
retransmissions, but leads to improvment in download time when
timestamps are not used (and the reverse effect when timestamps are
used, less retransmissions but bigger download times)? because I
couldnt figure it out,....also is there anywhere where the reordering
response of tcp linux described? (it seem dupthreshold is dynamically
adjusted based on the reordering history... but I was not able to find
out how...)...
Oumer Teyeb wrote:
Oumer Teyeb wrote:
Hi,
Alexey Kuznetsov wrote:
Condition triggering start of fast retransmit is the same.
The behaviour while retransmit is different. FACKless code
behaves more like NewReno.
Ok, that is a good point!! Now at least I can convince myself the
CDFs for the first retransmissions showing that SACK leads to earlier
retransmissions than no SACK are not wrong....and I can even convince
myself that this is the real reason behind sack/fack's performance
degredation for the case of no timestamps,:-)... ...
Actually, then the increase in the number of retransmissions and the
increase in teh download time from no SACK - SACK for timestamp case
seems to make sense also...my reasoning is like this...if there is
timestamps, that means there is reordering detection...hence the
number retransmissions are reduced because we avoid the time spent in
fast recovery.... when we introduce SACK on top of timestamps, we
enter fast retransmits earlier than no SACK case as we seem to agree,
and since the timestamp reduces the number of retransmission once we
are in fast recovery, the retransmissions we see are basically the
first few retransmissions that made us enter the false fast
retransmits, so we have a little increase in the retransmissions and a
little increase in the download times... but when no timestamps are
used, there is no reordering detection and so SACK leads to less
number of retransmissions because it retransmits selectively, but it
doesnt improve the download time because it enters fast retransmit
eralier than the no SACK and in this case the fast retransmits are
very costly because they are not detected lead to window reduction....
am I making sense?:-).... still the DSACK case is puzzling me....
Regards,
Oumer
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