Hi all,
I have some questions regarding Linux TCP in the presence of delays or
packet drops. It is somehow long mail, but the questions are two or
three, just wanted to provide a detailed information so that the problem
is clear. thanx for the patience!!
Best regards,
Oumer
Note that for the traces referred here, SACK,timestamps, and FRTO are
all disabled...
1) packet drops
================
I have a trace where the tcp sender window is flushed and then the
connection speed is changed from 1Mbps to 384kbps...
The trace files from both the client and the server side can be found at
http://kom.aau.dk/~oumer/drop_0_delay_SERVER.dat
http://kom.aau.dk/~oumer/drop_0_delay_CLIENT.dat
and the tcptrace time sequence curve can be found in
http://kom.aau.dk/~oumer/drop_0_delay.ps
as can be seen from the plot and the trace files at around
17:19:35.705733, the window was flushed (both the sender's and
receivers), and hence packets with seq numbers from
1840001135 upto 1840058075 were dropped (39 packets)...and also the ACK
for 1840001135 was also dropped (from the traces this can be seen as it
appears
in the client trace but not on the server trace)...
and since there were still packets to be sent the sender keeps sending a
few more packets
and when few of them are received (from the client side trace..)
17:19:35.938017 1840059535:1840060995(1460) ack 3059152863 win 5840 (DF)...
17:19:35.938028 ack 1840001135 win 62780 (DF) [tos 0x8]...first ACK
that is going to be received by the sender
17:19:35.969316 1840060995:1840062455(1460) ack 3059152863 win 5840 (DF)
17:19:35.969325 1840001135 win 62780 (DF) [tos 0x8]....first duplicate ACK
17:19:36.000519 1840062455:1840063915(1460) ack 3059152863 win 5840 (DF)
17:19:36.000528 ack 1840001135 win 62780 (DF) [tos 0x8]... second
duplicate ACK
when the server gets this 2nd duplicate ACK, it retransmits the packets
(this is clearly visible from the tcptrace curve.)..eventhough a 3rd
duplicate ACK soon follows.
so my first question "why is the second duplicate ACK triggering a
retransmission?"...
also after that, there are a couple of retransmissions triggerd by the
reception of the ACK for the new ACKs and at time instant (server side
trace)
17:19:36.057149 . 1840001135:1840002595(1460) ack 3059152863 win 5840
(DF)..first packet retransmitted
17:19:36.085569 ack 1840001135 win 62780 (DF) [tos 0x8] ...this is the
third duplicate ACK which should have caused the retrans, but lets
ignore it for now
17:19:36.248599 ack 1840002595 ...retransmitted packet acked
17:19:36.251382 1840002595:1840004055(1460) ack 3059152863 win 5840
(DF) ... next packet retransmitted
17:19:36.442831 ack 1840004055 win 61320 (DF) [tos 0x8]...2nd packet
acked also
17:19:36.445625 1840004055:1840005515(1460) ack 3059152863 win 5840
(DF) .. third packet retransmitted
17:19:36.637224 ack 1840005515 win 61320 (DF) [tos 0x8] ... third
packet acked
17:19:37.417022 1840005515:1840006975(1460) ack 3059152863 win 5840
(DF) ... fourth packet retransmitted
As you can see there is 0.8 second gap between the ack for the
reception of the ACK for the third packet and the sending of the fourth
packet...so my second question "why didnt the sender immediatly
send the fourth packet after the reception of the ack for the third?"
I generated the same scenario 20 times, and the same thing happens in
all of them...
2)packet delays
===============
in the second scenario, I have a 2 second delay, but no packet
drops...the downgrade in bandwidth also happens, but the packets in the
window are buffered for 2 seconds and released...
The trace files from both the client and the server side can be found at....
http://kom.aau.dk/~oumer/delay_0_drop_SERVER.dat
http://kom.aau.dk/~oumer/delay_0_drop_CLIENT.dat
and the tcptrace time sequence curve can be found in
http://kom.aau.dk/~oumer/delay_0_drop.ps
The delay is applied from 17:20:01.066725 to 17:20:03.067022
as can be seen from the traces and plot packets with seq number
1858561966 to 1858618906 ( a total of 40 packets) were queued at the
server and one packet from the receiver, which is an ACK for
pkt # 1858560506 ....
at around 17:20:03.15 this ack is received and sender thinks this is the
result of its retransmission (which actually was dropped, so at this
point the receiver hasnot got any retransmissions).. and the normal
retransmission is resumed (as well as sending of some new data, as the
window allows it) as can be seen from the server side trace upto time
instant 17:20:04.539682
...at which point we can see that on the client trace the
retransmissions actually start arriving at the receiver (so far the ACKs
that were triggering the retransmissions were acks to the reception of
the original
but delayed packets)...and this duplicate arrivals lead to multiple
duplicate ACKs... what I dont understand is why this duplicate ACKs
(there are 40 duplicate ACKs.), no fast retransmission was triggered..
so my third question "Why is it that the duplicate ACKs are not
tiggerring fast retransmissions?" this creates a 1.3 second gap
transmission gap...actually this is better than fast retransmission
because it is not leading to further retransmissions...so is the linux
TCP so clever that it can figure out the problem without using SACK,
timestamps or FRTO ? ...or is this a special "feature" :-)....
I have repeated this also twenty times and the traces are similar...
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