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Re: A&M Rattler and TCP performance



Please could you post a trace from tcpdump.

OK. Here are a few lines containing both a retransmit and a stall. Line wrapping will probably make this very ugly, but here goes :


13:26:53.073631 IP (tos 0x0, ttl 64, id 24003, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: P 34753:36201(1448) ack 1 win 5840 <nop,nop,timestamp 2762928138 6772731>
13:26:53.074012 IP (tos 0x0, ttl 64, id 29674, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 31857 win 17376 <nop,nop,timestamp 6772731 2762928137>
13:26:53.074022 IP (tos 0x0, ttl 64, id 24004, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: P 36201:37649(1448) ack 1 win 5840 <nop,nop,timestamp 2762928138 6772731>
13:26:53.074401 IP (tos 0x0, ttl 64, id 29675, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 31857 win 17376 <nop,nop,timestamp 6772731 2762928137>
13:26:53.074411 IP (tos 0x0, ttl 64, id 24005, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 37649:39097(1448) ack 1 win 5840 <nop,nop,timestamp 2762928139 6772731>
13:26:53.074781 IP (tos 0x0, ttl 64, id 29676, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 31857 win 17376 <nop,nop,timestamp 6772731 2762928137>
13:26:53.074793 IP (tos 0x0, ttl 64, id 24006, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 31857:33305(1448) ack 1 win 5840 <nop,nop,timestamp 2762928139 6772731>
13:26:53.075204 IP (tos 0x0, ttl 64, id 29677, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 39097 win 10136 <nop,nop,timestamp 6772731 2762928139>
13:26:53.075217 IP (tos 0x0, ttl 64, id 24007, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 39097:40545(1448) ack 1 win 5840 <nop,nop,timestamp 2762928140 6772731>
13:26:53.075226 IP (tos 0x0, ttl 64, id 24008, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 40545:41993(1448) ack 1 win 5840 <nop,nop,timestamp 2762928140 6772731>
13:26:53.075517 IP (tos 0x0, ttl 64, id 29678, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 39097 win 17376 <nop,nop,timestamp 6772731 2762928139>
13:26:53.075717 IP (tos 0x0, ttl 64, id 29679, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 41993 win 14480 <nop,nop,timestamp 6772731 2762928140>
13:26:53.075728 IP (tos 0x0, ttl 64, id 24009, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: P 41993:43441(1448) ack 1 win 5840 <nop,nop,timestamp 2762928140 6772731>
13:26:53.075737 IP (tos 0x0, ttl 64, id 24010, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 43441:44889(1448) ack 1 win 5840 <nop,nop,timestamp 2762928140 6772731>
13:26:53.075904 IP (tos 0x0, ttl 64, id 29680, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 41993 win 17376 <nop,nop,timestamp 6772731 2762928140>
13:26:53.172454 IP (tos 0x0, ttl 64, id 29681, offset 0, flags [DF], proto 6, length: 52) 192.168.123.124.12345 > 192.168.123.123.51054: . [tcp sum ok] ack 43441 win 17376 <nop,nop,timestamp 6772741 2762928140>
13:26:53.172465 IP (tos 0x0, ttl 64, id 24011, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 44889:46337(1448) ack 1 win 5840 <nop,nop,timestamp 2762928237 6772741>
13:26:53.172474 IP (tos 0x0, ttl 64, id 24012, offset 0, flags [DF], proto 6, length: 1500) 192.168.123.123.51054 > 192.168.123.124.12345: . 46337:47785(1448) ack 1 win 5840 <nop,nop,timestamp 2762928237 6772741>


This is not good. 10% packet getting loss it way too high. You should
investigate this. You might want to see if you are running out of mbuf
or clusters. This would cause a discard and so a retry.

I will try this. I think I can enable some diagnostic output when running out of mbufs. Also, I will try again to increase the number of buffers, and maybe verify somehow that my change is actually effective.


Is it trying to make the window smaller? My guess is it is. Making the
window smaller could indiate it is running out of buffers.

Actually, it seems to make the window larger. In this tcpdump, the window was increased from 14480 to 17376 bytes(?) by the target, right before the stall.


Best regards,
 Ola Bård Langlo

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