Re: Miscalculated TCP ACK ?

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 22 août 2011 à 13:05 -0500, Gabriel Beddingfield a écrit : 
> On Mon, Aug 22, 2011 at 12:19 PM, Daniel Baluta <daniel.baluta@gmail.com> wrote:
> >>> It appears to me that the ACK is off-by-one (should have been 27676).
> >>> The server replies again with retransmissions.  This is with several
> >>> kernel versions on local machines, ranging from 2.6.31 to 2.6.38.
> >
> >> Not a bug if frame carried a FIN
> 
> The server packet had the FIN bit set.
> 

then its fine.

> > Well then, why do retransmissions occur?
> > Gabriel could you post the full capture file?
> 
> Sure: http://gabe.is-a-geek.org/tmp/20110822-skyward-conversation-segment.tcpdump
> 
> I clipped the conversation to the vicinity of the problem.
> 

This sounds as a TSO bug, maybe driver/NIC related

What is the NIC/driver used on your server ?

try "ethtool -K eth0 tso off" on the server

$ tcpdump -p -n -S -r 20110822-skyward-conversation-segment.tcpdump
reading from file 20110822-skyward-conversation-segment.tcpdump, link-type EN10MB (Ethernet)

Sorry we miss a few frames just before.

14:48:31.368344 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], seq 401237709:401239157, ack 1404739303, win 64334, options [nop,nop,TS val 1663311 ecr 3624365496], length 1448
14:48:31.368410 IP 192.168.1.25.36015 > 50.84.128.30.443: Flags [.], ack 401239157, win 52128, options [nop,nop,TS val 3624365515 ecr 1663311], length 0
14:48:31.368556 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], seq 401239157:401240605, ack 1404739303, win 64334, options [nop,nop,TS val 1663311 ecr 3624365496], length 1448
14:48:31.369019 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], seq 401240605:401242053, ack 1404739303, win 64334, options [nop,nop,TS val 1663311 ecr 3624365496], length 1448
14:48:31.369099 IP 192.168.1.25.36015 > 50.84.128.30.443: Flags [.], ack 401242053, win 57920, options [nop,nop,TS val 3624365515 ecr 1663311], length 0
14:48:31.369357 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], seq 401242053:401243501, ack 1404739303, win 64334, options [nop,nop,TS val 1663311 ecr 3624365496], length 1448
14:48:31.369412 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [FP.], seq 401243501:401243519, ack 1404739303, win 64334, options [nop,nop,TS val 1663311 ecr 3624365496], length 18

following frames acknowledge all data sent from server and FIN flag.
14:48:31.369442 IP 192.168.1.25.36015 > 50.84.128.30.443: Flags [.], ack 401243520, win 57920, options [nop,nop,TS val 3624365516 ecr 1663311], length 0

Up to there, its OK.

But following frame makes no sense : this data was already acknowledged

14:48:31.373447 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], seq 401236261:401237709, ack 1404739303, win 64334, options [nop,nop,TS val 1663311 ecr 3624365496], length 1448

14:48:31.373532 IP 192.168.1.25.36015 > 50.84.128.30.443: Flags [.], ack 401243520, win 57920, options [nop,nop,TS val 3624365520 ecr 1663311,nop,nop,sack 1 {401236261:401237709}], length 0

client timeouts :
14:48:31.458943 IP 192.168.1.25.36015 > 50.84.128.30.443: Flags [F.], seq 1404739303, ack 401243520, win 57920, options [nop,nop,TS val 3624365605 ecr 1663311], length 0

14:48:31.475204 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], ack 1404739304, win 64334, options [nop,nop,TS val 1663321 ecr 3624365605], length 0
14:48:31.475351 IP 50.84.128.30.443 > 192.168.1.25.36015: Flags [.], ack 1404739304, win 0, length 0
14:48:31.475395 IP 192.168.1.25.36015 > 50.84.128.30.443: Flags [R], seq 1404739304, win 0, length 0