kern/159795: excessive duplicate ACKs and TCP session freezes

[Darren Baginski]

>Number:         159795
>Category:       kern
>Synopsis:       excessive duplicate ACKs and TCP session freezes
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Mon Aug 15 16:40:03 UTC 2011
>Closed-Date:
>Last-Modified:
>Originator:     Darren Baginski
>Release:        8.2-RELEASE
>Organization:
N/A
>Environment:
FreeBSD downloader1.lab.local 8.2-RELEASE FreeBSD 8.2-RELEASE #0: Tue Mar 15 09:14:03 UTC 2011     root at downloader1.lab.local:/usr/obj/usr/src/sys/GENERIC  amd64
>Description:
There is excessive duplicate ACKs and  session freezes while downloading huge files at speed over http or ftp.

tcpdump:

19:17:04.022367 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.022369 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.022553 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.022703 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.022707 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.022709 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.022856 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541894 ecr 783406220], length 0
19:17:04.023002 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023005 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023007 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023151 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023155 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023157 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023159 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023200 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0
19:17:04.023301 IP 10.178.56.18.19930 > 10.178.132.6.80: Flags [.], ack 27950968, win 16384, options [nop,nop,TS val 977541895 ecr 783406220], length 0

>How-To-Repeat:
Setup two boxes, connect the via 1Gbit switch and run sequential wget or fetch tests. Receiving box generates excessive ACKs and in about 40% cases tcp session freezes and then timeouts.
>Fix:
Unknown, but looks like the was similar report 

http://lists.freebsd.org/pipermail/freebsd-net/2011-February/027895.html

>Release-Note:
>Audit-Trail:
>Unformatted: