amd64/123603: tcp_do_segment and Received duplicate SYN

[John]

>Number:         123603
>Category:       amd64
>Synopsis:       tcp_do_segment and Received duplicate SYN
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    freebsd-amd64
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Mon May 12 07:50:01 UTC 2008
>Closed-Date:
>Last-Modified:
>Originator:     John
>Release:        FB7.0 (x64)
>Organization:
NULL
>Environment:
FreeBSD mail.mydomain.com 7.0-RELEASE FreeBSD 7.0-RELEASE #0: Thu Mar  6 12:04:57 HKT 2008     root at mydomain.com:/usr/src/sys/amd64/compile/FB7NEW  amd64
>Description:
A FreeBSD 7.0 (x64) Lighttpd Web Server with most-updated ports and patchs.
when a client connect and view a static HTML file, At the first time (before web server idle time), it needs to wait a long time to establish a connection, OR when this server try to download file from Internet, there are lots of logs messages just like below:

May 12 11:57:54 mail kernel: TCP: [55.66.77.88]:41792 to [11.22.33.44]:80 tcpflags 0x2<SYN>; syncache_add: Received duplicate SYN, 
resetting timer and retransmitting SYN|ACK
May 12 15:17:53 mail kernel: TCP: [193.166.3.2]:45979 to [11.22.33.44]:63372 tcpflags 0x10<ACK>; tcp_do_segment: FIN_WAIT_1: Received 1448 bytes of data after socket was closed, sending RST and removing tcpcb
May 12 15:17:53 mail kernel: TCP: [193.166.3.2]:21 to [11.22.33.44]:55007 tcpflags 0x18<PUSH,ACK>; tcp_do_segment: FIN_WAIT_2: Received 13 bytes of data after socket was closed, sending RST and removing tcpcb

>How-To-Repeat:
any type of  connection will generate above log messages.
>Fix:
no idea .

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