>Number:         20461
>Category:       kern
>Synopsis:       Incorrect handling of MSS with RFC 1323 timestamp option
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Feb 21 16:12:00 PST 2003
>Closed-Date:
>Last-Modified:
>Originator:     Iljitsch van Beijnum
>Release:        none
>Organization:
>Environment:
I don't use netbsd myself; I see the problem when I connect to www.netbsd.org.
>Description:
When the TCP timestamp option (RFC 1323) is used the extra 12 bytes aren't taken into account when creating a packet so the packet ends up being 12 bytes larger than the maximum segment size announced by the correspondent host + 40 bytes for IP+TCP headers:

00:41:28.787634 torreya.muada.com.49185 > www.netbsd.org.http: S [bad tcp cksum 7515!] 468191823:468191823(0) win 32768 <mss 1445,nop,wscale 0,nop,nop,timestamp 1831355730 0> (DF) (ttl 64, id 4031, len 60)
00:41:28.960017 www.netbsd.org.http > torreya.muada.com.49185: S [tcp sum ok] 3350810543:3350810543(0) ack 468191824 win 32768 <mss 1460,nop,wscale 0,nop,nop,timestamp 19359762 1831355730> (ttl 51, id 43843, len 60)
00:41:28.960208 torreya.muada.com.49185 > www.netbsd.org.http: . [bad tcp cksum 1e07!] ack 1 win 33304 <nop,nop,timestamp 1831355730 19359762> (DF) (ttl 64, id 4032, len 52)
00:41:28.967175 torreya.muada.com.49185 > www.netbsd.org.http: P 1:243(242) ack 1 win 33304 <nop,nop,timestamp 1831355730 19359762> (DF) (ttl 64, id 4033, len 294)
00:41:29.182218 www.netbsd.org.http > torreya.muada.com.49185: . [tcp sum ok] ack 243 win 33580 <nop,nop,timestamp 19359763 1831355730> (ttl 51, id 43852, len 52)
00:41:29.287983 www.netbsd.org.http > torreya.muada.com.49185: . 1:1446(1445) ack 243 win 33580 <nop,nop,timestamp 19359763 1831355730> (ttl 51, id 43855, len 1497)
00:41:29.373683 www.netbsd.org.http > torreya.muada.com.49185: . 1446:2891(1445) ack 243 win 33580 <nop,nop,timestamp 19359763 1831355730> (ttl 51, id 43856, len 1497)
>How-To-Repeat:
- set mtu smaller than 1488 bytes on a host and enable RFC 1323 extensions
- connect to www.netbsd.org
>Fix:

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