kern/42367: MTU adjustments causing wm(4) to drop carrier

[bharder <bharder%methodlogic.net@localhost>]

>Number:         42367
>Category:       kern
>Synopsis:       wm(4) interface dropping carrier when MTU too low (?)
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Nov 23 20:15:00 +0000 2009
>Originator:     bharder <bharder%methodlogic.net@localhost>
>Release:        NetBSD 5.0_STABLE
>Organization:
        Method Logic Digital (http://www.methodlogic.net)
>Environment:
System: NetBSD oxford 5.0_STABLE NetBSD 5.0_STABLE (oxford) #10: Mon Nov 23 
00:44:49 PST 2009 root@oxford:/usr/obj/sys/arch/i386/compile/oxford i386
Architecture: i386
Machine: i386
>Description:
        Noticed with latest dhcpcd update to 5.1.3, specifically "Default to 
requesting an MTU" behaviour.
        Excerpt from /var/log/messages (with IP addresses obfuscated):

        Nov 23 00:10:48 oxford dhcpcd: wm0: leased 123.45.67.89 for 3600 seconds
        Nov 23 00:10:49 oxford dhcpcd: wm0: MTU set to 576
        Nov 23 00:10:49 oxford dhcpcd: forking to background
        Nov 23 00:10:49 oxford dhcpcd: wm0: carrier lost
        Nov 23 00:10:49 oxford dhcpcd: wm0: MTU restored to 1500
        Nov 23 00:10:51 oxford dhcpcd: wm0: carrier acquired
        Nov 23 00:10:51 oxford dhcpcd: wm0: rebinding lease of 123.45.67.89
        Nov 23 00:10:51 oxford dhcpcd: wm0: acknowledged 123.45.67.89 from 
98.76.54.32
        Nov 23 00:10:51 oxford dhcpcd: wm0: checking for 123.45.67.89
        Nov 23 00:10:56 oxford dhcpcd: wm0: leased 123.45.67.89 for 3593 seconds
        Nov 23 00:10:56 oxford dhcpcd: wm0: MTU set to 576
        Nov 23 00:10:56 oxford dhcpcd: wm0: carrier lost
        Nov 23 00:10:56 oxford dhcpcd: wm0: MTU restored to 1500
        Nov 23 00:10:58 oxford dhcpcd: wm0: carrier acquired
        Nov 23 00:10:58 oxford dhcpcd: wm0: rebinding lease of 123.45.67.89
        Nov 23 00:10:58 oxford dhcpcd: wm0: acknowledged 123.45.67.89 from 
98.76.54.32
        Nov 23 00:10:58 oxford dhcpcd: wm0: checking for 123.45.67.89
        Nov 23 00:11:02 oxford dhcpcd: wm0: leased 123.45.67.89 for 3586 seconds
        Nov 23 00:11:03 oxford dhcpcd: wm0: MTU set to 576
        Nov 23 00:11:03 oxford dhcpcd: wm0: carrier lost

>How-To-Repeat:
        If I understand correctly, if the dhcp server is sending
        sufficiently small MTU, and dhcp client is honouring it,
        this behaviour is triggered in wm(4).
>Fix:
        Yes, please. My understanding of this problem was aided by
        discussion w/ Roy Marples in irc on #netbsd. He may be able
        to provide more details. Thanks, Roy.