Re: ICMP_UNREACH_NEEDFRAG returns iface MTU instead of route?

[Dave Huang <khym%azeotrope.org@localhost>]

On Sat, Dec 21, 2013 at 04:55:37PM -0600, Dave Huang wrote:
> On Sat, Dec 21, 2013 at 03:03:48PM -0500, Dennis Ferguson wrote:
> > The reason is that the only
> > use I've seen made of the route MTU field is to implement a cache of PMTUs
> > discovered by applications running on the local host

> It doesn't seem like NetBSD uses it as a PMTU cache--at least I've
> never seen "netstat -r" show any routes added due to PMTU discovery.

Huh, I guess I just haven't been looking at the right time (and also
probably because 1500-byte MTUs are so common?) After running with the
patch in
<http://mail-index.netbsd.org/tech-net/2013/12/20/msg004421.html> that
makes IPSEC send ICMP fragmentation needed, I do in fact see "netstat
-r" show entries for hosts on the other side of the tunnel, with
appropriately-reduced MTUs. E.g., after uploading a file to 10.2.1.20,
this showed up:

Destination        Gateway            Flags    Refs      Use    Mtu Interface
[ ... ]
10.2.1.20          10.1.1.67          UGHD        -        -   1420  fxp0

I still think the patch in kern/48472 improves the situation though...
if it's decided that discovered PMTUs shouldn't affect forwarded
packets (which I agree with), that's a separate topic from accurately
reporting the MTU that's actually being used.
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