ICMP_UNREACH_NEEDFRAG returns iface MTU instead of route?

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

It seems that the ICMP fragmentation needed packet contains the
interface MTU rather than the route MTU if the route MTU is lower
than the iface's: see sys/netinet/ip_input.c, ip_forward(), case
EMSGSIZE--around line 1337: destmtu = rt->rt_ifp->if_mtu;

Is that what it should be doing? It seems wrong and isn't what I
expected... if I artificially lower the MTU of a route, e.g.,
route add www.netbsd.org $my_gateway_ip -mtu 1200

Then ping -Ds 1300 www.netbsd.org from another machine that routes
through the above router, I get:
PING www.netbsd.org (149.20.53.86): 1300 data bytes
36 bytes from foxy.azeotrope.org (10.1.1.67): frag needed and DF set.  Next 
MTU=1500 for icmp_seq=0

Shouldn't Next MTU=1200, rather than 1500?

This is on a -current i386 kernel from October 2012, but the latest
ip_input.c (1.308) looks like it does the same thing, though I haven't
actually tried.
-- 
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