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Re: ICMP_UNREACH_NEEDFRAG returns iface MTU instead of route?



Dave Huang <khym%azeotrope.org@localhost> writes:

> On Sat, Dec 28, 2013 at 10:55:53AM -0500, Greg Troxel wrote:
>> I disagree; there seems to be no notion in the standards that discovered
>> MTUs for routes are to be propagated.   The entire notion of "route MTU"
>> is just an implementation detail to store PMTU-D information.
>
> Is it? That seems like the root of the problem then... the routing
> table is used to store routing information, including info on how to
> route forwarded packets. PTMU-D info should be stored elsewhere... or
> at least it should be marked with some flag tht the kernel can look at
> to know whether it's a PMTU cache entry or an actual route. AFAICT,
> neither Linux nor Windows uses the routing table for its PMTU cache.

That's a fair point, but a central design point of the 4.4BSD networking
code is to use (abuse) the (single) routing table for multiple things,
including ARP.  It's a reasonable notion that these PMTU-D entries
should be identifiable.

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