Re: Path MTU Hop-by-Hop Option

[Mikael Abrahamsson <[e-mail]>]

On Tue, 16 Oct 2018, Bob Hinden wrote:

> Please read the draft.  Links below.

I believe the router should not only report the MTU of the outgoing 
interface, it should also report MTU and MRU of the incoming interface 
(whatever is smallest, or both).

I have seen misconfigurations where each side of a link has different 
MTU/MRU, resulting in PMTUD blackhole with silently dropped packets. If 
we're serious about this option, we should report whatever is lowest of 
all MTU/MRU values of incoming and/or outgoing interfaces this packet 
touched. That would potentially involve including all interfaces this 
packet could have ECMP:ed egress interface.

It would also be interesting to have a mechanism along the lines of 
https://tools.ietf.org/html/draft-van-beijnum-multi-mtu-05 to actually 
have the routers probe the working MTU between each other, and report 
that. Whatever option you propose should take this kind of mechanism into 
account (and use the probed value if that's lower than the interface 
setting).

Also, what value should be reported in case of MPLS and varying other 
encapsulations. I guess the value of the IPv4/IPv6 packet that could fit 
into the egress interface after all necessary labels etc are put on there?

So if the egress encap path is two labels deep and the egress MTU is 9180, 
then 9172 should be reported as IPv4/IPv6 egress MTU as returned by the 
router when processing this proposed option?

-- 
Mikael Abrahamsson    email: [e-mail]