Thoughts on increasing MTUs on the internet

Fri Apr 13 17:31:36 UTC 2007

Thus spake "Mikael Abrahamsson" <swmike at swm.pp.se>
> The internet is a very diverse and complicated beast and if end
> systems can properly detect PMTU by doing discovery of this, it
> might work.  ... Make sure they can properly detect PMTU by
> use of nothing more than "is this packet size getting thru" (ie
> no ICMP-NEED-TO-FRAG) or alike, then we might see partial
> adoption of larger MTU in some parts and if this becomes a
> major customer requirement then it might spread.

PMTU Black Hole Detection works well in my experience, but unfortunately MS 
doesn't turn it on by default, which is where all of the L2VPN with <1500 
MTU issues come from; turn BHD on and the problems just go away...  (And, as 
others have noted, there's better PMTUD algorithms that are designed to work 
_with_ black holes, but IME they're not really needed)

Still, we have a (mostly) working solution for wide-area use; what's missing 
is the critical step in getting varying MTUs working on a single subnet. 
All the solutions so far have required setting a higher, but still fixed, 
MTU for every device and that isn't realistic on the edge except in tightly 
controlled environments like HPC or internal datacenters.

Perry Lorier's solution is rather clever; perhaps we don't even need a 
protocol sanctioned by the IEEE or IETF?

S

Stephen Sprunk      "Those people who think they know everything
CCIE #3723         are a great annoyance to those of us who do."
K5SSS                                             --Isaac Asimov 