Thoughts on increasing MTUs on the internet

Fri Apr 13 21:15:00 UTC 2007

Thus spake "Lasher, Donn" <DLasher at newedgenetworks.com>
>>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)
>
> I wish I'd had your experience. PMTU _can_ work well, but on the internet 
> as
> a whole, far too many ignorant paranoid admins block PMTU, mostly by
> accident, causing all sorts of unpleasantness.

You can't block PMTUD per se, just the ICMP messages that dumber 
implementations rely on.  And, as I noted, MS's implementation is dumb by 
default, which leads to the problems we're all familiar with.  "PMTU Black 
Hole Detection" is appropriately named; one registry change* and a reboot is 
all you need to solve the problem.  Of course, that's non-trivial to 
implement when there's hundreds of millions of boxes with the wrong 
setting...

> Clearing DF only takes you so far. Unless both ends are aware, and respond
> apppropriately to the squeeze in the middle, you're back to square one.

Smarter implementations still set DF.  The difference is that when they get 
neither an ACK nor an ICMP, they try progressively smaller sizes until they 
do get a response of some kind.  They make a note of what works and continue 
on with that, with the occasional larger probe in case the problem was 
transient.

In fact, one could consider Lorier's "mtud" to be roughly the same idea; 
it's only needed because the stack's own PMTUD code is typically bypassed 
for on-subnet destinations and/or not as smart as it should be.

S

* HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\
Parameters\EnablePMTUBHDetect=1

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