[NANOG] Microsoft.com PMTUD black hole?

Nathan Anderson/FSR nathana at fsr.com
Wed May 7 13:50:34 CDT 2008

Iljitsch van Beijnum wrote:

> No. This would add significant delay because you'd have to give the 
> other side enough time to respond to the large packet (also sending a 
> large packet on something like GPRS/EDGE is a waste of bandwidth and 
> battery power) while if there is ICMP filtering, there won't be a 
> response, which is exactly the reason why we're in this bind in the 
> first place

I admit the idea needs tweaking (at best), and it was just a stray 
thought :-), but 1) even if there is ICMP filtering happening way at the 
other end, I (the TCP initiator) will still get a response from the 
router in the middle (RITM) that is reducing the total path MTU if I try 
to send a packet through it larger than the actual path MTU, and 2) if I 
don't get a response to my single large packet (either from a RITM or 
the other end) in a timely fashion (less than a second?), then the 
client/initiator may just assume that path MTU == local MTU and will set 
its MSS accordingly (which is no different than what is happening now), 
until it has a reason to think differently.

Also, if there is already something in the local PMTU cache for a single 
host address, I'm not sure I follow why it would be a bad idea for the 
TCP initiator to consult that cache when preparing the SYN.  Although, 
on second thought, I suppose it is possible (and, in more than a few 
cases, likely) that in instances of route path asymmetry, the PMTU of 
the path from the initiator to the server may be different than the PMTU 
of the path back from the server to the client.  Hmmm.

Okay, scratch that idea then. :-P

Nathan Anderson
First Step Internet, LLC
nathana at fsr.com

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