[NANOG] Microsoft.com PMTUD black hole?

Wed May 7 19:43:49 UTC 2008

I'm not sure what the issue is here. 

Just about every modern firewall I've used has an option to enable PMTU
on interfaces, while blocking all other ICMP.

Is MS not running something manufactured in the last 10 years at their
perimeter?

> -----Original Message-----
> From: Nathan Anderson/FSR [mailto:nathana at fsr.com] 
> Sent: Wednesday, May 07, 2008 12:39 PM
> To: Valdis.Kletnieks at vt.edu
> Cc: nanog at merit.edu
> Subject: Re: [NANOG] Microsoft.com PMTUD black hole?
> 
> Valdis.Kletnieks at vt.edu wrote:
> 
> > The usual case where you get screwed over is when the 
> router trying to 
> > toss the ICMP FRAG NEEDED is *behind* the ICMP-munching 
> firewall.  And 
> > in case (2), you still can't assume that path MTU == local MTU, 
> > because your local MTU is likely 1500, and the fragging 
> router often 
> > trying to stuff your 1500 byte packet down an PPPoE tunnel 
> that's got an MTU of 1492....
> 
> Yes, but my point was precisely that one OR the other side (server OR
> client) is going to NOT have the ICMP-munching firewall in 
> between itself and the "RITM" as I have affectionately been 
> calling it (although it is definitely possible that there are 
> two ICMP-munchers on either side of the RITM).
> 
> And case #2 is exactly what is occurring right now _anyway_: 
> hosts assume that path MTU == local MTU even if there is 
> already an active PMTU cache entry from a recent earlier 
> communication with the remote host.  So I don't see how 
> making that assumption _after_ making an honest attempt at 
> actively determining whether or not it is actually the case 
> is any more broken than they way things are already being done.
> 
> The problem is that, as I realized at the end of the message 
> you quoted, there are potentially multiple paths between the 
> same two hosts, and the path that the packet takes in one 
> direction is not guaranteed to be the same path that the 
> packet takes in the opposite direction.
> 
> --
> Nathan Anderson
> First Step Internet, LLC
> nathana at fsr.com
> 
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