North American Network Operators Group

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Re: MTU path discovery and IPSec

  • From: Valdis.Kletnieks
  • Date: Wed Dec 03 11:50:17 2003

On Wed, 03 Dec 2003 16:05:39 GMT, [email protected]  said:

> 1) I assume MTU path discovery has to been in enabled on each router in the path in order for it work correctly?!

Actually, no.  All that's required is that:

a) The router handle the case of a too-large packet with the DF bit set by
sending back an ICMP 'Dest Unreachable - Frag Needed' packet.  I've never
actually encountered a router that didn't get this part right. (Has anybody ever
seen a router botch this, *other* than a config error covered in (b) below?)

b) said ICMP makes it back to the originating machine.  This is where all the
operational breakage I've ever seen on PMTU Discovery comes from. And in almost
all cases, one of two things is at fault.  Either some bonehead firewall admin
is "blocking all ICMP for security" (fixable by reconfiguring the firewall to
let ICMP Frag Needed error messages through), or some bonehead network provider
numbered their point-to-points from 1918 space and the ICMP gets ingress/egress
filtered (this one is usually not fixable except with a baseball bat).

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