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Re: MTU path discovery and IPSec

  • From: Owen DeLong
  • Date: Wed Dec 03 18:35:41 2003

--On Wednesday, December 3, 2003 11:39 AM -0500 [email protected] wrote:

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?)

When you consider that most firewalls are technically routers (albeit
somewhat pathological routers), yes... Many firewalls fail to send back
the ICMP and silently drop the DF packet.

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).

Yep... Those are definitely the most common PMTU-D problems.


If it wasn't crypto-signed, it probably didn't come from me.

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