The problem is described pretty clearly at http://www.cisco.com/warp/public/105/56.html. The issue I have experienced is that fragmentation can lead to performance impacts that are unacceptable. I wish we could start a clue campaign informing people why ICMP should not be summarily dumped at the firewall. Chris Proctor EPIK Communications
-----Original Message----- From: Valdis.Kletnieks@vt.edu [mailto:Valdis.Kletnieks@vt.edu] Sent: Wednesday, December 03, 2003 11:39 AM To: jgraun@comcast.net Cc: nanog@merit.edu Subject: Re: MTU path discovery and IPSec
On Wed, 03 Dec 2003 16:05:39 GMT, jgraun@comcast.net 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).