----- Original Message ----- > From: "Mark Smith" > <na...@85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org> > To: "Simon Leinen" <simon.lei...@switch.ch> > Cc: "Brzozowski" <john_brzozow...@cable.comcast.com>, "NANOG" > <nanog@nanog.org>, John > Sent: Thursday, 2 September, 2010 9:50:28 AM > Subject: Re: ICMPv6 rate limits breaking PMTUD (and traceroute) [Re: Comcast > enables 6to4 relays] > On Wed, 01 Sep 2010 23:18:55 +0200 > Simon Leinen <simon.lei...@switch.ch> wrote: > > > Jack Bates writes: > > > 1) Your originating host may be breaking PMTU (so the packet you > > > send > > > is too large and doesn't make it, you never resend a smaller > > > packet, > > > but it works when tracerouting from the other side due to PMTU > > > working > > > in that direction and you are responding with the same size > > > packet). > > > > Your mentioning PMTU discovery issues in connection with 6to4 > > prompts me > > to confess how our open 6to4 relay has probably contributed to the > > perception of brokenness of 6to4 for quite a while *blush*. > > > > The relay runs on a Cisco 7600 with PFC3 - btw. this is an excellent > > platform to run an 6to4 relay on, because it can do the encap/decap > > in > > hardware if configured correctly. > > > > At some point of the relay becoming popular (load currently > > fluctuates > > between 80 Mb/s and 200 Mb/s), I noticed that our router very often > > failed to send ICMPv6 messages such as "packet too big". > > > > That potentially starts to explain why I haven't noticed PMTUD issues > on my 6to4 tunnel that I've been running for a number of years, and > have > been quite surprised be and somewhat doubtful of people to say there > are > issues. I've pumped the MTU up to 1472 on it to suit my PPPoE 1492 > MTU, > instead of leaving it at 1280, and haven't had any issues that have > made me suspect PMTUD. I'm in Asia Pacific so I've probably either > been > using 6to4 gateways that are lightly used, or ones that have had this > parameter changed. > Well I have an issue with a MTU of 1434 on a 6to4 link, but on my return path (I'm a client using an airport).
The MTU seems real odd, and it fails all the time, but then I notice, some time of the day it is ok, so may be it coincide with load on the relay? I have the strong feeling this is exactly what I'm experiencing... I can provide extensive debug information if interested, but do not want to bore the list with details...
