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From: "Mark Smith" <nanog@85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org> To: "Simon Leinen" <simon.leinen@switch.ch> Cc: "Brzozowski" <john_brzozowski@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.leinen@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...