ICMPv6 rate limits breaking PMTUD (and traceroute) [Re: Comcast enables 6to4 relays]

Franck Martin franck at genius.com
Thu Sep 2 00:10:40 UTC 2010

----- Original Message -----
> From: "Mark Smith" <nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org>
> To: "Simon Leinen" <simon.leinen at switch.ch>
> Cc: "Brzozowski" <john_brzozowski at cable.comcast.com>, "NANOG" <nanog at 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 at 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...

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