Re: [v4tov6transition] [Softwires] IPv6 VPNs configured over 1280MTU tunnels

Brian E Carpenter <> Mon, 11 October 2010 21:24 UTC

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Date: Tue, 12 Oct 2010 10:25:34 +1300
From: Brian E Carpenter <>
Organization: University of Auckland
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To: "Templin, Fred L" <>
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Subject: Re: [v4tov6transition] [Softwires] IPv6 VPNs configured over 1280MTU tunnels
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On 2010-10-12 07:23, Templin, Fred L wrote:
>>> In the to-host direction, because the ISP network is all what 
>>> is left to traverse before reaching the CPE.
>> In what you call the to-host direction, any ICMPv4
>> returned from the ISP network might not have enough
>> information for stateless translation to ICMPv6.
> I should also say, any ICMPv4 returned from within
> the end user network (where MTUs might not be so well
> managed) might not be delivered to the tunnel endpoint
> in the ISP network.

Well, yes, and there were several years when I frequently found myself
in hotel rooms having to manually set the IPv4 MTU on my laptop to a
low value, when connecting through broken dial-up ISPs. No doubt
we'll also go through some years before all operators are providing
an adequately large MTU to cope with IPv6-in-foo tunnels. I don't
think any transition solution can hope to be 100% watertight
on this. We'll make the changes that Rémi mentioned, but
you're correct that IPv6-in6a44 tunnels might have MTU issues.