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Re: weird IPv6 packet dropping with 6to4

On Sat, Sep 05, 2009 at 12:28:16PM -0400, Taylor R Campbell wrote:
>    Date: Sat, 5 Sep 2009 13:25:30 +0100
>    From: Matthias Scheler <>
>    I'm not very convinced about the usefulness of PF's scrubbing in general.
>    But there is no reason to limit the MSS of TCP connections here.
>    IPv6 requires to handle fragmentation via ICMP.
> Why might NetBSD not be sending ICMP6 packet-too-big messages, then?

Why should it? Your HTTP request is small, the gateway routing to your
tunnel would to do that. The main point is that you should not use
MSS clamping.

> There aren't any in the packet traces.  These packet traces are not
> Something I do notice, though, is that when I omit MSS clamping from
> stf0, I sometimes get IPv4 fragmented packets (containing encapsulated
> IPv6 packets).  While I'm sure that the system should be able to cope
> with this and reassemble whole IPv6 packets, that doesn't seem right!

Well, that could be your main problem. With an MTU of 1280 on stf0 the
incoming packets should not exceed 1300 bytes which does fit into the
MTU of 1492 bytes of pppoe0.

It looks to me as if there's a broken 6to4 relay router somewhere
out there.

Does access to over IPv6 work better for you?
My A-DSL router has an outgoing 6to4 tunnel with an MTU of 1280
which should avoid fragementation problems.

        Kind regards

Matthias Scheler                        

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