Re: Re: 6to4/SIT and IP DF

Re: Re: 6to4/SIT and IP DF

[mika.penttila]

> 
> Lähettäjä: Pekka Savola <pekkas@netcore.fi>
> Päiväys: 2003/10/15 ke AM 09:28:28 GMT+03:00
> Vastaanottaja: David Stevens <dlstevens@us.ibm.com>
> Kopio: r.venning@telstra.com,  <nate@thebog.net>,  <davem@redhat.com>, 
>         <netdev@oss.sgi.com>
> Aihe: Re: 6to4/SIT and IP DF
> 
> On Tue, 14 Oct 2003, David Stevens wrote:
> > I was trying out 6to4 and noticed that the v4 encapsulating header has DF
> > set, which RFC3056 says should not be set.
> > 
> > Because ICMPv4 won't, in general, include enough packet to determine the
> > original v6 sender, end-to-end PMTU won't work. The possible use I could
> > see is if the tunnel MTU is modified based on the PTMU (I didn't check),
> > but  that's probably not a good idea for any tunnels  that have "any" as
> > the remote v4 address. Doing that would force all MTU's to the lowest of
> > any v4 destination's path.
> > 
> > So, I think it's appropriate to always clear IP DF in the IPv4 header
> > generated by SIT, but I thought I'd see if anyone else has a comment on
> > that before I submit the trivial patch. :-)
> > 
> > Any thoughts?
> 
> Seems like a good idea.  The only thing I'm worried about is when someone
> is attached to a network of at least 1500 MTU (at IPv6 level), and uses
> 6to4 -- then basically every IPv6 packet over 1480 bytes will be
> fragmented in the network, even though it could potentially be chopped to
> smaller pieces already in the end-nodes.
> 
> Just wondering how our 6to4 implementation handles this case at the 
> moment..
> 
> -- 

I think we send icmp TOOBIG by default for packets longer than (pmtu - iphdr).

--Mika