MTU handling in 6RD deployments

Templin, Fred L Fred.L.Templin at
Fri Jan 10 16:44:20 CET 2014

Hi Mikael,

> -----Original Message-----
> From: Mikael Abrahamsson [mailto:swmike at]
> Sent: Thursday, January 09, 2014 11:11 PM
> To: Templin, Fred L
> Cc: IPv6 Ops list
> Subject: RE: MTU handling in 6RD deployments
> On Thu, 9 Jan 2014, Templin, Fred L wrote:
> > I don't doubt that your experience is valid for the environment you are
> > working in. What I am saying is that there may be many environments
> > where setting IPv4 link MTUs to 1520+ is a viable alternative and then
> > the hosts can see a full 1500+ MTU w/o ICMPs. SEAL detects when such
> > favorable conditions exist and uses limited fragmentation/reassembly
> > only when they don't. Or, if fragmentation/reassembly is deemed
> > unacceptable for the environment, then clamp the MSS.
> 6RD relays can be made cheap because they are stateless. 6RD
> implementation in hosts can be bade cheap, because it's easy. SEAL isn't
> stateless (obviously, since it can do re-assembly), thus increasing cost
> and complexity both in host and relay.

I understand. But, SEAL is not heavy-duty and steady state fragmentation
and reassembly is not a desired end condition. Instead, it is a sign that
something is out of tune and needs to be tuned properly. Or, if it can't
be tuned, then fall back to MSS clamping and you are no worse off than
without SEAL.

> So while it might have a technical fit, it isn't really an operational or
> monetary fit right this minute. 6RD is widely implemented today, by the
> time any other mechanism is implemented, the use-case for IPv6 tunneled in
> IPv4 might be much less interesting, hopefully more are moving towards
> IPv4 over native IPv6 for new implementations.

There is an alpha implementation available at:

And, I don't know whether any of us can say what the timeframe is for
all native IPv6 everywhere given that we are close to 20yrs in and still
no end in sight for IPv4. Also, SEAL works for all tunnel combinations
of IPvX-over-IPvY and is not specific to 6rd. So, implementation should
be of interest in the general sense for the longer term.

Thanks - Fred
fred.l.templin at
> --
> Mikael Abrahamsson    email: swmike at

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