MTU handling in 6RD deployments

Templin, Fred L Fred.L.Templin at
Tue Jan 7 19:12:39 CET 2014

Hi Tore,

> -----Original Message-----
> From: Tore Anderson [mailto:tore at]
> Sent: Tuesday, January 07, 2014 9:57 AM
> To: Templin, Fred L; IPv6 Ops list
> Subject: Re: MTU handling in 6RD deployments
> * Templin, Fred L
> > 6RD could use SEAL the same as any tunneling technology. SEAL makes
> > sure that packets up to 1500 get through no matter what, and lets
> > bigger packets through (as long as they fit the first-hop MTU) with
> > the expectation that hosts sending the bigger packets know what they
> > are doing. It works as follows:
> >
> >   - tunnel ingress pings the egress with a 1500 byte ping
> >   - if the ping succeeds, the path MTU is big enough to
> >     accommodate 1500s w/o fragmentation
> >   - if the ping fails, use fragmentation/reassembly to
> >     accommodate 1500 and smaller
> >   - end result - IPv6 hosts always see an MTU of at least 1500
> In order for the BR to support reassembly it must maintain state. That's
> going to have a very negative impact on its scaling properties...

A couple of things about this. First, reassembly is used only for packets
in the range of 1280-1500 bytes (smaller and larger packets are passed
w/o fragmentation). Second (and more importantly) reassembly is not needed
for packets of any size if the path can pass a 1500 byte ping packet. So
(as Ole said a few messages back) if the 6rd domain MTU can be made to be
>=1520 the fragmentation and reassembly process is suppressed and only
whole packets are transmitted.

Thanks - Fred
fred.l.templin at

> Tore

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