MTU handling in 6RD deployments

Mikael Abrahamsson swmike at
Fri Jan 10 08:10:54 CET 2014

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.

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.

Mikael Abrahamsson    email: swmike at

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