MTU handling in 6RD deployments

Templin, Fred L Fred.L.Templin at
Fri Jan 17 20:05:53 CET 2014


> > You don't ping the BR, you ping yourself via the BR. The BR only forwards the packet.
> Precisely. The whole idea is to stay on the data plane.

I do not work for a network equipment manufacturer, so I'll take
your word that remaining in the data plane is critical for 6rd BRs
and that high data rate loopbacks are not a problem. So, a looped
back MTU test tests both the forward and reverse path MTUs between
the CE and BR. This is important to the CE, because if it were only
to test the forward path to the BR it would not know whether the
reverse path MTU is big enough and so allowing an IPv6 destination
outside of the 6rd site to discover a too-large MSS could result
in communication failures.

In terms of the BR's knowledge of the path MTU to the CE, if we
can assume that the BR will receive the necessary ICMPs from the
6rd site then it can passively rely on translating ICMPv4 PTB
messages coming from the 6rd site into corresponding ICMPv6 PTB
messages to send back to the remote IPv6 correspondent. So, the
BR should be able to set an infinite IPv6 MTU on its tunnel
interface and passively translate any PTB messages it receives.
That, plus the fact that the two IPv6 hosts have to agree on an
MSS excuses the BR from having to do any active probing itself.

So, take what is already in RFC5969, and add that a successful
test of a 1500 byte probe allows the CE to set an infinite IPv6
MTU with the understanding that IPv6 hosts that want to use
sizes larger than 1500 are expected to use RFC4821.

BTW, by "infinite" I mean 4GB minus the encapsulation overhead.

Thanks - Fred
fred.l.templin at

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