option 212 for 6RD

Mikael Abrahamsson swmike at swm.pp.se
Tue Jan 15 06:50:16 CET 2013

On Mon, 14 Jan 2013, Jeroen Massar wrote:

> As the tunnel should indeed be 1480 or lower, but the LAN can just stay 
> at 1500 and changing that would cause all kinds of other odd issues I 
> would think for hosts that don't take the info from the RA... and the 
> CPE should really be doing PTBs thus lowering the MTU is not needed as 
> it will send a PTB because of the tunnel interface having the lower MTU 
> (eg 1480).

This causes almost every destination to have to do PMTUD (6RD gateway 
sending PTB).

I see no reason to keep LAN at 1500 if the tunnel is 1480. "all" traffic 
will need to do PTB causing unneccessary delay for initial connections to 
most if not all destinations (web server sending 1500 packet, getting PTB 
from 6RD gateway, then sending 1480 packet).

Personally I have run my home with an IPv6 MTU of 1400 for several years. 
We had this discussion in IETF v6ops as well, but then it was mobile. The 
suggestion there was to advertise the same MTU on the LAN as was seen in 
RAs on the device<->mobile network GTP tunnel. This makes a lot of sense 
to me and I strongly supported it.

What's your reason to so much want 1500 on the LAN that you'd rather take 
that than the delay for most connection I'm describing above?

Mikael Abrahamsson    email: swmike at swm.pp.se

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