option 212 for 6RD

Jean-Francois.TremblayING at videotron.com Jean-Francois.TremblayING at videotron.com
Tue Jan 15 00:01:20 CET 2013

> Do you mean "lower the MTU to 1480" of the tunnel interface ("WAN") or
> do you mean of the LAN?

LAN of course. The tunnel is already 1480 AFAIK. 
> 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... 

What kind of odd issues do you have in mind? Operationnal experience
shows no issue so far. 

> 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).

I had this same discussion last year with Mark T. in v6ops I believe. 

PMTUD is a nice concept, but it remains nice only in concept from our 
experience. In the real world with 6RD, we see that: 
1) PTB is often sent from the relay to the content provider
   (this doesn't preclude having it on the client side)
2) Content providers likely have a firewall or a load-balancer 
   blocking PTB (quite a standard setup)
3) Clients experience a high level of brokeness, especially on 
   streaming sites and with larger transfers, typical of MTU issues
4) Setting the MTU to 1480 in RAs on the LAN removes all issues as far 
   as we can tell (TCP MSS is lower and everyone is happy everafter)

This is why both Linksys and D-Link both lower the MTU to 1480 on the 
LAN when 6RD is enabled. If there's a better way to do this that 
works in the real world, I'd be happy to try it. So far, in my 
opinion, IPv6 PMTUD seems to be a fail. There's no way to fix all 
the content out there to make sure PTB gets through. 


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