option 212 for 6RD

Mark Townsley mark at townsley.net
Tue Jan 15 01:24:25 CET 2013

On Jan 15, 2013, at 12:01 AM, Jean-Francois.TremblayING at videotron.com wrote:

>> 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. 

Technically, it's the MTU of the WAN link minus 20 (section 9.1 of RFC5969). If you can run jumbo frames of 1520 in your access network, the LAN side should automatically adjust to 1500 rather than 1480. The same goes for providers that run PPPoE and otherwise provide a 1492 MTU to hosts (RFC 4638). 

On World IPv6 Day ISOC reported ~6% of the launch participants reached from various test points had PMTU problems, by World Launch, ~3% (among a lot more participants). Sadly, many providers end up defaulting to 1280, but its hard to blame them. There was a dashboard that showed all this, but it seems to be down or dead now (link under "other measurement sites and tools" at http://www.worldipv6launch.org/measurements/ fails).

- Mark

> /JF

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