Re: [v6ops] new draft: draft-ietf-v6ops-6204bis

[Tassos Chatzithomaoglou <[e-mail]>]

Ole,

Ole Troan wrote on 19/10/2011 11:19:
> Tassos,
>
>> Do you think it would be possible to add something about the MTU under "4.3.  LAN-Side Configuration"?
>>
>> L-15: The IPv6 CE router SHOULD by default include the LAN MTU into its Router Advertisements.
>>       The activation of this behavior MAY be user-configurable. The LAN MTU SHOULD also be user-configurable.
>>
>> There had been a similar talk 1 month ago, but i would like to see a recommended approach included.
> should the MTU be different than the default LAN link MTU and if so why?
It could be, in order to avoid PTB (packet too big) messages from the CE to the hosts for 
WAN communication (1500=>PTB=>1492=>ok).

> should internal to the home communication also be restricted in MTU?
That might by already restricted by the default LAN MTU.
I don't want that to be restricted, that's why the LAN MTU should be user-configurable.
> if the MTU is set e.g. equal to the WAN MTU, what do you do in the case of a routed home? how do you propagate the MTU inside the network?
> what if there are conflicts?
Although, a routed home might not be a typical subscriber's home, i would expect to have 
routed components handle this by their own.
I'm not asking for LAN MTU to be equal to WAN MTU specifically. I'm asking of advertising 
a MTU to the hosts, in order to avoid PTB messages, in case of WAN communication.
I'm also asking to be able to able to disable this, in order to have the hosts decide by 
their own.
> unless you have good answers to these questions, I suggest we don't.
>
> cheers,
> Ole
Generally, i would like to have available as many options as possible, in order to cover 
most scenarios.
If you want to have 1492 everywhere in order to lower the fragmentation possibilities, 
then you should have this option.
If you want to have 1500 or 9000, knowing that you might need to do fragmentation, then 
you should also have these options.

I don't know how many IPv6 CPEs support (or will support) >1500 MTU on their LAN 
interfaces, but judging from the ones i have tried, none of them did (maybe it's because 
of their FE interfaces). In that case, i don't believe using a 1492 MTU vs a 1500 one will 
make a big difference for internal home communication.


Regards,
Tassos