On 2010-03-06 05:52, Templin, Fred L wrote:
> Remi,
>
>> -----Original Message-----
>> From: Rémi Després [mailto:[email protected]]
>> Sent: Friday, March 05, 2010 6:34 AM
>> To: Templin, Fred L; Ole Troan; Brian E Carpenter
>> Cc: [email protected]
>> Subject: Re: [Softwires] SOFTWIRE working group last call on 6rd
>>
>> Fred, Ole, Brian,
>>
>> Le 4 mars 2010 à 22:03, Templin, Fred L a écrit :
>>
>>>> -----Original Message-----
>>>> From: Ole Troan [mailto:[email protected]] On Behalf Of Ole Troan
>>>>
>>>> ...I can remove the sentence recommending advertising the tunnel MTU on
>>>> the LAN-side interface. or
>>>> change it from a SHOULD to a MAY. as you say this is just to minimize the
>>>> incidence of invoking
>>>> PMTUD.
>>> Removing the sentence might be best. The MAY would be OK,
>>> but then you might need to also say something like: "(Note
>>> that advertising the tunnel MTU on the LAN-side interface
>>> would cause all hosts attached to the link to use a reduced
>>> MTU even for link-local communications.)"
>> Fred, Ole, Brian,
>>
>> Until PMTUD is made reliable in IPv6 (btw an important objective), all
>> IPv6-enabled hosts should have
>> their MTU set to 1280 to avoid IPv6 blackholes.
>> This could in principle apply only to IPv6 packets that leave customer
>> sites, but as long as hosts
>> have only one MTU parameter, it has to apply to all packets.
>> Since typical hosts have longer default MTUs than 1280, advertising MTU=1280
>> on links that offer
>> paths to the global Internet appears to be the only available tool at hand.
>>
>> The proposal is then to replace:
>> "A 6rd CE SHOULD advertise the 6rd Tunnel MTU, whether determined
>> automatically or configured
>> directly, on the LAN side by setting the MTU option in Router Advertisements
>> [RFC4861] messages to
>> the 6rd Tunnel MTU."
>> by:
>> "As long as the path MTU discovery has not been made reliable, a 6rd CE
>> SHOULD advertise on the LAN
>> side, in the MTU option of Router Advertisements [RFC4861], an MTU of 1280
>> octets . This is to
>> prevent that longer packets that could have traversed the local 6rd domain
>> may be discarded, because
>> of their size, further in the Internet because of their size. Note that this
>> may cause all hosts
>> attached to the link to use a reduced MTU even for link-local
>> communications, IPv6 and IPv4."
>
> The 1280 would be disappointing; I'm sure we can do better.
As has been said before, in theory there is no difference between theory
and practice, but in practice, there is.
Right now today a number of ISPs are planning to deploy 6RD, and we have
already seen clear evidence from 6to4 deployment that setting the MTU to
1280 is a necessary evil at the moment. So I think this advice should be
in the specification, as a SHOULD. Otherwise we'd be misleading those
ISPs.
I don't like this any more than you do, from a long term perspective.
Brian
_______________________________________________
Softwires mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/softwires
