Hi Brian, > -----Original Message----- > From: Brian E Carpenter [mailto:[email protected]] > Sent: Friday, March 05, 2010 12:38 PM > To: Templin, Fred L > Cc: Rémi Després; Ole Troan; [email protected] > Subject: Re: [Softwires] SOFTWIRE working group last call on 6rd > > 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.
You leave us with little hope. > I don't like this any more than you do, from a long term perspective. It still nags me that I feel we can do better; in fact, I am sure we can... Thanks - Fred [email protected] > Brian _______________________________________________ Softwires mailing list [email protected] https://www.ietf.org/mailman/listinfo/softwires
