Hi Remi, > -----Original Message----- > From: Rémi Després [mailto:[email protected]] > Sent: Thursday, October 14, 2010 12:09 AM > To: Templin, Fred L > Cc: Softwires; [email protected] > Subject: Re: [Softwires] [v4tov6transition] 6a44 MTU issues > > > Le 13 oct. 2010 à 22:04, Templin, Fred L a écrit : > >>>> ... > >>>> If IPv6 packets longer than 1280 would be accepted by 6a44 > >>>> servers, hosts could receive them in fragmented IPv4 datagrams. > >>> > >>> Or they might be reassembled in the NAT(s) in front of > >>> the host. > >> They would indeed. > >> But they would then be forwarded across the customer network. > >> There, they may somewhere be fragmented to fit into fragments > >> shorter than the ISP MTU + 28. > >> This happens for example if the ISP IPv6 MTU would be 1500-28 > >> and that of some link in the customer site would be 1500-40 > >> (say, for an IPv4 in IPv6 tunnel). > >> Right? > > > > The fact of life is that we have the ISP managed network > > domain and the end user network unmanaged network domain, > > whether the ISP controls the CPE or not. The managed domain > > should be well-behaved, but any manner of MTU irregularities > > is possible within the unmanaged domain. For example, I can > > login to my Linksys home router and manually set the MTU to > > any value I want. > > > > Anything that can be configured can be mis-configured, and > > the ISP has no control of any misconfigurations that might > > occur in the end user network. As far as the ISP can tell, > > the end user network is just a black hole that silently > > consumes packets. > > The point isn't about misconfigurations. > It is that stateless tunnels (i.e. without SEAL ...) are > legitimate within customer sites. > They can lead some intra-site IPv4 PMTUs that are less than > 1500, and even less than 1500 - 28.
No; the point is that (without a deterministic MTU discovery scheme) you can't control the few end user networks that set a 1308 MTU so the vast majority of end user networks that set a 1500+ MTU are made to suffer. Fred [email protected] > Then, because the 6a44 design privileges robustness, it is > better to avoid more constraints on intra-site PMTUs than > those that are absolutely mandatory. > In order to statelessly support IPv6/UPD/IPV4 packets > intra-site IPv4 PMTUs of 1280 + 8 + 20 = 1308 octets IS the > obligation (one that should be satisfied with all realistic > stateless tunnels in customer sites). > > RD > > > > _______________________________________________ Softwires mailing list [email protected] https://www.ietf.org/mailman/listinfo/softwires
