On 9 jul 2011, at 15:51, Sabahattin Gucukoglu wrote: >>> You're invited to file a report with <http://bugreport.apple.com> about this. Be sure to explain why fixing the broken path MTU discovery in the network is not an option and requiring the AirPort user to know enough about IPv6 router advertisement MTU options to set the value properly is an appropriate mitigation. > This is bug #9739722, Airport: Configurable IPv6 RA link-MTU or MSS Clamping. Severity 2. THIS IS A BAD IDEA. MSS clamping is modifying packets exchanged between consenting hosts. As a rule the network doesn't get to do this. Unfortunately the situation with IPv4 is so bad that it's not realistically possible to avoid this when you have a path MTU smaller than 1500. But with IPv6 we have a new chance to punish the people creating the problem rather than the ones implementing PMTUD properly, so The Right Thing To Do is NOT fix any PMTUD breakage in properly behaving systems, but rather push people that configure their systems incorrectly to fix this. And it seems to be working, PMTUD black holes happen with IPv6, but they're relatively rare. Advertising MTUs smaller than 1500 when the uplink has an MTU smaller than 1500 is completely unnecessary because IPv6 has good path MTU discovery and doing this also limits the size of packets exchanged locally. Advertising an MTU of 1500 is also not proper behavior, BTW. However, I do think that it's a good idea for makers of home routers to allow users to set the MTU advertised in RAs to anything between 1280 and 1500 (inclusive, assuming ethernet), but this should not be done by default. I still plan to get http://tools.ietf.org/html/draft-van-beijnum-multi-mtu-03 published at some point and having widespread reduced MTUs advertising in RAs means one more hurdle to an internet with larger packets. This is something that's sorely needed: 100 gigabit ethernet transmits more than two average sized IP packets in the time 10 megabit ethernet sends one bit with the current average packet size of ~ 500 bytes. Despite the fact that virtually all hardware supports larger packets these days. But unfortunately our friends over at the IEEE aren't in the position to increase the ethernet maximum packet size because that would break backward compatibility (of course with every new standard they create new stuff that's going to break if they eventually do it, those NE2000 cards aren't an issue anymore today). So if anyone is going to do it it's going to be us here at the IETF because unlike ethernet, IP allows for some parameter exchange between hosts during neighbor discov ery (or ARP). _______________________________________________ Ietf mailing list Ietf@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf