Hi, > > You don't ping the BR, you ping yourself via the BR. The BR only forwards > > the packet. > > Precisely. The whole idea is to stay on the data plane.
I do not work for a network equipment manufacturer, so I'll take your word that remaining in the data plane is critical for 6rd BRs and that high data rate loopbacks are not a problem. So, a looped back MTU test tests both the forward and reverse path MTUs between the CE and BR. This is important to the CE, because if it were only to test the forward path to the BR it would not know whether the reverse path MTU is big enough and so allowing an IPv6 destination outside of the 6rd site to discover a too-large MSS could result in communication failures. In terms of the BR's knowledge of the path MTU to the CE, if we can assume that the BR will receive the necessary ICMPs from the 6rd site then it can passively rely on translating ICMPv4 PTB messages coming from the 6rd site into corresponding ICMPv6 PTB messages to send back to the remote IPv6 correspondent. So, the BR should be able to set an infinite IPv6 MTU on its tunnel interface and passively translate any PTB messages it receives. That, plus the fact that the two IPv6 hosts have to agree on an MSS excuses the BR from having to do any active probing itself. So, take what is already in RFC5969, and add that a successful test of a 1500 byte probe allows the CE to set an infinite IPv6 MTU with the understanding that IPv6 hosts that want to use sizes larger than 1500 are expected to use RFC4821. BTW, by "infinite" I mean 4GB minus the encapsulation overhead. Thanks - Fred fred.l.temp...@boeing.com