Mark Smith wrote: > Hi Iljitsch, Perry, > > I'll admit I haven't been fully following your thread of discussion, > I've been a bit busy on some other things (another reason why I like the > simple solution - less thinking :-) ) > > Something to keep in mind for your solution. TCP announces the maximum > segment size it can receive in the SYN or SYN/ACK segments, and I'm > pretty sure these days that is derived directly from the (current) MTU > value of the interface it is going to use at the time of the 3-way > handshake. I'm pretty sure that irrespective of if PMTUD (or another > local link method) discovers a larger possible PMTU value at a later > time during the TCP connection, existing TCP connections will not be > able to "burst" up to it.
The MSS option would be to the MTU of the outgoing interface[1], as always. The actual size sent will be limited to 1500 initially and will grow as the size is detected. The reason for this is that if you have this topology: Host A <-1500-> Switch <-9k-> Host B Then if Host B announces an MSS of 9k, host A will never try and send an 9k packet. Even in the case of: Host A <-9k-> Switch <-1500-> Switch <-9k-> Host B Host A will refuse to forward a packet to Host B thats 9k until it has fully probed the MTU up to 9k. MSS is what the machine is capable of "receiving", and the host is entirely capable of receiving a 9k MTU packet. It says nothing about the path MTU. The initial packets of a TCP connection (SYN/SYNACK) are all assured to be smaller than the IPv6 minimum maximum size (1280 bytes), by the time you want to send some real data you hope that you're a good way through your MTU probing. > Assuming that you're not planning to modify TCP as well, this means that > the knowledge of the "best" possible MTU that TCP can use must be known > at the time of the initial 3-way handshake. This limitation possibly > applies to other unicast or semi-unicast transport layer protocols too, > such as SCTP. I disagree. MSS only says what the maximum size you can receive is (Maximum Segment Size). It doesn't say "This is the maximum size I think I can receive, I might be able to receive larger" > > Thinking about these solutions another way, all they're really > attempting to do is to take the TCP-specific MSS mechanism, generalise > it so it can be used for all transport layer unicast connections, and > moving the mechanism to the network layer. Well kinda yeah. We're trying to determine the MTU of a neighbour, rather than the MTU of an interface. ---- [1]: Well, minus the size of the headers etc.. You know what I mean. -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org Administrative Requests: https://www1.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
