> Currently, routers can advertise an MTU for a link. That's nice. But > what we really need is a way for hosts to find out the MTU each > individual neighbor can handle. 100 Mbps and slower ethernet interfaces > can typically handle only the standard 1500 byte ethernet MTU, while > gigabit ethernet interfaces usually support a much larger MTU. > > However, in most cases hosts with different MTUs are present on the > same subnet, so simply advertising a larger MTU wouldn't solve this. > (Not that this would work anyway as hosts are instructed to only listen > to MTU advertisements where the MTU is between 1280 and 1500 (for > ethernet).) > > But if hosts can tell each other the MTU they support, each set of two > hosts is always able to use the largest possible MTU between them. > (This would also require a new link MTU option that conveys the maximum > MTU the lower layer equipment supports. Switches have their own MTU and > even some hubs start doing strange things when a larger than expected > MTU is used.)
This might be useful when the L2 doesn't have any MTU limitations. For instance, with IP over ATM the default MTU is 9k but AAL5 has a 16 bit length field (if I don't misremember). Thus a pair of nodes can agree to use e.g. 37.5k MTU. But for links where the L2 has tigher limits things are quite a bit more difficuly. Taking Ethernet as an example. Having two hosts say that they want to use a 9k MTU over Ethernet is fine as long as the bridges/switches on the path all support that. But how can the hosts know that? Even if the hosts can determine this (send a 9k packet and see if it gets through?), what happens when an addition Ethernet switch or wire comes up (or goes away) and spanning tree changes the L2 path between those two hosts so that the packets no longer go through the same set of bridges/switches? If one would want to pursue this I think the best way would be for IEEE 802 to define a "frame size discovery" protocol at L2 so that hosts can robustly determine the frame size a given destination MAC address and be notified when it changes. But since Ethernet jumboframes are non-standard (even though more and more commonly used it seems) and there is concern about the coverage of the Ethernet CRC for larger frames, the IEEE has been unvilling to look into this. Once you have such L2 capability then a ND option for "I can receive packets with this MTU" allowing the a larger value than the default link MTU, would make sense to me. But without the L2 capability it doesn't seem very useful. Erik -------------------------------------------------------------------- IETF IPv6 working group mailing list [EMAIL PROTECTED] Administrative Requests: https://www1.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
