I think this is a really good idea; although, it could be a bridge too far. It certainly won’t get implemented quickly. I still remember implementing path MTU discovery thinking about the big performance win I was going to get in IPv6 from 9K packets (vs 1500 byte packets) only to discover that certain router vendors had interpreted one of the RFCs to mean they didn’t have to send packet-too-big messages if the packet in question was larger then 1500 bytes (because one of the RFCs stated that IPv6 implementations only have to support 1500 bytes). Yes, the RFCs could be interpreted that way, but that interpretation clearly violates the intent (to make path MTU reliable). The real world upshot of that is that anything over 1500 bytes is still a black hole and using path MTU can really only get you a whopping 300 bytes (from 1200 to 1500). Not trying to be negative, but 4M parcels are really looking to the future and that's only a good thing if we realize what we are doing.
Getting into the details: Using "segment" as the term to describe the individual parcel contents is probably not a good idea, because TCP has segments but most other ULPs do not. I don't completely follow the description as to exactly how one forms a parcel. For example, does each segment include in IPvX header? I think I read no, but it could be a little clearer. I think that requiring all the segments to be exactly the same size (except the last one) is a problem. It's definitely a problem for UDP. Even with TCP, it becomes difficult to use exactly N bytes in a packet -- It involves a very fragile dance between the ULP and the IP layer to communicate the exact size of the options being used. Things like IPSec make it impossible to use some MTU values and one needs to go a little under (so does fragmentation; although, that doesn't apply here). Given how difficult and fragile is it for an implementation to completely fill up to the MTU (or "L" in the context of your document), a reasonable design choice would be to make worse case assumptions when they tell ULP how many bytes it has to work with. If a packet is longer than 1500 bytes, some routers will not return ICMPv6 messages. You shouldn't depend on that for performance. _______________________________________________ Int-area mailing list Int-area@ietf.org https://www.ietf.org/mailman/listinfo/int-area
