On Jun 10 2008, at 16:13, Pascal Thubert (pthubert) wrote: > we still call the ISA100.11a transport > UDP though it's not
OK, so this transport protocol would like to use an 8-byte header with a source port number, a destination port number, and two highly compressible fields (redundant length, and the now redundant checksum), but it has its own *end-to-end* checksum, which is better than 16-bit one's complement and *does* include a pseudo header. Two observations: 1) compression works best when it is done cross-layer. So one would want to compress the whole stack from IP upwards to the ISA100 transport. This of course could include knowledge about the layer above UDP, just as we have included knowledge about RTP in RFC 3095/5225. 2) Given this transport, the end system is likely to want back the feature of UDPv4 that allowed sending a zero checksum. This transport is a nice example for how it was a mistake to conflate policy and mechanism in UDPv6: Encoding the *policy* not to send packets without a checksum that includes a pseudo-header with the *mechanism* to never allow zero checksums in UDPv6. Of course, you should not have mechanism that a policy makes sure you will never use, but the mechanism is already there for IPv4, and the example is exactly one such case where the policy would not imply non- use of the mechanism. Obviously, if an end system does not want protection from UDP (and it shouldn't want it in the ISA100 case), it should say so by setting the checksum to zero. The mechanism/policy conflation mentioned above makes that impossible. Changing UDPv6 at this stage is a big step. If that is not what we (the IETF/the IPv6 implementers community) want to do, there is no way for the end system to make that statement. This does not mean that we shouldn't honor it. Gruesse, Carsten _______________________________________________ 6lowpan mailing list [email protected] https://www.ietf.org/mailman/listinfo/6lowpan
