At 09:11 PM 9/5/2002 +0800, Jason Gao wrote: >--- TCP with ECN extension > >has already been a practice of fuzzy-layering. > >TCP in the end system and IP in the intermediate systems share the two ECN >bits in the IP header. that is incorrect. First off, IP also is found in the end system, and uses the ECN bits. More important, though, is that TCP uses an IP service, through an IP-provided API. The TCPs negotiate whether they are willing to run ECN, and if they agree, they (on transmission) use the API feature that says "please tell my peer if this datagram experiences congestion", and (on reception) use the API feature that says whether or not congestion was experienced somewhere in the network. All other communication regarding ECN is via the transport header. SCTP also has a defined facility for the transport exchange relevant to ECN. If your implementation delivers the IP header to or from TCP or SCTP, then the implementation of the API in question is the passage of that header. I know of a number of implementations that do that; it certainly is a convenient approach. However, I don't see any requirement that the API take that form, and I know some very common implementations that don't. I don't see any significant difference between using a service of this type, and using a service that says "please send this message as urgent data" to TCP, or "please send this message with this DSCP" to IP, or "please send this message without permitting fragmentation" to IP. It's just a service accessed through the API.