Hi All, That was a very interesting discussion of service codes on Monday :-). Here are some opinions I've come away with. One of the interesting items in the discussion was that one of the purposes of service codes was to relieve port scarcity -- it was explicitly intended that multiple apps with different service codes could listen simultaneously on the same port. I'm going to use socket terminology here because it's too much work to think up something totally generic. DCCP sockets have some different characteristics from other socket types, but mostly they're the same. I see two normal socket call flows, one for initiated connections and one for received connections. For initiated connections you call open, bind and connect. For received connections you call open, bind, listen and accept. Bound DCCP sockets are slightly different from bound other sockets -- a bound DCCP socket has a destination address, destination port AND A SERVICE CODE associated with it. Given this I would like to suggest the following requirements: REQ: A DCCP implementation MUST require the using application to explicitly set the service code associated with a socket before it can be used in a listen or connect request. The service code for a socket MUST NOT default to 0. It could default to SC=xFFFF, which is not a valid value for SC and will be rejected if ever used. REQ: A DCCP application in development SHOULD NOT use SC=0. It SHOULD choose a code from the private use space (first ASCII character equal to '?'). REQ: A DCCP implementation MUST reject a DCCP-Request with SC=0 if no socket is listening on that port with SC=0 (SC=0 MUST NOT be interpreted as a wildcard). Justification: If we allow SC to default to 0, and allow apps to use it, we'll get everyone using SC=0 and we'll loose the ability to alleviate port scarcity. REQ: A DCCP implementation MUST allow multiple sockets with different service codes to listen on the same port. Justification: If some implementations allow multiple listening sockets and others don't, app writers are going to be forced to assume the lowest common denominator and we'll have port scarcity problems. I realize that none of the existing implementations work this way, but I think that's asking for long-term problems. Tom P.
