OK, now I see what is going on. For some applications, there is a special domain name subtree, and for NAPTR records within that subtree, the protocol/rs fields are defined entirely by that application and are constrained only by the application. URN resolution is probably like this.
For other applications, the domain names are used for other purposes as well, and so the protocol field has to implicitly specify the application. E.g., for SIP, the protocol field is SIP or SIPS, or rather, the protocol fields SIP and SIPS are to be used only by the SIP application. Once the protocol field has selected the application, the rs field is limited only the the application. In the case of SIP, the rs values allowed are "D2U", "D2T", and "D2S", which are arbitrary strings, but have the mnemonics "domain to UDP", "domain to TCP", and "domain to SCTP". Now that I know what to look for, I can see some of this logic described in the RFCs. E.g., "Collision Avoidance" in RFC 3403. Thanks for getting me oriented, Dale --- interop.pingtel.com -- the public SIP phone interoperability test server _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
