"Dale R. Worley" <[EMAIL PROTECTED]> wrote:
> On Fri, 2006-04-28 at 12:00 +0200, Frank Shearar wrote:
> > service_field = [ [protocol] *("+" rs)]
> > protocol = ALPHA *31ALPHANUM
> > rs = ALPHA *31ALPHANUM
> > ; The protocol and rs fields are limited to 32
> > ; characters and must start with an alphabetic.
> >
> > In other words, an optional protocol specification followed by 0 or
> > more resolution services. Each resolution service is indicated by an
> > initial '+' character.
>
> Where do I discover what "protocol specification" means, and what its
> valid values are? Ditto for "resolution service".
>
> For instance, "SIP+D2U" is a valid service_field value (per RFC 3261, if
> I did not make a mistake), but I've never seen an explanation of what
> "D2U" as a "resolution service" means. (Although RFC 3263 does explain
> that NAPTR records with "SIP+D2U" are to be used for contacting a SIP
> server via UDP, but it gives no explanation of why that value has that
> meaning.)
Ah, OK. RFC 3403 says that each application can define the protocol/rs
fields any way it likes. RFC 2483 (URI Resolution Services Necessary for URN
Resolution) covers some resolution services, with names like I2L (Identifier
to Location: maps a single URI to a single URL), I2Ls (Identifier to
Locations: give a single URI and receive a list of URLs) and so on.
But I suspect that "D2U" here isn't a resolution service, as such. The
resolution services like I2L have an algorithm as part of their definition,
and RFC 2483 says
2.2 Algorithm
The exact algorithm for the operation must either be specified
completely or it must be considered opaque and defined by the server
or application.
For instance, RFC 2648 appendix A.2 contains a perl implementation of a URN
to URL resolver for the ietf namespace.
Here, "D2U" isn't an algorithm - it's a statement to use UDP. I must say,
prior to researching your question, I wasn't aware of the full complexity of
the whole DDDS system. Thanks! So when I implemented my NAPTR-handling code
I just used the D2X values to select a transport. That sounds dumb. Let me
try again: are you asking why it is that SIP uses "resolution services"
called D2X (as opposed to some other token)? Sort've "what does the D mean"
or "what kind of operation is 'D2U'"?
frank
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