[Sip] GRUU and grid

[Dale.Worley at comcast.net]

I was distracted during part of the last phone call, so I think I
heard the following idea discussed and largely agreed on.  But maybe
not.  Can people fill me in, or explain the ramifications of this
idea?

In previous drafts, a UA could convert a GRUU into a series of GRUU
URIs that would all route to the UA by adding a URI-parameter
"grid=(value)":

	sip:gruu-6 at example.com --> sip:gruu-6 at example.com;grid=abcde

Then the proxy responsible for the GRUU would carry the grid parameter
into the contact URI that was the final request-URI:

	sip:gruu-6 at example.com;grid=abcde routes to
		sip:10.1.1.1;grid=abcde

EKR among others noted that this was rather clunky and non-orthogonal
to the rest of the SIP addressing structure.

But the same effect could be obtained by using a header (in RFC 3261
terminology):

	sip:gruu-6 at example.com --> sip:gruu-6 at example.com?grid=abcde

At whatever stage of processing (UAC or proxy) this URI is inserted as
a request-URI, the header will be removed from the URI and turned into
a header field (RFC 3261 terminology):

	INVITE sip:gruu-6 at example.com
	grid: abcde

The header field will be copied by all proxies and eventually reach
the UAS.

The UAS can thus test for the "grid" header field in the same way that
it could previously test for the "grid" URI-parameter in the previous
proposals.

This new scheme has a number of advantages:

- It requires no new mechanisms, as this processing of headers into
  header fields is specified by RFC 3261.

- It is as easy for the UA to use as the URI-parameter.

- It requires reserving one header field name vs. reserving one
  URI-parameter name.

- UAs (or different layers of processing) could use the same mechanism
  for multiple additional values (as long as they use different header
  field names).

Dale

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