[sr-dev] about contact params

[Iñaki Baz Castillo]

2010/11/26 Juha Heinanen <jh at tutpro.com>:
> inaki,
>
> based on what in rfc3261 syntax?
>
> Contact        = ("Contact" / "m" ) HCOLON
>                 ( STAR / (contact-param *(COMMA contact-param)))
> contact-param  = (name-addr / addr-spec) *(SEMI contact-params)
> name-addr      = [ display-name ] LAQUOT addr-spec RAQUOT
> addr-spec      = SIP-URI / SIPS-URI / absoluteURI
> display-name   = *(token LWS)/ quoted-string
> contact-params = c-p-q / c-p-expires
>                 / contact-extension
>
> SIP-URI = "sip:" [ userinfo ] hostport
>          uri-parameters [ headers ]
>
> my reading of the above is that the whole contact body can be an
> addr-spec.


Juha, I can ensure you that I'm right, I studied it a lot some time
ago. In fact, Contact has similar syntax than From header (but Contact
allows various entries separated by comma). It's very common to see
the following From format:

  From: sip:alice at domain.org;tag=qweqwe

For sure, Fromtag MUST be a header param and not a URI param (as
Fromtag is defined to be a header param in RFC3261).


RURI is a addr-spec, so any param in the RURI is, of course, a URI param.
But in case a From/Contact/To header contains a addr-spec then a param
is ALWAYS a header param. If you want to add a URI param in these
headers you MUST user name-addr format.


One more argument:

  contact-param  = (name-addr / addr-spec) *(SEMI contact-params)

Imagine you use addr-spec, you want a URI param ";uri-param=1234" and
a header param ";header-param=9999". How would it look?:

  Contact: sip:alice at dom.org;uri-param=1234;header-param=9999

If you were right, how to determine that ;uri-param is a URI param and
;header-param is a header param? There is no way!
So the only solution is using name-addr:

  Contact: <sip:alice at dom.org;uri-param=1234>;header-param=9999


This is a complex grammar in the RFC 3261, and there have been long
discussions about it in sip-implementors maillist, you can search for
them if you want.



> my reading of the above is that the whole contact body can be an
> addr-spec.

Yes, of course, but then it CANNOT contain URI params, just header params.

Regards.





-- 
Iñaki Baz Castillo
<ibc at aliax.net>