Subject: [Sip] RFC4235 Dialog Event package query

From: "Michael Procter" <[email-masked]>
Date: Fri, 3 Mar 2006 14:44:27 -0000

I've been reading this document, and also Dale Worley's draft containing
implementation guidelines (draft-worley-sip-dialog-00).  One thing that
has come up in the discussions I have had with Dale is the question of
what is meant by the mandatory "entity" attribute in the dialog-info
element.

>From RFC4235 Section 4.1:
   o  entity: This attribute contains a URI that identifies the user
      whose dialog information is reported in the remainder of the
      document.  This user is referred to as the "observed user".

There doesn't appear to be any further discussion of what the entity
value should be, but all the examples in the RFC are apparently AORs.
(Except for the example in 4.2, which is missing the element altogether!)

It is also unclear what use the element is.  I first thought it could
be used to distinguish between responses to a forked SUBSCRIBE, but
that doesn't make any sense if the entity is the AOR.  Maybe the
Contact (or better, GRUU) would be a more useful URI to use in those
circumstances.

Having the entity as the AOR does give the interesting side effect of
allowing a GRUU to be mapped back to an AOR, simply by subscribing
to the dialog package for the GRUU you are interested in.  I remember
this was a hot topic a couple of months ago - I don't recall what
the result was.

A problem with requiring the AOR to be present is that not all UAs
know their AOR.  There are various devices that are still statically
registered and so don't necessarily know their AOR.  There are
probably other reasons why a UA might not know its AOR aswell.
Under these circumstances, I suppose the request-URI might be the
best thing to put in the field.

So, it seems likely that the intention was that an AOR be used, but
there appear to be good and not-so-good reasons to use a contact
URI/GRUU instead.  I would like to understand what the entity
element is for (any use-cases?) and why it is mandatory.  Any
clarifications to the usage could be added to Dale's draft.

Regards,

Michael Procter

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