[Simple] GRUU in presence document

[Salvatore Loreto <salvatore.loreto at ericsson.com>]

Hi there,

we have noticed that currently there isn't any way to use presence to 
identify the unique device.
There was something mentioned in an earlier version of the gruu draft : 
http://tools.ietf.org/html/draft-ietf-sip-gruu-10#section-4.3
but it has been removed.

The definition of the device ID in RFC 4479 suites well with the gruu, 
however the <contact> element
in the <tuple> element is a URI and the question is how the URI 
parameters will be handled.

We have identified several possible ways to use presence to identify the 
unique device, that we would like to discuss in the wg
The possible way are the following:

1) Use the public GRUU as the value of the contact element of the 
presence document.

2) Include the sip-instance of the public GRUU as a SIP-instance 
attribute in the device element of the presence document.
The GRUU is obtained by concatenating the aor of the contact element and 
the SIP-instance attribute in the device element.

3) Define a new element in the service element to define the public GRUU.

4) Use the sip-instance as the device-id of the device element.
   The GRUU is obtained by concatenating the AoR of the contact element 
and the device-id attribute in the service element containing the sip 
instance.
   The device element does not have to be included if not needed for 
other purposes.


Here the drawbacks of the different proposal we have identified so far:

Alt. 1 will change the existing meaning (if any) of the contact element 
and does not allow concatenation of service elements in case of multiple 
devices.

Alt. 2 requires a new device attribute to be defined, but has the 
advantage that multiple devices are supported by one service element and 
if needed;
         the deviceID can be different from the sip-instance of the GRUU.

Alt. 3 requires an extension to the schema and does not really give any 
advantage to alt 4 except that sip-instance and deviceID may be different.

Alt. 4 is schema compliant and supports multiple devices using the same 
service element. Requires that the deviceID is set to the sip-instance 
of GRUU

So, our proposal is to go for alternative 4 since it has the best fit 
with current standard and is assumed to be proposed to standardize GRUU 
for presence.
The <contact> and <deviceID> of the <service> element are optional but 
are mandatory to identify the GRUU.


regards
Sal
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