Subject: [SIP] SIP Extensions for Presence

From: "Hisham Khartabil" <[email-masked]>
Date: Wed, 8 Nov 2000 10:55:29 +0200

Hi all,

Apologies for the long email, but I have few questions regarding SIP
Extensions for Presence.

3. Definitions

"Presence User Agent (PUA): A Presence User Agent manipulates presence
information for a presentity. In SIP terms, this means that a PUA generates
REGISTER requests, conveying some kind of information about the presentity."

"Presence Agent (PA): A presence agent is a SIP user agent which is capable
of receiving SUBSCRIBE requests, responding to them, and generating
notifications of changes in presence state."

Q: what about the sip user agent that generates SUBSCRIBE request and
receiving notifications? Should we have a definition for that? Are we
assuming s/he is the SUBSCRIBER?

4.2 Message Flow
...
"In fact, it is possible that two different presence clients register for
the same presentity. In this case, the presence server, acting as a proxy,
may fork the SUBSCRIBE, which means it sends multiple copies of the request,
one to each presence client. Each client will presumably query its principal
about whether the subscription is acceptable. In the common case where a
principal has left their presence tool running at work, and then gone home
and started the tool there, the principal will accept the subscription at
home. This causes a success response to be generated, and forwarded to the
presence server (which is acting as a proxy), and then back to the
subscriber. The presence tool at home then assumes responsibility for that
subscription. The tool at work can continue to support subscriptions it
received previously."

Since I am not at work, my status will not change so no notifications will
be generated.  This does not give the subscriber and up to date notification
of my presence.

Q: how about if I don't want my subscriptions at work to continue and I want
them to be transferred home?  I know this is achieved over time since
SUBSCRIBE has Expires: header.  But this could take hours (or one hour).
There should be a way to transfer subscriptions.

...

"As an optimization, notifications do not need to be sent if the subscriber
is not actually online. This improves scalability."

Q: is this assuming that A is subscribed to B and vice versa? Otherwise, who
would a Presence Agent Know that the subscriber on offline?

...

"A SUBSCRIBE request MUST contain a Contact header. This indicates the
address(es) (as a SIP URL) to which the client would like to receive
notifications. This MUST be be one or more SIP addresses, containing the
fully qualified domain names for the host generating the subscription, or
the IP address of the host generating the subscription. Other addresses are
possible, supporting third party subscriptions. If it contains multiple
addresses, notifications will be sent to each address. If no Contact header
is present, no notifications will be sent."

Q: since a SUBSCRIBE request MUST contain a Contact header, why wouldn't you
respond with 4xx if no Contact is present?

General Question:
If a client sends a REGISTER or a SUBSCRIBE with Expires: 3600.  The client
then re-Registers or re-subscribe with Expires: 3600 again, but before the
first one has expired (say 600 secs left). Does the new timer get reset to
3600 secs or 4200 secs (3600 + whatever is left from the first request. 600
in our case).

Regards,
Hisham

0=====================================================================0
o Hisham Khartabil  -  Hotsip Finland                                 o
o mailto:[email-masked]                                  o
o sip: [email-masked]                                    o
o mobile: +358 40 582 7484                                            o
o fax: +3589 856 642 10                                               o
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