A user agent can be configured to automatically register upon initialization,
at preset intervals, or whenever a new user signs on to the particular device.
Registration is not limited to a single URI. Multiple URIs can be used to list
a number of alternative locations in a preferred order, or may be used to list
multiple possible services such as SIP, PSTN, and e-mail. For example, con-
sider the following example REGISTERmessage:
SIP client to Registrar
REGISTER sip:registrar.here.com SIP/2.0
Via: SIP/2.0/UDP 4.3.2.1:5060;branch=z9hG4bK87ds
To: User A <sip:[email protected]>
From: User A <sip:[email protected]>;tag=34323kl12d
Call-ID: a532431277gfhd43gsfg3awrsad
Max-Forwards: 70
CSeq: 16 REGISTER
Contact: sip:[email protected];class=personal
Contact: tel:+1-314-555-1212
Contact: mailto:[email protected]
Content-Length: 0Registrar to SIP client
SIP/2.0 200 OK
Via: SIP/2.0/UDP 4.3.2.1:5060;branch=z9hG4bK87ds
To: User A <sip:[email protected]>;tag=9128394
From: User A <sip:[email protected]>;tag=34323kl12d
Call-ID: a532431277gfhd43gsfg3awrsad
CSeq: 16 REGISTER
Contact: sip:[email protected];class=personal
Contact: tel:+1-314-555-1212
Contact: mailto:[email protected]
Content-Length: 0The 200 OKresponse to a REGISTERechoes the three ContactURIs that
have been successfully registered. In this case, a query to the Location Service
for the SIP URI sip:[email protected] return the three ContactURIs
that were registered. The first is a SIP URIs that can be used to reach user A.
The second URI represents user A’s telephone number, which could be reached
via the PSTN (or through SIP and a gateway), and the e-mail address of user A.
The SIP URIs in this example may also contain parameter extensions not
shown here, such as Contactthat are defined in the Caller Preferences docu-
ment [25], which allows a user agent to identify information about the type of
device identified by the URI. For example, the first URI is identified as a per-
sonal URI, the second as voicemail, the third is for business, and the fourth is
a cell phone.
Normally, a SIP server would process a list of URIs by trying the first
Contactheader URI first, then moving to the second, and so on, assuming a
sequential search. The Reject-Contactheader works in a similar way, but
SIP Overview 125