Section 10.12 described how an IMS user becomes aware during registration of the public user identities he can use and which of them are currently registered. Subsequently, the users – in our example Theresa and Tobias – will use these identities for different purposes. For every kind of dialog within the IMS – in this example the INVITE dialog – two identities are essential:
A registered and authenticated public user identity of the calling user (Tobias) needs to be indicated in the request, in order to guarantee the identification of the user within his home network and for execution rights for extended services. This is provided in the P-Asserted-Identity header within the INVITE request.
A registered and authenticated public user identity of the called user (Theresa) needs to be indicated in the request, in order to be able to contact the user and to execute services for her. This is provided in the request Uniform Resource Identifier (URI) of the INVITE request and in the P-Asserted-Identity header of the first response.
The INVITE request that Tobias's UE sends toward Theresa includes the following headers that are related to either his or her identity:
INVITE sip:firstname.lastname@example.org SIP/2.0 From: "Your Brother" <sip:email@example.com>;tag=veli To: "My beloved Sister" <sip:Theresa@sister.com> P-Preferred-Identity: <sip:firstname.lastname@example.org> Privacy: None
Obviously, the From and To headers can be set ...