As each UE is usually attached to the IMS over a wireless link, bandwidth resources are scarce and need to be handled with care. In Section 11.5 we saw that both ends exchange their preferences on the media and codecs used during a session until they agree on a specific set of media streams, each media stream using a single codec.
As media streams will not be routed through any CSCF, the IMS network needs to authorize reservation of agreed resources. Consequently, the P-CSCF requests the Policy Decision Function (PDF) to generate a media authorization token (Figure 11.8) whenever the first SIP message for the dialog is sent toward the UE (see Section 3.11.1). This token is added to the INVITE request and sent to Theresa's UE. Tobias's UE will receive its local token in the 183 (Session in Progress) response.
During the SDP offer/answer process, the local P-CSCF can tell the UE to group specific media streams together within one media PDP context or to keep certain media streams in separate media PDP contexts: this is achieved by the SDP media line grouping mechanism.
In addition to all this, network operators might want to restrict the media types and codecs that each UE can use. To do so, the P-CSCFs and S-CSCFs check the SDP that is included in the INVITE request. If one of them determines that a certain media type or codec should not be used, this CSCF will send a rejection to the UE. In our example we do not assume that such a ...