The IMS: IP Multimedia Concepts And Services, Second Edition

11.4. Compression negotiation

11.4.1. Overview

The basic compression capabilities of the UE and the P-CSCF have already been negotiated during the registration procedures (see Section 10.9). Consequently, all requests and responses that are sent between the two sets of UE and their P-CSCFs will be compressed.

In this example we only show how compression parameters are basically set during session establishment and concentrate only on the compression between Theresa's UE and her P-CSCF. The procedures for Tobias's end are identical.

11.4.2. Compression of the initial request

We assume that Theresa has registered a contact address that included the comp=SigComp parameter at her S-CSCF. Therefore, Theresa's S-CSCF will include this parameter when it acts as a SIP registrar and re-writes the request URI of the INVITE request (see Section 11.3.3.5).

INVITE sip:[5555::5:6:7:8]:1006;comp=SigComp SIP/2.0

When the P-CSCF receives this request, it will route it toward Theresa's UE based on the request URI and, as the comp=SigComp parameter is included, it will send it compressed. Furthermore, the P-CSCF will:

• add the comp=SigComp parameter to its entry in the Via header, so that Theresa will send all responses to the INVITE request compressed;

• add the comp=SigComp parameter to its entry in the Record-Route header, so that Theresa will send all subsequent requests in this dialog compressed.

Our INVITE request now looks like:

INVITE sip:[5555::5:6:7:8]:1006;comp=SigComp SIP/2.0 Via: SIP/2.0/UDP ...