23.3. Compressing a SIP message in IMS
23.3.1. Initialization of SIP compression
During the registration phase the User Equipment (UE) and the Proxy Call Session Control Function (P-CSCF) announce their willingness to perform compression by providing details about their compression capabilities, such as memory size and processing power, upload states and compression instructions (see Section 12.13.15). Due to the strong security requirements in the Third Generation Partnership Project's (3GPP) Internet Protocol Multimedia Subsystem (IMS), announcements and state creations are only allowed after a security association has been established. Otherwise, a malicious user could upload false states that would make compression vulnerable.
23.3.2. Compressing a SIP message
When the UE or P-CSCF wants to send a compressed SIP message, it follows the framework described in [RFC3320], which states that a SIP application in the UE should pass a message to a compressor dispatcher. The compressor dispatcher invokes a compressor, fetches the necessary compression states using a compartment identified by its ID and supplied by the application, and uses a certain compression algorithm to encode the message. Finally, the compressor dispatcher relays the compressed message to the transport layer to be delivered to the remote end (the P-CSCF).
The compressor is responsible for ensuring that the remote end can decompress the generated message. It is possible to include all the needed information ...
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