- P-Access-Network-Info: the UE provides the access-type and access-info, related to the serving access network.
- To: a tag is added to the To header.
- Contact: contains a SIP URI with the IP address or FQDN of the UE; it includes the comp=sigcomp parameter.
- SDP: the SDP contains the set of codecs supported by UE; it requests a confirmation of the QoS preconditions for establishing the session.