of the Contact and Expires headers in the REGISTER request, the registrar server will take different action. Exam- ples of this are shown in Table 4.2. If no expires parameter or Expires header is present, a SIP URI will expire in 1 hour. The presence of an Expires header sets the expiration of Contacts with no expires parameter. If an expires parameter is present, it sets the expiration time for that Contact only. Non-SIP URIs have no default expiration time. The CSeq is incremented for a REGISTER request. The use of the Request-URI, To, From, and Call - ID headers in a REGISTER request is slightly different than for other requests. The Request-URI contains only the Table 4.1 Mandatory Headers in an INVITE Request Call-ID Contact Max-Forwards SIP Request Messages Table 4.2 Types of Registrar Actions and Contact Headers Request Headers Registrar Action Contact: * Cancel all registrations Expires: 0 Contact: sip:galvani@bologna.edu.it; Add Contact to current registra- expires=30 tions; registration expires in 30 minutes Contact: sip:galvani@bologna.edu.it Add Contact to current registra- Expires: 30 tions; registration expires in 30 minutes Contact: sip:galvani@bolognauni.edu; Add all Contacts to registrations expires=45 in preference order listed; first one expires in 45 minutes, sec- ond in 30 minutes Contact: sip:l.galvani@bologna.it Expires: 30 Contact: sip:galvani@bologna.edu.it Add Contacts to current registra- action=proxy ;q=0.9 tions using specified preference SIP requests Contact:mailto:galvani@bologna.edu.it; should be proxied; SIP URI q=0.1 expires in 60 minutes (default) mailto URL does not expire No Contact header present Return all current registrations in response domain of the registrar server with no user portion. The REGISTER request may be forwarded or proxied until it reaches the authoritative registrar server for the specified domain. The To header contains the SIP URI of the AOR of the user agent that is being registered. The From contains the SIP URI of sender of the request, usually the sa