6. - SIP Messages SIP messages are the communication units used by SIP endpoints to establish, modify, and terminate multimedia sessions. SIP messages are exchanged between SIP user agents, proxy servers, and redirect servers, and are used to initiate and control communication sessions between two or more parties. The messages consist of a request or response line, message headers, and a message body, and are used to exchange information about the session, including media types, codecs, addresses, and authentication credentials. SIP messages can be sent over UDP, TCP, or TLS transport protocols.   Here are some examples of a SIP message SIP Invite Message INVITE sip:user@example.com SIP/2.0 Via: SIP/2.0/UDP 192.168.1.100:5060;branch=z9hG4bK123456789 From: Alice ;tag=987654321 To: Bob Call-ID: 123456789@192.168.1.100 CSeq: 1 INVITE Contact: Max-Forwards: 70 Content-Type: application/sdp Content-Length: 100 v=0 o=- 123456789 123456789 IN IP4 192.168.1.100 s=Session c=IN IP4 192.168.1.100 t=0 0 m=audio 5004 RTP/AVP 0 a=rtpmap:0 PCMU/8000 SIP OK Message SIP/2.0 200 OK Via: SIP/2.0/UDP 192.168.1.100:5060;branch=z9hG4bK123456789;received=192.168.1.100 From: Alice ;tag=987654321 To: Bob ;tag=123456789 Call-ID: 123456789@192.168.1.100 CSeq: 1 INVITE Contact: Content-Type: application/sdp Content-Length: 100 v=0 o=- 123456789 123456789 IN IP4 192.168.1.101 s=Session c=IN IP4 192.168.1.101 t=0 0 m=audio 5004 RTP/AVP 0 a=rtpmap:0 PCMU/8000 SIP Notify Message NOTIFY sip:alice@example.com SIP/2.0 Via: SIP/2.0/UDP 192.168.1.101:5060;branch=z9hG4bK987654321 From: Bob ;tag=123456789 To: Alice ;tag=987654321 Call-ID: 987654321@192.168.1.101 CSeq: 1 NOTIFY Event: message-summary Subscription-State: terminated;reason=noresource Content-Length: 0 SIP Prack Message PRACK sip:bob@example.com SIP/2.0 Via: SIP/2.0/UDP 192.168.1.100:5060;branch=z9hG4bK123456789 From: Alice ;tag=987654321 To: Bob ;tag=123456789 Call-ID: 123456789@192.168.1.100 CSeq: 1 PRACK Max-Forwards: 70 Content-Length: 0