On non-REGISTER requests the Expires header is used to identify when that particular message expires. On an INVITE, for example the Expires value is used to determine the validity of the invitation. The device sending the request should send a CANCEL after this time has passed. The device receiving the request can also respond with a 487 error if the session hasn’t been established once the Expires time has elapsed. Allow: This is the list of defined SIP methods supported by the SIP endpoint. While it doesn’t give you a complete idea of the SIP endpoint’s capabilities it does give you a general idea. A strict, well-defined SIP trunk endpoint to the PSTN would almost never support SUBSCRIBE and MESSAGE, for example. Content-Length: This is the length of the SIP message body. What? So far we’ve only been talking about SIP headers because that’s all we’ve looked at. However, we have gone over how SIP is usually used to setup sessions. It does this by communicating session information in the message body. REGISTER messages will almost never have a message body because the headers are sufficient to communicate the relatively small amount of information to establish a registration binding. We’ll start looking at bodies soon enough. To authenticate or not? Earlier I pointed out that SIP endpoints do not require authentication. How do you know? Easy. This is what a SIP registration looks like without authentication: Request: REGISTER sip:38.101.17.146:5060 SIP/2.0 Via: SIP/2.0/TCP 10.16.5.144:63643;rport;branch=z9hG4bKPjFPbCK7muSTmhJI5Zh.1fr8ieTfKfQY71 Route: Max-Forwards: 70 From: ;tag=CBYHBQyftWiigDDBsFRoq6qHtX.vSWkU To: Call-ID: UQBTcWqDNKVOVFJ.uqsSDc2TIGohmn4I CSeq: 23154 REGISTER User-Agent: PJSUA v1.10.0 Darwin-11.3/x86_64 Supported: outbound, path Contact: ;reg-id=1;+sip.instance="" Expires: 300 Allow: PRACK, INVITE, ACK, BYE, CANCEL, UPDATE, SUBSCRIBE, NOTIFY, REFER, MESSAGE, OPTIONS Content-Length: 0 Response: SIP/2.0 200 OK Via: SIP/2.0/TCP 10.16.5.144:63643;received=72.64.129.133;rport=63643;branch=z9hG4bKPjaNOCrxABHA9ouOegtckmD0np1YdEtuKE From: ;tag=CBYHBQyftWiigDDBsFRoq6qHtX.vSWkU To: ;tag=as7212c7bd Call-ID: UQBTcWqDNKVOVFJ.uqsSDc2TIGohmn4I CSeq: 23155 REGISTER User-Agent: Asterisk PBX Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY Supported: replaces Expires: 300 Contact: ;expires=300 Date: Fri, 25 May 2012 15:56:23 GMT Content-Length: 0 Here it is again with authentication: Request: REGISTER sip:38.101.17.146:5060 SIP/2.0 Via: SIP/2.0/TCP 10.16.5.144:63643;rport;branch=z9hG4bKPjFPbCK7muSTmhJI5Zh.1fr8ieTfKfQY71 Route: Max-Forwards: 70 From: ;tag=CBYHBQyftWiigDDBsFRoq6qHtX.vSWkU To: Call-ID: UQBTcWqDNKVOVFJ.uqsSDc2TIGohmn4I CSeq: 23154 REGISTER User-Agent: PJSUA v1.10.0 Darwin-11.3/x86_64 Supported: outbound, path Contact: ;reg-id=1;+sip.instance="" Expires: 300 Allow: PRACK, INVITE, ACK, BYE, CANCEL, UPDATE, SUBSCRIBE, NOTIFY, REFER, MESSAGE, OPTIONS Content-Length: 0 Response: SIP/2.0 401 Unauthorized Via: SIP/2.0/TCP 10.16.5.144:63643;received=72.64.129.133;rport=63643;branch=z9hG4bKPjFPbCK7muSTmhJI5Zh.1fr8ieTfKfQY71 From: ;tag=CBYHBQyftWiigDDBsFRoq6qHtX.vSWkU To: ;tag=as7212c7bd Call-ID: UQBTcWqDNKVOVFJ.uqsSDc2TIGohmn4I CSeq: 23154 REGISTER User-Agent: Asterisk PBX Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY Supported: replaces WWW-Authenticate: Digest algorithm=MD5, realm="asterisk", nonce="4c701411" Content-Length: 0