Scalable voip mobility intedration and deployment- P3: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | 18 Chapter 2 Table SIP REGISTER response SIP 200 OK Via SIP UDP 5060 branch z9hG4bK1072017640 received From 7010 sip 7010@ tag 915317945 To 7010 sip 7010@ tag as7374d984 Call-ID 1422523958@ CSeq 1 REGISTER User-Agent My PBX Allow INVITE ACK CANCEL OPTIONS BYE REFER SUBSCRIBE NOTIFY Supported replaces Expires 3600 Contact sip 7010@ expires 3600 Date Tue 27 Jan 2008 00 25 14 GMT Content-Length 0 when the extension is not known by the registrar. That would happen when the phone really does not belong to this particular telephone network. The Via From and To fields serve the same purpose as before. Note that the To field now has a tag. The From field is still from the original caller even though this is a response and the To field is still from the called party. The Via line and the indented one following it are all one line in the packet the description will use to mark that the following line continues the current one. The CSeq field has not changed as this is still the REGISTER request response pair. The User-Agent is the vendor name of the registrar. In this case it is My PBX. Because you cannot go out and purchase My PBX you may have to settle for someone else s. The Allow header states what types of actions a caller can request from the registrar. We will look at the more important ones INVITE and BYE shortly. The Expires header states that the registrar agreed with the client and is going to let the registration live for 3600 seconds. The registrar can shorten this amount if it so chooses and the client will lose its registration if it does not come in before then. You will notice that the Expires information repeats as a separate field in the Contact header. There are unfortunately still multiple ways of encoding exactly the same information in SIP and there will be a lot of redundancy. The problem of course with redundancy is that no one really knows how a complex system will work if
