any.com 131.160.1.112 (4) Invitation to a session for (1) Invitation to a session for sip :Bob@131.160.1.112 sip :Bob.Johnson@company.com Chapter 4 For instance, if a new authentication mechanism is proposed in the IETF, SIP systems can use it without implementing SIP modifications. A perfect example is the work on SDP next generation (SDPng) that the MMUSIC working group is undertaking. When SDPng is finalized, today's SIP sys- tems will be able to carry SDPng session descriptions instead of the SDP descriptions that are carried today. SIP will be able to take advantage of new Quality of Service (QoS) mechanisms as well. We call that future-proof. Separation Between Establishing and Describing a Session SIP clearly distinguishes between session establishment and session de- scription. As part of session establishment, SIP locates users as bidden, but it is silent on the topic of what users can do once the session is established. It does not define how a session should be described or session types. SIP just provides connectivity; what users do with it is outside of the scope of This distinction makes SIP essentially cooperative. It can now be used together with SDP, for instance, to establish Voice over IP (VoIP) sessions, and it will soon be possible to combine SIP with new session description protocols to establish types of sessions that do not yet exist. The concept we're describing is not dissimilar to the one that character- izes the IP layer. We saw that the most valuable service provided by the Internet is IP connectivity. All the rest are implemented taking IP connec- tivity as a base. Again, how IP connectivity is used falls outside the scope of IP itself. Hence, IP service creation is modular, fast, and efficient. SIP does not even assume that the session it has established will take place in the Internet. For instance, if Bob wants to invite Laura to join a conference call that is taking place in the public-switched telephone network (PSTN), all he has to do is use SIP to deliver the telephone number 