SIP (IETF RFC 3261) – is a Voice Over IP protocol that runs over http. It’s an application layer protocol for session initiation and management.
SIP components include User Agents (the usual clients and servers). Gatewaother addresses, like ISDN or H.323. Gateways also tend to translate audio.
SIP is very lightweight to set up conversations, which can then enable SIP agents to communicate directly with each other.
Session Description Protocol (IETF RFC 2327) describes sessions – used for bodies of SIP messages. Used, for example, to specify which codecs are preferred, in which order of preference.
While the SIP REGISTER dialog includes authentication (using http digest authentication) that is typically used today to hard code your user name and password into the phone device, which provides the same level of identity as do traditional telephones, but don’t really identify the user. Denis note that the Pingtel phones register themselves as a device and that the future is likely to be in systems that maintain individual user accounts on the phone devices themselves. There’s a draft proposal for SIP authenticated identity management.