TABLE OF CONTENTS Skype does not use SIP or H.323, but we thought we would cover these two protocols so that you could understand two of the main VoIP protocols used and how they differ from Skype. Session Initiation Protocol, or SIP, is a text-based protocol that uses port 5060 on both UDP and TCP for call setup and teardown. Version 2.0 of SIP has been codified in RFC 3261, where the various details of SIP are included. SIP is a signaling protocol very much like the telco SS7 protocol suite, but unlike SS7, which is very centralized and requires a private network, SIP was designed with the Internet and TCP/IP in mind and the use of many different media types such as voice and video. SIP is also a peer-to-peer type of architecture, similar to Skype. But whereas Skype does not use any central servers aside from the user authentication, SIP does use centralized servers for authentication. Since SIP is a signaling protocol, it offers features such as call waiting, call transfer, and call hold, among many others:
Call forwarding, including the equivalent of 700, 800, and 900-type calls
Call forwarding, no answer
Call forwarding, busy
Call forwarding, unconditional
Other address-translation services
Callee and calling number delivery, where numbers can be any (preferably unique) naming scheme
