RFC 2543 SIP: Session Initiation Protocol March 1999 SIP invitations used to create sessions carry session descriptions which allow participants to agree on a set of compatible media types. SIP supports user mobility by proxying and redirecting requests to the user's current location. Users can register their current location. SIP is not tied to any particular conference control protocol. SIP is designed to be independent of the lower-layer transport protocol and can be extended with additional capabilities. Table of Contents 1 Introduction ........................................ 7 1.1 Overview of SIP Functionality ....................... 7 1.2 Terminology ......................................... 8 1.3 Definitions ......................................... 9 1.4 Overview of SIP Operation ........................... 12 1.4.1 SIP Addressing ...................................... 12 1.4.2 Locating a SIP Server ............................... 13 1.4.3 SIP Transaction ..................................... 14 1.4.4 SIP Invitation ...................................... 15 1.4.5 Locating a User ..................................... 17 1.4.6 Changing an Existing Session ........................ 18 1.4.7 Registration Services ............................... 18 1.5 Protocol Properties ................................. 18 1.5.1 Minimal State ....................................... 18 1.5.2 Lower-Layer-Protocol Neutral ........................ 18 1.5.3 Text-Based .......................................... 20 2 SIP Uniform Resource Locators ....................... 20 3 SIP Message Overview ................................ 24 4 Request ............................................. 26 4.1 Request-Line ........................................ 26 4.2 Methods ............................................. 27 4.2.1 INVITE .............................................. 28 4.2.2 ACK ................................................. 29 4.2.3 OPTIONS ............................................. 29 4.2.4 BYE ................................................. 30 4.2.5 CANCEL .............................................. 30 4.2.6 REGISTER ............................................ 31 4.3 Request-URI ......................................... 34 4.3.1 SIP Version ......................................... 35 4.4 Option Tags ......................................... 35 4.4.1 Registering New Option Tags with IANA ............... 35 5 Response ............................................ 36 5.1 Status-Line ......................................... 36 5.1.1 Status Codes and Reason Phrases ..................... 37 6 Header Field Definitions ............................ 39 6.1 General Header Fields ............................... 41 6.2 Entity Header Fields ................................ 42 6.3 Request Header Fields ............................... 43 Handley, et al. Standards Track