RFC 3261 SIP: Session Initiation Protocol June 2002 Table of Contents 1 Introduction ........................................ 8 2 Overview of SIP Functionality ....................... 9 3 Terminology ......................................... 10 4 Overview of Operation ............................... 10 5 Structure of the Protocol ........................... 18 6 Definitions ......................................... 20 7 SIP Messages ........................................ 26 7.1 Requests ............................................ 27 7.2 Responses ........................................... 28 7.3 Header Fields ....................................... 29 7.3.1 Header Field Format ................................. 30 7.3.2 Header Field Classification ......................... 32 7.3.3 Compact Form ........................................ 32 7.4 Bodies .............................................. 33 7.4.1 Message Body Type ................................... 33 7.4.2 Message Body Length ................................. 33 7.5 Framing SIP Messages ................................ 34 8 General User Agent Behavior ......................... 34 8.1 UAC Behavior ........................................ 35 8.1.1 Generating the Request .............................. 35 8.1.1.1 Request-URI ......................................... 35 8.1.1.2 To .................................................. 36 8.1.1.3 From ................................................ 37 8.1.1.4 Call-ID ............................................. 37 8.1.1.5 CSeq ................................................ 38 8.1.1.6 Max-Forwards ........................................ 38 8.1.1.7 Via ................................................. 39 8.1.1.8 Contact ............................................. 40 8.1.1.9 Supported and Require ............................... 40 8.1.1.10 Additional Message Components ....................... 41 8.1.2 Sending the Request ................................. 41 8.1.3 Processing Responses ................................ 42 8.1.3.1 Transaction Layer Errors ............................ 42 8.1.3.2 Unrecognized Responses .............................. 42 8.1.3.3 Vias ................................................ 43 8.1.3.4 Processing 3xx Responses ............................ 43 8.1.3.5 Processing 4xx Responses ............................ 45 8.2 UAS Behavior ........................................ 46 8.2.1 Method Inspection ................................... 46 8.2.2 Header Inspection ................................... 46 8.2.2.1 To and Request-URI .................................. 46 8.2.2.2 Merged Requests ..................................... 47 8.2.2.3 Require ............................................. 47 8.2.3 Content Processing .................................. 48 8.2.4 Applying Extensions ................................. 49 8.2.5 Processing the Request .............................. 49 Rosenberg, et. al. Standards Track