iptel.org offers in-depth resources on SIP, the Session Initiated Protocol, as well as related protocols. The resources found here are both focused on understanding SIP and on practical resources for the SIP practitioner.

SIP Introduction

The SIP Introduction is a practical introduction to the SIP protocol, including visual displays of how the protocol works.

SIP Tutorial

A comprehensive tutorial to the SIP protocol (downloadable in Adobe Acrobat format).

Product Database

The product database lists SIP-relatedi products. Any user or manufacturer can add a product to the database.

Free SIP

List of free SIP software provided by iptel.org and third parties.

Other Useful Sites

• SIP site -- maintained by SIP author, Henning Schulzrinne
  
• http://www.tech-invite.com/ -- excellent website with many detailed information including call-flows
  
• Packetizer's SIP site (very useful, still use with caution -- website maintained by advocates of formerly competing technology, H.323)
  
• SIP Phoenix Group --A good site with different categories of research papers on SIP
  
• Voip-Info -- large wiki around all topics VoIPi