A Session Initiation Protocol (SIP) user agent is an endpoint in a signaling network that can send or receive SIP messages. One can build a functional user agent in a few hundred lines of Java∗ code that sets up a call between two SIP phones. However, such a user agent will not fully comply with the protocol. Writing a compliant user agent is a complex undertaking involving thousands of lines of code. The iSURF framework greatly reduces the effort of this undertaking. iSURF uses a SIP transaction library called siptrans as its transaction processing layer. However, iSURF can use a different transaction library, and siptrans can be used in a different framework or even in a SIP proxy. In this paper, we describe the protocol requirements for a SIP user agent and how our framework facilitates building such an agent. We also describe the design and architecture of both iSURF and siptrans.