iptel.org SIP Express Router (SER) is a high-performance, configurable, free server implementing Session Initiation Protocol (SIP, RFC3216). SIP is a signaling protocol that allows Internet users to establish VoIP calls, advertise their presence status, send and receive instant messages, and maintain any kind of session including games and chats. A major benefit of SIP is it creates an open framework for composing services out of multiple components. SER features flexibility that allows it to act in many roles needed for implementation of such services. For example, it can act as registrar and location server to provide mobility to users. It can also act as an access control element that guards PSTN gateways or any other scare SIP resources. Its built-in configuration language along with use of "plug-in" modules allows to add more new functionality easily. Currently available plug-ins implement messaging gateways to SMS and Jabber, RADIUS accounting and authorization, message store, SIMPLE presence agent and ENUM. There is also an application interface, that provides effective coupling with SIP-unaware applications. The applications, like web interface or administrative tools, can easily watch and manipulate server status, initiate SIP transactions and build features such as "click-to-dial". The application-interface is language- independent and can be used along with a variety of effective programming languages. SER has been designed to be able to scale and deal with various stress conditions. These may include communication with broken network components, signaling over NATs, power-up reboot avalanches, large user population, and any SIP applications generating high volume of traffic. Currently, transactional throughput of the server reaches thousands of calls per second on an off the shelf PC. -- Technical information: ANSI C-Written. Ported to Linux, BSD and Solaris (Sun). Support for both IPv4 and IPv6. Small footprint size: 300k core, all common plug-ins (optional) up to 630k. Web-based user provisioning, serweb, available. More information: SER distribution and documentation is available from iptel.org's site at http://www.iptel.org/ser/. User forum is available at http://mail.iptel.org/mailman/listinfo/serusers/.