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.

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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/.