OpenSIPS

OpenSIPS is an open-source SIP server and platform for real-time communications, designed to manage signaling and control for voice, video, and instant messaging sessions. It originated as a fork of the SIP Express Router (SER) project and is developed and maintained by the OpenSIPS Project and its community.

The software is built around a modular, event-driven architecture. The core handles the SIP protocol and routing, while functionality is extended through a large set of modules. OpenSIPS is written in C and configured through a text-based routing script that describes how requests should be processed and routed.

Key capabilities include SIP proxy, registrar, redirect server, and session border controller–like features, location service for user registration, dialog tracking, presence, messaging, and call routing. It supports NAT traversal, load balancing, failover, and security features such as TLS and SRTP. It can operate with WebSocket for WebRTC, supports SIP over UDP, TCP, and TLS, and can integrate with databases such as MySQL and PostgreSQL for user location, registration data, and accounting.

Deployment wise, OpenSIPS is used as a SIP routing engine in carriers, hosted PBX systems, and enterprise unified communications, or as a module within a broader SIP/VoIP architecture acting as an SBC or core router. It can be deployed in front of legacy SIP servers or as the central signaling backbone in a UC stack.

OpenSIPS is released under the GNU General Public License; source code and documentation are publicly available. It is cross-platform on Linux and some BSD systems and has a sizable developer community.

Administration and ecosystem: OpenSIPS provides a command-line tool, opensipsctl, for runtime management and monitoring, along with extensive documentation and a modular plugin model that enables customization for different deployments.