Source: snapvoip.blogspot.com

Open source SIP stack OpenSIPStack has been rereleased under a triple licensing scheme to ensure that it can be used by the largest possible number of individuals and development communities. This tri-license aims to address the perceived incompatibilities between Mozilla Public License (MPL), GNU General public license (GPL) and GNU Lesser Public License (LGPL). The stack was previously distributed under MPL 1.0.

Open Source SIP project, openSBC is based on the OpenSIPStack, a fully compliant (RFC 3261) SIP stack designed for stability and scalability, and with a heritage of commercial usage. The project currently contains reference implementations of a session border controller (OpenSBC), Yeya and several components that are useful to developers wishing to use Solegy’s service deployment platform.
OpenSBC

OpenSBC is a reference implementation of a hybrid SIP proxy and B2BUA (back to back user agent) created from the Open SIP Stack core. It is well suited for a number of VoIP implementations. Among other things, it can be used as a Registrar for SIP endpoints, as an entry/egress point for SIP trunking applications, or as a far-end NAT traversal solution.

OpenSBC has been designed for scalability and flexibility. Deployments can grow incrementally with traffic needs because a primary instance can be configured to load balnce sessions across other instances. Each instance may be run on separate servers, or multiple instances may be run on a single server.

OpenSBC can perform the following functions:

Session Border Controller: Full back-to-back user agent (B2BUA) hides network topology with:
- Integrated web UI for basic configuration tasks
- far-end NAT traversal with RTP proxy
- Complete transparency for end-nodes with support for pass-thru of non-standard SDPs,
- Routing using static rules, ENUM or Solegy RTBE
- Comprehensive logs using syslog server
- Encryption of SIP and RTP packets with simple hash

Registrar: Fully standards-compliant with support for pass-through registrations (also referred to as upper registration) and integrated support for presence using SIP/SIMPLE or XMPP.

Proxy: Fully standard-compliant with multi-protocol support (UDP, TCP, TLS*), processing and relaying signals from remote (SIP) and local endpoints.

Presence: Compliant with SIP/SIMPLE and XMPP standards with support for PUBLISH as well as SUBSCRIBE/NOTIFY events.

Event Packages: Support for message-summary information about waiting messages (voicemail) and presence

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Links;
Open Source Sip website
OpenSIPStack web site
Solegy website