Some of the features that KAMAILIO (OpenSER) have:

• robust and performant SIP (RFC3261) Registrar server, Location server, Proxy server, SIP Application server and Redirect server
• small footprint - the binary file is small size, functionality can be stripped/added via modules
• plug&play module interface - ability to add new extensions, without touching the core, therefore assuring a great stability of core components
• stateless and transactional statefull SIP Proxy processing
• support for UDP/TCP/TLS/SCTP transport layers on IPv4 and IPv6
  
• support for SRV and NAPTR DNS lookups, SRV DNS failover
• IP level Blacklists
  
• multi-homed and multi-domain support
  
• scripting language for configurations file. With a syntax similar to sripting languages, the configuration offers a powerful and flexible way to deploy custom SIP services.
• management interface via FIFO file and unix sockets
• pseudo-variables to access and manage parts of the SIP messages and attributes specific to users and server
• transformations to modify existing pseudo-variables, accessing only the wanted parts of the information
  
• authentication, authorization and accounting (AAA) via database (MySQL, PostgreSQL, UnixODBC, BerkeleyDB, Oracle, text files), RADIUS and DIAMETER
• digest and IP authentication
• SIMPLE Presence Server (rich presence)
  
• Presence User Agent
• XCAP support
• Presence DialogInfo support - SLA/BLA
  
• Instant Messaging
  
• CPL - Call Processing Language (RFC3880)
• SNMP - interface to Simple Network Management Protocol
• XMLRPC -management interface via XMLRPC
  
• NAT traversal support for SIP and RTP traffic
• ENUM support
• PERL Programming Interface - embed your extensions written in Perl
• Java SIP Servlet Application Interface - write Java SIP Servlets to extent your VoIP services and integrate with web services
  
• load balancing with failover
  
• flexible least cost routing
• logging capabilities - can log custom messages including any header or pseudo-variable and parts of SIP message structure.
• modular architecture - plug-and-play module interface to extend the server's functionality
• gateway to sms or xmpp and other IM services
  
• multiple database backends - (MySQL, PostgreSQL, UnixODBC, BerkeleyDB, Oracle, text files) and other database types which have unixodbc drivers
• straightforward interconnection with PSTN gateways
  
• impressive extension repository - over 80 modules are included in the KAMAILIO (OpenSER) repository
• Extensive documentation for both users and developers
  

Scalability:

• KAMAILIO (OpenSER) can run on embedded systems, with limitted resources - the performances can be up to hundreds of call setups per second
• used a load balancer in stateless mode, KAMAILIO (OpenSER) can handle over 5000 call setups per second
• on systems with 4GB memory, KAMAILIO (OpenSER) can serve a population over 300 000 online subscribers
• system can easily scale by adding more KAMAILIO (OpenSER) servers
• KAMAILIO (OpenSER) can be used in geographic distributed VoIP platforms
• Kamailio least-cost-routing scales up to millions of routing rules
  
• straightforward failover and redundancy

New features in individual releases:

New features in Kamailio 1.5.0

New features in Kamailio 1.4.0

New features in OpenSER 1.3.0

New features in OpenSER 1.2.0

New features in OpenSER 1.1.0

New features in OpenSER 1.0.1

News in OpenSER 0.9.4 vs. SER 0.9.0

OpenSER History