SIP2SIP

h1. Connectivity Problems

Sometimes real-time communication between end-points fails. Real-time communications is a complex, for each sessions several ports and protocols are used in both directions. What are the possible reasons? Here is a short checklist to eliminate possible causes:

h2. Software version

 * Upgrade to the latest version, very often a problem may have already been solved.

h2. Local Computer

 * Firewall software installed. Check if no software blocks access to the ports used by your application or enable the use of the proper ports in the firewall configuration.

h2. Internet Router

 * SIP ALG enabled. Your router re-writes the content of your SIP packets. Check of the router has any so called SIP support and "disabled it":http://www.voip-info.org/wiki/view/Routers+SIP+ALG

 * Firewall enabled. Check if no software blocks access to the ports used by your application or enable the use of the proper ports in the firewall configuration.

h2. Internet Provider

 * Policy that blocks either SIP signalling or RTP media. You can try use another SIP service provider that provides access over TCP or TLS instead of UDP or is using custom ports. Otherwise use another ISP.

 * Often service provider do not allow use of ICE and force a relay instead. Turn of ICE in the client if this is the case.

h2. SIP Service Provider

 * Eequires use of STUN protocol so that the SIP devices uses a public IP address in the Contact header. This is wrong and unreliable, such behaviour is explicitly discouraged by the "standards":http://projects.ag-projects.com/projects/blinkc/wiki/MostEncounteredProblems#SIP-Provider-requires-STUN

 * Some codecs are not supported. Instead of rejecting such sessions with an appropriate code, they simply won't answer.