How to troubleshoot VOIP issues with Firewalls
First, VoIP traffic has two components:
1-Signalling:
Process of establishing and terminating calls Commonly used protocols are SIP, H.323, MGCP, Skinny, etc.
2-Media : Audio/Video Data transmission:
Audio is transferred using the Real-time Transport Protocol (RTP)
RTP message is encapsulated in a UDP datagram that is further encapsulated in an IP datagram for transmission
Initially, signaling session is established between the entities involved. Informations such as IP/ports for RTP/RTCP communication are sent through a signaling messages, after which the RTP/RTCP streams are used for actual data.
Because of complex multivendor implementations for VoIP solutions, it is hard to explain or predict the behavior of firewalls for all those solutions. However, there are general guidelines to help troubleshoot any VoIP Issues.
It is very crucial to understand the communication flow. Different signaling protocols have different message structures, and understanding the fields in the signaling communication is the key to diagnostic VoIP Issues with debugging.
For example:
1-SIP uses SDP (Session Description Protocol) to exchange information about RTP/RTCP Streams.
2-H.323 uses RAS and H.245 channels to exchange information about RTP/RTCP Streams.
