Hand in hand with SIP, SDP, or Session Description Protocol, is a fundamental protocol for VoIP and the RCTS VoIP network. In this article, we will explore the importance of SDP and how it is used to negotiate voice session characteristics.
In VoIP, the SDP is structured in the body of the SIP message (as we can see in the image below). Each line is assigned a section that defines different parameters of the session in question, such as codecs, connection time and security parameters.
Like SIP, there are certain mandatory sections, such as "v=0", which indicates the version of SDP used, and "o=", which describes who generated the session.
Parameter negotiation happens when both parties send their session description and the participants use this information to decide which options should be used in the session. For example, in the top image we can validate that in INVITE(SIP) the various codecs supported by the participant are described.
In conclusion, SDP, is our born negotiator that allows a flexibility of parameters to each session participant. Without this flexibility, there would most likely be several compatibility and interoperability issues in many of the calls traversing the RCTS VoIP network.