SDP Comparison

When using Sentinel SIP, the SDP offer answer model is used to negotiate the media streams and associated codecs for the session, and the addition and removal of streams and associated codecs while a session is in progress.

When codecs and the number and type of streams change, Sentinel may consult the SDP media codec configuration to determine if the change was a material change, from a rating conditions perspective. The SDP media codec configuration defines a set of equivalence classes. When the two SDPs being compared have equivalent codecs in the same number of streams, the comparison will return a match which will indicate an immaterial change. When the number of streams or the codecs in the streams are not matched based on the configured equivalence classes, a material change will be indicated.

If the difference between the previous and newly negotiated SDP is material from a rating condition perspective, then Sentinel will perform a client-initiated credit re-authorization towards the Online Charging System (OCS). The OCS will be notified that the reason is due to rating conditions changing. While the re-authorization is in progress, the outgoing message is suspended. On successful response from the OCS, the outbound message processing is resumed.

If the reauthorization fails with a ServiceDenied reason and the message was a SIP UPDATE or INVITE, the newly negotiated SDP is discarded and 401 error response is sent to the originator. A CCR for the previous SDP is sent then to the OCS, to reestablish charging for the session at the previous rate. If this also fails, the session is ended.

If a change is immaterial, then the current credit reservation remains.