We are implementing early session support in our PJSIP-based application. Examples for this can be found in https://tools.ietf.org/html/rfc3959#section-7 and in 3GPP TS 24.182 section A.4.3.
Currently we only care about the SIP signalling, not about the media flows.
I am wondering if
a) anyone has experience implementing this with PJSIP, and knows the pitfalls that we should avoid
b) there are any plans to have early session support in pjsip_inv_session
The tricky part seems to be that we need two SDP negotiators and we have to be able to send out and receive two SDP documents (which can be any combination of offers and answers) in one SIP message.
Our plan is that we use the SDP negotiator of pjsip_inv_session for the (not early) session as before and define another negotiator for the early session in the application. The RX/TX callbacks would manipulate this second negotiator and adjust the SIP messages being sent and received.
Any comments or suggestions would be welcome.
Thanks,
Zoltan
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