[pjsip] Eraly session support in pjsip_inv_session (2 SDPs in one SIP message)

[Z***@rohde-schwarz.com]

Hello,

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