[sr-dev] [kamailio/kamailio] Kemi: sdp_with_transport() missing and sdp_transport() doesn't always help (#2194)

[Sebastian Damm]

### Description

In plain Kamailio language, we used the following line:

```
if ( sdp_with_transport("RTP/SAVP") ) {
```

Now we wanted to convert that to Kemi. Turns out, the `sdp_with_transport()` function is not exported to Kemi. So we thought, we could use `sdp_transport()` to work around the missing function. This works as long as there is only one transport in SDP. 

But if a phone sends two media streams in SDP, one with RTP/AVP and one with RTP/SAVP, `sdp_transport()` will return `-2`.


### Troubleshooting

#### Reproduction

Send an INVITE to Kamailio and try to get the transport type into a variable with ` sdp_transport()`. This can be done for example with a snom phone where in the rtp section of the identity configuration the parameter "RTP/SAVP" is set to "optional".

#### Log Messages

```
Jan 10 13:08:13 hagi /usr/sbin/kamailio[7136]: DEBUG: app_jsdt [app_jsdt_api.c:996]: sr_kemi_jsdt_exec_func_ex(): param[0] for: sdp_transport is str: $avp(mediaTransport)
Jan 10 13:08:13 hagi /usr/sbin/kamailio[7136]: DEBUG: <core> [core/usr_avp.c:887]: parse_avp_ident(): Parsing 'mediaTransport'
Jan 10 13:08:13 hagi /usr/sbin/kamailio[7136]: DEBUG: <core> [core/pvapi.c:368]: pv_cache_add(): pvar [$avp(mediaTransport)] added in cache
Jan 10 13:08:13 hagi /usr/sbin/kamailio[7136]: DEBUG: sdpops [sdpops_mod.c:1214]: sdp_transport_helper(): stream 0 of 0 - transport [RTP/SAVP]
Jan 10 13:08:13 hagi /usr/sbin/kamailio[7136]: DEBUG: sdpops [sdpops_mod.c:1214]: sdp_transport_helper(): stream 1 of 0 - transport [RTP/AVP]
Jan 10 13:08:13 hagi /usr/sbin/kamailio[7136]: DEBUG: sdpops [sdpops_mod.c:1219]: sdp_transport_helper(): no common transport 
```

### Additional Information

We are still running Kamailio 5.1.x, but as far as I could see, the behavior has not changed since then.


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