Problems with Hold, Dynamic Payload, Media Direction

10 Dec 2021

      Hello,

I have encountered several problems while configuring and testing
FreeSWITCH. This is on a current master branch, but the same issues are
present in older versions. The tests are done on a minimal config with
these additions:

vars:
- remove auth
- global_codec_prefs=OPUS,PCMU,PCMA,VP8
- rtp_pass_codecs_on_stream_change=true # To be able to toggle video stream

profiles:
- inbound-late-negotiation (with and w/o inherit codec)

dialplan:
- Simple dial plan. External to Internal flow. Condition for ext, then
bridge call to the other side.

Unfortunately, this Sofia options are removed from FS :( Are there
alternatives for them (while using rtp_pass_codecs_on_stream_change)?
renegotiate-codec-on-hold|true,false
renegotiate-codec-on-reinvite|true,false

(a) -- (FS) -- (b)

*1st: Hold* - Break transcoding.

-- INVITE (Opus, PCMA) --> |
                              | -- INVITE (PCMA) -->
                              | <---- OK (PCMA) ----
<------ OK (Opus) -------- |
                              .
                              .
                              | <--- ReINVITE (Hold or video on/off)
<---- ReINVITE (PCMA) ---- |
                              x

I tried with absolute codec string, inbound/outbound codec prefs... it
seems that FS does not keep/honor previous channel state and does not
differentiate stream change from stream parameters change (like media flow).

*2nd: Dynamic Payload *- It will offer Opus to the other side, because both
have it. But FS will renegotiate with new PT number on reinvite. This break
stream for some endpoints.

-- INVITE (Opus, 111) --> |
                             | -- INVITE (Opus, 102) -->
                             | <---- OK (Opus, 102) ----
<---- OK (Opus, 111) ---- |
                             .
                             .
                             | <-- ReINVITE (Hold or new m=line)
<- ReINVITE (Opus, 102) - |
                             x

Is there a way to originate call with a codec that has a dynamic payload
number, but to choose PT number? Something like setting rfc2833-pt variable
for outbound channel before originating call?

*3rd Media direction* - FS does not propagate media direction parameter to
inbound channel/leg. Inbound side expect media and some endpoints will drop
call after awhile. I know that answering with recvonly does not have sense
for audio, but for a video it has.

-- INVITE (sendrecv) --> |
                            | -- INVITE (sendrecv) -->
                            | <---- OK (recvonly) ----
<---- OK (sendrecv) ---- |

For late negotiation this is wrong. Even for early it should be possible to
pass reinvite to the other side on media direction change.

Fuad Trle

Dragos Oancea

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