Solved Does 3CX support the 200OK/Invite without SDP?

[Grace Liu]

Does anyone meet with the problem like this?
After we connect 3CX with the FXO gateway(3CX does not support) by IP to IP, when the extension calls out through the gateway, the call failed. In the 3CX capture, after the gateway sends 200OK/invite without SDP to the 3CX, 3CX sends 480 to the extension and send the bye to the FXO gateway.

The gateway side said that the cause of call failure may be related to 200OK/Invite without SDP, can anyone ensure if 3CX supports the 200OK/Invite without SDP? do you have any suggestions?

Thanks
Regards
Grace

 

[StefanW]

once an offer is been made, the device MUST reply with any of the codecs offered.
On invite or re-invite it will work. Contact the vendor as this is a broken reply to what I can see from the flow chart.

 

[Grace Liu]

@StefanW Thanks for your reply.

The gateway side reply that the FXO gateway uses 183 to negotiate codecs, not 200OK.

When we connects the other IPPBX(not 3CX) or softphone with FXO gateway by IP to IP, the extension or softphone can call out through FXO gateway normally.
Here it is the capture when the softphone calls out through the FXO gateway after the softphone connects with FXO gateway by IP to IP:


Thanks
Regards
Grace

 

[YiannisH_3CX]

Hi @Grace Liu

Here is what I found after a quick search based on RFC6337 and RFC3261:
RFC6337:

When a UAC includes an SDP body in the INVITE request as an offer,
   only the first SDP in a reliable non-failure response to the INVITE
   request is the real answer.  No other offer/answer exchanges can
   occur within the messages (other responses and ACK) of the INVITE
   transaction.

   In [RFC3261] there are some descriptions about an offer/answer
   exchange, but those cause a little confusion.  We interpret those
   descriptions as follows,

   UAC behavior:

      1.  If the first SDP that the UAC received is included in an
          unreliable provisional response to the INVITE request,
          [RFC3261] (Section 13.2.1, second bullet) requires that this
          be treated as an answer.  However, because that same section
          states that the answer has to be in a reliable non-failure
          message, this SDP is not the true answer and therefore the
          offer/answer exchange is not yet completed.

RFC3261:

If the initial offer is in an INVITE, the answer MUST be in a
         reliable non-failure message from UAS back to UAC which is
         correlated to that INVITE.  For this specification, that is
         only the final 2xx response to that INVITE.

Basically this means that the UAC (in your case the PBX) can receive an SDP in the 183 message but the this is not the true answer. The SDP must also be included in the final 200 OK.

 

[Grace Liu]

@YiannisH_3CX Thanks for your information.

Could you please confirm it for me?
I tested the other IPPBX(not 3CX) with the FXO gateway, when the IPPBX received the 200OK without SDP, it sent the 200OK(after adding the SDP) to the extension.
Can 3CX realize the function like this?


Thanks
Regards
Grace

 

[YiannisH_3CX]

Hi Grace,

No we currently do not support this and since its not RFC compatible we probably never will. You could add this as a feature request in our Ideas forum but I would not have high hopes.

 

[Grace Liu]

@YiannisH_3CX Thanks for your confirmation, and I get it.

 

[Grace Liu]

@StefanW and @YiannisH_3CX Thanks for your help.

The gateway side changes the gateway firmware that adds the SDP in the 200OK/invite, then the outbound call of 3CX extension is normal now, and the capture of 3CX is here:


Thanks
Regards
Grace

 

[YiannisH_3CX]

I am glad to see were able to resolve the issue. That does look OK now.