SPA500 Invite with no SDP causes phone to terminate call

[TheBBS]

We have an SPA502G running 7.6.2c firmware.

When an invite is received by the phone with no SDP and the call is answered the handset sends its OK response with SDP but then terminates the call.

If the same handset receives an invite with SDP all is OK.

Handset processing the invite with no SDP

AUD_startRtpRx(). [0:0]. Port 16430. Ip 0x0. Tx txPayloadType -1. 

Message: AUD_startRtpRx(). [0:0]. Port 16430. Ip 0x0. Tx txPayloadType -1.

Normally now I would expect to see the handset iterating through codecs and setting up the receive and transmit streams but this doesn't happen.

The next step is the handset logging its response of 200 OK to accept the invite.

Message: Respond 200 OK in SIP_dlgSessCmdProc
Message: Respond 200 OK in SIP_dlgSessCmdProc

However what now follows is the handset terminating the call.

[0:0]AUD Rel Call

Possible bug?

[Dan Lukes]

INVITE with no SDP ? While it's not violation of RFC3264. what's the meaning of such invite in the context of "SIP phone call setup" ?

Normally now I would expect to see the handset iterating through codecs and setting up the receive and transmit streams but this doesn't happen.

Handset can select a codec offered in SDP only. There's no SDP, thus, no codec. I assume the call is rejected because there's no common codec to establish voice stream.

I can't judge it's bug. May be, there is a scenario where INVITE with no SDP is meaningful. If such scenario exist, and it's something the phone should support, then it's bug. Otherwise you are trying to use phone for a purpose the phone has not been designed for.

[TheBBS]

The handset is effectively a 3rd party extension from CUCM via a 3rd party application. The 3rd party application is transparent at this point as the SDP is relayed from the initial invite received from CUCM.

I don't know anymore about the CUCM installation at this location.

I believe an invite without SDP from CUCM could be the result of a trunk using Early Offer or possibly a H.323 trunk.

I've seen something similar with a Mitel PBX and also from an Oracle ACME ESBC.

[Dan Lukes]

So the INVITE with no SDP is sent by  CUCM ? What hardware we are speaking here about ? And firmware version ?

[Daniele Giordano]

Invite without SDP is possible. The call setup can be handle using different methods:

Forward Negotiation

1. INVITE with SDP-Offer
2. 180 Ringing
3. 200 OK with SDP-Answer
4. ACK

Forward Negotiation with Early Media

1. INVITE with SDP-Offer
2. 183 Session Progress with SDP-Answer
3. 180 Ringing with SDP-Answer
4. 200 OK with SDP-Answer
5. ACK

Reverse Negotiation

1. INVITE
2. 180 Ringing
3. 200 OK with SDP-Offer
4. ACK with SDP-Answer

Reverse Negotiation with Early-Media

1. INVITE
2. 183 Session Progress with SDP-Offer
3. PRACK with SDP-Answer
4. 180 Ringing
5. PRACK
6. 200 OK with SDP-Offer
7. ACK

Re-Negotiation on an established call

Forward Negotiation

1. INVITE with SDP-Offer
2. 200 OK with SDP-Answer
3. ACK

Reverse Negotiation

1. INVITE
2. 200 OK with SDP-Offer
3. ACK with SDP-Answer

* source innovaphone wiki

So, your PBX tries to handle the call using Reverse Negotiation. In my opinion the phone should be able to handle this scenario. On the PBX side the Reverse Negotiation is tipically due to internetworking between other protocols.

You can try to force forward negotiation on the PBX side or open a TAC to cisco for a bug on the phone.

Regards.

[Dan Lukes]

---

@Daniele Giordano wrote:

 So, your PBX tries to handle the call using Reverse Negotiation. In my opinion the phone should be able to handle this scenario. On the PBX side the Reverse Negotiation is tipically due to internetworking between other protocols.

---

Although I'm maintaining PBXs in seven countries around, with various kind of interconnect, I never seen Reverse Negotiation in real. It doesn't mean it's not allowed flavor of call setup, of course.

I suspect the SPA5xx is not capable to handle it. @TheBBS should turn on syslog&debug on SPA502G and catch them. Log will allow us to confirm or reject the hypothesis. Even Support Center will ask for logs.

open a TAC to cisco for a bug on the phone

SPA502G is Small Business kind of product. TAC rejects tickets related to it most of time (although success stories has been observer in the past). SMB Support center is dedicated to SPA502G support - but I hear no no positive reference to SMB SC so far (beyond ability to provide simple answer to trivial question).