[OpenSIPS-Users] T38 and changing media port for T38 SDP

[Julian Yap]

Any help greatly appreciated!

I'm having problems with a T38 UA which changes port when negotiating
T38 media.  All the other UA's I've encountered thus far use the same
RTP port throughout.

I'm also using RTPProxy.

In the final 200 OK SDP, the UA changes media port from 49200 to 49152
but this changeover isn't detected and the media is sent back to port
49200 so the call then fails to negotiate T38 properly.  Not sure how
to log the port changes to further debug this issue as well.

Here is the flow:
| UA                | OpenSIPS          | T38 GW            |
|         INVITE SDP ( g711U)           |                   |
|(5060)   ------------------>  (5060)   |                   |
|         100 Trying|                   |                   |
|(5060)   <------------------  (5060)   |                   |
|                   |         INVITE SDP ( g711U)           |
|                   |(5060)   ------------------>  (5060)   |
|                   |         100 Trying|                   |
|                   |(5060)   <------------------  (5060)   |
|                   |         180 Ringing SDP ( g711U)      |
|                   |(5060)   <------------------  (5060)   |
|                   |         200 OK SDP ( g711U)           |
|                   |(5060)   <------------------  (5060)   |
|                   |         RTP (g711U)                   |
|                   |(11392)  <------------------  (14110)  |
|         RTP (g711U)                   |                   |
|(49200)  <------------------  (10878)  |                   |
|         180 Ringing SDP ( g711U)      |                   |
|(5060)   <------------------  (5060)   |                   |
|         RTP (g711U)                   |                   |
|(49200)  ------------------>  (10878)  |                   |
|                   |         RTP (g711U)                   |
|                   |(11392)  ------------------>  (14110)  |
|         200 OK SDP ( g711U)           |                   |
|(5060)   <------------------  (5060)   |                   |
|         ACK       |                   |                   |
|(5060)   ------------------>  (5060)   |                   |
|         RTP (g711U)                   |                   |
|(49200)  ------------------>  (10878)  |                   |
|         RTP (g711U)                   |                   |
|(49200)  <------------------  (10878)  |                   |
|                   |         200 OK SDP ( g711U)           |
|                   |(5060)   <------------------  (5060)   |
|                   |         RTP (g711U)                   |
|                   |(11392)  ------------------>  (14110)  |
|                   |         ACK       |                   |
|                   |(5060)   ------------------>  (5060)   |
|         200 OK SDP ( g711U)           |                   |
|(5060)   <------------------  (5060)   |                   |
|         ACK       |                   |                   |
|(5060)   ------------------>  (5060)   |                   |
|         RTP (g711U)                   |                   |
|(49200)  ------------------>  (10878)  |                   |
|         RTP (g711U)                   |                   |
|(49200)  <------------------  (10878)  |                   |
|                   |         ACK       |                   |
|                   |(5060)   ------------------>  (5060)   |
|                   |         INVITE SDP ( t38)             |
|                   |(5060)   <------------------  (5060)   |
|         INVITE SDP ( t38)             |                   |
|(5060)   <------------------  (5060)   |                   |
|         200 OK SDP ( t38)             |                   |
|(5060)   ------------------>  (5060)   |                   |

This is where it sends the 200 OK with a different media port.

- Julian