andynewlands at gmail.com
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 Posted: Mon Feb 28, 2022 9:37 am Post subject: [Freeswitch-users] Spurious DTMF characters when using RFC28 |
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Hi,
We have a reproducible problem where spurious DTMF characters are introduced on calls from Freeswitch to a remote (3rd party) IVR, but ONLY when using SRTP (not with RTP)
When the spurious character is generated it is:
1. Only ever following a valid/legitimate character (one that was genuinely keyed)
2. Often has a very long duration (see below)
3. Appears to be a random character - often 0x00, but sometimes 0-9/A-F,#,* (and that's when we get problems with the IVR - FS automatically drops the 0x00 values).
We have been able to reproduce this from mobile/cell phones and traditional land-lines.
I added some code to: switch_rtp.c, in static handle_rfc2833_result_t handle_rfc2833(switch_rtp_t *rtp_session, switch_size_t bytes, int *do_cng), to try to address this.
It logs an error (with the character hex value and duration) for DTMF characters with "long" durations (over 10000) and then discards them. As you can see, below, this is a fairly frequent occurrence (although most of the time, the character value is null, so FS drps it, anyway - but sometimes, "real" characters are introduced).
2022-02-28 14:00:53.743665 [NOTICE] switch_vpx.c:599 VPX encoder reset (WxH/BW) from 0x0/0 to 352x288/1024
2022-02-28 14:00:56.103665 [INFO] switch_channel.c:522 RECV DTMF 4:2400
2022-02-28 14:00:56.303664 [ERR] switch_channel.c:557 Ignored invalid DTMF on Call-ID: 3b5d08d3-d898-40b6-ae00-5886713ca6dd, DTMF: 0x0 Duration: 5063
2022-02-28 14:00:56.383661 [INFO] switch_channel.c:522 RECV DTMF 4:2400
2022-02-28 14:00:56.583665 [INFO] switch_channel.c:522 RECV DTMF 5:2400
2022-02-28 14:00:57.063662 [INFO] switch_channel.c:522 RECV DTMF 5:2400
2022-02-28 14:00:57.123670 [INFO] switch_channel.c:522 RECV DTMF 8:2720
2022-02-28 14:00:57.323673 [ERR] switch_rtp.c:686 Discarded long-duration DTMF on Call-ID: 3b5d08d3-d898-40b6-ae00-5886713ca6dd, DTMF: 0x0 Duration: 54088
2022-02-28 14:00:57.603671 [INFO] switch_channel.c:522 RECV DTMF 9:2400
2022-02-28 14:00:57.824102 [INFO] switch_channel.c:522 RECV DTMF 8:2720
2022-02-28 14:00:57.903663 [ERR] switch_rtp.c:686 Discarded long-duration DTMF on Call-ID: 3b5d08d3-d898-40b6-ae00-5886713ca6dd, DTMF: 0x0 Duration: 57002
2022-02-28 14:00:58.183668 [INFO] switch_channel.c:522 RECV DTMF 6:2400
2022-02-28 14:00:58.363666 [ERR] switch_rtp.c:686 Discarded long-duration DTMF on Call-ID: 3b5d08d3-d898-40b6-ae00-5886713ca6dd, DTMF: 0x35 Duration: 57073
2022-02-28 14:00:58.503667 [INFO] switch_channel.c:522 RECV DTMF 9:2400
2022-02-28 14:00:58.643667 [INFO] switch_channel.c:522 RECV DTMF 1:2400
2022-02-28 14:00:58.963663 [INFO] switch_channel.c:522 RECV DTMF 6:2400
2022-02-28 14:00:59.283665 [INFO] switch_channel.c:522 RECV DTMF 1:2400
The first spurious character is 0x00 (which FS will drop anyway) but this could easily be a valid character (and would escape detection because its duration is not too long - it's long but not unreasonable). So, this is an imperfect "fix" as we occasionally see a spurious character with a legitimate value and a "sensible" duration - sufficiently often for users to complain. The last [ERR] entry shows a '5' being introduced but this is dropped because the duration is "crazy" (57073)
As mentioned, this does not happen with secure media disabled - only with SRTP. So, I am wondering if the code correctly processes a digit then, sometimes (somehow) incorrectly attempts to decode part of an audio packet as though it were RFC2833.
Has anyone else experienced this? Does anyone know what may be causing the problem and what the fix might be.
I can provide console output and tcp dumps if required, along with any other details that may help (I'm also comfortable making code changes - to for extra logging/diagnostics etc).
Thank you.
Kind regards,
Andy |
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