[Freeswitch-users] Freeswitch DTMF issue - wrong timestamp base

Dragos Oancea dragos at freeswitch.org
Wed Mar 10 13:15:24 UTC 2021

you could open a github issue:
Attach FS debug logs and pcap file.

On Wed, Mar 10, 2021 at 12:35 AM Arslan Saeed <
arslan.saeed at overthewire.com.au> wrote:

> Hi All,
> We have come across an issue where freeswitch ((Version 1.10.5
> -release-17-25569c1631 64bit)
> seems to be doing something wrong with regards to RFC4733 DTMF event
> packets generation.
> What we see is that freeswitch would start using a new timestamp base for
> the RTP event packets (sometimes even for a few RTP packets preceeding the
> DTMF packetS) and then at the end of the DTMF event revert to older
> timestamp base. Although the marker bit is set on when new timestamp base
> is used but as far as I know this is not RFC compliant and causes issues
> for some of the PBX devices.
> RFC 4733, section says:
> DTMF digits and other named telephone events are carried as part of
> the audio stream, and they MUST use the same sequence number and
> timestamp base as the regular audio channel to simplify the
> generation of audio waveforms at a gateway.```
> https://tools.ietf.org/html/rfc4733#section-2.5.1
> --
> I think if freeswitch were to change the timestamp for DTMF event packets
> then it must also change the RTP  SSRC at same time (which does not happen)
> Although I don't understand the need to change timestamp base at all for
> the start of DTMF event  packets or even with the few proceeding RTP
> packets.
> I have tried to use following parameter but that does not help
> <param name="auto-rtp-bugs" data="CHANGE_SSRC_ON_MARKER"/>
>  I have also tried to test with these additional parameters (in order to
> infleunce freeswitch to not use a new timestamp base for the start of dtmf
> event packets)  but no change in freeswitch behaviour.
>   <param name="pass-rfc2833" value="true"/>
> <param name="rtp-rewrite-timestamps"value="true"/>
> <param name="auto-rtp-bugs"value="CISCO_SKIP_MARK_BIT_2833"/>
> <param name="auto-rtp-bugs"value="clear"/>
> In addition I have tried to use some jitter related sofia profile
> parameters but no affect.
> Another DTMF related issue that I have seen with freeswitch is that
> it sends DTMF event packets in a short burst and not evenly spaced out per
> the codec sample rate.
> For example if RTP is being sent at 20ms, freeswitch would send dtmf event
> packets at different interval like it may send 8ms apart or 13 ms apat and
> at times it sends multiple event packets just a few ms apart.
> To fix that behaviour, when we explicity set the RTP-timer-name to "soft"
> using
> <param name="rtp-timer-name" value="soft"/>
>  that fixes it and ensures evenly spaced DTMF packets say 20 ms apart
> (that fixes interop with some of the devices)
> Any help is highly appreciated.
> Thanks
> Arslan
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