The keypad during a call (a voicemail menu, an IVR) needs DTMF, and on an
IMS network it travels as RFC 4733 telephone-event packets on the call's
RTP stream. imsd-media reads `dtmf <tones>` lines on its stdin: each
queues the tones (0-9, *, #, A-D), and a tone takes seven 20 ms slots in
place of the audio frames — the event's five (100 ms at -10 dBm0: the
marker bit on the first packet, the duration growing a slot at a time,
the end bit on the fifth packet) and two in which the end packet goes out
again (§2.5.1.4) — then three slots of audio before the next tone. Events
and audio share the stream's sequence and timestamp space (§2.1), and one
sender keeps it: the mic thread hands each captured frame to the DTMF
state, which under one lock sends either the event packet the slot
carries or the frame (the main loop's silence tick does the same without
a mic, and so does the latch burst), so one packet leaves per slot, the
clock stays the capture's, and no frame can land inside an event
whichever thread holds the slot. The payload type comes from DTMF_PT in
the environment; unset means no DTMF and a dtmf line is dropped with a
message; a line of more than 4096 bytes is dropped through its end and
the next line goes. Nothing in imsd supplies the lines or the variable at
this commit; a hand run or the loopback does here, and a hand run behind
a job-control terminal is not stopped by pending input (the reader blocks
SIGTTIN, its read fails and it ends). pw-record no longer inherits stdin.
The stats sidecar gains tx_dtmf and dtmf_pt.
Tested: --selftest, and the fp6 journal's utilities/media_loopback.py
--dtmf arm on a host build of this commit's tree
(journal/ims/captures/2026-10-15-dtmf-loopback-c1-tx.txt; the plain, the
two overflow, the no-mic and the early-tones runs beside it) and its
ttyin.py pty test of the background hand run
(journal/ims/captures/2026-10-16-dtmf-ttyin-hand-run/); the dev phone's
dtmf benches ran in builds of this series.
Jorijn van der Graaf
29d624fc55