imsd

Userspace IMS/VoLTE daemon for mainline Linux phones.

Modem firmware on typical mainline-Linux phones will bring up the IMS PDN but can't be used for actual call audio. imsd implements the whole IMS client in userspace instead: SIP registration with USIM AKA authentication over kernel IPsec (ESP), call signalling, and the AMR-WB RTP media plane wired into PipeWire so a stock Plasma Mobile dialer can place and receive real VoLTE calls on a commercial network.

Status

Call-capable, both directions. Outgoing VoLTE calls place, answer, carry full-duplex AMR-WB audio, and tear down; incoming calls ring, answer via Accept, and carry media the same way — driven end-to-end from a stock dialer over the frozen D-Bus ABI. Current state:

The engine core (imsd-core) is a GLib-free, pure-std static library whose every decision — SIP framing, SDP selection, the AKAv1-MD5 digest, the ip xfrm command sequence, and the whole call FSM — is unit-tested on a dev box with no bus, modem, or phone; several suites pin output byte-for-byte against recorded network traffic. GLib/GDBus, sockets, and subprocess control live only in the imsd daemon shell.

Architecture

C++26 modules, built with Crafter Build. Four build products:

• imsd-core (static library) — the engine, pure import std C++ with no GLib and no I/O side effects. Everything here is unit-testable on any dev box without a modem, a bus, or a phone; several suites pin output byte-for-byte against recorded network traffic.
• imsd (executable) — the daemon shell: GLib main loop, GDBus service, sockets, subprocess control (qmicli/mmcli/ip), and the registration sequence + run loop that pumps the core. All GLib and I/O live here.
• imsd-media (executable) — the RTP/AMR-WB data plane, spawned once per call. Pure import std + POSIX, no GLib; links -ldl and dlopen's the codecs. A separate process for crash isolation and the far-end-hangup exit-code contract.
• imsd-dialerd (executable) — the Plasma Mobile integration: a session daemon owning org.kde.telephony.{CallUtils,DeviceUtils,UssdUtils}, bridging them to imsd on the system bus. GDBus translation only, no core. It is a separate process because those are per-user session-bus names that the root system daemon cannot own; built with -- --product=dialerd, autostarted from an .desktop file in place of plasma-dialer's modem-daemon (whose autostart must be disabled).

Terminating-network dependency (incoming)

A UE only receives a VoLTE call as a SIP INVITE if the network's Terminating Access Domain Selection (T-ADS) routes it to the PS/IMS domain rather than paging the modem over CS. That requires the IMS registration to advertise voice capability — the MMTEL ICSI (+g.3gpp.icsi-ref=...mmtel) and +sip.instance media-feature tags in the REGISTER Contact, which Imsd:Messages emits. T-ADS may still select CS for reasons outside the client's control (operator policy, the modem's own radio-capability signalling, a competing registration). When it does, no INVITE reaches imsd; that is a network/modem-integration matter, not a daemon defect.

Supported hardware & carriers

imsd talks to the modem through ModemManager plus qmicli over QRTR (USIM AKA via a UIM logical channel), so in practice it currently needs a Qualcomm modem on a mainline kernel with working WWAN/MM integration, kernel ESP (ip xfrm), and PipeWire for audio.

Everything below has been verified by community members to be fully workking:

Device	OS	Carrier
The Fairphone (Gen. 6)	postmarketOS, Linux 7.1.2	KPN NL
The Fairphone (Gen. 6)	postmarketOS, Linux 7.1.2	Telekom Deutschland GER
The Fairphone (Gen. 6)	postmarketOS, Linux 7.1.2	Phonero
The Fairphone (Gen. 6)	postmarketOS, Linux 7.1.2	Telia Norge

Build

Crafter Build + gio-2.0 headers (glib2 on Arch, glib-dev on Alpine).

crafter-build                       # bin/imsd-<target>-<march>/imsd (the daemon)
crafter-build -- --product=media    # bin/imsd-media-.../imsd-media (the media leg)
crafter-build -- --product=dialerd  # bin/imsd-dialerd-.../imsd-dialerd (dialer backend)
crafter-build test                  # unit tests (Util, Aka, Ipsec, Messages, Engine, Sip, Sdp)

Building with make (distro packaging)

For environments without crafter-build, the Makefile mirrors the same clang module pipeline: GNU make, clang++ with libc++ and the libc++ std module sources (std.cppm — package llvm-runtimes on Alpine, libc++ on Arch), lld, and gio-2.0 headers.

make            # build/make/{imsd,imsd-media,imsd-dialerd}
make check      # the same 7 unit-test suites
make install    # DESTDIR/PREFIX staged install incl. the packaging/ files

project.cpp stays the canonical build description; the Makefile tracks it.

Cross-compiling for a phone (aarch64 Alpine/postmarketOS)

packaging/make-sysroot.sh                  # one-time: Alpine aarch64 sysroot from the CDN
crafter-build -- --target=aarch64-alpine-linux-musl \
    --sysroot=$HOME/.cache/imsd/sysroot-aarch64-alpine \
    --march=armv8.6-a+fp16fml+aes+sha3+sm4 --mtune=cortex-a720
# same flags after `crafter-build test --target=aarch64-alpine-linux-musl`
# run the suite under qemu-aarch64 against the sysroot

Run (development)

./bin/imsd-*/imsd --session   # own net.catcrafts.IMS1 on the session bus
busctl --user call net.catcrafts.IMS1 /net/catcrafts/IMS1 net.catcrafts.IMS1 GetStatus

On a phone the daemon runs as root on the system bus (policy in packaging/net.catcrafts.IMS1.conf — it grants the postmarketOS default account user access; adjust it alongside AUDIO_USER if your session account is named differently), started by packaging/imsd.service after packaging/ims-pdn-up.sh has brought up the ims PDN through ModemManager. It spawns /usr/libexec/imsd-media per call. Registration resumes an in-kernel IPsec SA when one is present (avoiding the network's fresh-SA throttle) and keeps itself alive with a re-REGISTER refresh at half the granted lifetime. packaging/APKBUILD builds the apk from source via the Makefile; packaging/APKBUILD.binary + make-bin-tarball.sh wrap a cross-compiled build into an apk instead.

Configuration

Everything is environment variables. The packaged unit reads /run/imsd.env (written by ims-pdn-up.sh: the connected ims netdev) and then /etc/imsd.env (your configuration, wins on conflict). Minimum viable /etc/imsd.env:

# REQUIRED until P-CSCF discovery from the PDN's PCO is implemented: your
# carrier's P-CSCF address. Find it in a stock-firmware capture or your
# carrier's IMS documentation.
PCSCF=2001:db8::105

imsd

Variable	Default	Meaning
PCSCF	(none — required)	P-CSCF address; without it registration fails with a clear error
PCSCF_PORT	5060	unprotected SIP port for the initial REGISTER
DEV	qmapmux0.0	ims-PDN netdev; the packaged service passes the real one via /run/imsd.env
LOCAL	(auto)	UE address; default = the global IPv6 on DEV
USER_AGENT	imsd/<version>	REGISTER User-Agent. Some networks fingerprint UAs; setting your device's stock build string reproduces the stock modem's registration exactly. Empty omits the header
STATE_FILE	/var/lib/imsd/imsreg.state (root) / $XDG_STATE_HOME/imsd/imsreg.state	persisted registration context for warm resume
RESUME	auto	0 forces a fresh registration (ignores a warm SA)
REFRESH_INTERVAL	(auto)	keepalive re-REGISTER period in s; default = half the granted expiry, clamped to [120, 1800]
EALG	aes-cbc	offered ESP cipher (aes-cbc, des-ede3-cbc, null)
EMERGENCY_NUMBERS	(empty)	comma-separated additions to the builtin 112/911 emergency numbers (e.g. a private test core's short code). SIM EF_ECC is not read yet
RTP_PORT	50004	local RTP port advertised in SDP
PRECOND	0	1 offers SDP QoS preconditions
DUMP_SIP	0	1 writes raw REGISTER-200/SUBSCRIBE-200/INVITE dumps (mode 0600) for debugging — they contain your IMSI/MSISDN and addresses
DUMP_DIR, OUT_DIR	state dir	where dumps / per-call media stats land
IMSD_MEDIA	(auto)	path to imsd-media (default: next to imsd, else /usr/libexec/imsd-media)

The protected client/server ports are fixed at 45061/45062 (imsd::util::kPortUc/kPortUs) — changing them in source also requires updating the firewall rules ims-pdn-up.sh installs.

imsd-media (per-call, set through imsd's environment)

Variable	Default	Meaning
MIC, PLAY	1, 1	uplink mic capture / downlink playout via PipeWire
AUDIO_USER	user	desktop user whose PipeWire session carries call audio (postmarketOS default account)
GAIN, PLAY_GAIN	10, 1.0	uplink / downlink gain
AMR_MODE	2	AMR-WB encoder mode (0–8)
DTX	0	encoder discontinuous transmission
MEDIA_TIMEOUT	6.0	seconds of downlink silence before exit 3 (far-end-hangup signal)
RTP_DUMP	0	1 captures raw downlink RTP next to the stats file

D-Bus ABI

net.catcrafts.IMS1 at /net/catcrafts/IMS1. The ABI is frozen — dialer integrations depend on it:

Member	Signature
Dial(number)	s → s (callUni)
HangUp(callUni)	s
Accept(callUni)	s
SendDtmf(callUni, tones)	ss
GetCalls()	→ aa{sv}
GetStatus()	→ a{sv}
CallAdded	s a{sv}
CallStateChanged	s s s (callUni, state, reason)
CallDeleted	s
RegistrationChanged	b

License

GPL-3.0-only — see LICENSE.

Copyright

Copyright (C) 2026 Catcrafts® catcrafts.net | CODECS | (empty — defaults) | comma-separated codec preference list over AMR-WB, AMR (or AMR-NB), PCMA, PCMU: restricts and orders both the codecs offered on an outgoing call and those accepted from an inbound offer (default: offer AMR-WB + AMR, accept all four in that order). A bench knob — a network whose gateway transcodes every caller up to AMR-WB otherwise never lets the narrowband path run |