imsd

Userspace IMS/VoLTE daemon for mainline Linux phones.

Warning

imsd currently cannot make emergency calls

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:

Piece	State
SIP message layer + TCP framing (Imsd:Sip)	done, tested
SDP offer/answer for AMR-WB (Imsd:Sdp)	done, tested
USIM AKA + AKAv1-MD5 digest, SIM parsers (Imsd:Aka)	done, tested
IPsec SA commands + warm-SA reader (Imsd:Ipsec)	done, tested
SIP request/response builders (Imsd:Messages)	done, byte-pinned
Outgoing + incoming call state machine (Imsd:Engine)	done, tested
Registration (fresh + warm resume + keepalive refresh, MMTEL feature tags)	done
RTP + AMR-WB media plane into PipeWire (imsd-media)	done
D-Bus service net.catcrafts.IMS1 (register, Dial, Accept, HangUp, live media)	done
Plasma Dialer session backend (imsd-dialerd, org.kde.telephony.*)	done
DTMF (SendDtmf, RFC 4733)	not yet implemented

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 on exactly one device / one carrier (n=1) — reports from other combinations are very welcome:

Device	OS	Modem	Carrier	Status
Fairphone 6 (SM7635 "milos")	postmarketOS, mainline kernel	Qualcomm (QRTR/QMI)	KPN NL (eSIM)	MO + MT calls, full-duplex AMR-WB

Carrier-side assumptions that held on this network but may differ elsewhere: IPsec sec-agree with AKAv1-MD5 over IPv6, AMR-WB audio, TCP for the protected flows. The SIP/SDP/AKA layers are carrier-agnostic and unit-tested; the bring-up sequencing is where new networks will differ.

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)

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-a --mtune=generic
# 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.binary + make-bin-tarball.sh wrap a cross-compiled build into an apk.

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)
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