imsd/README.md

<!-- SPDX-License-Identifier: GPL-3.0-only -->
<!-- SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts® -->

# imsd

Userspace IMS/VoLTE daemon for mainline Linux phones.

> ## ⚠️ No emergency calls
>
> **imsd cannot make emergency calls (112, 911, 999, …).** Emergency calling
> needs dedicated IMS emergency-registration procedures and CS-fallback paths
> that imsd does not implement — and on a phone whose modem voice stack is
> bypassed like this, there may be no other working call path either. **Do
> not rely on a device running imsd to reach emergency services.**

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).

### Modules (imsd-core)

| Module | Responsibility |
|---|---|
| `Imsd:Util` | random SIP tokens, host/port formatting, hex/base64 decode, MD5 (RFC 1321) |
| `Imsd:Sip` | SIP message text layer: header access, status lines, registration lifetime, UDP response routing (RFC 3261 18.2.2), TCP stream framing (Content-Length, RFC 5626 keepalives) |
| `Imsd:Sdp` | AMR-WB SDP: offer building (octet-aligned, IR.92 QoS preconditions), answer parsing (payload-type selection, octet-align detection) |
| `Imsd:Aka` | AKAv1-MD5 digest (RFC 3310), home-network identity, `qmicli`/`mmcli` text parsers, AUTHENTICATE APDU build/parse |
| `Imsd:Ipsec` | IMS security association: the `ip xfrm` setup command sequence + a reader that reconstructs SPIs/ports/Security-Server from a warm SA |
| `Imsd:Messages` | Every SIP request/response the daemon sends (REGISTER fresh/protected/refresh, INVITE, PRACK, ACK 2xx/non-2xx, CANCEL, BYE, 200) |
| `Imsd:Engine` | Call state machine as a pure reducer, both directions: each event returns a list of Actions for the shell. Outgoing — PRACK on reliable 18x, ACK of 2xx/non-2xx, CANCEL/answer races. Incoming (UAS) — 100/180 on the INVITE (reliable when the caller requires 100rel), 200-with-answer on Accept, 486/480/488 rejects, the remote-CANCEL 200+487 pair. Both — session-timer refreshes, remote BYE, media-plane far-end-hangup, teardown-cause logging |

The **daemon shell** (`implementations/main.cpp`) owns the GDBus service, the
protected SIP/TCP flow + protected-port listeners, the SIM/AKA and `ip xfrm`
subprocess calls, the fresh/resume/keepalive registration sequence, and the
engine run loop that executes the call machine's Actions and spawns the media
leg. An out-of-dialog INVITE arriving on a protected-port listener becomes a
new (incoming) call machine; the connection it arrived on is remembered as
the call's durable reply channel so the 200 OK that a later `Accept` produces
still reaches the caller. The **media leg** (`implementations/media.cpp`) is
the RTP socket, the dlopen'd AMR-WB encode/decode, the pw-record/pw-play
subprocesses, and the timestamp-driven playout-clock reconstruction (CNG
gap-fill for far-end DTX). The **dialer daemon**
(`implementations/dialerd.cpp`) maps imsd's `CallAdded`/`CallStateChanged`/
`CallDeleted`/`RegistrationChanged` signals to the kde-telephony CallData
vocabulary (incoming calls surface as `direction=incoming`, state
`RingingIn`, which is what triggers the answer UI and ringtone).

### Roadmap

Ordered so every phase yielded something independently testable; 1–7 done.

1. **Message plane** (done): SIP text + SDP, unit tests.
2. **Engine** (done): the call state machine as a pure Action-returning
   reducer, tested against recorded SIP dialogs before touching a socket.
3. **AKA + IPsec** (done): by shelling out to `qmicli` (UIM APDUs) and
   `ip xfrm`, with the digest, parsers, and command sequence all in the pure
   core. A future native swap (libqmi-glib, XFRM netlink) can be bisected
   against the subprocess version.
4. **Transport** (done): TCP client flow + protected-port TCP/UDP listeners.
5. **Media** (done): RTP + AMR-WB (libopencore-amrwb / libvo-amrwbenc) with
   pw-record/pw-play. The receive side reconstructs the media clock from RTP
   timestamps and decodes NO_DATA frames for DTX gaps, so playback stays
   real-time-paced through far-end silence; the uplink is paced by the audio
   clock. (Native PipeWire `pw_stream` is a possible later refinement.)
6. **Call capability end-to-end** (done): validated live on a commercial
   network — an answered full-duplex AMR-WB call driven from a stock dialer.
7. **Incoming calls** (done): the UAS side of the engine + `Accept`, the
   session-bus dialer backend, and the REGISTER MMTEL feature tags. The
   daemon-side path (INVITE on the listener → ring → `Accept` → 200 with an
   AMR-WB answer at the caller's payload types → ACK → media → BYE) is
   validated on-device. Note the terminating-network dependency below.

Not yet implemented: DTMF (`SendDtmf`, RFC 4733).

### 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.

### Design rules

- **The D-Bus ABI is frozen.** Dialer integrations must never need to know
  which daemon version owns the name.
- **Engine core stays GLib-free.** GLib types stop at the daemon shell;
  core modules take `std::string_view` in and give values out. (GDBus was
  chosen over sd-bus because the surrounding telephony stack — ModemManager,
  libqmi — is GLib; but that's a shell concern.)
- **Edge-case behavior is part of the spec.** Header matching, framing, and
  SDP selection rules are pinned by unit tests, including deliberately odd
  cases (empty header values, keepalive discards, payload-type preference
  order). A change that flips one of those tests is an ABI change, not a
  cleanup.

### History

imsd comes out of the Fairphone 6 mainline bring-up, where this design —
userspace SIP/AKA/IPsec plus an RTP media plane into PipeWire, driving an
unmodified Plasma Mobile dialer — was validated end-to-end on a commercial
network (KPN, NL) with a Python prototype before this implementation was
started. Design decisions that look oddly specific (protected-port listener
pairs, media-inactivity call teardown, DTX playout reconstruction) are
lessons from that bring-up, and the recorded dialogs used in tests come
from it.

## 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](https://forgejo.catcrafts.net/Catcrafts/Crafter.Build) +
gio-2.0 headers (`glib2` on Arch, `glib-dev` on Alpine).

```sh
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)
```

What each product is and why it is a separate process is covered under
[Architecture](#architecture) above.

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

```sh
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)

```sh
./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`:

```sh
# 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` |

## Development disclosure

imsd was developed with substantial use of a generative-AI coding assistant
(Anthropic's Claude), driven and reviewed by the maintainer; commits carry
`Co-Authored-By: Claude` trailers. Every wire-format decision is pinned by
the unit tests, and the call paths were validated end-to-end on a live
commercial network.

## License

GPL-3.0-only — see [LICENSE](LICENSE).