imsd 0.2.7 — initial public snapshot

Userspace VoLTE/IMS daemon for mainline Linux phones (developed on the Fairphone 6): a GLib-free core (SIP, SDP, USIM AKA, IPsec SA setup, RTP media, call engine) behind a GDBus control daemon, a standalone AMR-WB media leg, and a Plasma Dialer backend. C++26 modules built with Crafter Build; the tree is clean under the project's house-style linter (crafter-build lint) across all three build products. Assisted-by: Claude:claude-fable-5 Signed-off-by: Jorijn van der Graaf <jorijnvdgraaf@catcrafts.net>
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+<!-- 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).