imsd/AGENTS.md

AGENTS.md — working on imsd

This file is for coding agents and for anyone changing imsd: what the repository contains, how it is built, tested and packaged, how the daemon behaves at runtime, and the conventions a change has to follow. The README is for users: what imsd is, how to install and configure it. The configuration variables are documented there, not here (§10). Where this file and the code disagree, the code is right and this file has a bug — fix it in the same change.

1. What imsd is

imsd is a userspace IMS/VoLTE client for mainline Linux phones. The modem firmware on such phones brings up the IMS PDN but cannot carry call audio, so imsd does the whole IMS client itself: SIP registration with USIM AKA authentication over kernel IPsec (ESP), call signalling for outgoing and incoming calls, and the RTP media plane (AMR-WB, AMR, G.711) wired into PipeWire. A stock mobile dialer — Plasma Dialer, or GNOME Calls under Phosh and GNOME Mobile — places and receives real VoLTE calls on a commercial network through it.

It talks to the modem through ModemManager (mmcli) and qmicli over QRTR (the USIM AKA run goes through a UIM logical channel) and, for the P-CSCF list the network put in the ims PDN's PCO, through its own QMI WDS client (imsd-pcscf, a raw QRTR socket bound to the bearer's mux data port — §13), installs its security associations with ip xfrm, and plays and records audio with the PipeWire tools (pw-play, pw-record). In practice that means a Qualcomm modem on a mainline kernel with working WWAN/ModemManager integration, kernel ESP, and PipeWire. The verified device/carrier combinations are in the README.

2. Repository layout

Path	What it is
project.cpp	The canonical build description, for Crafter Build. Chooses the product by --product=.
interfaces/Imsd.cppm	The primary module interface of imsd-core: export imports the eight partitions below.
interfaces/Imsd-Util.cppm	imsd::util — SIP tokens, IPv6 host/port formatting, the RFC 5952 §4 IPv6 text (CanonicalIp6/CanonicalAddr; an IPv4-mapped address spelled in hex stays hex, where §5 and inet_ntop write it dotted) and the comma-separated address-list parser (ParseAddressList), hex/base64, MD5 (RFC 1321), the fixed protected ports PortUc/PortUs (45061/45062).
interfaces/Imsd-Sip.cppm	imsd::sip — SIP as a text layer plus TCP framing: header extraction, status codes, the granted lifetime, ResponseOf transaction matching. Not a full RFC 3261 parser, deliberately.
interfaces/Imsd-Sdp.cppm	imsd::sdp — SDP offer/answer: the AMR-WB + AMR + telephone-event offer (optional IR.92 preconditions; OfferedTelephoneEventPt names its telephone-event numbers), the parser the media plane needs (the codec, its payload type and mode, and the telephone-event payload type the m-line lists at that codec's clock), BuildAnswer for the terminating side, the CODECS override.
interfaces/Imsd-Aka.cppm	imsd::aka — USIM AKA glue: the AKAv1-MD5 digest (RFC 3310), IMPI/IMPU/realm from IMSI + MCC/MNC, parsers for qmicli --uim-get-card-status and mmcli output, the AUTHENTICATE APDU and its response.
interfaces/Imsd-Ipsec.cppm	imsd::ipsec — the two ESP SA pairs (TS 33.203) as ip xfrm argv lists, the Security-Server mechanism selection, and the reader that reconstructs SPIs/ports from ip xfrm state/policy text for a warm resume, and (1.0.0) SaPeer, the P-CSCF the kernel's pair is to, read from the dir out policies that carry imsd's fingerprint, OwnXfrmDeleteCommands, the deletes by exact key of what two rules select — the policies carrying imsd's fingerprint and the reqid-1/2 transport-mode ESP states in their direction or on the UE's ims-PDN address, whoever installed them (§12), and FileNamesAnotherPair, whether a state file's spi_uc/spi_us name the kernel's pair (unknown on either side decides nothing).
interfaces/Imsd-Messages.cppm	imsd::msg — every SIP request and response imsd sends, built from an explicit Context and per-request variables so the builders are pure functions; byte-pinned by tests/Messages. Also IsEmergency.
interfaces/Imsd-Engine.cppm	imsd::engine — the decisions as pure reducers: CallMachine (one call's dialog, returning Actions for the shell) and RegRecovery (the registration recovery policy, §7.4).
interfaces/Imsd-Qmi.cppm	imsd::qmi — the QMI WDS request bytes imsd-pcscf sends (Bind Mux Data Port, Set IP Family, Get Packet Service Status, Get Current Settings, Get Profile Settings), the message/TLV parser, the P-CSCF list readers (0x22 flag, 0x23 IPv4, 0x24 FQDN, 0x2E IPv6 — the last one libqmi does not model).
interfaces/ImsdBus.cppm	imsd::bus — the polkit gate in front of call control, shared by imsd and imsd-ofonod. GLib, so outside imsd-core.
implementations/main.cpp	The imsd daemon shell: GLib main loop, GDBus service, sockets, subprocesses, the registration sequence and the run loop that pumps the core.
implementations/media.cpp	imsd-media, the per-call RTP leg, DTMF included (RFC 4733 events out, queued by dtmf <tones> lines on its stdin; the far end's events in, played as tones). Pure std + POSIX, dlopen's the AMR codecs.
implementations/pcscf.cpp	imsd-pcscf, the P-CSCF reader: the AF_QIPCRTR socket, the WDS service lookup and the Bind Mux Data Port / Set IP Family / Get Current Settings sequence around Imsd:Qmi (plus Get Profile Settings with --profile, Get Packet Service Status with -v). std + POSIX + the Linux QRTR UAPI (<linux/qrtr.h>), no GLib; never Start/Stop Network.
implementations/dialerd.cpp	imsd-dialerd, the Plasma Mobile session-bus backend (org.kde.telephony.*).
implementations/ofonod.cpp	imsd-ofonod, the GNOME Calls system-bus backend (org.ofono).
tests/<Suite>/main.cpp	The eight unit-test suites: Util, Aka, Ipsec, Qmi, Messages, Engine, Sip, Sdp.
packaging/	systemd units, the bring-up and upgrade scripts, D-Bus and polkit policies, presets, the gschema override, APKBUILD.binary and the CI helpers (§9).
.forgejo/workflows/package.yml	CI: builds the aarch64 apks on every push to main and publishes them to the Forgejo Alpine registry (§9.4).
bin/, build/	crafter-build's outputs and intermediates. Ignored.

Also ignored by .gitignore, and never to be committed: *.raw, *.rtp, *.stats, imsreg.state, imsd-*.tar.gz — captures and runtime state carry the subscriber's IMSI/MSISDN and network addresses.

3. Build products

Seven products come out of the tree. C++26 modules throughout, import std; in every source but the re-exporting Imsd.cppm, clang + libc++ + lld.

Product	Kind	Built from	Links	Notes
imsd-core	static library	the Imsd module and its eight partitions	—	Pure import std C++, no GLib, no I/O side effects: every message, digest, SA command and call-state decision is here and unit-testable on any dev box without a bus, a modem or a phone.
imsd	executable	implementations/main.cpp	imsd-core, imsd-bus, gio-2.0, pthread	The daemon. Default product.
imsd-bus	static library	interfaces/ImsdBus.cppm	gio-2.0	The polkit gate (net.catcrafts.IMS1.Voice) imsd and imsd-ofonod both link. A translation daemon that runs as root on the system bus reaches imsd as root, which polkit always allows, so it gates its own callers through this or it is a bypass.
imsd-media	executable	implementations/media.cpp	-ldl -lpthread	The RTP/AMR data plane, spawned once per call. No GLib, no core. A separate process for crash isolation and for the far-end-hangup exit-code contract (§7.6). -- --product=media.
imsd-dialerd	executable	implementations/dialerd.cpp	gio-2.0	Plasma Mobile integration: a session daemon owning org.kde.telephony.{CallUtils,DeviceUtils,UssdUtils}, bridging to imsd. GDBus translation only. A separate process because those are per-user session-bus names a root system daemon cannot own. -- --product=dialerd.
imsd-ofonod	executable	implementations/ofonod.cpp	imsd-bus, gio-2.0	GNOME Calls integration: a system daemon owning org.ofono, presenting imsd as an oFono modem. GDBus translation plus the polkit gate on its own callers. -- --product=ofonod.
imsd-pcscf	executable	implementations/pcscf.cpp	imsd-core	The P-CSCF reader ims-pdn-up.sh runs once the ims PDN is up (§7.3): one QMI WDS client over a raw QRTR socket, bound to the bearer's mux data port, asking for the call's runtime settings. Prints the PCO's P-CSCF list; never sends Start Network or Stop Network (its writes are its own client's mux binding and IP family, §12), and the call stayed connected and the registration stood after it (§13). Built with -- --product=pcscf.

The rule behind the split: imsd-core stays GLib-free and free of I/O so that every decision in it has a unit test. New protocol logic goes into a partition with a test. The shell (main.cpp; from 1.0.0 also pcscf.cpp's QRTR control lookup and its matching of answers by type, transaction and message id) holds protocol decisions no test reaches — some older than 0.3.10, some added by 0.3.10, 0.3.11, 0.3.12 or 1.0.0: the P-CSCF in use (pcscf_: the kernel's SA peer at a resume, PCSCF= or the first of PCSCF_DISCOVERED= at a fresh registration, the move between them and back — §7.2); the reg-event subscription schedule (§7.4: its retry ladder, Retry-After clamp, Timer F, renewal point and RFC 6665 reasons); the registration sequence and its transactions (AwaitFinal/AwaitInitialFinal: which message is a REGISTER's final, the dispatch of requests arriving meanwhile and the routing of our reg-event SUBSCRIBE's final read there, the 12 s wait restarted on a provisional); the reregistration's headers and its answer to a challenge (Reregister: the stored challenge answer, or the empty digest for a pre-0.3.11 state file, the new SPIs, PortUcNext; a 401 thrown as a refusal carrying its status); the state-file round trip (PersistState/LoadState: the security_server key, written empty for a value with a quote, a backslash or a newline — the choice between that list and the entry rebuilt from the SA is Imsd:Ipsec's, with a test); the inputs to RegRecovery — SipFlow::Connect's split of failures into local refusals and network silence, and the catch sites that sort a failed refresh (the keepalive's, ReconnectCycle's) and a failed fresh attempt (FreshRecover, DeferBringUp) into Refused, Silent or Throttled with the status, the Retry-After and saReplaced_, the bring-up resume's catch (a refusal's Retry-After fed as a hold, the old registration written off after a failed resume that a fresh registration follows; with no fresh target only a pair nothing can resume is written off — a 403 once fresh is due or a binding a NOTIFY terminated under the attempt counts as one by the engine's rule, and so does a resume it gives up: a 401, a refusal with a Retry-After, the second refusal, the count travelling in the holds (Reconnectable() and Fresh(), decided once after both catch arms) — and taken out of the kernel, §7.2 step 1), the OnLink read that writes the old registration off when its address has left the netdev (FreshRecover after a failed move, AddressGone), and PacketServiceUp's read of ModemManager before a silent cycle is counted; the no-target reads and reactions (1.0.0): the kernel text behind SaPeer, the holds' old_gone and the script's PCSCF_DISCOVERY= — whether a pair is resumable (SaPeer finds one and the holds' old_gone does not write it off: !peer || writtenOff in main()) and the choice of the idle or the fatal are shell code — at main()'s idle before the bus name (a pair the holds wrote off taken out of the kernel on the way) and at the no-target bring-up's write-off of a dead pair, its removal and the idle or the fatal that follows (§7.3); and the xfrm deletes the SA setup and the dead pair's teardown run — OwnXfrmDeleteCommands builds them from the kernel text, with a test; the shell reads that text and runs them (§12); the shell's reaction to a NOTIFY terminating our binding (the flow closed at once, or kept under a call and closed when the call ends, a reconnect cycle's or the bring-up resume's wait ended by it — the reducer's verdict is pinned, the reaction is not); and the holds file's codec (PersistHolds/LoadHolds: aging by boot seconds, the pdn/boot match, defaults for absent keys — proven on the phone only, captures 45- and 70- of 2026-09-25-polkit-voice; the pdn compare in canonical form on both sides (1.0.0) is unbenched: no capture stages a holds file whose pdn is spelled unlike LOCAL). For the schedule, a reducer next to RegRecovery, with a Subscription-State parser in Imsd:Sip pinned by tests/Sip, is the follow-up that moves it; a pure format/parse pair in Imsd:Engine is the holds file's.

4. Building

4.1 Native, with crafter-build

crafter-build is the Crafter.Build Flatpak. It carries its own clang, lld and libc++, and its SDK (org.freedesktop.Sdk, from Flathub) has the gio-2.0 headers, so the host needs Flatpak and nothing else:

flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
flatpak remote-add --if-not-exists catcrafts https://flatpak.catcrafts.net/catcrafts.flatpakrepo
flatpak install catcrafts net.catcrafts.CrafterBuild//stable
mkdir -p ~/.local/bin
printf '#!/bin/sh\nexec flatpak run net.catcrafts.CrafterBuild//stable "$@"\n' > ~/.local/bin/crafter-build
chmod +x ~/.local/bin/crafter-build

crafter-build is a script and not an alias so that the name also resolves in a shell that has not defined the alias (a fresh bash -c, a CI step); ~/.local/bin has to be on PATH (any directory on it will do). The sandbox does not see the host's /tmp: keep the checkout under /home, or grant its directory with flatpak run --filesystem=<dir>.

crafter-build                       # bin/imsd-<target>-<march>-<mtune>-<id>/imsd
crafter-build -- --product=media    # bin/imsd-media-…/imsd-media
crafter-build -- --product=dialerd  # bin/imsd-dialerd-…/imsd-dialerd
crafter-build -- --product=ofonod   # bin/imsd-ofonod-…/imsd-ofonod
crafter-build -- --product=pcscf    # bin/imsd-pcscf-…/imsd-pcscf
crafter-build test                  # the eight unit-test suites
crafter-build lint                  # the house-style rules (§4.2, §6)

Output directories under bin/ are named <product>-<target>-<march>-<mtune>-<id>; the ./bin/imsd-*/imsd globs in §8.3 rely on that.

A host-native binary built this way links the Flatpak's libc++ through an rpath into /app/lib: run it inside the sandbox (flatpak run --command=sh net.catcrafts.CrafterBuild//stable, with --socket=session-bus for §8.3's daemons — the sandbox's own session bus is a filter that refuses their names; crafter-build test runs the suites there anyway). A sysroot build (§4.3) gets no such rpath — it links the sysroot's libc++.

4.2 The lint rules, a dependency of project.cpp

project.cpp imports the house-style rules (§6) from their own repository:

#pragma crafter dependency "https://forgejo.catcrafts.net/Catcrafts/Crafter.LintRules.git" commit "<sha>"
import Crafter.LintRules;

crafter-build clones that repository — and updates the clone on later loads — and builds it for the host whenever it loads project.cpp: every build does this, CI's included, not only lint. The commit is pinned, so a change to the rules reaches imsd when the pin moves. LintRules::AddRules registers the rules on each product's configuration.

crafter-build lint            # report every finding; exit 1 on any
crafter-build lint 'naming'   # one rule (the argument is a glob)
crafter-build format          # apply the transform rules in place

Six rules read the AST (naming, no-char-pointer, const-local, constexpr-constant, fixed-width-types, single-declaration) and need the module PCMs: build the five programs and the tests first. With crafter-build 2.1.0 they still do not run on every file — implementations/main.cpp is among the misses ('type_traits' file not found).

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

packaging/make-sysroot.sh                  # one-time: Alpine aarch64 sysroot from the CDN (edge)
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
# `crafter-build test` with the same flags fails: its runner is qemu-aarch64,
# which the Flatpak's sandbox does not have. Test host-native (§4.1).

make-sysroot.sh needs no root, qemu or device: it downloads apk-tools-static and the Alpine keys and populates a minimal rootfs (musl, libc++ with its std module source, glib and their headers, compiler-rt, libunwind, linux-headers for imsd-pcscf's <linux/qrtr.h>, gcc for its crtbeginS.o/libgcc, which clang's *-alpine-linux-musl driver links against). In a cross build project.cpp does not ask the host's pkg-config (it would answer for the wrong architecture); it adds -I=/usr/include/glib-2.0 and -I=/usr/lib/glib-2.0/include, which clang resolves inside the sysroot. crafter-build compiles the std module against the sysroot's libc++ headers from the sysroot's own source — Alpine's llvm-runtimes, the same release as its libc++, which make-sysroot.sh installs (libc++-dev ships none); run over an existing sysroot the script upgrades its packages (apk add --upgrade). Without that source it falls back to the toolchain's source for the same libc++ major release, then to the sources Crafter.Build carries for other releases. This is the flow CI runs (§9.4).

5. Tests

crafter-build test runs the eight suites. Each suite is a plain main() with a Check(bool, message) counter: failures print FAIL: …, success prints <Suite>: all tests passed, and the exit status is the failure count for seven suites — non-zero short of a multiple of 256 — and 1 for Engine. crafter-build test ending in 8 passed, on every commit of a range (§11), is the project's definition of green.

What the suites pin:

• Util — tokens, IPv6 detection and host/port formatting, the RFC 5952 §4 form (the RFC's §4.1–4.2.3 examples, idempotence on inet_ntop-shaped text, what is refused), the address-list parser, hex/base64, MD5.
• Aka — the AKAv1-MD5 digest, identity derivation, the qmicli/mmcli text parsers, the AUTHENTICATE APDU and its response parser.
• Ipsec — the exact ip xfrm argv lists for the SA pairs, the Security-Server mechanism selection, the SA reader, the SA peer (SaPeer: the one address a reqid-1 and a reqid-2 dir out transport policy both name with imsd's protected port as the selector's sport — the fingerprint —, the pair kept beside a VPN's policies; a lone or disagreeing policy names no peer), FileNamesAnotherPair (the pair's own SPIs, other SPIs, one differing, no keys, a 0 from a missing inbound state, zeros in the file), and OwnXfrmDeleteCommands (our pair's eight deletes byte-exact on the fixture in the registered FP6's shape (synthetic addresses and keys), a VPN's tunnel SA and policies beside it untouched, another transport-mode user's pair on other ports and addresses untouched — its reqid-1 ESP state on the UE's address deleted, its AH state left —, a policy-less state on the UE's address deleted, one between other addresses left).
• Qmi — the request bytes imsd-pcscf sends (four of the five the probe's, byte for byte; Get Profile Settings is the helper's own, answered by the modem on the dev phone in the probe capture's --profile 2 -v run and on the dev phone's bring-up discoveries (the 2026-10-11/12 KPN bench), which pass --profile when the ims profile index is known — the profile-pcscf-via-pco flag line), the parser on the modem's 163-byte Get Current Settings answer in its exact TLV shape (addresses re-encoded to 2001:db8::/32), the OutOfCall answer, the IPv4/FQDN list readers on synthetic lists, the tolerance of a count that disagrees with a length, short and overrunning buffers, the packet-service-status and profile PCSCF-via-PCO readers (present and absent TLVs) and the error names (OutOfCall, NoEffect, an unknown code as ?).
• Messages — every outgoing SIP message byte-for-byte, cross-generated from the Python reference stack imsd replaced.
• Sip — the text layer's edge cases (empty header values, line anchoring, keepalive discards), ResponseOf, refused finals with reason headers, reginfo documents. A change that flips one of those tests is a behaviour change, not a cleanup.
• Sdp — the offer's exact bytes, the parser (the telephone-event payload type at the chosen codec's clock among it), BuildAnswer at the offer's own payload types and octet-align mode, the CODECS override.
• Engine — the call state machine driven through recorded network dialogs (reliable 18x/PRACK, 2xx ACK + media start, far-end hangup, CANCEL/answer races, busy/error finals, the terminating side's 100/180/200/486/487/488, G.711-only and AMR-NB gateway offers, ring timeout, BYE both ways, the emergency chain, DTMF — sent to the leg on an active call that negotiated telephone-event, dropped with the reason before the answer, after the call, without the negotiation, for a character that is no tone or for a string past DtmfMaxTones; the receive payload type: our offer's on a renumbered answer, the answer's when the audio took ours, the echoed offer's on an incoming call) and RegRecovery (thresholds, backoffs, the throttle schedule, Retry-After, which waits a Dial may cut short, when the old registration counts as gone, the holds a restart takes over, a binding the network terminated).

Rules:

• A decision in imsd-core gets a test in the suite that owns the partition. Fixtures are recorded network traffic where any exists, inline fixtures only where no recording exists.
• Behaviour that lives in the shell (main.cpp: timers, the reg-event subscription schedule, the audio-session resolution, the hand-over scripts) is proven on a phone where a bench can reach it; the commit message says in one sentence that it was tested, and the arms no bench has reached are named here. The known gaps: the reg-event subscription schedule (the arming and KPN's noresource closure have run on hardware, the 60 s retry only on a pre-fix build that renewed that closure; the retry on this code, the renewal firing, Timer F, the doubling, a SUBSCRIBE Retry-After, the renew arm and the back-off's survival across a refresh have run nowhere — §3, §7.4); a Retry-After on a REGISTER has run in the field (one report, a screenshot of 2026-10-08, O2, 0.3.10: a refresh's Retry-After set the next step's time — in 7 min, then in 13 min — and the refusals counted; the wait's length went unmeasured, the journal lines on the screenshot carry no timestamps; KPN's refusals, all 403, carry none) and a Dial bringing a recovery step forward (imsd-ofonod refuses the Dial itself and imsd-dialerd forwards it) is pinned in tests/Engine only; a 401 to a reregistration (the refreshes KPN accepted were never challenged — consecutive CSeqs on every bench — and O2 has not run this shape) and the escalation by one refused refresh that carries a Retry-After — the escalation to a fresh registration is pinned in tests/Engine only; a resume echoing a multi-mechanism Security-Server list on the wire (the choice of the stored list is pinned in tests/Ipsec with O2's recorded list; KPN's has one mechanism, so on the bench the stored value and the rebuilt one are the same bytes; O2's six-entry list has not resumed on this code, and the state-file round trip is shell code no test reaches); a Security-Server list PersistState writes empty (one with a quote, a backslash or a newline — neither KPN's nor O2's list has one; the resume of such a file ran with a hand-edited empty key, 2026-10-11); a state write of a standing registration after a fresh attempt that aborted before its SA setup (RegState::securityServer, not the attempt's ss_ — on KPN the old SA's retry after a throttled attempt is refused 403, the throttled-attempt chain below, so no such write follows here); the window that leaves a state file naming another SA pair than the kernel's (a stop or a crash between a fresh attempt's SA setup and its 200's state write — the check is pinned in tests/Ipsec and ran on a hand-edited list, 2026-10-11; the window itself is not staged); a refused initial or fresh REGISTER (403, 5xx) at bring-up or in recovery (KPN has only throttled or stayed silent — pinned in tests/Engine only); the write-off of the old registration by a recovery's fresh attempt that fails past the SA setup (such attempts are on record only as a bring-up's fatal exit, which does not pass that arm — pinned in tests/Engine only); the escalation by two refused refreshes that carry no Retry-After (KPN refuses a refresh only with the 403 that says the registration is gone — pinned in tests/Engine only); of the throttled-attempt chain in recovery (a throttled fresh attempt, the old SA retried once meanwhile with the stored binding's Contact and refused 403, the deferred attempt accepted at the hold's end — run 2026-10-03 on the series as it then stood) the deferred attempt made by the same process, since a restart inside the hold made it from the holds file instead (the same process made it on 2026-09-24, on the build before the policy's rework); the address moves writing old_gone to the holds file (the move benches predate that field); a SUBSCRIBE final read inside a REGISTER wait (KPN answers within the second); the shell's handling of a NOTIFY that terminates our own binding (§3; every captured termination is a previous contact's O2 deactivated, and a deregistration during a call cannot be staged); SipFlow::Connect's errno arms — socket() failing, a UDP connect() error, a bind() refused while the address is still on the link — which no bench reaches without fault injection (§3); a coverage gap on the air (no bench stages a radio gap without an RF shield) — the packet-service read's rule has run through a fake mmcli only (2026-10-03: searching and detached not counted, connecting with unknown counted), its other states, ModemManager not answering and its second site (a refresh unanswered over a reconnected flow) not at all, and tests/Engine pin nothing of it; the REGISTER wait's diverting arms — a datagram skipped on the init port, a response to another transaction ignored, a provisional restarting the wait, a request (an INVITE among them) dispatched and answered under the wait; an incoming call answered from a session's dialer since the polkit gate — imsd-dialerd's Accept by the active session, or GNOME Calls' Answer through imsd-ofonod's gate (imsd's own Accept gate has run: refused for an account without a seat session and for nobody, accepted for root, 2026-10-04); a refused final carrying reason headers (KPN's 403s carry none — the rendering is pinned in tests/Sip only); an AUDIO_USER pin in imsd's own environment that names no account (the logind fallback) or an account without a PipeWire socket (a silent call) — a pin naming an account with one ran, 2026-10-04. DetectLocal's three-tier address ranking for a run without LOCAL (§10.1) ran through a fake ip (2026-10-05: the phone's listing of the netdev and two cut-down ones). The 1.0.0 arms no bench has reached: a fresh registration moving from the resumed SA peer to PCSCF= and back when the attempt fails before the SA setup (FreshRecover); the fresh-target tail of a state file another attempt left behind (its no-target tail ran on the tip's host build, bench 23f, with a file naming another SA pair; the check itself, FileNamesAnotherPair, is pinned in tests/Ipsec; the window itself is not staged on the phone); another transport-mode reqid 1 + 2 pair beside ours on other ports, and two addresses each carrying imsd's fingerprint (tests/Ipsec only — a VPN's tunnel policy and a transport reqid-1 policy beside the live pair, resumed and kept, and a lone one with no pair of ours, none found, ran on the dev phone, 2026-10-11/12); the SA setup's and the dead pair's deletes with another transport-mode user's pair in the kernel (tests/Ipsec only — with a VPN's tunnel policy and tunnel state there, both left in place through a fresh registration and a dead pair's removal, they ran on the dev phone, 2026-10-12); a no-target resume the network refuses (the reducer's side of the count's travel, Held → Restore, is pinned in tests/Engine; the holds file's refusals key — PersistHolds/LoadHolds, shell code — and the shell's giving the pair up on the engine's verdict are unbenched); FreshRecover's restart for want of a fresh target (a resumed no-target registration whose recovery needs a fresh one); an empty Service-Route value in the 200, ignored (KPN's 200 carries none); a no-target resume whose refresh goes unanswered (the connect case ran on the tip's host build, bench 23f: the pair kept for the restart, the restart tries the resume again); RESUME=0 at a no-target start (BringUp's removal of the untried pair — the same deletes ran before the bus name on the host's kernel, a holds write-off in front of imsd's pair, 2026-10-13); RemoveDeadPair's no-peer line (N xfrm entries of imsd's own with no pair to resume: removing them); RunOwnXfrmDeletes' line for a delete that failed (… -> rc N: an entry gone between the read and the delete); the holds file's pdn compare in canonical form (no capture stages a holds file whose pdn is spelled unlike LOCAL); the discovery arms beyond the plain found one — the v4/FQDN also gave line, the eight-address cap, the IPv4-only and no-mux-id skips, the comma-list PCSCF= refusal, the reader's rc-0 answer without an address — on the mock bench only (S13, S19–S22, S55): KPN's PCO carries two IPv6 addresses and nothing else, so on the real PCO the dev phone reached the found arm alone; the configured skip, the bracketed refusal and, through the IMSD_PCSCF seam, the reader's failure and its exit 2 ran on the dev phone too (2026-10-11/12); the script's idle arms (no SIM, a refused ims PDN) with the refusal count's reset by a coverage gap, a timeout and a transient cause, the no-coverage and locked-SIM retries and the APN-string connect of a modem without the profile, on the mock bench only (a SIM-out boot would reach the no-SIM idle alone; the others need a SIM-in run against a refusing network, an outage, a locked SIM, a modem without the profile) — the daemon's idle: exit for the script's keys ran on the host build (2026-10-12, exit 0), its own idle (nothing to register to), the discovery-failed exit 1, main()'s removal of a pair the holds wrote off and imsd-ofonod starting without waiting for imsd on the dev phone (2026-10-12). DTMF ran on the dev phone (KPN, 2026-10-14 to 16, the fp6 journal's 2026-10-14-dtmf-bench-1 to 2026-10-16-dtmf-bench-17-heard): outgoing calls to voicemail over the bus (AMR-WB, telephone-event 98 — three tones, a string with a character that is no tone, a wrong call uni: the two drops' lines; benches 1, 5 to 9 and 11 to 16), 13 keys from GNOME Calls' keypad on a call to a stock phone the network answered narrowband (AMR 99, telephone-event 100), heard there (bench 2, the first sender; 28 keys from Plasma Dialer's keypad on two such calls heard there on the one-sender shape, bench 17); on incoming calls from that phone (the offer's 111 at 16 kHz) 10 and 4 keys sent from the keypad (benches 3 and 4); on Plasma Mobile, 11 keys from Plasma Dialer's keypad on an incoming call and 3 on an outgoing narrowband one, through imsd-dialerd (bench 10); the stock phone's key presses heard as tones (on GNOME Calls: bench 3's twelve on the renderer before the gap fill, bench 4's four on it with the gap fill; on Plasma Mobile: bench 10's ten on an incoming wideband call and two on an outgoing narrowband one, every received event rendered by the leg's own count). Every event of ours in benches 1 to 3 and 5 to 17 is in the RFC 4733 shape on the wire with the clock shared with the audio, and so are three of bench 4's four: the third carried a mic frame after its first packet (seq 1761 between 1760 and 1762) — the race of the sender's first shape, a thread of its own beside the mic's, which the sender's commit closes by having the mic thread send the event packets in its own frames' slots (one event of bench 4's four raced, none of the 23 live-mic events of benches 2 and 3; the loopback's mic is a file, paced like a microphone, and has not raced it). That first shape also stamped the audio a slot early or late per event against the capture clock on the AMR-WB calls measured — there pw-record delivers frames in bursts of two (§13) and the thread's 140 ms window took six to eight of them: the two tones of 2#, 10 slots apart by construction, are 11, 9 and 10 slots apart in benches 5, 12 and 13, AMR-WB voicemail calls (analysis-v2-bench-{5,12,13}.txt beside bench 14) — which the one-sender shape of bench 14 cannot (10); on a narrowband call the first shape's stamping was not measured. Not reached on hardware: a network answering without telephone-event (KPN echoes it; an answer that keeps the rtpmap off its m-line is pinned in tests/Sdp), the drops before the answer and after the call and the DtmfMaxTones bound (these three pinned in tests/Engine), the letters A–D and lower case (pinned in tests/Engine and the selftest), an inbound offer without telephone-event at the codec's clock (pinned in tests/Sdp), the shell's refusal of a line over PIPE_BUF and its not sent: no control pipe line on a call with no media leg (nothing pins these two), a pipe() failure, a write to a dead leg's pipe (DTMF '…' not sent: …), a full pipe's EAGAIN, G.711 (the loopback only), a received event with a code past D (dropped unrendered) or one whose packet reports more than two seconds the clock has not covered (that packet skipped — a long key press renders slot by slot), a lost first packet of an event (the renderer starts where the clock stands), and an answer that renumbers telephone-event or gives the audio our number (DTMF_RX_PT — KPN echoes our numbers; pinned in tests/Engine and by the loopback's collision arm).
• imsd-pcscf ran on the dev phone (KPN) on 2026-10-11 against the live ims bearer: the --mux 1 --profile 2 -v sequence, a plain run, and OutOfCall on an unbound mux (the fp6 journal's 2026-10-11-pcscf-pco-probe capture, 04-…). Not reached on hardware: exit 2 (a PCO without an IPv6 list), an error answer to Bind Mux Data Port or Set IP Family, a lookup that finds no WDS service, the answer timeout, skipped strays and indications, the lookup's second send to a local node other than 1, and the v4/fqdn output lines (tests/Qmi pins their parsing on synthetic TLVs only). The QRTR control lookup and the matching of answers by type, transaction and message id are shell code (pcscf.cpp) no test reaches.
• imsd-media --selftest round-trips the RFC 4867 payload packing (both formats, both AMR codecs), the G.711 tables, and pins the RFC 4733 event payload bytes, the keypad-to-event codes and every event's Q.23 frequency pair. MIC_SRC=<file> (raw s16 PCM at the codec rate, paced in real time) and PCM_DUMP=1 are test seams for driving the leg against a synthetic RTP peer without a phone; the fp6 journal's utilities/media_loopback.py is that peer, and its --dtmf arm writes a dtmf <tones> line to the leg's stdin and checks the events on the wire (one timestamp per event, the marker, the growing duration, the end bit three times, the shared sequence and timestamp space with the audio; with the mic file, or without a mic — --no-mic, the silence tick carrying the events), and its --dtmf-rx arm has the peer send events and checks the decoded downlink for each tone's two frequencies where the event sat, the stream intact around it — the sender's shape is proven there and on the dev phone's wire (the fp6 journal's dtmf benches: their pcaps go through the same checks), the renderer's there — and on the dev phone by ear and by the leg's count of events rendered (bench 10); what a gateway does with the events is proven only by a call to one.
• imsd --session and imsd-ofonod --session own their names on the session bus for development without root, polkit or a phone (§8.3).

6. Code style and lint

The rules are the Catcrafts house style, Crafter.LintRules, pulled in by project.cpp (§4.2); that repository's README lists the seventeen of them. // lint-disable-file <rule> (by convention on line 4, after the SPDX header and a blank line) and // lint-disable-next-line <rule> suppress a rule where a file needs it; nearly every source file disables fixed-width-types.

The tree was written under an earlier, project-local copy of the rules and does not pass the shared ones yet: crafter-build lint reports findings in most files — naming first (constants and statics are PascalCase here, the shared rule wants camelCase), then format-concat, const-local, wrap-join, single-declaration and no-char-pointer (const-local and single-declaration the copy did not report) — and the count moves (72 on 2026-10-06, before the Qmi files; 108 in 16 of 20 files on the tree that added them, 2026-10-13; 109 on the 1.0.0 tree, 2026-10-13: naming 69, format-concat 23, const-local 10, wrap-join 3, single-declaration 2, no-char-pointer 1 and if-single-line 1, the last new since). CI does not run lint.

Beyond the rules:

• Comments explain constraints the code cannot show — a network's observed behaviour, a spec clause, the reason a shape was chosen — and name their evidence (a dated bench, a capture) when they rest on one. They do not narrate what the code does.
• Log lines are the operator's interface (§7.8). A new state the daemon can be in gets a journal line that names it; existing lines are grepped for by scripts (imsd-handover.sh reads a pre-0.3.10 daemon's bring-up deferred: … next fresh registration in N min), so changing one is an interface change.
• GLib only in main.cpp, dialerd.cpp, ofonod.cpp and ImsdBus.cppm. media.cpp is std + POSIX. The partitions are std only.

7. How the daemon works

7.1 Processes and buses

imsd runs as root on the system bus as net.catcrafts.IMS1 (object /net/catcrafts/IMS1, interface net.catcrafts.IMS1; the frozen ABI, §8). The bus policy packaging/net.catcrafts.IMS1.conf lets any local account reach it; placing, answering and ending a call is polkit's net.catcrafts.IMS1.Voice (packaging/net.catcrafts.IMS1.policy, shaped like ModemManager's Voice action: the active local session and root — not an ssh login, an inactive session or a system service account). Reads (GetStatus, GetCalls) stay open, as ModemManager's are to every local account: the call signals reach them all anyway, the status (the address, the P-CSCF, the phone number) travels in no signal.

Two threads: a dedicated engine thread runs the SIP loop (blocking framed receive on the protected client flow, the protected-port listeners for terminating requests, media reaping, the keepalive refresh); the GLib main loop owns the bus. D-Bus method calls push commands to the engine queue; engine events reach the bus through g_idle_add, where the calls table and the status snapshot live (main thread only).

Per answered call imsd forks imsd-media (IMSD_MEDIA, default next to the imsd binary, else /usr/libexec/imsd-media) with the addresses, ports and payload type as arguments, the codec and audio knobs in its environment (§7.6, §10.2) and its stdin on a pipe from imsd, the control pipe the DTMF tones travel down (§7.6).

The desktop daemons are translation layers: imsd-dialerd runs as the session user (autostarted from /etc/xdg/autostart/imsd-dialerd.desktop in place of plasma-dialer's modem-daemon, whose autostart the package hides through /etc/skel/.config/autostart/org.kde.modem.daemon.desktop); imsd-ofonod runs as root on the system bus (imsd-ofonod.service, Conflicts=ofono.service). Both consume the frozen ABI and add nothing to it.

7.2 Bring-up

imsd.service runs two ExecStartPre= steps before the daemon:

1. /usr/libexec/ims-pdn-up.sh brings the ims PDN up through ModemManager, reads the P-CSCF list the network put in the PDN's PCO (imsd-pcscf; skipped when PCSCF= is set), gives the first a host route (§7.3) and writes /run/imsd.env with DEV= (the connected netdev), LOCAL= (the PDN address ModemManager assigned), PCSCF_DISCOVERED= (the IPv6 addresses of that list, at most eight, comma-separated in the modem's order; IPv4 entries and names dropped) and PCSCF_DISCOVERY= (how it came to be: found, none, failed or skipped, §7.3) — or IMSD_IDLE=<reason> alone when the bring-up found nothing to do (§7.3).
2. /usr/libexec/imsd-handover.sh (ExecStartPre=-, failure-tolerant) hands a pre-0.3.10 daemon's throttle wait over (§7.5).

The unit then reads EnvironmentFile=-/run/imsd.env and EnvironmentFile=-/etc/imsd.env (the admin's file wins) and starts /usr/bin/imsd. systemd reads those files anew for every command it spawns, so each ExecStartPre= already sees the values the previous one wrote. TimeoutStartSec=900 covers the bring-up's worst case (a 2-minute modem wait, a 5-minute registration gate, connect retries). Restart=on-failure, RestartSec=120; KillMode=mixed with TimeoutStopSec=10; StateDirectory=imsd (/var/lib/imsd).

The daemon's own bring-up (BringUp() in main.cpp):

1. The P-CSCF a fresh registration goes to is PCSCF= when set, else the first address of PCSCF_DISCOVERED= (the bring-up's reading of the PDN's PCO, §7.3); the journal says which (pcscf: configured … / pcscf: discovered … (using …)). A resume needs neither: the kernel's SAs name its P-CSCF (step 3). With neither, a bring-up that did not resume ends in the fatal no P-CSCF for a fresh registration: … and the restart's retry when the resume merely failed or the discovery did (PCSCF_DISCOVERY=failed, §7.3), else in the clean idle: no P-CSCF … exit (§7.3) — and since no fresh one follows, a resumable pair whose attempt failed (a silent connect, an unanswered refresh, a first refusal without a Retry-After — the refusal count goes to the holds) is not written off and stays for the restart to resume, unless the engine's rule ends it: a 401, a refusal with a Retry-After or the second refusal (RefusalsBeforeFresh; the count travels in the holds, so the restart's refusal is the second) give the resume up, a 403 once a fresh registration is due or a binding a NOTIFY terminated under the attempt write the registration off — either way the pair counts as one nothing can resume. A pair nothing can resume (its UE address left the link, no state file, a file another attempt left behind, written off before this start or given up by the engine during the attempt) is written off (old_gone in the holds — written as well for a pair RESUME=0 leaves untried) and taken out of the kernel first (the SA pair to … is nothing to resume: removing it — by exact key, what two rules select — the fingerprinted policies and the reqid-1/2 transport-mode ESP states in their direction or on the UE's ims-PDN address, §12), so the next bring-up finds nothing to resume.
2. qmicli -d qrtr://0 --uim-get-card-status finds the ready USIM (slot, AID); mmcli -m any gives the primary SIM path and the IMEI (for the +sip.instance URN); mmcli -i <sim> gives IMSI and MCC/MNC, from which the IMPI/IMPU/realm are derived. mmcli --location-get fills P-Access-Network-Info.
3. Warm resume when an SA pair is already in the kernel and the state file (STATE_FILE, default /var/lib/imsd/imsreg.state) describes the registration: reconnect through the existing SA and send a refresh re-REGISTER (RESUME via existing SA, then REGISTERED (resumed + true-refreshed)). This avoids the network's fresh-SA throttle. RESUME=0 forces a fresh registration where there is a fresh target; with none the untried pair is taken out and the bring-up ends as step 1 says. A resume the network refuses is not fatal while there is a fresh target: the fresh registration follows and replaces its SAs — at once, or after the hold the reducer keeps (a Retry-After the refusal carries, the one refusal that reaches the reducer with a target; a binding a NOTIFY terminated under the resume; a fresh hold kept from before this start); with no target every refusal reaches the reducer, §7.4; with none it is the fatal and the restart, or the idle (step 1). The SA reader, the holds file's pdn key, LOCAL and PCSCF compare addresses in RFC 5952 §4 form (imsd::util::CanonicalAddr: lower-case hex, leading zeros dropped, the longest zero run compressed — what ip xfrm prints for a plain IPv6 address; an IPv4-mapped address is read in that hex form only, never with the dotted tail ip prints), and PCSCF= is accepted in that one spelling only: upper case, leading zeros or an uncompressed run are refused at bring-up with the one to use (PCSCF='…' is not written as ip prints it; use PCSCF=…), a dotted-quad tail with the rule but no spelling, a bracketed value by the bring-up script with the rule (write the bare address, as ip prints it (no brackets)), a comma-separated PCSCF= (a list) with its own message (must be one address; in the packaged unit the bring-up script, which runs first, fails a list before the daemon sees it) — one spelling, so the bring-up script, which routes the value as typed, and the daemon agree on it. Before 1.0.0 an upper-case, zero-padded or uncompressed PCSCF= was accepted and, compared as text against ip xfrm, matched no SA and registered fresh at every restart; an upgraded phone with such a setting stops at bring-up with the spelling to use. A resume follows the kernel's SA peer (SaPeer), not PCSCF=: a registration made to another P-CSCF than today's setting is resumed there (pcscf: resuming on …; imsd routes that peer itself once ParseExistingSa confirms the SAs) and moves at the next fresh registration (the fresh registration moves from P-CSCF X to Y; an attempt that fails before the SA setup goes back while the old registration still stands — not written off by its address leaving the link, a NOTIFY that terminated the binding or a 403 once a fresh registration was due: back to P-CSCF X: …). A state file written for another SA pair than the kernel's — its spi_uc/spi_us against the pair's (FileNamesAnotherPair), a 0.3.x file's too: an attempt that got through the SA setup and did not live to record, to this or another P-CSCF — is not used; the bring-up registers fresh (with no fresh target: the fatal or the idle, step 1). Every fresh registration goes to PCSCF=, else the first entry of PCSCF_DISCOVERED= that CanonicalIp6 accepts; with neither, a bring-up that cannot resume ends as step 1 says (the fatal after a resumable pair's failed attempt or a failed discovery, else the clean idle: no P-CSCF … exit — a resume the engine gave up or wrote off included) and a recovery step that needs one restarts the daemon through systemd.
4. Fresh registration otherwise (FRESH registration): the target's host route (imsd's own ip -6 route replace; step 3 says which target), an unprotected REGISTER on PCSCF_PORT (UDP, always), the 401 with the AKA challenge and the Security-Server list, the AKA run over a UIM logical channel (--uim-open-logical-channel, --uim-send-apdu), the SA pair installed with ip xfrm, the protected REGISTER over the negotiated transport (§10.1, SIP_TRANSPORT), REGISTERED (fresh). Then the reg-event SUBSCRIBE (§7.4).
5. A keepalive re-REGISTER refresh at half the granted lifetime, within 120–1800 s (REFRESH_INTERVAL), only while idle. The refresh of a registration this process made fresh is a reregistration as TS 24.229 5.1.1.4.2 has it: the same Call-ID and Contact, CSeq+1, the Authorization of the last successful authentication verbatim (the 401's nonce, our cnonce and response, kept in the state file), a Security-Client with new SPIs and a new client port (PortUcNext, 45063) for the SA pairs a re-authentication would set up — the server port stays (TS 33.203 7.4.0) — and the Security-Verify (the 401's list). A registration resumed from the kernel's SAs sends the same Security-Client as that refresh, the Authorization the state file holds (a file from before 0.3.11: the empty digest) and the Security-Verify from the state file (since 0.3.12); resumed from a file written before 0.3.12, from one whose list PersistState wrote empty, or from one whose list names another SA pair than the kernel holds (an attempt the process did not live to record: its SAs installed, its 200 not written — since 1.0.0 such a file is not resumed at all when its own spi_uc/spi_us are not the pair's, step 3), it sends — on the resume's re-REGISTER, the keepalive re-REGISTERs, the reg-event SUBSCRIBE and INVITEs, until its next fresh registration — the one entry rebuilt from the SA (Imsd:Ipsec, which makes the choice): KPN's one-entry answer to the default offer in the same bytes, not a list such as O2's six, which RFC 3329 §2.3.1 has the server answer with a 494 when it does not mirror its own; a refused resume registers fresh (with no fresh target: the fatal and the restart, or the idle, step 1). A 401 to a reregistration asks for re-authentication over temporary SAs on the proposed ports, which imsd does not implement: it is a refusal and the recovery is a fresh registration (§7.4). Every refresh of the pre-0.3.11 shape on record from O2 UK (an initial registration's empty digest, the in-use SPIs) was refused with 503 (§13).

The state file is JSON: callid, ftag, cseq, spi_uc, spi_us, expiry, route, ppi, aor, contact_user, transport, since 1.0.0 pcscf (the P-CSCF the registration was made to; named in the line that reports a file another attempt left behind — the check is the SPIs', FileNamesAnotherPair), since 0.3.12 security_server (the registration's Security-Verify: the 401's list, written empty when it holds a quote, a backslash or a newline; absent, empty or naming another SA pair than the kernel's — a file from before 0.3.12, such a list, or a file an unrecorded attempt left behind — the resume rebuilds one mechanism from the SA, logs which, and stores that), and since 0.3.11 auth_nonce, auth_cnonce, auth_response (a registration resumed from an older file has none: its reregistrations — the resume's and the keepalives', across restarts — carry the empty digest, as before 0.3.11, until its next fresh registration, and each such resume logs it).

A failed fresh bring-up exits non-zero and systemd retries after RestartSec — except a bring-up the network refused or throttled, which waits in process on the refusal schedule (§7.4) with nothing to reconnect over, then tries again (bring-up deferred: …). A registered daemon whose address leaves the netdev with no PDN address there to move to also exits (engine fatal: … is no longer on qmapmux0.0 — exiting for systemd restart) — not at once: the loss is noticed at the next recovery step, the refresh (every 1800 s on KPN's 7200 s grant), whose re-REGISTER gets no reply and whose reconnect cannot bind the address (dev phone 2026-10-02, the ims bearer disconnected under a registered daemon: 14 min from the disconnect to the exit). After RestartSec (120 s) ims-pdn-up.sh runs again and imsd registers on the PDN's address (the same bench: a new prefix, REGISTERED (fresh) six seconds into the new start).

7.3 The ims PDN (packaging/ims-pdn-up.sh)

Finds or creates the ims bearer through ModemManager, connects it and configures the muxed netdev. Idempotent; a non-zero exit fails the unit.

• Waits up to 120 s for a modem, then gates the connect attempts on network registration (IMS_REG_TIMEOUT, default 300 s): some carriers attach ~2 min after boot.
• Idles instead of retrying when there is nothing to do: a modem in the failed state with a SIM reason (sim-missing, esim-without-profiles, sim-error), checked once the modem appears and again while waiting for registration, writes IMSD_IDLE=no-sim to /run/imsd.env and exits 0; a profile-indexed connect series refused ten times in a row — each a persistent 3GPP cause in ModemManager's typed error (ServiceOptionNotSubscribed, MissingOrUnknownApn, OperatorDeterminedBarring, ServiceOptionNotAuthorizedInPlmn, UserAuthenticationFailed, ServiceOptionNotSupported, Ipv4OnlyAllowed, Ipv6OnlyAllowed) with packet service at the time — writes IMSD_IDLE=no-ims-pdn (the line carrying the last error). The script's own rule, stricter than the daemon's PacketServiceUp: a refusal counts only while the modem reads registered, connecting or connected with packet service not detached; any other read (searching, disconnecting, unknown, an unanswered mmcli), a D-Bus timeout, esm-proc-timeout, a transient cause (InsufficientResources, ServiceOptionOutOfOrder, NetworkFailureActivation) or any other error starts the count again, as a coverage gap does — an outage must not idle VoLTE until the next boot. Installing the package and inserting a SIM is the opt-in: /etc/imsd.env holds overrides only and its existence changes nothing here (before 1.0.0 the apk's drop-in made it the switch for the whole unit, §9.2). imsd reads the key, logs idle: … and exits 0; Restart=on-failure leaves a clean exit alone, so the unit stays inactive until the next boot, a systemctl start imsd or a start of imsd-ofonod, whose Wants=imsd.service starts an idle imsd again (the Phosh image: an imsd-ofono upgrade, its own restart after a crash) — one attempt per boot, or per such start. What is not a fact keeps the retry: ten failures without coverage — the network found only at the end or lost on the way — exit 1 like a disabled radio, as do a SIM still waiting for its PIN (the modem locked), a modem whose final profile list has no ims APN (the APN-string connect is tried; a connected APN-string bearer — a run whose lookup failed at boot — is reused and its registration resumed) and an APN-string series, which a walled carrier's modem refuses locally. The dead end after the PDN is up is the daemon's own, decided in main() before it takes the bus name with the same clean exit: nothing to register to — no PCSCF= and nothing discovered: PCSCF_DISCOVERY=none (no IPv6 P-CSCF read from the PCO — the reader's exit 2: the list absent, empty, unreadable, or IPv4/FQDN entries only), skipped (a netdev without an rmnet mux id) or no PCSCF_DISCOVERY= at all (a manual run) — and no registration of its own to resume (SaPeer finds no pair, or the holds for this PDN and boot say old_gone): idle: no P-CSCF — none configured (PCSCF=), none discovered (see the ims-pdn-up lines) — and nothing to resume; set PCSCF= in /etc/imsd.env to name one; a pair the holds wrote off is taken out of the kernel by exact key on the way (the SA pair to … is nothing to resume: removing it, §12), so no start of this boot finds it. The no-target bring-up decides the same after its resume attempt: a resumable pair whose attempt failed keeps the fatal and the restart; a dead pair is written off, taken out of the kernel (§12) and the daemon idles (RequestIdle). A reader that did not answer (PCSCF_DISCOVERY=failed) is no fact: the daemon exits 1 instead — before the bus name with no P-CSCF: discovery failed … — exiting for systemd restart, from the no-target bring-up with bring-up FAILED: no P-CSCF for a fresh registration: … and engine fatal: … — exiting for systemd restart — and the unit's restart runs discovery again.
• Never creates or connects a bearer while the modem is disabled — its radio is off (airplane mode): a modem still disabled after the wait, or disabled when an attempt is due, makes the script exit non-zero (modem … disabled … (radio off): not connecting) and systemd retries. A bearer connect on a disabled modem fails with no-service and leaves ModemManager's state enabled with the modem in low power; switching the radio back on then no longer brings it up (dev phone 2026-10-06: not registered after 150 s, against 5 s without the attempt; a ModemManager disable/enable recovered it).
• Connects by 3GPP profile index (IMS_PROFILE_ID, default: looked up by APN in qmicli --wds-get-profile-list=3gpp), not by APN string: a profile-indexed request is exempt from the modem's attach-family forcing that refuses an APN-string IPv6 request when the attach PDN was granted IPv4-only (Odido, O2 UK, Swisscom). An empty profile list is the modem still coming up; a list without the APN is final only once the modem is registered (the list can be partial while it enables). A leftover APN-string bearer is never reused and is deleted when connecting by index.
• Sets accept_ra_defrtr=0 on the netdev before link-up: the PDN's router advertisement must never make the ims PDN the phone's IPv6 default route (on IPv4-only wifi every AAAA-first client would send its SYNs into the IMS walled garden and hang). The RA's prefix and MTU still apply. An RA default route already present (an imsd restart) is flushed.
• Sets the PDN MTU ModemManager reports on the link (1280 on the FP6), adds the address and waits out DAD.
• Resolves the P-CSCF: PCSCF when set (one IPv6 address; it wins and discovery is skipped — a list or a non-IPv6 value exits non-zero), else imsd-pcscf --mux <the netdev's rmnet mux_id> [--profile <index>] reads the list the network put in the PDN's PCO from the modem (§13; the mux id from ip -d link show <netdev>), not for IMS_IP_TYPE=ipv4, which the bring-up does not support (it configures only the PDN's IPv6 address). The usable addresses — IPv6; IPv4 ones and names are logged and dropped — are handed over (at most 8) and the first, the daemon's fresh target, gets a host route (dev <netdev>, no gateway — a raw-IP netdev without header_ops); imsd routes the P-CSCF it uses itself as well — the fresh target before a fresh registration, the resumed SA peer once its SAs are confirmed (§7.2 steps 3 and 4) —, as each call's media peer is. Nothing usable is logged with its cause (nothing usable (…)), is not the script's failure, and is told apart in the hand-off: PCSCF_DISCOVERY=none when the reader found no IPv6 P-CSCF in the PCO (its exit 2: the list absent, empty or unreadable — tests/Qmi pins the tolerant read) — a fact of this PDN: a registration of imsd's own that can be resumed is resumed and stays up (a fresh registration later restarts the daemon for want of a target, and that start runs discovery again), a bring-up that did not resume ends in the daemon's clean idle (idle: no P-CSCF …, the idle rule above) or, after a resumable pair's failed attempt, in the fatal and the restart; PCSCF_DISCOVERY=failed when the reader did not answer (an error, a timeout, nothing printed) — no fact: a daemon with nothing to resume exits 1 and the unit's restart runs discovery again. Anything else on the IMS network (its DNS servers, a traceroute target) needs its own host route (ip -6 route add <addr> dev qmapmux0.0). imsd installs a host route per media peer itself, and one to the P-CSCF it registers to or resumes on (§7.2 step 3).
• Opens imsd's protected ports 45061–45062 (TCP and UDP) on qmapmux* in nftables when a inet filter table exists (pmOS's default INPUT chain drops inbound on qmapmux*, which silently killed every network-initiated request after ESP decapsulation). Best-effort.
• Writes /run/imsd.env: DEV=<netdev>, LOCAL=<PDN address>, PCSCF_DISCOVERED=<list> (comma-separated in the modem's order; empty when nothing was discovered or PCSCF is set) and PCSCF_DISCOVERY=found|none|failed|skipped (how the list came to be: the PCO's answer, the reader's failure, or discovery not run — PCSCF= set, IMS_IP_TYPE=ipv4, no rmnet mux id), and logs the line it wrote. imsd registers on the address the network assigned, not on the kernel's temporary SLAAC address from the RA, which the netdev lists first and retires after about two days (the SAs and the Contact would go with it). The script never reads PCSCF_DISCOVERED from its environment: the unit hands it the previous run's file, and a stale discovered value must not pass for the admin's PCSCF=.

Its own settings (read from the environment, else from the file IMSD_ENV names, /etc/imsd.env by default, for manual runs): IMS_APN (default ims), IMS_IP_TYPE (ipv6), IMS_PROFILE_ID (empty = look up; none = connect by APN string), IMS_REG_TIMEOUT (300) and PCSCF; from the environment only, IMSD_PCSCF (the reader's path, default /usr/libexec/imsd-pcscf — the mock bench's seam) and IMSD_ENV (the configuration file's path — the bench's seam too).

7.4 Registration recovery (RegRecovery, Imsd-Engine.cppm)

What the daemon does when the registration stops working after bring-up, as a pure reducer the shell feeds with the clock and the network's answers. Two kinds of failure reach it: a refusal (the network answered a refresh with a final other than 200 — a 401, re-authentication, included — or a fresh registration's REGISTER with a final other than the expected one: from 300 up on the unprotected one, whose 401 is the challenge, anything but 200 on the protected one) and silence (the flow died and reconnecting through the SA, or the refresh over it, got no answer). Neither is retried for long on the same registration (TS 24.229 5.1.1.4 and RFC 5626 have the UE register anew). Before this reducer existed the daemon looped reconnect + re-REGISTER on one Call-ID and SA forever (O2 UK, 2026-09-24: 21 × 503 over 70 min, then 33 unanswered reconnect cycles over 2 h 39 min).

The policy, with its constants:

Step	Rule
Reconnect	Through the existing SA, then refresh. Back-off 5 s → 5 min (ReconnectDelayMin/Max).
Escalate to a fresh registration	After one refused refresh whose Retry-After is at least a second (the network's considered answer; a Retry-After of 0 or one that does not parse counts as none; O2 2026-10-08: the refresh refused 503 with Retry-After 414 s, the refresh after that wait 503 with Retry-After 772 s) or a 401 (re-authentication, which imsd answers with a fresh registration rather than over temporary SAs), after two refused refreshes without one (RefusalsBeforeFresh), or two reconnect cycles that end without an answer (SilentCyclesBeforeFresh) while the modem reports packet service (PacketServiceUp: ModemManager's registration and packet-service state, as ims-pdn-up.sh reads them). A cycle that fails during a coverage gap is the phone's silence, not the network's: not counted, retried on the local-refusal pace, so a radio gap across a refresh does not escalate into a fresh registration, which KPN refuses while the registration is live and drops it for (§13). New Call-ID, new challenge, new SA pair (registering fresh (recovery attempt N); with no fresh target the daemon restarts instead, §7.2 step 3).
A refused or throttled fresh attempt	Waits on the throttle schedule: 21 min, growing ×1.5 to 60 (ThrottleMinutesMin/Max). KPN refuses a fresh SA pair on an address whose registration is live (§13), and drops that registration the moment an initial REGISTER arrives.
A silent fresh attempt	Backs off 30 s → 10 min (FreshDelayMin/Max).
Retry-After	Always wins over a shorter wait, honoured up to an hour (RetryAfterMax, the schedule's longest wait); a longer one is cut to an hour and the refusal heard again then. On a refused fresh attempt it replaces the schedule's wait, shorter or longer, never below 30 s (FreshDelayMin).
A 403	A refusal like the others: never hammered (the schedule), never abandoned (a SIM that is unbarred registers within the hour).
The old registration is gone	A 403 once a fresh registration is due, a fresh attempt that got past the SA setup, a binding the network terminated (reg-event), an older address that left the netdev, or a bring-up whose resume failed while a fresh registration follows (with no fresh target a resumable pair whose attempt failed is kept for the restart; a pair nothing can resume — given up or written off by the engine, its address off the link, no state file, a file another attempt left behind, left untried by RESUME=0 — is written off and taken out of the kernel, §7.2 step 1): no reconnect over the old SA from then on. Otherwise the existing SA is still retried on the reconnect back-off while a fresh registration is deferred.
A Dial while unregistered	Brings the next recovery step forward (the reconnect, or the fresh attempt once it is due) — never a wait the network imposed (a Retry-After, or the schedule after a refused or throttled fresh attempt). imsd-ofonod refuses a Dial then and imsd-dialerd lists no modem.
Success	Resets everything.

The network's waits are holds, and a restart of imsd (an upgrade restarts it) does not cut them short: PersistHolds writes them to the holds file (HOLDS_FILE, default /run/imsd.holds) as JSON — pdn (the PDN's own address), boot (the boot id), written_at (seconds of uptime), hold, fresh_hold, throttle_minutes (the schedule's place), old_gone (the old registration is gone: a 403 once a fresh registration is due, an attempt past the SA setup, a binding the network terminated, an older address that left the netdev, and every bring-up that goes on to register fresh — its resume failed or there was nothing to resume; a bring-up with no fresh target writes old_gone when nothing can be resumed — a pair nothing can resume: its UE address left the link, no state file, a file another attempt left behind, written off before this start, given up or written off by the engine during the attempt, left untried by RESUME=0 — never for a pair kept for the restart, §7.2 step 1); the reducer's refusal count travels too (refusals): a resume refused without a Retry-After at a start with no fresh target is one refusal, the restart's is the second and RefusalsBeforeFresh gives the resume up — without it the restart loop would re-REGISTER on the same Call-ID every RestartSec, the F16 shape — and LoadHolds takes them over at the next start for the same pdn and boot id, waiting them out before the resume (a refresh's Retry-After) or before the fresh registration (a fresh attempt's schedule or Retry-After, a terminated binding's wait); with old_gone set the start does not resume over the old SAs at all: with a fresh target it waits the fresh hold out and registers fresh, with none it removes the pair and ends as §7.2 step 1 says — also after a start during an outage, whose resume and fresh attempt both fail without an answer: the next start then registers fresh while the network still holds the old registration, which KPN refuses (§13; dev phone 2026-10-04: 21 min where 0.3.9's retried resume would have recovered at once). A file without the key reads as not gone; the 0.3.9 hand-over (imsd-handover.sh) writes the key as true, since the wait it hands over follows a throttled initial REGISTER, at which KPN drops the old registration (§13). A reboot or another PDN address starts clean. The schedule's place is kept even after the holds run out, so a restart does not start it over at 21 min. When nothing is held the file is removed — when it is a regular file; /dev/null switches the knob off and anything that is not a regular file is emptied, never unlinked. The recovery policy's waits and the holds are measured on the boot clock (CLOCK_BOOTTIME), so a suspend counts; the keepalive refresh gate, the CRLF keepalive and the reg-event retry and renewal timers run on the steady clock and count awake seconds only.

The reg-event subscription (RFC 3680; the network's own view of the binding) is one live dialog per registration, subscribed anew ahead of the lifetime the S-CSCF granted. Refused or unanswered, it is retried after 60 s doubling to 10 min (a Retry-After honoured up to an hour; a registration refresh neither brings a scheduled retry forward nor starts its back-off over — the back-off resets when a subscription starts afresh or is accepted). When the S-CSCF ends it for a reason that allows renewal (RFC 6665: deactivated, timeout, giveup, probation) it is renewed; after any other reason — noresource, rejected, a reason-less end — the next registration refresh subscribes anew. An earlier subscription's NOTIFY is still read for the binding's state, but its end is not ours to renew (KPN closes a registration's previous subscription with noresource when a new one replaces it). A NOTIFY that terminates the binding triggers a fresh registration where there is a fresh target (with none a running daemon restarts, and a bring-up whose resume the NOTIFY ended idles — exit 1 instead when PCSCF_DISCOVERY=failed — §7.2 steps 1 and 3) — at once, or after the network's retry-after (up to an hour); after the call, when one is up (one exception: a REGISTER wait that still gets its 200 after such a NOTIFY, a call having kept the flow, leaves that registration standing). After rejected and unregistered it waits on the refusal schedule, a wait no Dial cuts short: the binding is gone, so nothing is retried meanwhile. One termination counts once, however many NOTIFYs report it (a repeated probation report may extend its hold from its own retry-after). The schedule is shell code in main.cpp, not a reducer (§3); its hardware coverage per arm is in §5.

A registration resumed on an older address (a pre-0.3.10 temporary one) moves to the PDN's own address in place: with its next fresh registration, or when the older address leaves the netdev first, which takes that registration with it.

7.5 Upgrades and the 0.3.9 hand-over

An apk upgrade (or an apk add of a package whose daemon an apk del left running) restarts imsd and imsd-ofonod once the transaction is done: the packages' post-upgrade/post-install scripts mark the units needs-restart (systemctl set-property … Markers=+needs-restart, after a daemon-reload when the unit is not-found because its file was just reinstalled) and pmOS's systemd trigger runs enqueue-marked. A call in progress ends. The restart resumes the registration over its existing SAs, or goes on waiting out a hold the network had set (§7.4) — or, with an upper-case, zero-padded or uncompressed PCSCF= (accepted until 0.3.12), stops at bring-up with the spelling to use (§7.2 step 3). A stopped unit keeps no marker and enqueue-marked leaves it stopped (dev phone, systemd 262rc3, 2026-10-03) — so a phone that never had /etc/imsd.env (the unit never started before 1.0.0) keeps its stopped unit through the upgrade and starts at its next boot, registering where §7.3's rule lets it — unless imsd-ofonod runs there (the Phosh image, imsd-ofono): the upgrade restarts that daemon, and its Wants=imsd.service starts imsd at once (enqueue-marked queues the marked unit's try-restart, systemd collapses it to a restart for a running unit, and a restart job pulls in a start job for each Wants= — systemd 262rc3, job_type_collapse and transaction_add_job_and_dependencies; dev phone 2026-10-11: a stopped imsd was started in the same second an imsd-ofonod restart began, two seconds after it had been stopped, and resumed) — without waiting for it: since 1.0.0 imsd-ofonod.service has no After=imsd.service (that restart would otherwise wait behind the ims PDN bring-up — the whole ExecStartPre, up to TimeoutStartSec=900), so GNOME Calls' backend starts at once and republishes when imsd's name appears (OnImsdAppeared; dev phone 2026-10-12, bench 30).

A daemon older than 0.3.10 kept its bring-up throttle wait in memory only, so the restart registered fresh at once instead of waiting the schedule out. For that one hop: imsd.post-upgrade records the running daemon's InvocationID in /run/imsd.handover (when no holds file exists); at the next start imsd-handover.sh reads that finished invocation's last bring-up line from the journal and, when it is the throttle's (bring-up deferred: fresh-SA throttle active (spi-s=0) — … next fresh registration in N min; the old daemon's other deferrals, after a 401 it could not use, are not handed over), ages it on the monotonic clock (__MONOTONIC_TIMESTAMP against /proc/timer_list; the phone's wall clock can read 1970 until its first NTP sync, and the monotonic clock stops in a suspend, so the wait can only come out long, never short of the network's window), and writes what is left to the holds file as 0.3.10 keeps it — for the address this start registers on when that is the address the wait was earned on (the old daemon logged its LOCAL= once at start); on another address — the normal hop, 0.3.9 on the kernel's temporary address and 0.3.10 on the PDN's own, which holds no registration (§13) — only the verdict that the old registration is gone is written and the new start registers fresh at once (dev phone 2026-10-03: accepted, registered 16 s after 0.3.9's refused attempt) — LOCAL as systemd hands it to the unit's commands (the /etc/imsd.env pin if there is one, else what ims-pdn-up.sh just wrote), unquoted the way imsd itself gets it. Reading at the start rather than at the upgrade means a wait that ended in between — the old daemon making another attempt — is read as it ended. When the old address cannot be read the wait is handed over anyway; when the wait cannot be timed, or this start has no LOCAL, the journal says it was not handed over.

7.6 Calls and the media plane

CallMachine owns one call's SIP dialog, outgoing or incoming, and never touches a socket, a clock or a subprocess: each event returns Actions for the shell (send this on the client flow, answer that request on the channel it came in on, start/stop the media leg, hand it the tones a key press sent, emit this D-Bus state change, arm this deadline). States and reasons are the frozen D-Bus vocabulary: dialing / ringing / incoming / active / terminated, with reasons outgoing / incoming / accepted / local-hangup / remote-hangup / refused-or-busy / error.

• Outgoing: INVITE with the SDP offer (AMR-WB + AMR + telephone-event by default; CODECS restricts and orders; PRECOND=1 adds IR.92 preconditions), reliable 18x with PRACK, 2xx ACK and media start, CANCEL before answer, BYE after. Busy and error finals end the call with refused-or-busy or error.
• Incoming: an INVITE on a protected-port listener is validated before it is announced to the UI; 100 and 180 (reliable when the caller requires 100rel); Accept sends the 200 with the SDP answer at the offer's own payload types and octet-align mode, its telephone-event echoed when the offer's m-line lists one at the codec's clock; a remote CANCEL gets 200 + 487; an unusable offer 488 naming what was offered; ring timeout 480. A landline gateway's G.711-only or AMR-NB offer rings and is answered at its own payload types.
• Emergency: a number IsEmergency classifies (112, 911, plus EMERGENCY_NUMBERS; SIM EF_ECC is not read yet) is tried as urn:service:sos first, with one fallback to a plain INVITE of the digits; a plain INVITE answered 380 Alternative Service with an emergency body is re-attempted as urn:service:sos.
• DTMF (SendDtmf): the tones go out as RFC 4733 telephone-event packets on the call's RTP stream, at the payload type the SDP negotiated for the codec's clock (MediaLeg::dtmfPt: the answer's for an outgoing call, the offer's for an incoming one; the leg takes the far end's events at that number and at MediaLeg::dtmfRxPt, DTMF_RX_PT — our own SDP's at that clock: our offer's on a call we placed (the answerer sends with the offer's numbers, RFC 3264 §6.1), the echoed offer's on one we answered (the offerer with the answer's, §5.1); the answer's instead when the answer gave the audio our offer's number, which is then the leg's audio number — never at the audio's). The engine's OnDtmf lets them through on an active call that negotiated telephone-event and drops them otherwise, with a journal line that says why (before the answer, after the call, no telephone-event at the codec's clock in the SDP, a character that is no tone or a string past DtmfMaxTones (128) — then the whole string, never part of what was pressed; letters are upper-cased, a and A are one event). There is no second method: no SIP INFO, and no tone played into the microphone path. The shell writes one dtmf <tones> line per SendDtmf down the leg's control pipe.
• Incoming calls reach imsd only when the network's T-ADS routes the call to the IMS domain; the REGISTER Contact advertises the MMTEL ICSI and +sip.instance for that. When T-ADS still picks CS, no INVITE arrives — a network/modem-integration matter, not a daemon defect.

imsd-media is one process per call: imsd-media <local-ip> <rtp-port> <remote-ip> <remote-port> <pt> <secs> <out-base>. It binds the advertised RTP port (RTP_PORT, default 50004), sends uplink frames from pw-record through the encoder, captures the downlink, and with PLAY=1 reconstructs the media clock from RTP timestamps so pw-play stays real-time-paced through far-end DTX silence. Codecs: AMR-WB (16 kHz, the default), AMR narrowband and G.711 PCMA/PCMU (8 kHz, what a PSTN gateway offers); AMR frames ride RFC 4867 payloads, octet-aligned or bandwidth-efficient (OCTET_ALIGN, mirrored from the SDP). The AMR codecs are dlopen'd (libvo-amrwbenc + libopencore-amrwb for WB, libopencore-amrnb for NB); G.711 is a table. DTMF (RFC 4733 telephone-events; DTMF_PT names their payload type, unset = none; imsd sets it from the SDP and writes the lines down the control pipe, §7.1): each dtmf <tones> line on its stdin queues the tones, and a tone takes seven 20 ms slots of the uplink 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 (RFC 4733 §2.5.1.4) — then three slots of audio before the next tone; the audio's own sender carries the events — per captured frame (a silence tick without a mic) the event packet goes in the frame's place, one packet per slot — so events and audio share the stream's sequence and timestamp space (RFC 4733 §2.1), the clock stays the capture's, and the audio after an event is stamped as if it had run through it; a line of more than 4096 bytes is dropped through its end and the next line goes. The far end's events — taken at DTMF_RX_PT (our offer's 98 or 100 on a call we placed, the number an answerer sends with, RFC 3264 §6.1, whatever the answer renumbered ours to; the offer's number we echoed on a call we answered; the answer's telephone-event number instead when the answer gave the audio our offer's number, which is then the leg's audio number) and at DTMF_PT, never at the audio's — are played as the tones they stand for (ITU-T Q.23's frequency pairs, at the packet's volume), in the playout, in step with the audio clock: the gap before the event's start (a stock sender's DTX silence often precedes a key press — 0 to 7 slots on the dev phone, 2026-10-15) is filled as before an audio frame, each packet reports the event's duration so far and what the clock has not covered is synthesized, so a retransmitted end packet adds nothing and a lost update is caught up by the next; a far end whose audio resumes past the event (the stock phone's does, §13) meets the clock with the usual fill. Exit code 3 means the downlink dried up for MEDIA_TIMEOUT seconds (default 6.0) — the far-end-hangup signal, which on carriers whose BYE never reaches our SAs is the reliable teardown trigger. Per-call stats land in OUT_DIR (default the state dir); RTP_DUMP=1 captures the raw downlink next to them.

Audio session. Call audio goes through the PipeWire of one account. imsd resolves it for each call from logind: the user session active on seat0 (loginctl show-seat seat0; a greeter does not count) when it has a /run/user/<uid>/pipewire-0 socket, and passes it to imsd-media as AUDIO_USER. AUDIO_USER set in imsd's own environment pins an account instead; one that is not an account falls back to logind. imsd as packaged (root, on the system bus) logs the resolution at start and when a call's media starts (audio session: …), imsd-media once per call. An imsd-media started without the variable at all (by hand, or by an imsd older than 0.3.10 while an upgrade replaces it) tries the account user, its old default.

7.7 Runtime files

Path	Written by	Content
/etc/imsd.env	the admin	configuration (§10), none required since 1.0.0 where discovery finds a P-CSCF (§7.3): PCSCF= overrides what discovery found, and names the P-CSCF where discovery is skipped or finds none; the file's existence changes nothing. Before 1.0.0 the apk's drop-in ConditionPathExists=/etc/imsd.env made it the switch for the whole unit (§9.2)
/run/imsd.env	ims-pdn-up.sh	DEV=, LOCAL= of the connected PDN, PCSCF_DISCOVERED= the IPv6 addresses of the PCO's P-CSCF list in the modem's order, at most eight (IPv4 entries and names are dropped; empty when the PCO gave no IPv6 P-CSCF, when discovery did not run or failed, or when PCSCF= is set), `PCSCF_DISCOVERY=found
/run/imsd.holds (HOLDS_FILE)	imsd, imsd-handover.sh	the network's holds, the schedule's place and the reducer's refusal count (§7.4)
/run/imsd.handover	imsd.post-upgrade	the InvocationID of the daemon an upgrade replaces, written when no holds file exists (§7.5); consumed by imsd-handover.sh
/run/imsd-ofonod.started	imsd-ofono-restart-calls.sh	marker: imsd-ofonod has started once since boot
/var/lib/imsd/imsreg.state (STATE_FILE)	imsd	the registration context for a warm resume
/var/lib/imsd/*.stats, *.rtp, *.raw	imsd, imsd-media	per-call media stats, raw RTP (RTP_DUMP), SIP dumps (DUMP_SIP; mode 0600 — they carry the IMSI/MSISDN)
$XDG_STATE_HOME/imsd/ (else ~/.local/state/imsd/), $XDG_RUNTIME_DIR/imsd.holds (else /tmp/imsd.holds)	imsd	the same files for a non-root (--session) run

7.8 Journal vocabulary

Lines the operator (and the scripts) look for, all prefixed imsd: :

• LOCAL=… P-CSCF=…, SIP transport: …, SIM slot=… IMSI=… domain=… at start.

• RESUME via existing SA (cseq N), REGISTERED (resumed + true-refreshed), FRESH registration, Security-Server: …, REGISTERED (fresh).

• bring-up deferred: …; next: fresh registration in N s — one line; in N min from a minute (the pre-0.3.10 form bring-up deferred: … next fresh registration in N min is what imsd-handover.sh reads from an older daemon's journal), a hold the network set before this start stands (…), the refusal schedule stands at N min from before this start: ….

• …; next: fresh registration in N s (in N min from a minute, now), registering fresh (recovery attempt N), fresh registration deferred/refused/failed: …, reconnected + re-registered, recovered by a fresh registration.

• the registration's SAs are on X, not the PDN's own address Y: resuming there until the next fresh registration, the fresh registration moves from X to the PDN's own address Y, back on X: the old registration stands until the next fresh attempt, X has left DEV: the registration on it is gone; staying on Y; next: …, X is no longer on DEV: the registration on it is gone; on the PDN's own address Y now; next: ….

• pcscf: configured X / pcscf: configured X; discovery gave Y, Z (ignored: PCSCF= wins) / pcscf: discovered Y, Z (using Y) at start, pcscf: PCSCF_DISCOVERED entry '…' is not an IPv6 address; ignored; pcscf: resuming on X, the registration's SA peer; the next fresh registration goes to Y (+ (the peer is not in today's list), or … (no P-CSCF configured or discovered)), the fresh registration moves from P-CSCF X to Y, back to P-CSCF X: the old registration stands until the next fresh attempt, no reg state file; resume falls back to fresh and the state file names another SA pair (0x…/0x…[ to X]) than the kernel's (0x…/0x… to Y): another attempt's; resume falls back to fresh ( to X only when the file records a P-CSCF — a pre-1.0.0 file does not; with no fresh target both end in nothing can resume it: the pair is removed instead), no P-CSCF for a fresh registration: none configured (PCSCF=) and none discovered (PCSCF_DISCOVERED empty — see the ims-pdn-up lines).

• PCSCF='…' is not written as ip prints it; use PCSCF=… (under bring-up FAILED:) — one accepted spelling of the setting (§7.2 step 3); the bring-up script's ims-pdn-up: PCSCF '…': write the bare address, as ip prints it (no brackets) (its own prefix: a script line) is the same rule for a bracketed value.

• engine fatal: … — exiting for systemd restart.

• reg-event SUBSCRIBE sent, and the binding's state from NOTIFY.

• the SA pair to X is nothing to resume: removing it — a bring-up with no fresh target that found a pair nothing can resume (§7.2 step 1; N xfrm entries of imsd's own with no pair to resume: removing them when the removal finds entries of imsd's own but no pair SaPeer can name — policy-less states on the UE's address, a lone or disagreeing fingerprinted policy —, §12); a resumable pair whose attempt failed is kept for the restart (resume failed: … — the restart tries the resume again, resume connect failed; the restart tries the resume again, state file transport differs from SIP_TRANSPORT; the restart tries the resume again — the last at every restart while the forced transport stands; with a fresh target each ends in the next registration is a fresh one); a registration the engine's rule wrote off — a 403 once a fresh registration is due, a binding a NOTIFY terminated under the re-REGISTER wait — is removed instead (resume failed: … — written off by the network: the pair is removed; with a fresh target … — written off by the network: the next registration is a fresh one), and so is a resume the engine gives up with no fresh target — a 401, a refusal with a Retry-After, the second refusal, the count travelling in the holds (resume failed: … — the resume is given up (the engine's rule): the pair is removed).

• audio session: … at start and at each call's media start; CODECS override active: ….

• media <codec> pt=N octet=true|false -> [X]:P, telephone-event pt N|none at a call's media start (… pt N (theirs M) when DTMF_RX_PT differs from the one we send with); call X: DTMF '…' -> media leg (telephone-event pt N) for tones the engine lets through, call X: DTMF '…' dropped: the call is <state> / … dropped: no telephone-event negotiated for this call / … dropped: 'c' is not a tone / call X: DTMF of N tones dropped: more than 128 for tones not sent, SendDtmf(X, '…') ignored: no such call; DTMF '…' not sent: no control pipe to the media leg on a call whose control pipe could not be made or that has no media leg, DTMF '…' not sent: <error> when the control pipe's write fails (EPIPE: the leg died under the action; EAGAIN: it is alive, stopped reading, its pipe full), DTMF of N tones not sent: longer than the control pipe's atomic write (a line the engine's bound should have stopped), media control pipe: …; this call has no DTMF when the pipe could not be made.

• A refused REGISTER or SUBSCRIBE is logged with its status line and any Retry-After / Warning / Reason / Server headers; re-REGISTER challenged: 401 …; re-authentication asked, the next registration is a fresh one for a challenged refresh; state file holds no challenge answer (pre-0.3.11): the refresh carries an empty digest at a resume of a registration made before 0.3.11 — every reregistration of it carries the empty digest until its next fresh registration, and the line repeats at each later resume of it; Security-Verify from the state file (1 mechanism) / (N mechanisms), state file holds no Security-Server (a pre-0.3.12 file, or a list the file cannot hold): Security-Verify rebuilt from the SA as one mechanism or state file's Security-Server names another SA pair than the one in the kernel (spi-s N vs M): Security-Verify rebuilt from the SA as one mechanism at every resume.

• connect: no answer from the P-CSCF N times (…) when the protected connect series ends unanswered.

• idle: no SIM — nothing to do; systemctl start imsd when that changes (or idle: no ims PDN from the network — … from IMSD_IDLE; idle: no P-CSCF — none configured (PCSCF=), none discovered (see the ims-pdn-up lines) — and nothing to resume; set PCSCF= in /etc/imsd.env to name one — … the daemon's own, before the bus name or from the no-target bring-up), then exit 0; no P-CSCF: discovery failed (see the ims-pdn-up lines) and nothing to resume — exiting for systemd restart, exit 1, when the reader did not answer (PCSCF_DISCOVERY=failed).

From the scripts: ims-pdn-up: … — among them no SIM (sim-missing): not bringing the ims PDN up — systemctl start imsd once one is in, bearer connect refused 10 times in a row by profile index with packet service and a persistent cause (last: …): no ims PDN for this SIM here — systemctl start imsd to try again, wrote /run/imsd.env: IMSD_IDLE=…, P-CSCF discovered: a, b (pco-requested 1, profile-pcscf-via-pco 1), P-CSCF discovery also gave v4 … (not usable on the IPv6-only ims PDN), P-CSCF discovery: nothing usable (…), P-CSCF X configured: discovery skipped, host route to P-CSCF X dev Y (the configured address or the first discovered), wrote /run/imsd.env: DEV=… LOCAL=… PCSCF_DISCOVERED=… PCSCF_DISCOVERY=… —, imsd-handover: …, imsd-ofono-restart-calls: ….

8. D-Bus ABI and desktop integration

8.1 net.catcrafts.IMS1 (frozen)

Object /net/catcrafts/IMS1, interface net.catcrafts.IMS1. The ABI is frozen — the two translation daemons depend on it, and a new desktop needs a new translation daemon, not a change here.

Member	Signature
Dial(number)	s → s (callUni)
HangUp(callUni)	s
Accept(callUni)	s
SendDtmf(callUni, tones)	ss
GetCalls()	→ aa{sv}
GetStatus()	→ a{sv}: registered (b), local (s), pcscf (s), expiry (x), cseq (x), ppi (s)
CallAdded	s a{sv}
CallStateChanged	s s s (callUni, state, reason)
CallDeleted	s
RegistrationChanged	b

Dial, HangUp, Accept and SendDtmf are gated by polkit (§7.1); a refused caller gets org.freedesktop.DBus.Error.AccessDenied, and polkit not answering is logged with the sender. A translation daemon that runs as root on the system bus must gate its own callers through imsd-bus, or it is a bypass of net.catcrafts.IMS1.Voice.

8.2 Plasma Mobile (imsd-dialerd)

A drop-in for plasma-dialer's modem-daemon, owning the same session-bus names and objects (org.kde.telephony.CallUtils at /org/kde/telephony/CallUtils/tel/mm, likewise DeviceUtils and UssdUtils; the /tel/mm suffix is hard-coded in plasma-dialer's QML plugin and kde-telephony-daemon — keep it). plasma-dialer, kde-telephony-daemon and spacebar run unmodified. ABI details verified against plasma-dialer 6.7.3 are in the header of implementations/dialerd.cpp (CallData marshalled as a struct wrapping a{sv}, struct-wrapped int32 enums in MMCallState numbering, an empty callUni meaning the active call, callsChanged with the full vector on every add/delete). It lists no modem while imsd is unregistered.

8.3 GNOME Calls, Phosh, GNOME Mobile (imsd-ofonod)

Presents imsd as an oFono modem: org.ofono with a Manager at /, one Modem + VoiceCallManager at /imsd, and a VoiceCall object per live call — what GNOME Calls' bundled ofono provider plugin drives; Calls, Phosh and GNOME Shell run unmodified. org.ofono is the seam because it is the only one a third party can implement: org.gnome.Calls.Call is what Calls exports so a shell can observe and control a call that already exists (Accept, Hangup, SendDtmf, Silence — no Dial), and Phosh consumes it from Calls — its call UI needs nothing extra once Calls has a working origin; Calls' own backends are libpeas provider plugins (mm, ofono, sip, dummy) against private headers no distro installs. Consumer-shaped details verified against calls 50.0 are in the header of implementations/ofonod.cpp. Verified on a Fairphone 6 (postmarketOS, dbus-broker) with GNOME Calls 50.0: outgoing and incoming calls with audio both ways, answered and hung up through org.gnome.Calls.Call — the surface Phosh drives. The answered calls predate the polkit gate (§5).

Setup: apk add imsd-ofono enables imsd-ofonod.service by preset and ships a gschema override setting org.gnome.Calls autoload-plugins to ['ofono'] (the per-user equivalent is gsettings set org.gnome.Calls autoload-plugins "['ofono']"). Drop mm rather than adding ofono alongside it: the ModemManager provider would offer the modem's CS voice path as a second origin, and on a phone whose voice path is imsd that origin carries no audio. Never run it beside a real ofonod (one name, one owner; the unit has Conflicts=ofono.service and the daemon exits if it loses the name). Running imsd-dialerd at the same time is fine (different bus, different consumer). One dbus-broker detail is load-bearing and lives in packaging/org.ofono.conf: dbus-broker checks a broadcast against the sender's send policy for every name the receiver owns, so root (imsd) must be allowed send_destination="org.ofono" or imsd's call signals never reach the daemon.

Known gaps, all inherited rather than introduced:

• USSD is refused with org.ofono.Error.NotSupported and not advertised in the modem's Interfaces: the IMS stack has no USSD path.
• No hold, multiparty, transfer or call waiting (org.ofono.Error.NotImplemented): imsd is a single-call stack.
• Per-call caller-ID withholding is refused rather than silently ignored: imsd cannot honour a CLIR request, and failing is safer than dialling with the caller ID exposed.
• Phosh's emergency-call screen lists nothing: imsd-ofonod publishes org.ofono.VoiceCallManager.EmergencyNumbers (112, 911, plus EMERGENCY_NUMBERS), oFono's standard place for it, but Calls' ofono provider never reads it: it makes no GetProperties call on the VoiceCallManager and returns NULL for the origin's emergency-numbers, which is what org.gnome.Calls.EmergencyCalls.GetEmergencyContacts is built from (the ModemManager provider fills its equivalent via mm_sim_dup_emergency_numbers()). An upstream fix would close this. Dialling an emergency number from the dialpad is unaffected — that reaches imsd's own classifier.
• Restarting imsd-ofonod restarts GNOME Calls with it: when the modem's VoiceCallManager interface goes away and comes back, Calls 50.0 adds a new origin but leaves the old origin's CallAdded handler connected, so after one such cycle every call is added twice, and after a few the stale handler runs on freed memory and Calls dies with SIGBUS in g_hash_table_lookup (measured on the FP6). Calls-side; an upstream fix belongs in plugins/provider/ofono/calls-ofono-provider.c. To keep imsd's own restarts and registration drops from triggering it, the VoiceCallManager interface is listed permanently (Online tracks registration; Dial fails with "imsd is not registered" / "imsd is not running"), and only this daemon's own restart cycles the origin — after which Calls dials nothing until it restarts, so the unit's ExecStartPost (imsd-ofono-restart-calls.sh) restarts the Calls daemon of every user running one. The first start since boot leaves a running Calls alone.

Development without a phone or root (crafter-build's binaries run inside the sandbox: §4.1):

./bin/imsd-*/imsd --session &            # imsd's ABI on the session bus
./bin/imsd-ofonod-*/imsd-ofonod --session
busctl --user call org.ofono / org.ofono.Manager GetModems

9. Packaging, CI and releases

9.1 The APKBUILD

packaging/APKBUILD.binary is the packaging since 2026-09-02: it wraps a crafter-build binary cross-compiled per §4.3 (bundled by packaging/make-bin-tarball.sh together with the units, scripts and policies) into the apks CI publishes. pkgver is taken from the Version constant in implementations/main.cpp by build-package.sh. Subpackages: imsd-ofono (the GNOME Calls backend, opt-in) and imsd-systemd (the unit files; install_if systemd — ordered after imsd-ofono because systemd() amoves all of usr/lib/systemd/system). There is no from-source recipe: a build needs crafter-build (§4.1).

It declares provides="81voltd=$pkgver-r$pkgrel": this satisfies soc-qcom-modem's dependency and excludes the real package, because 81voltd serves the modem's own ims-PDN requests, races imsd for the PDN and flaps it with a new prefix every ~2.5 min — two IMS stacks cannot share one PDN. depends lists modemmanager, the AMR codec libraries, pipewire-tools and polkit (the gate fails closed without it). There is no OpenRC service: the unit's sequencing is only tested under systemd.

9.2 What the package installs and enables

imsd.service is enabled by preset (80-imsd.preset, plus the multi-user.target.wants link), so VoLTE survives reboots without a manual systemctl enable. Since 1.0.0 there is no condition on /etc/imsd.env: the P-CSCF comes from the PDN's PCO, so the unit starts on every phone with the package — installing it and inserting a SIM is the opt-in; the file holds overrides only — and a phone with nothing to do (no SIM, no ims PDN from the network, no P-CSCF in the PCO and nothing to resume) exits cleanly and stays inactive (§7.3) rather than failing and restarting. systemctl mask --now imsd opts out: a plain disable removes only the [Install] links, and imsd-ofonod's static Wants=imsd.service (the Phosh image) pulls a disabled imsd back in at every boot and at its every restart (dev phone 2026-10-11, the fp6 journal's 2026-10-11-0400-bench-kpn capture: an imsd-ofonod restart started a stopped and, in the control run, a disabled imsd); the mask held through an imsd-ofonod restart and a reboot on the dev phone (2026-10-11). systemctl unmask imsd opts back in. imsd-ofonod does not wait for imsd (no After=imsd.service since 1.0.0, §7.5). Until 0.3.12 the apk shipped a drop-in ConditionPathExists=/etc/imsd.env; the upgrade removes it, and an upgraded phone that never had the file starts imsd at its next boot — or during the upgrade itself where imsd-ofonod runs (§7.5) — and registers where §7.3's rule lets it. imsd-ofonod.service is enabled the same way by imsd-ofono. The preset, the multi-user.target.wants link and the drop-in no longer shipped are the apk's doing (APKBUILD.binary's package()); the idle, the Wants= pull-in and the opt-out are the bring-up script's and the daemon's, systemd's and the admin's. The main package hides kde-telephony's modem-daemon autostart for accounts created after it is installed (/etc/skel/.config/autostart/org.kde.modem.daemon.desktop) because imsd-dialerd owns those names.

9.3 Install scripts

imsd.post-upgrade, imsd-ofono.post-upgrade and imsd-systemd.post-upgrade mark their units needs-restart (§7.5); imsd.post-upgrade also records the old invocation for the 0.3.9 hand-over. Each *.post-install is a symlink to its post-upgrade: apk del leaves the daemon running, so a later apk add is an upgrade for that daemon and apk runs post-install for it. A first install finds no unit active and marks nothing. imsd-systemd marks imsd.service too, since it ships the unit file and apk installs it after imsd.

9.4 CI and what a push means

.forgejo/workflows/package.yml runs on every push to main (and on dispatch): on the flatpak runner label (a privileged runner registered to this repository — bubblewrap inside the job container needs the privilege), in an alpine:edge container, packaging/build-package.sh installs the Crafter.Build Flatpak (net.catcrafts.CrafterBuild//stable from flatpak.catcrafts.net — the branch Crafter.Build publishes from its release tags, where beta follows every push to its master; the build log names its commit) and llvm/abuild, builds the sysroot, cross-compiles the five products, runs the suite natively inside the Flatpak, wraps the result with APKBUILD.binary, and the publish step uploads the apks to the Forgejo Alpine registry (…/api/packages/Catcrafts/alpine/edge/fp6) with PACKAGE_TOKEN. The registry answers 409 for an already-published version and the step treats that as "nothing to do", so a push releases exactly when the Version constant changes. This is the only producer of the imsd apk: fp6-img images install the published package from the registry, and installed phones upgrade with plain apk upgrade. A push to main is a release to real users' phones. A release is a range of commits ending in a bump commit titled imsd X.Y.Z that changes only the Version constant.

10. Configuration reference

Everything is environment variables. The packaged unit reads /run/imsd.env (from ims-pdn-up.sh) and then /etc/imsd.env (the user's, wins on conflict; optional since 1.0.0). The variables are user documentation: the tables are in the README.

10.1 imsd

The README's first table. The protected client/server ports are fixed at 45061/45062 (imsd::util::PortUc/PortUs) — changing them in source also requires updating the firewall rules ims-pdn-up.sh installs. PCSCF and PCSCF_DISCOVERED are read at start (FreshPcscfFromEnv, for the start line and again by the engine): the first is one address in RFC 5952 form (§7.2 step 3: another spelling is refused at bring-up), and wins, the second a comma-separated IPv6 list whose first entry CanonicalIp6 accepts is the fresh target otherwise (an IPv4 address, a name, a literal with a zone or a dotted-quad tail is skipped — logged by the engine when the daemon goes on to register, not by the idle before the bus name). A resume goes to the kernel's SA peer whatever they say (and routes it itself); beyond the resume's pcscf: line, having a fresh target decides what a refused or failed resume leads to — the fresh registration (a refusal without a Retry-After is not counted) or, with none, the pair kept for the restart, removed, or the idle (§7.2 step 1) — and a comma-separated PCSCF= stops the bring-up before the resume.

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

The README's second table. These are not configuration and are left out there:

Variable	Default	Meaning
CODEC, OCTET_ALIGN, DTMF_PT, DTMF_RX_PT	(from the SDP)	set by imsd from the SDP (DTMF_PT: the telephone-event payload type at the codec's clock the leg sends with, empty when none was negotiated; DTMF_RX_PT: a second number the leg takes events at — our own SDP's at that clock, or the answer's when the answer gave the audio our offer's number, §7.6); not configuration
MIC_SRC, PCM_DUMP	(unset)	test seams (§5)

10.3 ims-pdn-up.sh and imsd-ofonod

IMS_APN, IMS_IP_TYPE, IMS_PROFILE_ID, IMS_REG_TIMEOUT, PCSCF, IMSD_PCSCF and IMSD_ENV (the configuration file's path, a bench seam) for the bring-up script (§7.3); IMSD_IDLE and PCSCF_DISCOVERY are the script's word to the daemon, not settings. imsd-ofonod reads EMERGENCY_NUMBERS (from /etc/imsd.env through its unit) and IMSD_BUS=session|system to override only the imsd side of its bus choice.

11. Making changes

• Commit messages. Subject area: what changes — the area is the module partition, program, script or packaging/ci/tests/README the change lives in (several joined by commas), the rest a short lowercase clause with no trailing period, imperative or declarative. The body says why — the field incident or the spec clause — and, where the change alters behaviour, in one sentence how it was tested. A version bump is a commit titled imsd X.Y.Z that changes only the Version constant and has no body.
• Versioning. MAJOR.MINOR.PATCH: the patch number counts releases, fixes and features alike; the minor number moves when a release changes what a user can rely on the daemon for (0.3 added emergency calling), not with the size of a release; the major number moves when a release changes what an installed phone does without being asked, and it is not an ABI change (§8.1: the ABI is frozen). 1.0.0 is the first to move it: the unit starts on every phone with the package, with no file to write, and registers where PCSCF= names the P-CSCF or, on the FP6's qmapmux0.0 (KPN, the one device and network it ran on), where the reader gets the PCO's IPv6 list.
• No AI attribution trailers. Commits carry no Assisted-by:, Co-Authored-By: or similar lines, whoever or whatever drafted them.
• Signed commits. Every commit is GPG-signed with the repository's configured identity (git config user.signingkey, commit.gpgsign). Do not override the identity from the environment.
• One change, one commit; a fix goes into the commit that introduced the problem while a range is unpublished (rebase), never as a follow-up on top. Every commit of a range builds and passes the eight suites on its own.
• Keep make-bin-tarball.sh's file list and APKBUILD.binary's package() in sync when a packaged file is added.
• The README and this file follow the code in the same commit. A new knob gets its row in the README's configuration tables; a new state its journal line in §7.8; a new file under /run or /var/lib its row in §7.7.
• Compatibility notes are kept where the mechanism lives, not in a changelog the repository does not have: the 0.3.9 hand-over is documented in §7.5 because imsd.post-upgrade ships in every later package and the path stays live for any direct upgrade from an older daemon.
• Never commit captures or state (*.raw, *.rtp, *.stats, imsreg.state): they carry the subscriber's identity.

12. Security considerations

• imsd runs as root (xfrm, qmicli). Its attack surface on the bus is the frozen ABI; the writes are polkit-gated (§7.1, §8.1). Do not add an ungated write member, and do not add a root translation daemon that forwards calls without gating its own callers through imsd-bus.

• imsd deletes by exact key what two rules select, whoever installed it. A fresh registration's SA setup and the dead pair's teardown (RemoveDeadPair) delete — ip xfrm state delete src dst proto esp spi, ip xfrm policy delete <selector> dir — what two rules select (OwnXfrmDeleteCommands): a transport-mode policy with reqid 1 or 2 whose selector carries the protected port 45061/45062 on the UE side (imsd's fingerprint), and a transport-mode reqid-1/2 ESP state running from such a policy's source to its destination or anchored on the UE's ims-PDN address (a state whose policy add failed goes too; an AH or IPComp state is not selected). Never ip xfrm state|policy flush: a policy-based VPN's (strongSwan's, libreswan's) SAs and policies, or another transport-mode user's pair on other ports and addresses, stay. Before 1.0.0 the setup flushed the host's whole xfrm state and policy. Residual: a transport-mode policy with reqid 1 and the UE-side port 45061, or reqid 2 and 45062 — alone or paired, between any addresses — goes, with the reqid-1/2 transport-mode ESP states in its direction, and a reqid-1/2 transport-mode ESP state on the UE's ims-PDN address goes whoever made it (tests/Ipsec pins both).

• The bus policies deliberately let any local account reach both names; authorization is polkit's, not the policy file's. Keep it that way: a policy that names an account breaks the moment the account is called something else.

• SIP dumps and the state file carry the IMSI, the MSISDN, SPIs and addresses, and the state file (since 0.3.11) the last challenge answer — nonce, cnonce and response — and (since 0.3.12) the P-CSCF's Security-Server list. Dumps are mode 0600 under the state dir, off by default (DUMP_SIP=0), and never leave the phone in a commit.

• The ims PDN is a walled garden: ims-pdn-up.sh keeps it from becoming the default route and gives the P-CSCF a host route — the configured one, or the first IPv6 address the modem reports from the PCO (the fresh target; at most eight are handed over, IPv4 entries and names dropped); imsd adds one per call for the media peer (§7.3) and one for the P-CSCF it registers to or resumes on (§7.2 step 3). Nothing else is routed over it. A change that routes anything else over it needs a reason and a bench.

• The fixed protected ports (45061/45062) are opened in nftables for qmapmux* only, by the bring-up script.

• imsd-pcscf holds a QMI WDS client of its own on the modem, as root, from ims-pdn-up.sh. It binds to the bearer's mux data port and reads the call's runtime settings; it never sends Start Network or Stop Network. Its only writes are its own client's mux binding and IP family; the call stayed connected and the registration stood after it (§13). One client socket, closed on exit (the service lookup's own socket closes when the lookup ends).

13. Carrier and platform facts the code relies on

Observed on real networks and encoded in the policy; cite them when touching the related code.

• KPN's fresh-SA throttle — the bring-up deferred: fresh-SA throttle active (spi-s=0) case — refuses a fresh SA pair on an address while the registration on that address is live, whatever its age or last refresh: refused 11 s, 23 min and 2 h 58 min into a registration, 4 and 6.6 min after its last refresh (2026-09-19, -20, -24, 2026-10-03). KPN drops that registration the moment the initial REGISTER arrives, throttled or not, and with it gone accepts a fresh pair there: 5 and 21 min after the throttled attempt (2026-10-03). An address that holds no registration is accepted while another address's is live (2026-09-26, 2026-10-03), and a new prefix's PDN address was accepted 15 s after a throttled attempt on the temporary one (the 0.3.9 → 0.3.10 hop, 2026-10-03). An address whose registration another address's had replaced was refused 26 s after, accepted 141 s and 10 min after (2026-10-03): how soon KPN accepts where a registration has just gone is not measured more closely. The schedule's 21 min is the wait that has never been refused, not a measured minimum; imsd resumes rather than registers fresh wherever it can, since on a live registration's address the attempt is refused and costs the registration.
• O2 UK, 2026-09-24: a refresh refused 21 × 503 over 70 min, then 33 unanswered reconnect cycles; a fresh registration was accepted in one second. The origin of RegRecovery. On 2026-10-08, under 0.3.10 (the first release to log a Retry-After), a refresh was refused 503 with Retry-After 414 s and the refresh after that wait 503 with Retry-After 772 s: the origin of the Retry-After escalation. Every refresh of the pre-0.3.11 shape — an initial registration's empty digest and the in-use SPIs in Security-Client — on record from O2 was refused with 503: 21 on 2026-09-24 under 0.3.9 and the same loop again that afternoon on a stationary phone, two on 2026-10-08 under 0.3.10, and the user never saw one succeed, while the fresh registrations that followed were accepted: the origin of the 5.1.1.4.2 refresh shape. KPN accepts both shapes without a challenge.
• KPN closes a registration's previous reg-event subscription with noresource when a new one replaces it: that end is not renewed.
• Modem attach-family forcing (Odido, O2 UK, Swisscom): an APN-string IPv6 bearer request is refused when the attach PDN was granted IPv4-only; a profile-indexed request is not.
• pw-record delivers 20 ms frames in bursts — of two on an AMR-WB call (1.8 frames per burst on average, the bursts ~37 ms apart; the dev phone, 2026-10-15/16: of the uplink's inter-packet gaps 42–45 % are under 4 ms and 50–52 % are 30–40 ms, 3–8 % lie between and under 1 % above — the fp6 journal's 2026-10-16-dtmf-bench-14/uplink-cadence.txt over benches 4, 5, 13 and 14, counted by the cadence.py beside it), of three or four on a narrowband one (3.3 on average, ~68 ms apart; 70 % under 4 ms, 28 % over 40 ms — 2026-10-16-dtmf-bench-17-heard/uplink-cadence-narrowband.txt, bench 17's two calls; the burst sizes of all six legs in burst-sizes.txt beside it). A sender on a clock of its own cannot count the frames a 140 ms window holds, so the events ride the frames' own slots (§7.6), and an event's packets leave in those bursts: the audio's own spacing, which RFC 4733 §2.5.1.2 calls the natural interval for event updates (its 50 ms recommendation is for a spacing of the sender's own).
• DTMF on KPN (2026-10-14 to 16): the answer to our offer echoes telephone-event at our payload types — 98 at 16 kHz on an AMR-WB call to voicemail, 100 at 8 kHz on a phone-to-phone call the network answered narrowband AMR at 99 — and a stock phone's terminating INVITE offers AMR-WB 107 with telephone-event 111; events of the sender's first shape (100 ms, a packet every 20 ms, at least 100 ms apart) were heard on that phone (bench 2 the first shape; bench 17 the one-sender shape — 28 keys on two narrowband calls, every one heard there). On its own AMR-WB calls to us (telephone-event 111 at 16 kHz) that phone sends one event of 20 packets per key press (340 ms, -10 dBm0, an update every 20 ms, the end packet three times), its audio resuming one to eight slots past the event's end and the event preceded by a DTX gap of up to seven slots (the fp6 journal's benches 2 to 4 and bench 10's incoming call, their far-end-events-* files); on a call we placed that the network answered narrowband (telephone-event 100 at 8 kHz) its two key presses arrived as events of 22 and 19 packets (400 and 340 ms), the audio resuming 15.8 and 19 slots (316 and 380 ms) past their ends, the first off the stream's 20 ms timestamp grid (bench 10's outgoing call) — whether that shape is the phone's or the network's interworking, nothing on disk says.
• pmOS's default nftables drops inbound on qmapmux*; without the protected-port rule every network-initiated request (NOTIFY, MT INVITE) dies after ESP decapsulation and MT calls fall back to CS.
• The kernel's temporary SLAAC address on the ims netdev is listed first and retired after about two days; a registration bound to it dies with it.
• systemd reads EnvironmentFile= per spawned command (systemd 262~rc3 on the FP6, 2026-10-03: an ExecStart saw what its ExecStartPre had just written), and unquotes a quoted value — which is why the hand-over script takes LOCAL from its environment rather than parsing the files itself.
• GNOME Calls 50.0 crashes after an oFono origin withdraw/re-add cycle and dials nothing after org.ofono restarts until it restarts itself.
• dbus-broker checks broadcasts against the sender's send policy for every name the receiver owns (the org.ofono.conf detail).
• The phone's wall clock reads 1970 until the first NTP sync; anything that ages a journal line uses the monotonic or boot clock.
• A WDS client bound to the bearer's mux data port, with IP family 6 set, reads the call's runtime settings; one that sent neither the binding nor Set IP Family got OutOfCall. ModemManager owns the ims bearer's call and never asks the modem for the P-CSCF fields; a second client that does Bind Mux Data Port (the netdev's rmnet mux_id, endpoint embedded/1 for the ipa driver — ModemManager's own binding), Set IP Family 6 and Get Current Settings gets them: TLV 0x22 = 1, the IPv6 list in TLV 0x2E (two addresses on KPN), no IPv4 list, an empty FQDN list. Get Current Settings from a client that sent neither Bind Mux Data Port nor Set IP Family answered OutOfCall (the control run), as did a client bound to a mux with no call (--mux 7). The bearer stayed connected and the registration stood afterwards (dev phone, 2026-10-11; the fp6 journal's 2026-10-11-pcscf-pco-probe capture). libqmi does not model 0x2E and qmicli cannot print any of it, hence imsd-pcscf.