The reg-event subscription is the network's own account of our binding
(TS 24.229 5.1.1.3), and the daemon opened a new dialog for it at every
refresh and reconnect and otherwise never looked back: a refused
SUBSCRIBE was forgotten, an unanswered one had no timeout, an ended
subscription got nothing until the next refresh's new dialog, and a
NOTIFY was logged whatever it said. O2 UK, 2026-09-24: a fresh
registration's SUBSCRIBE got 500, a NOTIFY arrived all the same and
showed our contact active, the flow died eight seconds after the 200 and
no connect over the SA succeeded after that; the 500 was logged and
nothing was done about it.
Keep the subscription alive: retry a refused or unanswered SUBSCRIBE on
a backoff, renew one the S-CSCF ends with deactivated, timeout, giveup
or probation (RFC 6665 4.1.3; after any other reason, or none, the next
refresh subscribes anew), and subscribe anew ahead of the lifetime the
S-CSCF granted. A refresh or a reconnect opens no dialog while imsd
holds one for the registration or has a retry scheduled.
Read the reginfo for our own contact and act on its state: a NOTIFY that
terminates our binding schedules a fresh registration — at once for
deactivated or expired, after the network's retry-after (up to an hour)
for probation, on imsd's own refusal schedule (21 min, growing to 60)
for rejected or unregistered (RFC 3680). A hold already running stands,
and a REGISTER wait a call kept open that still gets its 200 after such
a NOTIFY leaves that registration standing. The parser is in Imsd:Sip
and pinned by tests/Sip with the O2 document shapes; the policy is
RegRecovery's (OnBindingTerminated, added here with its tests in
tests/Engine).
Tested on the dev phone (KPN) in builds of this series; not every path
has run there.
A refused final was logged as its bare status code ("re-REGISTER failed
503", "reg-event SUBSCRIBE -> 500") — the unprotected REGISTER's as
"expected 401", without even the code — so a field journal could not say
which node refused or what it said. O2 UK answered a phone's 30-minute
refresh with 503 twenty-one times over seventy minutes and then refused
a fresh registration's reg-event SUBSCRIBE with 500 (alyx, 2026-09-24);
the journal holds nothing but the numbers.
Log the status line with the headers that carry the reason —
Retry-After, Warning, Reason, Server — for a REGISTER's refused final
(initial, protected, refresh) and for a refused SUBSCRIBE the run loop
reads, and dump the refused message under DUMP_SIP as its own fail dump
next to the 200s.
To tell a REGISTER's final from what else arrives, the final is now
matched by Call-ID and CSeq, and a request that arrives on the client
flow while one is awaited is dispatched: a refresh no longer reads such
a request as a failed REGISTER. An INVITE among those requests is a call
like any other, announced on the bus: a Dial whose reconnect it arrived
under meets the call-in-progress rule again before it places its own,
and when a resume, a refresh or a reconnect fails the call that arrived
under it ends with the flow it came in on (486 while the flow still
carries one, and the bus told). The unprotected REGISTER's reply is
matched the same way on the init port.
Tested on the dev phone (KPN) in builds of this series; not every path
has run there.
imsd's protected leg (the second REGISTER and everything after it) was
TCP only. A P-CSCF that never answers the TCP connect on its protected
server port (O2 UK: the SYNs leave ESP-protected, nothing comes back)
left the unit looping with no way forward, although the SAs, the
listener sockets and the firewall rule already covered UDP.
The client flow now carries a transport. UDP binds the same protected
client port and connect()s the datagram socket to the P-CSCF's protected
server port, so the kernel delivers that peer's datagrams to it ahead of
the unconnected listener on the same port. One datagram is one message
(RFC 3261 18.3: a Content-Length that fits truncates, one that does not
fit discards the datagram, none means the rest of the datagram); a
receive error is logged and marks the flow dead; a message over the
single-packet ESP budget at the ims PDN's MTU is logged once per flow,
since the kernel fragments it and a P-CSCF may drop the fragments.
Every protected request's Via follows the transport; the challenge
stays UDP.
SIP_TRANSPORT selects the policy: auto (default) registers over TCP and,
after two unanswered connects, registers again over UDP (new challenge,
new SA pair) - but only on a phone whose state file does not record a
successful TCP registration, so an outage on a TCP carrier never becomes
a second initial REGISTER; a phone whose last registration was UDP goes
straight to UDP. tcp and udp force one. A connect refused locally
(EADDRNOTAVAIL: the 4-tuple still in TIME_WAIT from the previous flow)
keeps retrying and is not counted as silence; SO_ERROR results are
logged by name. The registration's transport is persisted so a warm
resume reconnects the same way, and the resume gives up after two
silences.
Two things the change exposed and fixes: an engine-fatal event exited 0
("exiting for systemd restart" with Restart=on-failure never firing),
and a fresh registration refused with the P-CSCF's fresh-SA throttle
(Security-Server spi-s=0) must not be retried by a 120-s restart loop,
since every attempt re-arms the ~20-min window - it is now waited out
in-process, growing on repeats, with D-Bus commands still served. A
resume the network refuses falls through to a fresh registration. A
retransmitted 200 OK to our INVITE is ACKed again and no longer starts
a second media leg.
Verified on KPN: resume over TCP after a binary swap, MT and MO calls
with media both ways, the throttle deferral and its self-recovery, the
UDP client path up to KPN dropping the datagram. The UDP success path
is a field test; SIP retransmission timers over UDP are not in this
change.
Userspace VoLTE/IMS daemon for mainline Linux phones (developed on the
Fairphone 6): a GLib-free core (SIP, SDP, USIM AKA, IPsec SA setup, RTP
media, call engine) behind a GDBus control daemon, a standalone AMR-WB
media leg, and a Plasma Dialer backend. C++26 modules built with Crafter
Build; the tree is clean under the project's house-style linter
(crafter-build lint) across all three build products.
Assisted-by: Claude:claude-fable-5
Signed-off-by: Jorijn van der Graaf <jorijnvdgraaf@catcrafts.net>
