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sip: UDP for the protected leg, TCP first with a fallback

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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.
This commit is contained in:
Jorijn van der Graaf 2026-09-17 14:58:46 +02:00
commit 5c89dcb033
Signed by: jorijnvdgraaf
GPG key ID: 2937E59CDCC1BCFB
8 changed files with 292 additions and 29 deletions
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15
tests/Messages/main.cpp
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@ -192,6 +192,21 @@ int main() {
Reg2, "protected REGISTER");
}
// ---- protected-leg transport follows Context.transport (SIP_TRANSPORT=udp,
// or the auto fallback): every protected request's Via says UDP, the
// challenge's Via stays UDP either way, and TCP is the untouched default.
{
Context cu = c;
cu.transport = "UDP";
std::string auth = AuthAka(cu, "NONCEXYZ", "CNONCE0123456789", "RESP0000");
std::string r2u = BuildRegisterProtected(cu, "REGCALLID@2001:db8::db43", "FTAG5678", "REGBRANCH123456", 2, 7449812u, 176466735u, auth);
Check(r2u.contains("\r\nVia: SIP/2.0/UDP [2001:db8::db43]:45062;branch=z9hG4bKREGBRANCH123456;rport\r\n"), "protected REGISTER Via follows transport=UDP");
Check(!r2u.contains("SIP/2.0/TCP"), "no TCP token left in a UDP protected REGISTER");
Check(BuildSubscribeReg(cu, "SUBCALLID@2001:db8::db43", "STAG1234", "SUBBRANCH0000000000").contains("\r\nVia: SIP/2.0/UDP [2001:db8::db43]:45062;"), "SUBSCRIBE Via follows transport=UDP");
Check(BuildRegisterInitial(cu, "REGCALLID@2001:db8::db43", "FTAG5678", "REGBRANCH123456", 7449812u, 176466735u).contains("\r\nVia: SIP/2.0/UDP [2001:db8::db43]:5060;"), "initial REGISTER Via is UDP regardless");
Check(BuildRegisterProtected(c, "REGCALLID@2001:db8::db43", "FTAG5678", "REGBRANCH123456", 2, 7449812u, 176466735u, auth).contains("\r\nVia: SIP/2.0/TCP [2001:db8::db43]:45062;"), "default transport stays TCP");
}
// ---- RuriFor variants
Eq(RuriFor("1233", c.id.domain), "sip:1233;phone-context=ims.mnc001.mcc001.3gppnetwork.org@ims.mnc001.mcc001.3gppnetwork.org;user=phone", "ruri short code");
Eq(RuriFor("+31612345678", c.id.domain), "sip:+31612345678@ims.mnc001.mcc001.3gppnetwork.org;user=phone", "ruri E.164");