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engine/sdp: CODECS override for the codecs we offer and accept

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KPN's interconnect gateway transcodes every caller up: a G.711-only
fixed-line INVITE reached the phone offering PCMA, PCMU, AMR and AMR-WB
(bench call 2026-09-08), so the AMR-WB-only build rang and the narrowband
path — the one the Telia field report hit with 488 — cannot be reached on
the air through KPN by any caller.

CODECS=<list> (comma-separated over AMR-WB, AMR/AMR-NB, PCMA, PCMU) makes
the list both the codecs offered on an outgoing call and the acceptance
preference for an inbound offer, in that order: CODECS=PCMA takes G.711
A-law out of the mixed offer above, CODECS=PCMA,PCMU sends a G.711-only
offer toward the network. Unset, nothing changes — the default offer
bytes and the AMR-WB > AMR > PCMA > PCMU preference stay pinned. The
daemon logs an active override at startup.
This commit is contained in:
Jorijn van der Graaf 2026-09-08 17:53:26 +02:00
commit 37ec8660b7
6 changed files with 270 additions and 67 deletions
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156
interfaces/Imsd-Sdp.cppm
View file

@ -5,15 +5,18 @@
/*
Imsd:Sdp — SDP offer/answer building + parsing for the voice media plane.
The offer is a fixed AMR-WB (octet-aligned) + AMR + telephone-event shape
with an optional GSMA IR.92 QoS-precondition block; it is what commercial IMS
cores expect from a VoLTE UE, and its exact bytes are pinned by tests/Sdp. The
parser extracts only what the media plane needs: remote address/port, the
negotiated payload type, the codec and octet-align mode — it reads answers
to our offers and inbound offers alike (same fields either way). The codecs
it knows are the ones imsd-media can play: AMR-WB, AMR (narrowband) and
G.711 (PCMA/PCMU) — the last two are what a PSTN gateway offers when a
landline calls. BuildAnswer is the terminating side: it answers an inbound
The offer is by default an AMR-WB (octet-aligned) + AMR + telephone-event
shape with an optional GSMA IR.92 QoS-precondition block; it is what
commercial IMS cores expect from a VoLTE UE, and its exact bytes are pinned by
tests/Sdp. The parser extracts only what the media plane needs: remote
address/port, the negotiated payload type, the codec and octet-align mode — it
reads answers to our offers and inbound offers alike (same fields either way).
The codecs it knows are the ones imsd-media can play: AMR-WB, AMR (narrowband)
and G.711 (PCMA/PCMU) — the last two are what a PSTN gateway offers when a
landline calls. An explicit codec list (the daemon's CODECS override) replaces
both the offered set and the acceptance preference: it is how the narrowband
path is exercised against a network whose gateway transcodes every caller up
to AMR-WB. BuildAnswer is the terminating side: it answers an inbound
offer with the offer's own payload type numbers (RFC 3264) and mirrors its
octet-align mode (RFC 4867 requires both directions to use one mode). Pure
std C++.
@ -49,20 +52,52 @@ namespace imsd::sdp {
export namespace imsd::sdp {
// The codec names imsd-media plays, in the default acceptance order:
// wideband beats narrowband, AMR beats G.711 (a mobile-to-mobile call
// must never land on a G.711 leg just because the gateway listed it).
constexpr std::array<std::string_view, 4> kCodecs{"AMR-WB", "AMR", "PCMA", "PCMU"};
// A codec preference list from its textual form ("PCMA, amr-nb"):
// case-insensitive, spaces ignored, AMR-NB accepted for AMR, unknown
// names and repeats dropped. Empty in = empty out = the defaults.
std::vector<std::string> ParseCodecList(std::string_view spec) {
std::vector<std::string> out;
for (auto part : std::views::split(spec, ',')) {
std::string n;
for (char c : std::string_view(part))
if (c != ' ' && c != '\t') n += static_cast<char>(std::toupper(static_cast<unsigned char>(c)));
if (n == "AMR-NB") n = "AMR";
if (std::ranges::find(kCodecs, n) == kCodecs.end()) continue;
if (std::ranges::find(out, n) == out.end()) out.push_back(n);
}
return out;
}
struct Offer {
std::string local; // connection address (v4 or v6)
int rtpPort = 0;
std::uint64_t sessionId = 0; // o= sess-id/version; caller randomizes
bool precond = false; // IR.92 QoS preconditions block
std::string currRemote = "none"; // a=curr:qos remote <this>
std::vector<std::string> codecs; // offered codecs in order; empty = AMR-WB, AMR
};
// Our offer: AMR-WB (octet-aligned) first, AMR narrowband second — the
// 3GPP-mandatory codec every IMS core and PSTN gateway can answer, so a
// call we place TO a landline no longer depends on the far side
// transcoding up to wideband — each with telephone-event at its own
// clock. Byte layout is pinned by tests/Sdp.
// clock. Byte layout is pinned by tests/Sdp. An explicit `codecs` list
// offers exactly those, in that order, with fixed payload types (97
// AMR-WB, 99 AMR, 8 PCMA, 0 PCMU) and telephone-event only at the clocks
// in use. G.711 is not offered by default: a VoLTE UE offering PCMA
// deviates from stock shapes for no gain.
std::string BuildOffer(const Offer& o) {
static const std::vector<std::string> kDefaultOffer{"AMR-WB", "AMR"};
const std::vector<std::string>& codecs = o.codecs.empty() ? kDefaultOffer : o.codecs;
bool wb = std::ranges::find(codecs, "AMR-WB") != codecs.end();
bool amr = std::ranges::find(codecs, "AMR") != codecs.end();
bool nb = codecs.size() > (wb ? 1u : 0u); // anything besides AMR-WB is 8 kHz
auto pt = [](const std::string& c) { return c == "AMR-WB" ? 97 : c == "AMR" ? 99 : c == "PCMA" ? 8 : 0; };
std::string ipver = Is6(o.local) ? "IP6" : "IP4";
std::string s;
s += "v=0\r\n";
@ -70,18 +105,33 @@ export namespace imsd::sdp {
s += "s=-\r\n";
s += std::format("c=IN {} {}\r\n", ipver, o.local);
s += "t=0 0\r\n";
s += std::format("m=audio {} RTP/AVP 97 99 98 100\r\n", o.rtpPort);
s += "b=AS:41\r\n";
std::string pts;
for (const auto& c : codecs) pts += std::format(" {}", pt(c));
if (wb) pts += " 98";
if (nb) pts += " 100";
s += std::format("m=audio {} RTP/AVP{}\r\n", o.rtpPort, pts);
s += std::format("b=AS:{}\r\n", wb ? 41 : amr ? 30 : 80);
s += "b=RS:512\r\n";
s += "b=RR:1536\r\n";
s += "a=rtpmap:97 AMR-WB/16000/1\r\n";
s += "a=fmtp:97 octet-align=1;mode-change-capability=2;max-red=0\r\n";
s += "a=rtpmap:99 AMR/8000/1\r\n";
s += "a=fmtp:99 octet-align=1;mode-change-capability=2;max-red=0\r\n";
s += "a=rtpmap:98 telephone-event/16000\r\n";
s += "a=fmtp:98 0-15\r\n";
s += "a=rtpmap:100 telephone-event/8000\r\n";
s += "a=fmtp:100 0-15\r\n";
for (const auto& c : codecs) {
if (c == "AMR-WB") {
s += "a=rtpmap:97 AMR-WB/16000/1\r\n";
s += "a=fmtp:97 octet-align=1;mode-change-capability=2;max-red=0\r\n";
} else if (c == "AMR") {
s += "a=rtpmap:99 AMR/8000/1\r\n";
s += "a=fmtp:99 octet-align=1;mode-change-capability=2;max-red=0\r\n";
} else {
s += std::format("a=rtpmap:{} {}/8000/1\r\n", pt(c), c);
}
}
if (wb) {
s += "a=rtpmap:98 telephone-event/16000\r\n";
s += "a=fmtp:98 0-15\r\n";
}
if (nb) {
s += "a=rtpmap:100 telephone-event/8000\r\n";
s += "a=fmtp:100 0-15\r\n";
}
s += "a=ptime:20\r\n";
s += "a=maxptime:240\r\n";
if (o.precond) {
@ -144,16 +194,16 @@ export namespace imsd::sdp {
}
// Payload-type preference: AMR-WB with octet-align=1 > AMR-WB > AMR with
// octet-align=1 > AMR > PCMA > PCMU. Octet-aligned wins within a codec
// because it is the media leg's native, call-proven format
// (bandwidth-efficient is supported but was the s56 static-audio
// octet-align=1 > AMR > PCMA > PCMU (kCodecs order). Octet-aligned wins
// within a codec because it is the media leg's native, call-proven
// format (bandwidth-efficient is supported but was the s56 static-audio
// culprit) — when the peer offers both variants, taking the
// octet-aligned pt keeps real calls on the battle-tested path. Wideband
// beats narrowband, AMR beats G.711 (a mobile-to-mobile call must never
// land on a G.711 leg just because the gateway listed it). With no
// playable codec at all, the first pt is used and codec comes back EMPTY
// — pretending otherwise would defeat codec checks upstream.
Answer ParseAnswer(std::string_view body) {
// octet-aligned pt keeps real calls on the battle-tested path. A
// non-empty `prefs` replaces the codec order (and restricts acceptance
// to the codecs named) — the CODECS override. With no acceptable codec
// at all, the first pt is used and codec comes back EMPTY — pretending
// otherwise would defeat codec checks upstream.
Answer ParseAnswer(std::string_view body, std::span<const std::string> prefs = {}) {
Answer a;
if (auto at = Find(body, "c=IN IP6 ")) a.ip = TokenAt(body, *at + 9);
else if (auto at4 = Find(body, "c=IN IP4 ")) a.ip = TokenAt(body, *at4 + 9);
@ -184,39 +234,29 @@ export namespace imsd::sdp {
std::string_view line = body.substr(*fm, lineEnd == std::string_view::npos ? std::string_view::npos : lineEnd - *fm);
return line.contains("octet-align=1");
};
int wbOa = -1;
int wb = -1;
int nbOa = -1;
int nb = -1;
int pcma = -1;
int pcmu = -1;
struct Seen { int first = -1; int oa = -1; };
std::map<std::string, Seen> seen; // codec name -> first pt, first octet-aligned pt
for (int p : pts) {
std::string name = CodecName(body, p);
if (name == "AMR-WB") {
if (wb == -1) wb = p;
if (wbOa == -1 && hasOctetAlign(p)) wbOa = p;
} else if (name == "AMR") {
if (nb == -1) nb = p;
if (nbOa == -1 && hasOctetAlign(p)) nbOa = p;
} else if (name == "PCMA") {
if (pcma == -1) pcma = p;
} else if (name == "PCMU") {
if (pcmu == -1) pcmu = p;
}
if (std::ranges::find(kCodecs, name) == kCodecs.end()) continue;
Seen& s = seen[name];
if (s.first == -1) s.first = p;
if (s.oa == -1 && name.starts_with("AMR") && hasOctetAlign(p)) s.oa = p;
}
if (wbOa != -1 || wb != -1) {
a.payloadType = wbOa != -1 ? wbOa : wb;
a.codec = "AMR-WB";
} else if (nbOa != -1 || nb != -1) {
a.payloadType = nbOa != -1 ? nbOa : nb;
a.codec = "AMR";
} else if (pcma != -1) {
a.payloadType = pcma;
a.codec = "PCMA";
} else if (pcmu != -1) {
a.payloadType = pcmu;
a.codec = "PCMU";
auto pick = [&](std::string_view name) {
auto it = seen.find(std::string(name));
if (it == seen.end()) return false;
a.payloadType = it->second.oa != -1 ? it->second.oa : it->second.first;
a.codec = std::string(name);
return true;
};
bool found = false;
if (prefs.empty()) {
for (std::string_view n : kCodecs) if ((found = pick(n))) break;
} else {
for (const std::string& n : prefs) if ((found = pick(n))) break;
}
if (!found) {
a.payloadType = pts.front();
a.codec.clear();
}