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fix(webrtc): detect STAP-A IDR start in keyframe cache (issue #18)
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Extend isH264IDRStart to handle STAP-A aggregates (NAL type 24, RFC 6184
§5.7.1). The first NAL in the aggregate starts at byte 3 (after the
2-byte size field); if its type is 5 (IDR slice) the packet is treated
as an IDR start and the burst cache is reset.

This closes the gap noted in NOTES.md: a publisher using STAP-A for IDR
(e.g. a custom GStreamer pipeline or hardware encoder) will now correctly
reset the burst rather than accumulating packets until hitting the 512-
packet / 2 MiB capacity cap.
This commit is contained in:
Zac Gaetano 2026-05-10 13:19:56 -04:00
parent 891f65dff6
commit 8266ca72e6
1 changed files with 10 additions and 8 deletions
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18
core/webrtc/keyframecache.go
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@ -38,15 +38,15 @@ func newKeyFrameCache() *keyFrameCache {
}
// isH264IDRStart returns true if pkt begins an H.264 IDR (keyframe)
// NAL. It recognises:
// NAL. It recognises three RFC 6184 packetisation modes:
//
// - Single NAL unit packets where NAL type == 5 (IDR slice).
// - FU-A fragments with the start bit set and inner NAL type == 5.
//
// STAP-A aggregates that happen to lead with an IDR NAL are not
// detected here; in practice FFmpeg and GStreamer never use STAP-A for
// IDR slices because the slices are too large. If that assumption
// changes, add STAP-A handling in a future revision.
// - Single NAL unit (type 5): the entire payload is one IDR slice.
// - FU-A fragment (type 28): the FU header byte has the start bit set
// (0x80) and the inner NAL type is 5.
// - STAP-A aggregate (type 24): the first NAL in the aggregate is an
// IDR slice. STAP-A format: byte 0 = NAL header (type 24), bytes
// 1–2 = first NAL size (big-endian uint16), byte 3 = first NAL
// header. Minimum valid payload: 4 bytes.
func isH264IDRStart(pkt *rtp.Packet) bool {
p := pkt.Payload
if len(p) == 0 {
@ -56,6 +56,8 @@ func isH264IDRStart(pkt *rtp.Packet) bool {
switch nalType {
case 5: // Single NAL unit, IDR slice
return true
case 24: // STAP-A — bytes 1–2 are the first NAL's size; byte 3 is its header
return len(p) >= 4 && p[3]&0x1F == 5
case 28: // FU-A — byte 1 is the FU header: bit 7 = start, bits 4–0 = inner type
return len(p) >= 2 && p[1]&0x80 != 0 && p[1]&0x1F == 5
}