rtph264: accept non-compliant single FU-A frames (#649)

Some IP cameras (e.g. CostarHD) have been observed to emit single
fragment RTP FU-A packets, with start and end bit both set, for
sufficiently small H.264 P-frames. Rather than emit an error,
workaround such non-compliant frames.

pkg/format/rtph264/decoder.go

@@ -90,10 +90,6 @@ func (d *Decoder) decodeNALUs(pkt *rtp.Packet) ([][]byte, error) {
 		if start == 1 {
 			d.resetFragments()
 
-			if end != 0 {
-				return nil, fmt.Errorf("invalid FU-A packet (can't contain both a start and end bit)")
-			}
-
 			nri := (pkt.Payload[0] >> 5) & 0x03
 			typ := pkt.Payload[1] & 0x1F
 			d.fragmentsSize = len(pkt.Payload[1:])
@@ -101,6 +97,20 @@ func (d *Decoder) decodeNALUs(pkt *rtp.Packet) ([][]byte, error) {
 			d.fragmentNextSeqNum = pkt.SequenceNumber + 1
 			d.firstPacketReceived = true
 
+			// RFC 6184 clearly states:
+			//
+			//   A fragmented NAL unit MUST NOT be transmitted in one FU; that is, the
+			//   Start bit and End bit MUST NOT both be set to one in the same FU
+			//   header.
+			//
+			// However, some vendors camera (e.g. CostarHD) have been observed to nevertheless
+			// emit one fragmented NAL unit for sufficiently small P-frames.
+			if end != 0 {
+				nalus = [][]byte{joinFragments(d.fragments, d.fragmentsSize)}
+				d.resetFragments()
+				break
+			}
+
 			return nil, ErrMorePacketsNeeded
 		}
 

pkg/format/rtph264/decoder_test.go

@@ -79,6 +79,38 @@ func TestDecodeCorruptedFragment(t *testing.T) {
 	require.Equal(t, [][]byte{{0x01, 0x00}}, nalus)
 }
 
+func TestDecodeNoncompliantFragment(t *testing.T) {
+	d := &Decoder{}
+	err := d.Init()
+	require.NoError(t, err)
+
+	nalus, err := d.Decode(&rtp.Packet{
+		Header: rtp.Header{
+			Version:        2,
+			Marker:         true,
+			PayloadType:    96,
+			SequenceNumber: 18853,
+			Timestamp:      1731630255,
+			SSRC:           0x466b0000,
+		},
+
+		// FU-A with both start and end bit intentionally set
+		// While not compliant with RFC 6184, IP cameras from some vendors
+		// (e.g. CostarHD) have been observed to produce such FU-A payloads for
+		// sufficiently small P-frames.
+		Payload: mergeBytes(
+			[]byte{
+				0x3c,       // FU indicator
+				0xc1,       // FU header (start and end bit both intentionally set)
+				0xe7, 0x00, // DON
+				0xca, 0xfe, // Payload
+			},
+		),
+	})
+	require.NoError(t, err)
+	require.Equal(t, [][]byte{{0x21, 0xe7, 0x00, 0xca, 0xfe}}, nalus)
+}
+
 func TestDecodeSTAPAWithPadding(t *testing.T) {
 	d := &Decoder{}
 	err := d.Init()