snapshot of https://github.com/livekit/livekit.git @ 08c5433449c106943231037d0b892895f6e92d3b (2026-09-15)

pkg/sfu/packettrailer/packet_trailer.go

@@ -0,0 +1,48 @@
+// Copyright 2026 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package packettrailer
+
+var Magic = [4]byte{'L', 'K', 'T', 'S'}
+
+const (
+	xorByte = 0xFF
+
+	envelopeSize = 5 // 1B trailer_len + 4B magic
+)
+
+// StripTrailer returns the number of bytes to strip from the end of an RTP
+// payload if it contains an LKTS trailer. The trailer is located by checking
+// for the "LKTS" magic suffix and then reading the XORed trailer_len byte
+// immediately before it. isEndOfFrame must be set only for packets ending an
+// encoded frame, i. e. where a trailer could have been appended. Returns 0 if
+// absent or ineligible.
+func StripTrailer(payload []byte, isEndOfFrame bool) int {
+	if !isEndOfFrame || len(payload) < envelopeSize {
+		return 0
+	}
+
+	tail := payload[len(payload)-4:]
+	if tail[0] != Magic[0] || tail[1] != Magic[1] ||
+		tail[2] != Magic[2] || tail[3] != Magic[3] {
+		return 0
+	}
+
+	trailerLen := int(payload[len(payload)-5] ^ xorByte)
+	if trailerLen < envelopeSize || trailerLen > len(payload) {
+		return 0
+	}
+
+	return trailerLen
+}