Fix RTP header extension length calculation (RFC 3550 compliance) (#1556)

benjamin-hoeve · Copilot · web-flow · commit 1080fa968260 · 2026-04-15T13:20:43.000+01:00
* Fix RTP header extension length calculation (RFC 3550 compliance)

- The extension length field specifies the number of 32-bit words, not bytes
- Multiply the length field by 4 to convert from 32-bit words to bytes

* Review suggestion: fix for misleading variable name

Co-authored-by: Copilot Autofix powered by AI &lt;175728472+Copilot@users.noreply.github.com&gt;

---------

Co-authored-by: Copilot Autofix powered by AI &lt;175728472+Copilot@users.noreply.github.com&gt;
diff --git a/src/SIPSorcery/net/DtlsSrtp/Lib/SRTP/Readers/RtpReader.cs b/src/SIPSorcery/net/DtlsSrtp/Lib/SRTP/Readers/RtpReader.cs
@@ -45,12 +45,10 @@ public static int ReadExtensionsLength(ReadOnlySpan<byte> payload)
             int length = ReadHeaderLenWithoutExtensions(payload);
             if ((payload[0] & 0x10) == 0x10)
             {
-                return 4 + System.Buffers.Binary.BinaryPrimitives.ReadUInt16BigEndian(payload.Slice(length + 2, 2));
-            }
-            else
-            {
-                return 0;
+                var extensionLengthWords = System.Buffers.Binary.BinaryPrimitives.ReadUInt16BigEndian(payload.Slice(length + 2, 2));
+                return 4 + (extensionLengthWords * 4);
             }
+            return 0;
         }
 
         public static int ReadHeaderLenWithoutExtensions(ReadOnlySpan<byte> payload)

Original file line number	Diff line number	Diff line change
`@@ -45,12 +45,10 @@ public static int ReadExtensionsLength(ReadOnlySpan<byte> payload)`
`45`	`45`	`int length = ReadHeaderLenWithoutExtensions(payload);`
`46`	`46`	`if ((payload[0] & 0x10) == 0x10)`
`47`	`47`	`{`
`48`		`- return 4 + System.Buffers.Binary.BinaryPrimitives.ReadUInt16BigEndian(payload.Slice(length + 2, 2));`
`49`		`- }`
`50`		`- else`
`51`		`- {`
`52`		`- return 0;`
	`48`	`+ var extensionLengthWords = System.Buffers.Binary.BinaryPrimitives.ReadUInt16BigEndian(payload.Slice(length + 2, 2));`
	`49`	`+ return 4 + (extensionLengthWords * 4);`
`53`	`50`	`}`
	`51`	`+ return 0;`
`54`	`52`	`}`
`55`	`53`
`56`	`54`	`public static int ReadHeaderLenWithoutExtensions(ReadOnlySpan<byte> payload)`