diff --git a/mplex/README.md b/mplex/README.md
index 2bb1d36..f86bf0c 100644
--- a/mplex/README.md
+++ b/mplex/README.md
@@ -62,7 +62,8 @@ flag = header & 0x07
 id = header >> 3
 ```
 
-The maximum header length is 9 bytes (per the unsigned-varint spec). With 9 continuation bits and 3 message flag bits  the maximum stream ID is 60 bits (maximum value of `2^60 - 1`).
+- Implementations MUST support up to 28bit stream IDs (32bit headers pre-encoding, 5 bytes after varint encoding).
+- Implementations SHOULD support up to 60bit stream IDs (64bit headers pre-encoding, 9 bytes after varint encoding).
 
 ### Flag Values