The frame format in HTTP/2 looks like this (source: HTTP/2: Frame Format):
+-----------------------------------------------+
| Length (24) |
+---------------+---------------+---------------+
| Type (8) | Flags (8) |
+-+-------------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+=+=============================================================+
| Frame Payload (0...) ...
+---------------------------------------------------------------+
R: A reserved 1-bit field. The semantics of this bit are undefined, and the bit MUST remain unset (0x0) when sending and MUST be ignored when receiving.
Stream Indentifier: A stream identifier (see Section 5.1.1) expressed as an unsigned 31-bit integer. The value 0x0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream.
Is there a reason why they did not use a 32-bit unsigned integer? And why specifying that a reserved bit has to be set to 0 and must be ignored by the receiver?
Is it just a concession to languages like Java, which do not have a 32-bit unsigned integer?
