The meta data is encoded into the Variable Header to carry Values which
can be left to take a default value if they are not present. Initially it
will be described for the PUBLISH packet but might be extended to other
packets as well. Each metadata value has an Identifier and an associated Value.
The Identifier also describes the data type of the Value.
1) Create new data types in "1.5 Data representations"
- Make the Remaining Length construct, a defined data type so that
it can be used to encode the number of bytes containing the metadata,
as well as for its original purpose.
Move the description of Remaining Length from section "2 MQTT
Control Packet format" to section "1.5 Data representations" as a datatype
called Variable Byte Integer.
- Add a Four Byte Integer data type type. This will be used initially
to decribe expiry intervals. This is in addition to the existing
Two Byte Integer value.
2) Update the description of the PUBLISH Packet to include the
Identifier/Value pairs which can be present in the Variable Header.
- The Variable header contains the following fields in the
following order:
Topic Name, Packet Identifier, Length in bytes of Id/Value pairs,
Identifier/Value pairs.
- After the description of Packet Identifier, add:
x.x.x.x Length of Id/Value pairs.
The Length of the Identifier/Value pairs in the PUBLISH
Packet variable header encoded as a Variable Byte Integer.
The Length does not include the
bytes used to encode the Length. A Length of
zero indicates that there are no Identifier/Value pairs in the
PUBLISH packet. The Id/Value pairs can appear in any order.
x.x.x.x Identifier/Value pairs.
An Id/Value pair consists of an Id which defines the usage and data type,
followed by the value itself.
The Id is encoded as a Variable Byte Integer.
x.x.x.x.x Identifier: Payload Format Indicator.
If absent the PUBLISH payload consists of unspecified bytes.
Byte 1 (0x01) Identifier of the Payload Format Indicator. It is a protocol error
to include the Payload Format Indicator more than once.
Followed by the Value of the Payload Format Indicator, either of:
Byte 0 (0x00) Indicates that the payload is unspecified bytes, this is
equivalent to not sending a Payload Format Indicator.
Byte 1 (0x01) Indicates that the payload is UTF-8 Encoded Character Data.
Note that the UTF-8 Data in the payload does not include a
length prefix, nor is it subject to the restrictions
described in section 1.5.3.
A server MUST send the Payload Format Indicator unaltered to all
subscribers receiving the publication, using this version of MQTT.
The receiver is not required to validate that the payload is of
the format indicated, however it MAY close the network connection if it
discovers that the payload does not match the Payload Format
Indicator.
The server MUST send the publication to an MQTT client using the
MQTT V3.1.1 protocol with a matching subscription, in this case no
Payload Format Indicator is included.
On receipt of any other Identifier or Value, the receiver MUST close the
network connection.
- Update 3.3.2.3 Variable header non normative example to show zero
Id/Value pairs.
Non normative comment
In this version of the specification only one byte Id's are used.
Note:
1) Jira MQTT-288 will consider making a unified and organised set of Metadata Id's
for use across all packet types in a separate Jira once the complete set of
values is known.
2) Jira MQTT-282 will consider moving existing values into the optional
metadata in .
3) Further Message format types such as JSON and Base64 may also
be added in future.
4) Jira MQTT-236 will provide a better mechanism for indicating protocol
violations in metadata.
Critical