Message Header

IOTMP Message Header

The header is the mandatory first part of every IOTMP message. It provides the essential metadata required to parse the message:

• Message Type – identifies the purpose of the message.

• Message Size – defines the length of the message body in bytes (excluding the header).

The header is encoded using varint fields, resulting in a minimum size of 2 bytes.

Field	Type	Mandatory	Description
Message Type	varint	Yes	Specifies the Message Type.
Message Size	varint	Yes	Specifies the Message Size, without taking into account the header size.

Message Type

The Message Type field identifies the purpose of the IOTMP message. It is always present in the header and is encoded as a varint.

Message types define the control flow of the protocol, covering actions such as connection management, resource execution, error handling, and data streaming.

The protocol reserves specific numeric values for core operations, while leaving others available for future extensions. This ensures both compatibility and extensibility as new functionality is introduced.

The currently defined message types are:

Message Type	Value	Description
Reserved	0x00	Reserved field, not used
Ok	0x01	Acknowledges success
Error	0x02	Indicates an error occurred
Connect	0x03	Initiates a connection and specifies its parameters
Disconnect	0x04	Terminates the current connection
Keep Alive	0x05	Keep the connection open
Run Resource	0x06	Execute a specific resource on the client or server
Describe Resources	0x07	Request a description of available resources
Start Stream	0x08	Starts a stream on the specified resource
Stop Stream	0x09	Stops an active stream
Stream Data	0x0A	Transmits data for an active stream

Message Size

The Message Size field specifies the length of the message body, expressed in bytes. It does not include the size of the header itself.

• Encoded as a varint.

• Always present in the message header.

• A value of 0 indicates that the message has no body (only a header).

This field enables the receiver to determine exactly how many bytes to read for the payload and when a message is complete.