Every OPC UA Connection Protocol Message has a header with the fields defined in Table 68.
Table 68 – OPC UA Connection Protocol Message header
|
Name |
Type |
Description |
|
MessageType |
Byte [3] |
A three byte ASCII code that identifies the Message type. The following values are defined at this time: ACK an Acknowledge Message. RHE a ReverseHello Message. The SecureChannel layer defines additional values which the OPC UA Connection Protocol layer shall accept. |
|
Reserved |
Byte [1] |
Ignored. shall be set to the ASCII codes for ‘F’ if the MessageType is one of the values supported by the OPC UA Connection Protocol. |
|
MessageSize |
UInt32 |
The length of the Message, in bytes. This value includes the 8 bytes for the Message header. |
The layout of the OPC UA Connection Protocol Message header is intentionally identical to the first 8 bytes of the OPC UA Secure Conversation Message header defined in Table 53. This allows the OPC UA Connection Protocol layer to extract the SecureChannel Messages from the incoming stream even if it does not understand their contents.
The OPC UA Connection Protocol layer shall verify the MessageType and make sure the MessageSize is less than the negotiated ReceiveBufferSize before passing any Message onto the SecureChannel layer.