Every MessageChunk has a Message header with the fields defined in Table 53.
Table 53 – OPC UA Secure Conversation Message Header
|
Name |
Data Type |
Description |
|
MessageType |
Byte [3] |
A three byte ASCII code that identifies the Message type. The following values are defined at this time: MSGA Message secured with the keys associated with a channel. OPN OpenSecureChannel Message. CLO CloseSecureChannel Message. |
|
IsFinal |
Byte |
A one byte ASCII code that indicates whether the MessageChunk is the final chunk in a Message. The following values are defined at this time: C An intermediate chunk. F The final chunk. A The final chunk (used when an error occurred and the Message is aborted). This field is only meaningful for MessageType of ‘MSG’ This field is always ‘F’ for other MessageTypes. |
|
MessageSize |
UInt32 |
The length of the MessageChunk, in bytes. The length starts from the beginning of the MessageType field. |
|
SecureChannelId |
UInt32 |
A unique identifier for the SecureChannel assigned by the Server. If a Server receives a SecureChannelId which it does not recognize it shall return an appropriate transport layer error. When a Server starts the first SecureChannelId used should be a value that is likely to be unique after each restart. This ensures that a Server restart does not cause previously connected Clients to accidently ‘reuse’ SecureChannels that did not belong to them. |