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7.1.2.2 Message Header

Every OPC UA Connection Protocol Message has a header with the fields defined in Table 65.

Table 65 – 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:

HEL a Hello Message.

ACK an Acknowledge Message.

ERR an Error 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 50. 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.

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