Acknowledge

The Server replies to an hello message with an ack message.

The OPC UA Acknowledge message

The hello and acknowledge messages are used to negotiate buffer size, message size, and the maximum number of chunks per message to use. The fields are identical to the hello message, only the MessageType is now ‘ACK’ and the EndpointUrl is not needed.

So the ACK message contains the following fields:

Parameter	Datatype	Value	Comments
MessageType	3 ACII bytes	ACK	from acknowledge
IsFinal	1 ASCII byte	F	from final
MessageSize	ULInt32	32 (fixed size!)
ProtocolVersion	ULInt32	0	uncertified stack
ReceiverBufferSize	ULInt32	65536
SendBufferSize	ULInt32	65536
MaxMessageSize	ULInt32	0	unlimited
MaxChunkCount	ULInt32	0	unlimited

Construct

Using Construct we can create a struct, containing these fields.

from construct import *
c = Struct('OPC UA TCP Acknowledge Message',
        String('MessageType', 3),
        String('Reserved', 1),
        ULInt32('MessageSize'),
        ULInt32('ProtocolVersion'),
        ULInt32('ReceiveBufferSize'),
        ULInt32('SendBufferSize'),
        ULInt32('MaxMessageSize'),
        ULInt32('MaxChunkCount'),
    )