Figure 12 illustrates the structure of a Message placed on the wire. This also illustrates how the Message elements defined by the OPC UA Binary Encoding mapping (see 5.2) and the OPC UA Secure Conversation mapping (see 6.7) relate to the OPC UA Connection Protocol Messages.
Figure 12 – OPC UA Connection Protocol Message structure
Every OPC UA Connection Protocol Message has a header with the fields defined in Table 50.
Table 50 – 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 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 41. 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.
The Hello Message has the additional fields shown in Table 51.
Table 51 – OPC UA Connection Protocol Hello Message
|
Name |
Data Type |
Description |
|
ProtocolVersion |
UInt32 |
The latest version of the UACP protocol supported by the Client. The Server may reject the Client by returning Bad_ProtocolVersionUnsupported. If the Server accepts the connection is responsible for ensuring that it returns Messages that conform to this version of the protocol. The Server shall always accept versions greater than what it supports. |
|
ReceiveBufferSize |
UInt32 |
The largest MessageChunk that the sender can receive. |
|
SendBufferSize |
UInt32 |
The largest MessageChunk that the sender will send. |
|
MaxMessageSize |
UInt32 |
The maximum size for any response Message. The Server shall abort the Message with a Bad_ResponseTooLarge Error Message if a response Message exceeds this value. The mechanism for aborting Messages is described fully in 6.7.3. The Message size is calculated using the unencrypted Message body. A value of zero indicates that the Client has no limit. |
|
MaxChunkCount |
UInt32 |
The maximum number of chunks in any response Message. The Server shall abort the Message with a Bad_ResponseTooLarge Error Message if a response Message exceeds this value. The mechanism for aborting Messages is described fully in 6.7.3. A value of zero indicates that the Client has no limit. |
|
EndpointUrl |
String |
The URL of the Endpoint which the Client wished to connect to. The encoded value shall be less than 4 096 bytes. Servers shall return a Bad_TcpEndpointUrlInvalid Error Message and close the connection if the length exceeds 4 096 or if it does not recognize the resource identified by the URL. |
The EndpointUrl parameter is used to allow multiple Servers to share the same endpoint on a machine. The process listening (also known as the proxy) on the endpoint would connect to the Server identified by the EndpointUrl and would forward all Messages to the Server via this socket. If one socket closes, then the proxy shall close the other socket.
If the Server does not have sufficient resources to allow the establishment of a new SecureChannel it shall immediately return a Bad_TcpNotEnoughResources Error Message and gracefully close the socket. Client should not overload Servers that return this error by immediately trying to create a new SecureChannel.
The Acknowledge Message has the additional fields shown in Table 52.
Table 52 – OPC UA Connection Protocol Acknowledge Message
|
Name |
Type |
Description |
|
ProtocolVersion |
UInt32 |
The latest version of the UACP protocol supported by the Server. If the Client accepts the connection is responsible for ensuring that it sends Messages that conform to this version of the protocol. The Client shall always accept versions greater than what it supports. |
|
ReceiveBufferSize |
UInt32 |
The largest MessageChunk that the sender can receive. This value shall not be larger than what the Client requested in the Hello Message. |
|
SendBufferSize |
UInt32 |
The largest MessageChunk that the sender will send. This value shall not be larger than what the Client requested in the Hello Message. |
|
MaxMessageSize |
UInt32 |
The maximum size for any request Message. The Client shall abort the Message with a Bad_RequestTooLarge StatusCode if a request Message exceeds this value. The mechanism for aborting Messages is described fully in 6.7.3. The Message size is calculated using the unencrypted Message body. A value of zero indicates that the Server has no limit. |
|
MaxChunkCount |
UInt32 |
The maximum number of chunks in any request Message. The Client shall abort the Message with a Bad_RequestTooLarge StatusCode if a request Message exceeds this value. The mechanism for aborting Messages is described fully in 6.7.3. A value of zero indicates that the Server has no limit. |
The Error Message has the additional fields shown in Table 53.
Table 53 – OPC UA Connection Protocol Error Message
|
Name |
Type |
Description |
|
Error |
UInt32 |
The numeric code for the error. This shall be one of the values listed in Table 55. |
|
Reason |
String |
A more verbose description of the error. This string shall not be more than 4 096 bytes. A Client shall ignore strings that are longer than this. |
The socket is always closed gracefully by the Client after it receives an Error Message.
The ReverseHello Message has the additional fields shown in Table 54.
Table 54 – OPC UA Connection Protocol ReverseHello Message
|
Name |
Data Type |
Description |
|
ServerUri |
String |
The ApplicationUri of the Server which sent the Message. The encoded value shall be less than 4 096 bytes. Client shall return a Bad_TcpEndpointUrlInvalid error and close the connection if the length exceeds 4 096 or if it does not recognize the Server identified by the URI. |
|
EndpointUrl |
String |
The URL of the Endpoint which the Client uses when establishing the SecureChannel. This value shall be passed back to the Server in the Hello Message. The encoded value shall be less than 4 096 bytes. Clients shall return a Bad_TcpEndpointUrlInvalid error and close the connection if the length exceeds 4 096 or if it does not recognize the resource identified by the URL. This value is a unique identifier for the Server which the Client may use to look up configuration information. It should be one of the URLs returned by the GetEndpoints Service. |
For connection based protocols, such as TCP, the ReverseHello Message allows Servers behind firewalls with no open ports to connect to a Client and request that the Client establish a SecureChannel using the socket created by the Server.
For message based protocols the ReverseHello Message allows Servers to announce their presence to a Client. In this scenario, the EndpointUrl specifies the Server’s protocol specific address and any tokens required to access it.