FB_IotMqtt5ConnAckProperties

The function block enables the reception of status information from the message broker directly after the Connection Acknowledgement, which is part of the connection setup. This allows the message broker to communicate information to the client about potential constraints, such as a maximum packet size that the message broker can handle.

It is the client's task to query these values and to react to them if necessary. If the client has received ConnAck properties and makes them available at the output, this is indicated by bPropertiesAvailable=TRUE.

General description of the Connection Acknowledgement Properties:

Property	Description
Retain available	Specifies whether the message broker supports retain messages.
Server KeepAlive	Specifies the KeepAlive time assigned by the server.
Shared subscriptions available	Specifies whether the message broker supports shared subscriptions.
Wildcard subscriptions available	Specifies whether the message broker supports wildcard subscriptions, e.g. subscriptions to a # or + topic.
Authentication data	Contains the authentication data, depending on the authentication method used.
Maximum packet size	Specifies the maximum packet size of the control packet. If this value is not specified, then no maximum size is explicitly set.
Maximum QoS	Specifies the maximum QoS level that the message broker supports. For example, some message broker implementations do not support QoS 2.
Reason code	Optional reason code that the message broker can transmit to the client as part of the ConnAck procedure. In conjunction with the Server Reference (see below), the message broker can use this to inform the client, for example, that it is temporarily or permanently unavailable and/or can be reached at a new address.
Maximum receive	The server uses this property to limit the number of QoS1 and QoS2 publishes it is willing to process for the client simultaneously.
Session expiry interval	The server can use this to inform the client that it is using a different Session Expiry Interval than originally requested by the client.
Response info	Optional return information from the message broker to the client. A use case for this can be, for example, that the broker informs the client about a topic area which has been assigned to the client and which it can access.
Maximum topic alias	Specifies the maximum value that can be used for a topic alias.
Assigned client ID	Returns the ClientID assigned by the message broker if the client has not specified its own ClientID.
Server reference	Optional (address) reference to another message broker, e.g. for Reason Code 0x9C or 0x9D (Server unavailable, Server has moved).
User Properties	User Properties are key/value pairs that can be attached to the PublishProperties. This is done by means of the function block FB_IotMqtt5UserProperties. The meaning of the UserProperties is not part of the MQTT5 specification and therefore application specific.

Syntax

Definition:

FUNCTION_BLOCK FB_IotMqtt5ConnAckProperties EXTENDS FB_IotMqtt5UserProperties
VAR_OUTPUT
    bError           : BOOL;
    hrErrorCode      : HRESULT;
END_VAR

Outputs

Name	Type	Description
bError	BOOL	Becomes TRUE as soon as an error situation occurs.
hrErrorCode	HRESULT	Returns an error code if the bError output is set. An explanation of the possible error codes can be found in the Appendix.

Properties

Name	Type	Access	Description
bPropertiesAvailable	BOOL	Get	Indicates whether properties are present.
bRetainAvailable	BOOL	Get	Specifies whether the message broker supports retain messages.
bServerKeepAlive	BOOL	Get	Indicates whether a different KeepAlive value is used by the server than was originally requested by the client.
bSessionPresent	BOOL	Get	Indicates whether the client already has a session with the broker.
bSharedSubAvailable	BOOL	Get	Specifies whether the message broker supports shared subscriptions.
bSubIdAvailable	BOOL	Get	Specifies whether the message broker supports the use of Subscription Identifiers.
bWildcardSubAvailable	BOOL	Get	Specifies whether the message broker supports Wildcard Subscriptions.
nAuthDataSize	UINT	Get	Specifies the size of the Authentication Data.
nMaxPackateSize	UDINT	Get	Specifies the maximum packet size of the control packet.
nMaxQoS	BYTE	Get	Specifies the maximum QoS level that the message broker supports.
nReasonCode	BYTE	Get	Specifies an optional reason code that the server transmits to the client as part of the ConnAck procedure.
nReceiveMax	UINT	Get	Maximum number of simultaneous QoS 1 and 2 publishes that the server supports.
nResponseInfoSize	UINT	Get	Returns the size of the response information.
nSessionExpiryInterval	UDINT	Get	Value in seconds for the Session Expiry Interval supported by the server.
nTopicAliasMax	UINT	Get	Maximum value for the Topic Alias that the server supports.
sAssignedClientId	STRING	Get	Client ID assigned by the message broker.
sAuthMethod	STRING	Get	Returns the name of the authentication method used.
sReasonString	STRING	Get	Readable variant of the reason code.
sServerReference	STRING	Get	Optional Server Reference, the format is not defined by the specification.

Methods

Name	Description
GetAuthData	Returns the authentication data, which depends on the used authentication method.
GetResponseInfo	Allows the response information to be read out. This allows the message broker to transmit to the client a basis for generating the response topic in the Req/Res procedure. The format is not defined by the specification.
SetConnAckProperties	Enables the setting of the ConnAck properties.

Requirements

Development environment	Target platform	PLC libraries to include
TwinCAT v3.1.4026.0	IPC or CX (x86, x64, ARM)	Tc3_IotBase (>= v3.4.2.0)