CONNACK properties

The last field in the Variable header of CONNACK packet is a set of Properties. A set contains a Property Length followed by the Properties. A Property consists of an Identifier and a value.

This section lists all possible CONNACK Properties and describes their usage:

Table 1.6. CONNACK properties

Identifier	Value type	Description
session_expiry_interval	`uint32_t`	Represents the Session Expiry Internal in seconds.
receive_maximum	`uint16_t`	The maximum number of QoS 1 and QoS 2 publications that the Server is willing to process concurrently.
maximum_qos	`uint8_t`	The highest QoS the Server supports.
retain_available	`uint8_t`	A value of 0 means that retained message are not supported. A value of 1 means they are supported.
maximum_packet_size	`uint32_t`	The maximum `packet size` in bytes as defined by the specification that the Server is willing to accept.
assigned_client_identifier	`std::string`	The Client Identifier which was assigned by the Server because a zero length Client Identifier was found in the `CONNECT` packet
topic_alias_maximum	`uint16_t`	The highest value that the Server will accept as a Topic Alias sent by the Client.
reason_string	`std::string`	A UTF-8 Encoded String representing the reason associated with this response.
user_property	`std::pair<std::string, std::string>`	Name, value pair (`UTF-8 String Pair`) defining User Property. There can be multiple pairs in one packet. The meaning of these properties is not defined by the specification.
wildcard_subscription_available	`uint8_t`	A value of 0 means that Wildcard Subscriptions are not supported. A value of 1 means they are supported. If not present, they are supported.
subscription_identifier_available	`uint8_t`	A value of 0 means that Subscriptions Identifiers are not supported. A value of 1 means they are supported. If not present, they are supported.
shared_subscription_available	`uint8_t`	A value of 0 means that Shared Subscriptions are not supported. A value of 1 means they are supported. If not present, they are supported.
server_keep_alive	`uint16_t`	The Keep Alive time assigned by the Server.
response_information	`std::string`	A UTF-8 Encoded String which is used as the basis for creating a Response Topic.
server_reference	`std::string`	A UTF-8 Encoded String which can be used by the Client to identfy another Server to use.
authentication_method	`std::string`	A UTF-8 Encoded String containing the name of the authentication method used for extended authentication.
authentication_data	`std::string`	Binary Data containing authentication data. The contents of the data are defined by the authentication method.

Usage

After obtaining an instance of async_mqtt5::connack_props, the subscript operator can be used to access a Property.

The Identifiers listed in the table above are available within the async_mqtt5::prop namespace for Property access.

	Note
	When accessing a property value, the subscript operator will return a `std::optional` of the value type for all properties, except for `async_mqtt5::prop::user_property`, where it will return an instance of `std::vector<std::pair<std::string, std::string>>`.

Example

The following example shows how to set a Property value:

async_mqtt5::connack_props props;
props[async_mqtt5::prop::maximum_packet_size] = 65535;
props[async_mqtt5::prop::assigned_client_identifier] = "ClientID";
props[async_mqtt5::prop::user_property].emplace_back("name", "value");

The following example shows how to retrieve a Property value:

std::optional<std::string> auth_method = props[async_mqtt5::prop::authentication_method];
if (auth_method.has_value())
	// authentication method property was previously set
else
	// authentication method property was not set

std::vector<std::pair<std::string, std::string>>& user_props = props[async_mqtt5::prop::user_property];
if (!user_props.empty())
	// user property was previously set
else
	// user property was not set