The following new features are
added to MQTT v5.0
·
Session expiry
Split the Clean Session flag into a Clean Start flag which
indicates that the session should start without using an existing session, and
a Session Expiry interval which says how long to retain the session after a
disconnect. The session expiry interval can be modified at disconnect. Setting
of Clean Start to 1 and Session Expiry Interval to 0 is equivalent in MQTT
v3.1.1 of setting Clean Session to 1.
·
Message expiry
Allow an expiry interval to be set when a message is published.
·
Reason code on all ACKs
Change all response packets to contain a reason code. This include
CONNACK, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBACK, UNSUBACK, DISCONNECT, and
AUTH. This allows the invoker to determine whether the requested function
succeeded.
·
Reason string on all ACKs
Change most packets with a reason code to also allow an optional
reason string. This is designed for problem determination and is not intended
to be parsed by the receiver.
·
Server disconnect
Allow DISCONNECT to be sent by the Server to indicate the reason the
connection is closed.
·
Payload format and content type
Allow the payload format (binary, text) and a MIME style content
type to be specified when a message is published. These are forwarded on to the
receiver of the message.
·
Request / Response
Formalize the request/response pattern within MQTT and provide the
Response Topic and Correlation Data properties to allow response messages to be
routed back to the publisher of a request. Also, add the ability for the Client
to get configuration information from the Server about how to construct the
response topics.
·
Shared Subscriptions
Add shared subscription support allowing for load balanced
consumers of a subscription
·
Subscription ID
Allow a numeric subscription identifier to be specified on a SUBSCRIBE,
and returned on the message when it is delivered. This allows the Client to
determine which subscription or subscriptions caused the message to be
delivered.
·
Topic Alias
Decrease the size of the MQTT packet overhead by allowing the
topic name to be abbreviated to a small integer. The Client and Server
independently specify how many topic aliases they allow.
·
Flow control
Allow the Client and Server to independently specify the number of
outstanding reliable messages (QoS>0) they allow. The sender pauses sending
such messages to stay below this quota. This is used to limit the rate of
reliable messages, and to limit how many are in flight at one time.
·
User properties
Add User Properties to most packets. User properties on PUBLISH
are included with the message and are defined by the Client applications. The
user properties on PUBLISH and Will Properties are forwarded by the Server to
the receiver of the message. User properties on the
CONNECT, SUBSCRIBE, and UNSUBSCRIBE packets are defined by the Server
implementation. The user properties on CONNACK PUBACK, PUBREC, PUBREL, PUBCOMP,
SUBACK, UNSUBACK and AUTH packets are defined by the sender, and are unique to
the sender implementation. The meaning of user properties is not defined by
MQTT.
·
Maximum Packet Size
Allow the Client and Server to independently specify the maximum
packet size they support. It is an error for the session partner to send a
larger packet.
·
Optional Server feature availability
Define a set of features which the Server does not allow and
provide a mechanism for the Server to specify this to the Client. The features
which can be specified in this way are: Maximum QoS, Retain Available, Wildcard
Subscription Available, Subscription Identifier Available, and Shared
Subscription Available. It is an error for the Client to use features that the
Server has declared are not available.
It is possible in earlier versions of MQTT for a Server to not
implement a feature by declaring that the Client is not authorized for that
function. This feature allows such optional behavior to be declared and adds
specific Reason Codes when the Client uses one of these features anyway.
·
Enhanced authentication
Provide a mechanism to enable challenge/response style
authentication including mutual authentication. This allows SASL style
authentication to be used if supported by both Client and Server, and includes
the ability for a Client to re-authenticate within a connection.
·
Subscription options
Provide subscription options primarily defined to allow for
message bridge applications. These include an option to not send messages
originating on this Client (noLocal), and options for handling retained
messages on subscribe.
·
Will delay
Add the ability to specify a delay between the end of the
connection and sending the will message. This is designed so that if a
connection to the session is re-established then the will message is not sent.
This allows for brief interruptions of the connection without notification to
others.
·
Server Keep Alive
Allow the Server to specify the value it wishes the Client to use
as a keep alive. This allows the Server to set a maximum allowed keepalive and
still have the Client honor it.
·
Assigned ClientID
In cases where the ClientID is assigned by the Server, return the
assigned ClientID. This also lifts the restriction that Server assigned
ClientIDs can only be used with Clean Session=1 connections.
·
Server reference
Allow the Server to specify an
alternate Server to use on CONNACK or DISCONNECT. This can be used as a
redirect or to do provisioning.
