Incorrect handling of Topic Alias? #2199
Description
Am I right that here:
the server simply copies Topic Alias used by the message publisher to messages sent to subscribers? In other words, subscribers get the same Topic Alias property that was present in the original published message. Correct?
If so, I suspect such a behavior is against the MQTT v5 specs. If I understand it correctly, the sender (in this case a client that publishes a message) can set a Topic Alias in a PUBLISH and then in subsequent PUBLISH messages the Topic Alias can be used instead of the Topic Name.
The sender decides whether to use a Topic Alias and chooses the value. It sets a Topic Alias mapping by including a non-zero length Topic Name and a Topic Alias in the PUBLISH packet. The receiver processes the PUBLISH as normal but also sets the specified Topic Alias mapping to this Topic Name.
If a Topic Alias mapping has been set at the receiver, a sender can send a PUBLISH packet that contains that Topic Alias and a zero length Topic Name. The receiver then treats the incoming PUBLISH as if it had contained the Topic Name of the Topic Alias.
However, the spec also says:
Topic Alias mappings exist only within a Network Connection and last only for the lifetime of that Network Connection. A receiver MUST NOT carry forward any Topic Alias mappings from one Network Connection to another [MQTT-3.3.2-7].
This means to me that the Topic Alias mapping between the message publisher and the server (where the client is the sender and the server is the receiver) is NOT the same mapping as between the server and a subscriber (where the server is the sender and the subscriber is the receiver). The Topic Alias mapping is specific to a given pair of sender and receiver.
For example, client1 publishes a message on topicA setting a Topic Alias 1, while client2 publishes a message on the same topicA setting a Topic Alias 10. Messages on topicA forwarded from client1 to client2 will contain Topic Alias 1 even though for client2 such a Topic Alias is unknown.
Furthermore:
The Topic Alias mappings used by the Client and Server are independent from each other. Thus, when a Client sends a PUBLISH containing a Topic Alias value of 1 to a Server and the Server sends a PUBLISH with a Topic Alias value of 1 to that Client they will in general be referring to different Topics.
Am I missing anything?