# MQTT Protocol Guide ## Server or Broker A program or device that acts as an intermediary between Clients which publish Application Messages and Clients which have made Subscriptions. A Server Accepts Network Connections from Clients. Accepts Application Messages published by Clients. Processes Subscribe and Unsubscribe requests from Clients. Forwards Application Messages that match Client Subscriptions. Client ----> Broker(Server) ----> Client Publisher ----> Broker -----> Subscriber ## Subscription and Session ### Subscription A Subscription comprises a Topic Filter and a maximum QoS. A Subscription is associated with a single Session. A Session can contain more than one Subscription. Each Subscription within a session has a different Topic Filter. ### Session A stateful interaction between a Client and a Server. Some Sessions last only as long as the Network Connection, others can span multiple consecutive Network Connections between a Client and a Server. ## Topic Name and Filter An expression contained in a Subscription, to indicate an interest in one or more topics. A Topic Filter can include wildcard characters. ## Packet Identifier