What properties make MQTT have a high latency?

Question

The only possible reason that I could think of is the low overhead ie fixed header size of only 2 bytes minimum, leading to low packet size. Are there other factors in the design of the protocol?

EDIT:- I am sorry, I made a mental typo (?), as @Shashi pointed out, I actually meant high latency, low bandwidth.

Answer 1

MQTT is designed for devices with little memory footprint, low network bandwidth etc. Devices, for example sensors, energy meters, pace makers etc are ideal use cases for MQTT. Low latency means high speed. For low latency you require different protocol, like Reliable Multicast running over Gigabit Ethernet or InfiniBand networks.

Answer 2

One of the key factors is, that the TCP connection a MQTT client establishes is reused all the time. That means you don't have to establish a new connection all the time as it's the case with classic HTTP. Also, as you already suspected, the very low packet size is key here, typical MQTT messages don't have much overhead over the raw TCP packet.

To save more bandwidth on unreliable networks, the persistent session feature of MQTT allows clients to subscribe only once and on reconnect the subscriptions are retained for the client. For subscribing clients this can drastically reduce the overhead as the subscription message is only sent once.

Answer 3

Another reason, it seems is the Last Will and Testament feature, which is a useful to have feature in high latency network, low bandwidth and unreliable networks.