Which protocol is lightweight?

CoAP messages are also very small, which makes them ideal for use in constrained devices with limited memory and processing power.

CoAP is a very simple protocol, but it is still powerful enough to support a wide range of M2M and IoT applications. It is supported by a growing number of devices and platforms, and it is likely to become the de facto standard for M2M and IoT communication in the future.

Here is a brief explanation of each option:

• MQTT is a publish/subscribe messaging protocol that is often used in M2M and IoT applications. It is a more complex protocol than CoAP, but it is also more reliable.
• HTTP is a standard web protocol that is used to transfer data between clients and servers. It is a very powerful protocol, but it is not well-suited for use in constrained devices.
• SPI is a security protocol that is used to protect data in transit. It is not a communication protocol, and it is not well-suited for use in M2M and IoT applications.