What are the major differences between HTTP and COAP?

Question

HTTP is mainly used for viewing web pages. Coap is a simplified version of HTTP for IoT or WSNs. Although COAP is based on UDP, it should have ACK messages to emulate TCP. Since COAP is simpler than HTTP, it will have lower latency and draw less power.

Then, why browsers and web servers do not replace HTTP with COAP? Given the previous arguments, is it expected that COAP will completely replace HTTP? Is it just a matter of time? Are there any features which are supported only by HTTP?

Answer 1

CoAP and HTTP con be used for different purposes. CoAP has been implemented for IoT and M2M environment,in other words,to send short messagges using UDP. For instance:

A typical CoAP exchange consists of 2 messages, i.e., a request and a response. In contrast, an HTTP request first requires the client to establish a TCP connection and later terminate it. This results in at least 9 messages for only one request [11]. Note that this argument is not necessarily true for large payloads. After TCP’s slow-start phase, it is able to send multiple packets at once and acknowledge all of them with a single acknowledgement. CoAP’s blockwise transfer [8] though, requires an acknowledgement for each block and leads to more messages and higher transfer time. Since we expect the majority of CoAP messages to be rather short, this is of less importance. However, CoAP’s blockwise mechanism allows a constrained server to not only receive but also process a large request block-by-block. This would not be possible if we used HTTP and TCP. (Scalability for IoT CLoud Services by Martin Lanter)

Actually, Firefox can support CoAP using Copper(CU) plug-in. ;)