Comparing MQTT, CoAP, HTTP, XMPP, and SOAP: Which IoT Protocol Fits Your Needs?

For the Internet of Things, reliable communication with devices is essential, and the most widely used protocols include MQTT, CoAP, HTTP, XMPP, and SOAP.

MQTT

MQTT (Message Queuing Telemetry Transport) was originally developed by IBM for low‑bandwidth, unreliable networks and resource‑constrained sensors. It uses a publish/subscribe model and runs over TCP/IP.

Publish/subscribe messaging decouples producers and consumers.

Payload is opaque to the broker.

Operates over TCP/IP.

Three Quality of Service (QoS) levels:

QoS 0 – “at most once”, delivery depends on the underlying network.

QoS 1 – “at least once”, guarantees delivery but may duplicate messages.

QoS 2 – “exactly once”, ensures a single delivery.

Small overhead (fixed 2‑byte header).

Last Will and Testament feature notifies peers of unexpected client disconnects.

CoAP

CoAP (Constrained Application Protocol) is designed for devices with limited memory and processing power. It follows a RESTful model over UDP and 6LoWPAN.

CON – Confirmable request, requires an ACK.

NON – Non‑confirmable request, no ACK needed.

ACK – Acknowledgment for a CON message.

RST – Reset, sent when a message cannot be processed.

CoAP messages have a simple binary format: a fixed header, optional options, and an optional payload. The minimum size is 4 bytes.

Differences Between MQTT and CoAP

MQTT uses TCP (connection‑oriented) while CoAP uses UDP (connection‑less).

CoAP includes built‑in discovery and content negotiation; MQTT relies on the client to know the payload format.

MQTT maintains a long‑lived TCP connection between client and broker; CoAP may require NAT traversal techniques.

MQTT is a many‑to‑many, broker‑based system; CoAP is essentially a one‑to‑one request/response protocol.

HTTP

HTTP (HyperText Transfer Protocol) defines request and response messages.

Request line: <method> <request‑URL> <version>
Headers

Entity‑body (optional)

Response line: <version> <status> <reason‑phrase>
Headers

Entity‑body (optional)

Common methods include GET, POST, DELETE, PUT, CONNECT, HEAD, OPTIONS, and TRACE.

XMPP

XMPP (Extensible Messaging and Presence Protocol) is an XML‑based protocol originally known as Jabber. It supports real‑time messaging, presence, and structured data exchange.

<?xml version='1.0'?>
<stream:stream to='example.com' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams' version='1.0'>

<?xml version='1.0'?>
<stream:stream from='example.com' id='someid' xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams' version='1.0'>

SOAP

SOAP (Simple Object Access Protocol) is an XML‑based protocol for exchanging structured information over the Internet, often combined with HTTP, SMTP, or MIME.

It is suitable for enterprise applications but can be heavyweight for constrained IoT devices due to high overhead.

Conclusion

MQTT currently enjoys broad support from major cloud providers (Alibaba Cloud, AWS, Baidu Cloud, Azure, Tencent Cloud) and benefits from early adoption, making it a strong choice for many IoT scenarios. However, as IoT ecosystems evolve, platforms are increasingly adding support for CoAP, XMPP, SOAP, and other protocols to address specific requirements such as low‑power operation, RESTful interaction, or enterprise integration.

For further reading, see the arXiv paper “A Survey of Communication Protocols for Internet of Things and Related Challenges of Fog and Cloud Computing Integration”.