MQTT vs CoAP: Comparing Protocols for IoT Connectivity

Introduction

In today's rapidly evolving digital landscape, the Internet of Things (IoT) has emerged as a key area of interest. At the heart of IoT lies the critical role played by communication protocols. These protocols, in their simplest essence, are a set of rules that define how information is exchanged across a network. The protocols stipulate the format, timing, sequence, and error-checking methods employed in data transmission.

This blog specifically examines two such communication protocols: MQTT (Message Queuing Telemetry Transport) and CoAP (Constrained Application Protocol). We will provide a comprehensive comparison, covering their definitions, key features, advantages and disadvantages, and their variances in design and architecture, security features, and use cases.

What is MQTT?

MQTT(Message Queuing Telemetry Transport) is a lightweight messaging protocol that is widely used for IoT applications. Originating in the late 1990s, MQTT was designed to be a simple and reliable method of transferring telemetry data in situations where network bandwidth is limited and reliability cannot be guaranteed.

Key features of MQTT include:

• Publish-Subscribe Model: MQTT operates on a publish-subscribe model which is ideal for IoT applications. The publisher sends a message to a topic, and all subscribers to that topic receive the message.
• Lightweight: Due to its minimal data packet size, MQTT is extremely lightweight which makes it ideal for use in situations where network bandwidth is at a premium.
• Quality of Service: MQTT offers three levels of Quality of Service (QoS) ranging from "at most once", where messages are delivered according to the best efforts of the operating environment and no response is required, to "exactly once", where messages are assured to arrive exactly once and duplicates are not allowed.
• Retained Messages: MQTT allows for messages to be retained on a topic. The most recent message for a topic is stored and presented to any client that subscribes to that topic.
• Last Will and Testament: If an MQTT client disconnects ungracefully, a pre-defined "last will and testament" message is sent to all subscribers.
• Session Management: MQTT has built-in session management requirements. This means that if a connection is lost, the session can be re-established without loss of messages.

What is CoAP?

CoAP(Constrained Application Protocol) is a specialized web transfer protocol for use with constrained nodes and constrained networks in IoT. It is designed to easily translate to HTTP for simplified integration with the web, while also meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments.

Key features of CoAP include:

• UDP Based: Unlike MQTT which runs over TCP, CoAP is designed to use UDP and is thus better suited for limited network and resources,
• HTTP-like Semantics: CoAP employs HTTP-like semantics, using methods such as GET, POST, PUT, and DELETE for interactions. This makes it easy for developers who are familiar with HTTP to use CoAP.
• Confirmable Messages: CoAP provides a mechanism for confirmable message delivery. This ensures that messages are delivered to the recipient, and if an acknowledgment is not received, the message is retransmitted.
• Resource Observation: CoAP allows clients to "observe" resources, enabling them to automatically get updates whenever the state of a resource changes.
• Block-Wise Transfers: CoAP supports the transfer of larger payloads by splitting them into smaller blocks. This is useful for constrained networks where packet size matters.

MQTT vs CoAP: A Comparison

A summarized comparison table is shown below:

	MQTT	CoAP
Transport Layer	TCP	UDP
Header Size	2 bytes	4 bytes
Resource Overhead	Low	Very Low
Message Model	Pub/Sub	Request/Response RESTful
Message Reliability	Very High	Lower
Feature variety	Great	Lesser

Connectivity

MQTT operates on top of the TCP protocol, ensuring reliable data transmission but with higher overhead. In contrast, CoAP operates over UDP, providing lower overhead but less reliability. The MQTT protocol uses a flexible header with a minimal size of 2 bytes, while CoAP uses a fixed-size header of 4 bytes. MQTT is highly scalable and performs well in high-traffic environments but may consume more resources, whereas CoAP, being lighter, is more suitable for constrained environments but can be challenged in highly concurrent settings.

Both MQTT and CoAP provide connection security features, but their implementations differ:

• MQTT relies on the secure transmission offered by underlying protocols like SSL/TLS
• CoAP has built-in support for DTLS (Datagram Transport Layer Security).

Additionally, MQTT supports built-in authentication parameters, such as using a username and password in the CONNECT message. However, CoAP protocols do not provide such built-in authentication parameters. Users need to incorporate these mechanisms, such as the Authorization header in the HTTP protocol.

Message Model

MQTT operates on a publish-subscribe model, which makes it a great fit for scenarios where the sender and receiver are not synchronized. This is particularly useful for applications in the Internet of Things (IoT), where communication between devices often happens asynchronously. Devices can publish their data, and any other device interested in that information can subscribe to receive it. This allows for effective communication between devices without the need for them to be in sync.

On the other hand, CoAP operates on a request-response model with a RESTful resource management approach. Designed to mimic HTTP, CoAP follows a traditional request-response model, making it well-suited for scenarios where devices need to interact directly on a one-to-one basis. However, it also supports the Observer pattern, allowing devices to subscribe to resources and receive updates when they change, similar to MQTT's publish-subscribe model.

Message Reliability

Both MQTT and CoAP have been designed for reliable transport. MQTT has very high reliability and higher resource requirements, whereas CoAP implements a simple retransmission mechanism based only on UDP.

MQTT provides three levels of Quality of Service (QoS) to ensure message delivery:

• QoS0: "At most once" no response required, suitable when occasional message loss is acceptable.
• QoS1: "At least once" delivery, retransmits unacknowledged messages, ideal when message loss is unacceptable but duplicates are tolerable.
• QoS2: "Exactly once" delivery, no duplicates allowed, best for mission-critical applications requiring exact message reception.

CoAP only offers a mechanism for confirmable message delivery. Confirmable messages are retried until an acknowledgment is received, providing a level of reliability, like QoS1 in the MQTT protocol.

Additionally, MQTT offers the concept of sessions. It maintains the session state, which includes all the necessary information to continue a session if the connection gets disrupted. On the other hand, CoAP lacks a built-in session management feature. Consequently, if the connection is lost, the session cannot resume, potentially resulting in the loss of any messages in transit.

Feature Variety

MQTT includes more and richer built-in functionality. For example:

• Last Will Message: MQTT has a built-in mechanism to handle sudden disconnections. If a client disconnects ungracefully, a pre-defined "last will message" will be published.
• Retention of Messages: MQTT retains the most recent message on a topic, providing it to any new subscribers to that topic. This is useful in situations where late joiners need to know the latest state.
• Shared Subscription: MQTT supports shared subscriptions, where multiple clients can subscribe to the same topic and load balance the messages between them.

In contrast, CoAP focuses on simplicity and addresses the limitations of UDP-based protocols, such as:

• Block-Wise Transfer: CoAP facilitates the transfer of large payloads by breaking them into smaller blocks. This is especially beneficial in constrained networks where packet size is critical. Moreover, it prevents message segmentation issues at the IP layer caused by packets larger than 1500 bytes.

Use Cases and Industry Adoption

MQTT Use Cases

• Oil and Gas Pipeline Monitoring: MQTT can assure message delivery which is crucial in monitoring pipelines for any leaks or damage.
• Industrial Automation: In manufacturing industries, MQTT can be used to monitor and control different machinery and equipment in real time.
• Vehicle Telematics: MQTT is used for sending vehicle performance and diagnostic data from onboard devices to servers for real-time tracking.
• Telemedicine: MQTT enables reliable and real-time transmission of patient data from wearable medical devices to healthcare providers.

CoAP Use Cases

• Low-Power Sensors: Due to its low overhead, CoAP is ideal for IoT sensors operating on low-power and constrained networks.
• Agriculture: In smart farming, CoAP can be used for soil moisture monitoring, climate control in greenhouses, and livestock tracking.
• Environmental Monitoring: CoAP is used in devices that monitor environmental conditions like temperature, humidity, and air quality.
• Smart metering: Due to CoAP's low power consumption and ability to run on constrained devices. It has a huge advantage in data collection related to water, electricity, and gas meters.

Considerations for Choosing Between MQTT and CoAP

When deciding between MQTT and CoAP, several factors should be considered:

• Network Conditions: If your network is unreliable or has limited bandwidth, MQTT may be the better choice due to its lightweight design and ability to handle high latencies.
• Device Capabilities: If your devices have limited processing power or memory, CoAP's low overhead may make it the more suitable option.
• Message Delivery Assurance: If your messages must be delivered exactly once, MQTT's higher levels of QoS may be beneficial.
• Security Requirements: MQTT and CoAP provide security features, but their implementations differ. Your specific security requirements may make one protocol more suitable than the other.

EMQX: A Unified MQTT Platform with Multi-Protocol Support

EMQX is a scalable, distributed MQTT messaging platform that supports unlimited connections, offers seamless integration, and can be deployed anywhere.

EMQX not only fully supports MQTT 3.1, 3.1.1, and 5.0 but also supports various mainstream protocols for connectivity. It provides extensive connection capabilities to handle IoT devices for various scenarios and serves as a unified access platform and management interface for backend IoT management services, reducing the adaptation costs between heterogeneous protocols.

In EMQX v5, the Gateway architecture has been added to unify the model of multi-protocol access. We support the conversion of protocols such as MQTT-SN, Stomp, CoAP, LwM2M, etc. into a unified Pub/Sub messaging model with independent authentication configuration, and client management.

For more details refer to: Multi-Protocol Gateway | EMQX Enterprise Docs

Conclusion

MQTT and CoAP are two powerful protocols designed for the unique requirements of IoT devices. While they have many similarities, their differences in design and architecture make them suitable for different use cases. Understanding these differences can help in selecting the right protocol for your IoT project.

As the IoT continues to evolve, we can expect further advancements and possibly new protocols to emerge that cater to the diverse and growing needs of this field. It will be exciting to see the future developments in IoT communication protocols.

Related Resources

• CoAP Protocol: Key Features, Use Cases, and Pros/Cons
• Connecting CoAP Devices to EMQX
• What Is the MQTT Protocol: A Beginner's Guide
• Free MQTT Broker: Exploring Options and Choosing the Right Solution

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