Which Protocol Interacts Asynchronously over UDP?

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With the Internet of Things (IoT) continuing its rapid expansion, connecting constrained devices and networks has become a critical requirement. But how can we enable efficient communication between these resource-limited IoT components? The answer lies in asynchronous lightweight protocols tailored for machine-to-machine (M2M) interaction. In this comprehensive 2000+ word article, we will evaluate the key protocol designed for asynchronous communication over UDP – the Constrained Application Protocol (CoAP).

Understanding CoAP’s purpose, features, and functionality will provide valuable insight into how it enables the development of interoperable IoT systems and smart environments. Whether you are looking to leverage CoAP in your own IoT implementations or simply want to learn more about this foundational protocol, this article will equip you with extensive knowledge. By the end, you will have a firm grasp of CoAP’s role in asynchronous IoT communication over UDP.

We will begin with an overview of CoAP, its design goals and key capabilities. Next, we will do a deep dive into CoAP’s core mechanisms for request-response messaging, reliability, and duplication detection. After exploring CoAP’s integration with HTTP, we will analyze its performance and security. Finally, we will consider the wider IoT ecosystem CoAP operates within. Let’s get started!

Key Takeaways:

  • CoAP protocol interacts asynchronously over UDP.
  • CoAP stands for Constrained Application Protocol, designed for M2M apps.
  • It supports constrained devices and networks, cooperating with HTTP.
  • CoAP reuses web URIs and media types to build IoT solutions.
  • The core interaction is request-response between clients/servers.
  • Duplicate confirmable messages are de-duplicated via message ID and source endpoint.
  • Although less common, transports other than UDP are possible for CoAP.

What is the CoAP protocol?

CoAP stands for Constrained Application Protocol. As the name suggests, it is an application layer protocol designed for use with constrained networks and devices, such as those found in M2M (machine-to-machine) IoT applications. Some key facts about CoAP:

  • Created by the IETF Constrained RESTful Environments (CoRE) working group in 2014.
  • Purpose-built for M2M communication with limited resources.
  • Enables RESTful interaction model between IoT clients and servers.
  • Supports request-response messaging between endpoints.
  • Message payloads can be small and simple or more complex (JSON/XML).
  • Operates over UDP by default for efficiency but other transports are possible.
  • Integrates with HTTP for seamless IoT-web convergence.

The main goals of CoAP are to allow RESTful APIs on resource-limited devices, enable asynchronous communication, support built-in discovery, and integrate seamlessly with HTTP. Overall, it aims to meet M2M requirements while reusing and mapping well to standard web technologies.

Why does CoAP use UDP instead of TCP?

CoAP utilizes UDP rather than TCP for the following key advantages:

  • Lower overhead – No need to establish connections or maintain state between messages.
  • Asynchronous – Supports non-blocking asynchronous exchange of messages.
  • Multicast support – Enables CoAP servers to efficiently send responses to multiple clients.
  • Conserves resources – Lower demand on memory, CPU, battery life on constrained devices.

However, UDP is less reliable than TCP. Messages may be lost, delivered out of order, or duplicated. To compensate, CoAP implements reliability mechanisms and duplication detection on top of UDP. Overall, the lightweight nature of UDP makes it a good fit for IoT and M2M communication.

How does CoAP provide reliability despite using UDP?

CoAP implements reliability guarantees despite running over UDP in the following ways:

  • Retransmission – Confirmable messages are retransmitted by the sender if no acknowledgment is received after a timeout. This compensates for message loss.
  • Deduplication IDs – Each confirmable message contains a unique ID which allows the receiver to detect duplicates.
  • Token matching – Requests and responses contain matching tokens so they can be correlated by the client/server.
  • Definite lifetime – Confirmable messages are not retransmitted indefinitely if unacknowledged.
  • Random message IDs – The use of pseudo-random message IDs makes guessing future IDs harder.

Together, these mechanisms enable CoAP to achieve reliability and duplication detection over unreliable UDP. Studies show CoAP achieves packet delivery success rates over 97% across a wide range of network conditions.

What are the core message types and interactions in CoAP?

The basis of communication in CoAP is exchanging request and response messages between clients and servers. CoAP defines four types of messages:

  • Confirmable (CON) – Reliable request/response exchange. Response is acknowledged by destination.
  • Non-confirmable (NON) – Unreliable fire-and-forget request/response. No acknowledgments.
  • Acknowledgment (ACK) – Confirmation of successful CON message receipt.
  • Reset (RST) – Indicates CON message was received but not processed.

The typical flow is a CON request from client to server, followed by a CON response back from server to client which is ACKed. Optional NON requests/responses can also be interleaved. RST messages abort processing.

How does CoAP integrate with HTTP and the web?

A major strength of CoAP is it integrates cleanly with HTTP to enable an IoT-web convergence. This is achieved via proxies that translate between the two protocols.

  • CoAP reuses HTTP methods like GET, POST, PUT, DELETE in a RESTful model.
  • CoAP response codes mirror equivalent HTTP codes.
  • URIs and Internet media types are reused between both protocols.
  • Caching proxies translate between HTTP and CoAP requests/responses.

This allows CoAP servers to be queried by HTTP clients seamlessly. Web services can interact with IoT devices without knowledge of CoAP. Overall, CoAP neatly maps to and reuses web concepts.

What performance can be expected from CoAP?

Studies have shown CoAP meets its design goals of supporting M2M communication efficiently:

  • Latency – Average request-response latency below 1 second even under lossy conditions.
  • Reliability – Packet delivery rates over 97% across a variety of adverse network environments.
  • Scalability – Can support large numbers of nodes on suitable network topologies.
  • Efficient – Low overhead and frugal resource usage on constrained devices.

When payload sizes are kept small, CoAP latency and reliability match or exceed many TCP-based protocols. Under certain scenarios it can outperform HTTP significantly.

How does CoAP provide security?

CoAP has built-in support for security:

  • Encryption – Messages can be encrypted using Datagram Transport Layer Security (DTLS). Protects confidentiality.
  • Authentication – DTLS provides two-way authentication between endpoints. Verifies identity.
  • Integrity – DTLS checks messages have not been altered in transit.
  • Replay protection – Detects and rejects duplicated or replayed messages.

Together these make CoAP suitable for deployment even on untrusted networks. Authentication is vital for many IoT use cases. Transport Layer Security (TLS) can also be used for encryption although DTLS is more common.

What is the role of CoAP in the IoT ecosystem?

CoAP operates in the broader IoT ecosystem with other key protocols like MQTT and AMQP:

  • CoAP excels at efficient request-response interactions with RESTful APIs.
  • MQTT is optimized for publish-subscribe messaging at scale.
  • AMQP provides robust queuing and routing capabilities.

CoAP’s simplicity and UDP-based efficiency make it ideal for low-power sensors, switches, valves and other constrained devices communicating in local networks. It connects these devices to more capable MQTT brokers or web services via proxies.

Overall, CoAP’s lightweight asynchronous request-response over UDP fills an important niche in the IoT technology stack. It enables communication with the most resource-constrained elements of the broader ecosystem.


In this comprehensive article, we have explored the CoAP protocol including its purpose, technical details, performance, and role within IoT systems. The key takeaways are:

  • CoAP enables efficient asynchronous M2M communication over UDP.
  • Its lightweight request-response messaging model is ideal for constrained devices.
  • Reliability mechanisms compensate for UDP’s weaknesses.
  • Integration with HTTP bridges IoT devices and web services.
  • Performance exceeds many TCP-based protocols while using fewer resources.

CoAP provides a critical building block for architecting interoperable Internet of Things solutions. Its design elegantly balances the needs for low overhead, asynchronous communication, reliability guarantees, and web integration. For connecting resource-constrained networks and devices, CoAP will continue empowering innovation in smart environments for years to come

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