Constrained Application Protocol (CoAP) is a specialized Internet application protocol for constrained devices, as defined in RFC 7252. It enables those constrained devices called "nodes" to communicate with the wider Internet using similar protocols. CoAP is designed for use between devices on the same constrained network (e.g., low-power, lossy networks), between devices and general nodes on the Internet, and between devices on different constrained networks both joined by an internet. CoAP is also being used via other mechanisms, such as SMS on mobile communication networks.

CoAP is an application-layer protocol that is intended for use in resource-constrained Internet devices, such as wireless sensor network nodes. CoAP 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.^[1][2] Multicast, low overhead, and simplicity are important for Internet of things (IoT) and machine-to-machine (M2M) communication, which tend to be embedded and have much less memory and power supply than traditional Internet devices have. Therefore, efficiency is very important. CoAP can run on most devices that support UDP or a UDP analogue.

The Internet Engineering Task Force (IETF) Constrained RESTful Environments Working Group (CoRE) has done the major standardization work for this protocol. In order to make the protocol suitable to IoT and M2M applications, various new functions have been added.

Specification

The core of the protocol is specified in RFC 7252. Various extensions have been proposed, particularly:

• RFC 7641 (2015) Observing Resources in the Constrained Application Protocol
• RFC 7959 (2016) Block-Wise Transfers in the Constrained Application Protocol (CoAP)
• RFC 8323 (2018) CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
• RFC 8974 (2021) Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)

Message formats

CoAP makes use of two message types, requests and responses, using a simple, binary header format. CoAP is by default bound to UDP and optionally to DTLS, providing a high level of communications security. When bound to UDP, the entire message must fit within a single datagram. When used with 6LoWPAN as defined in RFC 4944, messages should fit into a single IEEE 802.15.4 frame to minimize fragmentation.

The smallest CoAP message is 4 bytes in length, if the token, options and payload fields are omitted, i.e. if it only consists of the CoAP header. The header is followed by the token value (0 to 8 bytes) which may be followed by a list of options in an optimized type–length–value format. Any bytes after the header, token and options (if any) are considered the message payload, which is prefixed by the one-byte "payload marker" (0xFF). The length of the payload is implied by the datagram length.

#define COAP_HEADER_VERSION(data)  ( (0xC0 & (data)[0]) >> 6      )
#define COAP_HEADER_TYPE(data)     ( (0x30 & (data)[0]) >> 4      )
#define COAP_HEADER_TKL(data)      ( (0x0F & (data)[0]) >> 0      )
#define COAP_HEADER_CLASS(data)    ( ((data)[1] >> 5) & 0x07      )
#define COAP_HEADER_CODE(data)     ( ((data)[1] >> 0) & 0x1F      )
#define COAP_HEADER_MID(data)      ( ((data)[2] << 8) | (data)[3] )

Implementations

Proxy implementations

• Squid 3.1.9 with transparent HTTP-CoAP mapping module
• jcoap Proxy
• Californium cf-proxy2
• CoAPthon
• FreeCoAP

CoAP group communication

In many CoAP application domains it is essential to have the ability to address several CoAP resources as a group, instead of addressing each resource individually (e.g. to turn on all the CoAP-enabled lights in a room with a single CoAP request triggered by toggling the light switch). To address this need, the IETF has developed an optional extension for CoAP in the form of an experimental RFC: Group Communication for CoAP - RFC 7390^[3] This extension relies on IP multicast to deliver the CoAP request to all group members. The use of multicast has certain benefits such as reducing the number of packets needed to deliver the request to the members. However, multicast also has its limitations such as poor reliability and being cache-unfriendly. An alternative method for CoAP group communication that uses unicasts instead of multicasts relies on having an intermediary where the groups are created. Clients send their group requests to the intermediary, which in turn sends individual unicast requests to the group members, collects the replies from them, and sends back an aggregated reply to the client.^[4]

Security

CoAP defines four security modes:^[5]

• NoSec, where DTLS is disabled
• PreSharedKey, where DTLS is enabled, there is a list of pre-shared keys, and each key includes a list of which nodes it can be used to communicate with. Devices must support the AES cipher suite.
• RawPublicKey, where DTLS is enabled and the device uses an asymmetric key pair without a certificate, which is validated out of band. Devices must support the AES cipher suite and Elliptic Curve algorithms for key exchange.
• Certificate, where DTLS is enabled and the device uses X.509 certificates for validation.

Research has been conducted on optimizing DTLS by implementing security associates as CoAP resources rather than using DTLS as a security wrapper for CoAP traffic. This research has indicated that improvements of up to 6.5 times none optimized implementations. ^[6]

In addition to DTLS, RFC8613^[7] defines the Object Security for Constrained RESTful Environments (OSCORE) protocol which provides security for CoAP at the application layer.

Security issues

Although the protocol standard includes provisions for mitigating the threat of DDoS amplification attacks,^[8] these provisions are not implemented in practice,^[9] resulting in the presence of over 580,000 targets primarily located in China and attacks up to 320 Gbit/s.^[10]

See also

• Internet of Things
• OMA Lightweight M2M
• Web of Things
• Static Context Header Compression (SCHC)

References

External links

Wikimedia Commons has media related to SSL and TLS.

• RFC 7252 "The Constrained Application Protocol (CoAP)"
• coap.me – CoAP test server run by University of Bremen