Constrained Application Protocol

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.

Internet protocol suite
Application layer
Transport layer
Internet layer
Link layer
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CoAP is a service 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 extremely important for Internet of Things (IoT) and Machine-to-Machine (M2M) devices, which tend to be deeply 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. The core of the protocol is specified in RFC 7252; important extensions are in various stages of the standardization process.

Features

Message formats

The smallest CoAP message is 4 bytes in length, if omitting Token, Options and Payload. CoAP makes use of two message types, requests and responses, using a simple, binary, base header format. The base header may be followed by options in an optimized Type-Length-Value format. CoAP is by default bound to UDP and optionally to DTLS, providing a high level of communications security.

Any bytes after the headers in the packet are considered the message body. The length of the message body is implied by the datagram length. 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.

CoAP Header
OffsetsOctet0123
OctetBit012345678910111213141516171819202122232425262728293031
432 VERTypeToken LengthRequest/Response CodeMessage ID
864 Token (0 - 8 bytes)
1296
16128 Options (If Available)
20160 11111111Payload (If Available)

CoAP Fixed Header: Version, Type, Token Length, Request/Response Code and Message ID.

The first 4 bytes are mandatory in all CoAP datagrams.

These fields can be easily extracted from these 4 bytes in C via these macros:

#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]) )

Version (VER) (2 bits)
Indicates the CoAP version number.

Type (2 bits)
This describes the datagram's message type for the two message type context of Request and Response.
  • Request
    • 0 : Confirmable : This message expects a corresponding Acknowledgement message.
    • 1 : Non-confirmable : This message does not expect a confirmation message.
  • Response
    • 2 : Acknowledgement : This message is a response that acknowledge a confirmable message
    • 3 : Reset : This message indicates that it had received a message but could not process it.

Token Length (4 bits)
Indicates the length of the variable-length Token field, which may be 0-8 bytes in length.

Request/Response Code (8 bits)
0 1 2 3 4 5 6 7
Class Code

The three most significant bits form a number known as the "class", which is analogous to the class of HTTP status codes. The five least significant bits form a code that communicates further detail about the request or response. The entire code is typically communicated in the form class.code .

You can find the latest CoAP request/response codes at , though the below list gives some examples:

    1. Method: 0.XX
      1. EMPTY
      2. GET
      3. POST
      4. PUT
      5. DELETE
      6. FETCH
      7. PATCH
      8. iPATCH
    2. Success: 2.XX
      1. Created
      2. Deleted
      3. Valid
      4. Changed
      5. Content
      6. Continue
    1. Client Error: 4.XX
      1. Bad Request
      2. Unauthorized
      3. Bad Option
      4. Forbidden
      5. Not Found
      6. Method Not Allowed
      7. Not Acceptable
      8. Request Entity Incomplete
      9. Conflict
      10. Precondition Failed
      11. Request Entity Too Large
      12. Unsupported Content-Format
    1. Server Error: 5.XX
      1. Internal Server Error
      2. Not Implemented
      3. Bad Gateway
      4. Service Unavailable
      5. Gateway Timeout
      6. Proxying Not Supported
    2. Signaling Codes: 7.XX
      1. Unassigned
      2. CSM
      3. Ping
      4. Pong
      5. Release
      6. Abort

Message ID (16 bits)
Used to detect message duplication and to match messages of type Acknowledgement/Reset to messages of type Confirmable/Non-confirmable.:Response messages will have the same Message ID as request.

Token

Optional field whose size is indicated by the Token Length field, whose values is generated by the client. The server must echo every token value without any modification back to the client. It is intended for use as a client-local identifier to provide extra context for certain concurrent transactions.

Option
Option Format
Bit Positions
0 1 2 3 4 5 6 7
Option Delta Option Length
Option Delta Extended (None, 8bit, 16bits)
Option Length Extended (None, 8bit, 16bits)
Option Value

Option Delta:

  • 0 to 12 : For delta between 0 to 12 : Small delta between the last option id and the desired option id
  • 13 : For delta from 13 to 268 : Option Delta Extended is 8bit that is the Option Delta value minus 13
  • 14 : For delta from 269 to 65,804 : Option Delta Extended is 16bit that is the Option Delta value minus 269
  • 15 : Reserved for Payload Marker, where the Options Delta/length are set together as 0xFF.

Option Length:

  • 0 to 12 : For Option Length between 0 to 12 : Small Option Length between the last option id and the desired option id
  • 13 : For Option Length from 13 to 268 : Option Length Extended is 8bit that is the Option Length value minus 13
  • 14 : For Option Length from 269 to 65,804 : Option Length Extended is 16bit that is the Option Length value minus 269
  • 15 : Reserved for future use. Is an error if Option Delta field is set to 0xFF.

Option Value:

  • Size of Option Value field is defined by Option Length value in bytes.
  • Semantic and format this field depends on the respective option.

Implementations
NameProgramming LanguageImplemented CoAP versionClient/ServerImplemented CoAP featuresLicenseLink
aiocoapPython 3RFC 7252Client + ServerBlockwise Transfers, Observe (partial)MIThttps://pypi.python.org/pypi/aiocoap
CaliforniumJavaRFC 7252Client + ServerObserve, Blockwise Transfers, DTLSEPL+EDLhttps://www.eclipse.org/californium
cantcoapC++/CRFC 7252Client + ServerBSDhttps://github.com/staropram/cantcoap
CanopusGoRFC 7252Client + ServerCoreApache License 2.0https://github.com/zubairhamed/canopus
Go-CoAPGoRFC 7252, RFC 8232, RFC 7641, RFC 7959Client + ServerCore, Observe, Blockwise, Multicast, TCP/TLSApache License 2.0https://github.com/go-ocf/go-coap
CoAP implementation for GoGoRFC 7252Client + ServerCore + Draft SubscribeMIThttps://github.com/dustin/go-coap
CoAP.NETC#RFC 7252, coap-13, coap-08, coap-03Client + ServerCore, Observe, Blockwise Transfers3-clause BSDhttps://github.com/smeshlink/CoAP.NET
CoAPSharpC#, .NETRFC 7252Client + ServerCore, Observe, Block, RDLGPLhttp://www.coapsharp.com
CoAPthonPythonRFC 7252Client + Server + Forward Proxy + Reverse ProxyObserve, Multicast server discovery, CoRE Link Format parsing, Block-wiseMIThttps://github.com/Tanganelli/CoAPthon
CoAP ShellJavaRFC 7252ClientObserve, Blockwise Transfers, DTLSApache License 2.0https://github.com/tzolov/coap-shell
CopperJavaScript (Browser Plugin)RFC 7252ClientObserve, Blockwise Transfers3-clause BSDhttps://github.com/mkovatsc/Copper https://addons.mozilla.org/firefox/addon/copper-270430/%5B%5D
eCoAPCRFC 7252Client + ServerCoreMIThttps://gitlab.com/jobol/ecoap
Erbium for ContikiCRFC 7252Client + ServerObserve, Blockwise Transfers3-clause BSDhttp://www.contiki-os.org/ (er-rest-example)
iCoAPObjective-CRFC 7252ClientCore, Observe, Blockwise TransfersMIThttps://github.com/stuffrabbit/iCoAP
jCoAPJavaRFC 7252Client + ServerObserve, Blockwise TransfersApache License 2.0https://code.google.com/p/jcoap/
libcoapCRFC 7252Client + ServerObserve, Blockwise Transfers, DTLSBSD/GPLhttps://github.com/obgm/libcoap
LibNyociCRFC 7252Client + ServerCore, Observe, Block, DTLSMIThttps://github.com/darconeous/libnyoci
lobaro-coapCRFC 7252Client + ServerObserve, Blockwise TransfersMIThttp://www.lobaro.com/lobaro-coap
microcoapCRFC 7252Client + ServerMIThttps://github.com/1248/microcoap
microCoAPyMicroPythonRFC 7252Client + ServerCoreApache License 2.0https://github.com/insighio/microCoAPy
nanocoapCRFC 7252Client + ServerCore, Blockwise TransfersLGPLhttps://api.riot-os.org/group__net__nanocoap.html
nCoapJavaRFC 7252Client + ServerObserve, Blockwise Transfers, CoRE Link Format, Endpoint-ID-DraftBSDhttps://github.com/okleine/nCoAP
node-coapJavascriptRFC 7252Client + ServerCore, Observe, BlockMIThttps://github.com/mcollina/node-coap
Ruby coapRubyRFC 7252Client + Server (david)Core, Observe, Block, RDMIT, GPLhttps://github.com/nning/coap
https://github.com/nning/david
Sensinode C Device LibraryCRFC 7252Client + ServerCore, Observe, Block, RDCommercialhttps://silver.arm.com/browse/SEN00
Sensinode Java Device LibraryJava SERFC 7252Client + ServerCore, Observe, Block, RDCommercialhttps://silver.arm.com/browse/SEN00
Sensinode NanoService PlatformJava SERFC 7252Cloud ServerCore, Observe, Block, RDCommercialhttps://silver.arm.com/browse/SEN00
SwiftCoAPSwiftRFC 7252Client + ServerCore, Observe, Blockwise TransfersMIThttps://github.com/stuffrabbit/SwiftCoAP
TinyOS CoapBlipnesC/Ccoap-13Client + ServerObserve, Blockwise TransfersBSDhttps://web.archive.org/web/20130312140509/http://docs.tinyos.net/tinywiki/index.php/CoAP
txThingsPython (Twisted)RFC 7252Client + ServerBlockwise Transfers, Observe (partial)MIThttps://github.com/mwasilak/txThings/
FreeCoAPCRFC 7252Client + Server + HTTP/CoAP ProxyCore, DTLS, Blockwise TransfersBSDhttps://github.com/keith-cullen/FreeCoAP
coap-rsRustRFC 7252Client + ServerMIThttps://github.com/Covertness/coap-rs
YaCoAPCMIThttps://github.com/RIOT-Makers/YaCoAP

Proxy implementations

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]

Security issues

Although the protocol standard includes provisions for mitigating the threat of DDoS amplification attacks[7], these provisions are not implemented in practice[8], resulting in the presence of over 580,000 targets primarily located in China and attacks up to 320Gbps[9].

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See also

References
  1. RFC 7252, Constrained Application Protocol (CoAP)
  2. "Integrating Wireless Sensor Networks with the Web" , Walter, Colitti 2011
  3. RFC 7390, Group Communication for CoAP
  4. "Flexible Unicast-Based Group Communication for CoAP-Enabled Devices" , Ishaq, I.; Hoebeke, J.; Van den Abeele, F.; Rossey, J.; Moerman, I.; Demeester, P. Sensors 2014
  5. RFC 7252, Constrained Application Protocol (CoAP)
  6. Capossele, Angelo; Cervo, Valerio; De Cicco, Gianluca; Petrioli, Chiara (June 2015). "Security as a CoAP resource: An optimized DTLS implementation for the IoT". IEEE: 529–554. doi:10.1109/ICC.2015.7248379.
  7. "TLS 1.3 is going to save us all, and other reasons why IoT is still insecure", Dani Grant, 2017-12-24
  8. "When Machines Can't Talk: Security and Privacy Issues of Machine-to-Machine Data Protocols", Federico Maggi and Rainer Vosseler, 2018-12-06
  9. "The CoAP protocol is the next big thing for DDoS attacks", Catalin Cimpanu, 2018-12-05
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