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.
CoAP fixed-size header The first 4 bytes are mandatory in all CoAP datagrams, they constitute the fixed-size header. These fields can be 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]
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 acknowledges 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) 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: • Method: 0.XX • Success: 2.XX • Client Error: 4.XX • Server error: 5.XX • Signaling Codes: 7.XX
Message ID (16 bits) :Used to detect message duplication and to match messages of type acknowledgement/reset to messages of type confirmable/non-confirmable.
Token Every request carries a token (but it may be zero length) whose value was generated by the client. The server must echo every token value without any modification back to the client in the corresponding response. It is intended for use as a client-local identifier to match requests and responses, especially for concurrent requests. Matching requests and responses is not done with the message ID because a response may be sent in a different message than the acknowledgement (which uses the message ID for matching). For example, this could be done to prevent retransmissions if obtaining the result takes some time. Such a detached response is called "separate response". In contrast, transmitting the response directly in the acknowledgement is called "piggybacked response" which is expected to be preferred for efficiency reasons.
Option Option delta: • 0 to 12: For delta between 0 and 12: Represents the exact delta value between the last option ID and the desired option ID, with no option delta extended value • 13: For delta from 13 to 268: Option delta extended is an 8-bit value that represents the option delta value minus 13 • 14: For delta from 269 to 65,804: Option delta extended is a 16-bit value that represents the option delta value minus 269 • 15: Reserved for payload marker, where the option delta and option length are set together as 0xFF. Option length: • 0 to 12: For option length between 0 and 12: Represents the exact length value, with no option length extended value • 13: For option length from 13 to 268: Option length extended is an 8-bit value that represents the option length value minus 13 • 14: For option length from 269 to 65,804: Option length extended is a 16-bit value that represents the option length value minus 269 • 15: Reserved for future use. It is an error for the option length field to be 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. ==Active protocol implementations==