DESCRIPTION

The CoAP PDU is of the form

--header--|--optional token--|--optional options--|--optional payload--

The terminology used is taken mainly from "RFC7252 1.2. Terminology".

The PDU must be built in the correct order, from left to right. In particular, the options need to be added in the correct numerical option order as they are stored in the PDU using relative numeric offsets from the previous option number.

There are option list functions available where options can be added to a chained list of options and then the chain list is sorted and then be added to the PDU.

Typically for clients, when creating a request, the PDU needs to be created before filling it with the appropriate information.

Typically with a server, the response PDU, with the optional token already added in, will already be created before the response handler is called, and the response PDU will need to be updated as appropriate starting with the optional options. Note that updating the response pdu’s code variable will cause the response pdu to get transmitted. If code does not get updated, and the PDU is of type CONFIRMABLE, then the response PDU is transmitted as an empty ACK packet. The response pdu is always freed off by the underlying library.

For handling situations where the data to be transmitted does not fit into a single packet, see coap_block(3).

PDU CREATE AND HEADER FUNCTIONS

Function: coap_new_pdu()

The coap_new_pdu() function returns a newly created PDU of type coap_pdu_t.

The type is one of the following

COAP_MESSAGE_CON Set the _PDU_ to be of type confirmable.
COAP_MESSAGE_NON Set the _PDU_ to be of type non-confirmable.
COAP_MESSAGE_ACK Set the _PDU_ to be of type acknowledge (for internal use).
COAP_MESSAGE_RST Set the _PDU_ to be of type reset.

The code is one of the following

COAP_EMPTY_CODE                               0.00
COAP_REQUEST_CODE_GET                         0.01
COAP_REQUEST_CODE_POST                        0.02
COAP_REQUEST_CODE_PUT                         0.03
COAP_REQUEST_CODE_DELETE                      0.04
COAP_REQUEST_CODE_FETCH                       0.05
COAP_REQUEST_CODE_PATCH                       0.06
COAP_REQUEST_CODE_IPATCH                      0.07
COAP_RESPONSE_CODE_OK                         2.00
COAP_RESPONSE_CODE_CREATED                    2.01
COAP_RESPONSE_CODE_DELETED                    2.02
COAP_RESPONSE_CODE_VALID                      2.03
COAP_RESPONSE_CODE_CHANGED                    2.04
COAP_RESPONSE_CODE_CONTENT                    2.05
COAP_RESPONSE_CODE_CONTINUE                   2.31
COAP_RESPONSE_CODE_BAD_REQUEST                4.00
COAP_RESPONSE_CODE_UNAUTHORIZED               4.01
COAP_RESPONSE_CODE_BAD_OPTION                 4.02
COAP_RESPONSE_CODE_FORBIDDEN                  4.03
COAP_RESPONSE_CODE_NOT_FOUND                  4.04
COAP_RESPONSE_CODE_NOT_ALLOWED                4.05
COAP_RESPONSE_CODE_NOT_ACCEPTABLE             4.06
COAP_RESPONSE_CODE_INCOMPLETE                 4.08
COAP_RESPONSE_CODE_CONFLICT                   4.09
COAP_RESPONSE_CODE_PRECONDITION_FAILED        4.12
COAP_RESPONSE_CODE_REQUEST_TOO_LARGE          4.13
COAP_RESPONSE_CODE_UNSUPPORTED_CONTENT_FORMAT 4.15
COAP_RESPONSE_CODE_UNPROCESSABLE              4.22
COAP_RESPONSE_CODE_TOO_MANY_REQUESTS          4.29
COAP_RESPONSE_CODE_INTERNAL_ERROR             5.00
COAP_RESPONSE_CODE_NOT_IMPLEMENTED            5.01
COAP_RESPONSE_CODE_BAD_GATEWAY                5.02
COAP_RESPONSE_CODE_SERVICE_UNAVAILABLE        5.03
COAP_RESPONSE_CODE_GATEWAY_TIMEOUT            5.04
COAP_RESPONSE_CODE_PROXYING_NOT_SUPPORTED     5.05
COAP_RESPONSE_CODE_HOP_LIMIT_REACHED          5.08
COAP_SIGNALING_CODE_CSM                       7.01
COAP_SIGNALING_CODE_PING                      7.02
COAP_SIGNALING_CODE_PONG                      7.03
COAP_SIGNALING_CODE_RELEASE                   7.04
COAP_SIGNALING_CODE_ABORT                     7.05

and session is used to set up other default values.

Function: coap_pdu_init()

The coap_pdu_init() function does the same work as coap_new_pdu() but gives the additional ability to define the default values for message_id and max_size that coap_new_pdu() creates.

The message_id must be unique per request (which is not the same as the token), and must not be reused within EXCHANGE_LIFETIME (usually 247 seconds). To automate this, the function coap_new_message_id(session) should be called.

At the CoAP protocol level, requests and responses are matched by message_id which is why it needs to be unique. At the application level, for "separate" responses, the initial empty ACK response matches the message_id of the request (handled by libcoap) but the actual response has the same token as the request and this must be used for the match. For "piggybacked" responses the token must still be used as the valid match for request and response. and the message_id just happens to match (but unsafe in case the server is sending back a "separate" response).

The max_size parameter defines the maximum size of a PDU and is usually determined by calling coap_session_max_pdu_size(session);

Function: coap_new_message_id()

The coap_new_message_id() function returns the next message id to use for sending a new request message.

NOTE: For reliable messages RFC8323, this will always return 0.

Function: coap_pdu_set_mid()

The coap_pdu_set_mid() function is used to set the message id mid in the PDU pdu.

Function: coap_pdu_set_code()

The coap_pdu_set_code() function is used to set the code code in the PDU pdu.

Function: coap_pdu_set_type()

The coap_pdu_set_type() function is used to set the type of the PDU pdu.

NOTE: A PDU does not need to be created by the server application to send back a response. The libcoap server logic creates the initial PDU with COAP_EMPTY_CODE, appropriate message_id, matching token and potentially some other options before calling the appropriate request handler (See coap_register_request_handler(3)).

PDU TOKEN FUNCTIONS

Function: coap_session_init_token()

The coap_session_init_token() function is used to initialize the starting token of length for the session.

NOTE: this only takes the first 8 bytes of the token if extended tokens are being used (RFC8974).

Function: coap_session_new_token()

The coap_session_new_token() function is used to obtain the next available token of length for the session. Note that the same token must be used for doing an observe cancellation that was used for doing the observe registration. Otherwise tokens should be unique for each request/response so that they can be correctly matched.

NOTE: Only a token of up to 8 bytes is returned.

Function: coap_add_token()

The coap_add_token() function adds in the specified token’s data of length length to the PDU pdu. The maximum (but impractical due to PDU sizes) length of the token is 65804 bytes. If anything more than 8, the remote peer needs to support extended tokens for this to work. Adding the token must be done before any options or data are added. This function must only be called once per pdu, and must not be called in the appropriate request handler as the token has already been added into the skeletal response PDU.

If a token is not added, then the token in the PDU is zero length, but still a valid token which is used for matching. The exception is an empty ACK packet.

NOTE: The token provided by the client application may not be the same as used internally by libcoap - for example when doing data transmission where the body of data is spread over multiple payloads (see coap_block(3)). However, when the data transfers complete, the application will receive the corrected token in the response PDU.

PDU OPTIONS FUNCTIONS

The following is the current list of options with their numeric value

/*
 * The C, U, and N flags indicate the properties
 * Critical, Unsafe, and NoCacheKey, respectively.
 * If U is set, then N has no meaning as per
 * https://rfc-editor.org/rfc/rfc7252#section-5.10
 * and is set to a -.
 * Separately, R is for the options that can be repeated
 *
 * The least significant byte of the option is set as followed
 * as per https://rfc-editor.org/rfc/rfc7252#section-5.4.6
 *
 *   0   1   2   3   4   5   6   7
 * --+---+---+---+---+---+---+---+
 *           | NoCacheKey| U | C |
 * --+---+---+---+---+---+---+---+
 *
 * https://rfc-editor.org/rfc/rfc8613#section-4 goes on to define E, I and U
 * properties Encrypted and Integrity Protected, Integrity Protected Only and
 * Unprotected respectively.  Integrity Protected Only is not currently used.
 *
 * An Option is tagged with CUNREIU with any of the letters replaced with _ if
 * not set, or - for N if U is set (see above) for aiding understanding of the
 * Option.
 */

COAP_OPTION_IF_MATCH        1 /* C__RE__, opaque,    0-8 B, RFC7252 */
COAP_OPTION_URI_HOST        3 /* CU-___U, String,  1-255 B, RFC7252 */
COAP_OPTION_ETAG            4 /* ___RE__, opaque,    1-8 B, RFC7252 */
COAP_OPTION_IF_NONE_MATCH   5 /* C___E__, empty,       0 B, RFC7252 */
COAP_OPTION_OBSERVE         6 /* _U-_E_U, empty/uint,  0 B/0-3 B, RFC7641 */
COAP_OPTION_URI_PORT        7 /* CU-___U, uint,      0-2 B, RFC7252 */
COAP_OPTION_LOCATION_PATH   8 /* ___RE__, String,  0-255 B, RFC7252 */
COAP_OPTION_OSCORE          9 /* C_____U, *,       0-255 B, RFC8613 */
COAP_OPTION_URI_PATH       11 /* CU-RE__, String,  0-255 B, RFC7252 */
COAP_OPTION_CONTENT_FORMAT 12 /* ____E__, uint,      0-2 B, RFC7252 */
/* COAP_OPTION_MAXAGE default 60 seconds if not set */
COAP_OPTION_MAXAGE         14 /* _U-_E_U, uint,      0-4 B, RFC7252 */
COAP_OPTION_URI_QUERY      15 /* CU-RE__, String,  1-255 B, RFC7252 */
COAP_OPTION_HOP_LIMIT      16 /* ______U, uint,        1 B, RFC8768 */
COAP_OPTION_ACCEPT         17 /* C___E__, uint,      0-2 B, RFC7252 */
COAP_OPTION_Q_BLOCK1       19 /* CU__E_U, uint,      0-3 B, RFC8177 */
COAP_OPTION_LOCATION_QUERY 20 /* ___RE__, String,  0-255 B, RFC7252 */
COAP_OPTION_BLOCK2         23 /* CU-_E_U, uint,      0-3 B, RFC7959 */
COAP_OPTION_BLOCK1         27 /* CU-_E_U, uint,      0-3 B, RFC7959 */
COAP_OPTION_SIZE2          28 /* __N_E_U, uint,      0-4 B, RFC7959 */
COAP_OPTION_Q_BLOCK2       31 /* CU_RE_U, uint,      0-3 B, RFC9177 */
COAP_OPTION_PROXY_URI      35 /* CU-___U, String, 1-1034 B, RFC7252 */
COAP_OPTION_PROXY_SCHEME   39 /* CU-___U, String,  1-255 B, RFC7252 */
COAP_OPTION_SIZE1          60 /* __N_E_U, uint,      0-4 B, RFC7252 */
COAP_OPTION_ECHO          252 /* _N__E_U, opaque,   0-40 B, RFC9175 */
COAP_OPTION_NORESPONSE    258 /* _U-_E_U, uint,      0-1 B, RFC7967 */
COAP_OPTION_RTAG          292 /* ___RE_U, opaque,    0-8 B, RFC9175 */

See FURTHER INFORMATION as to how to get the latest list.

Function: coap_new_optlist()

The coap_new_optlist() function returns a newly created optlist entry of type coap_optlist_t*. The number specifies which CoAP option is to be used, and is one of the COAP_OPTION_* definitions. The length is the length of the data of the option, and data points to the content of the option.

NOTE: Where possible, the option data needs to be stripped of leading zeros (big endian) to reduce the amount of data needed in the PDU, as well as in some cases the maximum data size of an option can be exceeded if not stripped and hence be illegal. This is done by using coap_encode_var_safe() or coap_encode_var_safe8().

Function: coap_insert_optlist()

The coap_insert_optlist() function adds the optlist entry onto the end of the optlist_chain. The initial optlist_chain entry needs to be set to NULL before this function is first called. The coap_delete_optlist() function has to be called to free off all the optlist_chain entries.

Function: coap_delete_optlist()

The coap_delete_optlist() function deletes and frees off all the optlist entries in the optlist_chain.

Function: coap_add_optlist_pdu()

The coap_add_optlist_pdu() function sorts all of the entries in optlist_chain into ascending option numeric order and adds all the entries to the pdu. This function does not free off the entries in optlist_chain. This function must be called after adding any token and before adding in the payload data.

Function: coap_add_option()

The coap_add_option() function adds in the specified option of type number with data of length length to the PDU pdu. It is important that options are added to the pdu with number either being the same or greater than the previous option number that was added.

NOTE: Where possible, the option data needs to be stripped of leading zeros (big endian) to reduce the amount of data needed in the PDU, as well as in some cases the maximum data size of an option can be exceeded if not stripped and hence be illegal. This is done by using coap_encode_var_safe() or coap_encode_var_safe8().

Function: coap_encode_var_safe()

The coap_encode_var_safe() function encodes value into buffer which has a size of size in bytes. Normally, the buffer size should be at least the sizeof(int) bytes unless you definitely know less space is required.

Function: coap_encode_var_safe8()

The coap_encode_var_safe8() function encodes 8 byte value into buffer which has a size of size in bytes. Normally, the buffer size should be at least 8 bytes unless you definitely know less space is required.

Function: coap_split_path()

The coap_split_path() function splits up path of length length and places the result in buffer which has a size of buflen with the nul character separating each path component. buflen needs to be preset with the size of buffer before the function call, and then buflen is updated with the actual size of buffer used. The return value indicates the number of components that individual COAP_OPTION_URI_PATH options need to be created for.

Function: coap_split_query()

The coap_split_query() function splits up query of length length and places the result in buffer which has a size of buflen with the nul character separating each path component. buflen needs to be preset with the size of buffer before the function call, and then buflen is updated with the actual size of buffer used. The return value indicates the number of components that individual COAP_OPTION_URI_QUERY options need to be created for.

PDU OPTIONS - LIBCOAP HANDLING

Most of the options are under the control of the applications, but the following are primarily used internally by libcoap.

COAP_OPTION_BLOCK1 and COAP_OPTION_BLOCK2

These Block options are used when a large body needs to be split up into multiple payloads. Following the introduction of coap_context_set_block_mode(3), libcoap can internally handle the setting of these options (see coap_block(3)). Applications can continue to include these options to set hint block size values.

It is recommended that coap_context_set_block_mode(context, COAP_BLOCK_USE_LIBCOAP|COAP_BLOCK_SINGLE_BODY) is used to reduce the programming requirements for block handling within the applications.

COAP_OPTION_ECHO

This option can be set by the server application to indicate that the state of the client’s freshness is confirmed. The libcoap client logic will detect the use of the Echo option by the server and reflect back the Echo value in the next request without involving the client application. The opaque option Echo may be seen by the client application.

COAP_OPTION_ETAG

This option is normally set by the server libcoap logic when sending back multiple payloads so that the (libcoap logic) client can re-assemble the correct body.

COAP_OPTION_HOP_LIMIT

When using proxy logic, the value of the Hop-Limit option is decremented by one for each proxy hop. If the count decrements to zero, then a 5.08 (Hop Limit Reached) error code is returned to the sender. The initial count is 16, unless the client application sets its own limit using the Hop-Limit option.

COAP_OPTION_RTAG

This option is set by the libcoap client logic when transmitting multiple bodies with multiple payloads so that the (libcoap logic) server can differentiate and re-assemble the correct body.

COAP_OPTION_SIZE1 and COAP_OPTION_SIZE2

These options are added by the libcoap logic to provide a size (Size1 by libcoap client logic, Size2 by libcoap server logic) indication to the recipient of the size of the large body that is to be transferred. See coap_block(3).

COAP_OPTION_OSCORE

This option is used during OSCORE enabled communications. It should not be set by any application, but is used internally. See coap_oscore(3) for further information on how to set up OSCORE.

PDU PAYLOAD FUNCTIONS

Function: coap_add_data()

The coap_add_data() function adds in the specified payload data of length length to the PDU pdu. Adding the payload data must be done after any token or options are added. This function must only be called once per pdu.

Function: coap_add_data_blocked_response()

The coap_add_data_blocked_response() function adds in the appropriate part of the payload data of length length to the PDU pdu. It should be used as a direct replacement for coap_add_data() if it is possible that the data will not fit into a single pdu. It also adds in the appropriate CoAP options to handle Block-Wise transfer. This function is usually used for a server’s GET / FETCH response. The request and response are the same parameters for the registered GET / FETCH resource handler. The media_type is for the format of the data and maxage defines the lifetime of the response. If set to -1, then the Max-Age option does not get included. This function must only be called once per pdu. It is the responsibility of the client to recognize that it has only received a part of the data and request the next block (with the appropriate Block options) from the server. Returning the next requested block is handled by this function.

NOTE: This function has been superseded by coap_add_data_large_response(). See coap_block(3).

PDU TRANSMIT FUNCTIONS

Function: coap_send()

The coap_send() function is used to initiate the transmission of the pdu associated with the session. The caller must not access or delete pdu after calling coap_send() - even if there is a return error.

NOTE: For request handlers, returning from the request handler will cause the response PDU to be transmitted as appropriate and there is no need to call coap_send() to do this.

RETURN VALUES

coap_new_pdu() and coap_pdu_init() return a newly created PDU or NULL if there is a malloc or parameter failure.

coap_new_optlist() returns a newly created optlist or NULL if there is a malloc failure.

coap_add_token(), coap_insert_optlist(), coap_add_optlist_pdu() and coap_add_data() return 0 on failure, 1 on success.

coap_encode_var_safe() returns either the length of bytes encoded (which can be 0 when encoding 0) or 0 on failure.

coap_encode_var_safe8() returns either the length of bytes encoded (which can be 0 when encoding 0) or 0 on failure.

coap_add_option() returns the size of option added, or 0 on failure.

coap_send() returns the CoAP message ID on success or COAP_INVALID_MID on failure.

coap_split_path() and coap_split_query() return the number of components found.

coap_new_message_id() returns the next CoAP message ID to use.

EXAMPLES

Setup PDU and Transmit

#include <coap3/coap.h>

static int
build_send_pdu(coap_context_t *context, coap_session_t *session,
uint8_t msgtype, uint8_t request_code, const char *uri, const char *query,
unsigned char *data, size_t length, int observe) {

  coap_pdu_t *pdu;
  uint8_t buf[1024];
  size_t buflen;
  uint8_t *sbuf = buf;
  int res;
  coap_optlist_t *optlist_chain = NULL;
  /* Remove (void) definition if variable is used */
  (void)context;

  /* Create the pdu with the appropriate options */
  pdu = coap_pdu_init(msgtype, request_code, coap_new_message_id(session),
                      coap_session_max_pdu_size(session));
  if (!pdu)
    return 0;

  /*
   * Create unique token for this request for handling unsolicited /
   * delayed responses
   */
  coap_session_new_token(session, &buflen, buf);
  if (!coap_add_token(pdu, buflen, buf)) {
    coap_log_debug("cannot add token to request\n");
    goto error;
  }

  if (uri) {
    /* Add in the URI options */
    buflen = sizeof(buf);
    res = coap_split_path((const uint8_t*)uri, strlen(uri), sbuf, &buflen);
    while (res--) {
      if (!coap_insert_optlist(&optlist_chain,
                               coap_new_optlist(COAP_OPTION_URI_PATH,
                        coap_opt_length(sbuf), coap_opt_value(sbuf))))
        goto error;
      sbuf += coap_opt_size(sbuf);
    }
  }

  if (query) {
    /* Add in the QUERY options */
    buflen = sizeof(buf);
    res = coap_split_query((const uint8_t*)query, strlen(query), sbuf, &buflen);
    while (res--) {
      if (!coap_insert_optlist(&optlist_chain,
                               coap_new_optlist(COAP_OPTION_URI_QUERY,
                        coap_opt_length(sbuf), coap_opt_value(sbuf))))
        goto error;
      sbuf += coap_opt_size(sbuf);
    }
  }

  if (request_code == COAP_REQUEST_GET && observe) {
    /* Indicate that we want to observe this resource */
    if (!coap_insert_optlist(&optlist_chain,
                             coap_new_optlist(COAP_OPTION_OBSERVE,
                               coap_encode_var_safe(buf, sizeof(buf),
                               COAP_OBSERVE_ESTABLISH), buf)
                             ))
      goto error;
  }

  /* ... Other code / options etc. ... */

  /* Add in all the options (after internal sorting) to the pdu */
  if (!coap_add_optlist_pdu(pdu, &optlist_chain))
    goto error;

  if (data && length) {
    /* Add in the specified data */
    if (!coap_add_data(pdu, length, data))
      goto error;
  }

  if (coap_send(session, pdu) == COAP_INVALID_MID)
    goto error;
  return 1;

error:

  if (pdu)
    coap_delete_pdu(pdu);
  return 0;

}

Resource Request Handler Response PDU Update

#include <coap3/coap.h>

#include <stdio.h>

static void
hnd_get_time(coap_resource_t *resource, coap_session_t *session,
const coap_pdu_t *request, const coap_string_t *query, coap_pdu_t *response) {

  unsigned char buf[40];
  size_t len;
  time_t now;

  /* ... Additional analysis code for resource, request pdu etc.  ... */

  /* After analysis, generate a suitable response */

  now = time(NULL);

  if (query != NULL && coap_string_equal(query, coap_make_str_const("secs"))) {
    /* Output secs since Jan 1 1970 */
    len = snprintf((char *)buf, sizeof(buf), "%lu", now);
  }
  else {
    /* Output human-readable time */
    struct tm *tmp;
    tmp = gmtime(&now);
    if (!tmp) {
      /* If 'now' is not valid */
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_NOT_FOUND);
      return;
    }
    len = strftime((char *)buf, sizeof(buf), "%b %d %H:%M:%S", tmp);
  }
  coap_pdu_set_code(response, COAP_RESPONSE_CODE_CONTENT);
  /*
   * Invoke coap_add_data_large_response() to do all the hard work.
   *
   * Define the format - COAP_MEDIATYPE_TEXT_PLAIN - to add in
   * Define how long this response is valid for (secs) - 1 - to add in.
   * Etag Option added internally with unique value as param set to 0
   *
   * Observe Option added internally if needed within the function
   * Block2 Option added internally if output too large
   * Size2 Option added internally
   */
  coap_add_data_large_response(resource, session, request, response,
                               query, COAP_MEDIATYPE_TEXT_PLAIN, 1, 0,
                               len,
                               buf, NULL, NULL);

  /*
   * Returning from the request handler will cause the response to be
   * sent off (assuming coap_pdu_set_code() has been called), unless
   * the response is confirmable and the code is COAP_EMPTY_CODE which
   * will cause an empty ACK packet to be returned).
   */

}

SEE ALSO

coap_block(3), coap_observe(3), coap_oscore(3), coap_pdu_access(3) and coap_resource(3)

FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

"RFC7959: Block-Wise Transfers in the Constrained Application Protocol (CoAP)"

"RFC8613: Object Security for Constrained RESTful Environments (OSCORE)"

"RFC8974: Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)"

for further information.

See https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#option-numbers for the current set of defined CoAP Options.

BUGS

Please report bugs on the mailing list for libcoap: libcoap-developers@lists.sourceforge.net or raise an issue on GitHub at https://github.com/obgm/libcoap/issues

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>