DESCRIPTION

Regular setting up of a PDU and transmission is covered in coap_pdu_setup(3) where all the payload data can fit in a single packet. This man page covers how to work with PDUs where the overall body of information may need to be split across several packets by using CoAP Block-Wise Transfers (RFC7959 and RFC9177).

The block-wise transfers can be controlled by the application, or libcoap is instructed to do all the requests for the next blocks and only present the final body of the result to the application. In summary, the following three ways handle processing a body of data that has to be split across multiple payloads (blocks).

This man page focuses on getting libcoap to do all the work, not how to do it all in the application.

However, if the client supplies a Block1 or Block2 Option in a GET request type PDU where the block number is not 0, this is assumed to be a random access request and any other blocks will not be requested by libcoap even if instructed otherwise.

The functions that are named _large are intended as replacements for the equivalent functions as described in coap_pdu_setup(3).

CALLBACK HANDLER

Callback Type: coap_release_large_data_t

/**
 * Callback handler for de-allocating the data based on @p app_ptr provided to
 * coap_add_data_large_*() functions following transmission of the supplied
 * data.
 *
 * @param session The session that this data is associated with
 * @param app_ptr The application provided pointer to the
 *                coap_add_data_large_*() functions
 */
typedef void (*coap_release_large_data_t)(coap_session_t *session,
                                          void *app_ptr);

FUNCTIONS

Function: coap_context_set_block_mode()

The coap_context_set_block_mode() function is used to set up the context level block_mode block handling bits for supporting RFC7959 block_mode flows down to a session when a session is created and if the peer does not support the respective block mode, an appropriate bit may get disabled in the session block_mode.

#define COAP_BLOCK_USE_LIBCOAP   0x01 /* Use libcoap to do block requests */
#define COAP_BLOCK_SINGLE_BODY   0x02 /* Deliver the data as a single body */
#define COAP_BLOCK_TRY_Q_BLOCK   0x04 /* Try Q-Block method */
#define COAP_BLOCK_USE_M_Q_BLOCK 0x08 /* (Client) Use M bit when recovering Q-Block2 */
#define COAP_BLOCK_NO_PREEMPTIVE_RTAG 0x10 /* (Client) Don't use pre-emptive Request-Tags */
#define COAP_BLOCK_STLESS_FETCH  0x20 /* (Client) Assume server supports stateless FETCH */
#define COAP_BLOCK_STLESS_BLOCK2 0x40 /* (Server) Server is stateless for Block2 transfers */
#define COAP_BLOCK_NOT_RANDOM_BLOCK1 0x80 /* (Server) Disable server handling random order block1 */

block_mode is an or’d set of zero or more COAP_BLOCK_* definitions.

If COAP_BLOCK_USE_LIBCOAP is not set, then everything works as per Option 1 above.

If COAP_BLOCK_SINGLE_BODY is set, then the entire body of data is presented to the receiving handler, otherwise each individual block is presented on arrival. To obtain the data, length and current offset, coap_get_data_large() must be used instead of coap_get_data(). It may be appropriate not to set COAP_BLOCK_SINGLE_BODY if there are RAM limitations.

NOTE: It is the responsibility of the receiving application to re-assemble the data as appropriate (e.g., using coap_block_build_body()) if COAP_BLOCK_SINGLE_BODY is not set.

NOTE: If COAP_BLOCK_SINGLE_BODY is not set, then the CoAP server on receiving request data that is split over multiple data blocks must respond with COAP_RESPONSE_CODE_CONTINUE 2.31 (Continue) response code if the received data is not for the final block, otherwise a COAP_RESPONSE_CODE_CREATED 2.01 (Created) or COAP_RESPONSE_CODE_CHANGED 2.04 (Changed) should be returned.

To indicate support for Q-Block-1 and Q-Block2, COAP_BLOCK_TRY_Q_BLOCK needs to be set on both the client and server. COAP_BLOCK_SINGLE_BODY is assumed to be set if using Q-Block as the data will always be presented as a single body. If COAP_BLOCK_USE_M_Q_BLOCK is defined, then the M bit version of recovery will be used if possible.

If COAP_BLOCK_USE_LIBCOAP is set, then any PDUs presented to the application handlers will get the tokens set back to the initiating token so that requests can be matched with responses even if different tokens had to be used for the series of packet interchanges. Furthermore, if COAP_BLOCK_SINGLE_BODY is set, then the PDU that presents the entire body will have any BlockX or Q-BlockX option removed.

NOTE: COAP_BLOCK_USE_LIBCOAP must be set if libcoap is to do all the block tracking and requesting, otherwise the application will have to do all of this work (the default if coap_context_set_block_mode() is not called).

If COAP_BLOCK_NO_PREEMPTIVE_RTAG is set, then Request-Tag options are only sent when a large amount of data is being sent to the server using the Block1 option. Otherwise, a Request-Tag option is sent with any request (apart from DELETE) on the off chance that there may be multiple Block2 based responses for multiple requests to the same resource that need to be differentiated between.

If COAP_BLOCK_STLESS_FETCH is set, then it is assumed that the server supports stateless support for FETCH requests - in other words, the FETCH request always includes the FETCH data when requesting the next block of data from the server when using Block2.

If COAP_BLOCK_STLESS_BLOCK2 is set, then the server does not maintain state for any data returned that uses Block2 to split up the data chunks. The application is called for every request to get the complete set of data which is then split into the separate Block2 responses as appropriate.

If COAP_BLOCK_NOT_RANDOM_BLOCK1 is set, then the server does not allow any Block1 packets arriving in the wrong block number order by returning 4.08 error codes. Normally this should not be set as networks can re-order the arrival of UDP packets, in particular this may happen if NON is being used for Block1 by the client.

NOTE: This is ignored if Q-Block1 is being used.

Function: coap_context_set_max_block_size()

The coap_context_set_max_block_size() function is used to set the max_block_size in the context that a server supports when the Block1 or Block2 options are used. This must be set before a server session is created.

Function: coap_add_data_large_request()

The coap_add_data_large_request() function is similar to coap_add_data(), but supports the transmission of data that has a body size that is potentially larger than can be fitted into a single client request PDU. The specified payload data of length length is associated with the session with the first block of data added to the PDU pdu along with the appropriate CoAP options such as (Q-)Block1, Size1 and Request-Tag if the data does not fit in a single PDU.

When the block receipt has been acknowledged by the peer, the library will then send the next block of data until all the data has been transmitted.

The data passed to the function coap_add_data_large_request() must exist until all blocks have been transmitted. The callback function release_func can be used to release storage that has been dynamically allocated to hold the transmit data. If not NULL, the callback function is called once the final block of data has been transmitted. The user-defined parameter app_ptr is the same value that was passed to coap_add_data_large_request(). Even if there is an error return, release_func (if set) is always called.

NOTE: This function must only be called once per pdu.

NOTE: Options cannot be added to the pdu after coap_add_data_large_request() is called.

Function: coap_add_data_large_response()

The coap_add_data_large_response() function is responsible for handling the server’s large responses to requests. coap_add_data_large_response() should be used as a direct replacement for coap_add_data() if it is possible that the length of data will not fit in a single server’s response pdu. This function adds in the initial part of the payload data of length length to the PDU pdu.

The data passed to the function coap_add_data_large_response() must exist until all blocks have been transmitted. The callback function release_func can be used to release storage that has been dynamically allocated to hold the transmit data. If not NULL, the callback function is called once the final block of data has been transmitted. The user-defined parameter app_ptr is the same value that was passed to coap_add_data_large_response(). Even if there is an error return, release_func (if set) is always called.

It also adds in the appropriate CoAP options such as Block2, Size2 and ETag to handle block-wise transfer if the data does not fit in a single PDU.

resource, query, session, request, and response are the same parameters as in the called resource handler that invokes coap_add_data_large_response(). If etag is 0, then a unique ETag value will be generated, else is the ETag value to use. The media_type is for the format of the data and maxage defines the lifetime of the response. If maxage is set to -1, then the Max-Age option does not get included (which indicates the default value of 60 seconds according to "RFC7252 5.6.1. Freshness Model").

The application request handler for the resource is only called once instead of potentially multiple times, unless if COAP_BLOCK_STLESS_BLOCK2 is set.

NOTE: This function must only be called once per pdu.

NOTE: Options cannot be added to the pdu after coap_add_data_large_request() is called.

Function: coap_get_data_large()

The coap_get_data_large() function is used abstract from the pdu information about the received data by updating length with the length of data available, data with a pointer to where the data is located, offset with where this block of data starts and total with the total amount of data. offset will always be zero if block_mode includes COAP_BLOCK_SINGLE_BODY. All of the body’s data has been received if "offset + length == total".

NOTE: total is potentially only an indication of the total size of the body and is only exact when all of the data has been received.

Function: coap_block_build_body()

The coap_block_build_body() function is used to re-assemble the received data as returned by coap_get_data_large() into a single blob of data. Data from data of length length starting from offset offset is added to body_data. The resultant state of body_data is returned. If body_data is NULL, or total is larger than the current size of body_data, then body_data is re-allocated and returned. If there is an error, body_data gets de-allocated.

If block_mode (as set by coap_context_set_block_mode()) includes COAP_BLOCK_SINGLE_BODY, then the request/response handler will only get called once with the entire body containing the data from all of the individual blocks. If there is a change of data during the blocks receipt (e.g., ETag value changes), then the entire set of data is re-requested and the partial body dropped.

Function: coap_q_block_is_supported()

The coap_q_block_is_supported() function is used to determine whether libcoap has been build with Q-Block support or not.

RETURN VALUES

coap_add_data_large_request(), coap_add_data_large_response(), and coap_get_data_large() return 0 on failure, 1 on success.

coap_block_build_body() returns the current state of the body’s data (which may have some missing gaps) or NULL on error.

coap_context_set_max_block_size() returns 0 on failure, 1 on success.

coap_q_block_is_supported() returns 0 on failure, 1 on success.

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 *path,
               const char *query, unsigned char *data, size_t length,
               int observe) {
  coap_pdu_t *pdu;
  uint8_t buf[8];
  size_t buflen;
  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 (path) {
    /* Add in the Uri-Path options */
    if (!coap_path_into_optlist((const uint8_t *)path, strlen(path),
                                COAP_OPTION_URI_PATH, &optlist_chain))
      goto error;
  }

  if (query) {
    /* Add in the Uri-Query options */
    if (!coap_query_into_optlist((const uint8_t *)query, strlen(query),
                                 COAP_OPTION_URI_QUERY, &optlist_chain))
      goto error;
  }

  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_large_request(session, pdu, length, data, NULL, NULL))
      goto error;
  }

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

error:

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

}

int
main(int argc, char *argv[]) {
  coap_context_t *context = NULL;
  coap_session_t *session = NULL;
  unsigned char *data = NULL;
  size_t data_length = 0;

  (void)argc;
  (void)argv;

  /* Initialize libcoap library */
  coap_startup();

  /* ... Set up context, session etc. ... */

  /* Set up using libcoap to do the block work */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);

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

  /* .. build data and define data_length ... */

  build_send_pdu(context, session, COAP_MESSAGE_CON, COAP_REQUEST_PUT,
                 "/example/uri", NULL, data, data_length, 0);

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

  coap_cleanup();
  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 failure response and return if needed */

  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.
   * [A good practice, even though ins this case, the amount of data is small]
   *
   * Define the format - COAP_MEDIATYPE_TEXT_PLAIN - to add in
   * Define how long this response is valid for (secs) - 1 - to add in.
   *
   * Observe Option added internally if needed within the function
   * Block2 Option added internally if output too large
   * Size2 Option added internally
   * ETag Option added internally
   */
  coap_add_data_large_response(resource, session, request, response,
                               query, COAP_MEDIATYPE_TEXT_PLAIN, 1, 0,
                               len,
                               buf,
                               NULL, NULL);
  /*
   * When request handler returns, the response pdu will get automatically
   * sent, unless the pdu code is not updated and this is a NON or TCP based
   * request.
   */
}

int
main(int argc, char *argv[]) {
  coap_context_t *context = NULL;
  coap_resource_t *r;
  coap_resource_t *time_resource;
  int not_exit = 1;

  /* Initialize libcoap library */
  coap_startup();

  (void)argc;
  (void)argv;

  /* ... Set up context etc. ... */

  /* Set up using libcoap to do the block work */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);

  /* Create a resource to return time */
  r = coap_resource_init(coap_make_str_const("time"),
                         COAP_RESOURCE_FLAGS_NOTIFY_CON);
  coap_resource_set_get_observable(r, 1);
  coap_register_request_handler(r, COAP_REQUEST_GET, hnd_get_time);

  /* Document resource for 'time' request */
  coap_add_attr(r, coap_make_str_const("ct"), coap_make_str_const("0"), 0);
  coap_add_attr(r, coap_make_str_const("title"),
                coap_make_str_const("\"Internal Clock\""), 0);
  coap_add_attr(r, coap_make_str_const("rt"), coap_make_str_const("\"secs\""),
                0);
  coap_add_attr(r, coap_make_str_const("if"), coap_make_str_const("\"clock\""),
                0);

  coap_add_resource(context, r);
  time_resource = r;

  /* ... Loop waiting for incoming traffic ... */
  while (!not_exit) {
    coap_io_process(context, 1000);

    /* Cause a notification to anyone Observing 'time' */
    coap_resource_notify_observers(time_resource, NULL);
  }

  /* Clean up */

  coap_free_context(context);
  coap_cleanup();

}

SEE ALSO

coap_init(3) coap_pdu_setup(3), coap_observe(3), and coap_resource(3)

FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

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

for further information.

BUGS

Please raise an issue on GitHub at https://github.com/obgm/libcoap/issues to report any bugs.

Please raise a Pull Request at https://github.com/obgm/libcoap/pulls for any fixes.

AUTHORS

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