DESCRIPTION

The CoAP Cache provides support for two opaque objects that can be used for tracking requests and responses.

The first is the ability to derive a Cache Key from the cacheable parts of a CoAP PDU as defined in "RFC7252 5.6. Caching" updated by "RFC7641 2. The Observe Option" and "RFC8132 2. Fetch Method".

The Cache Key is a SHA256 digest if libcoap was built with TLS support, otherwise it uses the internal coap_hash() function, using the information abstracted from the PDU and (optionally) the CoAP session.

This Cache Key can then be used to match against incoming PDUs and then appropriate action logic can take place.

There is support for excluding specific CoAP options from the Cache Key. Examples could be to exclude CoAP BLOCK1 and BLOCK2 Options for the client or server for ease of tracking a large PUT or GET response, but to not exclude these CoAP options in a proxy where it makes sense to cache the individual blocks.

The second is providing Cache Entries (which can be looked up by PDU and hence by Cache Key) which hold additional information to make information tracking simpler. These Cache Entries are automatically deleted when a session closes or a context is deleted. These Cache Entries are maintained on a hashed list for speed of lookup.

The following enums are defined.

typedef enum coap_cache_session_based_t {
  COAP_CACHE_NOT_SESSION_BASED,
  COAP_CACHE_IS_SESSION_BASED
} coap_cache_session_based_t;

typedef enum coap_cache_record_pdu_t {
  COAP_CACHE_NOT_RECORD_PDU,
  COAP_CACHE_RECORD_PDU
} coap_cache_record_pdu_t;

FUNCTIONS

Function: coap_cache_derive_key()

The coap_cache_derive_key() function abstracts all the non NoCacheKey CoAP options, ignores the CoAP Observe option and includes a FETCH body from pdu. If session_based is COAP_CACHE_IS_SESSION_BASED, then session pointer is also included. CoAP options can be specifically ignored by the use of coap_cache_ignore_options(). A digest is then built from all of the information and returned. NULL is returned on error.

Function: coap_cache_derive_key_w_ignore()

The coap_cache_derive_key_w_ignore() function abstracts all the non NoCacheKey CoAP options, ignores the CoAP Observe option and includes a FETCH body from pdu. Further options to ignore are specified by the ignore_count of ignore_options. If session_based is COAP_CACHE_IS_SESSION_BASED, then session pointer is also included. A digest is then built from all of the information and returned. NULL is returned on error.

Function: coap_delete_cache_key()

The coap_delete_cache_key() function deletes the cache_key that was returned from a coap_cache_derive_key() or coap_cache_derive_key_w_ignore() call.

Function: coap_cache_ignore_options()

The coap_cache_ignore_options() function is used to store in context a list of count options held in options. The specified options will not be included in the data used for the coap_cache_derive_key() function.

Function: coap_new_cache_entry()

The coap_new_cache_entry() function will create a new Cache Entry based on the Cache Key derived from the pdu, session_based and session. If record_pdu is COAP_CACHE_RECORD_PDU, then a copy of the pdu is stored in the Cache Entry for subsequent retrieval. The Cache Entry can also store application specific data (coap_cache_set_app_data() and coap_cache_get_app_data()). idle_timeout in seconds defines the length of time not being used before it gets deleted. If idle_timeout is set to 0, then the Cache Entry will not get idle expired. The created Cache Entry is returned, or NULL on error.

Function: coap_delete_cache_entry()

The coap_delete_cache_entry() function can be used to delete the Cache Entry cache_entry held within context. This will remove the Cache Entry from the hash lookup list and free off any internally held data. If the Cache Entry is session based, then it will automatically get deleted when the session is freed off or when the idle timeout expires.

Function: coap_cache_get_by_key()

The coap_cache_get_by_key() function will locate the Cache Entry held in the context environment that has Cache Key cache_key. Returns NULL if the Cache Key was not found.

Function: coap_cache_get_by_pdu()

The coap_cache_get_by_pdu() function will locate the Cache Entry held in the session environment that has a Cache Key derived from the pdu and whether session_based or not. This function calls coap_cache_derive_key() internally, and so normally coap_cache_ignore_options() would have previously been called with COAP_OPTION_BLOCK1 or COAP_OPTION_BLOCK2 to ignore the values held within these options.

Function: coap_cache_get_pdu()

The coap_cache_get_pdu() function returns the PDU that was stored with the Cache Entry when it was created with coap_new_cache_entry() and record_pdu was set to COAP_CACHE_RECORD_PDU. If a PDU was not initially stored, NULL is returned. NOTE: A copy of the returned PDU must be taken for use in sending a CoAP packet using coap_pdu_duplicate().

Function: coap_cache_set_app_data()

The coap_cache_set_app_data() function is used to associate data with the cache_entry. If callback is not NULL, it points to a function to free off data when the cache_entry is deleted. If any data has been previously stored in the cache_entry, the pointer to the old data will get overwritten, but the old data will not get freed off.

The callback handler function prototype is defined as:

/**
 * Callback to free off the app data when the cache-entry is
 * being deleted / freed off.
 *
 * @param data  The app data to be freed off.
 */
typedef void (*coap_cache_app_data_free_callback_t)(void *data);

Function: coap_cache_get_app_data()

The coap_cache_get_app_data() function is used to get the previously stored data in the cache_entry.

RETURN VALUES

coap_cache_derive_key() and coap_cache_derive_key_w_ignore() returns a newly created Cache Key or NULL if there is a creation failure.

coap_cache_ignore_options() returns 1 if success, 0 on failure.

coap_new_cache_entry(), coap_cache_get_by_key() and coap_cache_get_by_pdu() return the Cache Entry or NULL if there is a failure.

coap_cache_get_pdu() returns the PDU that is held within the Cache Entry or NULL if there is no PDU available.

coap_cache_get_app_data() returns the application data value previously set by the coap_cache_set_app_data() function or NULL.

EXAMPLES

PUT Handler supporting BLOCK1

#include <coap3/coap.h>

static coap_binary_t *example_data_ptr = NULL;
static int example_data_media_type = COAP_MEDIATYPE_TEXT_PLAIN;

static void
cache_free_app_data(void *data) {
  coap_binary_t *bdata = (coap_binary_t *)data;
  coap_delete_binary(bdata);
}

/*
 * Large Data PUT handler
 */

static void
hnd_put_example_data(coap_context_t *ctx,
                     coap_resource_t *resource,
                     coap_session_t *session,
                     coap_pdu_t *request,
                     coap_binary_t *token,
                     coap_string_t *query,
                     coap_pdu_t *response) {
  size_t size;
  const uint8_t *data;
  coap_opt_iterator_t opt_iter;
  coap_opt_t *option;
  size_t offset;
  size_t total;
  coap_binary_t *data_so_far;

  /* Remove (void) definition if variable is used */
  (void)ctx;
  (void)token;
  (void)query;

  if (coap_get_data_large(request, &size, &data, &offset, &total) &&
      size != total) {
    /*
     * A part of the data has been received (COAP_BLOCK_SINGLE_BODY not set).
     * However, total unfortunately is only an indication, so it is not safe to
     * allocate a block based on total.  As per
     * https://rfc-editor.org/rfc/rfc7959#section-4
     *   o  In a request carrying a Block1 Option, to indicate the current
     *         estimate the client has of the total size of the resource
     *         representation, measured in bytes ("size indication").
     *
     * coap_cache_ignore_options() must have previously been called with at
     * least COAP_OPTION_BLOCK1 set as the option value will change per block.
     */
    coap_cache_entry_t *cache_entry = coap_cache_get_by_pdu(session,
                                                            request,
                                                            COAP_CACHE_IS_SESSION_BASED);

    if (offset == 0) {
      if (!cache_entry) {
        /*
         * Set idle_timeout parameter to COAP_MAX_TRANSMIT_WAIT if you want
         * early removal on transmission failure. 0 means only delete when
         * the session is deleted as session_based is set here.
         */
        cache_entry = coap_new_cache_entry(session, request,
                                           COAP_CACHE_NOT_RECORD_PDU,
                                           COAP_CACHE_IS_SESSION_BASED, 0);
      } else {
        data_so_far = coap_cache_get_app_data(cache_entry);
        if (data_so_far) {
          coap_delete_binary(data_so_far);
          data_so_far = NULL;
        }
        coap_cache_set_app_data(cache_entry, NULL, NULL);
      }
    }
    if (!cache_entry) {
      if (offset == 0) {
        coap_log_warn("Unable to create a new cache entry\n");
      } else {
        coap_log_warn(
            "No cache entry available for the non-first BLOCK\n");
      }
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_INTERNAL_ERROR);
      return;
    }

    if (size) {
      /* Add in the new data to cache entry */
      data_so_far = coap_cache_get_app_data(cache_entry);
      data_so_far = coap_block_build_body(data_so_far, size, data,
                                          offset, total);
      /* Yes, data_so_far can be NULL if error */
      coap_cache_set_app_data(cache_entry, data_so_far, cache_free_app_data);
    }
    if (offset + size == total) {
      /* All the data is now in */
      data_so_far = coap_cache_get_app_data(cache_entry);
      coap_cache_set_app_data(cache_entry, NULL, NULL);
    } else {
      /* Give us the next block response */
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_CONTINUE);
      return;
    }
  } else {
    /* single body of data received */
    data_so_far = coap_new_binary(size);
    if (data_so_far) {
      memcpy(data_so_far->s, data, size);
    }
  }

  if (example_data_ptr) {
    /* pre-existed response */
    coap_pdu_set_code(response, COAP_RESPONSE_CODE_CHANGED);
    coap_delete_binary(example_data_ptr);
  } else {
    /* just generated response */
    coap_pdu_set_code(response, COAP_RESPONSE_CODE_CREATED);
  }

  example_data_ptr = data_so_far;
  if ((option = coap_check_option(request, COAP_OPTION_CONTENT_FORMAT,
                                  &opt_iter)) != NULL) {
    example_data_media_type =
        coap_decode_var_bytes(coap_opt_value(option),
                              coap_opt_length(option));
  } else {
    example_data_media_type = COAP_MEDIATYPE_TEXT_PLAIN;
  }

  coap_pdu_set_code(response, COAP_RESPONSE_CODE_CHANGED);
  coap_resource_notify_observers(resource, NULL);
}

int
main(int argc, char *argv[]) {
  coap_context_t *ctx = NULL;  /* Set up as normal */
  /* ... */
  uint16_t cache_ignore_options[] = { COAP_OPTION_BLOCK1,
                                      COAP_OPTION_BLOCK2
                                    };

  /* Initialize libcoap library */
  coap_startup();

  /* Remove (void) definition if variable is used */
  (void)argc;
  (void)argv;

  /* ... */

  /* Define the options to ignore when setting up cache-keys */
  coap_cache_ignore_options(ctx, cache_ignore_options,
                            sizeof(cache_ignore_options)/sizeof(cache_ignore_options[0]));

  /* ... */
  coap_cleanup();

}

SEE ALSO

coap_block(3), coap_init(3), coap_pdu_setup(3), coap_resource(3) and coap_string(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>