libcoap 4.3.1


#include <coap3/coap.h>

void coap_session_set_ack_random_factor(coap_session_t *session, coap_fixed_point_t value);

coap_fixed_point_t coap_session_get_ack_random_factor( const coap_session_t *session);

void coap_session_set_ack_timeout(coap_session_t *session, coap_fixed_point_t value);

coap_fixed_point_t coap_session_get_ack_timeout( const coap_session_t *session);

void coap_session_set_default_leisure(coap_session_t *session, coap_fixed_point_t value);

coap_fixed_point_t coap_session_get_default_leisure( const coap_session_t *session);

void coap_session_set_max_retransmit(coap_session_t *session, uint16_t value);

uint16_t coap_session_get_max_retransmit(const coap_session_t *session);

void coap_session_set_nstart(coap_session_t *session, uint16_t value);

uint16_t coap_session_get_nstart(const coap_session_t *session);

void coap_session_set_probing_rate(coap_session_t *session, uint32_t value);

uint32_t coap_session_get_probing_rate(const coap_session_t *session);

int coap_debug_set_packet_loss(const char *loss_level);

For specific (D)TLS library support, link with -lcoap-3-notls, -lcoap-3-gnutls, -lcoap-3-openssl, -lcoap-3-mbedtls or -lcoap-3-tinydtls. Otherwise, link with -lcoap-3 to get the default (D)TLS library support.


For CoAP Confirmable messages, it is possible to define the retry counts, repeat rate etc. for error recovery. Further information can be found in "RFC7272: 4.2. Messages Transmitted Reliably", with the default values defined in "RFC7272: 4.8 Transmission Parameters".

It is not recommended that the suggested default setting are changed, but there may be some special requirements that need different values and the consequences of changing these values is fully understood.

Some of the parameters or return values are in fixed point format as defined by the coap_fixed_point_t structure as below

typedef struct coap_fixed_point_t {
  uint16_t integer_part;    /* Integer part of fixed point variable */
  uint16_t fractional_part; /* Fractional part of fixed point variable
                               1/1000 (3 points) precision */
} coap_fixed_point_t;

The CoAP message retry rules are (with the default values to compute the time)

1st retransmit after 1 * ack_timeout * ack_random factor (3 seconds)
2nd retransmit after 2 * ack_timeout * ack_random factor (6 seconds)
3rd retransmit after 3 * ack_timeout * ack_random factor (12 seconds)
4th retransmit after 4 * ack_timeout * ack_random factor (24 seconds)
5th retransmit after 5 * ack_timeout * ack_random factor (48 seconds)

As max_retransmit (by default) is 4, then the 5th retransmit does not get sent,
but at that point COAP_NACK_TOO_MANY_RETRIES gets raised in the nack_handler
(if defined). Note that the sum of the seconds is 93 matching RFC7252.

It should be noted that these retries are separate from the DTLS or TLS encrypted session setup retry timeouts. For DTLS, the initial requesting packet will get sent max_retransmit times before reporting failure. For TLS the initial TCP connection will timeout before reporting failure.

It is also possible to set up packet losses, for both confirmable, and non-confirmable messages. This can be used for stress testing packet transmission recovery as well as application handling of lossy networks.

The following functions reflect the RFC7252 uppercase names in lowercase following the coap_session[sg]_ function prefix.

The coap_session_set_ack_random_factor() function updates the session ack random wait factor, used to randomize re-transmissions, with the new value. The default value is 1.5.

The coap_session_get_ack_random_factor() function returns the current session ack random wait factor.

The coap_session_set_ack_timeout() function updates the session initial ack or response timeout with the new value. The default value is 2.0.

The coap_session_get_ack_timeout() function returns the current session initial ack or response timeout.

The coap_session_set_default_leisure() function updates the session default leisure time with the new value. The initial default value is 5.0.

The coap_session_get_default_leisure() function returns the current session default leisure time.

The coap_session_set_max_retransmit() function updates the session maximum retransmit count with the new value. The default value is 4.

The coap_session_get_max_retransmit() function returns the current session maximum retransmit count.

The coap_session_set_nstart() function updates the session nstart with the new value. The default value is 1.

The coap_session_get_nstart() function returns the current session nstart value.

The coap_session_set_probing_rate() function updates the session probing rate with the new value. The default value is 1 byte per second.

The coap_session_get_probing_rate() function returns the current session probing rate value.

The coap_debug_set_packet_loss() function is uses to set the packet loss levels as defined in loss_level. loss_level can be set as a percentage from "0%" to "100%". Alternatively, it is possible to specify which packets of a packet sequence are dropped. A definition of "1,5-9,11-20,101" means that packets 1, 5 through 9, 11 through 20 and 101 will get dropped. A maximum of 10 different packet sets is supported. The packet count is reset to 0 when coap_debug_set_packet_loss() is called. To remove any packet losses, set the loss_level to "0%".


coap_session_get_ack_random_factor(), coap_session_get_ack_timeout(), coap_session_get_default_leisure(), coap_session_get_max_retransmit(), coap_session_get_nstart() and coap_session_get_probing_rate() return their respective current values.

coap_debug_set_packet_loss() returns 0 if loss_level does not parse correctly, otherwise 1 if successful.


The libcoap recovery/re-transmit logic will only work for confirmable requests.

To see what is happening (other than by sniffing the network traffic), the logging level needs to be set to LOG_DEBUG in the client by using coap_set_log_level(LOG_DEBUG) and coap_dtls_set_log_level(LOG_DEBUG).

The client needs to be sending confirmable requests during the test.

The server can either be stopped, or if packet loss levels are set to 100% by using coap_debug_set_packet_loss("100%") when receiving the client requests.

NOTE: If the server end of the connection is returning ICMP unreachable packets after being turned off, you will get a debug message of the form "coap_network_read: unreachable", so libcoap will stop doing the retries. If this is the case, then you need to make use of (on the server) coap_debug_set_packet_loss("100%") or put in some packet filtering to drop the packets.

The client should then restart transmitting the requests based on the ack_timeout, ack_random_factor and max_retransmit values. The client’s nack_handler will get called with COAP_NACK_TOO_MANY_RETRIES when the confirmable request cannot be successfully transmitted.


See "RFC7252: The Constrained Application Protocol (CoAP)" for further information.


Please report bugs on the mailing list for libcoap: or raise an issue on GitHub at


The libcoap project <>