libcoap  4.2.0


coap_recovery, coap_session_set_max_retransmit, coap_session_set_ack_timeout, coap_session_set_ack_random_factor, coap_session_get_max_transmit, coap_session_get_ack_timeout, coap_session_get_ack_random_factor, coap_debug_set_packet_loss — Work with CoAP packet transmissions


#include <coap2/coap.h>

void coap_session_set_max_retransmit(coap_session_t *session, unsigned int value);

void coap_session_set_ack_timeout(coap_session_t *session, coap_fixed_point_t value);

void coap_session_set_ack_random_factor(coap_session_t *session, coap_fixed_point_t value);

unsigned int coap_session_get_max_transmit(coap_session_t *session);

coap_fixed_point_t coap_session_get_ack_timeout(coap_session_t *session);

coap_fixed_point_t coap_session_get_ack_random_factor(coap_session_t *session);

int coap_debug_set_packet_loss(const char *loss_level);

Link with -lcoap-2, -lcoap-2-gnutls, -lcoap-2-openssl or -lcoap-2-tinydtls depending on your (D)TLS library type.


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".

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.

Changing the default values for multicast packets is not supported.

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

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_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_max_retransmit() function returns the current session maximum retransmit count.

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

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

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_max_retransmit(), coap_session_get_ack_timeout() and coap_session_get_ack_random_factor() 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:


The libcoap project <>