DESCRIPTION

There is CoAP Confirmable message transmission recovery as defined in RFC7252, as well CoAP Non-Confirmable split into blocks message transmission recovery support as defined in RFC9177.

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 Confirmable 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
"https://rfc-editor.org/rfc/rfc7252#section-4.8.2[RFC7252 4.8.2.
MAX_TRANSMIT_WAIT]".

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.

For CoAP Non-Confirmable messages with RFC9177 enabled at both ends of a session, it is possible to define the retry counts etc. for data recovery. Further information can be found in "RFC9177 7.2. Non-confirmable (NON)".

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.

FUNCTIONS

The following functions reflect the RFC7252 and RFC9177 uppercase names in lowercase following the coap_session_set_ or coap_session_get_ function prefix.

Function: coap_session_set_ack_random_factor()

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 (RFC7252).

Function: coap_session_get_ack_random_factor()

The coap_session_get_ack_random_factor() function returns the current session ack random wait factor (RFC7252).

Function: coap_session_set_ack_timeout()

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 (RFC7252).

Function: coap_session_get_ack_timeout()

The coap_session_get_ack_timeout() function returns the current session initial ack or response timeout (RFC7252).

Function: coap_session_set_default_leisure()

The coap_session_set_default_leisure() function updates the session default leisure time with the new value. The default value is 5.0 (RFC7252).

Function: coap_session_get_default_leisure()

The coap_session_get_default_leisure() function returns the current session default leisure time (RFC7252).

Function: coap_session_set_max_payloads()

The coap_session_set_max_payloads() function updates the session maximum payloads count with the new value. The default value is 10 (RFC9177).

NOTE: Both ends of the session must have the same maximum payloads value to minimize any recovery times.

Function: coap_session_get_max_payloads()

The coap_session_get_max_payloads() function returns the current session maximum payloads count (RFC9177).

Function: coap_session_set_max_retransmit()

The coap_session_set_max_retransmit() function updates the session maximum retransmit count with the new value. The default value is 4 (RFC7252).

Function: coap_session_get_max_retransmit()

The coap_session_get_max_retransmit() function returns the current session maximum retransmit count (RFC7252).

Function: coap_session_set_non_max_retransmit()

The coap_session_set_non_max_retransmit() function updates the session maximum non retransmit count with the new value. The default value is 4 (RFC9177).

Function: coap_session_get_non_max_retransmit()

The coap_session_get_non_max_retransmit() function returns the current session maximum non retransmit count (RFC9177).

Function: coap_session_set_non_receive_timeout()

The coap_session_set_non_receive_timeout() function updates the session non receive timeout with the new value. The default value is 4.0 (RFC9177).

Function: coap_session_get_non_receive_timeout()

The coap_session_get_non_receive_timeout() function returns the current session non receive timeout (RFC9177).

Function: coap_session_set_non_timeout()

The coap_session_set_non_timeout() function updates the session non timeout with the new value. The default value is 2.0 (RFC9177).

Function: coap_session_get_non_timeout()

The coap_session_get_non_timeout() function returns the current session non timeout (RFC9177).

Function: coap_session_set_nstart()

The coap_session_set_nstart() function updates the session nstart with the new value. The default value is 1 (RFC7252).

Function: coap_session_set_probing_rate()

The coap_session_get_nstart() function returns the current session nstart value (RFC7252).

Function: coap_session_set_probing_rate()

The coap_session_set_probing_rate() function updates the session probing rate with the new value. The default value is 1 byte per second (RFC7252).

Function: coap_session_get_probing_rate()

The coap_session_get_probing_rate() function returns the current session probing rate value (RFC7252).

Function: coap_debug_set_packet_loss()

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%".

RETURN VALUES

coap_session_get_ack_random_factor(), coap_session_get_ack_timeout(), coap_session_get_default_leisure(), coap_session_get_max_payloads(), coap_session_get_max_retransmit(), coap_session_get_non_max_retransmit(), coap_session_get_non_receive_timeout(), coap_session_get_non_timeout(), 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.

TESTING

The libcoap RFC7252 recovery/re-transmit logic will only work for confirmable requests or if RFC9177 support is enabled for non-confirmable large body transmissions that need to span multiple packets.

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

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.

For confirmable transmissions, 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.

For non-confirmable block transmissions (RFC9177 enabled), the client should then restart transmitting the missing requests based on the max_payloads, non_timeout, non_receive_timeout, and non_max_retransmit values. The client’s nack_handler will get called with COAP_NACK_TOO_MANY_RETRIES when the non-confirmable large body request cannot be successfully transmitted.

FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

"RFC9177: Constrained Application Protocol (CoAP) Block-Wise Transfer Options Supporting Robust Transmission"

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>