Difference between revisions of "Mqtt notes"

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m (MQTT demo basic TLS on Github.)
m (Add link to MQTT Last Will and Testament.)
 
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−
Notes on MQTT . . .
+
== Overview ==
 +
 
 +
Notes on MQTT
 +
 
 +
2021-12-20 - On devzone.nordicsemi.com creating public forum post with title "aws_iot MQTT subscriber keep alive", following suggested posts as solutions come up:
 +
 
 +
<ul>
 +
*  https://devzone.nordicsemi.com/f/nordic-q-a/47771/nrf9160-aws-iot-mqtt-keep-alive
 +
A possible Zephyr k_work_submit() solution:
 +
<ul>
 +
*  https://github.com/jtguggedal/fw-nrfconnect-nrf/commit/a21e6993050a8c992ed3d997a18bf11e184f7b29
 +
</ul>
 +
* 
 +
</ul>
 +
 
 +
<!-- comentario -->
 +
 
 +
== [[#top|^]] Tau and AT, iDRX and cDRX parameters ==
 +
 
 +
In response to post on Nordic Devzone:
 +
<ul>
 +
*  https://devzone.nordicsemi.com/f/nordic-q-a/52540/nrf9160-disconnects-after-4-5-minutes-when-keep-alive-time-is-60/213233#213233
 +
*  https://devzone.nordicsemi.com/power/w/opp/4/user-guide-lte
 +
</ul>
 +
Searching for reference document "3GPP":
 +
<ul>
 +
*  https://www.3gpp.org/technologies/keywords-acronyms/98-lte
 +
*  https://www.3gpp.org/component/itpgooglesearch/search?gsquery=UE
 +
</ul>
 +
A little further along with references to 3GPP Specification 27.007, and definition of User Equipment (UE):
 +
*  https://infocenter.nordicsemi.com/pdf/nrf91_at_commands_v1.4.1.pdf
 +
*  https://portal.3gpp.org/ChangeRequests.aspx?q=1&versionId=75070&release=192
 +
 
 +
<!-- comentario -->
 +
 
 +
== [[#top|^]] MQTT Persistent Sessions References ==
 +
 
 +
<ul>
 +
*  https://docs.aws.amazon.com/iot/latest/developerguide/mqtt.html#mqtt-persistent-sessions
 +
*  https://docs.aws.amazon.com/iot/latest/developerguide/mqtt.html#persistent-session-reconnect
 +
*  https://docs.aws.amazon.com/general/latest/gr/iot-core.html#message-broker-limits
 +
</ul>
 +
 
 +
<!-- comentario -->
 +
 
 +
== [[#top|^]] aws_iot Run-time Message Excerpts ==
 +
 
 +
Looks like in Nordic Semi's aws_iot sample app, the following enumerated events represent a firmware instance receiving subscribed messages from an MQTT broker:
 +
<pre>
 +
- 1108 DEV - aws_iot event handler called,
 +
AWS_IOT_EVT_READY
 +
- 1108 DEV - aws_iot event handler called,
 +
AWS_IOT_EVT_DATA_RECEIVED
 +
</pre>
 +
 
 +
s3://iot-poc/hello/world/
 +
 
 +
Ah, trouble when attempting to add MQTT topics:
 +
 
 +
<pre>
 +
*** Booting Zephyr OS build v2.6.0-rc1-ncs1  ***
 +
I: Modem library is not yet initialized, AT commands not sent
 +
I: Configuration of MAGPIO and COEX0 is left to drivers
 +
The AWS IoT sample started, version: v1.0.0
 +
ZZZ
 +
ZZZ
 +
ZZZ
 +
ZZZ - preparing to subscribe to hello/world topic,
 +
ZZZ
 +
ZZZ
 +
ZZZ - calling routine to subscribe to MQTT topics . . .
 +
ZZZ
 +
E: Application subscription list count mismatch
 +
aws_iot_subscription_topics_add, error: -122
 +
ZZZ
 +
ZZZ
 +
ZZZ
 +
ZZZ
 +
Adding application specific topics failed, error: -122
 +
Summary of callback pointers for 'set flag' events:
 +
</pre>
 +
 
 +
Appears to come from `./nrf/subsys/net/lib/aws_iot/src/aws_iot.c:1168: LOG_ERR("Application subscription list count mismatch");`
 +
 
 +
Note 3 - code excert from `nrf/subsys/net/lib/aws_iot/src/aws_iot.c`:
 +
<pre>
 +
689                if (!mqtt_evt->param.connack.session_present_flag ||
 +
690                    IS_ENABLED(CONFIG_MQTT_CLEAN_SESSION)) {
 +
691                        err = topic_subscribe();
 +
692
 +
693                        if (err < 0) {
 +
694                                aws_iot_evt.type = AWS_IOT_EVT_ERROR;
 +
695                                aws_iot_evt.data.err = err;
 +
696                                aws_iot_notify_event(&aws_iot_evt);
 +
697                                break;
 +
698                        }
 +
699                        if (err == 0) {
 +
700                                /* There were not topics to subscribe to. */
 +
701                                aws_iot_evt.type = AWS_IOT_EVT_READY;
 +
702                                aws_iot_notify_event(&aws_iot_evt);
 +
703                        } /* else: wait for SUBACK */
 +
704                } else {
 +
705                        /* pre-existing session:
 +
706                          * subscription is already established.
 +
707                          */
 +
708                        aws_iot_evt.type = AWS_IOT_EVT_READY;
 +
709                        aws_iot_notify_event(&aws_iot_evt);
 +
710
 +
711                        if (IS_ENABLED(
 +
</pre>
 +
 
 +
<pre>
 +
951 #if defined(CONFIG_AWS_IOT_LAST_WILL)
 +
952        static struct mqtt_topic last_will_topic = {
 +
953                .topic.utf8 = CONFIG_AWS_IOT_LAST_WILL_TOPIC,
 +
954                .topic.size = sizeof(CONFIG_AWS_IOT_LAST_WILL_TOPIC) - 1,
 +
955                .qos = MQTT_QOS_0_AT_MOST_ONCE
 +
956        };
 +
957
 +
958        static struct mqtt_utf8 last_will_message = {
 +
959                .utf8 = CONFIG_AWS_IOT_LAST_WILL_MESSAGE,
 +
960                .size = sizeof(CONFIG_AWS_IOT_LAST_WILL_MESSAGE) - 1
 +
961        };
 +
962
 +
963        client->will_topic = &last_will_topic;
 +
964        client->will_message = &last_will_message;
 +
965 #endif
 +
966
 +
967        static sec_tag_t sec_tag_list[] = { CONFIG_AWS_IOT_SEC_TAG };
 +
968        struct mqtt_sec_config *tls_cfg = &(client->transport).tls.config;
 +
969
 +
970        tls_cfg->peer_verify            = 2;
 +
971        tls_cfg->cipher_count          = 0;
 +
972        tls_cfg->cipher_list            = NULL;
 +
973        tls_cfg->sec_tag_count          = ARRAY_SIZE(sec_tag_list);
 +
974        tls_cfg->sec_tag_list          = sec_tag_list;
 +
975        tls_cfg->hostname              = CONFIG_AWS_IOT_BROKER_HOST_NAME;
 +
976        tls_cfg->session_cache = TLS_SESSION_CACHE_DISABLED;
 +
977
 +
978 #if !defined(CONFIG_NRF_MODEM_LIB)
 +
979        err = certificates_provision();
 +
980        if (err) {
 +
981                LOG_ERR("Could not provision certificates, error: %d", err);
 +
982                return err;
 +
983        }
 +
984 #endif /* !defined(CONFIG_NRF_MODEM_LIB) */
 +
985
 +
986        return err;
 +
987 }
 +
</pre>
 +
 
 +
<pre>
 +
z4-sandbox-kionix-work/nrf/subsys/net/lib/aws_iot/src$ grep -n 'struct mqtt_topic' ./*.*
 +
451: const struct mqtt_topic aws_iot_rx_list[] = {
 +
546: .list = (struct mqtt_topic *)&aws_iot_rx_list,
 +
952: static struct mqtt_topic last_will_topic = {
 +
</pre>
 +
 
 +
== edit point ==
 +
 
 +
<pre>
 +
 
 +
./zephyr/include/net/mqtt.h:178:struct mqtt_topic {
 +
 
 +
 
 +
 
 +
/** @brief Abstracts UTF-8 encoded strings. */
 +
struct mqtt_utf8 {
 +
        const uint8_t *utf8;      /**< Pointer to UTF-8 string. */
 +
        uint32_t size;            /**< Size of UTF string, in bytes. */
 +
};
 +
 
 +
 
 +
/** @brief Abstracts MQTT UTF-8 encoded topic that can be subscribed
 +
*        to or published.
 +
*/
 +
struct mqtt_topic {
 +
        /** Topic on to be published or subscribed to. */
 +
        struct mqtt_utf8 topic;
 +
 
 +
        /** Quality of service requested for the subscription.
 +
        *  @ref mqtt_qos for details.
 +
        */
 +
        uint8_t qos;
 +
};
 +
 
 +
 
 +
</pre>
 +
 
 +
A link to review:
 +
<ul>
 +
*  https://thenewstack.io/mqtt-protocol-iot/
 +
</ul>
 +
 
 +
 
 +
2021-12-14 Tue
 +
 
 +
*  https://forums.aws.amazon.com/index.jspa
 +
*  https://aws.amazon.com/blogs/iot/getting-started-mqtt-retained-messages-aws-iot-core/
 +
*  https://docs.aws.amazon.com/iot/latest/developerguide/mqtt.html#mqtt-retain
 +
 
 +
 
  
−
*  https://thenewstack.io/mqtt-protocol-iot/
 
  
 
2021-12-08
 
2021-12-08
  
 
  *  https://github.com/aws/aws-iot-device-sdk-embedded-C/blob/main/demos/mqtt/mqtt_demo_basic_tls/mqtt_demo_basic_tls.c
 
  *  https://github.com/aws/aws-iot-device-sdk-embedded-C/blob/main/demos/mqtt/mqtt_demo_basic_tls/mqtt_demo_basic_tls.c
 +
+++
 +
*  https://docs.aws.amazon.com/iot/latest/developerguide/topics.html
  
  
Line 24: Line 226:
 
Nordic AWS FOTA example:
 
Nordic AWS FOTA example:
 
  # https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.6.1/nrf/samples/nrf9160/aws_fota/README.html
 
  # https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.6.1/nrf/samples/nrf9160/aws_fota/README.html
 +
 +
 +
 +
== [[#top|^]] successful publish to hello/world topic ==
 +
 +
<pre>
 +
Created JSON message but not publishing to AWS IoT broker,
 +
- 1110 DEV - new file scoped flag for aws_iot holds 0,
 +
message is: {
 +
        "version":      "512",
 +
        "deviceType":  "PulseV2",
 +
        "deviceSerialNumber":  "352656101125105",
 +
        "state":        {
 +
                "reported":    {
 +
                        "ts":  1639641106680,
 +
                        "vrms": "0.406",
 +
                        "alert_vrms_threshhold_1":      "no_alert",
 +
                        "latitude":    "45.40494423760174",
 +
                        "longitude":    "-122.5310215439456",
 +
                        "altitude":    "0.0"
 +
                }
 +
        }
 +
}
 +
RRC mode: Connected
 +
- 1108 DEV - aws_iot event handler called,
 +
AWS_IOT_EVT_DATA_RECEIVED
 +
Data received from AWS IoT console:
 +
Topic: hello/world
 +
Message: {
 +
        "version":      "513",
 +
        "note": "test 010",
 +
        "KEYNAME001":  "*** **** *** **** ***",
 +
        "KEYNAME002":  "*** **** *** **** ***",
 +
        "KEYNAME003":  "*** **** *** **** ***"
 +
}
 +
adm: power_time_domain=0.003, power_freq_domain=0.002, diff=0.001, ratio=1.604
 +
- 1105 - calculated VRMS of 0.000000
 +
 +
 +
- MARK 7 - at every 100 CLI loop iterations
 +
button press count at 0
 +
adm: power_time_domain=0.002, power_freq_domain=0.001, diff=0.001, ratio=1.631
 +
- 1105 - calculated VRMS of 0.798562
 +
 +
 +
RRC mode: Idle
 +
</pre>
 +
 +
 +
== edit point ==
 +
 +
2021-12-21 Note:  parameter `aws_iot_config *const config` can be as the value NULL:
 +
 +
<pre>
 +
989 static int connect_client(struct aws_iot_config *const config)
 +
990 {
 +
991        int err;
 +
992
 +
993        err = client_broker_init(&client);
 +
994        if (err) {
 +
995                LOG_ERR("client_broker_init, error: %d", err);
 +
996                return err;
 +
997        }
 +
998
 +
999        err = mqtt_connect(&client);
 +
1000        if (err) {
 +
1001                LOG_ERR("mqtt_connect, error: %d", err);
 +
1002                err = connect_error_translate(err);
 +
1003                return err;
 +
1004        }
 +
1005
 +
1006        if (IS_ENABLED(CONFIG_AWS_IOT_SEND_TIMEOUT)) {
 +
1007                struct timeval timeout = {
 +
1008                        .tv_sec = CONFIG_AWS_IOT_SEND_TIMEOUT_SEC
 +
1009                };
 +
</pre>
 +
 +
 +
and in file [west_workspace]/nrf/ext/curl/lib/mqtt.c . . .
 +
 +
<pre>
 +
140 static CURLcode mqtt_connect(struct connectdata *conn)
 +
141 {
 +
142  CURLcode result = CURLE_OK;
 +
143  const size_t client_id_offset = 14;
 +
144  const size_t packetlen = client_id_offset + MQTT_CLIENTID_LEN;
 +
145  char client_id[MQTT_CLIENTID_LEN + 1] = "curl";
 +
146  const size_t clen = strlen("curl");
 +
147  char packet[32] = {
 +
148    MQTT_MSG_CONNECT,  /* packet type */
 +
149    0x00,              /* remaining length */
 +
150    0x00, 0x04,        /* protocol length */
 +
151    'M','Q','T','T',  /* protocol name */
 +
152    0x04,              /* protocol level */
 +
153    0x02,              /* CONNECT flag: CleanSession */
 +
154    0x00, 0x3c,        /* keep-alive 0 = disabled */
 +
155    0x00, 0x00        /* payload1 length */
 +
156  };
 +
157  packet[1] = (packetlen - 2) & 0x7f;
 +
158  packet[client_id_offset - 1] = MQTT_CLIENTID_LEN;
 +
159
 +
160  result = Curl_rand_hex(conn->data, (unsigned char *)&client_id[clen],
 +
161                          MQTT_CLIENTID_LEN - clen + 1);
 +
</pre>
 +
 +
 +
== MQTT LWT ==
 +
 +
*  https://www.hivemq.com/blog/mqtt-essentials-part-9-last-will-and-testament/

Latest revision as of 22:04, 24 October 2024

Overview

Notes on MQTT

2021-12-20 - On devzone.nordicsemi.com creating public forum post with title "aws_iot MQTT subscriber keep alive", following suggested posts as solutions come up:


^ Tau and AT, iDRX and cDRX parameters

In response to post on Nordic Devzone:

Searching for reference document "3GPP":

A little further along with references to 3GPP Specification 27.007, and definition of User Equipment (UE):


^ MQTT Persistent Sessions References


^ aws_iot Run-time Message Excerpts

Looks like in Nordic Semi's aws_iot sample app, the following enumerated events represent a firmware instance receiving subscribed messages from an MQTT broker: 

- 1108 DEV - aws_iot event handler called,
AWS_IOT_EVT_READY
- 1108 DEV - aws_iot event handler called,
AWS_IOT_EVT_DATA_RECEIVED

s3://iot-poc/hello/world/

Ah, trouble when attempting to add MQTT topics: 

*** Booting Zephyr OS build v2.6.0-rc1-ncs1  ***
I: Modem library is not yet initialized, AT commands not sent
I: Configuration of MAGPIO and COEX0 is left to drivers
The AWS IoT sample started, version: v1.0.0
ZZZ
ZZZ
ZZZ
ZZZ - preparing to subscribe to hello/world topic,
ZZZ
ZZZ
ZZZ - calling routine to subscribe to MQTT topics . . .
ZZZ
E: Application subscription list count mismatch
aws_iot_subscription_topics_add, error: -122
ZZZ
ZZZ
ZZZ
ZZZ
Adding application specific topics failed, error: -122
Summary of callback pointers for 'set flag' events:

Appears to come from `./nrf/subsys/net/lib/aws_iot/src/aws_iot.c:1168: LOG_ERR("Application subscription list count mismatch");`

Note 3 - code excert from `nrf/subsys/net/lib/aws_iot/src/aws_iot.c`: 

 689                 if (!mqtt_evt->param.connack.session_present_flag ||
 690                     IS_ENABLED(CONFIG_MQTT_CLEAN_SESSION)) {
 691                         err = topic_subscribe();
 692 
 693                         if (err < 0) {
 694                                 aws_iot_evt.type = AWS_IOT_EVT_ERROR;
 695                                 aws_iot_evt.data.err = err;
 696                                 aws_iot_notify_event(&aws_iot_evt);
 697                                 break;
 698                         }
 699                         if (err == 0) {
 700                                 /* There were not topics to subscribe to. */
 701                                 aws_iot_evt.type = AWS_IOT_EVT_READY;
 702                                 aws_iot_notify_event(&aws_iot_evt);
 703                         } /* else: wait for SUBACK */
 704                 } else {
 705                         /* pre-existing session:
 706                          * subscription is already established.
 707                          */
 708                         aws_iot_evt.type = AWS_IOT_EVT_READY;
 709                         aws_iot_notify_event(&aws_iot_evt);
 710 
 711                         if (IS_ENABLED(

 951 #if defined(CONFIG_AWS_IOT_LAST_WILL)
 952         static struct mqtt_topic last_will_topic = {
 953                 .topic.utf8 = CONFIG_AWS_IOT_LAST_WILL_TOPIC,
 954                 .topic.size = sizeof(CONFIG_AWS_IOT_LAST_WILL_TOPIC) - 1,
 955                 .qos = MQTT_QOS_0_AT_MOST_ONCE
 956         };
 957 
 958         static struct mqtt_utf8 last_will_message = {
 959                 .utf8 = CONFIG_AWS_IOT_LAST_WILL_MESSAGE,
 960                 .size = sizeof(CONFIG_AWS_IOT_LAST_WILL_MESSAGE) - 1
 961         };
 962 
 963         client->will_topic = &last_will_topic;
 964         client->will_message = &last_will_message;
 965 #endif
 966 
 967         static sec_tag_t sec_tag_list[] = { CONFIG_AWS_IOT_SEC_TAG };
 968         struct mqtt_sec_config *tls_cfg = &(client->transport).tls.config;
 969 
 970         tls_cfg->peer_verify            = 2;
 971         tls_cfg->cipher_count           = 0;
 972         tls_cfg->cipher_list            = NULL;
 973         tls_cfg->sec_tag_count          = ARRAY_SIZE(sec_tag_list);
 974         tls_cfg->sec_tag_list           = sec_tag_list;
 975         tls_cfg->hostname               = CONFIG_AWS_IOT_BROKER_HOST_NAME;
 976         tls_cfg->session_cache = TLS_SESSION_CACHE_DISABLED;
 977 
 978 #if !defined(CONFIG_NRF_MODEM_LIB)
 979         err = certificates_provision();
 980         if (err) {
 981                 LOG_ERR("Could not provision certificates, error: %d", err);
 982                 return err;
 983         }
 984 #endif /* !defined(CONFIG_NRF_MODEM_LIB) */
 985 
 986         return err;
 987 }

z4-sandbox-kionix-work/nrf/subsys/net/lib/aws_iot/src$ grep -n 'struct mqtt_topic' ./*.*
451:	const struct mqtt_topic aws_iot_rx_list[] = {
546:			.list = (struct mqtt_topic *)&aws_iot_rx_list,
952:	static struct mqtt_topic last_will_topic = {

edit point


./zephyr/include/net/mqtt.h:178:struct mqtt_topic {



/** @brief Abstracts UTF-8 encoded strings. */
struct mqtt_utf8 {
        const uint8_t *utf8;       /**< Pointer to UTF-8 string. */
        uint32_t size;             /**< Size of UTF string, in bytes. */
};


/** @brief Abstracts MQTT UTF-8 encoded topic that can be subscribed
 *         to or published.
 */
struct mqtt_topic {
        /** Topic on to be published or subscribed to. */
        struct mqtt_utf8 topic;

        /** Quality of service requested for the subscription.
         *  @ref mqtt_qos for details.
         */
        uint8_t qos;
};

A link to review:


2021-12-14 Tue



2021-12-08 

*  https://github.com/aws/aws-iot-device-sdk-embedded-C/blob/main/demos/mqtt/mqtt_demo_basic_tls/mqtt_demo_basic_tls.c

+++ 

*  https://docs.aws.amazon.com/iot/latest/developerguide/topics.html


Unsorted references to MQTT: 

# https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/samples/net/mqtt_publisher/README.html
# https://www.hivemq.com/blog/mqtt-essentials-part2-publish-subscribe/
# https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/samples/index.html
# https://www.circuito.io/blog/arduino-code/
# https://www.hivemq.com/blog/mqtt-essentials-part-7-persistent-session-queuing-messages/
# https://www.hivemq.com/blog/mqtt-essentials-part-3-client-broker-connection-establishment/



# https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.6.1/nrf/examples.html
# https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.6.1/nrf/ug_tfm.html#ug-tfm
# https://ci-builds.trustedfirmware.org/static-files/UxIoKYsDyPQ0ojvJOv9EX6-hZNDZd6IPGfcm05Bk1b0xNjM3MTA4NjMxODUxOjk6YW5vbnltb3VzOmpvYi90Zi1tLWJ1aWxkLWRvY3MtbmlnaHRseS9sYXN0U3RhYmxlQnVpbGQvYXJ0aWZhY3Q=/trusted-firmware-m/build/docs/user_guide/html/index.html

Nordic AWS FOTA example: 

# https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.6.1/nrf/samples/nrf9160/aws_fota/README.html


^ successful publish to hello/world topic

Created JSON message but not publishing to AWS IoT broker,
- 1110 DEV - new file scoped flag for aws_iot holds 0,
message is: {
        "version":      "512",
        "deviceType":   "PulseV2",
        "deviceSerialNumber":   "352656101125105",
        "state":        {
                "reported":     {
                        "ts":   1639641106680,
                        "vrms": "0.406",
                        "alert_vrms_threshhold_1":      "no_alert",
                        "latitude":     "45.40494423760174",
                        "longitude":    "-122.5310215439456",
                        "altitude":     "0.0"
                }
        }
}
RRC mode: Connected
- 1108 DEV - aws_iot event handler called,
AWS_IOT_EVT_DATA_RECEIVED
Data received from AWS IoT console:
Topic: hello/world
Message: {
        "version":      "513",
        "note": "test 010",
        "KEYNAME001":   "*** **** *** **** ***",
        "KEYNAME002":   "*** **** *** **** ***",
        "KEYNAME003":   "*** **** *** **** ***"
}
adm: power_time_domain=0.003, power_freq_domain=0.002, diff=0.001, ratio=1.604
- 1105 - calculated VRMS of 0.000000


- MARK 7 - at every 100 CLI loop iterations
button press count at 0
adm: power_time_domain=0.002, power_freq_domain=0.001, diff=0.001, ratio=1.631
- 1105 - calculated VRMS of 0.798562


RRC mode: Idle


edit point

2021-12-21 Note: parameter `aws_iot_config *const config` can be as the value NULL: 

 989 static int connect_client(struct aws_iot_config *const config)
 990 {
 991         int err;
 992 
 993         err = client_broker_init(&client);
 994         if (err) {
 995                 LOG_ERR("client_broker_init, error: %d", err);
 996                 return err;
 997         }
 998 
 999         err = mqtt_connect(&client);
1000         if (err) {
1001                 LOG_ERR("mqtt_connect, error: %d", err);
1002                 err = connect_error_translate(err);
1003                 return err;
1004         }
1005 
1006         if (IS_ENABLED(CONFIG_AWS_IOT_SEND_TIMEOUT)) {
1007                 struct timeval timeout = {
1008                         .tv_sec = CONFIG_AWS_IOT_SEND_TIMEOUT_SEC
1009                 };


and in file [west_workspace]/nrf/ext/curl/lib/mqtt.c . . . 

140 static CURLcode mqtt_connect(struct connectdata *conn)
141 {
142   CURLcode result = CURLE_OK;
143   const size_t client_id_offset = 14;
144   const size_t packetlen = client_id_offset + MQTT_CLIENTID_LEN;
145   char client_id[MQTT_CLIENTID_LEN + 1] = "curl";
146   const size_t clen = strlen("curl");
147   char packet[32] = {
148     MQTT_MSG_CONNECT,  /* packet type */
149     0x00,              /* remaining length */
150     0x00, 0x04,        /* protocol length */
151     'M','Q','T','T',   /* protocol name */
152     0x04,              /* protocol level */
153     0x02,              /* CONNECT flag: CleanSession */
154     0x00, 0x3c,        /* keep-alive 0 = disabled */
155     0x00, 0x00         /* payload1 length */
156   };
157   packet[1] = (packetlen - 2) & 0x7f;
158   packet[client_id_offset - 1] = MQTT_CLIENTID_LEN;
159 
160   result = Curl_rand_hex(conn->data, (unsigned char *)&client_id[clen],
161                          MQTT_CLIENTID_LEN - clen + 1);


MQTT LWT

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