Issue with closing a socket after sending a UDP message

Question

Dear All,

I need to develop a low-power application that sends a UDP message, waits for a certain amount of time (5 seconds) for a response and if there is no response it closes the socket and goes to sleep. I am using the nRF9160DK

This is the functions I use to connect send and receive data:

int blockingRecv(int fd, u8_t *buf, u32_t size, u32_t flags)
{
	int err;
    uint32_t start = (uint32_t)k_uptime_get();
	do {
		err = recv(fd, buf, size, flags);
 	} while (err < 0 && errno == EAGAIN && (uint32_t)k_uptime_get() - start < UDP_TIMEOUT_MS);

	return err;
}

int blockingSend(int fd, u8_t *buf, u32_t size, u32_t flags)
{
	int err;
    uint32_t start = (uint32_t)k_uptime_get();
	do {
		err = send(fd, buf, size, flags);
	} while (err < 0 && errno == EAGAIN && (uint32_t)k_uptime_get() - start < UDP_TIMEOUT_MS);

	return err;
}

int blockingConnect(int fd, struct sockaddr *local_addr, socklen_t len)
{
	int err;
    uint32_t start = (uint32_t)k_uptime_get();
	do {
		err = connect(fd, local_addr, len);
	} while (err < 0 && errno == EAGAIN && (uint32_t)k_uptime_get() - start < UDP_TIMEOUT_MS);

	return err;
}

And this is the way I am sending the message:

void sendUDPMessage(u8_t *buffer, ssize_t size, u8_t *recv_buffer, ssize_t recv_size)
{
    int err;

    struct addrinfo *dest_addr;
    err = getaddrinfo(thishost, NULL, NULL, &dest_addr);
    if (err < 0) {
#ifdef UDP_LOG
        printk("getaddrinfo err: %d\n\r", err);
#endif
    }
    if (err != -1) {
        ((struct sockaddr_in *)dest_addr->ai_addr)->sin_port = htons(thisport);
        ((struct sockaddr_in *)dest_addr->ai_addr)->sin_family = AF_INET;

        int socket_sender_fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
        if (socket_sender_fd < 0) {
#ifdef UDP_LOG
            printk("socket_sender_fd: %d\n", socket_sender_fd);
#endif
        }
#ifdef UDP_LOG
        printk("Sending UDP Message\n");
#endif
        blockingConnect(socket_sender_fd, (struct sockaddr *)dest_addr->ai_addr, sizeof((struct sockaddr *)dest_addr->ai_addr));
        err = blockingSend(socket_sender_fd, buffer, size, 0);
#if UDP_LOG
		printk("Bytes sent: %d\n", err);
#endif
        if (err < 0) {
#ifdef UDP_LOG
            printk("Sending response err: %d errno: %d\n", err, errno);
#endif
        }
        blockingRecv(socket_sender_fd, recv_buffer, recv_size - 1, MSG_DONTWAIT);

        if (socket_sender_fd != -1) {
            (void)close(socket_sender_fd);
        }
        freeaddrinfo(dest_addr);
    }
}

This is my prj.conf:

# Network
CONFIG_NETWORKING=y
CONFIG_NET_SOCKETS=y
CONFIG_NET_SOCKETS_OFFLOAD=y
# CONFIG_NET_SOCKETS_POSIX_NAMES=y
CONFIG_NET_RAW_MODE=y

# BSD library
CONFIG_BSD_LIBRARY=y

# Stacks and heaps
CONFIG_MAIN_STACK_SIZE=4096
CONFIG_HEAP_MEM_POOL_SIZE=1024

# NEW LIB
CONFIG_NEWLIB_LIBC=y

# SERIAL CONFIGURATION
CONFIG_SERIAL=n

# WATCHDOG CONFIGURATION
CONFIG_WATCHDOG=y

# I2C
CONFIG_I2C=y
CONFIG_I2C_NRFX=y
CONFIG_I2C_2=y
CONFIG_I2C_2_NRF_TWIM=y

# ADC
CONFIG_ADC=y
CONFIG_ADC_0=y
CONFIG_ADC_NRFX_SAADC=y

# FLASH MEMORY
CONFIG_FLASH=y
CONFIG_FLASH_PAGE_LAYOUT=y
CONFIG_MPU_ALLOW_FLASH_WRITE=y
CONFIG_NVS=y

# GPIO
CONFIG_GPIO=y

I have tried enabling and disabling the CONFIG_NET_SOCKETS_POSIX_NAMES option as well as commenting it out.

The issue that I am facing is the following:

Sometimes, after I send the message, the modem doesn't go entirely to sleep after the active time. Instead it behaves like this:

After the active time there are still some spikes that are happening that I don't know why they are there or what more do I need to do. Also the duration of those spikes is also inconsistent. I've noticed them to last for 2 minutes after the expiration of the active timer and I have seen them lasting for 30 seconds.

My PSM configuration is this: Periodic-TAU 00100001 (1 Hour) and Active Timer: 00000101 (10s).

Thank you very much for your help.