Sending the Probe

The event handler send_packet() sends the probe packet, which is defined in the following structure in the ipprobe-manager.h file:

typedef struct {
    u_short id;                     /** Probe ID */
    u_short type;                   /** Packet type */
    u_short seq;                    /** Packet sequence number */
    struct timeval tx_time;         /** Initiator transmit time */
    struct timeval target_rx_time;  /** Target receive time */
    struct timeval target_tx_time;  /** Target transmit time */
    struct timeval rx_time;         /** Initiator receive time */
    char pad[0];                    /** Pad section */
} probe_packet_data_t;


typedef struct {
    struct ip header;           /** IP header */
    union {
        struct icmphdr icmp;    /** ICMP header */
        struct udphdr udp;      /** UDP header */
    };
    probe_packet_data_t data;   /** Packet data section */
} probe_packet_t;

Setting Up Time Tracking

This code sets up time-interval tracking by first calling the FreeBSD function gettimeofday() and storing the value in the packet data field tx_time. The tx_time field is a FreeBSD timeval structure that represents an elapsed time. It is declared in sys/time.h and has the following members:

• long int tv_sec represents the number of whole seconds of elapsed time.

• long int tv_usec is the rest of the elapsed time (a fraction of a second), represented as the number of microseconds.

The receive_probe_packets() handler on the target router declares another timeval structure, rx_time, and populates it in the same way. The process_probe_packets() function then calculates the difference between the two values. For example:

packet_stats[i].sd_delay = time_diff(&data_buf[i].tx_time,
                    &data_buf[i].target_rx_time);

For more information about the application's statistics gathering, see Receiving Reply Packets and Calculating Statistics.

Sending the Packet

The code that sends the probe packet is:

static void
send_packet (evContext ctx UNUSED, void *uap UNUSED,
        struct timespec due UNUSED, struct timespec inter UNUSED)
{
    probe_packet_t *packet = (probe_packet_t *)tx_buf;
    int send_len;

    do {
        packet->data.seq = tx_count;
        gettimeofday(&packet->data.tx_time, NULL);

        if (probe_params.protocol == IPPROTO_ICMP) {
            packet->icmp.icmp_cksum = 0;
            packet->icmp.icmp_cksum = icmp_cksum(&packet->icmp,
                    probe_params.packet_size - sizeof(struct ip));
        } else if (probe_params.protocol == IPPROTO_UDP) {
            packet->udp.uh_sum = 0;
            packet->udp.uh_sum = udp_cksum(ntohs(packet->udp.uh_ulen),
                    &packet->header.ip_src.s_addr,
                    &packet->header.ip_dst.s_addr,
                    &packet->udp);
        }

        send_len = sendto(tx_socket, packet, probe_params.packet_size, 0,
                (struct sockaddr *)&target_addr, sizeof(target_addr));
        if (send_len < 0) {
            PRINT_ERRORNO("Send probe packet!\n");
        }

        if (++tx_count == probe_params.packet_count) {

            /* Disable the timer. */
            evClearTimer(ev_ctx, ev_tx_timer_id);
            printf("Sent %d probe packets to %s.\n", tx_count,
                    inet_ntoa(target_addr.sin_addr));
            break;
        }
    } while (probe_params.packet_interval == 0);
    return;
}

Receiving Reply Packets and Calculating Statistics

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