fastrace

Probe Structure

Each probe is tracked using a specialized structure with monotonic timing:

typedef struct {
    int ttl;                  /* Time-to-Live value */
    int probe;                /* Probe sequence number */
    struct timespec sent_time; /* Monotonic timestamp when sent */
    bool received;            /* Whether response was received */
    struct in_addr addr;      /* Address of responding hop */
    double rtt;               /* Round-trip time in ms */
    int port;                 /* UDP port used for this probe */
} probe_t;

Adaptive Route Discovery

Fastrace implements a configurable multi-TTL probing system with runtime-adjustable concurrency:

// Default concurrency window (tunable via -c flag)
config.max_active_ttls = 6;

// Probes per hop (tunable via -q flag)
config.num_probes = 3;

// Inter-probe delay in microseconds (tunable via -d flag)
config.probe_delay_us = 250;

Multiple TTLs stay in flight at once. Probes for a hop are batched under a single TTL setsockopt, cutting syscall overhead. Tuning -c, -q, and -d lets you balance speed and jitter.

Efficient Response Processing

The response processor uses a poll()-driven loop with non-blocking sockets to drain ICMP bursts. Time is monotonic throughout for stable RTT math:

int process_responses(int timeout_ms) {
    struct pollfd pfd = { .fd = recv_sock, .events = POLLIN | POLLERR | POLLHUP };
    if (timeout_ms > 0 && poll(&pfd, 1, timeout_ms) <= 0) return 0;
    return drain_icmp_socket();
}

static double timespec_diff_ms(const struct timespec *start, const struct timespec *end) {
    time_t sec = end->tv_sec - start->tv_sec;
    long nsec = end->tv_nsec - start->tv_nsec;
    if (nsec < 0) { sec -= 1; nsec += 1000000000L; }
    return (double)sec * 1000.0 + (double)nsec / 1000000.0;
}

Asynchronous DNS Resolution

Hostname lookups are offloaded to a dedicated background thread to prevent blocking the main packet loop:

static void *dns_worker(void *arg) {
    while (dns_running) {
        // ... wait for queue item ...
        if (getnameinfo(...) == 0) {
            host_cache_store(item->addr, host);
        }
    }
    return NULL;
}

Visual Path Representation

Fastrace provides a structured visual representation of network paths:

• Tree-like format shows branching at load-balanced routes
• Clear arrows indicate path progression
• Distinct formatting for primary and alternative routes

7   │→ 72.14.209.224    ( 60.76 ms)
      └→ 72.14.223.184   ( 61.65 ms)
8   │→ 142.251.244.109  ( 59.57 ms)
      └→ 216.239.62.49   ( 71.36 ms)
      └→ 142.250.210.95  ( 70.25 ms)