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;

While standard traceroute sequentially probes one TTL at a time, Fastrace processes multiple TTLs concurrently, dramatically reducing total trace time. The concurrency window can be adjusted at runtime to optimize for different network conditions.

Efficient Response Processing

The response processor relies on a poll()-driven event loop with non-blocking sockets to eagerly drain ICMP bursts:

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

RTT calculations use kernel-provided timestamps (SO_TIMESTAMP) when available, falling back to monotonic time:

struct cmsghdr *cmsg;
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
    if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SO_TIMESTAMP) {
        struct timeval *tv = (struct timeval *)CMSG_DATA(cmsg);
        // ... use kernel timestamp ...
    }
}

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)