fastrace

Probe Structure

Each probe is tracked using a specialized structure:

typedef struct {
    int ttl;                /* Time-to-Live value */
    int probe;              /* Probe sequence number */
    struct timeval sent_time; /* Timestamp when sent */
    int 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;

Concurrent Route Discovery

Fastrace implements a multi-TTL probing system that maintains multiple active TTL probes:

#define MAX_ACTIVE_TTLS 5   /* Maximum number of TTLs probed concurrently */

While standard traceroute sequentially probes one TTL at a time, Fastrace processes multiple TTLs concurrently, dramatically reducing total trace time.

Efficient Response Processing

The response processor uses select() with configurable timeouts to efficiently handle incoming packets without blocking:

int process_responses(int timeout_ms) {
    fd_set readfds;
    FD_ZERO(&readfds);
    FD_SET(recv_sock, &readfds);
    
    struct timeval timeout;
    timeout.tv_sec = timeout_ms / 1000;
    timeout.tv_usec = (timeout_ms % 1000) * 1000;
    
    int ret = select(recv_sock + 1, &readfds, NULL, NULL, &timeout);
    /* ... */
}

RTT calculations are performed with microsecond precision using the timersub macro:

struct timeval diff;
timersub(&recv_time, &probes[idx].sent_time, &diff);
double rtt = ((double)diff.tv_sec * 1000.0) + 
            ((double)diff.tv_usec / 1000.0);

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)