Method generateRetransmitTracepoint

shell/tracescript.go:350–418

generateRetransmitTracepoint creates the tcp_retransmit_skb tracepoint. This captures TCP retransmissions which indicate packet loss or congestion.

()

// generateRetransmitTracepoint creates the tcp_retransmit_skb tracepoint.
// This captures TCP retransmissions which indicate packet loss or congestion.
func (g *ScriptGenerator) generateRetransmitTracepoint() string {
	var sb strings.Builder

	sb.WriteString("tracepoint:tcp:tcp_retransmit_skb\n")
	sb.WriteString("{\n")

	// Only process IPv4 (family == 2)
	sb.WriteString(`    if (args->family != 2) { return; }

`)

	// Extract bytes into local vars first (required for BPF verifier)
	if g.hasIPFilter() {
		sb.WriteString(`    // Read IP bytes into local vars for filtering
    $s0 = args->saddr[0]; $s1 = args->saddr[1]; $s2 = args->saddr[2]; $s3 = args->saddr[3];
    $d0 = args->daddr[0]; $d1 = args->daddr[1]; $d2 = args->daddr[2]; $d3 = args->daddr[3];

`)
		// Add IP filter using local vars
		sb.WriteString(g.buildTCPIPFilterCheckFromLocalVars())
	}

	sb.WriteString(`    $saddr = ntop(2, args->saddr);
    $daddr = ntop(2, args->daddr);
    $sport = args->sport;
    $dport = args->dport;
    $state = args->state;

`)

	if g.config.OutputJSON {
		// JSON: use numeric tcp_state to avoid BPF verifier complexity with string variables
		// State values: 1=ESTABLISHED, 2=SYN_SENT, 3=SYN_RECV, 4=FIN_WAIT1, etc.
		sb.WriteString(`    printf("{\"time\":\"%s\",\"type\":\"RETRANS\",\"tcp_state\":%d,\"probe\":\"tcp_retransmit_skb\",\"src_ip\":\"%s\",\"src_port\":%d,\"dst_ip\":\"%s\",\"dst_port\":%d}\n",
           strftime("%H:%M:%S", nsecs),
           $state,
           $saddr, $sport,
           $daddr, $dport);
`)
	} else {
		// Table: decode TCP state to human-readable name (fixed values from include/net/tcp_states.h)
		sb.WriteString(`    $state_name = $state == 1  ? "ESTABLISHED" :
                  $state == 2  ? "SYN_SENT" :
                  $state == 3  ? "SYN_RECV" :
                  $state == 4  ? "FIN_WAIT1" :
                  $state == 5  ? "FIN_WAIT2" :
                  $state == 6  ? "TIME_WAIT" :
                  $state == 7  ? "CLOSE" :
                  $state == 8  ? "CLOSE_WAIT" :
                  $state == 9  ? "LAST_ACK" :
                  $state == 10 ? "LISTEN" :
                  $state == 11 ? "CLOSING" :
                  $state == 12 ? "NEW_SYN_RECV" :
                  "UNKNOWN";

    printf("%-12s %-10s %-18s %-18s %-18s %s:%-5d  ->  %s:%-5d\n",
           strftime("%H:%M:%S", nsecs),
           "RETRANS",