How to Trace TCP Socket TCP_INFO tcpi_retrans_synack Backoff Intervals in Linux Using eBPF

In high-performance Linux network stacks, connection establishment stalls often stem from exponential backoff delays during the initial TCP three-way handshake. When a client fails to return a timely ACK completing the handshake, the kernel retransmits the SYN-ACK packet and doubles the waiting interval according to binary exponential backoff algorithms. By deploying extended Berkeley Packet Filter (eBPF) tracing probes, systems engineers can observe these backoff sequences directly at kernel level without system call overhead.

Mechanisms of Exponential Backoff in TCP Handshakes

Upon receiving a SYN packet, the kernel creates an inet_request_sock and transmits a SYN-ACK. If no ACK arrives before the retransmission timer expires, inet_rtx_syn_ack() is triggered, incrementing the tcpi_retrans_synack counter. Each successive retry doubles the timeout interval (typically 1s, 2s, 4s, 8s, 16s) up to the limit configured by tcp_synack_retries. Tracking backoff timing exposes intermediate packet loss patterns and asymmetric network drops.

Constructing the bpftrace Exponential Backoff Monitor

The following bpftrace script captures each SYN-ACK retransmission event, calculates the duration of each backoff interval, and correlates attempts with client endpoints:

#!/usr/bin/env bpftrace

#include <net/sock.h>
#include <net/inet_connection_sock.h>

BEGIN
{
  printf("Tracing SYN-ACK exponential backoff intervals... Hit Ctrl-C to finish.
");
}

kprobe:inet_rtx_syn_ack
{
  $req = (struct request_sock *)arg1;
  $sk = (uint64)$req;
  $daddr = ntop($req->ir_rmt_addr);
  $dport = bswap($req->ir_rmt_port);
  $retry = $req->num_retrans;

  if (@prev_ts[$sk] > 0) {
    $delta_ms = (nsecs - @prev_ts[$sk]) / 1000000;
    printf("[%s] Host %s:%d | Attempt %d -> Interval: %u ms
",
      strftime("%H:%M:%S", nsecs), $daddr, $dport, $retry, $delta_ms);
    @backoff_intervals_ms = hist($delta_ms);
  }

  @prev_ts[$sk] = nsecs;
}

kprobe:tcp_check_req
{
  $sk = (uint64)arg1;
  delete(@prev_ts[$sk]);
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- SYN-ACK Backoff Interval Distribution (ms) ---
");
  print(@backoff_intervals_ms);
}

END
{
  clear(@prev_ts);
  clear(@backoff_intervals_ms);
}

Running and Interpreting Kernel Diagnostics

Run the script on the target Linux host with administrative privileges:

sudo bpftrace synack_backoff.bt

When handshake loss occurs, the diagnostic stream prints exponential interval doubling:

[16:46:12] Host 203.0.113.19:54210 | Attempt 1 -> Interval: 1004 ms
[16:46:14] Host 203.0.113.19:54210 | Attempt 2 -> Interval: 2008 ms
[16:46:18] Host 203.0.113.19:54210 | Attempt 3 -> Interval: 4016 ms

Remediation and Kernel Tuning Recommendations

To optimize handshake performance and minimize socket resource consumption:

• Tune SYN-ACK Retries: Set net.ipv4.tcp_synack_retries = 2 or 3 to release orphaned connection requests faster under sustained loss.
• Verify TCP Fast Open: Ensure net.ipv4.tcp_fastopen = 3 is configured to allow data transmission during the initial handshake where supported.
• Monitor Syn Flood Protections: Confirm that SYN cookies engage appropriately by checking netstat -s | grep -i cookie.