How to Trace TCP Socket TCP_INFO tcpi_retrans_synack Timeout Latencies in Linux Using eBPF

In high-throughput distributed architectures, understanding TCP connection handshake latencies is critical to troubleshooting tail-latency degradation. When a server emits a SYN-ACK packet that fails to be acknowledged promptly, the kernel initiates exponential backoff retransmissions, incrementing tcpi_retrans_synack. By employing extended Berkeley Packet Filter (eBPF) tracing probes, systems engineers can measure exact retransmission timeout durations at kernel level, pinpointing network degradations before clients abort connections.

Understanding SYN-ACK Retransmission Timers

When an inbound connection request arrives, the Linux networking stack creates an inet_request_sock and sets an initial retransmission timer governed by TCP_TIMEOUT_INIT (typically 1 second). If no final handshake ACK arrives from the client, the kernel retransmits the SYN-ACK and doubles the timeout interval (exponential backoff). Tracking elapsed nanoseconds between retries reveals whether latency is caused by client packet loss or asymmetric routing delays.

Authoring the bpftrace Latency Probe

The following bpftrace script hooks into inet_rtx_syn_ack to measure the inter-arrival latency between consecutive SYN-ACK retransmission attempts per socket:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing SYN-ACK retransmission timeout latencies... Press Ctrl-C to exit.
");
}

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 (@last_synack_ts[$sk] > 0) {
    $latency_ms = (nsecs - @last_synack_ts[$sk]) / 1000000;
    printf("[%s] Socket %p -> %s:%d | Retry: %d | Inter-RTX Latency: %u ms
",
      strftime("%H:%M:%S", nsecs), $sk, $daddr, $dport, $retry, $latency_ms);
    @synack_timeout_hist_ms = hist($latency_ms);
  }

  @last_synack_ts[$sk] = nsecs;
}

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

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- SYN-ACK Timeout Latency Distribution (ms) ---
");
  print(@synack_timeout_hist_ms);
}

END
{
  clear(@last_synack_ts);
  clear(@synack_timeout_hist_ms);
}

Running and Interpreting Kernel Diagnostics

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

sudo bpftrace synack_latency.bt

During traffic bursts, the trace logs precise timing between retry attempts:

[16:45:02] Socket 0xffff88012a4b8000 -> 198.51.100.44:49822 | Retry: 1 | Inter-RTX Latency: 1002 ms
[16:45:04] Socket 0xffff88012a4b8000 -> 198.51.100.44:49822 | Retry: 2 | Inter-RTX Latency: 2004 ms

Remediation Strategies and Configuration Auditing

When recurrent multi-second SYN-ACK timeouts are identified:

• Review Firewall SYN Ingress Limits: Ensure that hardware firewalls or stateful NAT gateways are not prematurely discarding handshake completions.
• Tune Initial Timeout: In low-latency intranet or data-centre clusters, evaluate TCP_RTO_MIN and route metrics to expedite handshake failure recovery.
• Audit SYN Backlog Size: Confirm that net.ipv4.tcp_max_syn_backlog is sufficiently scaled to absorb traffic surges during upstream path flapping.