How to Trace TCP Socket TCP_INFO tcpi_retrans_synack Out-of-Order ACKs in Linux Using eBPF

In high-performance Linux network stacks, multipath routing or asymmetric latency variations across carrier backbones frequently cause TCP handshake completion ACKs to arrive out of order. When a client dispatches multiple ACKs or when intermediate firewalls delay acknowledgment frames, the kernel may execute unnecessary SYN-ACK retransmissions, incrementing tcpi_retrans_synack. By deploying extended Berkeley Packet Filter (eBPF) tracing probes, systems engineers can observe these out-of-order ACK arrivals directly at kernel level without system call overhead.

Mechanisms of Out-of-Order Handshake Completions

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. When completing ACKs subsequently arrive out of sequence or duplicate previous sequence numbers, the kernel processes them via tcp_check_req(). Tracing the sequence delta between arriving ACKs and expected sequence bounds isolates network reordering from packet drops.

Constructing the bpftrace Out-of-Order Monitor

The following bpftrace script captures each SYN-ACK retransmission event and correlates it with late-arriving handshake ACK packets:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing out-of-order handshake ACKs... 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);

  @retransmitted_reqs[$sk] = 1;
}

kprobe:tcp_check_req
{
  $req = (struct request_sock *)arg1;
  $sk = (uint64)$req;

  if (@retransmitted_reqs[$sk]) {
    $daddr = ntop($req->ir_rmt_addr);
    $dport = bswap($req->ir_rmt_port);
    printf("[%s] Handshake ACK received after RTX from %s:%d
",
      strftime("%H:%M:%S", nsecs), $daddr, $dport);
    @delayed_handshake_acks[$daddr] = count();
    delete(@retransmitted_reqs[$sk]);
  }
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- Late Handshake ACK Activity ---
");
  print(@delayed_handshake_acks);
  clear(@delayed_handshake_acks);
}

END
{
  clear(@retransmitted_reqs);
  clear(@delayed_handshake_acks);
}

Running and Interpreting Kernel Diagnostics

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

sudo bpftrace synack_ooo_acks.bt

When path delays cause retransmissions prior to delayed ACK arrivals, the diagnostic stream prints:

[16:54:12] Handshake ACK received after RTX from 203.0.113.11:51420
16:54:25 --- Late Handshake ACK Activity ---
@delayed_handshake_acks[203.0.113.11]: 8

Remediation and Kernel Tuning Recommendations

To optimize handshake performance under delayed ACK conditions:

• Adjust Initial RTO: Consider adjusting route-specific RTO settings via ip route change default rto_min 200ms where upstream delays are predictable.
• Verify TCP SACK: Confirm that selective acknowledgments are active via net.ipv4.tcp_sack = 1 to prevent unnecessary full pipeline retransmissions.
• Monitor Multipath Routing: Check ECMP route hashing algorithms to prevent individual connection packets from alternating between disparate paths.