How to Trace TCP Socket TCP_INFO tcpi_retrans_synack Window Probes in Linux Using eBPF

In high-throughput Linux network infrastructure, subtle transport stalls frequently occur when downstream clients advertise near-zero receive windows during connection negotiation. When a client’s receive buffer fills up immediately upon or prior to handshake completion, the kernel dispatches zero-window probes to ascertain when buffer space clears. By deploying extended Berkeley Packet Filter (eBPF) tracing probes, systems engineers can observe zero-window probes and SYN-ACK retransmission anomalies directly at the kernel layer.

Understanding Zero-Window Handshake Anomalies

During the TCP three-way handshake, the client specifies its initial receive window in the SYN segment. If network devices or intermediate NAT gateways rewrite TCP window scale options or if the client application suffers from instantaneous memory pressure, the server may stall in SYN-ACK retransmissions. The kernel tracks these retries within tcpi_retrans_synack. Tracing socket buffer changes during handshake retransmissions isolates socket-level resource exhaustion from physical cable faults.

Constructing the bpftrace Window Probe Monitor

The following bpftrace script hooks into inet_rtx_syn_ack and socket window inspection functions to capture advertised receive window sizes during SYN-ACK retransmission events:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing SYN-ACK window probes and retransmissions... Hit Ctrl-C to finish.
");
}

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

  printf("[%s] SYN-ACK Retransmit to %s:%d (Attempt: %d)
",
    strftime("%H:%M:%S", nsecs), $daddr, $dport, $retries);

  @synack_probes_by_host[$daddr] = count();
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- SYN-ACK Window Probe Summary ---
");
  print(@synack_probes_by_host);
  clear(@synack_probes_by_host);
}

END
{
  clear(@synack_probes_by_host);
}

Running and Interpreting Kernel Measurements

Execute the script on your target server using administrative root capabilities:

sudo bpftrace synack_window_probes.bt

The diagnostic console outputs real-time retransmission notifications:

[16:48:10] SYN-ACK Retransmit to 198.51.100.77:52110 (Attempt: 1)
[16:48:12] SYN-ACK Retransmit to 198.51.100.77:52110 (Attempt: 2)
16:48:25 --- SYN-ACK Window Probe Summary ---
@synack_probes_by_host[198.51.100.77]: 6

Remediation Strategies and Kernel Tuning

When recurrent handshake stalls and window probes are identified:

• Verify Window Scaling: Ensure net.ipv4.tcp_window_scaling = 1 is enabled across all endpoints to avoid 64 KB window limits.
• Tune Initial RTO: Review net.ipv4.tcp_synack_retries to prevent orphaned request sockets from exhausting kernel memory during slow-client connections.
• Inspect Buffer Allocations: Check net.ipv4.tcp_rmem and net.ipv4.tcp_wmem to verify adequate memory headroom for dynamic window expansion.