How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Window Scaling Option Drops in Linux Using eBPF

TCP Window Scaling (RFC 7323) expands maximum receive windows from 64 KB up to 1 GB, enabling gigabit throughput over high-latency networks. However, legacy middleboxes and misconfigured security appliances frequently drop or zero out Window Scale options (Kind 3) during connection negotiation. When a client advertises window scaling and intermediate middleboxes reject SYN-ACK segments carrying scale factors, the handshake stalls. The server triggers repeated SYN-ACK retransmissions, tracked inside tcpi_retrans_synack. Extended Berkeley Packet Filter (eBPF) provides kernel-level visibility into these handshake stalls.

Mechanisms of TCP Window Scale Negotiation Failures

During the TCP three-way handshake, if net.ipv4.tcp_window_scaling = 1 is set, the kernel attaches window scale options to the SYN-ACK segment. If middleboxes drop segments with unrecognized options or clamp window scale values to zero, throughput collapses or connections drop completely. The server’s retransmission timer fires repeatedly, incrementing tcpi_retrans_synack. Tracing kernel window scale parsing routines isolates options corruption from physical packet loss.

Constructing the bpftrace Window Scale Monitor

The following bpftrace script captures window scale negotiation anomalies and correlates them with SYN-ACK retransmissions:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing TCP window scaling drops 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;

  if ($retries >= 1) {
    @synack_retries_under_wscale[$daddr] = count();
  }
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- TCP Window Scale Retransmission Summary ---
");
  print(@synack_retries_under_wscale);
  clear(@synack_retries_under_wscale);
}

END
{
  clear(@synack_retries_under_wscale);
}

Running and Interpreting Kernel Diagnostics

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

sudo bpftrace synack_wscale_drops.bt

When middleboxes drop window scaling segments, the diagnostic stream prints:

17:10:25 --- TCP Window Scale Retransmission Summary ---
@synack_retries_under_wscale[203.0.113.150]: 4

Remediation Strategies and Configuration Auditing

To optimize TCP window scaling behavior and eliminate middlebox interference:

• Verify Window Scaling Setting: Ensure net.ipv4.tcp_window_scaling = 1 is active in /etc/sysctl.conf.
• Check Middlebox Option Scrubbing: Verify that intermediate firewalls permit TCP Option Kind 3 without stripping or rewriting parameters.
• Inspect Buffer Allocations: Check net.ipv4.tcp_rmem and net.ipv4.tcp_wmem to verify adequate memory headroom for dynamic window expansion.