How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Thin Stream Linear Timeout Drops in Linux Using eBPF

In low-latency interactive applications such as real-time gaming, financial trading, and telemetry streaming, thin streams transmit small packets at irregular intervals without generating sufficient packet volume to trigger standard fast retransmit algorithms. Linux provides specialized thin-stream modifications (tcp_thin_linear_timeouts) to replace exponential backoff with aggressive linear retries. However, during initial connection establishment, if thin-stream optimizations interact unpredictably with middleboxes, connection negotiation can stall. 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 Thin-Stream Handshake Retransmissions

When an incoming SYN initiates a connection to a thin-stream socket, the kernel allocates request structures and arms initial retransmission timers. If thin-stream linear timeouts are enabled, backoff calculation differs from standard binary doubling. If intermediate network appliances filter rapid successive SYN-ACK bursts, connection negotiation fails. The kernel tracks retry accumulation in tcpi_retrans_synack. Tracing kernel thin-stream timer logic isolates middlebox filtering from client packet drops.

Constructing the bpftrace Thin-Stream Monitor

The following bpftrace script captures thin-stream handshake 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 thin-stream handshake 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 ($retries >= 1) {
    @synack_retries_thin[$daddr] = count();
  }
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- Thin-Stream Retransmission Summary ---
");
  print(@synack_retries_thin);
  clear(@synack_retries_thin);
}

END
{
  clear(@synack_retries_thin);
}

Running and Interpreting Kernel Diagnostics

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

sudo bpftrace synack_thin_drops.bt

When middleboxes drop thin-stream segments, the diagnostic stream prints:

17:20:25 --- Thin-Stream Retransmission Summary ---
@synack_retries_thin[203.0.113.188]: 4

Remediation Strategies and Configuration Auditing

To optimize thin-stream handling behavior and eliminate middlebox interference:

• Verify Thin-Stream Settings: Check sysctl parameters net.ipv4.tcp_thin_linear_timeouts = 0 or 1 based on middlebox tolerance.
• Audit Initial Handshake RTO: Ensure initial route RTO values allow sufficient grace period for initial handshakes.
• Monitor Socket Flags: Check netstat -s | grep -i retrans to confirm system-wide retransmission trends across thin-stream workloads.