How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Window Probe Zero Window Drops in Linux Using eBPF

In high-throughput Linux network communications, flow control relies on the receiver advertising available receive buffer space via the TCP window field. When receiver buffers are fully exhausted, the advertised window drops to zero, pausing sender transmissions. To detect when buffer space becomes available, the sender emits zero-window probes. However, if network firewalls discard single-byte probe segments or if early handshake buffer stalls lock the half-open queue, the kernel must fall back to conservative retransmissions, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) provide non-intrusive kernel tracing to isolate zero-window probe drops in real time.

Kernel Mechanics of Zero-Window Probing

When the receive window collapses to zero, the Linux kernel initiates the persist timer and calls tcp_send_probe0(). This function schedules periodic probe segments with an exponential backoff to elicit window updates from the remote endpoint.

If middleboxes drop zero-window probe segments or treat them as malformed packets, the persist timer repeatedly expires without response. During initial connection handshakes or early data negotiations, this deadlock forces listening sockets into repeated SYN-ACK retransmissions. Using eBPF attached to kernel persist timer functions gives infrastructure engineers exact per-socket visibility into receiver window stalls.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP zero-window probe generation and correlates them with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_window_probe_drops.bt - Trace Linux TCP zero-window probe drops */

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

tracepoint:tcp:tcp_retransmit_synack
{
    $sk = (struct sock *)args->skaddr;
    $saddr = ntop(args->saddr);
    $daddr = ntop(args->daddr);
    $sport = args->sport;
    $dport = args->dport;

    printf("SYN-ACK Retransmit: %s:%d -> %s:%d (Window Probe Candidate)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_send_probe0
{
    $sk = (struct sock *)arg0;
    $saddr = ntop($sk->__sk_common.skc_rcv_saddr);
    $daddr = ntop($sk->__sk_common.skc_daddr);

    printf("Zero-Window Probe Sent: %s -> %s
", $saddr, $daddr);
    @zero_window_probes[$saddr, $daddr] = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP Zero-Window Activity (5s Window) ---
");
    print(@zero_window_probes);
    print(@synack_retransmits);
    clear(@zero_window_probes);
    clear(@synack_retransmits);
}

Running the Trace on Production Linux Systems

Deploy the probe with root privileges on the target server:

# Install bpftrace and kernel dependencies
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Execute the trace
sudo bpftrace trace_window_probe_drops.bt

Kernel Optimization and Parameter Tuning

To inspect and tune TCP receive and send buffer allocations:

# Check system TCP window scaling parameter
sysctl net.ipv4.tcp_window_scaling

# Enlarge auto-tuning buffer maximums
sudo sysctl -w net.ipv4.tcp_rmem="4096 87380 16777216"
sudo sysctl -w net.ipv4.tcp_wmem="4096 65536 16777216"

By pairing eBPF kernel instrumentation with calibrated sysctl adjustments, network engineers can eliminate elusive SYN-ACK stalls while preserving essential throughput boundaries.