How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP PAWS Timestamp Drop Anomalies in Linux Using eBPF

In high-speed gigabit and multi-gigabit network environments, the Linux kernel employs Protection Against Wrapped Sequence Numbers (PAWS, RFC 7323) to discard old duplicate segments that fall within current sequence windows. However, when multiple client hosts hide behind a single NAT gateway with unsynchronized system clocks, returning SYN-ACK responses and client ACKs can appear to have receding timestamps. The Linux kernel perceives these as stale wrapped packets and discards them silently via PAWS validation, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate PAWS timestamp drop bottlenecks in real time.

Kernel Mechanics of PAWS Timestamp Validation

When TCP timestamps are negotiated during the handshake, incoming packets pass through tcp_validate_incoming(). The kernel compares the segment timestamp (tcp_tsval) against the recorded timestamp for the connection (tp->ts_recent). If tcp_tsval < tp->ts_recent and the timestamp is outside the rollover threshold, PAWS triggers:

The kernel drops the packet silently or sends a duplicate ACK, rejecting the handshake completion. Consequently, the listening socket retransmits SYN-ACK segments until timeout exhaustion, incrementing tcpi_retrans_synack in struct tcp_info. Using eBPF attached to kernel drop tracepoints gives infrastructure engineers exact per-socket PAWS drop visibility.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP PAWS drops and correlates them with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_paws_drops.bt - Trace Linux TCP PAWS timestamp validation 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 (PAWS Validation Suspect)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

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

    printf("PAWS Rejected Segment: %s <-> %s
", $saddr, $daddr);
    @paws_rejects[$saddr, $daddr] = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP PAWS Rejection Activity (5s Window) ---
");
    print(@paws_rejects);
    print(@synack_retransmits);
    clear(@paws_rejects);
    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_paws_drops.bt

When clients behind shared NAT gateways connect with out-of-order clocks, @paws_rejects increments rapidly alongside @synack_retransmits, exposing NAT timestamp collisions.

Kernel Optimization and Parameter Tuning

If PAWS collisions behind NAT gateways cause widespread connection failures, disable timestamps or adjust TCP TW recycle rules:

# Disable timestamps if client NAT cannot be corrected
sudo sysctl -w net.ipv4.tcp_timestamps=0

# Persist configuration
echo "net.ipv4.tcp_timestamps = 0" | sudo tee /etc/sysctl.d/99-tcp-paws.conf
sudo sysctl --system

Pairing kernel parameterization with real-time eBPF monitoring ensures reliable connection establishment during complex network routing scenarios.