How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP SYN-ACK ACK Timestamp Rollover Anomalies in Linux Using eBPF

In high-throughput Linux network infrastructure, TCP timestamp options (RFC 7323) provide millisecond-precision round-trip time (RTT) measurements and enable PAWS protection against wrapped sequence numbers. However, when server uptime spans hundreds of days or when clock frequency changes trigger 32-bit timestamp counter wraparound, timestamp comparisons can miscalculate segment recency. If the kernel misinterprets a valid returning client ACK as an obsolete timestamp segment, the ACK is dropped, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate timestamp rollover anomalies in real time.

Kernel Mechanics of Timestamp Wraparound Arithmetic

The Linux kernel implements signed modular arithmetic to evaluate incoming timestamps inside tcp_validate_incoming() and tcp_paws_check():

(s32)(tp->ts_recent - rx_opt->rcv_tsval) <= 0

Under extreme network conditions or asymmetric virtualization clock jumps, if the timestamp disparity exceeds 2^31 clock ticks, signed comparison inverts, leading the kernel to falsely categorize valid handshakes as obsolete. Consequently, the listening socket drops the returning ACK and retransmits SYN-ACK segments repeatedly. Extended Berkeley Packet Filters allow systems engineers to monitor exact sequence and timestamp disparities inside kernel tracepoints without overhead.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP timestamp validation and correlates rollover rejections with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_ts_rollover.bt - Trace Linux TCP timestamp rollover rejections */

#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 (Timestamp Rollover Suspect)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_validate_incoming
{
    $sk = (struct sock *)arg0;
    $skb = (struct sk_buff *)arg1;
    @ts_checks = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP Timestamp Activity (5s Window) ---
");
    print(@ts_checks);
    print(@synack_retransmits);
    clear(@ts_checks);
    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_ts_rollover.bt

Kernel Optimization and Parameter Tuning

To evaluate if system firewall rules or timestamp options are dropping valid handshakes, inspect Linux network statistics:

# Inspect kernel PAWS and timestamp reject counters
nstat -az TcpExtPAWSEstab
nstat -az TcpExtPAWSOpts

# Check current timestamp settings
sysctl net.ipv4.tcp_timestamps

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