How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Clamp Clock Skew Drops in Linux Using eBPF

High-throughput edge routing fabrics and enterprise backbone networks running Linux kernels frequently leverage TCP Authentication Option (TCP-AO, RFC 5925) to cryptographically safeguard BGP and data centre interconnect sessions. During asymmetric segment hardware key rollover events, subtle race conditions can emerge where severe window clamp conditions collide with microsecond-level clock skew between network peers. This timing divergence causes unexpected tcpi_retrans_synack anomalies and silent packet drops inside the kernel TCP subsystem. This tutorial provides an in-depth guide to diagnosing and tracing these clock skew drops in Linux using modern eBPF toolchains.

Understanding Hardware Key Rollover Window Clamp Clock Skew

TCP-AO replaces legacy MD5 authentication by supporting cryptographic master keys and automated key rollover without tearing down existing TCP connections. When hardware offload interfaces or hardware security modules (HSMs) coordinate dynamic key rotation, the kernel networking stack inspects inbound packet sequence spaces against active Message Authentication Codes (MACs).

However, when peer routers undergo rapid key transition while advertised receive windows are clamped, minor PTP or NTP clock drifts can cause one host to transmit packets with newly active key identifiers while the receiver’s hardware descriptor remains pinned to the expiring key. This discrepancy forces the kernel to reject segments, triggering retries and inflating tcpi_retrans_synack.

Developing the eBPF Kernel Probe

Standard networking diagnostic utilities such as netstat or ss -ti only show aggregate counters after failures occur. To isolate the exact instruction boundary where the clock skew collision takes place, engineers can deploy an eBPF program attached to the kernel’s TCP-AO ingress hooks. Below is a production-grade BCC/eBPF probe:

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

struct clock_skew_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 rcv_wnd;
    u64 ktime_delta_ns;
};

BPF_PERF_OUTPUT(clock_skew_events);

int trace_tcp_ao_clamp_clock_skew(struct pt_regs *ctx, struct sock *sk, struct sk_buff *skb) {
    if (!sk || !skb)
        return 0;

    struct tcp_sock *tp = tcp_sk(sk);
    struct clock_skew_event_t event = {};

    // Read IP header details
    struct iphdr *iph = (struct iphdr *)(skb->head + skb->network_header);
    event.saddr = iph->saddr;
    event.daddr = iph->daddr;

    // Read TCP socket info metrics
    event.retrans_synack = tp->total_retrans;
    event.rcv_wnd = tp->rcv_wnd;
    event.ktime_delta_ns = bpf_ktime_get_ns();

    // Evaluate clock skew condition during window clamp
    if (tp->rcv_wnd < 512 && event.retrans_synack > 0) {
        clock_skew_events.perf_submit(ctx, &event, sizeof(event));
    }

    return 0;
}

Compiling and Loading the Probe via Python

To capture and format kernel events in user space, write a lightweight Python wrapper using the bcc framework:

from bcc import BPF
import socket
import struct

bpf_source = """...""" # Insert C code above

b = BPF(text=bpf_source)
b.attach_kprobe(event="tcp_ao_verify_hash", fn_name="trace_tcp_ao_clamp_clock_skew")

def print_event(cpu, data, size):
    event = b["clock_skew_events"].event(data)
    src = socket.inet_ntoa(struct.pack("<I", event.saddr))
    dst = socket.inet_ntoa(struct.pack("<I", event.daddr))
    print(f"[TCP-AO CLOCK SKEW] Drop Detected | Src: {src} -> Dst: {dst} | RcvWnd: {event.rcv_wnd} | RetransSynack: {event.retrans_synack}")

b["clock_skew_events"].open_perf_buffer(print_event)
print("Monitoring TCP-AO hardware key rollover clock skew drops... Press Ctrl+C to halt.")

while True:
    try:
        b.perf_buffer_poll()
    except KeyboardInterrupt:
        break

Kernel Tuning and Mitigation Strategies

Once traces confirm clock skew clamp drops, adjust kernel socket buffer allocations and thread scheduling priorities to mitigate race conditions:

# Increase key rollover grace overlap period
sudo sysctl -w net.ipv4.tcp_ao_key_grace_period=5

# Optimise TCP receive window clamp thresholds
sudo sysctl -w net.ipv4.tcp_window_scaling=1
sudo sysctl -w net.ipv4.tcp_workaround_signed_windows=0

# Ensure PTP hardware clock synchronisation remains locked
sudo systemctl restart ptp4l

Implementing these kernel adjustments prevents sudden window collapses during key rotation, maintaining rock-solid TCP-AO tunnel stability across carrier-grade infrastructure.