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

Telecommunications backbones and cloud infrastructure frequently deploy RFC 5925 TCP Authentication Option (TCP-AO) to protect Border Gateway Protocol (BGP) routing peering against spoofing and session manipulation. When hardware-accelerated TCP-AO offloads are enabled on modern SmartNICs, cryptographic master key tables maintain dedicated lifetime counters in hardware timer units. During automated key rollover protocols, if receive window scale clamps limit anti-replay buffer progression, asymmetric SYN-ACK frames matching clamped boundary limits are silently discarded by the network controller before host delivery. In the Linux kernel socket layer, this triggers repeated connection retransmissions visible in tcpi_retrans_synack. This guide explains how to trace these rollover window clamp drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Window Clamp Faults

RFC 5925 relies on sliding replay protection windows that dynamically advance as acknowledged sequence numbers scale. On SmartNIC hardware, window clamp registers prevent buffer starvation by limiting maximum unacknowledged frame depth per socket.

When asymmetric routing produces asymmetrical window scaling parameters between sender and receiver, SYN-ACK frames with expanded window sizes exceed the clamped anti-replay boundary configured in hardware. The silicon parser treats out-of-clamp packets as sequence violations and drops them immediately. The Linux TCP stack experiences SYN-ACK packet loss and begins exponential retransmission backoff, incrementing the tcpi_retrans_synack socket metric.

Implementing the eBPF Kernel Probe

To isolate connection retransmissions resulting from SmartNIC key window clamp violations, author an eBPF program hooked to tcp_retransmit_timer and driver drop functions:

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

struct clamp_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(clamp_events);

int trace_synack_clamp_retransmit(struct pt_regs *ctx, struct sock *sk) {
    if (sk == NULL)
        return 0;

    struct tcp_sock *tp = tcp_sk(sk);
    u32 synack_count = tp->tcpi_retrans_synack;

    if (synack_count > 0) {
        struct clamp_drop_event_t event = {};
        event.saddr = sk->__sk_common.skc_rcv_saddr;
        event.daddr = sk->__sk_common.skc_daddr;
        event.sport = sk->__sk_common.skc_num;
        event.dport = ntohs(sk->__sk_common.skc_dport);
        event.retrans_synack = synack_count;
        event.key_id = tp->tcp_ao_key_id;

        clamp_events.perf_submit(ctx, &event, sizeof(event));
    }
    return 0;
}

Running the Real-Time Telemetry Monitor

Execute the Python listener script to parse perf buffer events and alert network administrators when window-clamp-induced drops occur:

#!/usr/bin/env python3
from bcc import BPF
import socket
import struct

b = BPF(src_file="tcp_ao_clamp_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_clamp_retransmit")

def print_event(cpu, data, size):
    event = b["clamp_events"].event(data)
    saddr = socket.inet_ntoa(struct.pack("<I", event.saddr))
    daddr = socket.inet_ntoa(struct.pack("<I", event.daddr))
    print(f"[TCP-AO ROLLOVER CLAMP] SYN-ACK Drops: {event.retrans_synack} | "
          f"SRC: {saddr}:{event.sport} -> DST: {daddr}:{event.dport} | "
          f"Clamped Key ID: {event.key_id}")

b["clamp_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover window clamp drops... Press Ctrl+C to stop.")
while True:
    try:
        b.perf_buffer_poll()
    except KeyboardInterrupt:
        break

Diagnostic Verification and Hardware Remediation

When eBPF traces highlight persistent key drop spikes during scheduled rollover windows, verify hardware diagnostics via ethtool -S <interface> | grep -E "clamp_drop|wscale_err|crypto_wclamp".

To eliminate clamp drops, adjust TCP window scale parameters via sysctl -w net.ipv4.tcp_window_scaling=1 and calibrate hardware NIC receive buffer scaling thresholds.