How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Underflow Counter 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 incoming asymmetric SYN-ACK frames present rolling counter increments that fall below hardware monotonically increasing thresholds, the network controller silently discards incoming frames 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 counter drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Monotonic Counter Faults

RFC 5925 implements anti-replay protection using monotonic counter sequences coupled with dynamic sliding windows. In SmartNIC hardware architectures, specialized silicon counters track highest-seen segment timestamps to prevent replay of captured cryptographic packets.

When asymmetric routing introduces severe jitter or routing flap across alternative BGP paths, valid SYN-ACK frames transmitted under a lagging counter sequence arrive out of order. If the hardware offload counter has already committed a higher sequence value, the trailing SYN-ACK frame is treated as a replay anomaly and dropped. 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 counter underflows, 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 counter_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(counter_events);

int trace_synack_counter_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 counter_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;

        counter_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 counter-induced drops occur:

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

b = BPF(src_file="tcp_ao_counter_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_counter_retransmit")

def print_event(cpu, data, size):
    event = b["counter_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 COUNTER] SYN-ACK Drops: {event.retrans_synack} | "
          f"SRC: {saddr}:{event.sport} -> DST: {daddr}:{event.dport} | "
          f"Counter Key ID: {event.key_id}")

b["counter_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover counter 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 "counter_drop|monotonic_err|crypto_seq_err".

To eliminate counter drops, expand the hardware anti-replay window depth via driver module parameters or configure path MTU and latency damping in your routing topology.