How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Expiry Timer 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. If asymmetric SYN-ACK frames reference key identifiers whose hardware expiration timers have lapsed while host kernel structures still recognise the key as active, 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 key timer drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Lifetime Timer Mismatches

SmartNICs supporting inline TCP-AO offload enforce strict temporal boundaries for cryptographic security. When Master Key Tuples (MKTs) are configured with hard expiration thresholds, hardware timer logic decrements remaining validity intervals independently of the host CPU.

When asymmetric routing paths or clock skew between host and SmartNIC management processors cause timer drift, the hardware offload engine can invalidate a key slot microseconds before the Linux kernel marks it as retired. Incoming SYN-ACK frames matching the newly expired hardware key slot are discarded without generating an ICMP unreachable response. 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 timer expiration, 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 timer_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(timer_events);

int trace_synack_timer_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 timer_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;

        timer_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 timer-induced drops occur:

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

b = BPF(src_file="tcp_ao_timer_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_timer_retransmit")

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

b["timer_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key expiry timer 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 associated with expiring key IDs, verify hardware diagnostics via ethtool -S <interface> | grep -E "key_expired|timer_drop|crypto_timeout".

To eliminate timer drift, synchronise the SmartNIC onboard clock with the host PTP hardware clock using phc2sys -s <interface> -m and introduce overlap grace periods during key renegotiations in BGP daemon configurations.