How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Underflow Drift 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 clock drift between host and network ASIC causes asymmetric SYN-ACK frames to reference keys outside acceptable hardware timestamp envelopes, 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 drift drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Clock Drift Faults

RFC 5925 incorporates temporal windowing to reject expired or prematurely issued cryptographic segments. When hardware offload silicon synchronizes with host clocks via PCIe timer interrupts, high PCIe bus latency or power-state transitions (ASPM) can introduce clock drift between the host kernel clock and the NIC hardware oscillator.

When an asymmetric SYN-ACK packet arrives during a key transition window, the hardware MAC engine checks packet timestamps against drifted silicon registers. If the packet appears to drift beyond allowable temporal limits, the frame is marked as invalid and purged 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 clock drift, 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 drift_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(drift_events);

int trace_synack_drift_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 drift_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;

        drift_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 drift-induced drops occur:

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

b = BPF(src_file="tcp_ao_drift_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_drift_retransmit")

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

b["drift_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover drift 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 "drift_drop|ts_mismatch|crypto_drift".

To eliminate drift drops, disable PCIe Active State Power Management (ASPM) on network adapters via pcie_aspm=off and tighten PTP hardware clock servo loops to sub-microsecond synchronization intervals.