How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Underflow 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 key sequence counters lower than the hardware replay-window threshold, 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 underflow drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Underflow Faults

RFC 5925 establishes dynamic sequence number replay protection to prevent malicious injection of replayed TCP segments. When hardware offload silicon manages key transitions, incoming TCP-AO Message Authentication Codes (MACs) are evaluated against internal sequence sliding windows.

When asymmetric routing paths cause variable packet propagation delays, a valid SYN-ACK packet generated under an older sequence window may arrive after the NIC has already advanced its replay window past the permissible underflow threshold. The silicon parser marks the frame as an anti-replay violation and purges the frame 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 rollover replay 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 underflow_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(underflow_events);

int trace_synack_underflow_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 underflow_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;

        underflow_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 underflow-induced drops occur:

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

b = BPF(src_file="tcp_ao_underflow_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_underflow_retransmit")

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

b["underflow_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover underflow 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 "replay_drop|underflow_err|crypto_seq".

To eliminate underflow drops, increase the anti-replay window tolerance on the SmartNIC via firmware parameter configuration or configure asymmetric delay buffering in your edge router interface.