How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Table Parity 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 reside in dedicated SRAM on the network processor. If asymmetric SYN-ACK frames reference key slots undergoing SRAM parity validation errors or ECC fault detection, 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 table parity drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Table Parity Faults

SmartNICs supporting inline TCP-AO offload maintain an internal Master Key Tuple (MKT) table indexed by Send/Receive Key IDs. To prevent compromised routing states due to bit flips in hostile environments, network silicon applies byte parity or ECC across key registers.

When asymmetric routing paths cause frequent key renegotiations or concurrent key rollovers across disparate cores, race conditions in SRAM write cycles can trigger transient parity check failures. When an incoming SYN-ACK frame contains a Key ID that fails hardware parity verification, the packet is purged at the MAC/crypto boundary. 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 parity faults, author an eBPF program hooked to tcp_retransmit_timer and the driver’s crypto error handlers:

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

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

BPF_PERF_OUTPUT(parity_events);

int trace_synack_parity_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 parity_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; // Extract active TCP-AO key index

        parity_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 parity-induced SYN-ACK drops occur:

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

b = BPF(src_file="tcp_ao_parity_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_parity_retransmit")

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

b["parity_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key table parity 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 a particular key ID, verify hardware diagnostics via ethtool -S <interface> | grep -E "crypto_err|key_parity|sram_ecc".

To mitigate drops while maintaining session security, re-synchronise the Master Key Tuple via Netlink socket options or disable hardware crypto offload temporarily using ethtool -K <interface> tls-hw-rx-offload off to allow the Linux kernel software crypto engine to handle TCP-AO validation safely.