How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key ID Exhaustion 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 IDs beyond the hardware table allocation limit, 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 exhaustion drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Table Exhaustion

SmartNICs supporting inline TCP-AO offload maintain an internal Master Key Tuple (MKT) table indexed by Send/Receive Key IDs with finite hardware slot capacity (commonly 128 or 256 keys per ASIC). To protect system integrity, silicon firmware strictly enforces slot boundaries.

When asymmetric routing environments deploy automated key rotation protocols that rapidly cycle key IDs without clearing expired tuples, new SYN-ACK frames bearing higher key identifiers cannot be accommodated in hardware memory. The silicon parser treats out-of-range key IDs as unauthenticated frames and drops them 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 slot exhaustion, 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 exhaustion_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(exhaustion_events);

int trace_synack_exhaustion(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 exhaustion_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;

        exhaustion_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 key exhaustion drops occur:

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

b = BPF(src_file="tcp_ao_exhaustion_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_exhaustion")

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

b["exhaustion_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key exhaustion 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 high key IDs, verify hardware diagnostics via ethtool -S <interface> | grep -E "key_overflow|mkt_full|crypto_drop".

To eliminate drops, purge stale key tuples using Netlink TCP-AO socket commands or configure key ID reuse policies in your routing daemon (such as BIRD or FRRouting) to keep active key identifiers within hardware bounds.