How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Overlap 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 hitless key rollover phases, both retiring and incoming keys must remain valid simultaneously. If asymmetric routing paths transmit SYN-ACK frames matching a retiring key slot that has been prematurely overwritten in hardware offload tables, 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 overlap drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Race Conditions

RFC 5925 mandates overlapping key validity intervals to prevent packet drops during key changes. However, when SmartNIC hardware offload engines receive Netlink key update messages, internal memory compaction algorithms may overwrite retiring Master Key Tuples before all in-flight asymmetric SYN-ACK frames arrive.

When an asymmetric SYN-ACK frame bearing the older Key ID reaches the NIC after table overwriting, the crypto validation engine detects an unregistered key ID 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 race conditions, 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 rollover_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(rollover_events);

int trace_synack_rollover_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 rollover_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;

        rollover_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 rollover-induced drops occur:

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

b = BPF(src_file="tcp_ao_rollover_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_rollover_retransmit")

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

b["rollover_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover overlap 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 "key_overlap|rollover_err|crypto_miss".

To eliminate rollover drops, increase the overlap buffer duration in your routing software (ensuring both old and new keys remain co-resident in hardware tables for at least 180 seconds) before retiring legacy key slots.