How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Underflow Bitmask 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 bitmasks that conflict with hardware active-key vector registers, 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 bitmask drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Bitmask Conflicts

SmartNICs supporting inline TCP-AO offload utilize bitmap vector registers to represent concurrent valid Master Key Tuples. During rolling transitions, bitmasks track which key slots can authenticate incoming traffic.

When asymmetric routing paths route traffic across heterogeneous interface cards or multi-path WAN links, delayed SYN-ACK packets may reference key positions whose bitmask flags have been cleared by hardware synchronization routines. The silicon parser treats cleared bitmask slots as invalid credentials 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 bitmask clearing, 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 bitmask_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(bitmask_events);

int trace_synack_bitmask_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 bitmask_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;

        bitmask_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 bitmask-induced drops occur:

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

b = BPF(src_file="tcp_ao_bitmask_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_bitmask_retransmit")

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

b["bitmask_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover bitmask 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 "mask_drop|bitmask_err|crypto_slot".

To eliminate bitmask drops, configure driver parameter flags to preserve active bitmask vectors across overlapping key cycles until socket close events are observed.