How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Clamp Collateral 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 multiple adjacent TCP connections share a hardware receive queue clamped for traffic isolation, collateral drops occur when legitimate SYN-ACK segments are rejected due to shared queue thresholds. In the Linux kernel socket layer, this triggers repeated connection retransmissions visible in tcpi_retrans_synack. This guide explains how to trace these rollover clamp collateral drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Collateral Queue Faults

SmartNICs supporting inline TCP-AO offload utilize shared descriptor ring queues to optimize silicon area. When dynamic key rollover routines throttle or clamp a congested queue, all concurrent cryptographic sessions mapped to that hardware bucket suffer collateral impact.

When an asymmetric SYN-ACK segment arrives for an unthrottled peer socket mapped to the same hardware ring index, the NIC ingress arbiter drops the packet because the composite queue quota is exceeded. 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 queue collateral drops, 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 collateral_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
    u32 rx_queue;
};

BPF_PERF_OUTPUT(collateral_events);

int trace_synack_collateral_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 collateral_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;
        event.rx_queue = sk->sk_rx_queue_mapping;

        collateral_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 collateral drops occur:

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

b = BPF(src_file="tcp_ao_collateral_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_collateral_retransmit")

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

b["collateral_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover collateral 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 across multiple independent peer sockets on the same RX queue, verify hardware diagnostics via ethtool -S <interface> | grep -E "collateral_drop|rx_queue_clamp|arbiter_drop".

To eliminate collateral drops, reconfigure RSS hash indirection tables or assign dedicated hardware RX queues to critical BGP peering sockets using ethtool -N <interface> rx-flow-hash tcp4 fn.