How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Deficit 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 asymmetric routing produces receive window deficit mismatches where available descriptor memory falls below minimum segment thresholds, incoming SYN-ACK frames are silently dropped by the NIC before kernel processing. In the Linux kernel socket layer, this triggers repeated connection retransmissions visible in tcpi_retrans_synack. This guide explains how to trace these rollover deficit drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Window Deficit Faults

SmartNICs supporting inline TCP-AO offload enforce strict deficit accounting across descriptor queues. When asymmetric cryptographic sessions exchange variable-length TCP-AO Message Authentication Codes, hardware memory managers allocate fixed headroom buffers.

When bursty network traffic depletes DMA head room, the hardware queue enters a deficit condition where incoming SYN-ACK segments cannot be paired with an available RX descriptor ring slot. The silicon drops the packet at ingress without interrupting host cores. 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 window deficit 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 deficit_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(deficit_events);

int trace_synack_deficit_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 deficit_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;

        deficit_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 deficit-induced drops occur:

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

b = BPF(src_file="tcp_ao_deficit_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_deficit_retransmit")

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

b["deficit_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover window deficit 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 "deficit_drop|rx_buf_starve|dma_deficit".

To eliminate deficit drops, expand hardware RX ring descriptors via ethtool -G <interface> rx 4096 and tune jumbo frame headroom thresholds.