How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Clamp Starvation 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 aggressive window clamping causes descriptor starvation in receive ring queues, asymmetric SYN-ACK frames matching starved ring allocations are discarded silently by the network controller 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 clamp starvation drop events using extended Berkeley Packet Filters (eBPF).

Architecture of SmartNIC Key Rollover Clamp Starvation Faults

SmartNICs supporting inline TCP-AO offload utilize queue clamp limiters to isolate misbehaving peer sockets from starving shared packet buffers. When cryptographic validation engines experience bursty ingress traffic across asymmetric links, clamp controllers throttle descriptor refills.

When an asymmetric SYN-ACK segment arrives while the target queue’s allocation quota is exhausted by clamp throttling, the silicon MAC receiver drops the segment before DMA mapping. 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 starvation, 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 starvation_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 key_id;
};

BPF_PERF_OUTPUT(starvation_events);

int trace_synack_starvation_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 starvation_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;

        starvation_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 starvation-induced drops occur:

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

b = BPF(src_file="tcp_ao_starvation_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_synack_starvation_retransmit")

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

b["starvation_events"].open_perf_buffer(print_event)
print("Listening for TCP-AO key rollover starvation 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 "starve_drop|clamp_throttle|dma_exhaust".

To eliminate starvation drops, relax hardware queue clamp thresholds and configure priority-based flow control (PFC) across the physical Ethernet interface.