How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Queue Descriptor Alignment Drops in Linux Using eBPF

High-throughput data centres and telecommunications networks frequently implement RFC 5925 TCP Authentication Option (TCP-AO) to safeguard core routing sessions against spoofing, replay attacks, and state-exhaustion exploits. When TCP-AO operates over asymmetric network interfaces paired with multi-queue Network Interface Cards (NICs), hardware ring buffers enforce strict memory alignment for incoming descriptor rings. If an asymmetric SYN-ACK segment carrying variable-length TCP-AO Message Authentication Codes (MACs) violates byte-alignment constraints, hardware DMA offloads can silently discard packets, triggering socket retransmissions visible in the Linux kernel socket telemetry tcpi_retrans_synack. This tutorial demonstrates how to trace these alignment drops using extended Berkeley Packet Filters (eBPF).

Architecture of Hardware Queue Descriptor Alignment in TCP-AO

Modern Linux network drivers distribute ingress network traffic across multiple receive queues (RX queues) using Receive Side Scaling (RSS) hashing. When hardware cryptographic offload engines or NIC scatter-gather DMA engines parse TCP headers, they require that packet buffer addresses and TCP header offsets adhere to 32-bit or 64-bit alignment boundaries.

Because TCP-AO options introduce an option length that varies depending on the Key Derivation Function (such as HMAC-SHA-1-96 or AES-128-CMAC-96), asymmetric network paths where forward and reverse packets use differing option layouts can cause SYN-ACK headers to misalign against receive ring descriptor boundaries. When this happens, the hardware pipeline drops the incoming frame before kernel sk_buff allocation, forcing the local TCP stack to repeatedly retransmit SYN-ACK segments. Monitoring tcpi_retrans_synack within struct tcp_info provides direct visibility into this failure mode.

Implementing the eBPF Kernel Probe

Using the BCC (BPF Compiler Collection) or libbpf framework, systems administrators can attach kernel probes to the internal TCP retransmit timer and driver-level ring buffer drop points. The following eBPF program hooks into tcp_retransmit_timer to detect SYN-ACK retransmission anomalies caused by TCP-AO alignment faults:

#include <uapi/linux/ptrace.h>
#include <net/sock.h>
#include <net/tcp.h>
#include <linux/tcp.h>

struct drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 queue_index;
};

BPF_PERF_OUTPUT(events);

int trace_tcp_retransmit(struct pt_regs *ctx, struct sock *sk) {
    if (sk == NULL)
        return 0;

    struct tcp_sock *tp = tcp_sk(sk);
    u32 synack_retrans = tp->tcpi_retrans_synack;

    // Filter only sockets experiencing recurrent SYN-ACK retransmissions
    if (synack_retrans > 0) {
        struct 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_retrans;
        event.queue_index = sk->sk_rx_queue_mapping;

        events.perf_submit(ctx, &event, sizeof(event));
    }
    return 0;
}

Compiling and Monitoring Drop Events via Python CLI

Once the eBPF probe is defined, execute a Python monitoring script to stream real-time alignment failure telemetry directly to the console. The user-space consumer formats the IPv4 addresses and identifies the offending hardware queue index:

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

b = BPF(src_file="tcp_ao_alignment_trace.c")
b.attach_kprobe(event="tcp_retransmit_timer", fn_name="trace_tcp_retransmit")

def print_event(cpu, data, size):
    event = b["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 ALERT] SYN-ACK Drops: {event.retrans_synack} | "
          f"SRC: {saddr}:{event.sport} -> DST: {daddr}:{event.dport} | "
          f"NIC Queue Index: {event.queue_index}")

b["events"].open_perf_buffer(print_event)
print("Listening for TCP-AO queue descriptor alignment drops... Press Ctrl+C to stop.")
while True:
    try:
        b.perf_buffer_poll()
    except KeyboardInterrupt:
        break

Driver Alignment Verification and Workarounds

When eBPF traces isolate repeated drops on specific queue indices, verify the hardware ring descriptor status using ethtool. Running ethtool -S <interface> | grep -E "rx_align|rx_drop|rx_no_buffer" will confirm hardware counter increments corresponding to the eBPF event timestamps.

To resolve persistent alignment drops without disabling TCP-AO authentication, adjust the driver receive ring buffer sizing using ethtool -G <interface> rx 4096, or modify the RSS indirection table to bypass buggy hardware hash parsers until vendor firmware patches can be applied.