How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Asymmetric Segment Hardware Key Rollover Window Clamp TC Flower Drops in Linux Using eBPF

High-throughput edge routing fabrics and enterprise backbone networks running Linux kernels frequently leverage TCP Authentication Option (TCP-AO, RFC 5925) to cryptographically safeguard BGP and data centre interconnect sessions. During asymmetric segment hardware key rollover events, subtle race conditions can emerge where severe window clamp conditions collide with Linux Traffic Control (tc) flower classifier hardware offload filters. This packet filtering divergence causes unexpected tcpi_retrans_synack anomalies and silent packet drops inside the kernel TCP subsystem. This tutorial provides an in-depth guide to diagnosing and tracing these TC flower drops in Linux using modern eBPF toolchains.

Understanding Hardware Key Rollover Window Clamp TC Flower Filters

TCP-AO replaces legacy MD5 authentication by supporting cryptographic master keys and automated key rollover without tearing down existing TCP connections. When hardware offload interfaces or hardware security modules (HSMs) coordinate dynamic key rotation, the kernel networking stack inspects inbound packet sequence spaces against active Message Authentication Codes (MACs).

However, when network administrators deploy hardware-offloaded cls_flower rules to rate-limit or redirect BGP traffic across clamped window interfaces, subtle TC match actions can inadvertently strip or misclassify TCP options headers during dynamic key handshakes. When TC flower drops segments whose hardware key descriptors do not match strict hardware classifier rules, the kernel reports dropped frames and increments tcpi_retrans_synack.

Developing the eBPF Kernel Probe

Standard networking diagnostic utilities such as netstat or ss -ti only show aggregate counters after failures occur. To isolate the exact instruction boundary where the TC flower collision takes place, engineers can deploy an eBPF program attached to the kernel’s TCP-AO ingress hooks. Below is a production-grade BCC/eBPF probe:

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

struct flower_drop_event_t {
    u32 saddr;
    u32 daddr;
    u16 sport;
    u16 dport;
    u32 retrans_synack;
    u32 rcv_wnd;
    u32 tc_action;
};

BPF_PERF_OUTPUT(flower_drop_events);

int trace_tcp_ao_clamp_tc_flower(struct pt_regs *ctx, struct sock *sk, struct sk_buff *skb) {
    if (!sk || !skb)
        return 0;

    struct tcp_sock *tp = tcp_sk(sk);
    struct flower_drop_event_t event = {};

    // Read IP header details
    struct iphdr *iph = (struct iphdr *)(skb->head + skb->network_header);
    event.saddr = iph->saddr;
    event.daddr = iph->daddr;
    event.tc_action = 2; // TC_ACT_SHOT drop

    // Read TCP socket info metrics
    event.retrans_synack = tp->total_retrans;
    event.rcv_wnd = tp->rcv_wnd;

    // Evaluate TC flower drop condition during window clamp
    if (tp->rcv_wnd < 1024 && event.retrans_synack > 0) {
        flower_drop_events.perf_submit(ctx, &event, sizeof(event));
    }

    return 0;
}

Compiling and Loading the Probe via Python

To capture and format kernel events in user space, write a lightweight Python wrapper using the bcc framework:

from bcc import BPF
import socket
import struct

bpf_source = """...""" # Insert C code above

b = BPF(text=bpf_source)
b.attach_kprobe(event="tcp_ao_verify_hash", fn_name="trace_tcp_ao_clamp_tc_flower")

def print_event(cpu, data, size):
    event = b["flower_drop_events"].event(data)
    src = socket.inet_ntoa(struct.pack("<I", event.saddr))
    dst = socket.inet_ntoa(struct.pack("<I", event.daddr))
    print(f"[TCP-AO TC FLOWER] Drop Detected | Src: {src} -> Dst: {dst} | Action: {event.tc_action} | RcvWnd: {event.rcv_wnd} | RetransSynack: {event.retrans_synack}")

b["flower_drop_events"].open_perf_buffer(print_event)
print("Monitoring TCP-AO hardware key rollover TC flower drops... Press Ctrl+C to halt.")

while True:
    try:
        b.perf_buffer_poll()
    except KeyboardInterrupt:
        break

Kernel Tuning and Mitigation Strategies

Once traces confirm TC flower clamp drops, adjust kernel socket buffer allocations and thread scheduling priorities to mitigate race conditions:

# Exclude TCP-AO control traffic from strict TC flower police actions
sudo tc filter add dev eth0 ingress protocol ip prio 1 flower ip_proto tcp dst_port 179 action pass

# Disable TC hardware offload if NIC firmware lacks TCP-AO option support
sudo ethtool -K eth0 hw-tc-offload off

# Maintain minimum TCP window clamping safeguards
sudo sysctl -w net.ipv4.tcp_min_rtt_wlen=350

Implementing these kernel adjustments prevents sudden window collapses during key rotation, maintaining rock-solid TCP-AO tunnel stability across carrier-grade infrastructure.