How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP TCP-AO Traffic Class Verification Drops in Linux Using eBPF

The TCP Authentication Option (TCP-AO, RFC 5925) provides robust cryptographic authentication for sensitive router peering connections. When configured with strict Master Key Tuples (MKT), TCP-AO can optionally include IP header fields—specifically Differentiated Services Code Point (DSCP) and Explicit Congestion Notification (ECN) bits—inside the cryptographic Message Authentication Code (MAC) calculation. However, if intermediate network carriers mutate DiffServ or QoS values during transit, the receiver’s computed MAC diverges from the header digest. The Linux kernel silently drops the packet, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate TCP-AO traffic class drops in real time.

Kernel Mechanics of TCP-AO Traffic Class Verification

Inside modern Linux kernels (Linux 6.7+), TCP-AO processes traffic class validation in tcp_ao_hash_header(). If the matching MKT specifies that the traffic class (IPv4 TOS / IPv6 Traffic Class) must be protected, the kernel includes the IP header octet in the SHA-1 or AES-CMAC hash:

When transit networks remark DSCP markings to zero or reclassify packets, the signature fails verification in tcp_ao_verify_hash(). Consequently, the listening socket repeatedly retransmits SYN-ACK segments. Using eBPF attached to kernel TCP-AO verification routines gives infrastructure engineers exact per-socket visibility into traffic class verification drops.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP-AO traffic class verification and correlates failed handshakes with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_tcp_ao_traffic_class.bt - Trace Linux TCP-AO traffic class drops */

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

tracepoint:tcp:tcp_retransmit_synack
{
    $sk = (struct sock *)args->skaddr;
    $saddr = ntop(args->saddr);
    $daddr = ntop(args->daddr);
    $sport = args->sport;
    $dport = args->dport;

    printf("SYN-ACK Retransmit: %s:%d -> %s:%d (TCP-AO Traffic Class Mismatch)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_ao_verify_hash
{
    $sk = (struct sock *)arg0;
    @ao_checks = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP-AO Verification Activity (5s Window) ---
");
    print(@ao_checks);
    print(@synack_retransmits);
    clear(@ao_checks);
    clear(@synack_retransmits);
}

Running the Trace on Production Linux Systems

Deploy the probe with root privileges on the target server:

# Install bpftrace and kernel dependencies
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Execute the trace
sudo bpftrace trace_tcp_ao_traffic_class.bt

Kernel Optimization and Parameter Tuning

To inspect TCP-AO socket states and verify if traffic class protection is enforced:

# Inspect socket TCP-AO configuration via iproute2
ss -ta --ao

# Check kernel SNMP network statistics
nstat -az TcpExtTCPAO*

By pairing eBPF kernel instrumentation with iproute2 diagnostics, network engineers can eliminate elusive SYN-ACK stalls while preserving strict cryptographic session security.