The TCP Authentication Option (TCP-AO, RFC 5925) provides cryptographic segment protection for mission-critical routing sessions such as BGP peering across wide area networks. Under RFC 5925, dynamic IP and TCP header bits subject to modification in flight—such as the Explicit Congestion Notification (ECN) bits (ECE and CWR flags)—must be zeroed out prior to cryptographic Message Authentication Code (MAC) calculation. When intermediate routers mark congestion by setting ECN flags in transit without the receiving kernel applying the corresponding zero-mask during verification, the segment is silently dropped.
Diagnosing these stealthy authentication failures requires low-overhead kernel tracing. By extracting metrics from struct tcp_sock and correlating the tcpi_retrans_synack counter from TCP_INFO, network engineers can pinpoint transient ECN bit anomalies using eBPF and bpftrace.
The ECN Zero-Masking Dynamics in the Linux Kernel
In Linux, tcp_ao_verify_header() copies the TCP header into a scratch buffer and applies a zero-mask to dynamic flag fields, specifically the ECE and CWR bits. If an unaligned hardware offload engine failed to perform this normalization before transmitting the initial segment, verification fails at tcp_ao_verify_header().
The key kernel parameters and monitoring offsets are outlined below:
| Kernel Symbol | Source Location | Data Structure Offset | Diagnostic Purpose |
|---|---|---|---|
tcp_ao_verify_header |
net/ipv4/tcp_ao.c |
struct tcp_ao_key * |
Validates AO digest against masked ECN bits |
tcp_get_info |
net/ipv4/tcp.c |
tcpi_retrans_synack |
Reports cumulative SYN-ACK retry count |
tcp_v4_rcv |
net/ipv4/tcp_ipv4.c |
skb->cb |
Traces inbound segment discard points |
Attaching bpftrace Probes to TCP-AO Verification
Run the following bpftrace script to monitor passive handshake retries and highlight ECN bit verification failures in real time:
sudo bpftrace -e '
#include <net/sock.h>
#include <linux/tcp.h>
kprobe:tcp_ao_verify_header {
$sk = (struct sock *)arg0;
$tp = (struct tcp_sock *)arg0;
$dport = $sk->__sk_common.skc_dport;
// Filter for BGP port 179
if ($dport == 179 || $sk->__sk_common.skc_num == 179) {
@monitored_synacks[$sk] = $tp->retrans_out;
}
}
kretprobe:tcp_ao_verify_header {
if (retval != 0) {
time("%H:%M:%S ");
printf("TCP-AO ECN bits verification failed with code %d
", retval);
}
}
END {
clear(@monitored_synacks);
}'Reading tcpi_retrans_synack via ss Diagnostic Tooling
Complement kernel probes by inspecting active socket stats using ss:
# Query active TCP-AO enabled sockets with extended TCP_INFO metrics
ss -ti 'sport = :179 or dport = :179'Examine the output for high retrans figures accompanied by stagnant RTT estimators. When correlated with eBPF tcp_ao_verify_header failure return codes, this confirms an asymmetric ECN bit discrepancy across peers.