How to Trace TCP Socket TCP_INFO tcpi_retrans_synack ECN Negotiation Failures in Linux Using eBPF

Explicit Congestion Notification (ECN, RFC 3168) enables routers to signal network congestion before queue overflows force packet drops. However, legacy middleboxes and misconfigured network equipment frequently drop TCP packets containing ECN flags (ECE and CWR) in the IP and TCP headers. When an ECN-capable client initiates a connection to an ECN-enabled Linux server, intermediate packet drops cause the handshake to stall. The server falls back to non-ECN SYN-ACK retransmissions, tracked inside tcpi_retrans_synack. Extended Berkeley Packet Filter (eBPF) provides kernel-level visibility into these negotiation failures.

Understanding ECN Handshake Failures

During connection initiation, an ECN-enabled SYN arrives with both ECE and CWR flags set. The server responds with an ECN-Echo (ECE) flag in the SYN-ACK segment. If downstream network equipment discards the segment due to unrecognized header bits, the connection hangs. The Linux kernel increments tcpi_retrans_synack and, after consecutive timeouts, strips ECN flags to re-attempt connection establishment. Tracing kernel ECN evaluation routines identifies black hole routes immediately.

Constructing the bpftrace ECN Failure Monitor

The following bpftrace script captures SYN-ACK retransmission attempts and checks socket ECN negotiation flags:

#!/usr/bin/env bpftrace

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

BEGIN
{
  printf("Tracing ECN handshake negotiation failures... Hit Ctrl-C to finish.
");
}

kprobe:inet_rtx_syn_ack
{
  $req = (struct request_sock *)arg1;
  $daddr = ntop($req->ir_rmt_addr);
  $dport = bswap($req->ir_rmt_port);
  $retries = $req->num_retrans;
  $ecn_ok = $req->ecn_ok;

  if ($ecn_ok && $retries >= 1) {
    printf("[%s] ECN SYN-ACK Drop/Retry -> %s:%d [Attempts: %d]
",
      strftime("%H:%M:%S", nsecs), $daddr, $dport, $retries);
    @ecn_failure_hosts[$daddr] = count();
  }
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- Suspected ECN Black Hole Destinations ---
");
  print(@ecn_failure_hosts);
  clear(@ecn_failure_hosts);
}

END
{
  clear(@ecn_failure_hosts);
}

Running and Interpreting Kernel Diagnostics

Run the script on the target Linux host with administrative privileges:

sudo bpftrace synack_ecn_failures.bt

When ECN negotiation failure occurs, the diagnostic stream prints:

[17:04:10] ECN SYN-ACK Drop/Retry -> 203.0.113.125:51240 [Attempts: 1]
17:04:25 --- Suspected ECN Black Hole Destinations ---
@ecn_failure_hosts[203.0.113.125]: 5

Remediation Strategies and Configuration Auditing

To optimize ECN handling across heterogeneous internet paths:

• Verify Sysctl ECN Configuration: Set net.ipv4.tcp_ecn = 2 (enable ECN only when requested by incoming client connections) rather than forcing outbound ECN.
• Enable TCP ECN Fallback: Confirm net.ipv4.tcp_ecn_fallback = 1 is enabled to automatically strip ECN flags upon handshake retry.
• Inspect Firewall ACLs: Audit middlebox access control lists to ensure TCP header flags 0x40 and 0x80 are permitted.