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

Explicit Congestion Notification (ECN, RFC 3168) allows routers to signal intermediate network congestion without dropping packets by marking bits in the IP header. During TCP connection establishment, endpoints negotiate ECN support by setting the ECE (ECN-Echo) and CWR (Congestion Window Reduced) flags in SYN and SYN-ACK packets. However, outdated middleboxes and legacy network firewalls often treat ECN flags as protocol violations, silently dropping SYN or SYN-ACK segments. When this occurs, the connection stalls into repeated retransmissions, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate ECN negotiation drop bottlenecks in real time.

Kernel Mechanics of ECN Negotiation

Inside the Linux network stack, the kernel evaluates incoming SYN flags inside tcp_ecn_create_request(). If net.ipv4.tcp_ecn = 1 is enabled, the kernel checks whether the incoming segment requests ECN:

When generating the SYN-ACK response, tcp_ecn_make_synack() sets the ECE flag. If upstream firewalls discard packets with ECE active, the client never receives the response, forcing the server to retransmit the segment repeatedly until timeout exhaustion. Extended Berkeley Packet Filters allow systems engineers to monitor exact sequence and flag disparities inside kernel tracepoints without overhead.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP ECN negotiation and correlates dropped handshakes with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_ecn_stalls.bt - Trace Linux TCP ECN handshake stalls */

#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 (ECN Negotiation Suspect)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

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

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP ECN Handshake Activity (5s Window) ---
");
    print(@ecn_synack_created);
    print(@synack_retransmits);
    clear(@ecn_synack_created);
    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_ecn_stalls.bt

Kernel Optimization and Parameter Tuning

To evaluate if ECN middlebox packet drops are impacting handshakes, inspect Linux network statistics:

# Check system ECN fallback counters
nstat -az TcpExtECN*

# Temporarily set ECN to fallback mode (2 = fallback to non-ECN on retransmit)
sudo sysctl -w net.ipv4.tcp_ecn=2

By pairing eBPF kernel instrumentation with calibrated sysctl adjustments, network engineers can eliminate elusive SYN-ACK stalls while preserving essential congestion notification features.