How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Nagle Algorithm Interlock Drops in Linux Using eBPF

The Nagle algorithm (RFC 896) inhibits senders from transmitting small TCP segments while previous unacknowledged data remains in flight. In request-response network protocols, Nagle frequently interacts with the receiver’s delayed ACK mechanism (the classic Nagle/Delayed-ACK deadlock), delaying small frame exchanges by up to 40ms. When application developers fail to set TCP_NODELAY on interactive sockets, this artificial latency can cause handshakes or early data negotiations to exceed timer thresholds, escalating tcpi_retrans_synack. Extended Berkeley Packet Filters (eBPF) enable non-intrusive kernel tracing to isolate Nagle interlock stalls in real time.

Kernel Mechanics of the Nagle Algorithm

Inside the Linux network stack, the Nagle algorithm is evaluated inside tcp_nagle_check(). If outgoing segment length is below the Maximum Segment Size (MSS) and unacknowledged bytes (tp->packets_out) are outstanding, the kernel holds transmission.

When combined with delayed ACKs from the peer, transmissions freeze until the remote delayed ACK timer expires or additional application data fills the MSS buffer. During connection handshakes involving small payload pipelining, these delays trigger retransmission timeouts. Using eBPF attached to kernel Nagle check functions gives infrastructure engineers exact per-socket visibility into interlock latencies.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP Nagle evaluations and correlates hold states with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_nagle_interlock.bt - Trace Linux TCP Nagle interlock 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 (Nagle Interlock Candidate)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

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

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP Nagle Check Activity (5s Window) ---
");
    print(@nagle_evaluations);
    print(@synack_retransmits);
    clear(@nagle_evaluations);
    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_nagle_interlock.bt

Kernel Optimization and Parameter Tuning

To inspect and disable Nagle delays programmatically:

# Verify socket nodelay flag via ss
ss -ti '( dport = :443 or sport = :443 )'

Enforcing setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one)) in application server code eliminates Nagle queuing, preventing sub-segment transmission delays.