How to Trace TCP Nagle Buffering Delays in Linux Using eBPF

In high-performance microservice ecosystems and distributed databases, communication protocols frequently transmit small payloads—such as JSON health checks, RPC coordination frames, or Redis queries. By default, the Linux networking stack enables Nagle’s algorithm (RFC 896) to coalesce small outbound writes into full Maximum Segment Size (MSS) packets. However, when senders buffer data waiting for outstanding segment acknowledgments, request pipelines experience severe latency serialization. Tracing Nagle buffering delays with eBPF exposes these stalls without altering application code.

Understanding Linux Kernel Nagle Algorithm Mechanics

The Linux TCP implementation evaluates outbound segment dispatch inside tcp_write_xmit():

• The Nagle Condition: If an outbound packet is smaller than MSS and the socket has unacknowledged data in flight (tp->packets_out > 0), transmission is inhibited until all outstanding segments are acknowledged or buffer space reaches MSS capacity.
• The Delayed ACK Interaction: When a receiver employs Delayed ACK, the sender’s Nagle timer and receiver’s delayed ACK timer dead-lock until the 40ms to 200ms delack timer expires, multiplying round-trip latencies by orders of magnitude.
• Socket Override: Applications bypass this buffering by setting the TCP_NODELAY socket option, but identifying which legacy microservices lack this flag historically required intrusive network taps.

Tracing Nagle Buffering Events in Real Time with bpftrace

Using bpftrace, intercept the kernel’s Nagle evaluation branch inside tcp_nagle_check():

sudo bpftrace -e '
#include <net/sock.h>
#include <linux/tcp.h>

kprobe:tcp_nagle_check {
    $sk = (struct sock *)arg0;
    $inet = (struct inet_sock *)arg0;
    $daddr = ntop($inet->inet_daddr);
    $dport = bswap($inet->inet_dport);

    @nagle_checks[comm, $daddr, $dport] = count();
}

kprobe:tcp_write_xmit {
    @xmit_attempts[comm] = count();
}

interval:s:10 {
    print(@nagle_checks);
    clear(@nagle_checks);
}'

This script tabulates exactly which processes and remote endpoints are subjected to kernel Nagle checks, highlighting candidates suffering payload serialization.

Correlating In-Flight Data and Socket Options

To inspect whether a socket has TCP_NODELAY disabled while accumulating packet delays:

sudo bpftrace -e '
kprobe:tcp_sendmsg_locked {
    $sk = (struct sock *)arg0;
    $tp = (struct tcp_sock *)arg0;
    $nonagle = $tp->nonagle;

    if ($nonagle == 0) {
        printf("Nagle Active on Socket: comm=%s pid=%d packets_out=%d\n",
               comm, pid, $tp->packets_out);
    }
}'

Capturing active Nagle sockets with pending packets out isolates the precise service layer responsible for cross-datacenter latency bottlenecks.