How to Trace TCP Socket TCP_TX_DELAY Pacing in Linux Using eBPF

High-speed networking environments and content delivery platforms rely on fine-grained transmission pacing to smooth bursty egress traffic, prevent downstream bufferbloat, and emulate realistic network latency in staging environments. In the Linux kernel, the TCP_TX_DELAY socket option allows applications to inject microsecond-level artificial transmission delays per socket. Tracing and verifying packet pacing dynamics using extended Berkeley Packet Filters (eBPF) provides kernel-level validation of transmission queues without disruptive packet captures.

Architecture of Kernel Transmission Delay Pacing

When an application configures TCP_TX_DELAY via setsockopt(), the kernel records the desired delay timestamp inside the socket structure (struct tcp_sock -> tcp_tx_delay). When outbound packets traverse the network stack, the Fair Queueing (FQ) queueing discipline (sch_fq) or Earliest Departure Time (EDT) packet scheduler checks the socket transmission delay.

Instead of dispatching the packet immediately to the network interface controller ring buffer, the kernel schedules departure at a specific future nanosecond timestamp. Verifying whether packets adhere to these pacing targets requires instrumenting kernel transmission functions.

Writing a Dedicated bpftrace Inspection Script

Using bpftrace, we can trace outbound socket scheduling and measure the exact time delta between socket submission and kernel packet departure:

#!/usr/bin/env bpftrace

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

kprobe:tcp_transmit_skb
{
  $sk = (struct sock *)arg0;
  $tp = (struct tcp_sock *)arg0;
  $skb = (struct sk_buff *)arg1;

  if ($tp->tcp_tx_delay > 0) {
    @configured_delay_us[$sk] = $tp->tcp_tx_delay;
    @tx_departures = count();
    @skb_delivery_time = hist($skb->tstamp);
  }
}

interval:s:5
{
  print(@configured_delay_us);
  print(@tx_departures);
  print(@skb_delivery_time);
  clear(@tx_departures);
  clear(@skb_delivery_time);
}

Save this script as trace_tx_delay.bt. It targets tcp_transmit_skb() and extracts active delay parameters alongside skb delivery timestamps.

Executing and Evaluating Kernel Tracing

Run the tracing tool with administrative privileges:

sudo bpftrace trace_tx_delay.bt

Initiate network traffic through an application configured with TCP_TX_DELAY. Evaluate the telemetry output for key indicators:

• Pacing stability: Check that @configured_delay_us reflects the microsecond duration requested by the user application.
• EDT timestamp accuracy: Ensure that $skb->tstamp increments consistently relative to kernel monotonic time, confirming that the packet scheduler holds frames until the designated departure slot.
• Egress queue health: Ensure departure count distributions remain stable without burst cliffs, which would indicate scheduler queue drops under heavy load.

Configuring Host Prerequisites for Accurate Pacing

To ensure TCP_TX_DELAY functions correctly, verify the host network configuration:

1. Configure the Fair Queueing scheduler on the primary network interface using sudo tc qdisc replace dev eth0 root fq, as default schedulers (such as pfifo_fast) do not honour EDT delivery timestamps.
2. Verify kernel support by checking sysctl net.ipv4.tcp_fackets and relevant TCP timestamp sysctls.
3. Ensure network interface hardware offloads do not strip departure timestamps prior to queueing.