How to Trace TCP Socket TCP_THIN_DUPACK in Linux Using eBPF

Thin-stream applications—such as online gaming protocols, SSH terminals, financial ticker feeds, and industrial sensor telemetry—transmit small payloads at irregular intervals. Because thin streams maintain very few packets in flight (often fewer than 4 packets per round trip), traditional TCP fast retransmission fails: an isolated packet loss cannot produce the standard three duplicate ACKs required to trigger fast recovery (RFC 5681). The Linux kernel provides the TCP_THIN_DUPACK socket option via setsockopt(), enabling dynamic fast retransmission on the very first duplicate ACK. Tracing thin-stream duplicate ACK heuristics using eBPF provides vital kernel-level visibility into low-latency packet recovery.

Mechanics of Thin-Stream Loss Recovery

In standard TCP, if a stream has only two packets in flight and the first is dropped, the arrival of the second packet triggers only one duplicate ACK. Without thin-stream optimizations, the sender must wait for an entire retransmission timeout (RTO, typically 200ms to 1s) to expire, causing devastating latency spikes for interactive users.

When TCP_THIN_DUPACK is enabled on a socket, the kernel monitors flight size. If the number of in-flight segments is below four (tp->packets_out < 4), the arrival of a single duplicate ACK immediately triggers fast retransmission, recovering lost packets within one RTT instead of waiting for RTO timers.

Authoring a Custom bpftrace Thin-Stream Telemetry Script

To inspect when thin-stream fast retransmissions occur and trace duplicate ACK evaluation thresholds, create an eBPF script using bpftrace:

#!/usr/bin/env bpftrace

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

kprobe:tcp_check_thin_dupack
{
  $sk = (struct sock *)arg0;
  $tp = (struct tcp_sock *)arg0;

  @thin_dupack_checks = count();
  if ($tp->thin_dupack) {
    @thin_dupack_enabled = count();
    time("%H:%M:%S: Thin dupack triggered on sock %p (packets_out=%u)\n",
         $sk, $tp->packets_out);
  }
}

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

interval:s:10
{
  print(@thin_dupack_checks);
  print(@thin_dupack_enabled);
  print(@fast_retrans_alerts);
  clear(@thin_dupack_checks);
  clear(@thin_dupack_enabled);
  clear(@fast_retrans_alerts);
}

Save this script as trace_thin_dupack.bt. It targets tcp_check_thin_dupack() to record thin-stream checks and fast retransmit activations.

Executing and Evaluating Kernel Telemetry

Run the bpftrace monitoring script with administrative privileges:

sudo bpftrace trace_thin_dupack.bt

Subject an interactive SSH session or gaming stream to synthetic packet drops. Review the output metrics:

• Thin dupack activations: Increments in @thin_dupack_enabled confirm that Linux is triggering fast recovery on the first duplicate ACK rather than waiting for RTO timeouts.
• Flight size validation: Ensure packets_out remains under 4, verifying that bulk downloads are not mistakenly classified as thin streams.
• System-wide counters: Check nstat -z | grep -i thin to inspect global kernel counters.

Configuring Host Sysctls for Thin Streams

To enable thin-stream optimizations system-wide or per-socket on Linux hosts:

1. Enable dynamic thin dupack system-wide via sysctl -w net.ipv4.tcp_thin_dupack=1.
2. Enable linear RTO timeouts for thin streams via sysctl -w net.ipv4.tcp_thin_linear_timeouts=1 to prevent exponential backoff penalties.
3. Alternatively, configure sockets directly in C/C++ applications using setsockopt(fd, IPPROTO_TCP, TCP_THIN_DUPACK, &val, sizeof(val)).