How to Trace TCP Socket TCP_CONGESTION Cubic-to-BBR Failover in Linux Using eBPF

Modern cloud infrastructure and content distribution networks frequently configure Google BBR (Bottleneck Bandwidth and RTT) to maximize transmission throughput across long-fat networks and lossy wireless links. However, if remote endpoints fail to report accurate ECN signals, or if network namespaces lack necessary kernel module dependencies, the Linux TCP stack automatically falls back to CUBIC or Reno. Tracing congestion algorithm failovers and socket initialization fallbacks using eBPF provides kernel-level visibility into throughput degradation.

Mechanics of Dynamic Congestion Algorithm Switching

When an application configures setsockopt(..., IPPROTO_TCP, TCP_CONGESTION, "bbr", ...), the kernel network stack invokes tcp_set_congestion_control():

1. The kernel searches the registered congestion control list (tcp_cong_list) for the requested algorithm identifier.
2. If the requested module (tcp_bbr.ko) is not loaded or fails capability validation, the kernel silently keeps the default congestion control (typically CUBIC) without returning an error to userland.
3. During active sessions, severe packet loss or watchdog timeouts can trigger emergency fallbacks.

Traditional network monitoring tools only observe drop in throughput, leaving administrators unaware that sockets are operating under legacy loss-based congestion models.

Authoring a Custom bpftrace Failover Telemetry Script

To detect when congestion control algorithm switches succeed or fail silently, write an eBPF script using bpftrace:

#!/usr/bin/env bpftrace

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

kprobe:tcp_set_congestion_control
{
  $sk = (struct sock *)arg0;
  $name = str(arg1);
  time("%H:%M:%S: Congestion algorithm '%s' requested for sock %p\n", $name, $sk);
  @requested_algo[$name] = count();
}

kretprobe:tcp_set_congestion_control
{
  if (retval != 0) {
    @switch_failures[retval] = count();
    time("%H:%M:%S: Congestion algorithm switch failed with error code %d\n", retval);
  } else {
    @switch_successes = count();
  }
}

interval:s:10
{
  print(@requested_algo);
  print(@switch_successes);
  print(@switch_failures);
  clear(@requested_algo);
  clear(@switch_successes);
  clear(@switch_failures);
}

Save this script as trace_cong_failover.bt. It instruments tcp_set_congestion_control() to capture active algorithm requests, successful switches, and rejection codes.

Executing and Evaluating Kernel Telemetry

Run the bpftrace monitoring script with administrative privileges:

sudo bpftrace trace_cong_failover.bt

Evaluate the output metrics during application startup and connection scaling:

• Algorithm requests: Verify that @requested_algo["bbr"] matches expected connection startup volumes.
• Rejection codes: Return code -2 (ENOENT) indicates that the BBR kernel module is missing from the running kernel image. Load it via sudo modprobe tcp_bbr.
• Default fallback verification: Inspect sysctl net.ipv4.tcp_congestion_control to ensure the host default reflects your intended standard.

Configuring Host Prerequisites for BBR

To ensure BBR operates reliably across all sockets:

1. Set the default queuing discipline to Fair Queueing: sysctl -w net.core.default_qdisc=fq (mandatory for BBR pacing).
2. Set the default congestion control to BBR: sysctl -w net.ipv4.tcp_congestion_control=bbr.
3. Verify module persistence by adding tcp_bbr and sch_fq to /etc/modules-load.d/modules.conf.