How to Trace TCP Congestion Window Reductions in Linux Using eBPF

Maintaining predictable throughput across distributed cloud storage clusters and microservice meshes requires steady TCP Congestion Window (CWND) scaling. However, when the Linux networking subsystem encounters packet loss, Duplicate ACKs, or Explicit Congestion Notification (ECN) congestion marks, congestion control algorithms (such as CUBIC or BBR) aggressively slash CWND size. These window reductions collapse transmission line rates and trigger application latency jitter. Tracing CWND reductions with eBPF isolates exact network congestion triggers.

Understanding Linux Kernel Congestion Control State Machines

The Linux TCP engine governs congestion response through a finite state machine:

• Open State: Normal state where acknowledgments continuously grow CWND according to the active algorithm’s growth curve.
• Disorder State: Triggered when Duplicate ACKs arrive. The kernel monitors the out-of-order gap while maintaining CWND.
• Recovery State: Entered when Duplicate ACKs exceed tcp_reordering thresholds. The kernel halves CWND (under CUBIC) and retransmits missing segments via tcp_fastretrans_alert().
• Loss State: Triggered upon Retransmission Timeout (RTO) timer expiry. CWND resets to 1 MSS, collapsing throughput.

Tracing Congestion State Transitions in Real Time with bpftrace

Using bpftrace, monitor state machine transitions directly inside tcp_set_ca_state():

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

kprobe:tcp_set_ca_state {
    $sk = (struct sock *)arg0;
    $new_state = arg1;
    $tp = (struct tcp_sock *)arg0;
    $inet = (struct inet_sock *)arg0;

    if ($new_state == 3) { // TCP_CA_Recovery
        printf("CWND Slashed (Recovery): comm=%s remote=%s:%d cwnd=%d ssthresh=%d\n",
               comm, ntop($inet->inet_daddr), bswap($inet->inet_dport),
               $tp->snd_cwnd, $tp->snd_ssthresh);
        @recovery_events[comm] = count();
    } else if ($new_state == 4) { // TCP_CA_Loss
        printf("CWND Collapsed (Loss): comm=%s remote=%s:%d\n",
               comm, ntop($inet->inet_daddr), bswap($inet->inet_dport));
        @loss_events[comm] = count();
    }
}'

This script logs every socket event where the kernel entered Recovery or Loss states, recording CWND and Slow Start Threshold (ssthresh) levels.

Evaluating Congestion Algorithms Across Network Paths

To inspect whether switching between CUBIC and BBR v2 improves throughput across high-latency WAN links:

sudo bpftrace -e '
tracepoint:tcp:tcp_probe {
    @cwnd_hist[args->daddr] = hist(args->snd_cwnd);
}

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

Analyzing CWND histograms highlights chronically degraded routes, guiding WAN optimization and queue discipline tuning across production infrastructure.