How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Selective ACK SACK Renege Drops in Linux Using eBPF

Selective Acknowledgement (SACK, RFC 2018) allows TCP receivers to inform senders about non-contiguous blocks of data received, enabling efficient retransmission of missing segments. However, under extreme memory pressure or aggressive socket pruning, a receiving host may be forced to renege on previously SACKed segments—discarding out-of-order buffers before the application layer reads them. When reneging occurs during connection handshakes or early data exchanges, the sender kernel must invalidate scoreboard states and trigger conservative timeouts, escalating tcpi_retrans_synack counters. Extended Berkeley Packet Filters (eBPF) provide non-intrusive kernel tracing to capture SACK renege events and drop bottlenecks in real time.

Kernel Mechanics of SACK Renege Handling

Inside the Linux network stack, the sender tracks out-of-order blocks in struct tcp_sock. When the receiver reneges on a previously acknowledged block, the incoming ACK segment advances snd_una without including expected SACK options, triggering tcp_check_sack_reneging().

When the kernel confirms reneging, it enters the recovery state, resets congestion windows, and falls back to slow retransmission timers. In half-open connection states, this unexpected state reversal stalls handshakes, triggering SYN-ACK retransmits. Using eBPF attached to kernel SACK recovery functions gives infrastructure engineers exact per-socket visibility into receiver memory pressure.

Authoring the Diagnostic Probe with bpftrace

Construct a dedicated bpftrace script that intercepts TCP SACK renege detection and correlates it with SYN-ACK retransmits:

#!/usr/bin/env bpftrace
/* trace_sack_renege.bt - Trace Linux TCP SACK reneging events */

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

tracepoint:tcp:tcp_retransmit_synack
{
    $sk = (struct sock *)args->skaddr;
    $saddr = ntop(args->saddr);
    $daddr = ntop(args->daddr);
    $sport = args->sport;
    $dport = args->dport;

    printf("SYN-ACK Retransmit: %s:%d -> %s:%d (SACK Renege Suspect)
",
           $saddr, $sport, $daddr, $dport);
    @synack_retransmits[$saddr, $daddr] = count();
}

kprobe:tcp_check_sack_reneging
{
    $sk = (struct sock *)arg0;
    $saddr = ntop($sk->__sk_common.skc_rcv_saddr);
    $daddr = ntop($sk->__sk_common.skc_daddr);

    printf("SACK Reneging Detected: %s <-> %s
", $saddr, $daddr);
    @sack_renege_events[$saddr, $daddr] = count();
}

interval:s:5
{
    time("%H:%M:%S ");
    printf("--- TCP SACK Activity (5s Window) ---
");
    print(@sack_renege_events);
    print(@synack_retransmits);
    clear(@sack_renege_events);
    clear(@synack_retransmits);
}

Running the Trace on Production Linux Systems

Deploy the probe with root privileges on the target server:

# Install bpftrace and kernel dependencies
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Execute the trace
sudo bpftrace trace_sack_renege.bt

Kernel Optimization and Parameter Tuning

To inspect system memory pressure that causes socket buffer pruning:

# Check TCP memory pressure counters
cat /proc/net/sockstat

# Inspect current socket read/write memory limits
sysctl net.ipv4.tcp_rmem
sysctl net.ipv4.tcp_wmem

By pairing eBPF kernel instrumentation with calibrated sysctl adjustments, network engineers can eliminate elusive SYN-ACK stalls while preserving essential memory boundaries.