How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Retransmission Ambiguity in Linux Using eBPF

In high-density Linux server environments, accurate round-trip time (RTT) calculation is vital for reliable congestion control. However, when a handshake SYN-ACK packet is retransmitted and an ACK eventually returns, Karn’s algorithm mandates that the kernel discard RTT measurements for that segment due to retransmission ambiguity—it is fundamentally impossible to discern whether the incoming ACK responds to the original SYN-ACK or the retransmitted segment. When timestamp options are disabled or corrupted, this ambiguity causes conservative retransmission timer backoffs that manifest as elevated tcpi_retrans_synack socket statistics. Extended Berkeley Packet Filters (eBPF) provide deep kernel introspection to isolate and resolve these ambiguity events.

Mechanisms of Karn’s Algorithm and Ambiguity in the Linux Kernel

Inside the Linux networking stack, tcp_ack() processes incoming acknowledgement packets. Under standard operation without TCP timestamps (RFC 7323), if tp->retrans_out is non-zero, the kernel cannot reliably match the returning ACK to a specific outgoing transmission timestamp. Consequently, the kernel invokes Karn’s rule: all RTT updates for the retransmitted packet are suppressed, and the retransmission timeout (RTO) is doubled.

When transient packet loss or asymmetric routing delays occur during client connections, repeated ambiguity triggers prevent the socket from recalibrating its smoothed round-trip time (srtt_us), trapping the connection in excessive RTO intervals. eBPF probes allow kernel engineers to capture the exact timestamp offset of the initial SYN-ACK and correlate returning ACKs precisely.

Developing the eBPF Diagnostic Probe with bpftrace

Construct a specialised bpftrace program that tracks initial SYN-ACK generation, subsequent retransmissions, and returning handshakes to detect ambiguity fallbacks:

#!/usr/bin/env bpftrace
/* trace_tcp_ambiguity.bt - Trace Linux TCP retransmission ambiguity */

#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;

    @synack_retrans[$sk] = nsecs;
    @retrans_counts[$saddr, $daddr, $sport, $dport] = count();
    printf("Ambiguity Candidate: SYN-ACK retransmitted for %s:%d -> %s:%d
",
           $saddr, $sport, $daddr, $dport);
}

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

    if (@synack_retrans[$sk] > 0) {
        $elapsed_us = (nsecs - @synack_retrans[$sk]) / 1000;
        $rto = $tp->icsk_rto;
        $retrans_synack = $tp->total_retrans;

        printf("ACK Processed Post-Retransmit | Latency: %u us | RTO: %u ms | Total Retrans: %u
",
               $elapsed_us, $rto, $retrans_synack);
        delete(@synack_retrans[$sk]);
    }
}

interval:s:10
{
    time("%H:%M:%S ");
    printf("--- Periodic SYN-ACK Ambiguity Summary ---
");
    print(@retrans_counts);
}

Running the eBPF Observability Script

Deploy the probe on your target Linux machine with administrative credentials:

# Install essential tracing packages on Debian/Ubuntu
sudo apt-get update && sudo apt-get install -y bpftrace linux-headers-$(uname -r)

# Execute the diagnostic probe
sudo bpftrace trace_tcp_ambiguity.bt

Observe the live terminal output. When handshakes complete following a retransmission event, the probe calculates the exact post-retransmit latency and highlights connections experiencing Karn timeout inflation.

Kernel Optimisation to Resolve Retransmission Ambiguity

To eliminate ambiguity at the protocol level, ensure that TCP timestamps (RFC 7323) are universally negotiated, allowing the kernel to timestamp each segment independently and bypass Karn’s restriction:

# Verify TCP timestamps state (1 indicates enabled)
sysctl net.ipv4.tcp_timestamps

# Enforce timestamp negotiation permanently
sudo sysctl -w net.ipv4.tcp_timestamps=1
echo "net.ipv4.tcp_timestamps = 1" | sudo tee /etc/sysctl.d/99-tcp-timestamps.conf
sudo sysctl --system

With TCP timestamps active, Linux accurately calculates RTT even during SYN-ACK retransmissions, keeping socket retransmission timers optimally calibrated and preventing connection latency spikes.