---
name: "tcpretrans.bt.bt"
section: "8"
platform: "linux"
package: "bpftrace"
version: "0.17.0-1"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/tcpretrans.bt.bt"
---

# tcpretrans.bt.bt(8)

Trace or count TCP retransmits. Uses Linux bpftrace/eBPF

## NAME

tcpretrans.bt - Trace or count TCP retransmits. Uses Linux bpftrace/eBPF

## SYNOPSIS

**tcpretrans.bt**

## DESCRIPTION

This traces TCP retransmits, showing address, port, and TCP state information, and sometimes the PID (although usually not, since retransmits are usually sent by the kernel on timeouts). To keep overhead very low, only the TCP retransmit functions are traced. This does not trace every packet (like [tcpdump(8)](/tcpdump/8.md) or a packet sniffer). Optionally, it can count retransmits over a user signalled interval to spot potentially dropping network paths the flows are traversing.

This uses dynamic tracing of the kernel tcp\_retransmit\_skb() and tcp\_send\_loss\_probe() functions, and will need to be updated to match kernel changes to these functions.

Since this uses BPF, only the root user can use this tool.

## REQUIREMENTS

CONFIG\_BPF and bcc. CONFIG\_BPF and bpftrace.

## EXAMPLES

- Trace TCP retransmits:

  \# **tcpretrans.bt**

-

  ## FIELDS

- TIME

  Time of the call, in HH:MM:SS format.

- PID

  Process ID that was on-CPU. This is less useful than it might sound, as it may usually be 0, for the kernel, for timer-based retransmits.

- LADDR

  Local IP address.

- LPORT

  Local port.

- RADDR

  Remote IP address.

- RPORT

  Remote port.

- STATE

  TCP session state.

## OVERHEAD

Should be negligible: TCP retransmit events should be low (\<1000/s), and the low overhead this tool adds to each event should make the cost negligible.

## SOURCE

This is from bpftrace

-

  https://github.com/iovisor/bpftrace

Also look in the bpftrace distribution for a companion \_examples.txt file containing example usage, output, and commentary for this tool.

This is a bpftrace version of the bcc tool of the same name. The bcc tool may provide more options and customizations.

-

  https://github.com/iovisor/bcc

## OS

Linux

## STABILITY

Unstable - in development.

## AUTHOR

Brendan Gregg, adapted for bpftrace by Dale Hamel

## SEE ALSO

[tcpconnect.bt(8)](/tcpconnect.bt/8.md), [tcpaccept.bt(8)](/tcpaccept.bt/8.md)