---
name: "tcpretrans-bpfcc"
section: "8"
platform: "linux"
package: "bpfcc-tools"
version: "0.31.0+ds-7"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/tcpretrans-bpfcc"
---

# tcpretrans-bpfcc(8)

Trace or count TCP retransmits and TLPs. Uses Linux eBPF/bcc.

## NAME

tcpretrans - Trace or count TCP retransmits and TLPs. Uses Linux eBPF/bcc.

## SYNOPSIS

**tcpretrans \[-h\] \[-s\] \[-l\] \[-c\] \[-4 \| -6\]**

## 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.

## OPTIONS

- \-h

  Print usage message.

- \-s

  Display TCP sequence numbers.

- \-l

  Include tail loss probe attempts (in some cases the kernel may not complete the TLP send).

- \-c

  Count occurring retransmits per flow.

- \-4

  Trace IPv4 family only.

- \-6

  Trace IPv6 family only.

## EXAMPLES

- Trace TCP retransmits:

  \# **tcpretrans**

- Trace TCP retransmits and TLP attempts:

  \# **tcpretrans -l**

- Trace IPv4 family only:

  \# **tcpretrans -4**

- Trace IPv6 family only:

  \# **tcpretrans -6**

## FIELDS

- TIME

  Time of the retransmit.

- 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.

- IP

  IP address family (4 or 6).

- LADDR

  Local IP address.

- LPORT

  Local port.

- T\>

  Type of event: R\> == retransmit, L\> == tail loss probe.

- RADDR

  Remote IP address.

- RPORT

  Remote port.

- STATE

  TCP session state.

- SEQ

  TCP sequence.

- RETRANSMITS

  Accumulated occurred retransmits since start.

## 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 bcc.

-

  https://github.com/iovisor/bcc

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

## OS

Linux

## STABILITY

Unstable - in development.

## AUTHOR

Brendan Gregg

## SEE ALSO

tcpconnect(8), tcpaccept(8)