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) 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