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/bpftrace/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), tcpaccept.bt(8)