.TH tcpretrans 8  "2016-02-14" "USER COMMANDS"
.SH NAME
tcpretrans \- Trace or count TCP retransmits and TLPs. Uses Linux eBPF/bcc.
.SH SYNOPSIS
.B tcpretrans [\-h] [\-s] [\-l] [\-c] [\-4 | \-6]
.SH 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.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
.TP
\-h
Print usage message.
.TP
\-s
Display TCP sequence numbers.
.TP
\-l
Include tail loss probe attempts (in some cases the kernel may not
complete the TLP send).
.TP
\-c
Count occurring retransmits per flow.
.TP
\-4
Trace IPv4 family only.
.TP
\-6
Trace IPv6 family only.
.SH EXAMPLES
.TP
Trace TCP retransmits:
#
.B tcpretrans
.TP
Trace TCP retransmits and TLP attempts:
#
.B tcpretrans \-l
.TP
Trace IPv4 family only:
#
.B tcpretrans \-4
.TP
Trace IPv6 family only:
#
.B tcpretrans \-6
.SH FIELDS
.TP
TIME
Time of the retransmit.
.TP
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.
.TP
IP
IP address family (4 or 6).
.TP
LADDR
Local IP address.
.TP
LPORT
Local port.
.TP
T>
Type of event: R> == retransmit, L> == tail loss probe.
.TP
RADDR
Remote IP address.
.TP
RPORT
Remote port.
.TP
STATE
TCP session state.
.TP
SEQ
TCP sequence.
.TP
RETRANSMITS
Accumulated occurred retransmits since start.
.SH 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.
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion _examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
tcpconnect(8), tcpaccept(8)