tcpretrans(8) System Manager's Manual tcpretrans(8)
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 in-
formation, and sometimes the PID (although usually not, since retrans-
mits 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.
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)
USER COMMANDS 2016-02-14 tcpretrans(8)
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