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92 lines
2.1 KiB
92 lines
2.1 KiB
4 months ago
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.TH tcpretrans 8 "2016-02-14" "USER COMMANDS"
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.SH NAME
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tcpretrans \- Trace or count TCP retransmits and TLPs. Uses Linux eBPF/bcc.
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.SH SYNOPSIS
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.B tcpretrans [\-h] [\-l]
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.SH DESCRIPTION
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This traces TCP retransmits, showing address, port, and TCP state information,
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and sometimes the PID (although usually not, since retransmits are usually
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sent by the kernel on timeouts). To keep overhead very low, only
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the TCP retransmit functions are traced. This does not trace every packet
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(like tcpdump(8) or a packet sniffer). Optionally, it can count retransmits
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over a user signalled interval to spot potentially dropping network paths the
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flows are traversing.
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This uses dynamic tracing of the kernel tcp_retransmit_skb() and
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tcp_send_loss_probe() functions, and will need to be updated to
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match kernel changes to these functions.
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Since this uses BPF, only the root user can use this tool.
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.SH REQUIREMENTS
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CONFIG_BPF and bcc.
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.SH OPTIONS
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.TP
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\-h
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Print usage message.
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.TP
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\-l
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Include tail loss probe attempts (in some cases the kernel may not
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complete the TLP send).
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.TP
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\-c
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Count occurring retransmits per flow.
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.SH EXAMPLES
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.TP
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Trace TCP retransmits:
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#
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.B tcpretrans
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.TP
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Trace TCP retransmits and TLP attempts:
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#
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.B tcpretrans \-l
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.SH FIELDS
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.TP
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TIME
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Time of the retransmit.
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.TP
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PID
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Process ID that was on-CPU. This is less useful than it might sound, as it
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may usually be 0, for the kernel, for timer-based retransmits.
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.TP
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IP
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IP address family (4 or 6).
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.TP
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LADDR
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Local IP address.
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.TP
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LPORT
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Local port.
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.TP
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T>
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Type of event: R> == retransmit, L> == tail loss probe.
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.TP
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RADDR
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Remote IP address.
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.TP
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RPORT
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Remote port.
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.TP
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STATE
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TCP session state.
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.TP
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RETRANSMITS
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Accumulated occurred retransmits since start.
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.SH OVERHEAD
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Should be negligible: TCP retransmit events should be low (<1000/s), and the
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low overhead this tool adds to each event should make the cost negligible.
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.SH SOURCE
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This is from bcc.
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.IP
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https://github.com/iovisor/bcc
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.PP
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Also look in the bcc distribution for a companion _examples.txt file containing
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example usage, output, and commentary for this tool.
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.SH OS
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Linux
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.SH STABILITY
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Unstable - in development.
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.SH AUTHOR
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Brendan Gregg
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.SH SEE ALSO
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tcpconnect(8), tcpaccept(8)
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