2011-03-30 13:49:47 +00:00
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.TH LOOPBACK 3
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.SH NAME
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loopback \- network link simulation
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.SH SYNOPSIS
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.nf
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2018-02-25 18:06:37 +00:00
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.B bind -a #λ /net
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2011-03-30 13:49:47 +00:00
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.BI /net/loopback n /[0-1]
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.BI /net/loopback n /[0-1]/data
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.BI /net/loopback n /[0-1]/ctl
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.BI /net/loopback n /[0-1]/status
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.BI /net/loopback n /[0-1]/stats
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.fi
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.SH DESCRIPTION
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The loopback interface,
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.BI /net/loopback n\f1,
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is a directory containing two subdirectories,
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one for each end of a simulated network link.
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The number
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.I n
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is the device number of the link, permitting multiple links to be used on a single machine.
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.PP
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Each directory contains files to control the associated connection,
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receive and send data,
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monitor the simulation parameters,
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and supply statistics.
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.PP
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The
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.B data
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files for the two directories are cross-connected.
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Writes to one are divided into packets of at most a certain size,
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typically 32768 bytes,
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written to a flow-controlled output queue,
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transferred across the link,
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and put into an input queue where it is readable from the other
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.B data
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file.
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.PP
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Options are set by writing to the
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.B ctl
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file for the receiving end of the link,
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and are reported in the same format by reading
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.BR status .
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The following options are supported.
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.TP
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.BI delay \ latency\ bytedelay
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Control the time a packet takes in the link.
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A packet
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.B n
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bytes long takes
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.I bytedelay
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.B *
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.B n
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nanoseconds to exit the output queue and
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is available for reading
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.I latency
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nanoseconds later.
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.TP
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.BI droprate \ n
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Randomly drop approximately one out of
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.B n
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packets.
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If zero drop no packets.
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.TP
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.BR indrop \ [01]
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Disallow or allow packets to be dropped if the input queue overflows.
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.TP
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.BI limit \ n
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Set the input and output queues to hold at most
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.I n
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bytes.
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.TP
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.B reset
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Clear all of the statistics recorded for the link.
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.PP
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Reading
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.B stats
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returns a list of 4 tagged numbers representing:
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.EX
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.ft 1
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packets sent to this receiver
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bytes sent to this receiver
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packets dropped due to droprate
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packets dropped due to input queue overflows
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.EE
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.SH SOURCE
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.B /sys/src/9/port/devloopback.c
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