TH LOOPBACK 3
SH NAME
loopback \- network link simulation
SH SYNOPSIS
nf
B bind -a #λ /net

BI /net/loopback n /[0-1]
BI /net/loopback n /[0-1]/data
BI /net/loopback n /[0-1]/ctl
BI /net/loopback n /[0-1]/status
BI /net/loopback n /[0-1]/stats

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

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