term% cat index.txt LOOPBACK(3) Library Functions Manual LOOPBACK(3)
NAME
loopback - network link simulation
SYNOPSIS
bind -a #X /net
/net/loopbackn/[0-1]
/net/loopbackn/[0-1]/data
/net/loopbackn/[0-1]/ctl
/net/loopbackn/[0-1]/status
/net/loopbackn/[0-1]/stats
DESCRIPTION
The loopback interface, /net/loopbackn, is a directory containing two
subdirectories, one for each end of a simulated network link. The num‐
ber n is the device number of the link, permitting multiple links to be
used on a single machine.
Each directory contains files to control the associated connection, re‐
ceive and send data, monitor the simulation parameters, and supply sta‐
tistics.
The 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 data file.
Options are set by writing to the ctl file for the receiving end of the
link, and are reported in the same format by reading status. The fol‐
lowing options are supported.
delay latency bytedelay
Control the time a packet takes in the link. A packet n bytes
long takes bytedelay * n nanoseconds to exit the output queue
and is available for reading latency nanoseconds later.
droprate n
Randomly drop approximately one out of n packets. If zero drop
no packets.
indrop [01]
Disallow or allow packets to be dropped if the input queue over‐
flows.
limit n
Set the input and output queues to hold at most n bytes.
reset Clear all of the statistics recorded for the link.
Reading stats returns a list of 4 tagged numbers representing:
packets sent to this receiver
bytes sent to this receiver
packets dropped due to droprate
packets dropped due to input queue overflows
SOURCE
/sys/src/9/port/devloopback.c
LOOPBACK(3)