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 number ­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, receive and send data, monitor the simulation parameters, and supply statistics. 

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 following 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 overflows.

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

Plan 9  —  July 26, 2000