LOOPBACK(3)                                           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 con-
          taining 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 con-
          nection, 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

     LOOPBACK(3)                                           LOOPBACK(3)

               bytes.

          reset
               Clear all of the statistics recorded for the link.

          Reading stats returns a list of 4 tagged numbers represent-
          ing:
               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