DUMMYNET(4)

NAME
dummynet - Flexible bandwidth manager and delay emulator

SYNOPSIS
#include <sys/types.h>
#include <sys/queue.h>
#include <netinet/in.h>
#include <netinet/ip_fw.h>
int
setsockopt(raw_socket, IPPROTO_IP, ipfw option, struct ipfw, size);

DESCRIPTION
dummynet is a system facility that permits the control of traffic going through the various network interfaces, by applying bandwidth and queue size limitations, and simulating delays and losses.

In its current implementation, packet selection is done with the ipfw program, by means of ``pipe'' rules. A dummynet pipe is characterized by a bandwidth, delay, queue size, and loss rate, which can be configured with the ipfw program. Pipes are numbered from 1 to 65534, and packets can be passed through multiple pipes depending on the ipfw configuration.

Dummynet operates at the ip level, but if bridging extensions are enabled, it is possible to pass bridged packets through pipes as well.

USAGE
Packets are sent to a pipe using the command

        ipfw add pipe NNN ....

and pipes are configured as follows:

        ipfw pipe NNN config bw B delay D queue Q plr P

where the bandwidth B can be expressed in bit/s, Kbit/s, Mbit/s, Bytes/s, KBytes/s, MBytes/s , delay in milliseconds, queue size in packets or Bytes, plr is the fraction of packets randomly dropped.

Getting ipfw to work right is not very intuitive, especially when the system is acting as a router or a bridge.

When acting as a router, the same ruleset is applied on both the input and the output path for routed packets, so you have to make sure that a packet does not go through the same pipe twice (unless this is what you really want).

When acting as a bridge, the ipfw filter is invoked only once, in the input path, for bridged packets.

Also, when simulating true full-duplex channels, be sure to pass traffic through two different pipes, depending on the direction. E.g. a suitable rule set for simulating an asymmetric bidirectional link would be the following:

       ipfw add pipe 1 ip from A to B out
       ipfw add pipe 2 ip from B to A in
       ipfw pipe 1 config bw 1Mbit/s delay 80ms
       ipfw pipe 2 config bw 128Kbit/s delay 300ms

OPERATION
The ipfw code is used to select packets that must be subject to bandwidth/queue/delay/losses, and returns the identifier of the ``pipe'' describing such limitations.

Selected packets are first queued in a bounded size queue, from which they are extracted at the programmed rate and passed to a second queue where delay is simulated. At the output from the second queue packets are reinjected into the protocol stack at the same point they came from (i.e. ip_input(), ip_output(), bdg_forward() ). Depending on the setting of the sysctl variable sys.net.inet.ipfw.one_pass Packets coming from a pipe can be either forwarded to their destination, or passed again through the ipfw rules, starting from the one after the matching rule.

dummynet performs its task once per timer tick. The granularity of operation is thus controlled by the kernel option

        options HZ

whose default value (100) means a granularity of 10ms. For an accurate simulation of high data rates it might be necessary to reduce the timer granularity to 1ms or less. Consider, however, that some interfaces using programmed I/O may require a considerable time to output packets. So, reducing the granularity too much might actually cause ticks to be missed thus reducing the accuracy of operation.

KERNEL OPTIONS
The following options in the kernel configuration file are related to dummynet operation:

      IPFIREWALL               - enable ipfirewall (required for dummynet).
      IPFIREWALL_VERBOSE       - enable firewall output.
      IPFIREWALL_VERBOSE_LIMIT - limit firewall output.
      DUMMYNET                 - enable dummynet operation.
      NMBCLUSTER               - set the amount of network packet buffers
      HZ                       - sets the timer granularity

Generally, the following options are required:

      options IPFIREWALL
      options DUMMYNET

additionally, one may want to increase the number of mbuf clusters (used to store network packets) according to the sum of the bandwidth-delay products and queue sizes of all configured pipes.

SYSCTL VARIABLES

        net.inet.ip.fw.one_pass

is set to 1 if we want packets to pass through the firewall code only once.

       net.link.ether.bridge_ipfw

is set if we want bridged packets to pass through the firewall code.

COMMANDS
The following socket options are used to manage pipes:

IP_DUMMYNET_CONFIGURE updates a pipe configuration (or creates a new one.

IP_DUMMYNET_DEL deletes all pipes having the matching rule number.

IP_DUMMYNET_GET returns the pipes matching the number.

IP_FW_FLUSH flushes the pipes matching the number.

When the kernel security level is greater than 2, only IP_DUMMYNET_GET is allowed.

SEE ALSO
setsockopt(2), ip(4), ipfw(8), sysctl(8).

BUGS
This manpage is not illustrating all the possible ways to use dummynet.

HISTORY
dummynet dummynet was initially implemented as a testing tool for TCP congestion control by Luigi Rizzo <luigi@iet.unipi.it>, as described on ACM Computer Communication Review, Jan.97 issue. Later it has been then modified to work at the ip and bridging level, and integrated with the IPFW packet filter.