freebsd-skq/share/man/man4/dummynet.4

.\"
.\" $FreeBSD$
.\"
.Dd Sep 28, 1998
.Dt DUMMYNET 4
.Os
.Sh NAME
.Nm dummynet
.Nd Flexible bandwidth manager and delay emulator
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/queue.h>
.Fd #include <netinet/in.h>
.Fd #include <netinet/ip_fw.h>
.Ft int
.Fn setsockopt raw_socket IPPROTO_IP "ipfw option" "struct ipfw" size
.Sh 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.
.Pp
In its current implementation,
packet selection is done with the
.Nm ipfw
program, by means of
.Nm ``pipe''
rules.
A dummynet
.Nm pipe
is characterized by a bandwidth, delay, queue size, and loss
rate, which can be configured with the
.Nm ipfw
program. Pipes are
numbered from 1 to 65534, and packets can be passed through multiple
pipes depending on the ipfw configuration.
.Pp
Dummynet operates at the ip level, but if bridging extensions are
enabled, it is possible to pass bridged packets through pipes as well.
.Sh USAGE
Packets are sent to a pipe using the command
.Bd -literal
    ipfw add pipe NNN ....
.Ed

and pipes are configured as follows:
.Bd -literal
    ipfw pipe NNN config bw B delay D queue Q plr P
.Ed

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.
.Pp
Getting ipfw to work right is not very intuitive, especially when
the system is acting as a router or a bridge.
.Pp
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).
.Pp
When acting as a bridge, the
.Nm ipfw
filter is invoked only once, in the input path,
for bridged packets.
.Pp
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:
.Bd -literal
   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
.Ed

.Pp
.Sh OPERATION
The
.Nm 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.
.Pp
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
.Nm ipfw
rules, starting from the one after the matching rule.
.Pp
.Nm dummynet
performs its task once per timer tick. The granularity of operation is
thus controlled by the kernel option
.Bd -literal
    options HZ
.Ed

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.

.Sh KERNEL OPTIONS
The following options in the kernel configuration file are related
to
.Nm dummynet
operation:
.Bd -literal
  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
.Ed
.Pp
Generally, the following options are required:
.Bd -literal
  options IPFIREWALL
  options DUMMYNET
.Ed

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.


.Sh SYSCTL VARIABLES
.Pp
.Bd -literal
    net.inet.ip.fw.one_pass
.Ed

is set to 1 if we want packets to pass through the firewall code only
once.
.Bd -literal
   net.link.ether.bridge_ipfw
.Ed

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

.Sh COMMANDS
The following socket options are used to manage pipes:
.Pp
IP_DUMMYNET_CONFIGURE updates a pipe configuration (or creates a
new one.
.Pp
IP_DUMMYNET_DEL deletes all pipes having the matching rule number.
.Pp
IP_DUMMYNET_GET returns the pipes matching the number.
.Pp
IP_FW_FLUSH flushes the pipes matching the number.
.Pp
When the kernel security level is greater than 2, only IP_DUMMYNET_GET
is allowed.
.Sh SEE ALSO
.Xr setsockopt 2 ,
.Xr ip 4 ,
.Xr ipfw 8 ,
.Xr sysctl 8 .
.Sh BUGS
This manpage is not illustrating all the possible ways to use
dummynet.
.Sh HISTORY
.Nm
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.