NAME

dnctl — Traffic shaper control program

SYNOPSIS

DESCRIPTION

The dnctl utility is the user interface for controlling the dummynet(4) traffic shaper.

dummynet operates by first using a packet filter to classify packets and divide them into flows, using any match pattern that can be used in dnctl rules. Depending on local policies, a flow can contain packets for a single TCP connection, or from/to a given host, or entire subnet, or a protocol type, etc.

Packets belonging to the same flow are then passed to either of two different objects, which implement the traffic regulation:

pipe

A pipe emulates a link with given bandwidth, propagation delay, queue size and packet loss rate. Packets are queued in front of the pipe as they come out from the classifier, and then transferred to the pipe according to the pipe's parameters.

queue

A queue is an abstraction used to implement the WF2Q+ (Worst-case Fair Weighted Fair Queueing) policy, which is an efficient variant of the WFQ policy.
The queue associates a weight and a reference pipe to each flow, and then all backlogged (i.e., with packets queued) flows linked to the same pipe share the pipe's bandwidth proportionally to their weights. Note that weights are not priorities; a flow with a lower weight is still guaranteed to get its fraction of the bandwidth even if a flow with a higher weight is permanently backlogged.

In practice, pipes can be used to set hard limits to the bandwidth that a flow can use, whereas queues can be used to determine how different flow share the available bandwidth.

The pipe and queue configuration commands are the following:

The following parameters can be configured for a pipe:

bw bandwidth | device

Bandwidth, measured in [K|M]{bit/s|Byte/s}.

A value of 0 (default) means unlimited bandwidth. The unit must immediately follow the number, as in

dnctl pipe 1 config bw 300Kbit/s

If a device name is specified instead of a numeric value, as in

dnctl pipe 1 config bw tun0

then the transmit clock is supplied by the specified device. At the moment no device supports this functionality.

delay ms-delay

Propagation delay, measured in milliseconds. The value is rounded to the next multiple of the clock tick (typically 10ms, but it is a good practice to run kernels with “options HZ=1000” to reduce the granularity to 1ms or less). Default value is 0, meaning no delay.

The following parameters can be configured for a queue:

pipe pipe_nr

Connects a queue to the specified pipe. Multiple queues (with the same or different weights) can be connected to the same pipe, which specifies the aggregate rate for the set of queues.

weight weight

Specifies the weight to be used for flows matching this queue. The weight must be in the range 1..100, and defaults to 1.

Finally, the following parameters can be configured for both pipes and queues:

buckets hash-table-size

Specifies the size of the hash table used for storing the various queues. Default value is 64 controlled by the sysctl(8) variable net.inet.ip.dummynet.hash_size, allowed range is 16 to 65536.

mask mask-specifier

Packets sent to a given pipe or queue by an dnctl rule can be further classified into multiple flows, each of which is then sent to a different dynamic pipe or queue. A flow identifier is constructed by masking the IP addresses, ports and protocol types as specified with the mask options in the configuration of the pipe or queue. For each different flow identifier, a new pipe or queue is created with the same parameters as the original object, and matching packets are sent to it.

Thus, when dynamic pipes are used, each flow will get the same bandwidth as defined by the pipe, whereas when dynamic queues are used, each flow will share the parent's pipe bandwidth evenly with other flows generated by the same queue (note that other queues with different weights might be connected to the same pipe).
Available mask specifiers are a combination of one or more of the following:

dst-ip mask, dst-ip6 mask, src-ip mask, src-ip6 mask, dst-port mask, src-port mask, proto mask or all,

where the latter means all bits in all fields are significant.

noerror

When a packet is dropped by a dummynet queue or pipe, the error is normally reported to the caller routine in the kernel, in the same way as it happens when a device queue fills up. Setting this option reports the packet as successfully delivered, which can be needed for some experimental setups where you want to simulate loss or congestion at a remote router.

plr packet-loss-rate

Packet loss rate. Argument packet-loss-rate is a floating-point number between 0 and 1, with 0 meaning no loss, 1 meaning 100% loss. The loss rate is internally represented on 31 bits.

queue {slots | sizeKbytes}

Queue size, in slots or KBytes. Default value is 50 slots, which is the typical queue size for Ethernet devices. Note that for slow speed links you should keep the queue size short or your traffic might be affected by a significant queueing delay. E.g., 50 max-sized ethernet packets (1500 bytes) mean 600Kbit or 20s of queue on a 30Kbit/s pipe. Even worse effect can result if you get packets from an interface with a much larger MTU, e.g. the loopback interface with its 16KB packets.

red | gred w_q/min_th/max_th/max_p

Make use of the RED (Random Early Detection) queue management algorithm. w_q and max_p are floating point numbers between 0 and 1 (0 not included), while min_th and max_th are integer numbers specifying thresholds for queue management (thresholds are computed in bytes if the queue has been defined in bytes, in slots otherwise). The dummynet(4) also supports the gentle RED variant (gred).

Three sysctl(8) variables can be used to control the RED behaviour:

net.inet.ip.dummynet.red_lookup_depth

specifies the accuracy in computing the average queue when the link is idle (defaults to 256, must be greater than zero)

net.inet.ip.dummynet.red_avg_pkt_size

specifies the expected average packet size (defaults to 512, must be greater than zero)

net.inet.ip.dummynet.red_max_pkt_size

specifies the expected maximum packet size, only used when queue thresholds are in bytes (defaults to 1500, must be greater than zero).

The following options are available:

-a

While listing, show counter values. The show command just implies this option.

-f

Don't ask for confirmation for commands that can cause problems if misused, i.e. flush. If there is no tty associated with the process, this is implied.

-h

Displays a short help.

-n

Only check syntax of the command strings, without actually passing them to the kernel.

-q

While adding, zeroing, resetlogging or flushing, be quiet about actions (implies -f). This is useful for adjusting rules by executing multiple dnctl commands in a script or by processing a file of many dnctl rules across a remote login session. If a flush is performed in normal (verbose) mode (with the default kernel configuration), it prints a message. Because all rules are flushed, the message might not be delivered to the login session, causing the remote login session to be closed and the remainder of the ruleset to not be processed. Access to the console would then be required to recover.

-s [field]

While listing pipes, sort according to one of the four counters (total or current packets or bytes).

-v

Be verbose.

To ease configuration, rules can be put into a file which is processed using dnctl as shown in the last synopsis line. An absolute pathname must be used. The file will be read line by line and applied as arguments to the dnctl utility.

Optionally, a preprocessor can be specified using -p preproc where pathname is to be piped through. Useful preprocessors include cpp(1) and m4(1). If preproc doesn't start with a slash (‘/’) as its first character, the usual PATH name search is performed. Care should be taken with this in environments where not all file systems are mounted (yet) by the time dnctl is being run (e.g. when they are mounted over NFS). Once -p has been specified, any additional arguments as passed on to the preprocessor for interpretation. This allows for flexible configuration files (like conditionalizing them on the local hostname) and the use of macros to centralize frequently required arguments like IP addresses.

CHECKLIST

Here are some important points to consider when designing your rules:

• Remember that you filter both packets going in and out. Most connections need packets going in both directions.
• Remember to test very carefully. It is a good idea to be near the console when doing this.
• Don't forget the loopback interface.

SYSCTL VARIABLES

A set of sysctl(8) variables controls the behaviour of the dummynet module. These are shown below together with their default value (but always check with the sysctl(8) command what value is actually in use) and meaning:

net.inet.ip.dummynet.expire: 1

Lazily delete dynamic pipes/queue once they have no pending traffic. You can disable this by setting the variable to 0, in which case the pipes/queues will only be deleted when the threshold is reached.

net.inet.ip.dummynet.hash_size: 64

Default size of the hash table used for dynamic pipes/queues. This value is used when no buckets option is specified when configuring a pipe/queue.

net.inet.ip.dummynet.max_chain_len: 16

Target value for the maximum number of pipes/queues in a hash bucket. The product max_chain_len*hash_size is used to determine the threshold over which empty pipes/queues will be expired even when net.inet.ip.dummynet.expire=0.

net.inet.ip.dummynet.red_lookup_depth: 256

 

net.inet.ip.dummynet.red_avg_pkt_size: 512

 

net.inet.ip.dummynet.red_max_pkt_size: 1500

Parameters used in the computations of the drop probability for the RED algorithm.

EXAMPLES

The following rules show some of the applications of for simulations and the like by using dummynet rules in pf.conf(8) configuration files.

To drop random incoming IPv4 and IPv6 ICMP packets with a probability of 5%, create a pipe:

and add these rules in your pf.conf file:

Should we want to simulate a bidirectional link with bandwidth limitations, the correct way is to create a pipe for each direction:

and add these rules in your pf.conf file:

The above can be very useful, e.g. if you want to see how your fancy Web page will look for a residential user who is connected only through a slow link. You should not use only one pipe for both directions, unless you want to simulate a half-duplex medium (e.g. AppleTalk, Ethernet, IRDA).

Note that with the above rules the pipes receive traffic for both the IPv4 and IPv6 protocols.

Should we want to verify network performance with the RED queue management algorithm, create this pipe:

and then add these rules to you pf.conf file:

Another typical application of the traffic shaper is to introduce some delay in the communication. This can significantly affect applications which do a lot of Remote Procedure Calls, and where the round-trip-time of the connection often becomes a limiting factor much more than bandwidth:

and add these rules in your pf.conf file:

Per-flow queueing can be useful for a variety of purposes. A very simple one is counting traffic:

and add these statements in your pf.conf file:

The above set of rules will create queues (and collect statistics) for all traffic. Because the pipes have no limitations, the only effect is collecting statistics. Note that we need six rules, not just the last two one, because when dnctl tries to match IP packets it will not consider ports, so we would not see connections on separate ports as different ones.

SEE ALSO

cpp(1), dummynet(4), m4(1), ip(4), pfctl(8), pf.conf(5), protocols(5), services(5), sysctl(8)

AUTHORS

Ugen J. S. Antsilevich,
Poul-Henning Kamp,
Alex Nash,
Archie Cobbs,
Luigi Rizzo.

API based upon code written by Daniel Boulet for BSDI.

Work on dummynet(4) traffic shaper supported by Akamba Corp.

HISTORY

The dnctl utility first appeared in FreeBSD 2.0. dummynet(4) was introduced in FreeBSD 2.2.8. Stateful extensions were introduced in FreeBSD 4.0.