Tc qdisc example

Tc qdisc example

This page is meant as a quick reference for how to use these tools, and as admittedly elementary validation for how accurate each function may or may not be. I was able to test how accurate this was with a simple ping test.

I would have expected a need for a larger sample set, but this shows that the tool is fairly accurate in this regard. An optional correlation may also be added I did not test this. This causes the random number generator to be less random and can be used to emulate packet burst losses. So if you had:. These numbers suggest there is a small amount of overhead to the delay injection.

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So I ran the test again with a larger value to verify that the overhead is fixed, rather than a percentage. These numbers look good, suggesting that the amount of overhead for delayed packets is minimal, being noticeable on the low end only. The other explanation could be that our delay is limited by the clock resolution of the kernel, and 3ms is an invalid interval. But based on these tests, I conclude that adding fixed delay is accurate enough for testing purposes.

Note that this is an approximation, not a true statistical correlation. However, it is more common to use a distribution to describe the delay variation. The tc tool includes several tables to specify a non-uniform distribution normal, pareto, paretonormal. Again, I would conclude that the approximation is just that, an approximation. This should be good enough for testing purposes. When using the limit parameter with a token bucket filter, it specifies the number of bytes that can be queued waiting for tokens.

tc qdisc example

From the man page:. You can also specify this the other way around by setting the latency parameter, which specifies the maximum amount of time a packet can sit in the TBF. The latter calculation takes into account the size of the bucket, the rate and possibly the peakrate if set.

These two parameters are mutually exclusive. Unfortunately, the netem discipline does not include rate control.

The first command sets up our root qdisc with a handle named which is equivalent to 1: since the minor number of a qdisc is always 0 and a packet delay of ms.

The second command creates a child class with 1: as the parent since has the same major number. This child class could now be referenced using its handle,and its children would be, etc. The buffer value tells us the size of the bucket in bytes.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. It only takes a minute to sign up.

I've wonder why rate in all cases is Please could you recommend me some documentation that explains tc show output? Also, the show output is somewhat cryptic, please could you recommend me some documentation that explain something like: backlog. I found this list of traffic control resources which might prove helpful to you in gaining enough domain knowledge about the topic.

The article is titled: Traffic Control. Specifically this section on Configuration. They discuss how Traffic Control works, how to configure it, and also provide a good basis for the terminology along with examples, which should prove helpful in getting a better understanding of how everything works. Lastly, I also found this article, titled: HTB Linux queuing discipline manual - user guidewhich makes the following comment about rate. Here they were talking about class rate but I think the two are similar enough:.

It is the same rate as used by gating. It seems that from I think the rate estimators in these distros are not enabled by default because, if you have too much htb classes it could consume much CPU resources. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Asked 6 years, 6 months ago. Active 6 years, 1 month ago. Viewed 23k times. Active Oldest Votes.Jump to navigation.

Rigorously testing a network device or distributed service requires complex, realistic network test environments. Linux Traffic Control tc with Network Emulation netem provides the building blocks to create an impairment node that simulates such networks.

This three-part series describes how an impairment node can be set up using Linux Traffic Control. In the first postLinux Traffic control and its queuing disciplines were introduced. This second part shows which traffic control configurations are available to impair traffic and how to use them. The third and last part will describe how to get an impairment node up and running!

The previous post introduced Linux traffic control and the queuing disciplines that define its behavior. It also described what the default qdisc configuration of a Linux interface looks like.

Finally, it showed how this default configuration can be replaced by a hierarchy of custom queuing disciplines. Our goal is still to create an Impairment Node device that manipulates traffic between two of its Ethernet interfaces eth0 and eth1while managing it from a third interface e. To impair traffic leaving interface eth0we replace the default root queuing discipline with one of our own.

Note that for a symmetrical impairment, the same must be done on the other interface eth1! Deleting a custom configuration using tc qdisc del actually replaces it with the default. Caveat : It is important to note that Traffic Control uses quite odd units.

This means that one kbps equals bytes per second instead of the expected bits per second. On the other hand, kibibit for data and kibibit per second for data rate are both represented by the unit kbit. To limit the outgoing traffic on an interface, we can use the Token Bucket Filter or tbf qdisc man pageread morepicture.

These tokens are refreshed at the desired output rate.

How QoS Works (Part 4 of 4) - Shaping and Policing

Tokens are saved up in a bucket of limited size, so smaller bursts of traffic can still be handled at a higher rate.The Linux kernel's network stack has network traffic control and shaping features. The iproute2 package installs the tc command to control these via the command line. The goal of this article is to show how to shape the traffic by using queueing disciplines. For instance, if you ever had to forbid downloads or torrents on a network that you admin, and not because you were against those services, but because users were "abusing" the bandwidth, then you could use queueing disciplines to allow that kind of traffic and, at the same time, be sure that one user cannot slowdown the entire network.

This is an advanced article; you are expected to have certain knowledge of network devices, iptables, etc. Queuing controls how data is sent ; receiving data is much more reactive with fewer network-oriented controls.

tc qdisc example

There are more relevant details, but they do not touch directly on queuing logic. In order to be the ones fully controlling the shape of the traffic, we need to be the slowest link of the chain.

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That is, if the connection has a maximum download speed of k, if you do not limit of the output to k or below it is going to be the modem shaping the traffic instead of us. Each network device has a root where a qdisc can be set.

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Classful qdiscs allow you to create classes, which work like branches on a tree. You can then set rules to filter packets into each class. Each class can itself have assigned other classful or classless qdisc.

Before starting to configure qdiscs, first we need to remove any existing qdisc from the root. This will remove any qdisc from the eth0 device:.

These are queues that do basic management of traffic by reordering, slowing or dropping packets. This qdiscs do not allow the creation of classes. This was the default qdisc up until systemd This way, no package gets special treatment. It works by creating a virtual bucket and then dropping tokens at certain speed, filling that bucket.

tc qdisc example

Each package takes a virtual token from the bucket, and uses it to get a permission to pass. If too many packets arrive, the bucket will have no more tokens left and the remaining packets are going to wait certain time for new tokens.Each of these queuing disciplines can be used as the primary qdisc on an interface, or can be used inside a leaf class of a classful qdiscs.

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These are the fundamental schedulers used under Linux. It performs no shaping or rearranging of packets. It simply transmits packets as soon as it can after receiving and queuing them. This is also the qdisc used inside all newly created classes until another qdisc or a class replaces the FIFO.

Emulating Network Latency And Packet Loss In Linux

A real FIFO qdisc must, however, have a size limit a buffer size to prevent it from overflowing in case it is unable to dequeue packets as quickly as it receives them. Linux implements two basic FIFO qdisc s, one based on bytes, and one on packets.

Regardless of the type of FIFO used, the size of the queue is defined by the parameter limit. For a pfifo the unit is understood to be packets and for a bfifo the unit is understood to be bytes.

Specifying a limit for a packet or byte FIFO. Based on a conventional FIFO qdisc, this qdisc also provides some prioritization.

It provides three different bands individual FIFOs for separating traffic.

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The highest priority traffic interactive flows are placed into band 0 and are always serviced first. Similarly, band 1 is always emptied of pending packets before band 2 is dequeued. The SFQ qdisc attempts to fairly distribute opportunity to transmit data to the network among an arbitrary number of flows. It accomplishes this by using a hash function to separate the traffic into separate internally maintained FIFOs which are dequeued in a round-robin fashion.

Because there is the possibility for unfairness to manifest in the choice of hash function, this function is altered periodically. Perturbation the parameter perturb sets this periodicity. Unfortunately, some clever software e. Kazaa and eMule among others obliterate the benefit of this attempt at fair queuing by opening as many TCP sessions flows as can be sustained.

In many networks, with well-behaved users, SFQ can adequately distribute the network resources to the contending flows, but other measures may be called for when obnoxious applications have invaded the network.

Conceptually, this qdisc is no different than SFQ although it allows the user to control more parameters than its simpler cousin.

Netem (Network Emulator)

This qdisc was conceived to overcome the shortcoming of SFQ identified above. By allowing the user to control which hashing algorithm is used for distributing access to network bandwidth, it is possible for the user to reach a fairer real distribution of bandwidth.

Theory declares that a RED algorithm is useful on a backbone or core network, but not as useful near the end-user. See the section on flows to see a general discussion of the thirstiness of TCP. This qdisc is built on tokens and buckets.

It simply shapes traffic transmitted on an interface. To limit the speed at which packets will be dequeued from a particular interface, the TBF qdisc is the perfect solution. It simply slows down transmitted traffic to the specified rate.

Packets are only transmitted if there are sufficient tokens available. Otherwise, packets are deferred. Delaying packets in this fashion will introduce an artificial latency into the packet's round trip time. Classless Queuing Disciplines qdisc s.

Note This is not the default qdisc on Linux interfaces. Specifying a limit for a packet or byte FIFO [root leander] cat bfifo. Creating an SFQ [root leander] cat sfq. ESFQ usage UsageFor any host connected to a network, there is the possibility of network congestion. The network bandwidth is always limited.

tc qdisc example

As the data flow on a network link increases, a time comes when the quality of service QoS gets degraded. New connections are blocked and the network throughput deteriorates.

The incoming and outgoing packets are queued before these are received or transmitted respectively. The queue for incoming packets is known as the ingress queue. Similarly, the queue for outgoing packets is called the egress queue. We have more control over the egress queue as it has packets generated by our host. We can re-order these packets in the queue, effectively favoring some packets over the rest. The ip -s link command gives the queue capacity qlen in number of packets.

If the queue is full and more packets come; these are discarded and are not transmitted. The ingress queue has packets which have been sent to us by other hosts.

We can not reorder them; the only thing we can do is to drop some packets, indicating network congestion by not sending the TCP ACK to the sending host. The sending host gets the hint and slows down transmission of packets to us. For UDP packets, this does not work. Shaping involves delaying the transmission of packets to meet a certain data rate.

This is the way we ensure that the output data rate does not exceed the desired value. Shapers can also smooth out the bursts in traffic. Shaping is done at egress. Scheduling is deciding which packet would be transmitted next.

This is done by rearranging the packets in the queue. The objectives are to provide a quick response for interactive applications and also to provide adequate bandwidth for bulk transfers like downloads initiated by remote hosts. Scheduling is done at egress. Policing is measuring the packets received on an interface and limiting these to a particular value.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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Is there a simple way to do this? This is actually what Mark's answer refers to, by a different name. The examples on their homepage already show how you can achieve what you've asked for:.

This is the simplest example, it just adds a fixed amount of delay to all packets going out of the local Ethernet. Now a simple ping test to host on the local network should show an increase of milliseconds. The delay is limited by the clock resolution of the kernel Hz. On most 2. Network delay variation isn't purely random, so to emulate that there is a correlation value as well.

This isn't true statistical correlation, but an approximation. Typically, the delay in a network is not uniform. It is more common to use a something like a normal distribution to describe the variation in delay.

The netem discipline can take a table to specify a non-uniform distribution. Random packet loss is specified in the 'tc' command in percent.

The smallest possible non-zero value is:. An optional correlation may also be added. This causes the random number generator to be less random and can be used to emulate packet burst losses. This will cause 0. Note that you should use tc qdisc add if you have no rules for that interface or tc qdisc change if you already have rules for that interface. For dropped packets I would simply use iptables and the statistic module.

Be careful, anything above about 0. One of my colleagues uses tc to do this. Refer to the man page for more information.