Overview

A queue is a collection of data packets collectively waiting to be transmitted by a network device using a pre-defined structure methodology. Queuing works almost on the same methodology used at banks or supermarkets, where the customer is treated according to its arrival.

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Queue implementation in MikroTik RouterOS is based on Hierarchical Token Bucket (HTB). HTB allows to the creation of a hierarchical queue structure and determines relations between queues. These hierarchical structures can be attached at two different places, the the Packet Flow diagram illustrate illustrates both input and postrouting chains.

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• /queue simple menu - designed to ease the configuration of simple, every day queuing tasks (such as single client upload/download limitation, p2p traffic limitation, etc.).
• /queue tree menu - for implementing advanced queuing tasks (such as global prioritization policy, and user group limitations). Requires marked packet flows from from  /ip firewall mangle facility.

RouterOS provides a possibility to configure queue in 8 levels -  the first level is an interface queue from the "/queue interface" menu and the other 7 are lower-level queues that can be created in Queue Simple and/or Queue Tree.

Rate limitation principles

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Rate limiting is used to control the rate of traffic flow sent or received on a network interface. Traffic which with rate that is less than or equal to the specified rate is sent, whereas traffic that exceeds the rate is dropped or delayed.

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1. discard all packets that exceed rate limit – rate-limiting (dropper or shaper) (100% rate limiter when queue-size=0)
2. delay packets that exceed the specific rate limit in the queue and transmit its them when it is possible – rate equalizing (scheduler) (100% rate equalizing when queue-size=unlimited)

Next The next figure explains the difference between rate limiting and rate equalizing:

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As you can see in the first case all traffic exceeds a specific rate and is dropped. In another case, traffic exceeds a specific rate and is delayed in the queue and transmitted later when it is possible, but note that the packet can be delayed only until the queue is not full. If there is no more space in the queue buffer, packets are dropped.

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A simple queue is a plain way how to limit traffic fora for a particular target. Also, you can use simple use simple queues to build advanced QoS applications. They have useful integrated features:

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In the following example, we have one SOHO device with two connected units PC and Server.

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We have a 15 Mbps connection available from ISP in this case. We want to be sure the server receives enough traffic, so we will configure a simple queue with a limit-at parameter to guarantee a server to receive receives 5Mbps:

/queue simple
add limit-at=5M/5M max-limit=15M/15M name=queue1 target=192.168.88.251/32

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The queue tree creates only a one-directional queue in one of the HTBs. It is also the only way how to add a queue on the a separate interface. This way it is possible to ease mangle configuration - you don't need separate marks for download and upload - only the upload will get to the Public interface and only the download will get to a Private interface. The main difference from Simple Queues is that the Queue tree is not ordered - all traffic passes it together.

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This sub-menu list by default created queue types and allows to add the addition of new user-specific ones.

By default, RouterOS creates the following pre-defined queue types:

[admin@MikroTik] > /queue type print
Flags: * - default 
 0 * name="default" kind=pfifo pfifo-limit=50 

 1 * name="ethernet-default" kind=pfifo pfifo-limit=50 

 2 * name="wireless-default" kind=sfq sfq-perturb=5 sfq-allot=1514 

 3 * name="synchronous-default" kind=red red-limit=60 red-min-threshold=10 red-max-threshold=50 red-burst=20 red-avg-packet=1000 

 4 * name="hotspot-default" kind=sfq sfq-perturb=5 sfq-allot=1514 

 5 * name="pcq-upload-default" kind=pcq pcq-rate=0 pcq-limit=50KiB pcq-classifier=src-address pcq-total-limit=2000KiB pcq-burst-rate=0 pcq-burst-threshold=0 pcq-burst-time=10s pcq-src-address-mask=32 
     pcq-dst-address-mask=32 pcq-src-address6-mask=128 pcq-dst-address6-mask=128 

 6 * name="pcq-download-default" kind=pcq pcq-rate=0 pcq-limit=50KiB pcq-classifier=dst-address pcq-total-limit=2000KiB pcq-burst-rate=0 pcq-burst-threshold=0 pcq-burst-time=10s pcq-src-address-mask=32 
     pcq-dst-address-mask=32 pcq-src-address6-mask=128 pcq-dst-address6-mask=128 

 7 * name="only-hardware-queue" kind=none 

 8 * name="multi-queue-ethernet-default" kind=mq-pfifo mq-pfifo-limit=50 

 9 * name="default-small" kind=pfifo pfifo-limit=10

All RouterBOARDS MikroTik products have the default queue type "only-hardware-queue" with "kind=none". "only-hardware-queue" leaves the interface with only hardware transmit descriptor transmit descriptor ring buffer which acts as a queue in itself. Usually, at least 100 packets can be queued for transmit in the transmit descriptor ring buffer. Transmit descriptor ring buffer size and the number of packets that can be queued in it varies for different types of ethernet MACs.  Having Having no software queue is especially beneficial on SMP systems because it removes the requirement to synchronize access to it from different CPUs/cores which is resource-intensive. Having the possibility to set "only-hardware-queue" requires support in an ethernet driver so it is available only for some ethernet interfaces mostly found on RouterBOARDs.

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MQ-PFIFO is pfifo with support for multiple transmit queues. This queue is beneficial on SMP systems with ethernet interfaces that have support for multiple transmit queues and have a Linux driver support for multiple transmit queues (mostly on x86 platforms). This kind uses the mq-pfifo-limit parameter.

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Random Early Drop is a queuing mechanism that tries to avoid network congestion by controlling the average queue size. The average queue size is compared to two thresholds: a minimum (min_th) and a maximum (max_th) threshold. If the average queue size (avg_q) is less than the minimum threshold, no packets are dropped. When the average queue size is greater than the maximum threshold, all incoming packets are dropped. But if the average queue size is between the minimum and maximum thresholds packets are randomly dropped with probability P_d where probability is exact a function of the average queue size: P_d = P_max(avg_q – min_th)/ (max_th - min_th). If the average queue grows, the probability of dropping incoming packets grows too. P_max - ratio, which can adjust the packet discarding probability abruptness, (the simplest case P_max can be equal to one. The 8.2 diagram shows the packet drop probability in the RED algorithm.

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SFQ

Stochastic Fairness Queuing (SFQ) is ensured by hashing and round-robin algorithms. SFQ is called "Stochastic" because it does not really allocate a queue for each flow, it has an algorithm that divides traffic over a limited number of queues (1024) using a hashing algorithm.

Traffic flow may be uniquely identified by 4 options (src-address, dst-address, src-port, and dst-port), so these parameters are used by the SFQ hashing algorithm to classify packets into one of 1024 possible sub-streams. Then round-robin algorithm will start to distribute available bandwidth to all sub-streams, on each round giving sfq-allot bytes of traffic. The whole SFQ queue can contain 128 packets and there are 1024 sub-streams available. The 8.3 diagram shows the SFQ operation:

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PCQ

PCQ algorithm is very simple - at first, it uses selected classifiers to distinguish one sub-stream from another, then applies individual FIFO queue size and limitation on every sub-stream, then groups all sub-streams together and applies global queue size and limitation.

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 It is possible to assign a speed limitation to sub-streams with the pcq-rate option. If "pcq-rate=0" sub-streams will divide available traffic equally.

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For example, instead of having 100 queues with 1000kbps limitation for download, we can have one PCQ queue with 100 sub-streams

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• pcq-burst-rate (number): maximal upload/download data rate which can be reached while the burst for substream is allowed
• pcq-burst-threshold (number): this is the value of burst on/off switch
• pcq-burst-time (time): a period of time (in seconds) over which the average data rate is calculated. (This is NOT the time of the actual burst)

PCQ also allows using different size IPv4 and IPv6 networks as sub-stream identifiers. Before it was locked to a single IP address. This is done mainly for IPv6 as customers from an ISP point of view will be represented by /64 network, but devices in customers network will be /128. PCQ can be used for both of these scenarios and more. PCQ parameters:

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CoDel (Controlled-Delay Active Queue Management) algorithm uses the local minimum queue as a measure of the persistent queue, similarly, it uses a minimum delay parameter as a measure of the standing queue delay. Queue size is calculated using packet residence time in the queue.

Properties

FQ-Codel

CoDel - Fair Queuing (FQ) with Controlled Delay (CoDel) uses a randomly determined model to classify incoming packets into different flows and is used to provide a fair share of the bandwidth to all the flows using the queue. Each flow is managed using CoDel queuing discipline which internally uses a FIFO algorithm.

Properties

CAKE

CAKE - Common Applications Kept Enhanced (CAKE) implemented as a queue discipline (qdisc) for the Linux kernel uses COBALT (AQM algorithm combining Codel and BLUE) and a variant of DRR++ for flow isolation. In other words, Cake’s fundamental design goal is user-friendliness. All settings are optional; the default settings are chosen to be practical in most common deployments. In most cases, the configuration requires only a bandwidth parameter to get useful results,

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Properties

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/queue interface

Interface Queue

/queue interface

Before sending data over an interface, it is processed Before sending data over an interface, it is processed by the queue. This sub-menu lists all available interfaces in RouterOS and allows to change queue type for a particular interface. The list is generated automatically.

[admin@MikroTik] > queue interface print
Columns: INTERFACE, QUEUE, ACTIVE-QUEUE
# INTERFACE QUEUE ACTIVE-QUEUE
0 ether1 only-hardware-queue only-hardware-queue
1 ether2 only-hardware-queue only-hardware-queue
2 ether3 only-hardware-queue only-hardware-queue
3 ether4 only-hardware-queue only-hardware-queue
4 ether5 only-hardware-queue only-hardware-queue
5 ether6 only-hardware-queue only-hardware-queue
6 ether7 only-hardware-queue only-hardware-queue
7 ether8 only-hardware-queue only-hardware-queue
8 ether9 only-hardware-queue only-hardware-queue
9 ether10 only-hardware-queue only-hardware-queue
10 sfp-sfpplus1 only-hardware-queue only-hardware-queue
11 wlan1 wireless-default wireless-default
12 wlan2 wireless-default wireless-default 

Queue load visualization in GUI

In Winbox and Webfig, a green, yellow, or red icon visualizes each Simple and Tree queue usage based on max-limit.