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AlliedWare PlusTM OS

Overview of |

Introduction

This How To Note describes the main features of QoS on switches running the

AlliedWare Plus OS. The main features include:

zPrioritisation and marking

Right at the point of ingress into the QoS process, packets classified to particular class

maps can have values written to one or more of their associated “markers”. The markers

can be externally visible fields (DSCP value, 802.

p value) and/or internally visible fields

(bandwidth class and queue number). These markers are explained in "Packet markers" on

page 4.

zPolicing

Packets belonging to any given class map can be assigned a colour (bandwidth class) based

on whether they are inside or outside the bandwidth limits set for that class map. The

packets are marked with the colour that was applied to them, and at various points in the

QoS process, decisions on the packets' fate can be made on the basis of what colour they

have been marked with.

zRemarking

After policing, remarking can update packets’ QoS markers depending on how well the

flow conforms to its bandwidth limits. For example, if a flow exceeds its bandwidth

requirements, QoS can update the packets’ DSCP values.

zPer-port control over egress queue parameters

Queue lengths, scheduling process, relative weights, etc can be set on all queues on a per-

port basis.

zHighly configurable default class map

All the parameters that can be set on a normal class map can also be set on the default

class map (the catch-all class map that matches all traffic that does not explicitly match any

other class map).

Quality of Service Features on x900-

x900-24, and SwitchBlade x908 Switches

Page 2 | AlliedWare Plus™ OS: Overview of QoS

zAbility to see the current state of egress queues

There are commands that enable you to see statistics relating to every egress queue on

every port.

Each of these features is discussed in much more detail later on in this document.

Contents

Introduction .............................................................................................................................................. 1

Which products and software version does this Note apply to? ................................................. 3

The process flow and methodology of the QoS system ................................................................ 3

Packet markers ................................................................................................................................. 4

Outline of the QoS processing flow ................................................................................................... 5

Initial mapping to an egress queue, based on 802.

p value .................................................... 5

Classification ...................................................................................................................................... 6

Premarking ......................................................................................................................................... 6

Policing ................................................................................................................................................ 7

Limiting or remarking (dropping non-conformant packets) ................................................... 7

Queue shaping .................................................................................................................................. 8

Scheduling .......................................................................................................................................... 9

Details of the component processes, and how to configure them ............................................ 10

QoS elements: policy maps, class maps, policers, matches .................................................. 10

Diagram of the overall QoS process flow ................................................................................ 11

Enabling QoS globally .................................................................................................................... 12

Initial mapping to queue based on tag ....................................................................................... 12

Classification .................................................................................................................................... 13

Premarking ....................................................................................................................................... 13

Policing .............................................................................................................................................. 16

Remarking ........................................................................................................................................ 22

Queue shaping—queue sets, RED, and tail-drop .................................................................... 23

Scheduling ........................................................................................................................................ 27

Egress bandwidth limiting .................................................................................................................... 29

Policing Examples ................................................................................................................................... 30

: Policing separate traffic types on separate ports ............................................................... 30

2: Policing one traffic type on combined ports ....................................................................... 32

3: Policing one traffic type on separate ports, and another traffic type on the same

ports combined ....................................................................................................................... 33

4: Policing combined traffic types on separate ports ............................................................. 35

5: Policing combined traffic types on combined ports .......................................................... 37

Fabric QoS .............................................................................................................................................. 38

Mapping the queues in the switching instances to queues in the fabric ............................ 39

Scheduling the queues within the fabric ................................................................................... 40

Page 3 | AlliedWare Plus™ OS: Overview of QoS

Which products and software version does this

Note apply to?

zProducts: SwitchBlade x908, x900-12XT/S, and x900-24 series switches

zSoftware versions: AlliedWare Plus version 5.2.1-0.1 and above

The process flow and methodology of the QoS

system

Before discussing the details of the various processes that comprise the QoS system, it is

desirable to first get a picture of what the processes are, and the order in which they are

applied to the packets passing through the system.

Therefore, this section discusses what the QoS system is really trying to do to packets, and

how it keeps track of what it has decided about any given packet.

The QoS system does the following things:

zdecides which egress queue to send a packet to

zdecides whether to drop the packet or attempt to forward it

zupdates markers in the packet for downstream devices to use

zcontrols the relative priorities of the egress queues

In general, the main aim of all the processes in the QoS system is to work out which egress

queue a particular packet should be put into.

There are several factors that can affect this choice of egress queue, so packets need to be

put through several processes, so that each of the competing factors has its opportunity to

exert its influence on the final choice of egress queue.

In some cases, the system can decide to simply discard certain packets at some steps in the

process.

Additionally, the QoS system often has an obligation to update certain fields within a packet,

to indicate to downstream devices how they should deal with the packet when it gets to

them.

So, we have this multi-stage process, and the eventual fate of a packet will depend on the sum

total of the various decisions that were made about it at various stages in the process. In

order to keep track of the outcomes of those decisions, a packet needs to be marked so that

at any point in the process it is possible to know the net effect of the decisions that have been

made on it so far.

Page 4 | AlliedWare Plus™ OS: Overview of QoS

Packet markers

There are four items that are used to mark packets as they pass through the QoS system.

zTwo markers that are carried within fields of the packet itself:

z802.

p: The 802.

p or User Priority field in the VLAN tag of an Ethernet frame. This

is a 3-bit number, so it can have a value in the range 0-7.

zDSCP: The Differentiated Services Code Point within the TOS field of an IP packet

header. This is a 6-bit number, so it can have a value in the range 0-63.

zTwo items that are just used within the switch chip. These are not fields within the packets,

but are extra parameters that the packets carry with them as they pass through the QoS

system:

zBandwidth Class: This parameter can take on the values green, yellow, or red.

Essentially it is an indicator of whether the packet is deemed to have been within the

acceptable bandwidth limit set for any particular traffic flow, or whether the packet's

traffic flow had already overflowed its acceptable limit by the time this particular

packet arrived.

A value of green indicates that the flow was within the acceptable limit when the

packet arrived, a value of yellow indicates that the flow was slightly outside its

acceptable limit when the packet arrived, and a value of red means that the flow was

well outside the limit when the packet arrived.

zEgress Queue: This indicates the egress queue that the packet is currently slated to be

placed into, if and when it finally negotiates its way through all the steps in the QoS

process and lines up in one of the queues at its eventual egress port.

Page 5 | AlliedWare Plus™ OS: Overview of QoS

Outline of the QoS processing flow

Let's look at each QoS process in the order that they are applied to a packet. The following

figure gives a quick view of the QoS features we are about to discuss.

Initial mapping to an egress queue, based on 802.

value

Immediately after ingress, a VLAN-tagged Ethernet frame can be assigned to the appropriate

egress queue on the basis of the value of its VLAN Tag User Priority. This means that

incoming frames that already carry meaningful priority information can be forwarded on the

basis of that information. The mapping of the User Priority value to an egress queue is

configurable, so the administrator can decide, for example, to send frames with a Priority

value of 7 to queue 3 and frames with a Priority of 2 to queue 7.

Untagged frames don't have a VLAN Tag User Priority, so these frames can be assigned to a

default queue of the administrator's choice.

The net effect of this process is to set a value on the Egress Queue marker that the packet is

carrying.

Ingress port

Premarking

Remarking

Limiting (dropping non-conformant)

Queue shaping

Classification using ACLs

Packet

Egress

Queue emptying and egress

Tagged: priority mapped to queue

Untagged: mapped to default queue

Ingress

Policing

QoS4.eps

Page 6 | AlliedWare Plus™ OS: Overview of QoS

Classification

Classification is simply a method of dividing the incoming traffic into traffic flows so that

packets of one type can be treated differently to packets of another type. To do this, you

create class maps and if desired ACLs. Incoming packets are inspected and may be classified

on a very broad range of criteria.

The classification process does not update any of the four marker values on the packet, but

does dictate the path that the packet will subsequently take through the QoS system.

Premarking

The “pre” part of premarking means this process happens before any bandwidth policing

takes place. The “marking” part refers to attaching QoS information to packets.

One possible use for this is to apply a DSCP value to a traffic stream. For example, packets

coming from a database server could require assured forwarding treatment, and so could be

marked with DSCP=

8 at ingress to the switch.

Recall that packets can be marked in four ways:

zthe VLAN tag user priority

zthe Differentiated Services Code Point (DSCP)

zthe bandwidth class the packet is assigned to

zthe egress queue the packet is assigned to.

A packet can have new values assigned for each of these marking values by the premarking

process. There are two mutually exclusive methods available for premarking:

zsetting the new values explicitly for all packets that match a certain class map, or

zlooking up the mark-dscp map and applying the map’s values to the packets. The mark-dscp

map is a user-defined table that maps particular DSCP values to particular sets of 802.

DSCP, bandwidth class, and egress queue values. See "Premarking" on page 13 for a table

that shows the mark-dscp map structure.

If premarking uses the mark-dscp map, there are two ways to choose the DSCP value to use

in looking up entries in the mark-dscp map:

zuse the existing DSCP value of the packet (different packets within the class map may well

have different DSCP values)

zspecify a single DSCP value that QoS will use for look-ups for all packets that match the

class map.

Whichever of these two criteria is used, the value is used to index the mark-dscp map.

Page 7 | AlliedWare Plus™ OS: Overview of QoS

Policing

Policing involves measuring the bandwidth used by a policer and comparing the measurement

to the bandwidth limits that have been set for the policer.

The policing process allocates a temporary bandwidth class value to packets. It is important

to note that the policing process does not overwrite the bandwidth class value that the

packet is already carrying around with it. Instead, an extra, temporary, bandwidth class

marker is attached to the packets.

When traffic first enters the switch, it is all marked with bandwidth class green, simply

because it has not been policed yet. Packets can be assigned a new bandwidth class at the

Premarking stage, but this is not done on the basis of actual measurement of bandwidth use.

At the policing stage, a policer's bandwidth usage is constantly monitored to see how well it

conforms to the limits set for it, and the individual packets within the flow are assigned to a

temporary bandwidth class depending on the policer's conformance to its limits at that time.

So, while a policer is still within its bandwidth limit, all the packets that have been classified to

that policer are marked with a temporary bandwidth class of green. If a policer starts to

exceed its limit, then the packets in that policer are given a temporary bandwidth class of

yellow. If it starts seriously exceeding its limits, then the packets’ temporary marking is

bandwidth class red.

The actual algorithms used to determine whether a policer is slightly exceeding its bandwidth

limit or seriously exceeding the limit are described later in this document.

Limiting or remarking (dropping non-conformant

packets)

Based on the temporary bandwidth class assigned to a packet at the policing stage, one of

two actions can be taken:

zthe packet can be dropped if it is was assigned to bandwidth class red by the policing

process, or

zthe packet can be remarked with new QoS property values.

The first of these two actions is straightforward; the user can choose to simply drop packets

if the policer exceeds the bandwidth limits set for it to the extent that packets are assigned

to bandwidth class red.

Remarking is a little more complex as it is not done solely on the basis of the bandwidth class

that the packet has been assigned to; the packet's current DSCP value, and its temporary

bandwidth class are used to determine the new values for all four QoS properties for the

packet (that is, new values for the DSCP, VLAN tag user priority, bandwidth class, and egress

queue can be specified). The new values are taken from the user-configurable policed-dscp

map.

Page 8 | AlliedWare Plus™ OS: Overview of QoS

Queue shaping

Each egress port has eight egress queues, which are numbered 0-7 with 7 being the highest

priority queue. Unfortunately, the queues are of a limited length, so packets cannot be added

to them indefinitely; if the switch is congested, the queues may fill up and no more packets

can be added. In this case, packets will inevitably be dropped from the end of the queues,

even if they are high-priority packets. Queue shaping is a general term to describe how the

egress queues can be managed to prevent the indiscriminate dropping of packets from the

tails of the egress queues.

Queue shaping can use Random Early Detection/Discard (RED). RED is a congestion

avoidance mechanism that allows some packets to be dropped before the average egress

queue exceeds the allocated maximum queue length. Lower priority packets are dropped

when severe congestion occurs, with progressively more and higher priority packets dropped

until congestion is eased. This is useful for TCP flows, because the sender will slow the rate

of transmission when it detects a packet loss. Note that using RED on UDP traffic flows is

not recommended because UDP does not reduce the rate of transmission and will simply

retransmit the dropped packets, which will add to the congestion.

The Random Early Discarding of packets from egress queues will typically be configured to

drop more packets with bandwidth class red than those with bandwidth class yellow, and to

drop even less of the packets with bandwidth class green.

RED curves are not the only queue shaping mechanism available. You can instead choose to

use a relatively simple tail-drop scheme. Using this method, you nominate a queue length at

which any further packets will be dropped. This is done for each of the three bandwidth

classes. Obviously, the queue-length threshold for bandwidth class red should be set at a

relatively low value, with the other bandwidth classes having progressively higher values.

Page 9 | AlliedWare Plus™ OS: Overview of QoS

Scheduling

In addition to managing the way in which packets can be dropped when the egress queues for

a given port start to fill up, you can also configure the method that is used to allocate

bandwidth to each of the queues to transmit packets onto the line.

There are two ways that the queues can be scheduled for transmission:

zStrict Priority Scheduling

Higher-priority queues are emptied before any packets are transmitted from lower-

priority queues. This means that queue 7 must be totally empty before any packets from

queue 6 are transmitted, and so on.

zWeighted Round-Robin Scheduling

The queues share bandwidth on the basis of user-defined weights. Using this method,

packets from a lower-priority queue can be transmitted even when packets are waiting in

a higher-priority queue. The weights can be configured to ensure that more packets per

second are sent from the higher-priority queues than from the lower-priority queues.

To allow for flexibility in scheduling, it is possible to use different scheduling methods for

different queues. For a given port, you can create up to three groups of egress queues, one

that uses Strict Priority Scheduling and two separate groups that each use Weighted Round-

Robin Scheduling. For example, consider this case:

zqueues 7, 6 & 5 are configured to use Strict Priority Scheduling

zqueues 4, 3 & 2 are in Weighted Round-Robin group

zqueues

& 0 are in Weighted Round-Robin group 2

Queues 7, 6 & 5 will be emptied using Strict Priority, that is, queue 7 will be emptied before

any packets from queue 6 can be transmitted and queue 6 must be completely emptied

before any packets from queue 5 are transmitted.

When queues 7, 6 & 5 are all completely empty, queues 4, 3 & 2 will be emptied concurrently

based on their respective weights.

Queues

& 0 will be emptied only when there are no packets awaiting transmission in any of

the other queues.

Page 10 | AlliedWare Plus™ OS: Overview of QoS

Details of the component processes, and how to

configure them

QoS elements: policy maps, class maps, policers,

matches

Some aspects of QoS are configured globally, such as default mapping of CoS to egress queue.

However, most aspects are configured on a per-port basis, mostly as part of the port’s policy

map.

The policy map contains QoS settings for a port, and is made of class maps—one class map

for each type of traffic you want to control on the port. Class maps have match commands to

specify what traffic the class map applies to, and policers to set the bandwidth parameters for

that type of traffic. Class maps can also have other settings, such as whether to premark

traffic.

The following figure summarises these configuration elements.

The default class map

Packets that do not match any configured class map are matched by the default class map.

These packets can still be subjected to premarking, policing and remarking. To configure

these features for the default class map, simply go into policy map class map mode for the

default class map, by using the following commands:

awplus(config-cmap)#policy-map <name>

awplus(config-pmap)#class default

Port

policy-map

class-map

match

policer

class-map

match policer

class-map

match match

qos-elements.eps

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