6_1 Quality of Service

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Section 6 Module 1 Page 1

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Service Router Operating System (SROS)

Section 6Quality of Service


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All Rights Reserved Alcatel-Lucent 2010Service Router Operating System (SROS)

Quality of Service 6 1 2

Objectives

Upon successful completion of this module, you will beable to explain: The need for Quality of Service in todays Service Providers

Network The difference between Integrated Services and Differentiated

Services Model The Alcatel-Lucent 7750 Service Router Quality of Service Model The Classifying mechanisms available (BA and MF) The (Re)Marking methods and rules The Buffer Acceptance Decision techniques (WRED and HPO) The different queue types and modes The Scheduling priorities, Single and Multi Tier


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Network-Wide View On QoS

Service QoS Policies

Network QoS PoliciesPE PE

SAPSAP

ServiceIngress

ServiceEgress

NetworkIngress

NetworkEgress

1 2 3 4

Classification: mapping oftraffic into forwardingclasses according L2/L3/L4header bits.

Buffer acceptance: acceptpackets according queue

filling levels.Scheduling: schedulepackets out of queuesaccording queueparameters. Defines in orout of profile traffic.

(Re)Marking: Mark theDSCP bits

Buffer acceptance:accept packetsaccording queue fillinglevels and in/out ofprofile.

Scheduling: schedulepackets out of queuesaccording queueparameters.

(Re)Marking: map theforwarding class andin/out profile into IPDSCP bits, MPLS EXPbits and/or dot1p bits

Classification: mapping oftraffic into forwardingclasses according IPDSCP, MPLS EXP bits ordot1p bits

Buffer acceptance: accept

packets according queuefilling levels and in/out ofprofile.

Scheduling: schedulepackets out of queuesaccording queueparameters.

Buffer acceptance: acceptpackets according queuefilling levels and in/out ofprofile.

Scheduling: schedulepackets out of queues

according queueparameters.

(Re)Marking: mark thedot1p bits.

MAP1 MAP2

IP or application match criteria for service ingress traffic:

Source IP address/prefix

Destination IP address/prefix

Source port/range Destination port/range

Protocol type (TCP, UDP, etc.)

DSCP value

IP fragment


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ServiceIngress

ServiceEgress

NetworkIngress

NetworkEgress

1

Service (SAP) Ingress

Service ingress/egress point can be delimited by a physical port orencapsulation

SAP-ingress QoS policy:

Defines traffic queues (type, en-queuing and de-queuing parameters, mode,etc.)

Assigns forwarding classes to queues

Maps traffic to a forwarding class based on user-defined match criteria and

assigns in or out-of-profile status to the packets Applied to SAPs

SAP Ingress Buffer Acceptance

Forwarding classes into Queues

Packet acceptance into Queues at SAP ingress Buffer pool allocation

Queue buffer management

HPO: High Priority Only

WRED: Weighted Random Early Discard

High priority versus low priority packets


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Network Egress

ServiceIngress

ServiceEgress

NetworkIngress

NetworkEgress

2

Network-Queue QoS Policy

Each network egress port supports a queue for each of the 8 internal forwardingclasses; queues are created automatically when a port is configured as a network port

Network queue policy defines the mapping of FCs to the queues

Queue parameters are defined in a network-queue QoS policy; on networkegress the policy is applied to an entire port

Network QoS Policy

Marks EXP or dot1p according to the FC of the traffic

The marking will be used by the Network Ingress of the PE on the other side of theIP/MPLS Network to classify the traffic

The IP DSCP bits are set at network egress by a network QoS policy

The DSCP markings at network egress are user configurable

The bits are marked according to which FC the outgoing packet belongs to

The marking is used at network ingress of the PE on the other side of the IP/MPLS network to classify thetraffic; any default network egress DSCP marking changes should be consistent across the network


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ServiceIngress

ServiceEgress

NetworkIngress

NetworkEgress

3

Network Ingress

Network QoS policy

Defines the mapping of EXP bits to one of the internal forwarding classes

Network-queue QoS policy

Specifies the queue to host the traffic of one or more paricular FCs

Network ingress queues are created automatically when a port is placed inthe network mode

Queue parameters are defined in a network-queue QoS policy; on ingress the

policy is applied at the MDA level to all network ingress ports on that MDA


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SAP-egress QoS policy defines:

Forwarding class to output queue mapping

In/Out-of-profile marking to high/low priority mapping

Service (SAP) Egress

ServiceIngress

ServiceEgress

NetworkIngress

NetworkEgress

4


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Classifying Traffic on SAP Ingress

Network Control

High Priority 1

Expedited Forwarding

High Priority 2

Low Priority 1

Assured Forwarding

Low Priority 2 Best Effort

DATA

SAP 1

Classification order

1. IP criteria | IPv6 criteria | mac-criteria

2. DSCP bits (IP Header)

3. Precedence bits (IP Header)

4. Dot1p bits (Ethernet Header)

5. Default settings

(The search is stopped when a match is made)

Access Ingress At SAP 1, purple traffic arrives on a port. As the traffic enters the ingress forwarding complex,

incoming datagrams of purple traffic are classified into one or more forwarding classes as determined by SAP

ingress QoS policies. The mapping of traffic to forwarding classes is based on the multi-field classification

rules in the SAP ingress policy.

Classification order

1. IP criteria | IPv6 criteria | mac-criteria

2. DSCP bits (IP Header)

3. Precedence bits (IP Header)

4. Dot1p bits (Ethernet Header)

5. Default settings

For a port configured in access mode, two buffer pools an access ingress pool and an access egress pool are

allocated for that ports exclusive use. A queue has the following three buffer-related parameters:

1. Committed burst size (CBS)

2. Maximum buffer size (MBS)

3. High priority only (HPO) buffer size

A flow cannot exceed its queues peak information rate (PIR) and maximum buffer size (MBS) limit. (Discussed in

detail a little later)


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qossap- i ngr ess 10 creat e

queue 3 pr i or i t y- mode cr eat eexi t

queue 2 profile-mode creat er at e 2000 cir 1000

exi tf c ef cr eat equeue 3exi t

f c "af" createqueue 2exi t

dot 1p 0 f c "af"dot 1p 5 f c "ef" pr i or i t y hi ghdscp ef f c "ef" pr i or i t y hi ghi p- c r i t er i a

ent r y 10 cr eat emat ch

dst - i p 10. 0. 0. 0/ 8exi tact i on f c "ef " pr i or i ty l owexi t

exi tdef aul t - f c "be

exi t

Forwarding Classes into Queues

serviceepipe 100 customer 1 create

sap 1/2/5:100 createingress

qos 10exit

exitspoke-sdp 12:100 createexitno shutdown

exit

4. Creating a policy-id 10

3. Declaring Queues

2. Mapping Traffic into Queues

1. Classifying Traffic

Applying policy to a

SAP on Ingress

TrafficFlow

Classifying Traffic:

Incoming traffic is identified by looking into the datagram and viewing the priority that has been set. Depending

on the priority of the ingress datagrams, they will be assigned to the appropriate classification.Network Control

High Priority 1

Expedited Forwarding

High Priority 2

Low Priority 1

Assured Forwarding

Low Priority 2

Best Effort

Mapping Traffic into Queues:

The classified incoming traffic is then placed into the appropriate queue

Declaring Queues:

The queues themselves are configured with priority-mode or profile-mode.

Priority Mode (Default) recognizes the queuing priority of the ingress classification and will ignore the explicitlyspecified profile state of the forwarding class or subclass.

Profile Mode ignores the queuing priority of the ingress traffic classification. All the packets are either markedexplicitly as in-profile, or out-of-profile based on the Committed Information Rate. Out-of-profile packets aretreated as low priority.


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Some packets are droppedbecause the peak size is largerthan Maximum Burst Size (MBS)

No drop because the peaksize is smaller than theCommitted Burst Size (CBS)

In Profile and Out of Profile Traffic

Time

OfferedBW

PIR

CIR

in-profile

out-of-profileThis traffic will be droppedfirst in case of congestionfurther in the network.

This traffics throughput isguaranteed throughout thenetwork.

In-Profile Traffic < Committed Information Rate

Out-Of-Profile Traffic >Committed Information Rate

queue 2 profile-mode create

rate 2000 cir 1000

exit

CIR: This is the minimum guaranteed service rate of a queue. Its functions are to prioritize the queue schedulingfor in-profile and out-of-profile, and to determine the markings of datagrams based on their profiles. Forexample, a queue that is below the CIR is services as in-profile and marked as such. If a queue rate exceedsthe CIR, then it is marked out-of-profile and serviced as such.

PIR: This is the maximum rate at which datagrams can exit a queue.

Committed Burst Size (CBS)

The CBS parameter specifies the amount of buffer space reserved for use by a given ingress or egress queue. Oncethe reserved buffer space for a queue has been used, the queue contends with other queues for additionalbuffer resources up to the Maximum Burst Size (MBS) of the queue. The CBS for a given queue can beconfigured or the system can assign a default size.

Maximum Burst Size (MBS)

The Maximum Burst Size parameter specifies the maximum size to which a queue can grow. This parameterensures that customers that are exceeding the PIR of a queue will not consume all the available bufferresources. The MBS for a given queue can be configured or will be assigned a default value by the system.

Service ingress and egress QoS policies define CBS and MBS for each queue separately, and the network QoS policydefines CBS and MBS for each network queue.


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Buffering Traffic

Configured as a% of total

buffer space

0xMax

High-Priority-OnlyWater Mark

Shared Buffers

Queues are created and use ReservedAND/OR Shared memory

Every Buffer Pool receives

Shared and Reserved portionof memory

Reserved

Buffers

MBS

HPO Water Mark identifies the

percentage that is reserved for HighPriority Traffic

Highpriority

onlyCBS

QueueMBS

Buffer Pools

Reserved Buffers

Reserved buffer space is a percentage of the total buffer space allocated to a queue. You can configure the

amount of reserved buffer space allocated to each queue, the default being fifty percent of the total buffer

space of the queue. The amount of reserved buffer space for a queue is known as the committed burst size

(CBS) of the queue.

Shared Buffers

Shared buffer space is the buffer space that is not reserved for use by specific queues.

Shared Buffers = Total Buffer Pool Reserved Buffers

A Committed Burst Size (CBS) of 0 means there is no guaranteed buffer space for a queue, and all the buffer

space for a queue is drawn from the shared portion of the buffer pool. In order to guarantee buffer space for

queues within a buffer pool, ensure that the sum of the CBS of queues created with the pool does not

oversubscribe the reserved portion of the pool.

HPO defines the amount of queue space reserved for high-queuing priority (in-profile) traffic. It is defined as apercentage of MBS. For example, as long as low-queuing priority traffic is less than MBS*(1-HPO), then it will be

admitted. If it goes above that threshold, it will be discarded since the rest of the queue is reserved for high-

queuing priority traffic.

Best practices suggest that all high priority traffic classes be in-profile and all best effort classes be out-of-

profile. That makes HPO unnecessary for these classes. In addition, if HPO is configured greater than 0 for out-

of-profile classes, then buffer space is wasted. Only assured forwarding classes can benefit from the

configuration of HPO.


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HPO Examples: exists per queue

0xMax

MBS

Highpriority

onlyCBS

QueueMBS

High Priority Packet

Tail Drop (Buffer full => Drop)

0xMax

Highpriority

onlyCBS

QueueMBS

Low Priority Packet

Buffer not full up to HPO => Accept

0xMax

Highpriorityonly

CBSQueue

MBSLow Priority Packet

Buffer full over HPO => Drop

Buffer not full => Accept


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Weighted Random Early Detection (WRED)

If the utilization of a buffer exceeds a certain threshold, WRED discardsincoming datagrams randomly using a given drop probability

The low slop manages shared buffer access of the low priority traffic,

and the high slope manages the shared buffer of the high priority traffic

Low Slope High Slope

Average Shared Buffer Utilization

Disc

ard

Probability

0

1

Slope Knee

Slope Start

Shared BuffersReservedbuffers

Slope policies can be used to manage the shared portion of buffers using the RED buffer management technique. The defaultis to drop the tail of any traffic that does not fit in the buffer.

When buffers are neither available/allowed for a given traffic stream, datagrams get discarded. Generally, discards happen

using the tail drop buffer management technique. Under some circumstances, random early detection (RED) buffermanagement is ideal. For example, for applications that use TCP (eg. HTTP, SSH, FTP), using RED provides better bandwidthmultiplexing because of TCPs slow-start algorithm. With tail drop, TCP behavior of multiple simultaneous streams maybecome sub-optimal due to a condition called global synchronization of the streams. RED avoid this condition and providesoptimal behavior for TCP streams. RED can be configured on interfaces using Slope Policies.

qos

slope-policy "myred" create

high-slope

start-avg 55

max-avg 100

max-prob 100

no shutdown

exitlow-slope

start-avg 40

max-avg 50

max-prob 100

no shutdown

exit


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WRED Examples: On Shared Buffer Pool

Average Shared Buffer Utilization

Dis

card

Probability

0

1

Slope Knee

Slope Start

Low Priority Packet + Random Number below Low Slope => Drop

Low/High Priority Packet => Accept

High Priority Packet + Random Number below High Slope => Drop

Low Priority Packet + Random Number above Low Slope => Accept

High Priority Packet + Random Number above High Slope => Accept

x

x

x

x

For packet congestion control, RED and WRED mechanisms are available for congestion management of the

service and network interfaces. A RED slope can be user configured to manage this feature

The 77x0 SR ATM MDAs support PPD. PPD will detect if an ATM cell is dropped from an AAL5 frame, and drop all

subsequent cells up to the tail cell of the AAL5 frame.

ATM PVCs are terminated on a Layer 3 service. Backpressure mechanisms are applied from each per-VC queue in

the ATM MDA to the Forwarding Class queues in the IOM, where congestion management is applied. As a result,

discards can only occur in the IOM queues and not in the per-VC ATM queues.


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WRED CLI example

qosslope-policy "myred" create

high-slopestart-avg 55max-avg 100max-prob 100no shutdown

exitlow-slope

start-avg 40max-avg 50max-prob 100no shutdown

exit

Discard

Probability

0

1

Max-avg

Start-avg

Max-prob

show pools network-egress 9/2/1

Pool Information======================================================Port : 9/2/1

Application : Net-Egr Pool Name : defaultResv CBS : Sum-------------------------------------------------------------------------------Utilization State Start-Avg Max-Avg Max-Prob-------------------------------------------------------------------------------High-Slope Up 55% 100% 100%Low-Slope Up 40% 50% 100%

Once a queue exceeds its reserved buffer allocation and starts using shared buffers, each service packet mappedto that queue is subject to the probability based drop function. The above slide depicts WRED slopeconfigurable parameters. Each slope is configured with thresholds describing points on the slope that intersectthe average utilization of the shared portion of the buffer pool (X-axis or horizontal plot with utilizationincreasing from 0 to 100%) and the probability of discard (Y-axis or vertical plot with probability rising from 0 to1).

Configurable parameters are:

Start-avg

Sets the low priority or high priority WRED slope position for the shared buffer average utilization value where thepacket discard probability starts to increase above zero.

Max-avg

Sets the low priority or high priority WRED slope position for the shared buffer average utilization value where thepacket discard probability rises directly to one.

Max-prob

Sets the low priority or high priority WRED slope position for the maximum non-one packet discard probabilityvalue before the packet discard probability rises directly to one.


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Default Single Tier Scheduler

Per MDA a fast moving arm takes the packets out of all the queues, calledthe scheduler

To differentiate traffic streams, the scheduler will prioritize some queues overother The highest priority is given to the Expedited Forwarding Queues that

operate at a speed below (within) the Committed Information Rate (CIR) The second highest priority is given to the Best Effort Queues that operate

at a speed below (within) CIR The least priority is given to the Expedited and Best Effort Queues that

operate at a speed above CIR

Best Effort Queue

Schedulingorder

1. Expedited < CIR

2. Best E ffort < CIR

3. Expedited and Best Effort > CIR

Expedited Queue

Best Effort Queue

Expedited Queue

Best Effort Queue

Expedited Queue

1

2

3

3

3

3

Single Tier Schedulers

Single-tier schedulers are implemented in hardware and are the default method of

scheduling queues in the 7750 SR. Queues are scheduled with single-tier

scheduling if no explicit hierarchical scheduler policy is defined or applied.

There are no explicit configurable parameters for single-tier scheduling other

than a queues CIR and PIR.

Single tier schedulers schedule queues based on the forwarding class of the queue

and the operational state of the queue relative to the queues CIR and PIR.

Queues operating within their CIR values are serviced before queues operating

above their CIR values with expedited forwarding class queues given

preference over non-expedited forwarding class queues.

Pairs of schedulers send traffic to a switch fabric port, service access port, or

network interface.

Queues of the same type are serviced in a round robin fashion.


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Tier 2 to 3

Tier 1

Hierarchical Scheduling

Hierarchical Scheduling involves creating layers of scheduling policieswhich are then applied to services ingress and service egress queues

Up to three tiers of schedulers are supported

Dynamic bandwidth allocation allows for greater use of availablebandwidth while still assuring high priority traffic throughput

Expedited

Best Effort

Best Effort

SAP 1Ingress Traffic

Tier 3 Tier 2

Tier 1 to 2

Dynamic Bandwidth Allocation

Hierarchical Schedulers (H-QoS)

The 7750 SR makes use of hierarchical virtual schedulers to implement hierarchical QoS (H-QoS).

H-QoS creates a hierarchy of schedulers configured in a series of parent-child relationships. The levels ofscheduler policies are then treated according to their assigned priority. This design allows the 7750 SR to

implement very granular QoS policies consisting of combinations of strict priority queuing and weighted fair

queuing for bandwidth management.

The Virtual scheduler acts as parent for child associated members (queues or other schedulers)

Each child provisioned with two sets of parameters:

within CIR strict level and weight

above CIR strict level and weight

The Scheduler loop makes two passes to distribute available bandwidth:

within CIR pass based on each childs within CIR strict level and weight within level

above CIR pass based on each childs above CIR strict level and weight within level

The Scheduler itself may be root or child of another virtual scheduler


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QoS Summary: Where to apply the policies

Port

SAP

Port

SAP

Ing. MDA

Ing. MDAEgr. MDA

Egr. MDA

Port

i/f

Port

i/f

SAPIngressPolicy

SAPEgressPolicy

Scheduler

Policy

Network Policy

Slope Policy

Network Queue

Policy

Lab 11

Buffer Acceptance (WRED/HPO) @ SAP 1, NP 2, NP 3 and SAP 4

Service Classifying (Precedence/DSCP/dot1p/IP and MAC Criteria) @ SAP 1

Network Classifying (Precedence/DSCP/dot1p/EXP) @ NP 3

Default Scheduling @ SAP 1, NP 2, NP 3, SAP 4Hierarchical Scheduling @ SAP 1 and SAP 4

Marking and Remarking (DSCP/Precedence/EXP) @ NP 2

Marking and Remarking (dot1p) @ SAP 4

Marking and Remarking (DSCP) @ SAP 1


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All Ri h R d 2010 Al l



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