Arquitectura de Redes QoS 14

8/13/2019 Arquitectura de Redes QoS 14

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Congestion Avoidance

Introducing Congestion Avoidance

Introducing RED

Configuring Class-Based Weighted RED Configuring Explicit Congestion Notification

Introducing Congestion Avoidance

Behavior of a TCP Sender

Sender sends N bytes (as much as credit

allows) Start credit (window size) is small

To avoid overloading network queues

Increases credit exponentially

To gauge network capability


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Behavior of a TCP Receiver

Receiver schedules an ACK on receipt of next

message.

TCP acknowledges the next segment it

expects to receive, not the last segment it

received.

In the example, N+1 is blocked, so the

receiver keeps acknowledging N+1 (the next

segment it expects to receive).

TCP Slow Start

If ACK acknowledges something

Updates credit and sends

If not, presumes it indicates a lost packet

Sends first unacknowledged messageright away

Halves current credit (slows down)

Increases slowly to gauge networkthroughput

Multiple Drops in TCP

If multiple drops occur in the same session:

Current TCPs wait for time-out

Selective acknowledge may be a

workaround

New fast retransmit phase takes several

round-trip times to recover


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Managing Interface Congestion

with Tail Drop

Router interfaces experience congestion when the output queue

is full:

Additional incoming packets are tail-dropped

Dropped packets may cause significant application

performance degradation

Tail drop has significant drawbacks

Tail Drop Limitations

Tail drop should be avoided because it contains significant

flaws:

TCP synchronization

TCP starvation

No differentiated drop

TCP Synchronization

Multiple TCP sessions start at different times.

TCP window sizes are increased.

Tail drops cause many packets of many sessions to be dropped

at the same time.

TCP sessions restart at the same time (synchronized).


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TCP Delay, Jitter and Starvation

Constant high buffer usage (long queue) causes delay.

More aggressive flows can cause other flows to starve.

No differentiated dropping occurs.

Introducing RED

Random Early Detection

Tail drop can be avoided if congestion is prevented.

RED is a mechanism that randomly drops packets before aqueue is full.

RED increases drop rate as the average queue size increases.

RED result:

TCP sessions slow down to the approximate rate of output-link bandwidth.

Average queue size is small (much less than the maximumqueue size).

TCP sessions are desynchronized by random drops.


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RED Profiles

RED Modes

RED has three modes:

No drop: When the average queue size is between 0 and theminimum threshold

Random drop: When the average queue size is between theminimum and the maximum threshold

Full drop (tail drop):When the average queue size is atmaximum threshold or above

Random drop should prevent congestion (prevent tail drops)

TCP Traffic Before RED

TCP synchronizationprevents average link utilization close to

the link bandwidth.

Tail dropscause TCP sessions to go into slow-start.


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TCP Traffic After RED

Average link utilization is much closer to link bandwidth.

Random dropscause TCP sessions to reduce window sizes.

Applying Congestion Avoidance

Configuring Class-Based Weighted RED


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WREDcan use multiple different RED profiles.

Each profile is identified by:

Minimum threshold

Maximum threshold Maximum drop probability

WRED profile selection is based on:

IP precedence(8 profiles)

DSCP(64 profiles)

WRED drops less important packets more aggressively thanmore important packets.

WRED can be applied at the interface, VC, or class level.

Weighted Random Early Detection

Class-Based WRED

Class-based WRED is available when configured in combination

with CBWFQ only.

Using CBWFQ with WRED allows the implementation of

DiffServ Assured Forwarding PHB.

Class-based configuration of WRED is identical to standalone

WRED.

WRED Building Blocks


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WRED Profiles

WRED profiles can be manually set.

WRED has 8 default value sets for precedence-based WRED.

WRED has 64 default value sets for DSCP-based WRED.

IP Precedence and

Class Selector Profiles

DSCP-Based WRED

(Expedited Forwarding)


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DSCP-Based WRED

(Assured Forwarding)

Configuring CB-WRED

class-map [match-any | match-all] class-name

router(config)#

policy-mappolicy-name

router(config)#

service-policy {input | output}policy-map-name

router(config-if)#

1. Create class map: Used for matching packets to a specified class

2. Create policy map (service policy):Specifies a traffic policy that canbe attached to one or more interfaces

3. Attach service policy:Associates the policy map to an outputinterface or VC

Configuring CB-WRED (Cont.)

random-detect

router(config-pmap-c)#

Enables IP precedence-based WRED in the selected classwithin the service policy configuration mode.

Default service profile is used.

Command can be used at the interface, per-VC (with random-detect-group), or the class level (service-policy).

Precedence-based WRED is the default mode.

WRED treats non-IP traffic as precedence 0.


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Changing the WRED Traffic Profile

random-detect precedenceprecedence min-threshold

max-threshold mark-prob-denominator

Changes WRED profile for specified IP precedence value.

Packet drop probability at maximum threshold is:

1 / mark-prob-denominator

Non-weighted RED is achieved by using the same WRED profile

for all precedence values.


WRED takes the average queue size to determine the current

WRED mode (no drop, random drop, full drop).

High values of N allow short bursts.

Low values of N make WRED more burst-sensitive.

Default value (9) should be used in most scenarios.

Average output queue size with N=9 is

Qave(t+1) = Qave(t) * 0.998 + Q t* 0.002

Changing WRED Sensitivity to Bursts

random-detect exponential-weighting-constant n


Example: CBWFQ Using IP Precedence with CB-WRED

Enable CBWFQ to prioritize traffic according to these

requirements: Class mission-criticalis marked with IP precedence values

3 and 4 (3 is high-drop, 4 is low-drop) and should get 30%

of interface bandwidth.

Class bulkis marked with IP precedence values 1 and 2 (1

is high-drop, 2 is low-drop) and should get 20% of interface

bandwidth.

All other traffic should be per-flow fair -queued.

Use differentiated WRED to prevent congestion in all three

classes.


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Example: CBWFQ Using IP Precedence with CB-WRED

Configuring DSCP-Based CB-WRED

random-detect dscp-based

Enables DSCP-based WRED.

Command can be used at the interface, per-VC (with random-detect-group), or the class level (service-policy).

Default service profile is used.

The WRED random-detect command and the WFQ queue-limitcommand are mutually exclusive for class policy.


Changing the WRED Traffic Profile

random-detect dscp dscpvalue min-threshold max-thresholdmark-prob-denominator


Changes WRED profile for specified DSCP value

Packet drop probability at maximum threshold is:1 / mark-prob-denominator


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Example: CB-WRED Using DSCP with CBWFQ

Enable CBWFQ to prioritize traffic according to theserequirements:

Class mission-criticalis marked using DSCP AF2 and should

get 30% of interface bandwidth. Class bulk is marked using DSCP AF1 and should get 20% of

interface bandwidth.

All other traffic should be per-flow fair-queued.

Use differentiated WRED to prevent congestion in all threeclasses.

Make sure the new configurations still conform to the designand implementation from the previous example.

Example: CB-WRED Using DSCP with CBWFQ (Cont.)

Monitoring CB-WRED

show policy-map interface interface-name

router#

Displays the configuration of all classes configured for all servicepolicies on the specified interface

router#show policy-map interface Ethernet 0/0

Ethernet0/0

Service-policy output: Policy1

Class-map: Mission-critical (match-all)

0 packets, 0 bytes 5 minute offered rate 0 bps, drop rate 0 bps

Match: ip precedence 2 Match: ip dscp 18 20 22

Weighted Fair Queueing

Output Queue: Conversation 265

Bandwidth 30 (%) Bandwidth 3000 (kbps)

(pkts matched/bytes matched) 0/0

(depth/total drops/no-buffer drops) 0/0/0

exponential weight: 9

mean queue depth: 0

Dscp Transmitted Random drop Tail drop Minimum Maximum Mark

(Prec) pkts/bytes pkts/bytes pkts/bytes threshold threshold probability

0(0) 0/0 0/0 0/0 20 40 1/10

1 0/0 0/0 0/0 22 40 1/10

2 0/0 0/0 0/0 24 40 1/10

...


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Configuring Explicit Congestion Notification

Explicit Congestion Notification

TCP congestion controls are not suited to applications that

are sensitive to delay or packet loss.

ECN (RFC 3168) removes need to rely on packet loss as a

congestion indicator.

ECN marks packets instead of dropping them when the

average queue length exceeds a specific threshold value.

Routers and end hosts can use ECN marking as a signal that

the network is congested and send packets at a slower rate.

ECN Field Defined


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ECN and WRED

ECN is an extension to WRED.

Congestion in WRED is indicated based on the average

queue length exceeding a specific threshold value.

If the number of packets in the queue is below the minimum

threshold, packets are transmitted.

Treatment is identical to a network using only WRED.

If the number of packets in the queue is above the maximum

threshold, packets are tail-dropped.

Treatment is identical to a network using only WRED.

ECN and WRED (Cont.)

If the number of packets in the queue is between the minimum and

maximum threshold, one of three scenarios can occur:

ECN-capable endpoints and WRED determine that the packet

should be dropped based on the drop probability:

ECT and CE bits for the packet are changed to 1 and the

packet is transmitted.

Non ECN-capable endpoints:

The packet may be dropped based on the WRED drop

probability.

The network is experiencing congestion:

Both the ECT and CE are already set. The packet istransmitted and no further marking is required.

Configuring ECN-Enabled WRED

random-detect ecn


Enables ECN.

ECN can be used whether WRED is based on the IP precedence

or DSCP value.

ECN must be configured through MQC.

router(config)# policy-map MyPolicy

router(config-pmap)# class class-defaultrouter(config-pmap-c)# bandwidth percent 70

router(config-pmap-c)# random-detect

router(config-pmap-c)# random-detect ecn


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Monitoring ECN-Enabled WRED

show policy-map [policy-map]

router#

Displays the configuration of all classes for a specified servicepolicy map or all classes for all existing policy maps

router#show policy-map

Policy Map MyPolicy

Class class-default


Bandwidth 70 (%)

exponential weight 9

explicit congestion notification

class min-threshold max-threshold mark-probability

----------------------------------------------------------

----------------------------------------------------------

0 - - 1/10

1 - - 1/10

2 - - 1/10

3 - - 1/10

. . .

show policy-map interface interface-name

router#

Displays the configuration of all classes configured for all service

policies on the specified interface

router#show policy-map interface Serial4/1

Serial4/1

Service-policy output:policy_ecn

Class-map:prec1 (match-all)

1000 packets, 125000 bytes

30 second offered rate 14000 bps, drop rate 5000 bps

Match:ip precedence 1


Output Queue:Conversation 42

Bandwidth 20 (%)

Bandwidth 100 (kbps)(pkts matched/bytes matched) 989/123625

(depth/total drops/no-buffer drops) 0/455/0

exponential weight:9

explicit congestion notification

Monitoring ECN-Enabled WRED (Cont.)

mean queue depth:0class Transmitted Random drop Tail drop Minimum Maximum Mark

pkts/bytes pkts/bytes pkts/bytes threshold threshold probability0 0/0 0/0 0/0 20 40 1/10

1 545/68125 0/0 0/0 22 40 1/102 0/0 0/0 0/0 24 40 1/103 0/0 0/0 0/0 26 40 1/104 0/0 0/0 0/0 28 40 1/105 0/0 0/0 0/0 30 40 1/106 0/0 0/0 0/0 32 40 1/107 0/0 0/0 0/0 34 40 1/10rsvp 0/0 0/0 0/0 36 40 1/10

class ECN Markpkts/bytes

0 0/01 43/53752 0/03 0/04 0/0

5 0/06 0/07 0/0

rsvp 0/0

Monitoring ECN-Enabled WRED (Cont.)

Documents

Arquitectura de Redes QoS 14