atm_traffic_management_and_control.pdf

7/27/2019 ATM_traffic_management_and_control.pdf

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CRIHAN ATM Course Version 2.2 - March 19992 . 5 Tr a f f ic M a n a g e m e nt a n d C o n t r ol - P a g e 1CRIHAN

ATM technology

ATMATM

Tr a f f ic M a n a g e m e n tTr a f f ic M a n a g e m e n ta n d C o n t r o la n d C o n t r o l


2/97


Topics

ATM Network Objectives

Services Categories

Traffic Contract

Traffic Management Mechanisms

Available Bit Rate (ABR)

Early Packet Discard (EPD)

Quantum Flow Control (QFC) The GFR Service


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ATM Applications

Circuit Emulation

Video

Video On Demand Distance Learning

Voice

Teleconferencing Distributed Supercomputing

Large File Transfers

Etc.


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ATM Applications and Network Objectives

All traffic carried via the same network elements

Optimize use of network resources

Meet stringent QoS requirements specific to each application Protects networks and users to achieve network performance

objectives


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Traffic Management

Problem: Providing Quality of Service How should ATM network resources be allocated to ensure good performances

including preventing congestion, e.g., how many virtual channels should be

assigned to a particular transmission link?

Solution: Traffic Management Specify a traffic contract on each virtual channel/path

Route (including rejecting setup request) each channel/path along a path with

adequate resources (Admission Control) Mark for loss all cells that violate the contract (Traffic Policing)

ATM architecture defines 5 services (QoS) categories


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ATM Quality of Service (QoS)

QoS per VP or VC

Desired bandwidth & QoS defined by end-user

Parameters set when PVC/SVC setup Defines endpoint-to-endpoint characteristics

QoS determines switch buffer and resource allocation


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CRIHAN ATM Course Version 2.2 - March 1999

2 . 5 Tr a f f ic M a n a g e m e nt a n d C o n t r ol - P a g e 8CRIHAN

ATM Service Categories (QoS Categories)

CBR - Constant Bit Rate Continuous flow of data with tight bounds on delay and delay variation

rt-VBR - Real-Time Variable Bit Rate Variable Bandwidth with tight bounds on delay and delay variation (traffic pattern is

known & variable)

nrt-VBR - Non Real-Time Variable Bit Rate

Variable bandwidth with tight bound on cell loss (traffic pattern is known & variable& can tolerate moderate cell delay)

UBR - Unspecified Bit Rate No guarantees (i.e. best effort delivery)

ABR - Available Bit Rate Flow Control on source with tight bound on cell loss (traffic more variable & bursty

than VBR & is delay tolerant)


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2 . 5 Tr a f f ic M a n a g e m e nt a n d C o n t r ol - P a g e 9CRIHAN

ATM Categories (QoS) and AAL attributes

Class

AAL

End-to-endTiming

Bit Rate

ConnectionMode

AssociatedATM Service Categories

AAL 3/4AAL 5

AAL 2AAL 1

1 2 3 U

Required Not Required

Constant Variable

Connection orientedConnection-

less

rt-VBR nrt-VBRCBR

ABR

UBR


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2 . 5 Tra f f i c M a n a gem ent a nd Con t ro l - P a g e 1 0CRIHAN

Topics


Services Categories

Traffic Contract




Quantum Flow Control (QFC)

The GFR Service


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ATM Traffic Contract

Traffic contract of a connection includes:

Service Category

QoS Requirements Traffic Descriptors

Conformance Definition

Definition of a Compliant Connection


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ATM Traffic Contract (cont)

Traffic Contract

Traffic ParametersQuality of Service

Conformance

Definition

ComplianceConnectionDefinition

UNIUser Network


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ATM Traffic Parameters

PCR: Peak Cell Rate

MBS (BT): Maximum Burst Size (Burst Tolerance)

SCR: Sustained Cell Rate ACR: Allowed Cell Rate

MCR: Minimum Cell Rate


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CBR Traffic Control

Peak

Cell Rate(PCR)

Time

Cell

Rate


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VBR Traffic Control

SustainedCell Rate

(SCR)

Time

Cell

Rate

BurstTolerance

(BT)

Cell Loss

Peak

Cell Rate(PCR)


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ABR Traffic Control

PeakCell Rate

(PCR)

Time

Cell

Rate Minimum

Cell Rate(MCR)

AllowedCell Rate

(ACR)


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Bandwidth Usage

TIME

BANDW

IDTH

CBR

VBR

BandwidthAvailable

UBR

ABR


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ATM Performance (QoS) parameters

CER: Cell Error Ratio (Accuracy)

SECBR: Severely-Errored Cell Block Ratio (Accuracy)

CLR: Cell Loss Ratio (Dependability) CMR: Cell Misinsertion Rate (Accuracy)

CTD: Cell Transfer Delay - Mean & Max (Speed)

CDV: Cell Delay Variation (Speed)


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Severely-Errored Cell Block Ratio

Severely-Errored Cell Blocks / Total Transmitted Cell Blocks

Cell Block is a sequence of N cells transmitted consecutively on agiven connection

Cell Block is usually the number of user information cellstransmitted between successive OAM cells


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Cell Loss Ratio (CLR)

Cells discarded due tocell/header errors, buffer

overflow, or UPC action.

Cell Bucket

ATM Switch SuccessfulReceived Cells

Difference betweenReceived & Successful

Lost Cells / Total Transmitted Cells


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Cell Misinsertion Rate (CMR)

Misinserted Cells / Time Interval

ATM SwitchTransmitted CellsReceived Cells

Cell Switched

To Wrong VPI / VCICell Stream


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Cell Transfer Delay (CTD)

Elapsed time between a transmitted cell exit event at MP1 and areceived cell exit event at MP2 for a particular connection

Takes into account: Propagation delay

Transmission delay Switching delay

Queuing delay

MP1

Cell Exit Event

MP2

Cell Entry Event


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Cell Delay Variation (CDV)

Fluctuation in cell transfer

delay may differ betweenindividual cells of the sameconnection

ATM SwitchRe-Timed CellSteam SequenceReceived Cells


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Service Category Parameters and Attributes

*

ABR

S 3

n/a

S

S 6

UBR

S 2

S

U

Unspecified

nrt-VBR

Unspecified

rt-VBR

S

Specified

*

CBR

Specified

n/a

Attribute

Tr a

fficPa

ram

s

PCR and CDVT4,5

SCR, MBS, CDVT4,5

MCR 4

Peak-to-peak CDV

MaxCTD

CLR4

Feedback

Q oS

Pa

ram

s

Note 1

n/a

UnspecifiedSpecified

Specified

ATM Layer Service Category


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Service Category Params & Attribs Notes

1: CLR is low for sources that adjust cell flow in response tocontrol information. Whether a quantitative value for CLR isspecified is network specific

2: May not be subject to CAC and UPC procedures

3: Represents the maximum rate at which the ABR source may

ever send. The actual rate is subject to the control information

4: These params are either explicitly or implicitly specified forPVCs or SVCs

5: CDVT refers to the Cell Delay Variation Tolerance (See Traffic

Contract). Does not need to be a unique value for a connection.Not signaled, network specific.

6: See ABR description


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Traffic Contract: Conformance

Guarantees are only valid if the traffic conforms to the negotiatedTraffic Contract

For CBR, VBR and UBR, conformance is defined by the GenericCell Rate Algorithm (GCRA)

For ABR, conformance is defined by the source and destination

behavior, but a dynamic GCRA is still a useful example of how to

test conformance Example causes of a non-conformance:

Excessive rate

Excessive burst

Non-conforming cells may be discarded or, when permitted,tagged with CLP=1 for lower priority


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Conformance : Generic Cell Rate Algorithm

Given the source traffic parameters, the networkpolicies the traffic using the Generic Cell Rate

Algorithm (GCRA):

For a sequence of cell arrival times, {tk},determines which cells conform to the traffic

contract

A counter scheme based on two parameters

denoted GCRA(I,L) Increment parameter: I (affects the cell rate)

Limit parameter: L (affects cell burst)

Leaky bucket

A cell that would cause the bucket tooverflow is non-conforming

L + I

One unit leak per unitof time

I units for each cell arrival


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Conformance : GCRA (cont)

GCRA is a continuous Leaky Bucket Algorithm. As each cellarrives, GCRA checks its conformance to an agreed rate

The GCRA has two parameters: I is the Increment per time unit

L is the Limit or capacity

When checking a rate with GCRA:

I is the reciprocal of the rate L is the tolerance


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Bucket fill just before (t-) and just after (t+) cell transmit time

2

1

2

1

2

1

2

1

2

1

GCRA(1.5,0.5)cell cell cell cell

t- t+ t- t+ t- t+ t- t+ t- t+

time

no cell


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Bucket fill just before (t-) and just after (t+) cell transmit time

2

1

2

1

2

1

2

1

2

1

GCRA(1.5,0.5)cell cell cell cell

t- t+ t- t+ t- t+ t- t+ t- t+

cell


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Conformance: Dual Leaky Bucket

R0

R1

CLP=1

CLP=1 & CLP=0

Drop

cells entering network


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Conformance tests

CBR Must conform to GCRA (1/PCR, CDVT)

VBRMust conform to both:

GCRA (1/PCR, CVDT)

GCRA (1/SCR, CVDT+BT)

ABRMust conform to DGCRA (1/ACR, t) where: ACR is the network specified rate ( PCR > ACR > MCR)

t is a dynamically variable burst tolerance

UBRWhether UBR must conform to GCRA (1/PCR, CVDT) is network specific

CDVT is the Cell Variation

Delay Tolerance - network

specific.

BT is the Burst Tolerancefor SCR.


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Ex. conformance definition at network ingress

Non-

confor-ming

Cells

PCR0+1

GCRA

(1/PCR,

CVDT)

GCRA

(1/SCR,

BT+CVDT)

CLP0+1

CLP1

CLP0

SCR0

Conforming Cells


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Traffic Contract: Compliance

Conformance is an attribute of the cell

Compliance is an attribute of the connection

QoS is guaranteed for all conforming cells of a compliantconnection

A connection is defined as compliant when the number of non-conforming cells is below a threshold set by the network provider

The threshold for compliance is specified in the traffic contract bythe network provider


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Topics


Services Categories

Traffic Contract


Available Bit Rate (ABR) Early Packet Discard (EPD)



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Traffic Management Components

Traffic Contract

Traffic ParametersQuality of Service

Parameters

Application

Traffic Management

Mechanisms

ATM

Layer


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Generic Functions (ATMF Spec. version 4.0)

Connection Admission Control (CAC)

Feedback Controls

Usage Parameter Control (UPC)

Cell Loss Priority ControlExamples: Selective Cell Discard and Explicit Forward Congestion Indication (EFCI)

Traffic Shaping

Network Resource Management (NRM)

Generic Flow Control

Frame Discard

ABR Flow Control

Other techniques


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Connection Admission Control (CAC)

Set of actions taken by the network during the set-up phase todetermine whether a connection request can be accepted orshould be rejected The connection must NOT affect the QoS of already established connections.

Bandwidth based on PCR? Waste of resources

Bandwidth based on minimum bandwidth and resources to meet the QoS?

(technique called source equivalent capacity or equivalent bandwidth). Only

effective if PCR/SCR > 1 Need for real-time mechanisms

Need accurate prediction of performance metrics

Use of multistate Markovian models CAC algorithms are proprietary mechanisms, not to be

standardized


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Feedback Controls

Set of actions taken by the network and by end-systems toregulate the traffic on an ATM connection according to the state ofnetwork elements

U P C l (UPC)


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Usage Parameter Control (UPC)

Also Called Traffic Policing

Set of actions taken by the network to monitor and control traffic,in terms of traffic offered and validity of the ATM connection, at the

end-system access

Main purpose is to protect the network, and to protect the QoS of

other already established connections

Detect violations of negotiated parameters and take appropriateactions (cell tagging and cell discard)

Monitoring performed for VCCs and VPCs Checks the validity of VPI/VCI

UPC function is usually placed at the ingress switch of the network

and is implemented as a simple or dual leaky bucket

C ll L P i it C t l


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Cell Loss Priority Control

For some service categories, the end-system may generate traffic

flows of cells with Cell Loss Priority (CLP) marking Network may follow models which treat this marking as

transparent or as significant

If treated as significant, network may selectively discard cells

marked with a low priority to protect, as far as possible, the QoSobjectives of cells with high priority

VPI VCIPTI

CLP

HEC

Payload

ATM Header (5 Bytes) ATM Payload (48 Bytes)

8 164 3 1 8

GFC

E li it F d C ti I di ti (EFCI)


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Explicit Forward Congestion Indication (EFCI)

The PTI field in the cell header contains a congestion notificationfunction. These bits may be turned on by network components toindicate congestion

This happens in the forward direction only

It is up to end-systems to react to this congestion indication

A technique available in ABR flow control enables backwards

compatibility with older switches: If a switch turns on the EFCI bits in any of the data cells, the ABR destination returns

an RM cell with a congestion notification which will instruct the source to reduce its

cell input rate

S l ti C ll Di d (SCD)


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Selective Cell Discard (SCD)

PTI In the cell header Bit 2: Used to indicate congestion in data cells (and Bit 3=0 (no RM cell))

A congested network component may discard cells that arenonconforming or if their CLP=1. This protect cells with CLP=0

VPI VCIPTI

CLP

HEC

Payload

3 1

UNI

NNI

VPI VCIPTI

CLP

HEC

Payload

ATM Header (5 Bytes) ATM Payload (48 Bytes)

8 164 3 1 8

GFC

T affic Sha ing


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Traffic Shaping

Traffic shaping alter the characteristics of cell traffic to achievebetter network efficiency and still meet the QoS requirements

Traffic shaping ensures compliance

PCR reduction,

Burst-length reduction,

CDV removal,

Cell spacing,

Etc.

Traffic shaping is optional and can be performed anywhere in the

network

Traffic Shaping (cont)


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Traffic Shaping (cont)

Limited by the delay that can be tolerated

S



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Proper and effective management of ATM virtual paths (VP) can beused to maximize network resource allocation and reduce thechance of congestion

All traffic management can be performed at the VP level rather thanthe VC level

UNI signaling 4.0 allows Switched Virtual Paths (SVP)

Can be used for a site backup



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Used for UNI only, not NNI

Used to indicate two classes of traffic: controlled or uncontrolled

Control traffic is subject to control mechanism while uncontrolledis not

Currently undefined

Set to 0000 (bit)

Proposed future uses: Flow Control

Shared media multiple access

Frame Discard


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Frame Discard

It is sometime more efficient to discard a frame than a cell

Frame means in this case an AAL Protocol Data Unit

Network detects frame boundaries by examining the SDU-type inthe payload type field in the ATM cell header

Implementation specific

Also called Early Packet Discard (EPD). See chapter IP over ATM

ABR Flow Control


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ABR Flow Control

May be used to adaptively share the available bandwidth amongparticipating users

Described later in this chapter

Forward Error Correction (FEC)


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Forward Error Correction (FEC)

Cell loss can result in degraded performance or even aggravatedcongestion situation

FEC is a technique which enables frames to be recovered from lost

or corrupted cells

FEC is implemented at the SSCS level

Source higher-layer service passes data down to the FEC-SSCS

which passes to the ATM layer. Destination FEC-SSCS recoversfrom any bit errors, cell, or frame loss and then presents the datato the higher-layer service

FEC is suitable for both real-time VBR video (where data loss is

damaging) and to an ABR LAN data (reliability and performanceare important)

Forward Error Correction (cont)


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Forward Error Correction (cont)

Physical

ATM

Physical

ATM

Physical

ATM

SAR

CPCS

FEC-SSCS

A

AL

SAP

Higher-Layer

Application

SAR

CPCS

FEC-SSCS

A

AL

SAP

Higher-Layer

Application

ATM Traffic Management Processes


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ATM Traffic Management Processes

DesiredQoS

BandwidthRequirements

ConnectionAdmission

Control

PVC/SVCConnectRequest

ResourceCheck

ForwardConnectRequest

Generic

Cell RateAlgorithm

UPC

Discard

or TagTraffic

Shaping

VP / VCBuffering

CLP

Control

Cell Stream

InformationFlow

TrafficDescriptorParameters- PCRMBSACRMCR

PerformanceParameters- CER- SECBRCLRCMRCTDCDV

QoS

ATM Host

TrafficShaping

Traffic Policing

ATMSwitch

Topics


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Topics


Services Categories

Traffic Contract





ABR - Flow Control Requirement


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ABR Flow Control Requirement

Applications like video or voice that use CBR or VBR connectionsknow how much bandwidth they will require. The resources arethen allocated by the network (traffic params, QoS). But...

Flow control plays an important role in avoiding congestioncollapse in data networks

The Internet, router networks, and pre-TM4.0 ATM networks have

no internal flow control; they depend on end-to-end flow control(e.g. TCP) at a higher layer

This type of traffic (e.g. LAN) is characterized by the following: Transmissions are bursty

The cell input rate of a source will vary over time

Variable length frames of sizes are considerably greater than 53 bytes

It is bandwidth greedy, using whatever bandwidth is available at the time

ABR - Flow Control Requirement (cont)


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ABR Flow Control Requirement (cont)

It is transported in connectionless layer-3 datagrams independent of the link-leveloperation (e.g. Ethernet)

They can increase or decrease their transmission rates based on changing networkconditions

ABR Flavors


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ABR Flavors

Rate based scheme Enables source to adapt its cell input rate based on feedback from the network

Credit Based scheme Link-by-link approach that enables the sender to transmit cells if there are available

buffers (credits) in the receiver switch

Number of credits is conveyed to the upstream neighbor via feedback

ATMF Traffic Management WG decided in 1994 to standardize a

rate-based ABR mechanism Best for WAN because switch do not need potentially large buffers

Backward compatible with EFCI

Several vendors decided to propose anyway a credit-schememode: QFC (Quantum Flow Control) Simple to implement

Could work ok on LANs

ABR - Flow Control Global View


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ABR Flow Control Global View

TCP flow control is the current basis of the Internet ABR provides a vastly improved basis for future networks

LAN LAN

WAN

Higher Layer Flow Control Loop

Large Delay - IMPLICIT BINARY FEEDBACK

Retransmission and Low Utilization

ABR Flow ControlMinimal Delay - EXPLICIT FEEDBACK

Near Zero Retransmission and High Utilization

ABR Service Model


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User is given a dynamic rate between MCR and PCR

Allowed rate varies based on network load

Uses feedback to deliver very low cell loss

Worst case delay is on order of round trip delay

Makes use of excess bandwidth in the network

TIME

BANDWIDTH

CBR

VBR

BandwidthAvailableUBR

ABR

ABR Basic Concept: End-to-End Flow Control


60/97



p

At the Source End-Station a set of rules called the Source Behaviordetermine the sending rate, based on control information from thenetwork

At each network element, a set of rules called the Switch Behaviordefine how feedback may be provided to control the source rate.The decision process for deciding the content of the feedback isunspecified

At the Destination End-Station a set of rules called the Destination

Behavior combine two functions: They define how a destination that receives forward congestion indicators from the

network should reflect these back to the source, and They allow a destination to provide its own feedback

ABR Operations


61/97



p

E

ndSystem

EndSyste

m

Switch Switch

NIC

Card

NIC

Card

Source and Destination Behavior

are implemented at the end-system,

for example in the NIC card

Switch Behavior

is implemented in the network switches

Closed Loop Congestion Control


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p g

Congestion control can be performed in open loop or closed loopsystems

Open loop congestion control is when network takes unilateral

action to avoid or relieve a congestion situation without notifyingthe traffic source (e.g. applications using CBR & VBR services)

Closed loop congestion control is when network provides a

feedback so the source can modify its cell input rate. Twonotifications techniques can be used: FECN and BECN (Forward

Explicit Congestion Notification, Backward Explicit CongestionNotification)

FECN / BECN


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FECN is a method that enables network components (e.g.switches) to signal congestion in either data or control cells asthey flow from the source to the destination

FECN serves as a signal to the destination to take action (notify thesource)

BECN flows in the opposite direction and can be sourced directly

from the point of congestion BECN: When a congestion is detected by the network, a

notification is sent back to the traffic source

BECN: Based on this information, the traffic source will either

increase or decrease the rate of input into the network

Closed Loop Congestion Control


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BECN

FECN

Closed Loop

Source Dest.

ABR Network Feedback Concepts


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The type of feedback from the network can be binary (bits flippedto indicate a congestion, increase/decrease rate) or explicit

If explicit, it can be a new cell input rate computed by the network

or the exact number of receive buffers in the adjacent switch The size of the feedback can be end-to-end or hop-by-hop (the

smaller the feedback loop, the quicker the source can be throttled

back) Tradeoff: resources are required to close a loop (e.g. send BECN)

and that the BECN messages still have to flow over what may be a

congested area on the way back to the source . A network can be

segmented in a series of concatenated closed loops with differentfeedback schemes

ABR RM-Cells


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In order to provide a mechanism for the network to providecongestion and rate change feedback to the source, the ABRsource is required to insert Resource Management (RM) cells

periodically within the data flow The RM-cells are turned around by the destination and returned to

the source along the return path

Each RM-cell contains an Explicit Rate (ER) at which the sourcewhishes to operate, and this rate may be reduced by any network

element to the rate it can currently support

Each RM-cell also includes binary fields that the source can use to

request permission to increase its rate, and that network elementsmay modify to prevent an increase or force a decrease

ATM RM-Cell Format


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ATM Header Payload

RM Protocol

Identifier (1 byte)

Function Specific

Field (45 bytes) CRC-10

Reserved

(6 bits)

Standard header

with PTI=6 (110)

ABR RM-Cell Flow


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RM-cells are sent from source to destination and back every Nrmcells, typically a 3% overhead per direction.

Source Destination

Switch Switch Switch Switch

RM

CRC10MCRER CCRDIR

BN, CI

NI

IDStandard ATM Header

with PTI = 6 (110)

Byte 1-5 6 8-97 10-11 12-13

Switch adjust ER, CI and NI to control source rate

RM Cell Format

ATM ABR RM-Cell Fields


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DIR = Direction (0=forward, 1=backward). Bit #8 of Octet #7

BN = BECN RM-cell (1=Non-Source Generated, 0=Sourcegenerated). Bit #7 of Octet #7

CI = Congestion Indication (1=Congestion, 0 otherwise). Bit #6 ofOctet #7

NI = No Increase (1=no additive increase allowed, 0 otherwise). Bit

#5 of Octet #7 RA = Not used for ABR

ER = Explicit Cell Rate. All bits of Octets #8-9

CCR = Current Cell Rate. All bits of Octets #10-11 MCR = Minimum Cell Rate. All bits of Octets #12-13

ABR Modes Of Switch Operations


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EFCI Binary Rate Operation (Mark EFCI in data cells) Workgroups

Relative Rate Binary Operation (Mark CI, NI bits in RM-cells)

Departmental LANs

Explicit Rate Operations (Mark ER field in RM-cell) Power Workgroups

Departmental Backbones

Corporate Backbones

Virtual Source, Virtual Destination WANs

Corporate Backbones

ABR Modes Of Switch Operations (cont)


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Com

plexity

Performance

EFCI Binary

Rate Operation

Relative Rate

Binary Operation

Explicit Rate

Operation

Virtual Source,

Virtual Destination

ABR Transient Exposure


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When a new or idle source become active, network feedback willnot take effect for one round trip time (RTT)

BECN can be used to shorten feedback delay

ACR

Time

ICR

RTT

ER Feedback Delay

ACR

Time

ICR

RTT

CI Feedback Delay

ABR Signaled Parameters


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PCR (Peak Cell Rate). Negotiation: Down. Default: mandatory

MCR (Minimum Cell Rate). Neg: down to MCRmin if signaled, elseno. Default: 0

ICR (Initial Cell Rate). Neg: Down. Default: PCR

TBE (Transient Buffer Exposure). Neg: Down. Default: 16777215

FRTT (Fixed Round Trip Delay). Neg: Accumulated. No default: set

by source to fixed source delay and then accumulated during thecall setup. FRTT used to determine other parameters (e.g. ICR)

RIF (Rate Increase Factor). Neg: Down. Default: 1/16

RDF (Rate Decrease Factor). Neg: Down (RIF/RDF ratio). Default:1/16

Other optional parameters (see ATMF TM 4.0 spec)

ABR Typical Signaled Parameters


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End-Stations need only to specify PCR and MCR

Switches need only: Negotiate PCR and MCR, based on conditions

Negotiate TBE, RDF, and RIF down to preset values

ICR will then be computed as TBE/FRTT

ABR Binary Rate Operation


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Sourc

eCellRate

Time

MCR

Minimum Cell Rate

ICR

Initial Cell Rate

PCRPeak Cell Rate

Average

Source Rate

ACRAllowed Cell Rate

RDF * ACRRate Decrease Factor

RIF * PCRRate Increase Factor

Network notcongested (CI=0)

Networkcongested (CI=1)

FRTTFixed Round Trip Time

TBE

Network Capacity

ABR Explicit Rate Operation


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Sourc

eCellRate

Time

MCRMinimum Cell Rate

ICR

Initial Cell Rate

PCRPeak Cell Rate

ACRAllowed Cell Rate

FRTTFixed Round Trip Time

TBE

RIF * PCRRate Increase Factor

Network Capacity

Explicit Rate Flow Control vs TCP


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ER is 100 times faster than TCP

Source: Lawrence G. Roberts, 06/97

WAN

TCP Control Loop - 1sec to control

200ms typical round trip - 20ms if uncongested

Explicit Rate Control Message

6ms to control

Congestion

ABR Virtual Source / Virtual Destination


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It is permissible to insert Virtual Source / Virtual Destination

modules into a network so as to protect one segment of thenetwork from another , and to shorten the control loops

VS/

VD

VS/

VDSource Dest

Control Loop Control Loop Control Loop

Rate-based segmented Control Loop

ABR Source Behavior, part 1


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What to do with a returning RM-Cell:

If CI = 1, decrease rate proportionally: ACR = (1 - RDF)ACR

else if NI = 0, allow linear rate increase ACR = ACR + RIF * PCR

Also look at explicit rate feedback ACR = Min (ACR, ER)

Also keep ACR between MCR and PCR

ABR Source Behavior, part 2


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Send a forward RM-Cell once per Nrm cells (N power of 2, from 2 to256) or more frequently if ACR is low

When sending a forward RM-Cell, do two implicit tests:

1) If source is coming out of idle, reduce ACR to ICR 2) If feedback pipe is broken, reduce ACR: ACR = (1 - CDF)ACR

(ICR = Initial Cell Rate, CDF = Cutoff Decrease Factor). See ATMFTM 4.0 Spec

ABR Destination Behavior


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Data Cell RM-Cell

RM-Cell

RM-Cell Received

Return RM-Cell as B-RM-Cell

If Saved EFCI=1 then CI=1, saved EFCI=0

If congested then adjust ER or CI

Data Cell Received

Save EFCI Indicator

ABR Switch Behavior


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Modes of Operation A: EFCI Binary Rate Operation

B: Relative Rate Binary Operation

C: Explicit Rate Operation

Data Cell RM-Cell

RM-Cell

RM-Cell Received

If B and congested then may set CI = 1 (probably on B-RM)

If C then reduce ER as needed (probably on B-RM),

Send RM Cell

Data Cell Received

If A and congested then set EFCI Send Cell

Topics


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Services Categories

Traffic Contract Traffic Management Mechanisms



The GFR Service



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Absence of ABR flow control standard have forced vendors to finda simple solution for UBR congestion control:

Support for LAN Traffic is potentially important for ATM

If no flow control, source will transmit up to PCR, congestion or nocongestion and cells will be discarded due to switch buffer

overflow

If there is a cell loss, the destination AAL will not be able toreconstruct the packet. The surviving cells will continue to flowand to fill the switch buffers

Meanwhile, the higher-layer protocol (e.g. TCP) at the UBR source

recognize that a packet has been lost and retransmits the packetinto the network: more and more congestion...

Early Packet Discard (cont)


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The simple solution is to discard the remaining cells from the lostpacket = Early Packet Discard

The switch maintain an EPD buffer threshold (% of all the switch

buffers) When the EPD buffer threshold is exceeded, the switch will drop all

remaining cells making the AAL5 PDU until the last cell (last cellhas PTI field marked)

EPD works well and is VERY simple to implement

UNI 4.0 ABR sources can signal their desire for EPD support atconnection setup using frame discard code point

Topics


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Services Categories




The GFR Service


Cl i l di b d fl l h i


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Classical credit-based flow control mechanism

Sender cannot transmit cells to a receiver unless there areavailable buffers in receiver

Operates on a link-by-link basis Feedback loop is small, link-speed propagation delay

Guarantees zero cell loss due to network congestion

Instantaneous access to available network bandwidth Configures automatically

Compatibility with UNI 3.0/3.1 With modification of IEs

Quantum Flow Control (cont)

QFC VC ti f f i


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QFC uses a per-VC accounting for fairness Credit balance

When N cells are transmitted to receiver, balance is decremented by N

When receiver forwards cells over to next link, credit is returned

- Credits

Credit Balance

+ Credits

- Credits

Credit Balance

+ Credits

Topics

ATM N t k Obj ti


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Services Categories




The GFR Service

Guaranteed Frame Rate Service (GFR)

Currently an ATM Forum Draft


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Currently an ATM Forum Draft Included in [ATM Forum Traffic Management 4.1]

Described also in [ATM Forum Traffic Management Baseline Text Document]

Intended to support non-real-time applications Requires that user data cells are organized in the form of cells that

can be delineated at the ATM layer

Guaranteed Frame Rate Service (cont)

Why a GFR Service ?


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Why a GFR Service ?

ATM Forum has defined 5 Service Categories CBR, rt-VBR, nrt-VBR, UBR and ABR

However, it is expected that many users will not be able to specifythe range of traffic parameters needed to request most ATMservices, or will not be equipped to comply with the (source)behavior rules of ABR The only access those users have to ATM networks would be through UBR

connections, which provide no service guarantees

Guaranteed Frame Rate Service (cont)

Goal of GFR is to bring the benefits of ATM performance and service


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Goal of GFR is to bring the benefits of ATM performance and serviceguarantees to users that are not able to take advantage of it

GFR service requires minimal interaction between users and ATM

networks But provides users some level of service guarantees

Simplicity of GFR Service spec comes at some cost in terms of therequirements imposed on the network

The GFR Service Model

The GFR Service provides the user with a:


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The GFR Service provides the user with a:

Minimum Cell Rate (MCR) guarantee

under the assumption of a given:

Maximum Frame Size (MFS)

and a given:

Maximum Burst Size (MBS)

(MFS and MBS expressed in units of cells)

The GFR Service Model (cont)

User can send frames in excess of the MCR and associated MBS


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User can send frames in excess of the MCR and associated MBSwith no guarantee, spec says that each user should have access tofair share of available resources (fair is inplementation specific)

User can send frame marked or unmarked CLP must be the same for all cells in a frame

CLP indicates to the network the importance of a frame

Network is allowd to tag cells in unmarked frames *only* if the user

has requested the tagging option Via signaling for SVCs or subscription for PVCs

GFR Service do not give the user explicit feedback

Currently, GFR Service only applies to VC connections (VCC),because frame delineation is not generally visible in a VP

connection

Service Category Parameters and Attributes



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O the r

GFR

S

S 8

S

U*

U

S

ABR

S3

n/a

S

S 6

UBR

S2

S

U

Unspecified

nrt-VBR

Unspecified

rt-VBR

S

Specified

*

CBR

Specified

n/a

Attribute

Tr a

ffi c

Pa

ram

s PCR and CDVT4,5

SCR, MBS, CDVT4,5

MCR 4

MFS9

Peak-to-peak CDV

MaxCTD

CLR 4

Feedback

Q

oS

Par a

ms

Note 1 Note 7

n/a

n/a

U

UnspecifiedSpecified

Specified


Service Category Params & Attribs Notes

1: CLR is low for sources that adjust cell flow in response to


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1: CLR is low for sources that adjust cell flow in response tocontrol information. Whether a quantitative value for CLR isspecified is network specific

2: May not be subject to CAC and UPC procedures 3: Represents the maximum rate at which the ABR source may

ever send. The actual rate is subject to the control information

4: These params are either explicitly or implicitly specified forPVCs or SVCs

5: CVDT refers to the Cell Delay Variation Tolerance (See TrafficContract). Does not need to be a unique value for a connection.

Not signaled, network specific. Different values may apply at eachinterface along the path of a connection

Service Category Params & Attribs Notes (c)



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7: CLR is low for frames that are eligible for a service guarantee

8: For GFR, MBS and CDVT are associated with MCR. SCR is not

applicable for GFR

atm_traffic_management_and_control.pdf

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