chapter 12. traffic and congestion control in atm networks

Chapter 12.Traffic and Congestion Control In ATM Networks

By Sanghyun Ahn, Dept. of Computer Science and Statistics, University of Seoul

Related Standards

ITU-T’s I.371 ATM Forum’s Traffic Management Spec.

Version 4.0


Requirements for ATM Traffic and Congestion Control

no flow control-based congestion control no feedback based congestion control

• implicit congestion control

= no explicit congestion notification

= source deduces the presence of congestion

by the loss of data

= too late reaction in high-speed ATM

(Latency/Speed Effect) inadequate for ATM


Requirements (cont’d) CBR support

• related to cell delay variation (CDV)• V(0) = an estimate of the amount of CDV that an application can tolerate V(I) = V(I-1) - [t(I) - (t(I-1) + )]• If V(I) is negative, then that cell is discarded• CDV can be reduced by increasing the data rate at the UNI

relative to the load and by increasing resources within the network

• Sources of CDV– queuing delay due to congestion– segmentation, interleaving, OAM cells, SDH overhead ...


ATM Traffic-Related Attributes Traffic Descriptors

• describe the traffic characteristics of a source and of a connection

• network establishes a connection only if sufficient resources are available

QoS Parameters• characterize the performance of an ATM connection in terms of

QoS that it provides Other

• feedback attribute for ABR


Traffic Descriptors Source Traffic Descriptor

• source characteristics of an ATM flow

• peak cell rate (PCR)

• sustainable cell rate (SCR)

• maximum burst size (MBS)

• minimum cell rate (MCR)

Connection Traffic Descriptor• characteristics of an ATM flow over an ATM connection

• source traffic descriptor

• cell delay variation tolerance (CDVT)

• conformance definition


Source Traffic Descriptor PCR

• an upper bound on the traffic that can be submitted by a source on an ATM connection

• PCR = 1/T, where T: min spacing between cells• for CBR and VBR

SCR• an upper bound on the average rate of an ATM connection• calculated over a time scale that is large relative to T• for VBR only• SCR < PCR


Source Traffic Descriptor (cont’d)

MBS• max number of cells that can be sent continuously at

PCR

• for VBR only MCR

• min commitment requested of the network

• for ABR only


Connection Traffic Descriptor

CDVT• a measure of the amount of variation in cell delay

that is introduced by network interface (e.g., SDH) and at UNI

Conformance Definition• used to specify unambiguously the conforming cells

of a connection at UNI

• net may enforce conformance by dropping or marking cells that exceed the conformance definition

• GCRA (Generic Cell Rate Algorithm)


QoS Parameters

Peak-to-peak Cell Delay Variation• maxCTD - fixed delay

• CDV is negotiated during connection establishment

whereas CDVT is normally explicitly set at UNI

and is not negotiated

• CDVT is the variation introduced by the source traffic itself (= an upper bound on CDV at UNI)

• CDV is the difference between the best- and worst-case expected end-to-end CTD


QoS Parameters (cont’d)

Maximum Cell Transfer Delay (maxCTD)• CTD

– time between transmission of the last bit of a cell at source UNI and the receipt of the first bit of a cell at the destination UNI

– variable due to buffering and cell scheduling

• maxCTD– max requested delay

Cell Loss Ratio (CLR)• ratio of lost cells to total transmitted cells


Traffic Management Framework Cell Insertion Time

• react immediately to cells as they are transmitted Round-Trip Propagation Time

• respond within the lifetime of a cell in net and may provide a feedback info to source

Connection Duration• determine whether a new connection at a given QoS can be

accommodated and what performance level will be agreed to Long Term

• affect more than one ATM connection and established for long-term use


Traffic Control

ATM Traffic Control Functions set of actions taken by the network to avoid congestion conditions or to minimize congestion effects Resource management using VPs Connection Admission Control (CAC) Usage Parameter Control (UPC) Traffic shaping


Resource Management using VPs Net provides aggregate capacity and performance

characteristics on VP and these are shared by VCs in the VP

QoS of a VPC = max QoS of VCCs in the VPC statistical multiplexing

• capacity of VPC average data rates of all VCCs• capacity of VPC < aggregate peak demand• efficient utilization of capacity• difficult to provide fair access• preferable to group VCCs into VPCs on the basis of similar

traffic characteristics and similar QoS requirements


Connection Admission Control

Net accepts a connection only if it can commit the resources necessary to support that traffic level while at the same time maintaining the agreed QoS of existing connections

if accepted, traffic contract between net & user net continues provide the agreed QoS as long as

the user complies with the traffic contract traffic contract parameters (table 12.3)


Usage Parameter Control (UPC)

Monitors a connection to determine whether the traffic conforms to the traffic contract once it has been accepted by CAC

Protect net resources from an overload on one connection by detecting violations of assigned parameters and taking appropriate actions


UPC: Two Separate Functions

Control of PCR & CDVT• a traffic flow is compliant if the peak rate of cell

transmission does not exceed the agreed peak cell rate, subject to the possibility of cell delay variation within the agreed bound

Control of SCR & burst tolerance


UPC: Generic Cell Rate Algorithm

Used both for PCR & SCR controls GCRA(I, L), where I: Increment, L: Limit peak cell rate algorithm sustainable cell rate algorithm


UPC: Peak Cell Rate Algorithm GCRA(T, ) T: average interarrival time between cells at PCR if there is no CDVT : CDVT limit Virtual Scheduling Algorithm

• fig. 12.6 (a), 12.7 (a)

• max # of conforming back-to-back cells

= 1 + / (T - ) Leaky Bucket Algorithm

• fig. 12.6 (b), 12.7 (b)


UPC: Sustainable Cell Rate Algorithm

GCRA(Ts, s)

Ts: interarrival time between cells at SCR if there is no burstiness s: burst tolerance

MBS = 1 + s / (Ts - T) s = (MBS - 1)(Ts - T)


UPC: Actions

compliant cells are passed along and noncompliant cells are discarded or tagged (CLP = 1)

fig. 12.10


Selective Cell Discard

at some point beyond the UPC function, net discards (CLP = 1) cells in case of congestion

discard lower-priority cells to protect the performance for higher-priority cells


Traffic Shaping smooth out a traffic flow and reduce cell clumping result in a fairer allocation of resources and a reduced

average delay time Token Bucket

• token generator produces tokens at a rate of tokens per sec and places these in token bucket

• to transmit a cell through the server, one token must be removed from the bucket

• if bucket is empty, cell is queued waiting for the next token• if there is a backlog of cells and an empty bucket, cells are emitted at

a smooth flow of cells per second with no cell delay variation until the backlog is cleared


ATM Traffic Control Approaches

Open-Loop Control• for CBR, rt-VBR, nrt-VBR

• based on traffic contract and UPC

• no feedback to source concerning congestion

Best-Effort• for UBR

• share the unused capacity in a relatively uncontrolled fashion

• inefficient: dropped cells cause retransmissions


ATM Traffic Control Approaches (cont’d)

Closed-Loop Control• for ABR

• provide feedback to sources to adjust the load dynamically and avoid cell loss and share the capacity fairly


ABR: Characteristics of ABR Service

ABR connections share available capacity The share of available capacity used by a single

ABR connection is dynamic and varies between MCR and PCR

The net provides feedback to ABR sources so that ABR flow is limited to available capacity

For ABR sources that adapt their transmission rate to the provided feedback, a low cell loss ratio is guaranteed


ABR: ABR Connection Characterization Parameters

ACR (Allowed Cell Rate)• current rate at which source is permitted to transmit cells

MCR (Minimum Cell Rate)• min value that ACR may take (I.e., net will not restrict a

source’s flow less than MCR) PCR (Peak Cell Rate)

• max value that ACR may take ICR (Initial Cell Rate)

• initial value assigned to ACR


ABR: Feedback Mechanism

Feedback is provided periodically in the form of a sequence of RM (Resource Management) cells

RM cell• contains CI (Congestion Indication) bit, NI (No

Increase) bit, ER (Explicit cell Rate) field

• source transmits one FRM (Forward RM) cell for every (Nrm - 1) data cells

• for each received FRM, destination transmits it back to source as a BRM (Backward RM) cell


ABR: Feedback Mechanism ACR Control

• Initially, ACR = ICR• linear increase, exponential decrease• NI=0, CI=0: ACRmax[MCR,min[ER,PCR,ACR+RIFPCR]]• NI=0, CI=1: ACRmax[MCR,min[ER,ACR(1-RDF)]]• NI=1, CI=0: ACRmax[MCR,min[ER,ACR]]• NI=1, CI=1: ACRmax[MCR,min[ER,ACR(1-RDF)]]


ABR: Feedback Mechanism (cont’d)

Source• set CI = 0, NI = 0 or 1• set ER equal to some desired transmission rate

Intermediate ATM switch• EFCI marking: cause dest to set CI = 1 in BRM• relative rate marking: set CI or NI• explicit rate marking: reduce ER value

Destination• under normal: if EFCI is marked in the previous data cell, set CI• under congestion: set CI or NI, or reduce ER


ABR: RM Cell Format

Header• PT = 110

• for VC rate control: VCI = 6 Protocol ID = 1 Message Type

• FRM (DIR = 0), BRM (DIR = 1)

• BECN cell– initially generated by source (BN = 0)

– generated by a switch or dest (BN =1)


ABR: Capacity Allocation

Binary Feedback Scheme• use EFCI, CI, NI bits

Explicit Rate Feedback Scheme• use ER field


ABR: Capacity Allocation (cont’d)

Binary Feedback Scheme when congestion occurs, switch performs a

binary notification either• by setting the EFCI on a forward data cell or

• by setting CI or NI on a FRM or BRM



Binary Feedback Scheme (cont’d) FIFO Queue

• dedicate the buffer at each output port to a single FIFO queue

• if buffer occupancy exceeds a threshold, switch issues binary notifications until buffer occupancy falls below the threshold

• may use two threshold

• may unfairly penalize connections passing through a number of switches



Binary Feedback Scheme (cont’d) Multiple Queues

• allocate a separate queue to each VC or to each group of VCs

• a separate threshold for each queue

• Adv:– a misbehaving source will not affect other VCs

– delay and loss behavior of individual VCs are decoupled



Binary Feedback Scheme (cont’d) Fair Share Notification

• selective feedback or intelligent marking

• allocate a fair share of capacity dynamically

• Fairshare = Target rate / Number of connections

• when congested, switch marks cells on any VC which satisfies CCR > Fairshare



Explicit Rate Feedback Scheme General Functions

• compute the fair share of the capacity for each VC that can be supported

• determine the current load, or degree of congestion

• compute an ER for each connection and send that ER to source



Explicit Rate Feedback Scheme (cont’d) Example Schemes

• EPRCA (Enhanced Proportional Rate Control Algorithm)

• ERICA (Explicit Rate Indication for Congestion Avoidance)

• CAPC (Congestion Avoidance using Proportional Control)



Explicit Rate Feedback Scheme (cont’d) EPRCA

• switch keeps track of MACR (Mean ACR)

• MACR(I) = (1-)MACR(I-1) + CCR(I)

• if congested, ER min[ER, DPFMACR]

• react to congestion by lowering ERs of VCs that are consuming more than their fair share of capacity


ABR: Capacity Allocation (cont’d)Explicit Rate Feedback Scheme (cont’d)

ERICA• selectively adjust VC rates so that the total ER allocated

to connections equals the target rate and is allocated fairly• Load Factor

– LF = Input rate / Target rate

• Fairshare = Target rate / Number of connections• VCshare = CCR / LF• ER = min[oldER, max[Fairshare, VCshare]• improve fairness under overload conditions


ABR: Capacity Allocation (cont’d)Explicit Rate Feedback Scheme (cont’d)

CAPC• if LF > 1, Fairshare = Fairshare min[ERU, 1+(1-LF)Rup]• if LF < 1, Fairshare = Fairshare min[ERF, 1-(LF-1)Rdp]

– ERU determines the max increased allowed in the allotment of fair share; ERU > 1

– Rup = a slop parameter between 0.025 and 0.1

– ERF determines the max decrease allowed in the allotment of fair share; ERF = 0.5

– Rdn = a slope parameter between 0.2 and 0.8



Explicit Rate Feedback Scheme (cont’d) CAPC (cont’d)

• if the calculated Fiarshare is lower than ER in RM cell, then set ER to Fairshare

• simpler to implement than ERICA,

very large rate oscillations if RIF is set too high,

sometimes lead to unfairness

chapter 12. traffic and congestion control in atm networks

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