Data Communications

Congestion in Data Networks

What Is Congestion?Congestion occurs when the number of

packets being transmitted through the network approaches the packet handling capacity of the network

Congestion control aims to keep number of packets below level at which performance falls off dramatically

Data network is a network of queuesGenerally 80% utilization is criticalFinite queues mean data may be lost

Figure 23.5 Incoming packet

Interaction of Queues

Effects of CongestionPackets arriving are stored at input buffers

(not ATM)

Routing decision madePacket moves to output bufferPackets queued for output transmitted as

fast as possibleStatistical time division multiplexing

If packets arrive too fast to be routed, or to be output, buffers will fill

Can discard packetsCan use flow control

Can propagate congestion through network

Ideal Performance

Top: As load increases, throughput directly increases

Middle: As load increases, delay increases

Bottom: Power is ratio of throughput to delay

Practical Performance

Ideal assumes infinite buffers and no overhead

Unfortunately, buffers are finiteAdditional overhead occurs in exchanging

congestion control messages

Effects of Congestion -No Control

Congestion Control ObjectivesIn general, we want to:Minimize discardsMaintain agreed QoS (if applicable)Minimize probability of one end user monopoly over

other end usersSimple to implement

Little overhead on network or user

Create minimal additional trafficDistribute resources fairlyLimit spread of congestionOperate effectively regardless of traffic flowMinimum impact on other systemsMinimize variance in QoS

Basic Mechanisms for Congestion Control

Open-Loop Congestion Control (rely on other layers for feedback and control) Retransmission policy - a good policy can reduce congestion Window policy - sel-reject better than go-back-N; use a bigger

window size Acknowledgment policy - don’t ack each packet individually Discard policy - a good policy by routers may prevent

congestion and at the same time may not harm the integrityof the transmission

Admission policy - QOS mechanism

Basic Mechanisms for Congestion Control

Closed-Loop Congestion Control Backpressure - when a router is congested, it informs the

previous upstream router to reduce the rate of outgoingpackets

Choke packet of choke point - sent by router to source, similarto ICMP’s source quench packet

Implicit signaling - look for delay in some other action Explicit signaling - router sends an explicit signal Backward signaling - bit is set in packet moving in the

direction opposite to the congestion Forward signaling - bit is set in packet moving in the direction

of congestion. Receiver can use policy such as slowingdown acks to alleviate congestion

Basic Mechanisms for Congestion Control (visual examples)

BackpressureIf node becomes congested it can slow down

or halt flow of packets from other nodesMay mean that other nodes have to apply

control on incoming packet ratesPropagates back to sourceCan restrict to logical connections

generating most trafficUsed in connection oriented that allow hop

by hop congestion control (e.g. X.25)Not used in ATM or frame relayOnly recently developed for IPv6 (PRI field)

Choke Packet

Control packet Generated at congested nodeSent to source nodee.g. ICMP source quench

From router or destinationSource cuts back until no more source

quench messageSent for every discarded packet, or

anticipated

Rather crude mechanism

Implicit Congestion Signaling

Transmission delay may increase with congestion

Packets may be discardedSource can detect these as implicit indications

of congestion (source is responsible, not network)

Useful on connectionless (datagram) networkse.g. IP based (TCP includes congestion and flow

control)

Used in frame relay LAPF

Explicit Congestion Signaling

Network alerts end systems of increasing congestion

Used on connection-oriented networksEnd systems take steps to reduce offered loadBackwards

Congestion avoidance info sent in opposite direction of packet travel

ForwardsCongestion avoidance info sent in same direction

as packet travel - when end system receives info, either sends it back to source or hands it to higher layer to take action

Categories of Explicit SignalingBinary

A bit set in a packet indicates congestion

Credit basedIndicates how many packets source may sendCommon for end to end flow control

Rate basedThe source may transmit data at a rate up to the

set limitAny node along the path of the connection can

reduce the data rate limit in a control message to the source

e.g. ATM

How Does TCP Handle / Avoid Congestion? (details in TDC 365 and TDC 463)

To Handle: TCP has a sender window size. Sender window size is minimum of receiver window size or network congestion window size.

To Avoid: TCP can use Slow Start and Additive Increase - at beginning, TCP sets congestion window size to maximum segment size, then increases window size with each ack.

Can also use Multiplicative Decrease - after timeout, threshold set to 1/2 previous threshold, and congestion window size reset to 1 (then slow start)

Figure 23.8 Multiplicative decrease

How Does Frame Relay HandleCongestion?

Connection management, coupled with

Discard strategy

Explicit signaling

Implicit signaling

In more detail:

Connection ManagementBefore a frame relay network allows a user to

transmit data, they agree on a connectionSome call this an SLA (service level

agreement)Frame relay calls it the CIR (committed

information rate)Committed burst size - max amount of data

the network agrees to transfer, under normal conditions

Excess burst size - max amount of data in excess of committed burst size

Different frame relay companies have different agreements

Connection ManagementWhat happens if you exceed your CIR and the

network experiences congestion?Frame relay may start discarding your frames

(Discard Eligible bit = 1) (Discard Strategy)Does frame relay tell you that your frames are

being tossed?No. Frame relay assumes a higher layer protocol

(such as TCP) will monitor lost or missing framesFrame relay could discard arbitrarily with no regard

for source, but then no reward for restraint so end systems transmit as fast as possible

CIR not 100% guaranteed, but network tries hardAggregate CIR should not exceed physical data rate

Figure 23.1 Traffic descriptors

Relationship Among Congestion Parameters

Explicit SignalingNetwork alerts end systems of growing

congestionBackward explicit congestion notification

Notifies the user that CA procedures should be initiated for traffic in the opposite direction; simpler

Forward explicit congestion notificationNotification goes forward, so end user has to

somehow get signal back to other end to tell them to slow down

Frame handler monitors its queuesMay notify some or all logical connectionsUser response

Reduce rate

Figure 23.9 BECN

Figure 23.10 FECN

Figure 23.11 Four cases of congestion

Implicit Signaling

Implicit Congestion Notification Telecom Definition

In frame relay, inference by user equipment that congestion has occurred in the network. The inference is triggered by realization of the receiving frame relay access device (FRAD) of transmission delays. Based on block, frame or packet sequence numbers, another protocol may recognize that one or more frames have been lost in transit. Control mechanisms at the upper protocol layers of the end devices then deal with frame loss by requesting retransmissions.

From: YourDictionary.com

What About ATM?

High speed, small cell size, limited overhead bits

Requirements (difficult)Majority of traffic not amenable to flow controlFeedback slow due to reduced transmission time

compared with propagation delayWide range of application demandsDifferent traffic patternsDifferent network servicesHigh speed switching and transmission increases

volatility

Latency/Speed EffectsConsider a typical ATM transmission speed of

150Mbps~2.8x10-6 seconds to insert single cellTime to traverse network depends on

propagation delay, switching delayAssume propagation at two-thirds speed of

lightIf source and destination on opposite sides of

USA, propagation time ~ 48x10-3 secondsGiven implicit congestion control, by the time

dropped cell notification has reached source, 7.2x106 bits have been transmitted

So, this is not a good strategy for ATM

Cell Delay Variation

For ATM voice/video, data is a stream of cells

Delay across network must be shortAND Rate of delivery must be constantThere will always be some variation in

transitDelay cell delivery to application so that

constant bit rate can be maintained to application

Time Re-assembly of CBR CellsD(i)=end to end delay of ith cellV(0)= estimate of amount of cell delay variation that an application can tolerate

Various Network Contributions to Cell Delay VariationPacket switched networks

Queuing delaysRouting decision time

Frame relayAs above but to lesser extent

ATMLess than frame relayATM protocol designed to minimize processing

overheads at switchesATM switches have very high throughputOnly noticeable delay is from congestionMust not accept load that causes congestion

Cell Delay Variation At The UNI in ATM

Even if application produces data at fixed rate, processing at (potentially) three layers of ATM causes delayInterleaving cells from different connectionsOperation and maintenance signals need to be

interleavedIf using synchronous digital hierarchy frames,

potential delays here are inserted at physical layer

Can not predict these delays(See figure next slide)

Origins of Cell Delay Variation

Traffic and Congestion Control Objectives for ATM

ATM layer traffic and congestion control should support QoS classes for all foreseeable network services

ATM layer traffic and congestion control should not rely on AAL protocols that are network specific, nor on higher level application specific protocols

Any traffic and congestion controls should minimize network and end to end system complexity

Traffic Management and Congestion Control Techniques

ITU-T and ATM Forum have defined a range of traffic management functions to maintain the QoS of ATM connections:Resource management using virtual paths - separate traffic flow according to service characteristics (1)Connection admission control (2)Usage parameter control (3)Traffic shaping (4)

Let’s examine these in more detail

Resource Management Using Virtual Paths (1)

ATM network can use the virtual path to group similar virtual channels

Connection Admission Control (2)

Good first line of defenseUser specifies traffic characteristics for new

connection (VCC or VPC) by selecting a QoSNetwork accepts connection only if it can

meet the demandTraffic contract

Peak cell rate - upper bound, CBR and VBRCell delay variation - CBR and VBRSustainable cell rate - average rate, VBRBurst tolerance - VBR

Usage Parameter Control (3)

Monitor established connection to ensure traffic conforms to contract

Protection of network resources from overload by one connection

Done on VCC and VPCControl of peak cell rate and cell delay

variation, orControl of sustainable cell rate and burst

toleranceDiscard cells that do not conform to traffic

contractCalled traffic policing

Traffic Shaping (4)

Smooth out traffic flow and reduce cell clumping

Token bucket and leaky bucket are examples of traffic shaping

Token bucket allows bursts, while leaky bucket maintains an even flow

(See figures next slides)

Figure 23.18 Token bucket

Token Bucket

Figure 23.16 Leaky bucket

Figure 23.17 Leaky bucket implementation

Leaky bucket keeps an average flow moving. Queue overflows?Discard packets. Unlike token bucket, no credit for no transmissions.

ATM’s Real-time Traffic Management

QoS provided for CBR, and rt-VBR is based on a traffic contract (connection admission control) and UBC (usage parameter control)

There is no feedback in these systems. Cells are simply discarded. This is called open-loop control.

This is not used for ABR or UBR traffic.

Non-real-time Traffic Management

Some applications (Web, file transfer) do not have well defined traffic characteristics

Best effortsAllow these applications to share unused capacity

If congestion builds, cells are dropped, eg UBR

Closed loop controlABR connections share available capacityShare varies between minimum cell rate (MCR) and

peak cell rate (PCR)ABR flow limited to available capacity by feedback

Buffers absorb excess traffic during feedback delay

Low cell loss

Feedback Mechanisms

Transmission rate characteristics:Allowed cell rateMinimum cell ratePeak cell rateInitial cell rate

Start with ACR=ICRAdjust ACR based on feedback from network

Resource management cellsCongestion indication (CI) bitNo increase (NI) bitExplicit cell rate (ER) field

Variations in Allowed Cell Rate

Cell Flow (RM = resource management)

23.7 Integrated Services23.7 Integrated Services

Integrated Services (IntServ) is a modelused to provide QoS in the Internetat the IP layer.

IntServ is a flow-based model, in thata user needs to create a flow or virtualcircuit between source and destination.

But IP is connectionless. How do youcreate a connection? Use RSVP.

Figure 23.19 Path messages

Path messages are sent from sender (S1) to all receivers(multiple if multi-cast). This establishes the path.

Figure 23.20 Resv messages

Once the path is set, receivers return Resv messages. Notehow reservations are merged (next slide).

Figure 23.21 Reservation merging

R3 takes the larger of the two reservations and sends thatupstream.

Figure 23.22 Reservation styles

Wild card filter - the router creates a single reservation for allthe senders, based on the largest request.Fixed filter - the router creates a distinct reservation for eachflow.Shared explicit - the router creates a single reservation which can be shared by a set of flows.

23.8 Differentiated Services23.8 Differentiated Services

An alternative to Integrated Services.

Produced by the IETF to create a class-based QoS modelfor IP.

Beyond the scope of this class.

Figure 23.24 Traffic conditioner

Meter checks to see if incoming flow matches negotiated trafficprofile. Marker can re-mark a packet that is using best-effortdelivery or down-mark a packet based on info received from meter.Shaper uses the info received from meter to reshape the traffic.Dropper works like a shaper with no buffer, discarding packets if the flow severely violates the negotiated profile.