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QoSQoS Provision in an MPLS/Provision in an MPLS/DiffServDiffServ NetworkNetworkΧάρης Κωνσταντινίδης

Νοέµβριος 2004

Summary

MPLS Architectural description and basic concepts

QoS Management general aspects

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MPLS Architectural description and basic concepts

Definition

An improved method for forwarding packets through a network using information, contained in labels attached to IP packets. It combines the performance and capabilities of Layer 2 switching with the proven scalability of Layer 3 routing, thus creating flexible networks that provide performance and stability.

MPLS Architectural description and basic concepts

Why MPLS?MPLS addresses the main concerns with traditional IP routing concerns:

Winner-takes-allRely on coarse attributes picking the best pathForwarding process can be rather complex -processing the entire IP header

RouterHost

A

MPLS Domain

Router

RouterRouterA

RouterC

RouterF

RouterG

RouterC

RouterB

RouterD

HostB

HostC

Path 1Path 2

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MPLS Architectural description and basic concepts

MPLS Operation [1]Central concept behind MPLS: the labelPackets are assigned a label when they enter an MPLS network and the network uses that label, rather than an IP address to deliver packets to the destination.

IP P

acke

tLa

bel

IP P

acke

tLa

bel

IP P

acke

tLa

bel

LSRIngress LSR

Egress LSR

Sender

ReceiverMPLS Domain

Forwardingbased on

IP Address

Forwardingbased onLabels

Forwardingbased on

IP Address

IP P

acke

t

IP P

acke

t

MPLS Architectural description and basic concepts

MPLS Operation [2]Label vs IP address

Labels are numbers. Numbers that are used to forward packets. They are little like IP addresses. What is then the difference?Their scope: A legitimate IP address is unique in all the world while an MPLS label has only localsignificance. A given label value is only significant on a particular link between two LSRs. Label values can change as a packet traverses an MPLS network.

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MPLS Architectural description and basic concepts

MPLS Operation [3]Label size: 32 bits Schematic representation

X

LSR AIngress

Y

Z

LSR DIngress

LSR CIngress

LSR B

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18

22

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MPLS Architectural description and basic concepts

MPLS Operation [4]Forwarding Equivalence Class (FEC)A group of IP packets which are forwarded in the same manner (over the same path with the same forwarding treatment)

Characteristics:Set of IP packets Is eventually encoded as the label Is not a route or path. However, packets in a FEC and originating at a given point follow a route (or one set of routes)

FEC gives greater control over the forwarding behavior of the network.

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MPLS Architectural description and basic concepts

MPLS Operation [5]Unlike traditional IP forwarding which is generally based strictly on IP addresses and possibly the diffserv Codepoint, forwarding equivalence classes can take into account many different factors:

Packet’s application protocolPacket’s source hostLink on which the packet arrivedQuality of service constraintsService levels agreementsCurrent network conditionsVirtual private network requirements

MPLS Architectural description and basic concepts

MPLS Operation [6]Label Switched Routers (LSRs):Mapping <incoming interface, label> to <outgoing interface, label> Label Switched Paths (LSPs): Even though the actual label value may change as a packet travels across a network,the packet’s path through the network is completely determined by the initial label the ingress LSR assigns it. This complete path is known as the label switched path (LSP).

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MPLS Architectural description and basic concepts

MPLS Operation [7]Mapping table:Each router along the path maintains a mapping table. The table takes an incoming interface and label value, which then maps it to an outgoing interface and label value.Selecting Labels:When the ingress router assigns an initial label to a packet, that label determines the packet’s full path through the MPLS network. Ingress routers select a label by determining the packet’s forwarding equivalence class or FEC.

MPLS Architectural description and basic concepts

MPLS Operation [8]Distributing labels

Label distribution is the process by which the upstream and the downstream router reach the agreement on the meaning of all MPLS labels they exchange. General principles of label distribution protocols:The most important principle is that the downstream router picks the label value because it is the only way to ensure that a label value for an incoming link is unique.

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MPLS Architectural description and basic concepts

MPLS Operation [9]Even though downstream routers pick label values, the trigger that generates a new label can come from either router:

Downstream unsolicited label distributionDownstream on demand label distribution

MPLS Architectural description and basic concepts

MPLS Operation [10]Label stacks:

Label stacks allow the creation of nested label switched paths, in which one large LSP uses several smaller LSPs on the way to the destination. MPLS supports LIFO (last in first out) for label stacks. However, now LSRs have to do some more than mapping <incoming interface, label> to <outgoing interface, label>.LSRs must take into account stack processing.“Penultimate hop popping”

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QoS Management

Quality of service (QoS):QoS is defined as those mechanisms that give network administrators the ability to manage traffic’s bandwidth, delay and congestion throughout the network. To realize true QoS, its architecture must be applied end to end, and not just at the end or at selected network devices. It is that feature of the network by which it can differentiate between different classes of traffic and treat them differently.

QoS Management

Resilience Capabilities [1]The quality that a customer should receive when using a service is specified by SLATypical SLA QoS-parameters for packet switched networks:

packet or cell loss, delay, delay-jitteravailability of the service

Deterioration of service due to failures of network equipment (IP/MPLS routers, SDH equipment).During that time service is unavailable.Today’s customers put high demands.

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QoS Management

Resilience Capabilities [2]Availability of a network:The percentage of time that it actually can be used.

Network congestion availability (NCA)The percentage of time that the network between two points is available.Service availabilityThe percentage of time that the service can be used.

Gradations of availability:complete, partial availability, not available

QoS Management

Resilience Capabilities [3]Survivability of a network:

The ability of providing essential services in the presence of failures and recover full services in a timely manner.Availability is the result of survivability.Goal in network design:To provide end-to-end IP services with high availability at the lowest possible cost.

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QoS Management

Resilience Capabilities [4]Protection mechanisms:Used to increase the availability

Physical protection (use protected physical links)Consists of routing each of the protected IP links over two disjoint physical paths (primary and protection path with the required capacity).Duplicated physical required capacity.Low cost but provides only protection against link failures due to fibre cut (not against routers or router’s interfaces failures).

QoS Management

Resilience Capabilities [5]IP layer protection (duplication of routers and physical links)

Requires that the two IP links be routed over non-protected but disjoint physical paths.Same physical capacity as the previous method but duplication of routers and router’s interfaces as well.Significantly higher cost due to the cost of router’s interfaces (full protection implies duplication of transit routers).Drawbacks (not efficient utilization of the network, long reaction times – IP layer protocols).

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QoS Management

Resilience Capabilities [6]MPLS protection (by using redundant topology and MPLS Tunnels for link protection).

Pre-establish backup MPLS tunnels to protect critical links and to enable MPLS link protection with fast restoration on those links.Very fast reaction times (comparable to detection time of IP protocols). Keep the effect of the failure within a small portion of the network.

QoS Management

Resilience Capabilities [7]Without MPLS the failure would cause updating of the routing tables in the whole network.With MPLS full de-loading the LD2_LD1 path, which would take place in case of IP layer protocol, will be avoided.

LD1.1 LD2.1

T1.1 T2.1

LD1.2 LD2.2

Failedlink

Backup MPLS tunnels(one for each direction)

LD1.1 LD2.1

T1.1 T2.1

LD1.2 LD2.2

Failedlink

Backup MPLS tunnels(one for each direction)

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QoS Management

Resilience Capabilities [8]Global Repair Model (backup LSP utilization)

The ingress node is responsible for resolving the restoration.One backup path per working path (cost in terms of recovery time-continuity test for detection)

Working path

Recovery path

LSR1 LSR3

LSR4

LSR5

LSR2

LSR6

QoS Management

Resilience Capabilities [9]Local Repair Model

The restoration procedure starts from the point of failure.Multiple backup paths and a priori reservation of resources leads to inefficient utilization.

Working path

Recovery path

LSR1

LSR2 LSR4

LSR3 LSR5 LSR6

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QoS Management

Resilience Capabilities [10]Reverse Backup

Redirection of traffic back to the sender and use of alternate LSP.Suitable in network scenarios where the traffic streams are very sensitive to packet losses.Drawback the time needed to reverse.

LSR1 Working path

Recovery path

LSR3 LSR5 LSR6

LSR2 LSR4

QoS Management

Resilience Capabilities [11]MPLS vs optical protectionDrawbacks

MPLS protection switching uses more IP ports which is expensive.

BenefitsBetter utilization of the fibre capacity. More equipment is protected.

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QoS Management

Network Dimensioning[1]:Refers to that part of the network planning process responsible for the evaluation of resources required in the network to support the expected amount of traffic with the requested QoS.Network elements taken into account:

RoutersSwitches Buffers Transmission capacity

QoS Management

Network Dimensioning[2]:Design issues taken into account:

Protection scheme to be appliedTraffic demandRouting scheme to be appliedTraffic classifications…

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QoS Management

Traffic and QoS measurements[1]How and which parameters should be monitored to provide QoS in an MPLS network.QoS deployment intends to provide a connection with certain performance bounds from the network by measuring the following key parameters:

BandwidthEnd-to-end delayPacket Delay and JitterPacket Loss

QoS Management

Traffic and QoS measurements[2]Bandwidth: describes the rated throughput capacity of a given medium, protocol or connection. It describes the required “size of the pipe”.End to end delay: is the average time it takes for a network packet to traverse the network from one endpoint to the other and is consisted of serializationdelay, propagation delay and switching (queuing-influence when network is congested) delay.Jitter: is the variation in the end-to-end delay of sequential packets.Packet loss: is measured as the percent of transmitted packets that never reach the intended destination.

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QoS Management

Traffic and QoS measurements[3]MMC (measuring, monitoring, control) framework in the QoS field.It is the means to provide differentiated service and to ensure that traffic profiles and SLAs are followed. Traffic monitoring is the process of observing traffic characteristics at a given point in the network and collect traffic information for analysis.

Investigates which metrics and properties of the network are the most vital.Find appropriate way of measuring these properties without getting misleading results.Evaluate the results and apply appropriate policies.

QoS Management

Control Actions[1]Real time QoS management by analyzing the different control actions that can be activated when congestion is detected.Control actions can be invoked for various reasons:

High load on the linkNew LSPs with higher priority are set up over a shared resource path pre-empting existing LSPswith lower priority.Equipment or link failure

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QoS Management

Control Actions[2]Possible control actions:

Protection switching (switching to a backup LSP in case of failure) ~msAutomatic LSPs Rerouting ~secManually controlled LSPs rerouting ~minOSPF weights reconfiguration ~min/hoursLSPs characteristics modification ~min

QoS Management

Control Actions[3]Information required for performing control actions.Control actions could be triggered by one or more congestion indicators crossing a threshold value. Control action is useful only if the duration of the congestion is significantly longer than the control reaction time.Two main questions:

When is the network congested?How long this situation is likely to continue?

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QoS Management

Control Actions[4]

Parameters used for Congestion detectionPacket loss ratioMaximum packet delay (for real time traffic)Individual flow throughput (for data traffic)

which congestionindicator ??

first detection time

Congestion threshold

congestion??

Congestion threshold

congestion??

time

congestion Time-span

QoS Management

Control Actions[5]Estimation of congestion duration

External information: congestion appears after automatic protection switching and apply of rerouting mechanisms → equipment failure.On the basis of the present and past status of the network using some predictive models.

Exponential smoothing techniquesPredictive models (short term trends)

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QoS Management

Control Actions[6]Suitability of the different control actions:

LSPs re-routing: move some traffic from the congested link to under utilized links.LSPs policing activation: if the overload is merely due to MPLS tunnels exceeding their administrativebandwidth.LSPs characteristic modification: modification of the administrative bandwidth of an MPLS tunnel (useful to find the actual traffic before rerouting).Schedulers re-configuration: tuning of the link bandwidth to the actual characteristics of the offered traffic (severe congestion conditions of valuable traffic).

Requirements for QoS management systems

MPLS VPNs specific requirementsA VPN is a set of administrative policies that control both connectivity and QoS among sites.Area of QoS: the challenge is to support a wide range of VPN customers:

Multiple classes of service per VPNDecision on which classes of service per VPMA class of service provided to an application in a VPN could be different from the class of service that the same application uses in another VPN.

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Conclusions

QoS Aspects to be considered:Resilience: Resilience is an important aspect of the network. Besides that a network should provide the promised QoS when all network elements are functioning and should also be able to provide service while failures occur.Proper network dimensioning of network resources is the first step required to ensure that the network is able to fulfill the QoS requirements of the different services under different operating conditions.

Conclusions

QoS Aspects to be considered:Traffic and QoS measurement:Investigation of the most important metrics and properties of the network is vital. Control actions:Congestion, defined as a situation in which some of the supported services experience a certain level of performance degradation. Several control actions exist to detect and handle these situations.

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Conclusions

Why MPLS?SpeedScalableSimpleTraffic engineeringQoSSupport of services

References

IP switching and routing essentials, Stephen A. Thomas [WILEY,2002].MPLS and Label Switching Networks,Uyless Black [Prentice Hall PTR, 2001].Selected QoS provision in an MPLS/DiffServ Internet – Saltmamontes, [Eurescom, 2003].QoS Online Routing and MPLS Multilevel Protection: A Survey.Jose L. Marzo, Eusebi Calle, Caterina Scoglio, Tricha Anjali, IEEE Communications Magazine , October 2003.

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