QoS Provision in an MPLS/DiffServ. Summary

Transcription

1 QoS Provision in an MPLS/DiffServ Network Χάρης Κωνσταντινίδης Νοέµβριος 2004 Summary MPLS Architectural description and basic concepts general aspects 1

2 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-all Rely on coarse attributes picking the best path Forwarding process can be rather complex - processing the entire IP header Host A Router Router B MPLS Domain Path 1 Path 2 Router A Router C Router D Router Host C Host B Router Router C Router F Router G 2

3 MPLS Architectural description and basic concepts MPLS Operation [1] Central concept behind MPLS: the label Packets 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. Forwarding based on IP Address Forwarding based on Labels Forwarding based on IP Address Sender Ingress LSR LSR Egress LSR MPLS Domain Receiver IP Packet IP Packet Label IP Packet Label IP Packet Label IP Packet 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 local significance. 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. 3

4 MPLS Architectural description and basic concepts MPLS Operation [3] Label size: 32 bits Schematic representation LSR D Ingress LSR A Ingress 8 18 Z 22 LSR B X 37 LSR C Ingress Y 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. 4

5 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 protocol Packet s source host Link on which the packet arrived Quality of service constraints Service levels agreements Current network conditions Virtual 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). 5

6 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. 6

7 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 distribution Downstream 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 7

8 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. Resilience Capabilities [1] The quality that a customer should receive when using a service is specified by SLA Typical SLA QoS-parameters for packet switched networks: packet or cell loss, delay, delay-jitter availability 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. 8

9 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 availability The percentage of time that the service can be used. Gradations of availability: complete, partial availability, not available 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. 9

10 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). 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). 10

11 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. 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 LD1.2 LD2.1 LD2.2 Failed link Backup MPLS tunnels T1.1 (one for each direction) T2.1 11

12 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) LSR2 Recovery path LSR4 LSR1 Working path LSR3 LSR5 LSR6 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. LSR2 LSR4 Recovery path LSR1 Working path LSR3 LSR5 LSR6 12

13 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. LSR2 Recovery path LSR4 LSR1 Working path LSR3 LSR5 LSR6 Resilience Capabilities [11] MPLS vs optical protection Drawbacks MPLS protection switching uses more IP ports which is expensive. Benefits Better utilization of the fibre capacity. More equipment is protected. 13

14 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: Routers Switches Buffers Transmission capacity Network Dimensioning[2]: Design issues taken into account: Protection scheme to be applied Traffic demand Routing scheme to be applied Traffic classifications 14

15 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: Bandwidth End-to-end delay Packet Delay and Jitter Packet Loss 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 serialization delay, propagation delay and switching (queuinginfluence 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. 15

16 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. 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 link New LSPs with higher priority are set up over a shared resource path pre-empting existing LSPs with lower priority. Equipment or link failure 16

17 Control Actions[2] Possible control actions: Protection switching (switching to a backup LSP in case of failure) ~ms Automatic LSPs Rerouting ~sec Manually controlled LSPs rerouting ~min OSPF weights reconfiguration ~min/hours LSPs characteristics modification ~min 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? 17

18 Control Actions[4] which congestion indicator?? congestion?? Congestion threshold congestion Time-span first detection time Parameters used for Congestion detection Packet loss ratio time Maximum packet delay (for real time traffic) Individual flow throughput (for data traffic) 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 techniques Predictive models (short term trends) 18

19 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 administrative bandwidth. 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 requirements A 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 VPN Decision on which classes of service per VPM A 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. 19

20 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. 20

21 Why MPLS? Speed Scalable Simple Traffic engineering QoS Support of services Conclusions 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

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