Internet Routing - MPLS. By Richard Harris

Transcription

1 Internet Routing - MPLS By Richard Harris

2 MPLS Presentation Outline Introduction Problems of Internet size Methods for overcoming potential problems What is MPLS? Overview MPLS terminology MPLS Architecture The label stack Label Switching Protocols MPLS Signalling Protocols Traffic Management in MPLS Post Graduate Certificate in Professional Development Slide 2

3 Objectives To be able to describe the MPLS mode of operation To explain what is meant by traffic engineering of MPLS paths Post Graduate Certificate in Professional Development Slide 3

4 Introduction Huge growth in the Internet Figures vary from 2x to 10x per annum for traffic growth! Major ISPs (Internet Service Providers) have coped with this growth in a variety of different directions, viz: Network architecture Capacity expansion Traffic management methods ( Traffic Engineering ) Post Graduate Certificate in Professional Development Slide 4

5 Network Architecture This approach entails Development of scalable network architectures Defining suitable network topologies and structures Identification of abstract objects and classes within the network. Post Graduate Certificate in Professional Development Slide 5

6 Capacity Expansion Most ISPs started with backbone capacities in the range of 44Mbps (in the USA this is DS3 ) during By 1999, the capacities used were about 622Mbps A recent announcement shows that ISPs are now introducing capacities in the region of 9.9Gbps (referred to as OC-192c) and using DWDM (Dense WDM). Post Graduate Certificate in Professional Development Slide 6

7 Traffic Management/Engineering This direction is required because the previous two approaches are insufficient to solve the growth problem mentioned in the introduction! Architectures and capacity expansion cannot provide the high quality Internet services that are now being demanded by the many different customers of this network. Traffic engineering deals with performance optimisation issues and it has been realised how important this aspect is for operating such a huge multipurpose network. Post Graduate Certificate in Professional Development Slide 7

8 Traffic Engineering Limitations of the existing Internet - 1 Historically difficult to perform effective traffic engineering and management of Internet traffic. IP was conceived as a best effort service. Shortcomings of IP technologies: Lack of measurement capabilities Limitations of intra-domain routing control functions. Routing protocols like OSPF, EIGRP, IGP etc work on topology-driven per-packet progressive connection control. Each router makes independent decisions using a local version of a routing area link state database. Post Graduate Certificate in Professional Development Slide 8

9 Traffic Engineering Limitations of the existing Internet 2 Route selection is based on shortest path calculations using simple additive link metrics. This approach is highly distributed and scalable, but flawed, because these protocols do not consider the characteristics of the offered traffic and the network capacity constraints when making routing decisions. Subsets of network resources become congested. Alternate paths become under-utilised. This suggests that there is poor resource allocation taking place. Traffic engineering is required to fix these problems. MPLS to the rescue! Post Graduate Certificate in Professional Development Slide 9

10 What will MPLS do? Multi Protocol Label Switching (MPLS) is expected to improve the price/performance of network layer routing, improve the scalability of the network layer, provide greater flexibility in the delivery of (new) routing services (by allowing new routing services to be added without a change to the forwarding mechanism). Post Graduate Certificate in Professional Development Slide 10

11 What is MPLS? - 1 MPLS is a technique for combining the best features of IP and ATM, viz: IP routing ATM throughput It borrows heavily from methods used in ATM to rapidly transfer cells across the network. Supports destination oriented, multicast and hierarchical routing Supports explicit routing Label-transport, Label-stack Quality of Service (QoS) MPLS is not a specific layer 3 protocol it appears to lie between layers 2 and 3. MPLS supports many other layer 2 technologies (Frame Relay, ATM, Ethernet, Token Ring) Post Graduate Certificate in Professional Development Slide 11

12 What is MPLS? - 2 A key concept of MPLS is the separation of an IP router s functions into two parts: Forwarding Control Forwarding: Responsible for how data packets are relayed between the network routers. It uses label swapping similar to ATM switches that use Virtual Path/Virtual Circuit identifiers although MPLS may be regarded as more general than VPI/VCI. A label is a short fixed length number, independent of the network layer. Label swapping essentially involves a table lookup of a packet s label to determine its route and new label value. It is much simpler than previous methods. Post Graduate Certificate in Professional Development Slide 12

13 Some MPLS Terminology LDP: Label Distribution Protocol LSP: Label Switched Path FEC: Forwarding Equivalence Class LSR: Label Switching Router LER: Label Edge Router Post Graduate Certificate in Professional Development Slide 13

14 What is MPLS? - 3 A router capable of MPLS is known as a Label Switching Router (LSR). A set of Label Switching Routers traversed by a packet is called a Label Switched Path (LSP). A contiguous set of LSPs under a single administration is known as an MPLS domain. A packet is transmitted across an MPLS domain based only on its label. Packet Label Post Graduate Certificate in Professional Development Slide 14

15 The label can: MPLS Label use an existing layer 2 header label (eg the VPI/VCI field in the ATM cell header) or inserted between the layer 2 and IP headers as a small shim label like the following: Label Value S Time-to-Live (TTL) 20-bits: Label value used by LSR to lookup either next-hop, operation to perform, or outgoing data-link encapsulation Type of service 1-bit: "Bottom of Label Stack" Flag 8-bits: TTL decremented by each LSR Post Graduate Certificate in Professional Development Slide 15

16 Route at edge, switch in core IP IP #L1 IP #L2 IP #L3 IP IP Forwarding LABEL SWITCHING IP Forwarding Post Graduate Certificate in Professional Development Slide 16

17 Label Switched Path (LSP) Intf In Label In Dest Intf Out Label Out Intf In Label In Dest Intf Out Intf In Dest Intf Out Label Out IP IP Post Graduate Certificate in Professional Development Slide 17

18 Explicitly routed or ER-LSP Route= {A,B,C} #216 B #14 #972 A #14 #972 C #462 - ER-LSP follows a route that the source chooses. In other words, the control message to establish the LSP (label request) is source routed. Post Graduate Certificate in Professional Development Slide 18

19 Explicitly Routed LSP (ER-LSP) In tf In D e s t In tf O u t L a b e l O u t IP Intf In Label In Dest Intf Out Label Out Intf In Label In Dest Intf Out IP Post Graduate Certificate in Professional Development Slide 19

20 ER LSP - advantages Operator has routing flexibility (policy-based, QoS-based) Can use routes other than shortest path Can compute routes based on constraints in exactly the same manner as ATM based on distributed topology database. (traffic engineering) Post Graduate Certificate in Professional Development Slide 20

21 Stack is LIFO MPLS Label Stack Depth n stack illustrated below: Label 1 1 Last label Push stack Label 2 Label n-1 Label n Pop stack Post Graduate Certificate in Professional Development Slide 21

22 MPLS: Label Stack; Hierarchical Routing Company Network Company Network S D Leased Virtual Path Post Graduate Certificate in Professional Development Slide 22

23 Forwarding Equivalence Classes More general than Virtual Circuits in ATM. Key features: All packets divided into subsets called FECs based on: IP source address IP destination address IP protocol TCP/UDP source/destination ports Time to Live (TTL) Type of service fields Granularity can be varied if desired for packet forwarding criteria. Mapping of FEC done only once at entry to MPLS domain. Post Graduate Certificate in Professional Development Slide 23

24 Packet Processing Forwarding strictly according to labels! Label is removed at the egress LSR. Next Hop Label Forwarding Entries NHLFEs: Outgoing interface (next hop) Outgoing label (push/pop label stack) Data link encapsulation (optional) Information about resources (optional) Packet handling policies (optional) Post Graduate Certificate in Professional Development Slide 24

25 Quality of Service Routing The control part of MPLS consists of Network layer routing protocols to distribute routing information between LSRs; Label binding procedures for converting this routing information into the forwarding tables needed for label switching. MPLS is designed to work with existing routing protocols like OSPF, BGP etc. Major interest in QoS routing instead of just shortest path routing. Signalling is required to establish the routes. Post Graduate Certificate in Professional Development Slide 25

26 Signalling Methods in MPLS Two signaling options proposed in the standards: CR-LDP, RSVP extensions: CR-LDP = LDP + Explicit Route RSVP ext = Traditional RSVP + Explicit Route + Scalability Extension ITU has decided on LDP/CR-LDP for public networks. Survival of the fittest not such a bad thing although RSVP has lots of work in scalability to do. Post Graduate Certificate in Professional Development Slide 26

27 ER-LSP setup using RSVP 5. When LER A receives Resv, the ER established. 1. Path message. It contains ER path < B,C,D> 2. New path state. Path message sent to next node 4. New reservation state. Resv message propagated upstream 3. Resv message originates. Contain the label to use and the required traffic/qos para. Per-hop Path and Resv refresh unless suppressed LER A LSR B LSR C LER D Per-hop Path and Resv refresh unless suppressed Per-hop Path and Resv refresh unless suppressed Post Graduate Certificate in Professional Development Slide 27

28 Constraint Routing LDP (CR-LDP) CR = Constraint based Routing eg: USE: (links with sufficient resources AND (links of type somecolor ) AND (links that have delay less than 200 ms) & & = Post Graduate Certificate in Professional Development Slide 28

29 Requirements for Constraint Based Routing 1) A topology database that knows about link attributes. {a,b,c} z ANSWER: OSPF/ISIS + attribs{a,b,c} {a,b,c} 2) A label distribution protocol that goes where it s told. ANSWER: LDP + Explicit Route{x,y,m,z} z x y m z Post Graduate Certificate in Professional Development Slide 29

30 Traffic Engineering B C Demand A D Traffic engineering is the process of mapping traffic demand onto a network Network Topology Traffic engineering key to optimizing cost/performance Post Graduate Certificate in Professional Development Slide 30

31 Purpose of traffic engineering : Maximize utilization of links and nodes throughout the network Engineer links to achieve required delay, gradeof-service Spread the network traffic across network links, minimize impact of single failure Ensure available spare link capacity for re-routing traffic on failure Meet policy requirements imposed by the network operator Post Graduate Certificate in Professional Development Slide 31

32 MPLS Traffic Engineering Methods MPLS can use the source routing capability to steer traffic on desired path Operator may manually configure these in each LSR along the desired path - analogous to setting up PVCs in ATM switches Ingress LSR may be configured with the path, RSVP used to set up LSP - some vendors have extended RSVP for MPLS path setup Ingress LSR may be configured with the path, LDP used to set up LSP - many vendors believe RSVP is not suitable. Ingress LSR may be configured with one or more LSRs along the desired path, hop-by-hop routing may be used to set up the rest of the path - loose source routing, less configuration required. If desired for control, routes discovered by hop-by-hop routing can be frozen ( route pinning ) In the future, constraint-based routing will off-load traffic engineering tasks from the operator to the network itself. Post Graduate Certificate in Professional Development Slide 32