MPLS

1. MPLS Topics: Introduction to MPLS Tutorial Questions and Recommended Reading

2. Routing Without MPLS • Large organizational networks are heavily subnetted for structuring purposes • Packets crossing such a network could transit many routers • Connectionless nature of IP poses two challenges • Per-packet processing • Distributed routing (Per hop behaviour PHB) • Core routers needs to forward tens of millions of packets per second • Must process each packet in a few nanoseconds • Potential for congestion in router forwarder (packet switch) • Packets follow the best path according to the routing table in each router • No opportunity for setting end-to-end path • Could override with static routes • But this approach doesn’t scale

3. BGP Routers to Service Provider Service Provider’s Transit Network Enterprise Network Enterprise Network Normal Subnet Routers Normal IP Transit Network • Connecting widely separated parts of an enterprise network • ‘Enterprise’: a large (national, international) company, organization • a number of sites (campuses, branches, offices)

4. IP vs. MPLS Transit Networks Service Provider’s Transit Network IP Normal IP routers Enterprise Network Enterprise Network Routed Path MPLS MPLS-enabled routers MPLS transit network Enterprise network (routed normally) Enterprise network (routed normally) Label Switched Path

5. Benefits of MPLS • Lower transit delay • Core routers switch not route • Traffic engineering • Packets can take non-standard path(OSPF path) • Scalability • Labels can be nestedto facilitate networkhierarchy • Flexibility • Can be used over • LANs • PPP tunnels • [ATM & Frame Relay backbones]

6. MPLS Goal: Dynamic Switched Flows • Original goal: bypass routing table lookup for as many packets as possible • Dynamically detect packet flows • Identified by unique pairs of IP addresses and port numbers • Switch, rather than route, packets on known flows • Cisco called this “route once, switch many” • Original approach based on two TCP-related assumptions • Majority of IP packets belong to TCP sessions • Rather than UDP datagram streams • TCP sessions have (relatively) long duration • File transfers, conferencing • Increasing popularity of Web browsing undermined this goal • Uses short-duration sessions • Per-flow path setup doesn’t scale

7. MPLS Goal: Dynamic Switched Flows(continued) MPLS transit network MPLS-enabledrouters Enterprise network (routed normally) Enterprise network (routed normally) Packet flowfollowing switched path Flow-detecting MPLS routers

8. MPLS Goal: Traffic Engineering • Determine end-to-end path for given packet flows • Override routing protocol decision where administratively appropriate • Allows routing policy to be set • Reflect service offerings • Low-delay path for voice traffic • More secure path for certain customers • Now seen as most important reason for using MPLS MPLS transit network MPLS routers Predetermined path

9. Multiprotocol Label Switching • Overview • Label Switching • Operation

10. Label Switching • Labels packets for faster switching through network • Connection-oriented protocols use virtual circuit ID • Frame relay DLCI • ATM VPI/VCI • Connectionless protocols need to add label • VLAN identifier (802.1Q trunking) • MPLS label (added to Ethernet and PPP) • Switches set up paths as required • Associate labels with paths • Use label as route-table lookup • Labels often have only link-by-link significance • Allows switch to differentiate incoming flows • Each switch maps label values predictably for outgoing flows DLCI = data-link connection identifierVCI = virtual channel identifierVPI = virtual path identifier

11. Label Switching Routers • MPLS-enabled devices are called Label Switching Routers (LSRs) • Multilayer switches enhanced with MPLS protocols • MPLS identifies two roles for LSRs • Edge LSRs and Core LSRs • Edge LSRs often calledLabel Edge Routers (LERs) • Edge LSRs • Determine packet path and perform flow classification • Assign unique labels to each flow • Core LSRs • Use label values to switch packets over cut-through paths • Layer 2 forwarding bypasses normal routing function Edge LSR Edge LSR Core LSRs

12. Multiprotocol Label Switching • Overview • Label Switching • Operation

13. Operational Overview I • Identify groups of packets of packets travelling over a common path • For example, towards the same destination network or host • Called a forwarding equivalence class • Assume they have common forwarding requirements and assign a label to each group • Encapsulate with label header carrying same label value • Communicate label settings to downstream router • Downstream router assigns label to outgoing FEC and communicates downstream • And so on

14. Operational Overview II • Once LSP has been set up routers process FEC accordingly • Ingress LSR (ingress LER) adds label to packet • Core LSRs match incoming labels to route table, which gives output port • Outgoing label map applies downstream label value • As previously communicated to downstream router • Bypassing conventional packet-by-packet, hop-by-hop L3 processing • Egress LSR (egressLER) removes it • Set of label mappings for a group constitutes the label switched path (LSP) for that FEC

15. Label Switching r c c q b b b b 5 1 s x s x 6 2 y y z z s m m 7 3 a p a 8 4 Port 5label map Port 5label map Port 1route table

16. Label Switched Path (LSP) • MPLS identifies two types of label switched path • Destination- based(follow the OSPF path) • Explicitly routed( follow the way u determined) • (Cisco terminology; descriptive) • Destination-based LSP follows conventional forwarding path • As determined by IP routing table • Originally set up from destination LER source LER • (Why is that?) • Explicitly routed LSP use source-specified path (source routing) • Path set up from source LER destination LER • Useful for overriding normal route selectionbased on least cost path • E.g. for enforcing route selection (‘routing policy’)

17. LSP Set-Up • LSR is essentially an MPLS-enabled multi-layer switch • Routing database constructed in normal way • Using, for example, OSPF • Then made available to MPLS switching engine • Switching engine assigns labels to forwarding paths • Sends route/label mappings to next-hop neighbour using a/the Label Distribution Protocol (LDP)( the path are renewed periodically) • Once LSP set up, packets are label-switched(packets can flow) • Conventional, in most cases, for router prior to egress LSR to remove label • Avoids processing load on LSR • Called penultimate hop-popping

18. May pop label Edge LSR Edge LSR Label Switching Path (LSP) Core LSRs How MPLS Works(continued) Normal IP packet Normal IP packet Labelled packet

19. MPLS Header • Header is 32 bits (4 octets) • Label field is 20 bits • Three-bit EXPerimental field can be used to carry (some) DiffServ markings through MPLS network • S = 1 indicates bottom of stack • Time To Live (TTL) is decremented by LSRs to maintain usual packet hop count 3 1 8 Number of bits 20 Label S EXP TTL

20. Label in FR DLCI field CRC CRC LAN/PPPheader Label in ATM VPI/VCI fields IP part-payload IP payload IP payload Label inserted as additional header Label Encapsulation Schemes • Format: 32 bits added to beginning of packet (prepended) • 20-bit label • Remaining bits used for variety of purposes • Two ways of inserting label • Add as protocol shim to existing header • PPP and switched LANs • usual method • Map onto existing virtual circuit identifier • ATM or Frame Relay • rare: included for completeness • Once the label is in place, established schemes can be used • E.g. label multiplexing and switching

21. Label Stacking I • Can add further label in front of (‘on top of’) the existing one(s) • Nested labels treated as stack • Hence use of term ‘pop’ • Network switches on top-most label • Allows several LSPs to be grouped for forwarding purposes • Provided they can be treated as a single FEC • E.g. all heading to same edge-point • Can continue the process, grouping groups together into a further group, with a new label • Each LSP marked by pair of label edge routers and a label in the stack • Ingress LER pushes new label onto stack • Egress LER pops label off the stack

22. Label Stacking II LERs LSP LSP LERs

23. MPLS Summary I • Multiprotocol Label Switching (MPLS) • Applies label switching to IP networks • Facilitates • Unequal cost load balancing • Setting routing policies (traffic engineering) • Virtual Private Networks (VPNs) • Bypasses potential bottlenecks causes by large route table look-ups • Allows provider network nesting though label stacking • Allows label mapping to be communicated in variety of ways • How do u communicate label route information across the network • LDP(label distribution protocol) • OSPF and BGP enhancements • RSVP • Details of how to recover from link failure still being finalized • Generalised MPLS: paths over SONET/SDH & wavelengths (‘s’) in WDM networks

24. Tutorial topics: MPLS • What are the similarities between MPLS label switching and FR/ATM switching? • What (if any) are the differences? • Look up “penultimate hop popping”. What is it and what does it achieve? • Why is a ‘destination-based’ MPLS path set up from destination LSR back towards source LSR? • Hint Remember path is unidirectional: think about label mapping

25. STUDY QUESTION Q1(i) What does the term label switching mean? Explain, using appropriate diagrams, how MPLS applies this principle to IP packets and the main advantages it gives. [5] (ii) Figure 1 shows part of an OSPF network. The network administrator notices that traffic from the remote site LANs frequently congests the route to Head Office. Explain why this is, and describe, with examples, how MPLS could be used to overcome this problem. [5]

26. FIGURE 1

27. Q 2. Explain what is meant by a Forwarding Equivalence Class and a Label Switched Path in MPLS. What is meant by 'DiffServ-enabled MPLS'? Explain briefly what MPLS labelling arrangements you would expect to see for a set of enterprise VPNs, where each VPN supports its own two DiffServ per-hop behaviours (PHBs). Would this change, and if so, how, if within each VPN, an enterprise also used MPLS to traffic engineer the routing of traffic with a different per-hop behaviour? Explain your answer. [6]