Introduction to MPLS and Traffic Engineering

1. Introduction to MPLS and Traffic Engineering Zartash Afzal Uzmi

2. Outline • Traditional IP Routing • Forwarding and routing • Problems with IP routing • Motivations behind MPLS • MPLS Terminology and Operation • MPLS Label, LSR and LSP, LFIB Vs FIB • Transport of an IP packet over MPLS • More MPLS terminology • Traffic Engineering [with MPLS] • Nomenclature • Requirements • Examples Lahore University of Management Sciences

3. Outline • Traditional IP Routing • Forwarding and routing • Problems with IP routing • Motivations behind MPLS • MPLS Terminology and Operation • MPLS Label, LSR and LSP, LFIB Vs FIB • Transport of an IP packet over MPLS • More MPLS terminology • Traffic Engineering [with MPLS] • Nomenclature • Requirements • Examples Lahore University of Management Sciences

4. Forwarding and routing • Forwarding: • Passing a packet to the next hop router • Routing: • Computing the “best” path to the destination • IP routing – includes routing and forwarding • Each router makes the forwarding decision • Each router makes the routing decision • MPLS routing • Only one router (source) makes the routing decision • Intermediate router make the forwarding decision Lahore University of Management Sciences

5. IP versus MPLS routing • IP routing • Each IP datagram is routed independently • Routing and forwarding is destination-based • Routers look at the destination addresses • May lead to congestion in parts of the network • MPLS routing • A path is computed “in advance” and a “virtual circuit” is established from ingress to egress • An MPLS path from ingress to egress node is called a label switched path (LSP) Lahore University of Management Sciences

6. How IP routing works Searching Longest Prefix Match in FIB (Too Slow) Lahore University of Management Sciences

7. Problems with IP routing • Too slow • IP lookup (longest prefix matching) “was” a major bottleneck in high performance routers • This was made worse by the fact that IP forwarding requires complex lookup operation at every hop along the path • Too rigid – no flexibility • Routing decisions are destination-based • Not scalable in some desirable applications • When mapping IP traffic onto ATM Lahore University of Management Sciences

8. IP routing rigidity example D 1 1 A S A B B 1 2 C • Packet 1: Destination A • Packet 2: Destination B • S computes shortest paths to A and B; finds D as next hop • Both packets will follow the same path • Leads to IP hotspots! • Solution? • Try to divert the traffic onto alternate paths Lahore University of Management Sciences

9. IP routing rigidity example D 1 4 A S A B B 1 2 C • Increase the cost of link DA from 1 to 4 • Traffic is diverted away from node D • A new IP hotspot is created! • Solution(?): Network Engineering • Put more bandwidth where the traffic is! • Leads to underutilized links; not suitable for large networks Lahore University of Management Sciences

10. Motivations behind MPLS • Avoid [slow] IP lookup • Led to the development of IP switching in 1996 • Provide some scalability for IP over ATM • Evolve routing functionality • Control was too closely tied to forwarding • Evolution of routing functionality led to some other benefits • Explicit path routing • Provision of service differentiation (QoS) Lahore University of Management Sciences

11. IP routing versus MPLS routing Traditional IP Routing Multiprotocol Label Switching (MPLS) 1 2 S D 3 4 5 MPLS allows overriding shortest paths! Lahore University of Management Sciences

12. Outline • Traditional IP Routing • Forwarding and routing • Problems with IP routing • Motivations behind MPLS • MPLS Terminology and Operation • MPLS Label, LSR and LSP, LFIB Vs FIB • Transport of an IP packet over MPLS • More MPLS terminology • Traffic Engineering [with MPLS] • Nomenclature • Requirements • Examples Lahore University of Management Sciences

13. MPLS label • To avoid IP lookup MPLS packets carry extra information called “Label” • Packet forwarding decision is made using label-based lookups • Labels have local significance only! • How routing along explicit path works? Label IP Datagram Lahore University of Management Sciences

14. Routing along explicit paths • Idea: Let the source make the complete routing decision • How is this accomplished? • Let the ingress attach a label to the IP packet and let intermediate routers make forwarding decisions only • On what basis should you choose different paths for different flows? • Define some constraints and hope that the constraints will take “some” traffic away from the hotspot! • Use CSPF instead of SPF (shortest path first) Lahore University of Management Sciences

15. 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Label | Exp|S| TTL Label = 20 bits Exp = Experimental, 3 bits S = Bottom of stack, 1bit TTL = Time to live, 8 bits Label, LSP and LSR • Label • Router that supports MPLS is known as label switching router (LSR) • An “Edge” LSR is also known as LER (edge router) • Path which is followed using labels is called LSP Lahore University of Management Sciences

16. LFIB versus FIB • Labels are searched in LFIB whereas normal IP Routing uses FIB to search longest prefix match for a destination IP address • Why switching based on labels is faster? • LFIB has fewer entries • Routing table FIB has very large number of entries • In LFIB, label is an exact match • In FIB, IP is longest prefix match Lahore University of Management Sciences

17. Mpls Flow Progress D R1 LSR4 R2 LSR1 D destination LSR6 LSR3 LSR2 R1 and R2 are regular routers LSR5 1 - R1 receives a packet for destination D connected to R2 Lahore University of Management Sciences

18. Mpls Flow Progress D R1 LSR4 R2 LSR1 D destination LSR6 LSR3 LSR2 LSR5 2 - R1 determines the next hop as LSR1 and forwards the packet (Makes a routing as well as a forwarding decision) Lahore University of Management Sciences

19. Mpls Flow Progress R1 LSR4 R2 LSR1 31 D D destination LSR6 LSR3 LSR2 LSR5 3 – LSR1 establishes a path to LSR6 and “PUSHES” a label (Makes a routing as well as a forwarding decision) Lahore University of Management Sciences

20. Mpls Flow Progress R1 LSR4 R2 LSR1 D destination LSR6 LSR3 17 D LSR2 Labels have local signifacance! LSR5 4 – LSR3 just looks at the incoming label LSR3 “SWAPS” with another label before forwarding Lahore University of Management Sciences

21. MPLS Flow Progress R1 LSR4 R2 LSR1 D destination LSR6 LSR3 17 D LSR2 Path within MPLS cloud is pre-established: LSP (label-switched path) LSR5 5 – LSR6 looks at the incoming label LSR6 “POPS” the label before forwarding to R2 Lahore University of Management Sciences

22. MPLS and explicit routing recap • Who establishes the LSPs in advance? • Ingress routers • How do ingress routers decide not to always take the shortest path? • Ingress routers use CSPF (constrained shortest path first) instead of SPF • Examples of constraints: • Do not use links left with less than 7Mb/s bandwidth • Do not use blue-colored links for this request • Use a path with delay less than 130ms Lahore University of Management Sciences

23. CSPF • What is the mechanism? • First prune all links not fulfilling constrains • Now find shortest path on the rest of the topology • Requires some reservation mechanism • Changing state of the network must also be recorded and propagated • For example, ingress needs to know how much bandwidth is left on links • The information is propagated by means of routing protocols and their extensions Lahore University of Management Sciences

24. Data More MPLS terminology Upstream Downstream 172.68.10/24 LSR1 LSR2 Lahore University of Management Sciences

25. Use label 5 for destination 171.68.32/24 MPLS Data Packet with label 5 travels Label advertisement • Always downstream to upstream label advertisement and distribution Downstream Upstream 171.68.32/24 LSR2 LSR1 Lahore University of Management Sciences

26. Sends label ONLY after receiving request Sends label Without any Request Request For label Label advertisement • Label advertisement can be downstream unsolicited or downstream on-demand Downstream Upstream 171.68.32/24 LSR2 LSR1 Downstream Upstream 171.68.32/24 LSR1 LSR2 Lahore University of Management Sciences

27. Egress LSR Ingress LSR Label distribution • Label distribution can be ordered or unordered • First we see an example of ordered label distribution Label Lahore University of Management Sciences

28. Egress LSR Ingress LSR Label distribution • Label distribution can be ordered or unordered • Next we see an example of unordered label distribution Label Label Lahore University of Management Sciences

29. Label retention modes • Label retention can be conservative or liberal ? Destination Label LSR1 Label Lahore University of Management Sciences

30. Label operations • Advertisement • Downstream unsolicited • Downstream on-demand • Distribution • Ordered • Unordered • Retention • Liberal • Conservative Lahore University of Management Sciences

31. Outline • Traditional IP Routing • Forwarding and routing • Problems with IP routing • Motivations behind MPLS • MPLS Terminology and Operation • MPLS Label, LSR and LSP, LFIB Vs FIB • Transport of an IP packet over MPLS • More MPLS terminology • Traffic Engineering [with MPLS] • Nomenclature • Requirements • Examples Lahore University of Management Sciences

32. Traffic Engineering Traffic Engineering with MPLS (Application of CSPF)

33. What is traffic engineering? • Performance optimization of operational networks • optimizing resource utilization • optimizing traffic performance • reliable network operation • How is traffic engineered? • measurement, modeling, characterization, and control of Internet traffic • Why? • high cost of network assets • service differentiation Lahore University of Management Sciences

34. Traffic engineering • Recall the IP hotspot problem • The ability to move traffic away from the shortest path calculated by the IGP (such as OSPF) to a less congested path • IP: changing a metric will cause ALL the traffic to divert to the less congested path • MPLS: allows explicit routing (using CSPF) and setup of such explicitly computed LSPs Lahore University of Management Sciences

35. MPLS-TE: How to do it? • LSPs are set up by LSRs based on information they learn from routing protocols (IGPs) • This defeats the purpose! • If we were to use “shortest path”, IGP was okay Lahore University of Management Sciences

36. MPLS TE: How we actually do it? • MPLS TE Requires: • Enhancements to routing protocols • OSPF-TE • ISIS-TE • Enhancement to signaling protocols to allow explicit constraint based routing • RSVP-TE and CR-LDP • Constraint based routing • Explicit route selection • Recovery mechanisms defined Lahore University of Management Sciences

37. Signaling mechanisms • RSVP-TE • Extensions to RSVP for traffic engineering • BGP-4 • Carrying label information in BGP-4 • CR-LDP • A label distribution protocol that distributes labels determined based on constraint based routing • RSVP-TE and CR-LDP both do label distribution and path reservation – use any one of them! Lahore University of Management Sciences

38. RSVP-TE Basic flow of LSP set-up using RSVP Lahore University of Management Sciences

39. RSVP-TE PATH Message • PATH message is used to establish state and request label assignment • R1 transmits a PATH message addressed to R9 Lahore University of Management Sciences

40. RSVP-TE RESV Message • RESV is used to distribute labels after reserving resources • R9 transmits a RESV message, with label=3, to R8 • R8 and R4 store “outbound” label and allocate an “inbound” label. They also transmits RESV with inbound label to upstream LSR • R1 binds label to forwarding equivalence class (FEC) Lahore University of Management Sciences

41. Rerouting LSP tunnels • When a more “optimal” route/path becomes available • When a failure of a resource occurs along a TE LSP • Make-before-break mechanism • Adaptive, smooth rerouting and traffic transfer before tearing down the old LSP • Not disruptive to traffic Lahore University of Management Sciences

42. Recovering LSP tunnels LSP Set-up Lahore University of Management Sciences

43. Protection LSP set up Lahore University of Management Sciences

44. Protection LSP Lahore University of Management Sciences

45. References • RFC 2702 “Requirements for Traffic Engineering Over MPLS” • RFC 3031 “Multiprotocol Label Switching Architecture” • RFC 3272 “Overview and Principles of Internet Traffic Engineering” • RFC 3346 “Applicability Statement for Traffic Engineering with MPLS” • MPLS Forum (http://www.mplsforum.org) Lahore University of Management Sciences