2. This teaching material is a part of e-Photon/ONe Master study in Optical Communications and Networks Course and module: Author(s): This tutorial is licensed under the Creative Commons creativecommons.org/licenses/by-nc-sa/3.0/ http://www.e-photon-one.org Optical Core Networks MPLS - basics Piero Castoldi, Scuola Superiore Sant’Anna, castoldi@sssup.it

3. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 2 (47) Outline MPLS fundamentals Label Encapsulation Label Distribution methods CREDIT: some figures are taken from the presentation “MPLS tutorial” by Peter Ashwood- Smith Bilel N. Jamoussi

4. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 3 (47) What is MPLS?  MPLS stands for “Multi-Protocol Label Switching”  MPLS is an IETF–specified framework that provides for the efficient control of traffic flows through the network regardless of transport media.  MPLS controls the way of mapping Layer 3 data flow onto Layer 2 traffic between adjacent network nodes without concern how Layer 2 or Layer 3 traffic is transported (That’s why it called ‘Multiple Protocol’)  MPLS supports the IP, ATM, and frame-relay Layer-2 protocols, even though it is appreciate as a more effective means of deploying IP networks across ATM-based WAN backbones.  MPLS incorporate best properties in both packet routing (IP) and circuit switching (ATM) Packet Routing Hybrid Circuit switching IP ATM MPLS + IP

5. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 4 (47) Multi Protocol Label Switching (MPLS) fundamentals

6. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 5 (47) “Label Substitution”, what is it? (1) BROADCAST : Go everywhere, stop when you get to B, never ask for directions. HOP BY HOP ROUTING : Continually ask who’s closer to B go there, repeat … stop when you get to B. “Going to B? You’d better go to X, it is on the way”. SOURCE ROUTING : Ask for a list (that you carry with you) of places to go that eventually lead you to B. “Going to B? Go straight 5 blocks, take the next left, 6 more blocks and take a right at the lights”. One of the many ways of getting from A to B:

7. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 6 (47) Label Substitution, what is it? (2) Have a friend go to B ahead of you using one of the last two techniques. At every road (link) he reserves a lane just for you. At every intersection (node) they post a big sign that says for a given lane which way to turn and what new lane to take. LANE#1 LANE#2 LANE#1 TURN RIGHT USE LANE#2

8. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 7 (47) A label by any other name... There are many examples of label substitution protocols already in existence. ATM - label is called VPI/VCI and travels with cell. Frame Relay - label is called a DLCI and travels with frame. TDM - label is called a timeslot its implied, like a lane. X25 - a label is an LCN Proprietary TAG etc.. GMPLS allows to use a “color substitution” where label is a light frequency (color)..

9. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 8 (47) What is a “LABEL”? A property that uniquely identifies a flow on a logical or physical interface

10. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 9 (47) Label Switched Path (LSP) #7 #99 #9 #3 Right #7 #99 RIGHT #9 #7 LEFT #99 #9 LEFT #4072 #3 IP #4072 IP

11. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 10 (47) Optical or Generalized Label Switched Path (G-LSP) RED RIGHT BLUE WHITE RIGHT ORANGE BLUE LEFT WHITE ORANGE LEFT RED IP RED BLUE WHITE ORANGE RED

12. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 11 (47) Label concept Value: Label value 20 bits Exp: Experimental Use, 3 bits S: Bottom of stack, 1 bit TTL: Time To Live, 8 bits Value: Label value 20 bits Exp: Experimental Use, 3 bits S: Bottom of stack, 1 bit TTL: Time To Live, 8 bits Total: 32 bit = 4 byte ValueExpS TTL IP packet MPLS label MPLS generates a short fixed-length label that acts as a shorthand representation of an IP packet’s header The label is attached in front of a IP packet. ! Packets are switched, not routed, based on labels

13. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 12 (47) Basic operation LER LSR Relative meaning of label (only within the link): each MPLS-capable router (LSR) changes the packet label LSR: Label Switching Router LER: Label Edge Router (Useful term not in standards) Ingress Router and Egress Router LSR: Label Switching Router LER: Label Edge Router (Useful term not in standards) Ingress Router and Egress Router IP forwarding Label Switching IP1 #L1 IP1 #L2 IP1 #L3 IP1

14. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 13 (47) FEC Forwarding Equivalence Class IP1 #L1 FEC = “A subset of packets that are all treated the same way by an edge router” The concept of FECs provides for a great deal of flexibility and scalability In conventional routing, a packet is assigned to a FEC at each hop (i.e. L3 look-up), in MPLS it is only done once at the network ingress Packets are destined for different address prefixes, but can be mapped to common path Packets are destined for different address prefixes, but can be mapped to common path IP2 #L1 IP1 #L2 IP2 #L2 IP1 #L3 IP2 #L3 LER LSR IP1 IP2

15. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 14 (47) Label stacking  Hierarchical use of the labels  Only outer label is used to forward packets  Creation of tunnel between non-neighbouring router => MPLS Domain  Scalability: the expansion of the network doesn’t increase the number of labels => This drastically reduces the size of routing tables in LSRs IPL1L2L3 … MPLS Domain 1 MPLS Domain 2 MPLS Domain 3

16. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 15 (47) MPLS features  Label swapping:  Bring the speed of layer 2 switching to layer 3  Separation of forwarding plane and control plane  Forwarding hierarchy via Label stacking  Increase the scalability  Constraint-based routing  Traffic Engineering  Fast reroute  Facilitate the virtual private networks (VPNs)  Enables Traffic Engineering and QoS  Provides an opportunity for mapping DiffServ fields onto an MPLS label  Facilitate the elimination of multiple layers  Resolve the problems of IP over ATM, in particular: Complexity of control and management and scalability issues

17. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 16 (47) So what is MPLS? Hop-by-hop or source routing to establish labels Possible use of labels native to the media (colors) Multi level label substitution transport

18. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 17 (47) Routers Do Both Routing and Switching Routing  Deciding the next hop based on the destination address.  A Layer 3 (L3) function. Switching  Moving a packet from an input port to an output port and out.  A layer 2 function. INPUT PORTSOUTPUT PORTS So we can avoid performing the layer 3 function. What benefit does this provide? In what situations would this benefit not be very significant?

19. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 18 (47) MPLS: Flexible Forwarding LSP to IP LABEL SWITCHING IP to LSP IP #L1IP#L2IP#L3 IP DA IP : Packets are forwarded based on Destination Address (DA) MPLS: Route at edge and switch in core Map packets to LSP based on (Source Address, Destination Address, protocol, port, DSCP, interface, etc.) and forward packets based Label IPDAIPDAIPDAIPDA

20. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 19 (47) MPLS-based Solutions IP Traffic Engineering  Constraint-based Routing making routing adapt to latest network loading Virtual Private Networks  Controllable tunneling mechanism L2/L3 Integration  Easy software implementation in current routers L1/L3 Integration  Use of MPLS to control Optical Cross Connects (OXC) -> GMPLS Enable QoS in IP Networks  Support IP Diffserv + ATM-style QoS

21. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 20 (47) MPLS Terminology LDP: Label Distribution Protocol LSP: Label Switched Path FEC: Forwarding Equivalence Class LSR: Label Switching Router LER: Label Edge Router (useful term not in standards), can be Ingress Router, Egress Router, Transit Router

22. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 21 (47) #216 #612 #5 #311 #14 #99 #963 #462 - An LSP is actually part of a tree from every source to that destination (unidirectional). - LDP builds that tree using existing IP forwarding tables to route the control messages. #963 #14 #99 #311 Label Switched Path (LSP)

23. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 22 (47) Topology dissemination in standard IP 47.1 47.2 47.3 1 2 3 1 2 3 1 2 3 Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.

24. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 23 (47) IP forwarding using hop-by-hop control 47.1 47.2 47.3 IP 47.1.1.1 1 2 3 1 2 1 2 3

25. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 24 (47) MPLS Label Distribution use-case 47.1 47.2 47.3 1 2 3 1 2 1 2 3 3 Mapping: 0.40 Request: 47.1 Mapping: 0.50 Request: 47.1

26. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 25 (47) Label Switched Path (LSP) 47.1 47.2 47.3 1 2 3 1 2 1 2 3 3 IP 47.1.1.1

27. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 26 (47) Benefits and Limitations Why might this approach be better than normal IP forwarding that does not use MPLS?  Remember, all packets still travel the same paths. ANSWER: The label look-up allows ultra-fast forwarding of FEC What else might we be able to do with MPLS that could be even more powerful?  See next two slides

28. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 27 (47) #216 #14 #462 - ER-LSP follows route that source chooses. In other words, the control message to establish the LSP (label request) is source routed. #972 #14 #972 A B C Route= {A,B,C} Explicited Routed LSP or ER-LSP (1)

29. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 28 (47) 47.1 47.2 47.3 1 2 3 1 2 1 2 3 3 IP 47.1.1.1 Explicited Routed LSP or ER-LSP (2)

30. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 29 (47) 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)

31. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 30 (47) ER LSP - discord! Two signaling options proposed in the standards: CR-LDP, RSVP extensions: — CR-LDP = LDP + Explicit Route — RSVP ext = Traditional RSVP + Explicit Route + Scalability Extensions Little difference in mechanisms, but RSVP is the winner (in terms of market). Survival of the fittest not such a bad thing.

32. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 31 (47) Label encapsulation

33. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 32 (47) Label Encapsulation MPLS Encapsulation is specified over various media types. Outermost labels may use existing format (VPI/VCI, etc.), while inner label(s) use a new “shim” label format. ATMFR EthernetPPP VPIVCIDLCI“Shim Label” Medium Label “Shim Label” ……. IP or other non-IP PAYLOAD Optical λ

34. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 33 (47) MPLS Link Layers MPLS is intended to run over multiple link layers Specifications for the following link layers currently exist: PPP/LAN: uses ‘shim’ header inserted between L2 and L3 headers ATM: label contained in VCI/VPI field of ATM header Frame Relay: label contained in DLCI field in FR header Translation between link layers types must be supported MPLS intended to be “multi-protocol” below as well as above

35. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 34 (47) MPLS Encapsulation - PPP & LAN Data Links LabelExp. S TTL Label: Label Value, 20 bits (0-16 reserved) Exp.: Experimental, 3 bits (was Class of Service) S:Bottom of Stack, 1 bit (1 = last entry in label stack) TTL:Time to Live, 8 bits Layer 2 Header (eg. PPP, 802.3) Network Layer Header and Packet (eg. IP) 4 Octets MPLS ‘Shim’ Headers (1-n) 1 n Network layer must be inferable from value of bottom label of the stack Note: The label at the bottom of the stack is the “top” label. MPLS on PPP links and LANs uses ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers MPLS on PPP links and LANs uses ‘Shim’ Header Inserted Between Layer 2 and Layer 3 Headers Label Stack Entry Format

36. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 35 (47) MPLS Encapsulation -> ATM ATM LSR constrained by the cell format imposed by existing ATM standards VPIPT CLP HEC 5 Octets ATM Header Format VCI AAL-5 Trailer Network Layer Header and Packet (eg. IP) 1 n AAL-5 PDU Frame (nx48 bytes) Generic Label Encap. (PPP/LAN format) ATM SAR ATM Header ATM Payload 48 Bytes Label Option 1 Option 2 Combined Label Option 3LabelATM VPI (Tunnel) Top 1 or 2 labels are contained in the VPI/VCI fields of ATM header - Option 1 uses two labels. - One in each or single label in combined field, negotiated by LDP Further fields in stack are encoded with ‘shim’ header in PPP/LAN format

37. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 36 (47) MPLS Encapsulation -> Frame Relay n 1 DLCI C/ R EAEA DLCI FE CN BE CN DEDE EAEA Q.922 Header Generic Encap. (PPP/LAN Format) Layer 3 Header and Packet DLCI Size = 10, 17, 23 Bits Current label value carried in DLCI field of Frame Relay header Can use either 2 or 4 octet Q.922 Address (10, 17, 23 bytes) Generic encapsulation contains n labels for stack of depth n - top label contains TTL (which FR header lacks), ‘explicit NULL’ label value

38. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 37 (47) Label distribution methods

39. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 38 (47) Label Distribution Protocol (LDP) - Purpose Label distribution ensures that adjacent routers have a common view of FEC label bindings Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR2 Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR2 LSR1 LSR2 LSR3 IP Packet 47.80.55.3 Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR3 Routing Table: Addr-prefix Next Hop 47.0.0.0/8 LSR3 For 47.0.0.0/8 use label ‘17’ Label Information Base: Label-In FEC Label-Out 17 47.0.0.0/8 XX Label Information Base: Label-In FEC Label-Out 17 47.0.0.0/8 XX Label Information Base: Label-In FEC Label-Out XX 47.0.0.0/8 17 Label Information Base: Label-In FEC Label-Out XX 47.0.0.0/8 17 Step 1: LSR creates binding between FEC and label value Step 2: LSR communicates binding to adjacent LSR Step 3: LSR inserts label value into forwarding base Common understanding of which FEC the label is referring to! Label distribution can either piggyback on top of an existing routing protocol, or a dedicated label distribution protocol (LDP) can be created Label distribution can either piggyback on top of an existing routing protocol, or a dedicated label distribution protocol (LDP) can be created

40. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 39 (47) Label Distribution - Methods LSR1 LSR2 Label Distribution can take place using one of two possible methods Downstream (unsolicited) Label Distribution Label-FEC Binding LSR2 and LSR1 are said to have an “LDP adjacency” (LSR2 being the downstream LSR) LSR2 discovers a ‘next hop’ for a particular FEC LSR2 generates a label for the FEC and communicates the binding to LSR1 LSR1 inserts the binding into its forwarding tables If LSR2 is the next hop for the FEC, LSR1 can use that label knowing that its meaning is understood LSR1 LSR2 Downstream-on-Demand Label Distribution Label-FEC Binding LSR1 recognizes LSR2 as its next-hop for an FEC A request is made to LSR2 for a binding between the FEC and a label If LSR2 recognizes the FEC and has a next hop for it, it creates a binding and replies to LSR1 Both LSRs then have a common understanding Request for Binding Both methods are supported, even in the same network at the same time For any single adjacency, LDP negotiation must agree on a common method

41. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 40 (47) #963 #14 #99 #311 Downstream (unsolicited) Label Distribution #462 D #311 D #963 D #14 D #99 D #216 D #612 D #5 D

42. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 41 (47) #963 #14 #99 #311 Downstream on-demand Label Distribution #462 D #311 D #963 D #14 D #99 D #216 D #612 D #5 D D?

43. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 42 (47) Distribution Control: Ordered vs. Independent Independent LSP Control Ordered LSP Control Next Hop (for FEC) Outgoing Label Incoming Label MPLS path forms, as associations are made between FEC next-hops and incoming and outgoing labels Each LSR makes independent decision on when to generate labels and communicate them to upstream peers Communicate label-FEC binding to peers once next-hop has been recognized LSP is formed as incoming and outgoing labels are spliced together Label-FEC binding is communicated to peers if: - LSR is the ‘egress’ LSR to particular FEC - label binding has been received from upstream LSR LSP formation ‘flows’ from egress to ingress Features Comparison Labels can be exchanged with less delay Does not depend on availability of egress node Granularity may not be consistent across the nodes at the start May require separate loop detection/mitigation method Requires more delay before packets can be forwarded along the LSP Depends on availability of egress node Mechanism for consistent granularity and freedom from loops Used for explicit routing and multicast Both methods are supported in the standard and can be fully interoperable

44. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 43 (47) #963 #14 #99 #311 Independent mode #462 D #311 D #963 D #14 D #99 D #216 D #612 D #5 D

45. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 44 (47) Label Retention Methods LSR1 LSR2 LSR3 LSR4 LSR5 Binding for LSR5 Binding for LSR5 Binding for LSR5 An LSR may receive label bindings from multiple LSRs Some bindings may come from LSRs that are not the valid next-hop for that FEC Liberal Label Retention Conservative Label Retention LSR1 LSR2 LSR3 LSR4 Label Bindings for LSR5 Valid Next Hop LSR4’s Label LSR3’s Label LSR2’s Label LSR1 LSR2 LSR3 LSR4 Label Bindings for LSR5 Valid Next Hop LSR4’s Label LSR3’s Label LSR2’s Label LSR maintains bindings received from LSRs other than the valid next hop If the next-hop changes, it may begin using these bindings immediately May allow more rapid adaptation to routing changes Requires an LSR to maintain many more labels LSR only maintains bindings received from valid next hop If the next-hop changes, binding must be requested from new next hop Restricts adaptation to changes in routing Fewer labels must be maintained by LSR

46. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 45 (47) Liberal retention mode #462 D #311 D #963 D #14 D #99 D #216 D #612 D #5 D #422 D #622 D These labels are kept incase they are needed after a failure.

47. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 46 (47) Conservative retention mode #462 D #311 D #963 D #14 D #99 D #216 D #612 D #5 D #422 D #622 D These labels are released the moment they are received.

48. Authors: Piero Castoldi Course: Optical Core Networks Module: MPLS basics Revision: 16/2/2008 47 (47) Suggested reading B. Davie, Y. Rekhter, “MPLS – Technology and Applications”, Morgan Kaufmann, 2000, ISBN 1-55860-656-4. E. Gray, “MPLS: Implementing the Technology”, Addison-Wesley, Reading, MA, 2001, ISBN 0-201-65762-7.