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Multiprotocol Label Switching

Multiprotocol Label Switching. Overview. L3 Switching vs Routing What is MPLS? MPLS on ATM: Issues Traffic Engineering Using MPLS MPLS Simulation Results. Introduction. MPLS stands for “Multiprotocol” label switching techniques applicable to any L3 protocol

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Multiprotocol Label Switching

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  1. Multiprotocol Label Switching Engineering Internet QoS

  2. Overview • L3 Switching vs Routing • What is MPLS? • MPLS on ATM: Issues • Traffic Engineering Using MPLS • MPLS Simulation Results Engineering Internet QoS

  3. Introduction • MPLS stands for “Multiprotocol” label switching • techniques applicable to any L3 protocol • A router supporting MPLS is know as Label Switched Router (LSR) • IP routing/control software is combined with label swapping • similar to ATM VCI/VPI swapping Engineering Internet QoS

  4. Proprietary Protocols • The Cell Switching Router (CSR) • ftp://ftp.wide.toshiba.co.jp/pub/csr/ • Ipsilon’s IP Switching • www.ipsilon.com • Cisco’s Tag Switching • www.cisco.com • IBM’s Aggregate Route-Based IP Switching (ARIS) • www.networking.ibm.com/isr/ip/ • IETF MPLS working group • www.ietf.org/html.charters/mpls-charter.html Engineering Internet QoS

  5. Why label switching • Scalablity of L3 routing and simplicity of L2 forwarding • high performance • use of ATM hardware • faster than IP lookup • integration of IP and ATM • new Services such as VPNs • hierarchy of Routing Engineering Internet QoS

  6. Terminologies • flow : a single instance of an application to application flow of data • forwarding equivalence class: a group of IP packets which are forwarded along same path (or same treatment) • label: a short fixed length physically contiguous identifier which is used to identify a FEC (local significance) • label Switched Router: an MPLS node which is capable of forwarding L3 packets Engineering Internet QoS

  7. Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com MPLS Domain LSR: label switching router LER: label edge router Engineering Internet QoS

  8. Conventional IP Routing • packet travels from one router to the next • an independent forwarding decision made at each hop • At each hop packets assigned to a Forwarding Equivalence Class (FEC) • packets considered to be in same FEC if the routing table contains some prefix X such that X is the longest match for each packet’s destination address Engineering Internet QoS

  9. MPLS Approach • assignments of packets to FEC done once • as the packet enters the network • FEC to which packet is assigned is encoded with a label • packet is forwarded to next hop with label • no further analysis of packet header at next hop Engineering Internet QoS

  10. label exp s ttl Label encoding • MPLS encapsulation should contain • the label stack field • time-to-live (TTL) field • a 3 bit Experimental field (earlier COS) • MPLS uses 32 bits for label encoding Label: Label Value, 20 bits (0-16 reserved) Exp.: Experimental, 3 bits (earlier Class of Service) S: Bottom of Stack, 1 bit (1 = last entry in label stack) TTL: 8 bit Time to Live Engineering Internet QoS

  11. Reserved Labels • Multiprotocol = IP, IPX, etc. Protocol is inferred from the last label in the stack. • 0 = IPv4 Explicit Null Label Þ Pop this last label. Use IPv4 header. • 1 = Router Alert LabelÞ Looked by the router software.Packet forwarded based on the next label in stackSimilar to “Router Alert Option” in IP Packets • 2 = IPv5 Explicit Null Label • 3 = Implicit Null LabelUsed only for assignment and distributionShould not appear in any label stack in the packets Engineering Internet QoS

  12. TTL Handling • At the beginning TTL = TTL from IP header • At every hop: TTL = TTL -1 • Drop the packet if TTL=0 • At the exit: TTL in IP Header = TTL from label • In some cases, entire MPLS domain may be considered one hop, e.g., ATM Engineering Internet QoS

  13. MPLS Encapsulation • For ATM VPI/VCI field and for Frame Relay DLCI field used • a “shim” layer between link layer and network layer headers for non-ATM/Frame Relay network • protocol independent • shim layer may consist of sequence of label stack entries • If PPP used, protocol field may identify frames that carry labels Engineering Internet QoS

  14. Label Processing • label used as index into a table which specifies next hop and a new label • longest match calculation eliminated at subsequent hops • old label replaced with new label and packet forwarded to next hop • label may also contain class of service (COS) • for scheduling/ discarding packets etc • again saves header processing Engineering Internet QoS

  15. Label Distribution • Who assigns labels for communication between A and B? • A, B, or someone else? • Downstream, upstream, ... • Where is the control for the entire path?A, B, ingress or egress LSR? • Separate protocol or existing route distribution mechanisms? • IETF’s Label Distribution Protocol (LDP) • based on Cisco’s TDP and IBM’s ARIS protocol Engineering Internet QoS

  16. Sample Network Topology Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  17. 192.35.10 162.25.8 162.25.8 192.35.10 1 1 0 1 Table at R4 Routing Tables Table at R2 17 Index of table 18 19 Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  18. Label Binding Example Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  19. Table at R2 In label In label out label out label Addr prefix Addr prefix Interface Interface 17 ? 17 19 162.25.8 162.25.8 1 1 ? 18 16 18 192.35.10 192.35.10 1 0 Label allocation Table at R4 Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  20. Label Switching Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  21. Hierarchy Routing BGP: Border Gateway Protocol (inter-domain) IGP: Interior Gateway Protocol (intra-domain) IGP Routers (R3, R4) in transit domain A maintain all the routes provided by Interdomain routing - necessary to forward transit traffic to/from domains B & C Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  22. IGP entries out label next hop in label Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Hierarchy Routing BGP entries out label next hop in label ? R3 52 21 …. R1 …. R2 52 R4 62 R2 31 R3 21 R5 62 R5 R5 31 R6 72 R4 41 72 …. 41 …. ? R5 R6 R5 Engineering Internet QoS

  23. Stack of labels • Packets carry several labels organised as a label stack • packet forwarded from one domain to other contains one label • packet forwarded through a transit domain contains two labels • LSRs use label from top of stack • labels pushed (ingress) and poped (egress) at domain boundaries Engineering Internet QoS

  24. IGP entries out label next hop in label Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com push/pop on stack swap label 21 to 31 R5 not directly connected Find direct conn. and label push label52 and send to R3 next hop R5 (self) pop off the top label of the stack swap this label 31 to 41 (BGP) BGP entries out label next hop in label push ? R3 52 21 …. R1 …. R2 52 R4 62 R2 31 R3 21 R5 pop 62 R5 R5 31 R6 72 R4 41 72 …. 41 …. ? R5 R6 R5 Engineering Internet QoS

  25. MPLS over ATM • ATM switches performing label switching are called ATM-LSRs • labels need to be encoded in VCI/VPI fields • MPLS forwarding similar to “label swapping” in ATM • same ATM user plane may be used • control plane to be changed (MPLS) • protocols BGP, OSPF .. replaces UNI, PNNI Engineering Internet QoS

  26. MPLS over ATM • Packets have a shim with Label=0. Actual label is encoded in VPI/VCI. • No TTL decrement • n-to-1 merge or n-to-n multipoint not supported • VCI = 0 through 32 should not be used in labels • If LSRs are connected via SVCs, VCI/VPIs cannot be used as labels. LSRs exchange VCIDs. Engineering Internet QoS

  27. Encoding labels in ATM header • SVC Encoding • VPI/VCI field to encode the label from top of stack • ATM signaling can be used • SVP Encoding • VPI encodes top label • VCI encodes 2nd label • SVP Multipoint Encoding Engineering Internet QoS

  28. Cell Interleave Problem Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  29. R1 ATM R2 in label addr prefix out label in interface interface 0 162.25 1 21 51 2 1 21 41 162.25 Request Separate labels get a label for 162.25 request separate labels for R1 and R2 from R3 162.25 if2 if0 R3 if1 get a label for 162.25 Predefined VPI/VCI used for Label binding Engineering Internet QoS

  30. VC - Merge Delay cells from one frame (AAL5 EOF marker) Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  31. MPLS on ATM: Issues • VCI field is sufficient for one level taggingVPI may be used for the 2nd level • LSR switches need to participate in network layer routing protocols (OSPF, BGP) • Multiple tags per destination may be used to avoid frame merging • VPI/VCI space may be segmented for label switching and normal ATM switching Engineering Internet QoS

  32. Traffic Engineering Using MPLS • Traffic Engineering • Performance Optimization • Efficient resource allocation • Constrained routing / Load balancing • Maximum throughput, Min delay, min lossÞ Quality of service • Meet policy requirements of operators Engineering Internet QoS

  33. R3 R1 R5 R2 R4 R6 Shortest-Path Routing MPLS Backbone A C B R2->R5->R6 link 45Mbps R2->R4->R6 link 15Mbps Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  34. Constrained Routed LSP • Hard to do non-shortest path routing with connectionless forwarding • Constrained Routed label-switched paths allows selective non-shortest path routing • load balancing across path possible • Signaling protocols for such path establishment being developed by IETF • M-RSVP is based on Resource Reservation Protocol RSVP (soft-state model) • CR-LDP defines hard-state signaling protocols Engineering Internet QoS

  35. R1 R2 R5 R6 R4 R3 CR-LSP Example MPLS Backbone A C B Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Engineering Internet QoS

  36. Traffic Trunk • In MPLS networks: “Traffic Trunks” = SVCs • A set of traffic parameters can be specified to determine the “Forwarding Equivalence Class (FEC)” or set of packets assigned to that trunk • Features of trunks • Multiple trunks can be used in parallel to the same egress. • Traffic trunks are routable entities like VCs • Each traffic trunk can have a set of associated characteristics, e.g., priority, preemption, policing, overbooking Engineering Internet QoS

  37. Traffic Trunks Features • Traffic Trunk Features • Trunk paths are setup based on policies or specified resource availability. • A traffic trunk can have alternate sets of paths in case of failure of the main path. Trunks can be rerouted. • Some trunks may preempt other trunks. A trunk can be preemptor, non-preemptor, preemptable, or non-preemptable. • Each trunk can have its own overbooking rate Engineering Internet QoS

  38. DL Label Exp S TTL IP TCP Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com Flows, Trunks, LSPs, and Links • Label Switched Path (LSP): All packets with the same label • Trunk: Same Label+Exp • Flow: Same MPLS+IP+TCP headers Engineering Internet QoS

  39. MPLS Simulation Results • Total network throughput improves significantly with proper traffic engineering • Congestion-unresponsive flows affect congestion- responsive flows • Separate trunks for different types of flows • Trunks should be end-to-end • Trunk + No Trunk = No Trunk Engineering Internet QoS

  40. Summary • Simplified forwarding based on exact match of fixed length label • Separation of routing and forwarding in IP networks • Facilitates the integration of ATM and IP • Enables the use of explicit routing/source routing in IP networks • Improved routing scalability (hierarchy) through stacking of labels • Traffic Engineering Benefits Engineering Internet QoS

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