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MPLS Architecture

MPLS Architecture

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MPLS Architecture

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  1. MPLS Architecture CSE 8344

  2. Internet IP LER LER LER LSR LSR LSR LSR MPLS IP MPLS Network Model MPLS LSR = Label Switched Router LER = Label Edge Router CSE 8344

  3. MPLS Benefits Comparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits: • The performance characteristics of layer 2 networks • The connectivity and network services of layer 3 networks • Improves the price/performance of network layer routing • Improved scalability CSE 8344

  4. MPLS Benefits (cont’d) • Improves the possibilities for traffic engineering • Supports the delivery of services with QoS guarantees • Avoids need for coordination of IP and ATM address allocation and routing information CSE 8344

  5. Necessity of L3 Forwarding • For security • To allow packet filtering at firewalls • Requires examination of packet contents, including the IP header • For forwarding at the initial router - used when hosts don’t do MPLS • For Scaling • Forward on a finer granularity than the labels can provide CSE 8344

  6. MPLS Architecture • Down stream label assignment for unicast traffic • On demand • Unsolicited • Path selection • Hop by hop • Explicit • Ordered vs. independent control • Loop detection and prevention mechanisms CSE 8344

  7. Label Distribution Protocol (LDP) • Set of procedures used by LSRs to establish LSPs • Mapping between network-layer routing information directly to data-link layer switched paths • LDP peers: • Two LSRs which use LDP to exchange label/stream mapping • Information exchange known as “LDP Session” CSE 8344

  8. LDP Messages • Discovery messages • Used to announce and maintain the presence of an LSR • Session/Adjacency messages • Used to establish, maintain and terminate sessions between LDP peers • Advertisement messages • Used to create, change, and delete label mappings • Notification messages • Used to provide advisory information and to signal error information CSE 8344

  9. Forwarding Equivalence Class (FEC) • Introduced to denote packet forwarding classes • Comprises traffic • To a particular destination • To destination with distinct service requirements CSE 8344

  10. LSP - FEC Mapping • FEC specified as a set of two elements • IP Address Prefix - any length from 0 – 32 • Host Address - 32 bit IP address • A given packet matches a particular LSP if and only if IP Address Prefix FEC element matches packet’s IP destination address CSE 8344

  11. Label Spaces • Useful for assignment and distribution of labels • Two types of label spaces • Per interface label space: Interface-specific labels used for interfaces that use interface resources for labels • Per platform label space: Platform-wide incoming labels used for interfaces that can share the same label space CSE 8344

  12. LDP Discovery • A mechanism that enables an LSR to discover potential LDP peers • Avoids unnecessary explicit configuration of LSR label switching peers • Two variants of the discovery mechanism • Basic discovery mechanism: used to discover LSR neighbors that are directly connected at the link level • Extended discovery mechanism: used to locate LSRs that are not directly connected at the link level CSE 8344

  13. LDP Discovery (Cont’d) • Basic discovery mechanism • To engage - send LDP Hellos periodically • LDP Hellos sent as UDP packets for all routers on that subnet • Extended discovery mechanism • To engage - send LDP targeted Hellos periodically • Targeted Hellos are sent to a specific address • Targeted LSR decides whether to respond or to ignore the targeted Hello CSE 8344

  14. Session Establishment • Exchange of LDP discovery Hellos triggers session establishment • Two step process • Transport connection establishment • If LSR1 does not already have a LDP session for the exchange of label spaces LSR1:a and LSR2:b, it attempts to open a TCP connection with LSR2 • LSR1 determines the transport addresses at its end (A1) and LSR2’s end (A2) of the TCP connection • If A1>A2, LSR1 plays the active role; otherwise it is passive • Session initialization • Negotiate session parameters by exchanging LDP initialization messages CSE 8344

  15. Label Distribution and Management • Two label distribution techniques • Downstream on demand label distribution: An LSR can distribute a FEC label binding in response to an explicit request • Downstream Unsolicited label distribution: Allows an LSR to distribute label bindings to LSRs that have not explicitly requested them • Both can be used in the same network at the same time; however, each LSR must be aware of the distribution method used by its peer CSE 8344

  16. Label Distribution Control Mode • Independent Label Distribution Control • Each LSR may advertise label mappings to its neighbors at any time • Independent Downstream on Demand mode - LSR answers without waiting for a label mapping from next hop • Independent Downstream Unsolicited mode - LSR advertises label mapping for a FEC whenever it is prepared • Consequence: upstream label can be advertised before a downstream label is received CSE 8344

  17. Distribution Control Mode (cont’d) • Ordered Label Distribution Control • Initiates transmission of label mapping for a FEC only if it has next FEC next hop or is the egress • If not, the LSR waits till it gets a label from downstream LSR • LSR acts as an egress for a particular FEC, if • Next hop router for FEC is outside of label switching network • FEC elements are reachable by crossing a domain boundary CSE 8344

  18. Label Retention Mode • Conservative Label Retention Mode • Advertised label mappings are retained only if they are used for forwarding packets • Downstream on Demand Mode typically used with Conservative Label Retention Mode • Advantage: only labels required are maintained • Disadvantage: a change in routing causes delay • Liberal Retention Mode • All label mappings are retained regardless of whether LSR is next hop or not • Faster reaction to routing changes CSE 8344

  19. Label Information Base • LSR maintains learned labels in Label Information Base (LIB) • Each entry of LIB associates an FEC with an (LDP Identifier, label) pair • When next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB CSE 8344

  20. Hierarchical Routing in MPLS • External Routers A,B,C,D,E,F - Talk BGP • Internal Routers 1,2,3,4,5,6 - Talk OSPF Domain #2 C D 1 Domain #1 6 Domain #3 2 3 4 5 B F A E Note: Internal routers in domains 1 and 3 not shown CSE 8344

  21. Hierarchical Routing (cont’d) • When IP packet traverses domain #2, it will contain two labels, encoded as a “label stack” • Higher level label used between routers C and D, which is encapsulated inside a lower level label used within Domain #2 • Operation at C • C needs to swap BGP label to put label that D expects • C also needs to add an OSPF label that 1 expects • C therefore pushes down the BGP label and adds a lower level label CSE 8344

  22. Explicit Routing in MPLS • Two options for route selection: • Hop by hop routing • Explicit routing • Explicit Routing (Source Routing) is a very powerful technique • With pure datagram routing, overhead of carrying complete explicit route is prohibitive • MPLS allows explicit route to be carried only at the time the LSP is setup, and not with each packet • MPLS makes explicit routing practical CSE 8344

  23. Explicit Routing (Cont’d) • In an explicitly routed LSP • LSP next hop is not chosen by the local node • Selected by a single node, usually the ingress • The sequence of LSRs may be chosen by • Configuration (e.g., by an operator or by a centralized server) CSE 8344

  24. Loops and Loop Handling • Routing protocols used in conjunction with MPLS are based on distributed computation which may contain loops • Loops handling - 3 categories • Loop Mitigation/Survival • Loop Detection • Loop Prevention CSE 8344

  25. Loop Mitigation • Minimizes the impact of loops by limiting the amount of resources consumed by the loop • Method • Based on use of TTL field which is decremented at each hop • Use of dynamic routing protocol converging rapidly to non-looping paths CSE 8344

  26. Loop Detection • Loops may be setup but they are subsequently detected • The detected loop is then broken by dropping label relationship • Broken loops now necessitates packets to be forwarded using L3 forwarding CSE 8344

  27. Loop Detection (Cont’d) • Method is based on transmitting a Loop Detection Control Packet (LDCP) whenever a route changes • LDCP is forwarded towards the destination until • Last MPLS node along the path is reached • TTL of the LDCP expires • It returns to the node which originated it CSE 8344

  28. Loop Prevention • Ensures that loops are never set up • Labels are not used until it is sure to be loop free • Methods • Labels are propagated starting at the egress switch • Use source routing to set up label bindings from the egress switch to each ingress switch CSE 8344

  29. QoS in MPLS CSE 8344

  30. Strategy • To support end-to-end QoS as in IP • MPLS not an end-to-end protocol • Efficient ways of mapping QoS to LSPs • Traffic Engineering key to QoS CSE 8344

  31. QoS Models • Best effort • Original IP service • Int-serv. • Fist IP effort to support QoS • Diff-serv. • Simple, scalable • Future • Int+ Diff+ TE with e2e SLAs CSE 8344

  32. VoIP Mission Critical Services Multimedia Video Conference, Collaborative Computing VPNs IntServ DiffServ MPLS Hybrid Frame Relay PPP HDLC SDLC ATM, POS FE,Gig.E 10GE Wireless Fixed,Mobile BroadBand Cable,xDSL CISCO QoS Framework POLICY-BASED NETWORKING PROVISIONING & MONITORING Signaling Techniques (RSVP, DSCP*, ATM (UNI/NNI)) Classification & Marking Techniques (DSCP, MPLS EXP, NBAR, etc.) Congestion Avoidance Techniques (WRED) Traffic Conditioners (Policing, Shaping) Congestion Management Techniques (WFQ, CBWFQ, LLQ) Link Efficiency Mechanisms (Compression, Fragmentation) CSE 8344

  33. Support of RSVP • Very similar to tag switching • Bind labels to reserved flows • Label object inside the RESV message • Labels propagate upstream • Only the edge router need to know the packet to flow mapping • Can aggregate flows instead of micro-flows CSE 8344

  34. RSVP Scalability • Aggregation • Refresh reduction • Use acknowledgements for refresh • Once received, increase the refresh time • Summary refresh CSE 8344

  35. Diff-Serv Support • E-LSP • “Queue” inferred from Label and EXP field • “Drop priority” inferred from label and EXP field • L-LSP • Queue” inferred exclusively from Label • “Drop priority” inferred from EXP field CSE 8344

  36. LSR LDP/RSVP LDP/RSVP E-LSP AF1 EF E-LSP • E-LSPs established by various label binding protocols (LDP or RSVP) • no new Signalling needed. • EF and AF1 on a single E-LSP • EF and AF1 packets travel on single LSP (single label) but are enqueued in different queues (different EXP values) • Queue & Drop Precedence is selected based on EXP CSE 8344

  37. E-LSP Referred to as Packet Classification or Coloring Standard IPV4: Bits 0-2 Called IP Precedence (Three MSB) (DiffServ Uses Six ToS bits…: Bits 0-5, with Two Reserved) Version Length ToS 1 Byte Len ID offset TTL Proto FCS IP-SA IP-DA Data CSE 8344

  38. IP Precedence to Label EXP CSE 8344

  39. PHB from EXP No additional signaling EXP->PHB configured Shim header required Up to 8 PHBs per LSP PHB from label + Exp/CLP Signaled at LSP setup Label->PHB mapped Shim or link layer header used Arbitrarily large E-LSP vs. L-LSP CSE 8344

  40. Explicit Congestion Notification(ECN) • TCP approach – based on packet drop • May not reflect the status • Resources could have been wasted • Early notification • Mark packets • Receiver conveys information to sender • Two bits used to deal with deployment disparity (CE & ECT) CSE 8344

  41. MPLS Support of ECN • Could use two bits as before • May not be available • Usually 1 bit available • LSRs should have the understanding on mapping CSE 8344

  42. Traffic Engineering in MPLS CSE 8344

  43. Traffic Engineering Objectives • Traffic Engineering (TE) concerned with performance optimization • The key performance objectives • traffic oriented e.g. minimization of packet loss • resource oriented - optimization of resource utilization e.g. efficient management of bandwidth CSE 8344

  44. Objectives (cont’d) • Minimizing congestion is a major traffic and resource oriented performance objective • Congestion manifest under two scenarios • Network resources insufficient or inadequate • Solved by capacity expansion or classical congestion control techniques • Inefficient mapping of traffic streams onto available resources • Reduced by adopting load balancing policies CSE 8344

  45. MPLS and Traffic Engineering • Main components used • Traffic Trunk - aggregation of traffic flows of the same class which are placed inside a Label Switched Path • Induced MPLS Graph • Analogous to a virtual topology in an overlay model • Logically mapped onto the physical network • Set of LSRs as nodes of the graph • Set of LSPs providing logical point to point connectivity between LSRs as edges CSE 8344

  46. Constraint Based Routing (CBR) • Associate each path with set of constraints • Performance, administrative • Local information • Routing algorithms • Optimizes various metrics • Ensures that the constraints are not violated CSE 8344

  47. Can IP Routing Do CBR? • Plain IP routing cannot • CBR has to be source based – each source may have different constraint to same destination • Link attributes need to be distributed • Need explicit routing instead of “destination-based” • Can be augmented to support CBR • Usually a combination is used CSE 8344

  48. CBR Components • Mechanism for source based path computing • Mechanism to collect necessary information • Constraints (local), attributes, topology • Support forwarding along the computed paths • Notification of residual resources after allocation CSE 8344

  49. Constrain-Based SPF 7 2 150 45 4 1 150 150 150 150 5 3 6 150 CSE 8344

  50. CSPF • Uses the following inputs • Link attributes • Topology state information • Path constraints • Basic approach • Prune resources that do not meet the constraints • Run a shortest path algorithm on the residual graph CSE 8344