MPLS and label switching networks

multiprotocol label switching (mpls) architecture overview and multiprotocol label switching (mpls) technologies
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Dr.NeerajMittal,India,Teacher
Published Date:19-07-2017
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Label Switching and MPLS Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1IP: ―Best-Effort Philosophy‖  Well architected, not necessarily worked out in detail  Realization: can‘t predict the future  Architectural decisions: stuff above  Make it reasonable transport  Make it flexible network  Make it extensible stuff below Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 3IP Control Plane Evolution  Again, just good enough (best-effort) …  But again, flexible, extensible  Distance Vector routing was fine for quite a while  Just in time, along came link state (OSPF and IS-IS)  Now a burning question in OSPF/IS-IS is:  Convergence ―in a few seconds‖ is not good enough?  See NANOG June 2002 for interesting videos and papers on how to fix LS-routing for fast convergence  Goal: ―Business‖ IP for service providers…  Make me money – new services, GoS  Don‘t lose me money – uptime, SLAs  OSPF/BGP not originally designed to support QoS or multiple services (eg: VoIP, VPNs) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 4ATM – Perfectionist‘s Dream  Connection-oriented  Does everything and does it well stuff above  Anticipated all future uses and transport factored them in network  Philosophical mismatch with IP ATM Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 5Overlay Model for IP-over-ATM Internetworking  Goal: Run IP over ATM core networks  Why? ATM switches offered performance, predictable behavior and services (CBR, VBR, GFR…)  ISPs created ―overlay‖ networks that presented a virtual topology to the edge routers in their network  Using ATM virtual circuits, the virtual network could be reengineered without changing the physical network  Benefits  Full traffic control  Per-circuit statistics  More balanced flow of traffic across links Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 6Overlay Model (Contd)  ATM core ringed by routers  PVCs overlaid onto physical network A Physical View B C A Logical C View B Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 7Issue 1: Mapping IP data-plane to ATM: Address Resolution Woes  A variety of server-based address resolution servers:  ATMARP (RFC 1577), LANE server, BUS server, MPOA server, NHRP server….  Use of separate pt-pt and pt-mpt VCs with servers  Multiple servers + backup VCs to them needed for fault tolerance  Separate servers needed in every LOGICAL domain (eg: LIS)  Mismatch between the notion of IP subnet and ATM network sizing  Cut-through forwarding between nodes on same ATM network hard to achieve Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 8Issue 2: Mapping IP control-plane (eg: OSPF) to ATM  Basic OSPF assumes that subnets are pt-pt or offer broadcast capability.  ATM is a Non-Broadcast Multiple Access (NBMA) media  NBMA ―segments‖ support multiple ―routers‖ with pt-pt VCs but do not support data-link broadcast/mcast capability  Each VC is costly = setting up full mesh for OSPF Hello messages is prohibitively expensive  Two ―flooding adjacency‖ models in OSPF:  Non-Broadcast Multiple Access (NBMA) model  Point-to-Multipoint (pt-mpt) Model  Different tradeoffs… Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 9Partial Mesh: NBMA model 1. Neighbor discovery: manually configured 2. Dijkstra SPF views NBMA as a full mesh Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 10Partial Mesh: pt-mpt model Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 11NBMA vs Pt-Mpt Subnet Model  Key assumption in NBMA model:  Each router on the subnet can communicate with every other (same as IP subnetmodel)  But this requires a ―full mesh‖ of expensive PVCs at the lower layer  Many organizations have a hub-and-spoke PVC setup, a.k.a. ―partial mesh‖  Conversion into NBMA model requires multiple IP subnets, and complex configuration (see fig on next slide)  OSPF‘s pt-mpt subnet model breaks the rule that two routers on the same network must be able to talk directly  Can turn partial PVC mesh into a single IP subnet Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 12OSPF Designated Routers (DRs): NBMA Case  Instead of sending a separate router-LSA for each router, one ―designated router‖ can create a network-LSA for the subnet Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 13OSPF Designated Router (DR): NBMA Case  One router elected as a designated router (DR)  Each router in subnet maintains ―flooding adjacency‖ with the DR, I.e., sends acks of LSAs to DR  DR informs each router of other routers on LAN  DR generates the network-LSA on subnet‘s behalf after synchronizing with all routers  Complex election protocol for DR in case of failure Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 14DR and BDR in OSPF NBMA model  In NBMA model: DR and BDR only maintain VCs and Hellos with all routers on NBMA Flooding in NBMA always goes through DR Multicast not available to optimize LSA flooding. DR generates network-LSA Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 15Summary: IP-to-ATM Overlay Model Drawbacks  IP-to-ATM: control-plane mapping issues  Need a full mesh of ATM PVCs for mapping IP routing  Both NBMA and Pt-Mpt mapping models have drawbacks  IP-to-ATM: data-plane mapping issues  Address resolution (eg: LANE, RFC 1577, MPOA, NHRP) requires a complex distributed server and multicast VC infrastructure  Segmentation-and-Reassembly (SAR) of IP packets into ATM cells can have a multiplier-effect on performance even if one cell in a packet is lost  ATM SAR has trouble scaling to OC-48 and OC-192 speeds  Packet-over-SONET (POS) emerged as an alternative at the link layer  ATM + AAL5 overhead (20%) deemed excessive Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 16Re-examining Basics: Routing vs Switching Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 17IP Routing vs IP Switching Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 18MPLS: Best of Both Worlds PACKET CIRCUIT HYBRID ROUTING SWITCHING IP MPLS ATM TDM +IP Caveat: one cares about combining the best of both worlds only for large ISP networks that need both features Note: the ―hybrid‖ also happens to be a solution that Shivkumar Kalyanaraman bypasses IP-over-ATM mapping woes Rensselaer Polytechnic Institute 19History: Ipsilon‘s IP Switching: Concept Hybrid: IP routing (control plane) + ATM switching (data plane) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 20Ipsilon‘s IP Switching ATM VCs setup when new IP “flows” seen, I.e., “data-driven” VC setup Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 21