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Introduction to the Path Computation Element

Introduction to the Path Computation Element

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Introduction to the Path Computation Element

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  1. Introduction to thePath Computation Element Adrian Farrel Old Dog Consulting

  2. Agenda • Domains, Path Computation, and TE Abstraction • The Path Computation Element • Basic Architectural Possibilities • PCE-Based Path Computation Examples • Applying PCE to The ASON Architecture

  3. Path Computation • The selection of the route through the network taken by a connection • Full precomputation • On-demand full computation • Segment-by-segment computation • Hop-by-hop computation

  4. Domains • “Any collection of network elements within a common sphere of address management or path computational responsibility.” • Classic examples… • IGP Areas • Autonomous Systems • ASON examples… • Sub-networks • Routing levels • Layers

  5. Interdomain Routing Issues • TE visibility is (completely) limited • Which domain contains the destination? • Which domain to go to next? • Which connection to the next domain? • How to guarantee path diversity for protection • Questions apply to: • Reachability • Acceptable paths • Optimal paths

  6. Virtual Link “You can reach this destination across this domain with these characteristics” BGP-TE model Requires large amount of information Needs frequent updates Virtual Node Hierarchical abstraction Presents subnetwork as a virtual switch Can be very deceptive No easy way to advertise “limited cross-connect capabilities” TE Abstraction – A Rejected Option Both rely on crankback signaling and high CPU aggregation

  7. Path Computation Element (PCE) • A new functional component • Performs path computation • Preserves confidentiality • Avoids abstraction/aggregation issues • Off-loads computational complexity • THE IETF’s PCE working group defines… • Path Computation Element: an entity (component, application or network node) that is capable of computing a network path or route based on a network graph and applying computational constraints.

  8. PCE TED PCE as an NMS Tool • PCE can be a tool used by the NMS • May be built into NMS or separate • Traffic Engineering Database (TED) • Fed by configuration, inventory, routing protocols NMS Service Request Config.IGP RequestResponse ServiceRequest Signalling Protocol

  9. PCE TED CC CC PCE in the Connection Controller • PCE can be collocated with the Connection Controller • Easy implementation matches existing code • Request/response is not a formal interface Config.IGP ServiceRequest Request/Response Signalling Protocol

  10. PCE TED CC CC The PCE Server • PCE implemented as a separate server • Dedicated server or powerful NE • Requires formal request/response protocol Config.IGP Request/Response ServiceRequest Signalling Protocol

  11. PCE PCE TED CC CC CC TED Cooperating PCE Servers • PCEs in different domains may cooperate • Use formal request/response protocol • Increase quality of computed path Config.IGP Request/Response Request/Response SignallingProtocol ServiceRequest

  12. End-to-end Independent PCEs • Computation on entry to each domain • Sub-optimal end-to-end paths • Paths may be blocked • Use crankback to recover

  13. End-to-end : Cooperating PCEs 2. Thinks… “A looks best” 3. How should I reach the Egress? 4. Thinks… “D would be best” PCE PCE 1. I want to reach the Egress 7. I want to reach the Egress 5. Route thru’ B 8. Route thru’ Y 6. Route thru’ X and B A C Ingress X Y Egress B D

  14. Diverse Path Cooperating PCEs 2. Thinks… “Need to exit through A and B” 4. Thinks… “(D or E) and F” 3. I want disjoint paths to the Egress 7. I want to reach the Egress Cookie PCE PCE 1. I want disjoint paths to the Egress 5. Route thru’ (A or B) and C Cookies 8. Route thru’ Z 9. I want to reach the Egress Cookie 6. Route thru’ A and XYC A D 10. Routethru’ PQ Ingress X Z Egress B E Q Y P C F

  15. PCE in the ASON Architecture • PCE is applicable to the ASON Architecture • One PCE per sub-network in peer routing • The model is exactly as just demonstrated • One PCE per level in multi-level routing • Hierarchical interaction between PCEs • One PCE per layer in multi-layer routing • Hierarchical interaction between PCEs • New work starting… • G.7715.2 : ASON routing architecture and requirements for remote path query

  16. Questions? • Background reading • draft-ietf-pce-architecture-04.txt • Free download from • Follow-up off-line •