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Introduction to the Path Computation Element

Introduction to the Path Computation Element Adrian Farrel Old Dog Consulting 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 Path Computation

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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 http://www.ietf.org/internet-drafts/ • Follow-up off-line • adrian@olddog.co.uk

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