Introduction to the Path Computation Element

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Introduction to thePath Computation Element

• Adrian Farrel
• Old Dog Consulting

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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

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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

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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

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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

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TE Abstraction A Rejected Option

• 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

Both rely on crankback signaling and high CPU
aggregation
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Path Computation Element (PCE)

• A new functional component
• Performs path computation
• Preserves confidentiality
• Avoids abstraction/aggregation issues
• Off-loads computational complexity
• THE IETFs 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.

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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
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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
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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
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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
ServiceRequest
SignallingProtocol
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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

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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
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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
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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

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Questions?

• Background reading
• draft-ietf-pce-architecture-04.txt
• Free download from http//www.ietf.org/internet-dr
  afts/
• Follow-up off-line
• adrian_at_olddog.co.uk