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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

Introduction to thePath Computation Element

Adrian Farrel

Old Dog Consulting

agenda
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
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
domains
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
interdomain routing issues
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
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”

TE Abstraction – A Rejected Option

Both rely on crankback signaling and high CPU aggregation

path computation element pce
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.
pce as an nms tool

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

pce in the connection controller

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

the pce server

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

cooperating pce servers

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

end to end independent pces
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
end to end cooperating pces
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

diverse path cooperating pces
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

pce in the ason architecture
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
questions
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