Introduction to the path computation element
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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 l.jpg

Introduction to thePath Computation Element

Adrian Farrel

Old Dog Consulting


Agenda l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg

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 l.jpg
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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Questions?

  • Background reading

    • draft-ietf-pce-architecture-04.txt

    • Free download from http://www.ietf.org/internet-drafts/

  • Follow-up off-line

    • [email protected]


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