ITU-T Workshop “NGN and its Transport Networks“Kobe, 20-21 April 2006

International Telecommunication UnionITU-

T

Introduction to theIntroduction to thePath Computation ElementPath Computation Element

Adrian FarrelOld Dog Consulting

ITU-T Workshop “NGN and its Transport Networks“Kobe, 20-21 April 2006 2

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Agenda

o Domains, Path Computation, and TE Abstraction

o The Path Computation Elemento Basic Architectural Possibilitieso PCE-Based Path Computation Exampleso Applying PCE to The ASON Architecture

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Path Computation

o The selection of the route through the network taken by a connection

1. Full precomputation2. On-demand full computation3. Segment-by-segment computation4. Hop-by-hop computation

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Domains

o “Any collection of network elements within a common sphere of address management or path computational responsibility.”

o Classic examples…1. IGP Areas2. Autonomous Systems

o ASON examples…1. Sub-networks2. Routing levels3. Layers

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Interdomain Routing Issues

o TE visibility is (completely) limitedo Which domain contains the

destination?o Which domain to go to next?o Which connection to the next domain?o How to guarantee path diversity for

protectiono Questions apply to:

1. Reachability2. Acceptable paths3. Optimal paths

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TE Abstraction – A Rejected Option

Virtual Linko “You can reach this

destination across this domain with these characteristics”

o BGP-TE modelo Requires large amount

of informationo Needs frequent updates

Virtual Nodeo Hierarchical abstractiono Presents subnetwork as

a virtual switcho 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)

o A new functional component1. Performs path computation2. Preserves confidentiality 3. Avoids abstraction/aggregation issues4. Off-loads computational complexity

o 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.

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PCE as an NMS Tool

o PCE can be a tool used by the NMS• May be built into NMS or separate

o Traffic Engineering Database (TED)• Fed by configuration, inventory, routing

protocols

PCE TED

Signalling Protocol

Service Request

ServiceRequest

NMSRequest

ResponseConfig.

IGP

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PCE in the Connection Controller

o PCE can be collocated with the Connection Controller• Easy implementation matches existing

code• Request/response is not a formal

interfacePCE

TED

CC CC

ServiceRequest

Config.IGP

Request/Response

Signalling Protocol

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The PCE Server

o PCE implemented as a separate server• Dedicated server or powerful NE

o Requires formal request/response protocol

PCE

TED

CC CCServiceRequest

Config.IGP

Request/Response

Signalling Protocol

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Cooperating PCE Servers

o PCEs in different domains may cooperate• Use formal request/response protocol• Increase quality of computed path

PCE

TED

CC CCServiceRequest

Config.IGP

Request/Response

SignallingProtocol CC

PCE

TED

Request/Response

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End-to-end Independent PCEs

o Computation on entry to each domaino Sub-optimal end-to-end pathso Paths may be blocked

• Use crankback to recover

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End-to-end : Cooperating PCEs

Ingress

Egress

PCE

A

D

C

B

2. Thinks…“A looks best”

PCE

4. Thinks…“D would be best”

6. Route thru’ X and B

1. I want to reach the Egress

3. How should I reach the Egress?

X

5. Route thru’ B

Y

7. I want to reach the Egress

8. Route thru’ Y

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Diverse Path Cooperating PCEs

Ingress

Egress

PCE

A

E

D

B

2. Thinks…“Need to exit through A and B”

PCE

4. Thinks…“(D or E) and F”

6. Route thru’ A and XYC

1. I want disjoint paths to the Egress

3. I want disjoint paths to the Egress

X

5. Route thru’ (A or B) and C

Cookies

Z

7. I want to reach the Egress

Cookie

8. Route thru’ Z

CF

YQ

P

9. I want to reach the Egress

Cookie

10. Routethru’ PQ

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PCE in the ASON Architecture

o PCE is applicable to the ASON Architectureo One PCE per sub-network in peer routing

• The model is exactly as just demonstratedo One PCE per level in multi-level routing

• Hierarchical interaction between PCEso One PCE per layer in multi-layer routing

• Hierarchical interaction between PCEso New work starting…

• G.7715.2 : ASON routing architecture and requirements for remote path query

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Questions?

o Background reading• draft-ietf-pce-architecture-04.txt• Free download from

http://www.ietf.org/internet-drafts/

o Follow-up off-line• adrian@olddog.co.uk