Introduction to the Introduction to the Path Computation Element - - PowerPoint PPT Presentation

I TU-T W orkshop “NGN and its Transport Netw orks“ Kobe, 2 0 -2 1 April 2 0 0 6

I nternational Telecom m unication Union

ITU-T

Introduction to the Introduction to the Path Computation Element Path 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 AS

ON Architecture

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Path Com putation

• The selection of the route through the

network taken by a connection

1.

Full precomputation

2.

On-demand full computation

3.

S egment-by-segment computation

4.

Hop-by-hop computation

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Dom ains

• “ Any collection of network elements

within a common sphere of address management or path computational responsibility.”

• Classic examples…

1.

IGP Areas

2.

Autonomous S ystems

• AS

ON examples…

1.

S ub-networks

2.

Routing levels

3.

Layers

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I nterdom ain Routing I ssues

• 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:

1.

Reachability

2.

Acceptable paths

3.

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 Com putation Elem ent ( PCE)

• A new functional component

1.

Performs path computation

2.

Preserves confidentiality

3.

Avoids abstraction/ aggregation issues

4.

Off-loads computational complexity

• THE IETF’ s PCE working group defines…
• Path Computat ion Element: an entit y

(component, applicat ion or network node) that is capable of comput ing a network path or route based on a network graph and applying computat ional constraints.

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PCE as an NMS Tool

• PCE can be a tool used by the NMS
• May be built into NMS
• r separate
• Traffic Engineering Database (TED)
• Fed by configuration, inventory, routing

protocols

PCE TED Signalling Protocol Service Request Service Request NMS Request Response Config. IGP

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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

PCE TED CC CC Service Request Config. IGP 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

PCE TED CC CC Service Request Config. IGP Request/Response 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

PCE TED CC CC Service Request Config. IGP Request/Response Signalling Protocol CC PCE TED Request/Response

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End-to-end I ndependent PCEs

• Computation on entry to each domain
• S

ub-optimal end-to-end paths

• 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

C F Y Q P

• 9. I want to reach

the Egress Cookie

• 10. Route

thru’ PQ

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PCE in the ASON Architecture

• PCE is applicable to the AS

ON Architecture

• One PCE per sub-network in peer routing
• The model is exactly as j ust 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 : AS

ON 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-drafts/

• Follow-up off-line
• adrian@
• lddog.co.uk