Path Computation Element (PCE)

PCE

Index 1.  What is QoS? 2.  Why it’s necessary in networks? 3.  Qualities of traffic

1.  An example of IGP Cost, OSPF

4.  Adding QoS in OSPF 1.  Problem: Adding QoS in IGP(OSPF)

5.  Solution: PCE 6.  How PCE works? 7.  PCE and TED 8.  PCE and Multi-Domains 9.  PCE Architecture 10.  PCECP Protocol

1.  PCRequest & PCReply

11.  Multi-Domain Path computation 1.  Per-Domain Path Computation 2.  Cooperating PCEs 3.  Backward Recursive Path Computation 4.  BRPC: Mesh Domain Problems

12.  Conclusions

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What is QoS?   QoS(Quality of Service): is the collective effect of service

performance which determines the degree of satisfaction of a user of the service

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Why it’s necessary in networks?   Sometimes there is stricter service demands that can’t be

provided by the default systems

  For example: o  Video Streaming o  VoIP o  Cloud Computing

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Qualities of traffic

  Low throughput   Dropped Packets   Errors   Latency   Jitter   Out-of-order delivery   Green Routing

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An example of IGP Cost, OSPF   The cost of an interface in OSPF is an indication of the

overhead required to send packets across a certain interface   The cost of an interface is inversely proportional to the

bandwidth of that interface

  Cost= 10↑8 /𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ  in bps

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Adding QoS in OSPF   In practice, this OSPF routing cost is not enough

  It’s necessary improve a new next-hope calculation

  Cost = α  BW + β  Delay + χ  Jitter + … + ζ  Losses

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Problem: Adding QoS in IGP(OSPF)   Adding new parameters, seriously complicates the dynamics

of flooding because the flooding convergence time can be longer than the change rate of some metrics like available bandwidth

  Adding new parameters increments the network overhead   Creates inaccuracy

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Solution: PCE   Definition: 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 constrains (RFC4655)

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How PCE works?

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  PCC-PCE communication protocol (PCEP)   PCE

 The remote server that provides path computation   Path computation capabilities

  PCE Responds with a path (or failure)

  PCC  The network element that request computation services

  PCC Requests a computation   From where to where   Type of path? (Constraints)

PCE and TED

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  Path computation requires knowledge of available network resources

  TED: Traffic Engineering Database   The TED may be built from

  Information distributed by a routing protocol (OSPF-TE)   Information configured directly

How PCE works?

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  A single PCE cannot compute a multi-domain path

PCE

PCE and Multi-Domains

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  A PCE only see inside its domain then   Computation of a multi-domain path may use cooperating

PCEs  Communication between PCEs  This communication is exactly as PCE-PCC communication

How PCE works?

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  A single PC cannot compute a multi-domain path

PCE

PCE PCE

PCE

Domain 1

Domain 2 Domain 3

Domain 4

PCE Architecture

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1.  Composite PCE:   PCC and PCE are located in the same node

2.  External PCE:   PCC and PCE are located in different nodes

3.  Management-based PCE:   PCC is a NMS

4.  Multiple PCE Path Computation:   Multiple PCEs are used to compute the path   No inter PCE communication

5.  Multiple PCE Path Computation with Inter-PCE communication:

  Multiple PCEs collaborate to compute the path   Inter PCE communication

PCE Architecture

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PCE

TED NMS

LSR

Signalling Engine

LSR

Signalling Engine

LSR

Signalling Engine

LSR

Signalling Engine

LSR

Signalling Engine

LSR

Signalling Engine

LSR

Signalling Engine

LSR

Signalling Engine

PCE

TED

PCE

TED

Server

PCE

TED

Server

Type 3 Type 1 Type 2 & 5

PCEP Protocol

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  Operates over TCP   Session-based protocol   Seven messages

 Open  Keepalive  Request  Response  Notify   Error  Close

PCRequest & PCReply

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  PCReq message asks for a path computation   From and To  Constraints (bandwidth, etc…)  Metric to optimize

  IGP metric (ospf)   TE metric   Hop count

  PCRep reports the computed path (or failure)   Explicit route   Failure

Multi-Domain Path computation

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  Three main methods have already been defined 1.  Per-domain path computation 2.  Simple cooperating PCEs 3.  Backward Recursive Path Computation

Per-Domain Path Computation

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  PCE calculates the path for it’s own domain.   Pro:

  Simple   Cont:

  It’s not the optimal path route, because you can’t see all the domains.

PCE1

PCE2

1 2

3

4

Cooperating PCEs

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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

Backward Recursive Path Computation

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  PCE calculates the path for all domains.   Pass the set of paths to the neighboring PCEs   Pro:

  Compute the optimal global path   Cont:

  It’s slow and expensive.   It’s a point of failures.   The destination must be known.

A

TQ

L

M

J I

GEC

B VSD

PCE1 PCE2

PCE3

BRPC: Mesh Domain Problems

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  BRPC uses recursive backward algorithm  The root it’s the destination   That means every PCE-AS calculates it’s own domain part of

path and then delegates route tree to the backward crossed AS to find the optimum path

 This algorithm generates death loops

23

A UT

Q

R

L

M

N

K

J

I H

GE

FC

B

SP

DPCE1

PCE2

PCE3

PCE4

Conclusions

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  PCE is a logical functional component  May be centralized within a domain or distributed   It is not an all-seeing oracle

  PCEs may cooperate to determine end-to-end multidomain path

  It may too complex for large multidomain paths   Loops, erased by knowing sequences of domains

Bibliography   http://people.ac.upc.edu/careglio/teaching/XC2/XCII-Tema2e.pdf   http://en.wikipedia.org/wiki/Quality_of_service   http://tools.ietf.org/html/rfc5088   http://docwiki.cisco.com/wiki/Main_Page   http://www.cisco.com/en/US/docs/ios_xr_sw/iosxr_r3.7/routing/

configuration/guide/rc37ospf.html

  http://es.scribd.com/yerenial/d/89531987-Cisco-PCE   http://www.itg523.de/oeffentlich/07maerz/PCE-Concept.pdf   http://www.rfc-editor.org/rfc/rfc4655.txt   http://www.olddog.co.uk/Farrel_PCE_Tutorial.ppt

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Path Computation Element (PCE)

PCE