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Path Computation Element (PCE) PCE

PCE · 2013-11-25 · 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

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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

2

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

4

Qualities of traffic

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

5

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

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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