october 8, 2004mpls: te and restoration1 mpls: traffic engineering and restoration routing basics...

October 8, 2004 MPLS: TE and Restoration 1

MPLS: Traffic Engineering and Restoration Routing Basics

Zartash Afzal UzmiComputer Science and Engineering

DepartmentLahore University of Management

Sciences


Outline Background

IP Routing and related problems MPLS Routing Basics

Labels and label switched paths Traffic Engineering Restoration Routing Our Research Conclusions


Application Scenario A service provider (ISP) with several points

of presence (PoPs) geographically distributed

ISP provisions applications with “strict” network requirements (e.g., VoIP service)

Two major requirements: Guaranteed minimum bandwidth between a

source and a destination Less then 50ms recovery time in the event of

any network element failure


Traditional (IP) Routing Characterized by best effort service Individual nodes (routers) take routing

and forwarding decisions Usually based on a pre-computed shortest

path Forwarding is destination based

When routers forward packets, they only look at the destination address

May lead to congestion in some parts of the network


IP Routing Example

Packet 1: Destination A Packet 2: Destination B S computes shortest paths to A and B; finds D as next hop Both packets will follow the same path

Leads to IP hotspots! Solution?

Try to divert the traffic onto alternate paths

1 1

1 2

A B

C

A

B

S

D


IP Routing Example

Increase the cost of link DA from 1 to 4 Traffic is diverted away from node D A new IP hotspot is created! Solution(?): Network Engineering

Put more bandwidth where the traffic is! Leads to underutilized links; not suitable for large networks

1 4

1 2

A B

C

SA

B

D


IP Routing Vs MPLSTraditional IP RoutingMultiprotocol Label Switching (MPLS)

S D

543

21

MPLS allows overriding shortest paths!


Routing Along Parallel Paths

Idea: Let the source make the complete routing

decision; source decides the complete path for each flow

How this may be accomplished? Attach a label to the IP packets; let everyone make

forwarding decision on that label On what basis should you choose different

paths for different flows? Define some constraints and hope that the constraints

will take “some” traffic away from the hotspot! Use CSPF instead of SPF (shortest path first)


MPLS: Basics

How did they route along parallel paths? They did use a label They also decided to use a new label at

each hop to save on label space

Terminology LSP: Label switched path LSR: Label switch router

IP DatagramLabel


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4D

1 - R1 receives a packet for destination D connected to R2

R1 and R2 areregular routers

D

destination


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4D

2 - R1 determines the next hop as LSR1 and forwards the packet(Makes a routing as well as a forwarding decision)

D

destination


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

3 – LSR1 establishes a path to LSR6 and “PUSHES” a label(Makes a routing as well as a forwarding decision)

D

destination

31


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

4 – LSR3 just looks at the incoming labelLSR3 “SWAPS” with another label before forwarding

D

destination

17

Labels have localsignifacance!


Mpls Flow Progress

LSR1

LSR2

LSR3

LSR5

LSR6

R1 R2LSR4

D

5 – LSR6 looks at the incoming labelLSR6 “POPS” the label before forwarding to R2

D

destination

17

Path within MPLS cloudis pre-established:LSP (label-switched path)


TE Capability Recap Who establishes the LSPs in advance?

Ingress routers How do ingress routers decide not to

always take the shortest path? Ingress routers use CSPF (constrained

shortest path first) instead of SPF Examples of constraints:

Do not use links left with less than 7Mb/s bandwidth

Do not use links with blue color for this request Use a path with delay less than 130ms


MPLS Routing

S D

543

21

MPLS allows routing on pre-established paths!


IP versus MPLS: Summary In IP Routing, each router makes its own

routing and forwarding decisions In MPLS, source makes the routing decision Intermediate routers make forwarding decisions

In IP Routing, packets usually follow the SPF In MPLS packets follow the CSPF

In IP Routing, restoration takes few seconds In MPLS, restoration can be of the order of 10ms


CSPF What is the mechanism?

First prune all links not fulfilling constrains Now find shortest path on the rest of the

topology Requires some Reservation mechanism Changing state of the network must also

be recorded and propagated For example, ingress needs to know how

much bandwidth is left on links The information is propagated by means of

routing protocols and their extensions


Restoration Routing

Application of Traffic Engineering


Restoration in IP network

In traditional IP, what happens when a link or node fails? Information needs to be disseminated

in the network During this time, packets may go in

loops Restoration latency is in the order of

seconds


Restoration in MPLS

S 1 2 3 D

Primary Path

Backup Path

Path Protection

This type of “path Protection” still takes 100s of ms.


Restoration in MPLS

S 1 2 3 D

Primary Path

Backup Path

Element Local Protection

Local Protection takes of order of 10ms


Opportunity Cost Fast restoration requires that backup

paths are established “in advance” Backup provisioning requires bandwidth

reservation along the backup paths Backup bandwidth is taken from the

primary bandwidth Fewer primary LSPs can be established

Can we do something to avoid “wasting” so much bandwidth in backup paths? Try to share the backup bandwidth!


BW Sharing in Backup Paths Assumption:

Two primary paths, whose backups are sharing bandwidth, must not fail together

Is this assumption realistic? Failure is a low probability event Once failure occurs, new primary paths

with new backups are computed Failure of another element in that time is

unlikely


BW Sharing in Backup Paths

Example:-

S1 D1

S2 D2

4 53

b1

b2

max(b1, b2)

= LSR


Creation of Backup Paths


Types of Backup Paths

primary path

next-hop backup pathingress node

egress nodenext-next-hop backup path

s

d

s d


Backup Paths: Definitions

A next-hop (nhop) backup path that spans link(i,j) is a backup path which: Originates at node i Merges with the primary at node j Provides restoration for one or more

primary LSPs that traverse link(i,j) when:

link(i,j) fails


Backup Paths: Definitions A next-next-hop (nnhop) backup path

that spans link(i,j) and link(j,k) is a backup path which: Originates at node i Merges with the primary at node k Provides restoration for one or more primary

LSPs that traverse link(i,j) and link(j,k) when either:

Node j fails Link(i,j) fails


Activation Sets When an element fails, a number

of backups are activated “simultaneously” Such backups are in the activation set

of that protected element Backups is a single activation set

can not share the bandwidth Backups in different activation sets

may share the bandwidth


Activation Set for node j

What paths are activated when node j fails? NNhop paths that span link(x,j) and

link(j,y) for all x,y

Note that a node is protected by nnhop paths only!


Activation Set for node j

next-next-hop backup path

l

j

k

i


Activation Set for link(i,j)

What paths are activated when link(i,j) fails: Nhop path that spans link(i,j) Nhop path that spans link(j,i) NNhop paths that span link(i,j) and

link(j,x) for all x not equal to i,j NNhop paths that span link(j,i) and

link(i,x) for all x not equal to i,j


Activation Set for link(i,j)

g

lh

k

ji

next-hop backup path

next-next-hop backup path


Providing Protection Suppose link(i,j) is traversed by a new primary

LSP with bandwidth demand b A backup path “around” the link(i,j) can either be:

Nhop path (if node j is egress) NNhop path (if node j is not egress) In either case, point of local repair (PLR) is node i

We are protecting the LSP that traverses the triplet(PLR, facility, MP)

PLR is always node i Facility is the entity being protected: link(i,j) or node j MP is either node j or some other node adjacent to node

j


Providing Protection

Let the bandwidth corresponding to previously established LSPs traversing the triplet (PLR, facility, MP) is bold

The backup path is recomputed with bandwidth demand bnew = bold+b

Various computation algorithms can be deployed and have been studied


Computing the Backups How much bandwidth can be shared?

Depends upon the routing information propagated

Aggregate information scenario: Fij: BW reserved on link(i,j) for primary LSPs Gij: BW reserved on link(i,j) for backup LSPs Rij: Residual BW on link(i,j) Link(i,j) will propagate above information

Note: total primary BW on link(i,j) is Fij+Fji


Computing the Backups When the new backup path is nhop, how much is

shareable on link(u,v)? Fij+Fji-bold is the maximum bandwidth that will

simultaneously be active with new backup The bandwith shareable on link(u,v) is:

Suv = max(0, Guv – (Fij+Fji-bold)) When the new backup path is nnhop, how much is

shareable on link(u,v)? Note that nnhop is protecting against a link as well as a

node. Thus, the bandwidth required for both the activation sets must be computed

Max(Fij+Fji-bold, Fxj-bold) is the maximum that will simultaneously be active with the new backup

The bandwidth shareable on link(u,v) is: Suv = max(0, Guv – max(Fij+Fji-bold, Fxj-bold))


Computing the Backups PLR knows Ruv and Suv for all links PLR computes total bandwidth Ruv+Suv available

to route the new backup path on each link(u,v) All links for which Ruv+Suv < bnew are pruned For each remaining link(u,v), the additional

bandwidth required is given by max(0, bnew-Suv)

PLR computes the route that requires minimum additional bandwidth

Note: The computed path is sub-optimal


Simulation Parameters 20 node ISP network Each link with capacity 120 units 380 possible pairs LSP requests arrive one by one

Ingress/Egress chosen randomly Bandwidth demand for each request is

uniformly distributed between 1 and 6 Call holding time is infinite 10 experiments with randomly selected

ingress/egress pairs and traffic demands


Schemes Compared Kini’s scheme

Signalled path is suboptimal Reservations made are corrective

Facility Optimal path is signalled Static pools for primary and backups

NPP Primary and backups dynamically allocated Optimal path is signalled


Results

0

200

400

600

800

1000

250 400 550 700 850 1000

Total Number of LSP Requests

Nu

mb

er o

f L

SP

s P

lace

d

NPP

FAC

KINI


Results

0

500

1000

1500

2000

2500

3000

250 400 550 700 850 1000Total Number of LSP Requests

To

tal

BW

Pla

ced

NPP

FAC

KINI

october 8, 2004mpls: te and restoration1 mpls: traffic engineering and restoration routing basics...

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