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MPLS: Traffic Engineering and
Restoration Routing
Zartash Afzal Uzmi
Computer Science and Engineering
Lahore University of Management Sciences
December 20, 2004
MPLS: TE and Restoration
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Outline
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Background


MPLS Routing




IP Routing and related problems
Labels and label switched paths
Traffic Engineering
Restoration Routing
Our Research
December 20, 2004
MPLS: TE and Restoration
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Application Scenario
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

A service provider (ISP) with geographically
distributed points of presence (PoPs)
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
December 20, 2004
MPLS: TE and Restoration
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Traditional (IP) Routing
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Individual nodes (routers) take

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A routing decision, and
A forwarding decision
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
December 20, 2004
MPLS: TE and Restoration
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IP Routing Example
D
1
A
1
S
B




1
B
2
C
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


A
Leads to IP hotspots!
Solution?

Try to divert the traffic onto alternate paths
December 20, 2004
MPLS: TE and Restoration
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IP Routing Example
D
1
A
4
S
B




A
1
C
B
2
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
December 20, 2004
MPLS: TE and Restoration
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IP Routing Vs MPLS
Traditional IP Label
Routing
Multiprotocol
Switching (MPLS)
1
2
S
D
3
4
5
MPLS allows overriding shortest paths!
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MPLS: TE and Restoration
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Routing Along Explicit Paths
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Idea:
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
Let the source make the complete routing decision;
source decides the complete path for each flow
How this may be accomplished?
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
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?

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Define some constraints and hope that the constraints will
take “some” traffic away from the hotspot!
Use CSPF instead of SPF (shortest path first)
December 20, 2004
MPLS: TE and Restoration
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MPLS: Basics

How did they route along parallel paths?

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They decided to use a label
They also decided to use a new label at each hop
to save on label space
Label

IP Datagram
Terminology
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
LSP: Label switched path
LSR: Label switch router
December 20, 2004
MPLS: TE and Restoration
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MPLS Flow Progress
D
R1
LSR4
R2
LSR1
D
LSR6
destination
LSR3
LSR2
R1 and R2 are
regular routers
LSR5
1 - R1 receives a packet for destination D connected to R2
December 20, 2004
MPLS: TE and Restoration
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MPLS Flow Progress
R1
D
LSR4
R2
LSR1
D
LSR6
destination
LSR3
LSR2
LSR5
2 - R1 determines the next hop as LSR1 and forwards the packet
(Makes a routing as well as a forwarding decision)
December 20, 2004
MPLS: TE and Restoration
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MPLS Flow Progress
R1
LSR4
LSR1
31
R2
D
D
LSR6
destination
LSR3
LSR2
LSR5
3 – LSR1 establishes a path to LSR6 and “PUSHES” a label
(Makes a routing as well as a forwarding decision)
December 20, 2004
MPLS: TE and Restoration
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MPLS Flow Progress
R1
LSR4
R2
LSR1
D
LSR6
LSR3
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destination
D
LSR2
LSR5
Labels have local
signifacance!
4 – LSR3 just looks at the incoming label
LSR3 “SWAPS” with another label before forwarding
December 20, 2004
MPLS: TE and Restoration
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MPLS Flow Progress
R1
LSR4
R2
LSR1
D
LSR6
LSR3
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destination
D
LSR2
LSR5
Path within MPLS cloud
is pre-established:
LSP (label-switched path)
5 – LSR6 looks at the incoming label
LSR6 “POPS” the label before forwarding to R2
December 20, 2004
MPLS: TE and Restoration
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TE Capability Recap
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Who establishes the LSPs in advance?
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Ingress routers
How do ingress routers decide not to always
take the shortest path?
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Ingress routers use CSPF (constrained shortest
path first) instead of SPF
Examples of constraints:
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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
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MPLS: TE and Restoration
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IP versus MPLS: Summary
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In IP Routing, each router makes its own
routing and forwarding decisions

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In IP Routing, packets usually follow the SPF

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In MPLS, source makes the routing decision
Intermediate routers make forwarding decisions
In MPLS packets follow the CSPF
In IP Routing, restoration takes few seconds

In MPLS, restoration can be of the order of 10ms
December 20, 2004
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CSPF
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What is the mechanism?
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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
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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
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MPLS: TE and Restoration
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Restoration Routing
Application of Traffic Engineering
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MPLS: TE and Restoration
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Restoration in IP network

In traditional IP, what happens when a
link or node fails?
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Failure information must be disseminated
in the network
During this time, packets may go in loops
Restoration latency is in the order of
seconds
December 20, 2004
MPLS: TE and Restoration
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Restoration in MPLS
Path Protection
S
1
2
3
D
This type of “path Protection”
still takes 100s of ms.
Primary Path
Backup Path
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MPLS: TE and Restoration
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Restoration in MPLS
Element Local Protection
S
1
Primary Path
2
3
D
Local Protection takes order of 10ms
Backup Path
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MPLS: TE and Restoration
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Opportunity Cost
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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

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Fewer primary LSPs can be accommodated
Can we do something to avoid “wasting” so
much bandwidth in backup paths?

Try to share the backup bandwidth!
December 20, 2004
MPLS: TE and Restoration
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BW Sharing in Backup Paths
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Assumption:
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Two primary paths, whose backups are
sharing bandwidth, must not fail together
Is this assumption realistic?
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Failure is a low probability event
Once failure occurs, new primary paths
with new backups are computed
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Failure of another element in that time is
unlikely
December 20, 2004
MPLS: TE and Restoration
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BW Sharing in Backup Paths
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Example
b1
S1
D1
= LSR
max(b1, b2)
3
S2
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4
b2
5
D2
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Activation Sets
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When an element fails, a number of
backups are activated “simultaneously”
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Such backups are in the activation set of
that protected element
Backups in a single activation set can
not share the bandwidth
Backups in different activation sets may
share the bandwidth
December 20, 2004
MPLS: TE and Restoration
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Activation Set for node j
i
j
k
l
next-next-hop backup path
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MPLS: TE and Restoration
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Activation Set for link(i,j)
g
k
i
j
h
l
next-hop backup path
next-next-hop backup path
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MPLS: TE and Restoration
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Considerations
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What constitutes a backup sharing scheme?
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How much information is propagated through
routing protocols
Who propagates that information
Who maintains that information
Design an algorithm which provides
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Maximum bandwidth sharing
Minimum information propagation
December 20, 2004
MPLS: TE and Restoration
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Simulation Parameters
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20 node ISP network
Each link with capacity 120 units
380 possible pairs
LSP requests arrive one by one
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Ingress/Egress chosen randomly
Bandwidth demand for each request is
uniformly distributed between 1 and 6
Call holding time is infinite
100 experiments with randomly selected
ingress/egress pairs and traffic demands
December 20, 2004
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Schemes Compared
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Kini’s scheme
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Facility
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Signaled path is suboptimal
Reservations made are corrective
Optimal path is signaled
Static pools for primary and backups
NPP
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Primary and backups dynamically allocated
Optimal path is signaled
December 20, 2004
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Results
Number of LSPs Placed
1000
800
600
400
NPP
200
FAC
KINI
0
250
400
550
700
850
1000
Total Number of LSP Requests
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Results
3000
Total BW Placed
2500
2000
1500
1000
NPP
FAC
500
KINI
0
250
December 20, 2004
400
550
700
850
Total Number of LSP Requests
MPLS: TE and Restoration
1000
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