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MPLS: Traffic Engineering and
Restoration Routing Basics
Zartash Afzal Uzmi
Computer Science and Engineering Department
Lahore University of Management Sciences
October 8, 2004
MPLS: TE and Restoration
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Outline
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Background
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MPLS Routing Basics


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IP Routing and related problems
Labels and label switched paths
Traffic Engineering
Restoration Routing
Our Research
Conclusions
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Application Scenario
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

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:

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Guaranteed minimum bandwidth between a
source and a destination
Less then 50ms recovery time in the event of any
network element failure
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Traditional (IP) Routing

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Characterized by best effort service
Individual nodes (routers) take routing and
forwarding decisions
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Usually based on a pre-computed shortest path
Forwarding is destination based

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When routers forward packets, they only look at
the destination address
May lead to congestion in some parts of the
network
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IP Routing Example
D
1
A
1
S
B


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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
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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
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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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Routing Along Parallel 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)
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MPLS: Basics

How did they route along parallel paths?
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They did use a label
They also decided to use a new label at each hop
to save on label space
Label
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IP Datagram
Terminology
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LSP: Label switched path
LSR: Label switch router
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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
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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)
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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)
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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
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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
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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 Routing
1
2
S
D
3
4
5
MPLS allows routing on pre-established paths!
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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
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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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Restoration Routing
Application of Traffic Engineering
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Restoration in IP network
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In traditional IP, what happens when a
link or node fails?
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Information needs to be disseminated in
the network
During this time, packets may go in loops
Restoration latency is in the order of
seconds
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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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Restoration in MPLS
Element Local Protection
S
1
Primary Path
2
3
D
Local Protection takes of order of 10ms
Backup Path
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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 established
Can we do something to avoid “wasting” so
much bandwidth in backup paths?
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Try to share the backup bandwidth!
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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
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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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b2
5
D2
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Creation of Backup Paths
b
a
i
j
c
p
k
l
backup path
primary LSP
Backup path a protects primary LSPs traversing link(i,j) and link(j,k)
Backup path b protects primary LSPs traversing link(i,j) and link(j,l)
Backup path c protects primary LSPs traversing link(j,k)
Backup path d protects primary LSPs traversing link(j,l)
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Types of Backup Paths
s
d
primary path
next-hop backup path
next-next-hop backup path
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s
ingress node
d
egress node
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Backup Paths: Definitions
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A next-hop (nhop) backup path that
spans link(i,j) is a backup path which:
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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
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Backup Paths: Definitions

A next-next-hop (nnhop) backup path that
spans link(i,j) and link(j,k) is a backup path
which:
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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:
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Node j fails
Link(i,j) fails
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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 is a single activation set can
not share the bandwidth
Backups in different activation sets may
share the bandwidth
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Activation Set for node j
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What paths are activated when node j
fails?
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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!
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Activation Set for node j
i
j
k
l
next-next-hop backup path
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Activation Set for link(i,j)
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What paths are activated when link(i,j)
fails:
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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
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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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Providing Protection
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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:
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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)
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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
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Providing Protection
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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
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Computing the Backups
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How much bandwidth can be shared?
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Aggregate information scenario:
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Depends upon the routing information propagated
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
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Computing the Backups
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When the new backup path is nhop, how much is shareable on
link(u,v)?
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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)?
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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))
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Computing the Backups
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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
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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
10 experiments with randomly selected
ingress/egress pairs and traffic demands
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Schemes Compared
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Kini’s scheme
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Facility
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Signalled path is suboptimal
Reservations made are corrective
Optimal path is signalled
Static pools for primary and backups
NPP
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Primary and backups dynamically allocated
Optimal path is signalled
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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
October 8, 2004
400
550
700
850
Total Number of LSP Requests
MPLS: TE and Restoration
1000
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