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Multiprotocol Label Switching (MPLS)
and its Applications
Network Architecture
Spring 2009
Lecture 17
1
Sources for this Material
• MPLS presentation by Philip Matthews,
Nortel Networks,
April 2000, prepared by Dr. Bilel Jamoussi
and
Peter Ashwood-Smith
• "Simplified Operations Through Resilient IP
Network Design" presented by Hadriel
Kaplan, Avici Systems, IPOM 2003 Tutorial
2
“Label Substitution” what is it?
Have a friend go to B ahead of you. At every road they
reserve a lane just for you. At every intersection they post
a big sign that says for a given lane which way to turn and
what new lane to take.
Your job in getting to the destination is now easy.
LANE#1 TURN RIGHT USE LANE#2
LANE#1
LANE#2
3
Label Switched Path
#3 Right #7
IP
#3
#7
#7 LEFT #99
#99 RIGHT #9
#99
#9
#9 LEFT #4072
#4072
IP
4
Routers Do Both Routing and Switching
• Routing
— Deciding the next hop based on
the destination address.
— A Layer 3 (L3) function.
• Switching
— Moving a packet from an input
port to an output port and out.
— A layer 2 function.
— Usually a switching decision is a
simple table lookup.
INPUT PORTS
OUTPUT PORTS
5
STANDARD IP
Dest
47.1
47.2
47.3
Dest
47.1
47.2
47.3
Out
1
2
3
1 47.1
1
Dest
47.1
47.2
47.3
Out
1
2
3
IP 47.1.1.1
2
IP 47.1.1.1
3
Out
1
2
3
2
IP 47.1.1.1
1
47.2
47.3 3
2
IP 47.1.1.1
6
Label Switched Path (LSP)
Intf Label Dest Intf Label
In In
Out Out
3
50
47.1 1
40
Intf Dest Intf Label
In
Out Out
3
47.1 1
50
3
1
47.3 3
Label Dest Intf
In
Out
40
47.1 1
IP 47.1.1.1
1 47.1
3
1
Intf
In
3
2
2
47.2
2
IP 47.1.1.1
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MPLS: Flexible Forwarding
IP: Packets are forwarded based on Destination Address (DA). We can
call this “destination based routing”.
IP DA
IP DA
IP DA
IP DA
IP DA
MPLS:
• Map packets to LSP based on (Source Address, Destination Address,
protocol, port, DSCP, interface, etc.)
• Forward packets based on the Label
IP
IP
IP to LSP
#L1
IP
#L2
LABEL SWITCHING
IP
#L3
IP
LSP to IP
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What is MPLS?
• MPLS = Multi-Protocol Label Switching
• MPLS is an IETF Standardized mechanism for
controlling packet routing.
• MPLS Framework and Architecture
— Defines the scope, the various components and their interactions
• Encapsulations
— Labels are used at the data plane to make forwarding decisions
• Signaling Protocols
— Distribute Labels to establish Label Switched Paths
• Routing Protocol Traffic Engineering Extensions
— Distribute Bandwidth and other link attributes to make routing
decisions
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MPLS Terminology
• LDP: Label Distribution Protocol
• LSP: Label Switched Path
• LER: Label Edge Router (edge of an area that
supports MPLS)
• LSR: Label Switching Router (inside an area that
supports MPLS)
• FEC: Forwarding Equivalence Class
— Which packets are on which LSPs – based on
destination, source, QoS, application, UDP or TCP, etc.
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MPLS Encapsulation
MPLS ‘Shim’ Headers (1-n)
n
•••
1
Network Layer Header
and Packet (eg. IP)
Layer 2 Header
(eg. PPP, 802.3)
4 Octets
Label Stack
Entry Format
Label
Exp.
S
TTL
Label: Label Value, 20 bits (Values 0 through 16 are reserved)
Exp.:
Experimental, 3 bits (was Class of Service)
S:
Bottom of Stack, 1 bit (1 = last entry in label stack)
TTL:
Time to Live, 8 bits
• Network layer must be inferable from value of bottom label of the stack
MPLS on LANs uses a ‘Shim’ Header Inserted
Between Layer 2 and Layer 3 Headers
(other technologies use different approaches)
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MPLS Turns Routing into Switching
• So we can avoid performing the layer 3 function.
— Use labels to decide next hops.
• What benefit does this provide?
• In what situations would this benefit not be very
significant?
12
Solutions Enabled by MPLS
• Virtual Private Networks
— Connect two or more separate sites over the Internet
— Label switched paths can be created to be “virtual links” between
routers.
— This can create something that looks like a network for a customer.
— Key Features: Security, control over performance, management ability.
• Enable QoS in IP Networks
— Support Diffserv using connection-oriented QoS
— “Connections” can be flows or large aggregates
• IP Traffic Engineering
— Use constraint-based routing to adapt to latest network loading and
QoS performance
• L2/L3 Integration
— Integrate with L1 and L2 technologies like Optical Cross Connects
(OXC’s) and ATM
•
Resilient Network Design
— Automatic Failover and Backup
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BEST OF BOTH WORLDS
PACKET
Forwarding
IP
HYBRID
MPLS
+IP
CIRCUIT
SWITCHING
ATM
• MPLS + IP forms a middle ground that combines
the best of IP and the best of circuit switching
technologies.
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EXPLICITLY ROUTED LSP ER-LSP
This entry gives the
longest prefix match.
Intf Label Dest Intf Label
In In
Out Out
3
50
47.1 1
40
Intf Dest
In
3
47.1.1
3
47.1
Intf
Out
2
1
Label
Out
33
50
Intf
In
3
Label Dest Intf
In
Out
40
47.1 1
IP 47.1.1.1
1 47.1
3
3
2
1
1
47.3 3
2
47.2
2
IP 47.1.1.1
Explicitly Routed
RoutingLSP
LSP
that does not follow
the standard IP path.
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ER LSP - Advantages
• Operator has routing flexibility
— Can establish LSP’s based on policy, QoS, etc.
— Can have pre-established LSP’s that can be used in case
of failures.
• Can use routes other than the shortest path
• Can compute routes based on dynamic
constraints (available bandwidth, delay, etc.)
based on a distributed topology database.
(traffic engineering)
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Traffic Engineering
B
C
Demand
A
D
Traffic engineering is the process of mapping traffic demand onto a network
Network
Topology
Purpose of traffic engineering:
• Maximize utilization of links and nodes throughout the network
• Engineer links to achieve required delay, grade-of-service
• Spread the network traffic across network links to minimize impact of failure
• Ensure available spare link capacity for re-routing traffic on failure
• Meet policy requirements imposed by the network operator
Traffic engineering is key to optimizing cost/performance
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The need for MPLS protection
• Layer 3 recovery is too slow.
— OSPF, RIP, etc. require a redistribution of updated link status
information in response to a fault.
— Then routers must recompute their routes.
— Takes on the order of seconds.
— Can have looping and lost packets in the meantime.
• Other technologies are very fast.
— SONET can establish an alternate route around a failure within
50 milliseconds.
— By having active backup resources immediately available.
• It would be good to have millisecond failovers with
MPLS.
18
Pre-signaled Standby LSP’s
• Planning occurs before failure
— Then LSP ingress learns of the failure
— Moves traffic to use standby LSP
• Ingress must first know about the failure
— Must receive failure notifications.
— The farther away from the failure, the longer it will take to start
the reroute.
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MPLS Fast Reroute
— A merge node joins traffic back onto the primary LSP.
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Summary of Motivations for MPLS
• Simplified forwarding based on an exact match of a fixed
length label
— Initial driver for MPLS was based on the existence of cheap, fast
switches from previous ATM technology
• Separation of routing and forwarding in IP networks
— Facilitates evolution of routing techniques by fixing the forwarding
method
— New routing functionality can be deployed without changing the
forwarding techniques of every router in the Internet
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Summary of Motivations for MPLS
• Enables the use of explicit routing/source routing in IP
networks
— Can easily be used for such things as traffic management, QoS
routing
• Promotes the partitioning of functionality within the
network
— Moves detailed processing of packets to the edge; restricts core to
simple packet forwarding
— Assists in maintaining scalability of IP protocols in large networks
• MPLS can enable fast restoration from failures.
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Summary of Motivations for MPLS
• Applicability to multiple layers
— Can be deployed at Layer 2 on Ethernet, Wireless, or legacy ATM and
Frame Relay technologies.
— Can be deployed at Layer 1 for Fiber, Wireless, etc.
• But MPLS is much more complex than traditional IP
forwarding
— Routers need to be able to forward based on labels (in addition to their
normal functions).
— LSP’s must be signalled and maintained.
— Some ISP’s have said they are not using MPLS and do not plan to.
– This will continue to be true if overprovisioning remains effective.
– But some of these ISP’s are realizing that their customers want
MPLS to provide more assurance about their IP-based services.
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