MPLS and its Applications CS 520

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MPLS and its ApplicationsCS 520 Winter
2007Lecture 17
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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

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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.
LANE1
LANE2
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Label Switched Path
3 Right 7
7 LEFT 99
99 RIGHT 9
9 LEFT 4072
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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
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STANDARD IP
47.1
1
IP 47.1.1.1
2
IP 47.1.1.1
1
3
2
IP 47.1.1.1
1
47.2
3
47.3
2
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Label Switched Path (LSP)
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
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MPLS Flexible Forwarding
IP Packets are forwarded based on Destination
Address (DA). We can call this destination
based routing.

• MPLS
• Map packets to LSP based on (Source Address,
  Destination Address, protocol, port, DSCP,
  interface, etc.)
• Forward packets based on the Label

IP
IP
LSP to IP
IP to LSP
LABEL SWITCHING
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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?

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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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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 (OXCs) and ATM
• Resilient Network Design
• Automatic Failover and Backup

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BEST OF BOTH WORLDS
CIRCUITSWITCHING
PACKETForwarding
HYBRID

• MPLS IP forms a middle ground that combines the
  best of IP and the best of circuit switching
  technologies.

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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

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EXPLICITLY ROUTED LSP ER-LSP
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
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ER LSP - Advantages

• Operator has routing flexibility
• Can establish LSPs based on policy, QoS, etc.
• Can have pre-established LSPs 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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MPLS Encapsulation
MPLS Shim Headers (1-n)

n
1
Network Layer Header and Packet (eg. IP)
Layer 2 Header (eg. PPP, 802.3)

• 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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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.

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Pre-signaled Standby LSPs

• 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).
• LSPs must be signalled and maintained.
• Some ISPs 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 ISPs are realizing that their
  customers want MPLS to provide more assurance
  about their IP-based services.