1. MPLS and its Applications CS 520 – Winter 2006 Lecture 17

2. 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

3. “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.
LANE#1 TURN RIGHT USE LANE#2
LANE#1
LANE#2

4. Label Switched Path #3 #7 #99 #9 #4072 IP IP #3 Right #7 #7 LEFT #99

5. Routers Do Both Routing and Switching
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

6. STANDARD IP 1
47.1 IP 1 2 IP 3 2 IP 1 3
47.2
47.3 2 IP

7. Label Switched Path (LSP)IP 1
47.1 3 3 2 1 1 2
47.3 3
47.2 2 IP

8. 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
#L1 IP
#L2 IP
#L3 IP
IP to LSP
LABEL SWITCHING
LSP to IP

9. 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?

10. 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

11. 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

12. IP MPLS+IP ATM BEST OF BOTH WORLDS
PACKET Forwarding
CIRCUIT SWITCHING
HYBRID IP
MPLS+IP
ATM
MPLS + IP forms a middle ground that combines the best of IP and the best of circuit switching technologies.

13. 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

14. EXPLICITLY ROUTED LSP ER-LSP
This entry gives the longest prefix match. IP 1
47.1 3 3
Explicitly Routing LSP that does not follow the standard IP path. 2 1 1 2
47.3 3
47.2 2 IP

15. 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)

16. MPLS Encapsulation •••
MPLS ‘Shim’ Headers (1-n) n
••• 1
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
4 Octets
Label Stack
Entry Format
Layer 2 Header
(eg. PPP, 802.3)
Network Layer Header
and Packet (eg. IP)
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)

17. Traffic Engineering Purpose of traffic engineering: B C A D
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

18. 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.

19. 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.

20. MPLS Fast Reroute
A merge node joins traffic back onto the primary LSP.

21. 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

22. 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.

23. 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.