1. Introduction to MPLS and Traffic Engineering
Zartash Afzal Uzmi

2. CS 573: Network Protocols and Standards
Feb 14, 2008
Outline
Traditional IP Routing
Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Terminology and Operation
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]
Nomenclature
Requirements
Examples
Feb 14, 2008
CS 573: Network Protocols and Standards

3. CS 573: Network Protocols and Standards
Feb 14, 2008
Outline
Traditional IP Routing
Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Terminology and Operation
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]
Nomenclature
Requirements
Examples
Feb 14, 2008
CS 573: Network Protocols and Standards

4. Forwarding and routing
Passing a packet to the next hop router
Routing:
Computing the “best” path to the destination
IP routing – includes routing and forwarding
Each router makes the forwarding decision
Each router makes the routing decision
MPLS routing
Only one router (source) makes the routing decision
Intermediate routers make the forwarding decision
Feb 14, 2008
CS 573: Network Protocols and Standards

5. CS 573: Network Protocols and Standards
IP versus MPLS routing
IP routing
Each IP datagram is routed independently
Routing and forwarding is destination-based
Routers look at the destination addresses
May lead to congestion in parts of the network
MPLS routing
A path is computed “in advance” and a “virtual circuit” is established from ingress to egress
An MPLS path from ingress to egress node is called a label switched path (LSP)
Feb 14, 2008
CS 573: Network Protocols and Standards

6. CS 573: Network Protocols and Standards
How IP routing works
Searching Longest Prefix Match in FIB (Too Slow)
Feb 14, 2008
CS 573: Network Protocols and Standards

7. Problems with IP routing
Too slow
IP lookup (longest prefix matching) “was” a major bottleneck in high performance routers
This was made worse by the fact that IP forwarding requires complex lookup operation at every hop along the path
Too rigid – no flexibility
Routing decisions are destination-based
Not scalable in some desirable applications
When mapping IP traffic onto ATM
Feb 14, 2008
CS 573: Network Protocols and Standards

8. IP routing rigidity example1 1 A S A B B 1 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
Leads to IP hotspots!
Solution?
Try to divert the traffic onto alternate paths
Feb 14, 2008
CS 573: Network Protocols and Standards

9. IP routing rigidity example1 4 A S A B B 1 2 C
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
Feb 14, 2008
CS 573: Network Protocols and Standards

10. Motivations behind MPLS
Avoid [slow] IP lookup
Led to the development of IP switching in 1996
Provide some scalability for IP over ATM
Evolve routing functionality
Control was too closely tied to forwarding
Evolution of routing functionality led to some other benefits
Explicit path routing
Provision of service differentiation (QoS)
Feb 14, 2008
CS 573: Network Protocols and Standards

11. IP routing versus MPLS routing
Multiprotocol Label Switching (MPLS)
Traditional IP Routing 1 2 S D 3 4 5
MPLS allows overriding shortest paths!
Feb 14, 2008
CS 573: Network Protocols and Standards

12. CS 573: Network Protocols and Standards
Feb 14, 2008
Outline
Traditional IP Routing
Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Terminology and Operation
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]
Nomenclature
Requirements
Examples
Feb 14, 2008
CS 573: Network Protocols and Standards

13. CS 573: Network Protocols and Standards
MPLS label
To avoid IP lookup MPLS packets carry extra information called “Label”
Packet forwarding decision is made using label-based lookups
Labels have local significance only!
How routing along explicit path works?
Label
IP Datagram
Feb 14, 2008
CS 573: Network Protocols and Standards

14. Routing along explicit paths
Idea: Let the source make the complete routing decision
How is this accomplished?
Let the ingress attach a label to the IP packet and let intermediate routers make forwarding decisions only
On what basis should you choose different paths for different flows?
Define some constraints and hope that the constraints will take “some” traffic away from the hotspot!
Use CSPF instead of SPF (shortest path first)
Feb 14, 2008
CS 573: Network Protocols and Standards

15. CS 573: Network Protocols and Standards
Label, LSP and LSR
Label
Router that supports MPLS is known as label switching router (LSR)
An “Edge” LSR is also known as LER (edge router)
Path which is followed using labels is called LSP
Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits
Label | Exp|S| TTL
Feb 14, 2008
CS 573: Network Protocols and Standards

16. CS 573: Network Protocols and Standards
LFIB versus FIB
Labels are searched in LFIB whereas normal IP Routing uses FIB to search longest prefix match for a destination IP address
Why switching based on labels is faster?
LFIB has fewer entries
Routing table FIB has larger number of entries???
In LFIB, label is an exact match
In FIB, IP is longest prefix match
Feb 14, 2008
CS 573: Network Protocols and Standards

17. CS 573: Network Protocols and Standards
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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CS 573: Network Protocols and Standards

18. CS 573: Network Protocols and Standards
Mpls Flow Progress D R1
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)
Feb 14, 2008
CS 573: Network Protocols and Standards

19. CS 573: Network Protocols and Standards
Mpls Flow Progress R1
LSR4 R2
LSR1 31 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)
Feb 14, 2008
CS 573: Network Protocols and Standards

20. CS 573: Network Protocols and Standards
Mpls Flow Progress R1
LSR4 R2
LSR1 D
LSR6
destination
LSR3 17 D
LSR2
Labels have local
signifacance!
LSR5
4 – LSR3 just looks at the incoming label
LSR3 “SWAPS” with another label before forwarding
Feb 14, 2008
CS 573: Network Protocols and Standards

21. CS 573: Network Protocols and Standards
MPLS Flow Progress R1
LSR4 R2
LSR1 D
LSR6
destination
LSR3 17 D
LSR2
Path within MPLS cloud
is pre-established:
LSP (label-switched path)
LSR5
5 – LSR6 looks at the incoming label
LSR6 “POPS” the label before forwarding to R2
Feb 14, 2008
CS 573: Network Protocols and Standards

22. MPLS and explicit routing recap
Who establishes the LSPs in advance?
Ingress routers
How do ingress routers decide not to always take the shortest path?
Ingress routers use CSPF (constrained shortest path first) instead of SPF
Examples of constraints:
Do not use links left with less than 7Mb/s bandwidth
Do not use blue-colored links for this request
Use a path with delay less than 130ms
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CS 573: Network Protocols and Standards

23. CS 573: Network Protocols and Standards
CSPF
What is the mechanism? (in typical cases!)
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
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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CS 573: Network Protocols and Standards

24. CS 573: Network Protocols and Standards
More MPLS terminology
Upstream
Downstream
/24
LSR1
LSR2
Data
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CS 573: Network Protocols and Standards

25. Label advertisement
Always downstream to upstream label advertisement and distribution
Downstream
Upstream
Use label 5 for destination /24
/24
MPLS Data Packet
with label 5 travels
LSR2
LSR1
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CS 573: Network Protocols and Standards

26. CS 573: Network Protocols and Standards
Label advertisement
Label advertisement can be downstream unsolicited or downstream on-demand
Sends label
Without any Request
Downstream
Upstream
/24
LSR1
LSR2
Sends label ONLY after
receiving request
Downstream
Upstream
/24
LSR1
Request For label
LSR2
Feb 14, 2008
CS 573: Network Protocols and Standards

27. CS 573: Network Protocols and Standards
Label distribution
Label distribution can be ordered or unordered
First we see an example of ordered label distribution
Ingress LSR
Egress LSR
Label
Feb 14, 2008
CS 573: Network Protocols and Standards

28. CS 573: Network Protocols and Standards
Label distribution
Label distribution can be ordered or unordered
Next we see an example of unordered label distribution
Ingress LSR
Egress LSR
Label
Label
Feb 14, 2008
CS 573: Network Protocols and Standards

29. CS 573: Network Protocols and Standards
Label retention modes
Label retention can be conservative or liberal ?
Destination
Label
LSR1
Label
Feb 14, 2008
CS 573: Network Protocols and Standards

30. CS 573: Network Protocols and Standards
Label operations
Advertisement
Downstream unsolicited
Downstream on-demand
Distribution
Ordered
Unordered
Retention
Liberal
Conservative
Feb 14, 2008
CS 573: Network Protocols and Standards

31. CS 573: Network Protocols and Standards
Feb 14, 2008
Outline
Traditional IP Routing
Forwarding and routing
Problems with IP routing
Motivations behind MPLS
MPLS Terminology and Operation
MPLS Label, LSR and LSP, LFIB Vs FIB
Transport of an IP packet over MPLS
More MPLS terminology
Traffic Engineering [with MPLS]
Nomenclature
Requirements
Examples
Feb 14, 2008
CS 573: Network Protocols and Standards

32. Traffic Engineering with MPLS (Application of CSPF)

33. What is traffic engineering?
Performance optimization of operational networks
optimizing resource utilization
optimizing traffic performance
reliable network operation
How is traffic engineered?
measurement, modeling, characterization, and control of Internet traffic
Why?
high cost of network assets
service differentiation
Feb 14, 2008
CS 573: Network Protocols and Standards

34. CS 573: Network Protocols and Standards
Traffic engineering
Recall the IP hotspot problem
The ability to move traffic away from the shortest path calculated by the IGP (such as OSPF) to a less congested path
IP: changing a metric will cause ALL the traffic to divert to the less congested path
MPLS: allows explicit routing (using CSPF) and setup of such explicitly computed LSPs
Feb 14, 2008
CS 573: Network Protocols and Standards

35. CS 573: Network Protocols and Standards
MPLS-TE: How to do it?
LSPs are set up by LSRs based on information they learn from routing protocols (IGPs)
This defeats the purpose!
If we were to use “shortest path”, IGP was okay
Feb 14, 2008
CS 573: Network Protocols and Standards

36. MPLS TE: How we actually do it?
MPLS TE Requires:
Enhancements to routing protocols
OSPF-TE
ISIS-TE
Enhancement to signaling protocols to allow explicit constraint based routing
RSVP-TE and CR-LDP
Constraint based routing
Explicit route selection
Recovery mechanisms defined
Feb 14, 2008
CS 573: Network Protocols and Standards

37. CS 573: Network Protocols and Standards
Signaling mechanisms
RSVP-TE
Extensions to RSVP for traffic engineering
BGP-4
Carrying label information in BGP-4
CR-LDP
A label distribution protocol that distributes labels determined based on constraint based routing
RSVP-TE and CR-LDP both do label distribution and path reservation – use any one of them!
Feb 14, 2008
CS 573: Network Protocols and Standards

38. CS 573: Network Protocols and Standards
RSVP-TE
Basic flow of LSP set-up using RSVP
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CS 573: Network Protocols and Standards

39. CS 573: Network Protocols and Standards
RSVP-TE PATH Message
PATH message is used to establish state and request label assignment
R1 transmits a PATH message addressed to R9
Feb 14, 2008
CS 573: Network Protocols and Standards

40. CS 573: Network Protocols and Standards
RSVP-TE RESV Message
RESV is used to distribute labels after reserving resources
R9 transmits a RESV message, with label=3, to R8
R8 and R4 store “outbound” label and allocate an “inbound” label. They also transmits RESV with inbound label to upstream LSR
R1 binds label to forwarding equivalence class (FEC)
Feb 14, 2008
CS 573: Network Protocols and Standards

41. CS 573: Network Protocols and Standards
Rerouting LSP tunnels
When a more “optimal” route/path becomes available
When a failure of a resource occurs along a TE LSP
Make-before-break mechanism
Adaptive, smooth rerouting and traffic transfer before tearing down the old LSP
Not disruptive to traffic
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CS 573: Network Protocols and Standards

42. Recovering LSP tunnels
LSP Set-up
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CS 573: Network Protocols and Standards

43. CS 573: Network Protocols and Standards
Protection LSP set up
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CS 573: Network Protocols and Standards

44. CS 573: Network Protocols and Standards
Protection LSP
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CS 573: Network Protocols and Standards

45. CS 573: Network Protocols and Standards
References
RFC 2702 “Requirements for Traffic Engineering Over MPLS”
RFC 3031 “Multiprotocol Label Switching Architecture”
RFC 3272 “Overview and Principles of Internet Traffic Engineering”
RFC 3346 “Applicability Statement for Traffic Engineering with MPLS”
MPLS Forum (
Feb 14, 2008
CS 573: Network Protocols and Standards