Introduction to MPLS and Traffic Engineering

1
Introduction to MPLS and Traffic Engineering

• Zartash Afzal Uzmi

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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

3
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

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Forwarding and routing

• Forwarding
• 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 router make the forwarding decision

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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)

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How IP routing works
Searching Longest Prefix Match in FIB (Too Slow)
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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

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IP routing rigidity example
D
1
1
A
A
B
S
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

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IP routing rigidity example
D
1
4
A
A
B
S
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

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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)

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IP routing versus MPLS routing
Traditional IP Routing
Multiprotocol Label Switching (MPLS)
2
1
S
D
3
5
4
MPLS allows overriding shortest paths!
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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

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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?

IP Datagram
Label
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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)

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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

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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 very large number of
  entries
• In LFIB, label is an exact match
• In FIB, IP is longest prefix match

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Mpls Flow Progress
D
R1
R2
LSR4
LSR1
D
destination
LSR6
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
D
R1
R2
LSR4
LSR1
D
destination
LSR6
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
R2
LSR4
LSR1
D
31
D
destination
LSR6
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
R2
LSR4
LSR1
D
destination
LSR6
LSR3
D
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LSR2
Labels have local signifacance!
LSR5
4 LSR3 just looks at the incoming label LSR3
SWAPS with another label before forwarding
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MPLS Flow Progress
R1
R2
LSR4
LSR1
D
destination
LSR6
LSR3
D
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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
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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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CSPF

• What is the mechanism?
• 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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More MPLS terminology
Upstream
Downstream
172.68.10/24
LSR1
LSR2
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Label advertisement

• Always downstream to upstream label advertisement
  and distribution

Downstream
Upstream
171.68.32/24
LSR2
LSR1
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Label advertisement

• Label advertisement can be downstream unsolicited
  or downstream on-demand

Downstream
Upstream
171.68.32/24
LSR2
LSR1
Downstream
Upstream
171.68.32/24
LSR1
LSR2
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Label distribution

• Label distribution can be ordered or unordered
• First we see an example of ordered label
  distribution

Label
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Label distribution

• Label distribution can be ordered or unordered
• Next we see an example of unordered label
  distribution

Label
Label
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Label retention modes

• Label retention can be conservative or liberal

?
Destination
Label
LSR1
Label
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Label operations

• Advertisement
• Downstream unsolicited
• Downstream on-demand
• Distribution
• Ordered
• Unordered
• Retention
• Liberal
• Conservative

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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

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Traffic Engineering

• Traffic Engineering with MPLS
• (Application of CSPF)

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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

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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

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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

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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

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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!

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RSVP-TE
Basic flow of LSP set-up using RSVP
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RSVP-TE PATH Message

• PATH message is used to establish state and
  request label assignment
• R1 transmits a PATH message addressed to R9

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RSVP-TE RESV Message

• RESV is used to distribute labels after reserving
  resources
• R9 transmits a RESV message, with label3, 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)

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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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Recovering LSP tunnels
LSP Set-up
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Protection LSP set up
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Protection LSP
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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 (http//www.mplsforum.org)