MPLS and Traffic Engineering

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MPLS and Traffic Engineering

• Zartash Afzal Uzmi
• Department of Computer Science
• Lahore University of Management Sciences (LUMS)

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Outline

• Traditional IP Routing
• IP Routing Operation and Problems
• Motivation behind MPLS
• MPLS Terminology and Operation
• MPLS Label, LSR and LSP, LFIB Vs FIB
• Transport of an IP packet over MPLS
• Traffic Engineering with MPLS
• Nomenclature
• Requirements
• Examples

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Traditional IP Routing

• IP forwarding is done independently at every hop
• IP forwarding decisions are made using
• Destination IP address (in packet header!)
• Routing table (updated by routing algorithms!)
• Each IP router runs its own instance of the
  routing algorithm
• Each IP router makes its own forwarding decisions

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How IP Routing Works?
Searching Longest Prefix Match in FIB (Too Slow)
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Problems with IP Routing

• 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

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Motivation behind MPLS

• Avoid slow IP lookup
• Provide traffic differentiation (QoS)
• Voice is really different from data!
• Evolve routing functionality
• Control was too closely tied to forwarding!
• Simplify deployment of IPv6

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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!

IP Datagram
Label
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LSR and LSP

• Router that supports MPLS is known as label
  switching router (LSR)
• Path which is followed by using labels is called
  label switched path (LSP)

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LFIB Vs 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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Transport of IP over MPLS
Label Pushing
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Transport of IP over MPLS
Label Swapping
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Transport of IP over MPLS
Label Swapping
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Transport of IP over MPLS
Label Popping
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Transport of IP over MPLS
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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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Hyperaggregation Problem

• Routing Protocols Create A single "Shortest Path"

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Hyperaggregation Problem
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Nomenclature

• Network Engineering
• Put the bandwidth where the traffic is!
• Physical cable deployment
• Virtual connection provisioning
• Traffic Engineering
• Put the traffic where the bandwidth is!
• Optimization of routes
• Ability to explicitly route traffic

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Traditional Traffic Engineering
Traffic sent to A or B follows path with lowest
metrics!
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Traditional Traffic Engineering

• Demerits of IGP-based traffic engineering
• Changing traffic metric causes ALL the traffic to
  shift to the new path
• Can not shift traffic destined only for A or only
  for B to the new path (through C)
• Result is under or over utilization of some links

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Traffic Engineering IGP vs. MPLS

• Traditional TE (IGP based)
• The ability to move traffic away from the
  shortest path calculated by the IGP to a less
  congested path
• MPLS TE
• Allows explicit routing and setup of LSPs
• Provides recovery mechanisms failure
• Enables Value added services
• VPNs, SLAs, VoIP, etc.

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MPLS TE How we may do it?
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MPLS TE How we may 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 and 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

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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)

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Upstream and downstream LSR
Upstream
Downstream
172.68.10/24
LSR1
LSR2
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How MPLS Works
Searching Longest Prefix Match in FIB (Too Slow)
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Label Distribution
ALWAYS, Downstream to upstream label distribution
171.68.32/24
LSR2
LSR1
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Downstream Un-solicited
Upstream
Upstream
171.68.32/24
LSR2
LSR1
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Downstream On Demand (DoD)
Down Stream
Upstream
171.68.32/24
LSR2
LSR1
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Ordered Label Distribution
Label
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Unordered Label Distribution
Label
Label
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Label Retention Modes
1. Liberal Retention Mode
2. Conservative Retention Mode
?
Destination
Label
LSR1
Label
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Label Distribution Modes
Label distribution modes
Advertisement
Distribution
Downstream-on-Demand
Downstream-Unsolicited
Independent
Ordered
Retention
Conservative
Liberal
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Hierarchical LSP
Ingress LSR for LSP3