MultiProtocol Label Switching

1
MultiProtocol Label Switching

• Ron Bonica
• vBNS Engineering
• Worldcom
• May 15, 2000

2
MPLS Benefits

• Traffic Engineering
• Restoration (order seconds)
• Virtual Private Networks
• Reusable Technology
• MPLS signaling to configure optical cross connects

3
MPLS Non-Benefits

• Forwarding Speed
• Vendors demonstrating OC-192 line rate forwarding
  without MPLS
• Forwarding Simplicity
• Label lookup is simpler than longest match
• That isnt visible to customer
• Quality of Service
• No service contracts associated with an LSP

4
MPLS Concepts

• Label Switching Path (LSP)
• Label Switching Router (LSR)
• Ingress, Transit, Egress
• LSP Path Selection
• LSP Setup and Restoration
• Resource Reservation Protocol (RSVP)
• Label Distribution Protocol (LDP)

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Label Switching Path

• Tunnel through a routing domain
• Ingress LSR makes IP routing decision
• Encapsulates datagram in MPLS header
• Label has local significance only
• Forwards datagram through LSP
• Transit LSRs forward based on MPLS header
• No L3 routing decision
• Label pushing and swapping
• Egress LSR pops MPLS label

6
Label Switching Path Disclaimers

• LSP does not appear as an interface to the
  ingress or egress router
• Ingress and egress routers do not exchange link
  state information across LSP
• For networks using PIM, multicast traffic does
  not traverse LSPs
• LSP does not preclude non-MPLS traffic from
  traversing interface via L3 forwarding

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Label Switched Path
LSR1
LSR3
Switched
Label
Path
LSR2
LSR5
LSR4
LSR6
Ingress
Egress
LSR8
LSR7
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Ingress Router Assigning Datagram to LSP

• By BGP next-hop
• Typical
• Not very granular
• Encourages creative uses of BGP next-hop
• By IP destination
• Administratively configured at ingress router
• By other IP header fields
• Various offerings from various vendors

9
Creative Uses for BGP Next-hop

• Egress LSR has two loopback addresses
• One LSP terminates at first loopback address
• Another LSP terminates at second loopback address
• Each loopback address is BGP next-hop for
  selected sub-networks
• Administratively configured at egress

10
LSP Path Setup

• Static
• Manually configured at each node
• Explicitly Routed LSP
• Signaled from ingress LSR
• Path strictly or loosely defined
• Constraint Based Shortest Path First
• Signaled from ingress LSR
• Link attributes and LSP constraints

11
ER-LSP Strict Definition
LSR1
LSR3
Switched
Label
Path
LSR2
LSR5
LSR4
LSR6
Ingress
Egress
LSR8
LSR7
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ER-LSP Loose Definition
LSR1
LSR3
Switched
Label
Path
LSR2
LSR5
LSR4
LSR6
Ingress
Egress
LSR8
LSR7
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Constraint Based Routing

• Links administratively assigned attributes
• Bandwidth, color
• Link attributes distributed by IGP
• TE extensions to OSPF and ISIS
• LSP routing constraints administratively defined
• Bandwidth, maximum hops, color
• LSR calculates routes that satisfies constraints

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Constraint Based Routing
LSR1
LSR3
Switched
Label
Path
LSR2
LSR5
LSR4
LSR6
Ingress
Path
Label
Egress
Switched
LSR8
LSR7
15
LSP Setup Signaling

• Resource Reservation Protocol (RSVP)
• Distribute Information about paths and labels
• Label Distribution Protocol (LDP)
• Distribute information about paths, labels and
  Forwarding Equivalence Class mappings

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LSP Setup RSVP

• RSVP messages propagated from ingress LSR to
  egress LSR
• Reverse of direction described in RFC 2205
• Soft state established at each intermediate node
• Represents LSRs behavior with regard to a
  incoming label on a specified interface
• Forwarding Interface
• Push, Pop, or Swap labels

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RSVP Messages

• PATH
• Ingress to egress
• Integrity, label request and constraint objects
• RESV
• Egress to ingress
• Integrity, label and record objects
• TEAR
• Either direction
• Label object

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LSP Setup LDP

• Protocol Between LDP Peers
• Discovery Messages
• UDP
• General (broadcast on segment)
• Targeted (can be multiple hops away, unicast)
• Session Control, Advertisement and Notification
  Messages
• TCP

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LDP Modes

• Downstream Unsolicited
• Downstream On Demand

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LDP Messages

• Discovery
• Hello
• Session Control
• Initialization
• Keep Alive
• Notification

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LDP Messages (continued)

• Advertisement
• Address /Address Withdraw
• Label Request
• Label Abort Request
• Label Mapping / Withdraw / Release
• Label Mapping contains FEC/Label binding

22
Restoration Alternatives

• Fail-over to secondary path
• Setup in advance
• Physically diverse from primary
• Recalculate Path
• Signal back to source (slow)
• Circumvent failure (better)
• Fast Reroute (best)

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Restoration Hot Standby
LSR1
LSR3
Primary
LSR2
LSR5
LSR4
LSR6
Ingress
Egress
Secondary
LSR8
LSR7
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Restoration Signal to Source
Tear
LSR1
LSR3
Setup
X
LSR2
LSR5
LSR4
LSR6
Ingress
Egress
LSR8
LSR7
25
Restoration Circumvent Failure
LSR1
LSR3
Setup
X
LSR2
LSR5
LSR4
LSR6
Ingress
Egress
LSR8
LSR7
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Restoration Time Hot Standby LSP Versus IGP
Reroute
MPLS1.25 Sec
OSPF1.25 Sec
x
R1
R2
Preferred Route or Primary LSP
MPLS1.5 Sec
x
OSPF3.25 Sec
Other Route or Standby LSP
R4
R3
Destination
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Circuit Cross Connects

• Juniper proprietary
• Makes LSP look like ATM or Frame Relay PVC
• Associate ATM or frame relay UNI with endpoint of
  MPLS LSP
• All traffic from LSP is routed to UNI
• All traffic from UNI is routed to LSP

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Circuit Cross Connects (continued)

• Endpoints must be like (ATM-ATM, Frame-Frame)
• AAL5 encapsulated IP over ATM
• AAL5 frame reassembled at ingress
• Ingress sends MPLS encapsulated AAL5 frame
  through network
• Egress breaks frame into cells and send through
  ATM UNI
• No ATM cell tax across backbone

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What is Traffic Engineering

• Isnt that why we did MPLS in the first place?

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Traffic Engineering

• Goals
• Meet customer SLAs
• Minimize expenditure required to meet SLAs
• Methods
• Direct traffic to devices that can provide
  required resources
• Divert traffic from devices that provide scarce
  resources, when required and possible.

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Traffic Engineering Alternatives Before MPLS

• Layer 2 Traffic Engineering
• Full layer 2 mesh connects routers
• Two infrastructures to purchase and manage
• Same TE problems need to be solved at layer 2
  (e.g., path calculation, configuration
  management)
• Traffic Engineering using IGP metrics
• Control not sufficiently granular

32
TE Using IGP Granularity Problem
Using IGP metrics, steer traffic from R1 to R6
through R4. Also steer traffic from R2 to R6
through R5.
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Traffic Engineering Methods

• Define edge-to-edge service requirements
• Bandwidth, delay, reliability
• Input from customer SLAs and performance data
• Catalogue link capabilities, network wide
• Calculate paths based on service requirements and
  link capabilities

34
Path Calculation Alternatives

• Off-line path calculation
• Model network and calculate paths off-line
• Transfer paths to routers (configure ER-LSP)
• On-line path calculation
• Configure routers with attributes of directly
  connected links
• IGP distributes link attributes with link state
  announcements
• Configure ingress router with LSP requirement
• Routers calculate path (distributed calculation)

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Path Calculation Observations

• Path calculation is challenging
• On-line (distributed) path calculation is very
  challenging
• Order in which routers reserve resources becomes
  significant
• Distributed path calculation for large networks
  is very, very challenging

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Difficult Paths to Calculate
600 mbps
200 mbps
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Off-Line Path Calculation

• Pros
• Opportunity to review calculated paths before
  they go into the network

• Cons
• Configuration management effort
• Impractical to re-calculate for each potential
  link failure

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On-Line Path Calculation

• Pros
• Reduced configuration management effort
• Automatic re-calculation after link failure, with
  information about which link failed

• Cons
• No opportunity to review calculated paths before
  they go into the network