Introduction of MultiProtocol Label Switching

1
Introduction of Multi-Protocol Label Switching

• Yi-Lun Chen

2
Outline

• Label Switching
• MPLS Architecture
• Label Distribution Protocols
• Summary

3
Whats Label Switching?
Some ways from A to B

• Broadcast
• Go everywhere, stop when arriving B
• Hop by Hop routing
• Continuously find the closest way to B
• Source Routing
• A to B using the intermediate node list carried
  by the packet

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Label Switching

• Pre-reserve a path from A to B, identify packet
  with label, and sign this path in all
  intermediate nodes

C
A
Packet with labeled 1
B
Packet with labeled 2
5
Route at edge,switch in core
Continually, Route at Edge
Switch in core
Route at Edge
IP
IP
IP Forwarding
IP Forwarding
LABEL SWITCHING
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Why use Label Switching?

• Speed and Delay
• Routing Table lookup need time
• Scalability
• The scope of a label is between two routing peers
• Label can be reused
• Simplicity
• Just do it Forward a packet based on its label
• Resource Control
• We can reserve path with QoS conditions
• Route Control
• Policy-based Routing and Traffic Engineering
  could be done

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MPLS just do

• Pre-build path
• Use labels to be the transport media
• Multi-level label switching

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Outline

• Label Switching
• MPLS Architecture
• Label Distribution Protocols
• Summary

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Hybrid of two mechanisms

• Build label paths and use them
• MPLS combine the packet routing and circuit
  switching

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Path Distribution example

• We want to bind labels from domain 12.3 to
  domain 45.6

1
45.6
3
Router decide by itself
2
3
1
Router decide by itself
2
1
3
45.7
12.3
2
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Use the path

• Packet from domain 12.3 to domain 45.6

1
45.6
3
2
3
1
2
1
3
45.7
12.3
2
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LSP Label Switched Path

• A tree rooted by destination
• Unidirectional
• Could be built by existing IP Routing Table

Destination
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Label Encapsulation

• May use existing format or new shim label

IP PAYLOAD
Upper Layer
For multi-level label stack
Shim Label .
VPI
VCI
DLCI
Shim Label
Label
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ATM

• Top level labels use VPI/VCI fields
• Further fields in stack are encoded with shim
  header

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PPP Ethernet Data Links

• Add shim label between Layer 2 and Layer 3
• Upper layer protocol is inferable from value of
  the bottom level of the stack

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Do label switching in LSR
IP
IP

• Encapsulate in Ingress LSR
• Forwarding in intermediate LSR
• Decapsulate in Egress LSR

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Make switching decisions in Ingress LSR

• FEC (Forwarding Equivalence Class)
• A subset of packets that are all treated the same
  way by a router
• NHLFE (Next Hop Label Forwarding Entry)
• Lookup when doing switch
• Containing label push/pop/swap and next hop
  information
• FTN (FEC-To-NHLFE map)
• Map FEC to a (set of) NHLFEs

Make Decisions in Ingress LSR

• Match the corresponding FEC
• Find NHLFE using FTN and FEC
• Process packet with the information containing in
  the NHLFE

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Make switching decisions in Intermediate/Egress
LSR

• ILM (Incoming Label Map)
• Lookup when doing switch
• Containing label push/pop/swap and next hop
  information

Make Decisions in Intermediate/Egress LSR

• Find ILM using incoming label
• Map the result to the NHLFE
• Process packet with the information containing in
  the NHLFE

Must consider TTL and MTU
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Build LSP

• Using Label Distribution Protocols (LDP)
• 2 methods to build LSP
• By Hop-by-Hop
• By Explicitly Routed

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Outline

• Label Switching
• MPLS Architecture
• Label Distribution Protocols
• Summary

• LDP Concept
• CR-LDP and RSVP-TE

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What does LDP want to do?

• Negotiation of binding label between LSRs

OK! Label 10 For 12.0.0.0/8
To 12.0.0.0/8 Use Label 10

• Can distribute Label Switched Network in two ways
• Piggyback on existing routing protocol
• Create dedicated LDP

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Label mapping between peers
FEC-Label Binding
FEC-Label Binding
Binding Request

• 2 Mapping methods

• 2.Downstream-on-demand
• Upstream LSR send explicit request to downstream
  LSR for binding
• Downstream LSR start binding after receiving
  Request

• 1.Downstream Unsolicited
• Downstream LSR can initiate binding without
  Explicit Request

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Path distribution control

• 2 Control methods

• 2.Ordered Control
• LSR do mapping and return Label mapping message
  when
• Receiving Label mapping message from next-hop
• This LSR is the Egress

• 1.Independent Control
• LSR return label binding at any time it desires
• Dont need to wait for Label Mapping message from
  the next hop

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Label retention control

• 2 methods

Valid Next Hop

• 2.Conservative Label Retention
• Only maintain binding from the valid next hop
• Fewer labels need to maintain
• When route change, it must rebinding label from
  new next-hop

• 1.Liberal Label Retention
• Maintains all binding from LSRs other than the
  valid next hop
• Maintains many more labels
• Can use these binding immediately when next hop
  change

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LDP Standard messages

• Defined in MPLS-LDP RFC 3036
• TLV (Type-Length Value) Encoding for many
  information
• 4 different message areas

• Peer discovery
• Session management
• Label distribution
• Notification

• Address
• Address Withdraw
• Label Request
• Label Mapping
• Label Withdraw
• Label Release
• Label Abort Request

• Notification

• Hello

• Initialization
• KeepAlive

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CR-LDP

• Constraint-based Routing LDP

• Setup path with explicit route
• Defined in RFC 3213, RFC 3214
• Use LDP messages
• Request
• Map
• Notification
• Add some parameters for more purposes
• ER (Explicit Route)
• Traffic Parameters etc
• Have no any dependency with non-MPLS protocols
• Can inter-work with other LDPs

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LSP Setup
LSR B
LSR C
LSR D
LSR A
ER Label Switched Path
Ingress
Egress
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QoS conditionCR-LDP Traffic parameters

• Using Token Bucket to achieve QoS control

• CDR and CBS
• CDR is the max rate that LSP commits to be
  available
• CBS is the max burst size allowed at CDR

• Frequency for the period of CDR
• Imply the granularity of resource reservation

• PDR and PBS
• PDR is the max rate could sent to the LSP
• PBS is the max burst size allowed at PDR

• Weight means the path priority
• Determines the relative priority of multiple
  LSPs when congestion or excess bandwidth

• EBS
• As an additional limit on the CDRs token bucket

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RSVP-TE

• Resource Reservation Protocol Extensions for
  Traffic Engineering

• Extension of RSVP protocol
• To support Label distributing
• Defined in RFC 3209
• Do label mapping process when exchange RSVP
  messages
• PATH message for label request
• RESV message for label binding
• QoS support from RSVP capability

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LSP setup with RSVP

• Similar to the conventional RSVP message flow

LSR B
LSR C
LSR D
LSR A
Label Switched Path
Ingress
Egress
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Outline

• Label Switching
• MPLS Architecture
• Label Distribution Protocols
• Summary

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Label Switching
Routing
OSPF, IS-IS, BGP, RIP
Forwarding
MPLS

• Build path in upper layer , use path in MPLS
  layer
• More efficiency

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Traffic Engineering

• Traffic Engineering provide more flexibility
• MPLS is suitable to TE

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VPN in core

• Its a TE application
• Bring industries more security and convenient

MPLS Tunnel
MPLS Core Network
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Upper Layer Consistency with different data link
layer

• Network can across multiple layer 2 technologies

IPetc
MPLS

Frame Relay
PPP (SONET, DS-3 etc.)
ATM
Ethernet
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Summary

• MPLS brings us many benefits

• MPLS is between Layer 2 and Layer 3
• It is faster than traditional Layer 3 routing
• It is Layer 2 independent
• ATM, Frame Relay, Ethernet etc could co-work in
  MPLS core network
• MPLS is a mixed technology of packet routing and
  circuit switching
• It is suitable to support QoS condition
• It is suitable to achieve Traffic Engineering

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Reference

• 1 RFC 3031, Multiprotocol Label Switching
  Architecture, E. Rosen, A. Viswanathan, R.
  Collon, Jan 2001
• 2 RFC 3032, MPLS Label Stack Encoding, E.
  Rosen, D.Tappan, G.Fedorkow, Y.Rekhter,
  D.Farinacci, T. Li, A. Conta, Jan 2001
• 3 RFC 3036, LDP Specification, L.Andersson,
  P.Doolan, N.Feldman, A.Fredette, B.Thomas , Jan
  2001
• 4 Uyless Blank, MPLS and Label Switching
  Networks, Prentice Hall
• 5 Osama Aboul-Magd, Bilel Jamoussi, QoS and
  Service Interworking Using Constraint-Route Label
  Distribution Protocol(CR-LDP), IEEE
  Communications Magazine, May 2001
• 6 Interworking, Traffic Engineering, and
  Quality of Service in Carrier Networks, Nortel
  Networks, 2000
• 7 Peter Ashwood-Smith, Bilel N. Jamoussi, MPLS
  Tutorial, Nortel Networks, 1999