CS 520 Lecture 9 MPLS and its Applications with modifications for CS 520

1
CS 520 Lecture 9MPLS and its Applications(with
modifications for CS 520)

• Philip MatthewsNortel NetworksApril 2000
• (Material prepared byDr. Bilel Jamoussi and
  Peter Ashwood-Smith)

2
What This Lecture is About

• What MPLS is
• What MPLS is good for
• MPLS protocol/mode comparisons opinions
• Some applications of MPLS
• MPLS future
• Good tutorial information at www.mplsrc.com.

3
What This Lecture is NOT About

• General networking
• Product comparisons
• Marketing

4
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint-Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

5
What is Standards-based MPLS?

• Framework and Architecture
• Define the scope, the various components and
  their interactions
• Encapsulations
• Labels used at the data plane to make forwarding
  decisions
• Signaling Protocols
• Distribution of Labels to establish Label
  Switched Paths
• Routing Protocol Traffic Engineering Extensions
• Distribution of Bandwidth and other link
  attributes

6
Label Substitution what is it?
Have a friend go to B ahead of you. At every road
they reserve a lane just for you. At every
intersection they post a big sign that says for a
given lane which way to turn and what new lane to
take.
LANE1
LANE2
7
Label Switched Path (LSP)
1 RIGHT 2
1 RIGHT 2
2 LEFT 1
2 LEFT 1
8
What is a LABEL?A property that uniquely
identifies a flow on a logical or physical
interface Labels may be platform wide unique or
more commonly interface wide unique.
9
A label by any other name ...

• There are many examples of label substitution
  protocols already in existence

• ATM - label is called VPI/VCI and travels with
  cell
• Frame Relay - label is called a DLCI and travels
  with frame
• TDM - label is called a timeslot - label is
  implied, like a lane
• Frequency substitution where label is a light
  frequency via DWDM, OXC etc.

10
Label Switched Path
3 Right 7
7 LEFT 99
99 RIGHT 9
9 LEFT 4072
11
Optical Label Switched Path
RED RIGHT BLUE
RED
BLUE
BLUE LEFT WHITE
WHITE RIGHT ORANGE
WHITE
ORANGE
ORANGE LEFT RED
RED
12
SO WHAT IS MPLS ?

• Hop-by-hop or source routing to establish labels
• Uses label native to the media
• In packet header for packet switched networks
• Time slots in TDM networks
• Wavelengths in WDM networks
• Multiple levels of labels (stacks of labels
  within labels).

13
ROUTE AT EDGE, SWITCH IN CORE
IP
IP
IP Forwarding
IP Forwarding
LABEL SWITCHING
14
Routers Do Both Routing and Switching

• Routing
• Deciding the next hop based on the destination
  address.
• A Layer 3 (L3) function.
• Switching
• Moving a packet from an input port to an output
  port and out.
• A layer 2 function.

INPUT PORTS
OUTPUT PORTS
15
Routers Do Both Routing and Switching

• So we can avoid performing the layer 3 function.
• What benefit does this provide?
• In what situations would this benefit not be very
  significant?

16
MPLS Flexible Forwarding
IP Packets are forwarded based on Destination
Address (DA)

• MPLS
• Map packets to LSP based on (Source Address,
  Destination Address, protocol, port, DSCP,
  interface, etc.)
• Forward packets based Label

IP
IP
LSP to IP
IP to LSP
LABEL SWITCHING
17
MPLS-based Solutions

• IP Traffic Engineering
• Constraint-based Routing making routing adapt to
  latest network loading
• Virtual Private Networks
• Controllable tunneling mechanism
• L2/L3 Integration
• Leverage ATM hardware
• L1/L3 Integration
• Use of MPLS to control Optical Cross Connects
  (OXC)
• Enable QoS in IP Networks
• Support Diffserv ATM-style QoS

18
BEST OF BOTH WORLDS
CIRCUITSWITCHING
PACKETForwarding
HYBRID

• MPLS IP form a middle ground that combines the
  best of IP and the best of circuit switching
  technologies.

19
MPLS Terminology

• LDP Label Distribution Protocol
• LSP Label Switched Path
• FEC Forwarding Equivalence Class
• LER Label Edge Router (edge of an area that
  supports MPLS)
• LSR Label Switching Router (inside an area that
  supports MPLS)

20
Forwarding Equivalence Classes
LSR
LSR
LER
LER
LSP
Packets are destined for different address
prefixes, but can be mapped to common path

• FEC A subset of packets that are all treated
  the same way by a router
• The concept of FECs provides for a great deal of
  flexibility and scalability
• In conventional routing, a packet is assigned to
  an FEC at each hop (i.e. L3 look-up), in MPLS it
  is only done once at the network ingress.

21
LABEL SWITCHED PATH (vanilla)
- A Vanilla LSP creates MPLS paths for standard
IP routing (from IP routing tables). - A Vanilla
LSP is actually part of a tree from every source
to that destination (unidirectional).
22
MPLS BUILT ON STANDARD IP
47.1
1
2
1
3
2
1
47.2
3
2
Network 47.3

• Destination based forwarding tables as built by
  OSPF, IS-IS, RIP, etc.

23
IP FORWARDING USED BY HOP-BY-HOP CONTROL
47.1
1
IP 47.1.1.1
2
IP 47.1.1.1
1
3
2
IP 47.1.1.1
1
47.2
3
47.3
2
24
MPLS Label Distribution
1
47.1
3
2
3
1
1
2
47.3
3
47.2
2
25
Label Switched Path (LSP)
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
26
Benefits and Limitations

• Why might this approach be better than normal IP
  forwarding that does not use MPLS?
• Remember, all packets still travel the same
  paths.
• What else might we be able to do with MPLS that
  could be even more powerful?

27
EXPLICITLY ROUTED OR ER-LSP
B
C
A
- ER-LSP follows route that source chooses. In
other words, the control message to establish the
LSP (label request) is source routed.
28
EXPLICITLY ROUTED LSP ER-LSP
1
47.1
3
3
2
1
1
2
47.3
3
47.2
2
29
ER LSP - Advantages

• Operator has routing flexibility
• Can establish LSPs based on policy, QoS, etc.
• Can have pre-established LSPs that can be used
  in case of failures.
• Can use routes other than shortest path
• Can compute routes based on dynamic constraints
  (available bandwidth, delay, etc.) in exactly the
  same manner as ATM based on a distributed
  topology database.(traffic engineering)

30
ER LSP - Discord!

• Two signaling options proposed in the standards
  CR-LDP, RSVP extensions
• CR-LDP Label Distribution Protocol (LDP)
  Explicit Routing
• RSVP-TE Traditional Resource Reservation
  Protocol (RSVP) Explicit Route Scalability
  Extensions
• RSVP was established several years ago to be able
  to reserve resources along a path.
• To ensure QoS by making sure each flow had enough
  resources.
• Had significant scalability problems.
• ITU has decided on LDP/CR-LDP for public
  networks.

31
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

32
Upper Layer Consistency Across Lower Layers

GigEthernet
Optical Cross Connect (OXC)
Frame Relay
ATM

• MPLS is multiprotocol below (link layer)
• Provides for consistent operations, engineering
  across multiple technologies
• Allows operators to leverage existing
  infrastructure
• Co-existence with other protocols is provided for

33
Label Encapsulation
IP or other non-IP PAYLOAD
Shim Label .
VPI
VCI
DLCI
Shim Label
?
Label
ATM
FR
Ethernet
PPP
Optical
Medium
MPLS Encapsulation is specified over various
media types. Outermost labels may use existing
format (VPI/VCI, etc.), while inner label(s) use
a new shim label format.
34
MPLS Link Layers

• MPLS is intended to run over multiple link layers
• Specifications for the following link layers
  currently exist
• ATM label contained in VCI/VPI field of ATM
  header
• Frame Relay label contained in DLCI field in FR
  header
• PPP (Point-to-Point Protocol for dial-up links)
  uses shim header inserted between L2 and L3
  headers
• LAN also uses shim header
• Translation between link layer types must be
  supported

MPLS intended to be multi-protocol below as
well as above.
35
MPLS Encapsulation - PPP LAN Data Links
MPLS Shim Headers (1-n)

n
1
Network Layer Header and Packet (eg. IP)
Layer 2 Header (eg. PPP, 802.3)
4 Octets
Label Stack Entry Format
TTL
Label
Exp.
S
Label Label Value, 20 bits (0-16
reserved) Exp. Experimental, 3 bits (was Class
of Service) S Bottom of Stack, 1 bit (1
last entry in label stack) TTL Time to Live, 8
bits

• Network layer must be inferable from value of
  bottom label of the stack
• Note The label at the bottom of the stack is
  the top label.

MPLS on PPP links and LANs uses Shim Header
Inserted Between Layer 2 and Layer 3 Headers
36
MPLS Encapsulation - ATM
ATM LSR constrained by the cell format imposed by
existing ATM standards
5 Octets
ATM Header Format
VPI
PT
HEC
VCI
CLP
Label
Label
Option 1
Combined Label
Option 2
Option 3
Label
ATM VPI (Tunnel)

• Top 1 or 2 labels are contained in the VPI/VCI
  fields of ATM header
• - Option 1 uses two labels.
• - One in each or single label in combined field,
  negotiated by LDP
• Further fields in stack are encoded with shim
  header in PPP/LAN format

37
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint-Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

38
Label Distribution Protocols

• Label Distribution Protocol (LDP)
• Constraint-based Routing LDP (CR-LDP)
• Extensions to RSVP

39
Label Distribution Protocol (LDP) - Purpose
Label distribution ensures that adjacent routers
have a common view of FEC lt-gt label bindings
Routing Table Addr-prefix Next
Hop 47.0.0.0/8 LSR3
Routing Table Addr-prefix Next
Hop 47.0.0.0/8 LSR2
LSR1
LSR3
LSR2
IP Packet
47.80.55.3
Common understanding of which FEC the label is
referring to!
Label distribution can either piggyback on top of
an existing routing protocol, or a dedicated
label distribution protocol (LDP) can be created.
40
Label Distribution - Methods
Label Distribution can take place using one of
two possible methods
Both methods are supported, even in the same
network at the same time For any single
adjacency, LDP negotiation must agree on a common
method
41
Downstream Mode Making Shortest Path First (SPF)
Tree Copy In Hardware
42
Downstream On Demand Making SPF Tree Copy In
Hardware
43
Label Assignment Example

• Given the following network. Link cost metrics
  are assumed to be simple hop counts.
• This network supports traffic from three traffic
  classes as follows.
• From A1 (Access 1) to A3, Audio traffic
• From A1 to A3, Data traffic
• From A1 to A4, Data traffic
• From A2 to A4, Data traffic
• "Hot spots" might exist in the network using
  native IP routing, depending on how much load is
  coming from each traffic class. What possible
  hot spots in the network could be alleviated
  somewhat using MPLS? What possible hot spots
  could not be alleviated? Assume each traffic
  class has equal load.

44
Label Assignment Example

• If each of these traffic classes were to be put
  into separate MPLS FEC's, what information from
  the IP header would the Label Edge Routers (that
  is to say, R1 and R2) use to classify traffic
  into FEC's?

45
Label Assignment Example

• If MPLS LSP's were created just to follow default
  IP routing, list the labels that would be used
  for each FEC on each hop (according to numbering
  as you wish). Assume that all data traffic is
  treated that same out of each router and all
  audio traffic is treated that same out of each
  router. Merge labels whenever possible.

46
Label Assignment Example
47
Label Assignment Example

• Now, according to a plan you devise, show how you
  would allocate labels to alleviate (at least
  partially) hot spots as much as possible. List
  the labels (according to numbering as you wish)
  that would be used for each FEC on each hop.
  Merge labels whenever possible.
• Multiple correct answers exist. Just give one
  that meets the requirements given above

48
Label Assignment Example
49
Label Distribution Protocols

• Overview of Hop-by-hop Explicit
• Label Distribution Protocol (LDP)
• Constraint-based Routing LDP (CR-LDP)
• Extensions to RSVP

50
Traffic Engineering Requirements

• Constraint-Based Routing is one method of Traffic
  Engineering. Traffic Engineering seeks to
  engineer the best use of capacity.
• RFC 2702
• Strict Loose ER
• Specification of QoS
• Specification of Traffic Parameters
• Route Pinning
• Preemption
• Failure Recovery

51
Signalling Requirements

• Dynamic Establishment of an MPLS Label Switched
  Path (LSP)
• LSP may span multiple domains
• LSP follows an Explicit Route
• LSP follows a route that meets other constraints
• Constraint-Routed LSP
• Protocol has to be
• Simple
• Scalable
• Reliable

52
Constraint Based Routing using LDP (CR-LDP)

• Built on existing LDP messages over TCP.
• Defines an Explicit Route
• Detailed path that can traverse any links
  supporting CR-LDP.
• Defines a set of constraints for LSP computation
  and admission
• Expectation and Allocation of resources
• Peak burst rate, Committed burst rate,
• Excess burst, Frequency, Weight.
• Preemption Level
• Setup and Holding Priority with respect to other
  LSPs.
• Resource Class
• Color of traffic inclusion, exclusion rules for
  links.

53
CR-LDP Preemption

• A CR-LSP carries an LSP priority. This priority
  can be used to allow new LSPs to bump existing
  LSPs of lower priority in order to steal their
  resources.
• This is especially useful during times of failure
  and allows you to rank the LSPs such that the
  most important obtain resources before less
  important LSPs.
• These are called the setup-Priority and a
  holding-Priority and 8 levels are provided.

54
CR-LDP Preemption

• When an LSP is established its setup-Priority is
  compared with the holding-Priority of existing
  LSPs, any with lower holding-Priority may be
  bumped to obtain their resources.
• This process may continue in a domino fashion
  until the lowest holding-Priority LSPs either
  clear or are on the worst routes.

55
Preemption A.K.A. Bumping
B
C
A
56
Preemption A.K.A. Bumping

• In general, preemption is a controversial topic.
• Not allowed for telephone calls in the United
  States, even during emergencies.
• So, how could preemption be useful?

57
Preemption A.K.A. Bumping

• What is bad about preemption?
• How might these bad effects be counteracted?

58
Label Distribution Protocols

• Overview of Hop-by-hop Explicit
• Label Distribution Protocol (LDP)
• Constraint-based Routing LDP (CR-LDP)
• Extensions to RSVP

59
ER-LSP setup using RSVP-TE

• Built on RSVP messages over IP.
• In RSVP, a source requests resources along a
  path.
• Then the source regularly sends refresh messages
  to keep the reservations active.
• Extensions to RSVP
• Explicit Route Object
• Label Request
• Label Object
• Session Attribute
• Record Route Object
• Defines a set of constraints for LSP computation
  and admission
• Expectation and Allocation of resources Uses
  Inserv-style reservations
• Preemption Level Setup and Holding Priority with
  respect to other LSPs.

60
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

61
MPLS and ATM

• MPLS is being used to carry existing ATM traffic
• ATM network is removed
• ATM traffic is sent over MPLS LSPs
• ATM VCs are mapped to MPLS LSPs
• One virtual circuit becomes another
• Various levels of Interoperation are Possible
• Full interoperation (Peer Model)
• ATM and MPLS send all messages back and forth
• Overlay Model
• ATM cells are just encapsulated with MPLS shim
  headers
• MPLS may support ATM VCs, may not

62
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

63
IP Follows a tree to destination

Desta.b.c.d
a.b.c.d
Desta.b.c.d
Desta.b.c.d
- IP will over-utilize best paths and
under-utilize less good paths.
64
HOP-BY-HOP (A.K.A Vanilla) LDP
216
963
14
612
462
311
99
5
- Ultra fast, simple forwarding a.k.a switching -
Follows same route as normal IP datapath - So
like IP, LDP will over-utilize best paths and
under-utilize less good paths.
65
Label Switched Path (Two Types)
427
216
819
77
18
963
14
612
462
311
99
5

• Two types of Label Switched Paths
• Hop by hop (Vanilla LDP)
• Explicit Routing (LDPER)

66
CR-LDP
CR Constraint based Routing Example USE
(links with sufficient resources) AND
(links of type someColor) AND
(links that have delay less than 200 ms)
67
Traffic Engineering
B
C
Demand
A
D
Traffic engineering is the process of mapping
traffic demand onto a network
Network Topology
Purpose of traffic engineering

• Maximize utilization of links and nodes
  throughout the network
• Engineer links to achieve required delay,
  grade-of-service
• Spread the network traffic across network links,
  minimize impact of single failure
• Ensure available spare link capacity for
  re-routing traffic on failure
• Meet policy requirements imposed by the network
  operator

Traffic engineering is key to optimizing
cost/performance
68
MPLS Traffic Engineering Methods

• MPLS can use the source routing capability to
  steer traffic on desired paths
• Operator may manually configure these in each LSR
  along the desired path
• Analogous to setting up PVCs in ATM switches

69
MPLS Traffic Engineering Methods

• Ingress LSR may be configured with one or more
  LSRs along the desired path, hop-by-hop routing
  may be used to set up the rest of the path
• a.k.a. loose source routing, less configuration
  required
• If desired for control, route discovered by
  hop-by-hop routing can be frozen
• a.k.a route pinning
• In the future, constraint-based routing will
  offload traffic engineering tasks from the
  operator to the network itself

70
WHEN TO USE TE?

• When the following is not acceptable Simply
  ranking things so that you throw away the least
  important traffic first.
• When traffic is being thrown away but you have
  other viable routes that are unused or
  underutilized.
• Dont use TE if it is not necessary. In fact
  dont use MPLS if vanilla IP is working for you.
  Use LDP, CR-LDP and RSVP-TE if/when they are
  needed.

71
Reactive traffic engineering
Wait till you have a problem and then patch
around it. 1- Identify a flow to move Q
how? 2- Establish tunnel on some other route
Q what route?
72
How to identify a flow to move? Good Statistics!
A) move the flow that is being discarded. For
this you need to have stats that show src, dest,
protocol that are being thrown away. (note this
is T.E. of least important traffic) B) move some
other high priority user on the link somewhere
else. For this you need to have stats that show
src, dest, protocol of high users. (note, this
is T.E. of more important traffic)
73
What route to use for tunnel? Not shortest path!
A) Explicitly route without help of constraint
based routing. B) Use constraint not this link
so that MPLS can pick all the other links
dynamically but is not allowed to pick the
congested one. Neither of these approaches will
result in shortest paths and both are hard to
administer as things scale up.
74
Pro-active traffic engineering (plan ahead)
S1
S2
S3
D
S4
S5
S6
1- Start with rough idea on Si, D B/W
requirements. 2- Establish constraint based
tunnels Si -gt D 3- repeat forever at low
frequency 3a- Remeasure Si -gt D B/W
utilization. 3b- Adjust reservations on Si
-gtD to be closer to actual utilization
using hot swaps.
75
Pro-active traffic engineering
S1,tcp
S2,udp
S3,tcp
D
S4,tcp
S5,udp
S6,tcp
This approach will favor badly behaved
Si,D pairs. For example UDP-like streaming
video will get better reservations than TCP which
backs off. Possible solution is to break it
down into Si,D,Protocol triples and slightly
increase TCP reservations but decrease UDP.
76
MPLS Traffic engineering

• Imperative to be able to monitor flow rates to
  the granularity of source, dest, protocol .
• Use MPLS constraint based routing to assign paths
  to flows based on a reservation.
• Try to adjust the reservations periodically to
  reflect changes in utilization.
• MPLS aims to do a really good job of placing
  routes given the reservations are accurate.
• MPLS allows dynamic changes to reservations so
  they can slowly converge on reality over time.

77
MPLS Traffic engineering interactions with
vanilla IP.

• There are non trivial interaction issues to deal
  with when some of the traffic (MPLS) is traffic
  engineered and the rest (vanilla IP) is not.

78
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

79
MPLS Provides Benefits for Establishing Virtual
Private Networks

• Virtual Private Network (VPN)
• Connects two or more separate sites over the
  Internet
• Allows them to function as if they were a single,
  private network.
• Key Features Security, control over performance,
  management ability.
• Use of MPLS for VPNs
• MPLS can set up one or more LSPs between sites.
• Organizations can choose how they use the LSPs.
• Can view the set of LSPs as a network.
• Significant debate is in progress on how to use
  MPLS for VPNs.
• Will study VPNs more in a later lecture.

80
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

81
Generalized MPLS

• The label switching concept is very powerful.
• Especially the ability to nest labels within
  labels in a label stack.
• Why is this capability helpful?

82
Generalized MPLS

• Idea can be extended to more than just streams of
  packets.
• Therefore, a concept has been developed called
  Generalized MPLS
• Being developed in the IETF ccamp working group.
• Common Control and Measurement Plane (ccamp)
• Goal is to use Generalized MPLS (GMPLS) to
  dynamically provision resources and to provide
  network survivability using protection and
  restoration techniques.
• For elements such as routers, switches, DWDM
  systems, Add-Drop Multiplexors (ADMs), photonic
  cross-connects (PXCs), optical cross-connects
  (OXCs), etc .
• Encompassing time-division (e.g. SDH/SONET, PDH,
  G.709), wavelength (lambdas), and spatial
  switching (e.g. incoming port or fiber to
  outgoing port or fiber).
• Overview document draft-ietf-ccamp-gmpls-architec
  ture-03.txt

83
OXC
F1
F4
l2
l1
t0..47
t48..
Logical Label Stack
48
0
1..
49....

• Extend MPLS (and CR-LDP and RSVP-TE) to support
  new labels
• fiber number N in bundle of M fibers.
• fiber wavelengths as a label (lambda)
• position in time within a wavelength (TDM)

84
Generalized MPLS

• Now CR-LDP or RSVP-TE can be used to request
  these new labels.
• Request a fiber over which to send the packet.
• Request the wavelength within the fiber.
• Request the timeslot within the wavelength.
• Request the IP LSP.
• GMPLS was called MP(Lambda)S at first
• Because it was first applied to optical
  wavelengths.

85
(No Transcript)
86
Generalized MPLS Labels

• GMPLS uses the MPLS label stack.
• An LSP must start and end on similar types of
  devices.
• How would the effective label stack look for the
  previous figure?
• Keep in mind that all labels may not actually be
  used, they may be implicit.
• There is no fiber label. The packet stays on
  that fiber until a termination point for that
  fiber.

87
Generalized MPLS

• GMPLS allows two modes of operation.
• 1. Overlay Model
• Allows separate control mechanisms to be used.
• One network is over a lower level technology.
• MPLS LSPs send traffic through this lower level
  technology.
• Similar to concepts we have seen all semester.
• 2. Peer Model
• Uses MPLS control functions at all levels.
• To control fiber selection, wavelength selection,
  up to packet level.
• Creates a unified control plane - no more IP over
  ATM over SONET, etc.
• A combination of the two can be used, depending
  on the location in the network, creating a
  hybrid model

88
Tutorial Outline

• Overview
• Label Encapsulations
• Label Distribution Protocols
• MPLS ATM
• Constraint Based Routing
• Operational Experiences with Similar Protocols
• MPLS and VPNs
• Generalized MPLS
• Summary

89
Summary of Motivations for MPLS

• Simplified forwarding based on an exact match of
  a fixed length label
• Initial driver for MPLS was based on the
  existence of cheap, fast ATM switches
• Separation of routing and forwarding in IP
  networks
• Facilitates evolution of routing techniques by
  fixing the forwarding method
• New routing functionality can be deployed without
  changing the forwarding techniques of every
  router in the Internet
• Facilitates the integration of ATM and IP
• Allows carriers to leverage their large
  investment of ATM equipment

90
Summary of Motivations for MPLS

• Enables the use of explicit routing/source
  routing in IP networks
• Can easily be used for such things as traffic
  management, QoS routing
• Promotes the partitioning of functionality within
  the network
• Move detailed processing of packets to the edge
  restrict core to simple packet forwarding
• Assists in maintaining scalability of IP
  protocols in large networks
• Improved routing scalability through stacking of
  labels
• Removes the need for full routing tables from
  interior routers in transit domain only routes
  to border routers are required

91
Summary of Motivations for MPLS

• Applicability to both cell and packet link-layers
• Can be deployed on both cell (eg. ATM) and packet
  (eg. FR, Ethernet) media
• Common management and techniques simplifies
  engineering
• But MPLS is much more complex than traditional IP
  forwarding
• Routers also need to be able to forward based on
  labels.
• LSPs must be signalled and maintained.
• Many ISPs are not using MPLS and do not plan to.
• This will continue to be true if overprovisioning
  remains cost effective.
• But MPLS is more seriously being considered to
  carry legacy ATM and Frame Relay traffic
  (connection-oriented traffic).

92
MPLS Partitioning Routing and Forwarding
Based on Classful Addr. Prefix? Classless Addr.
Prefix? Multicast Addr.? Port No.? ToS Field?
Routing
OSPF, IS-IS, BGP, RIP
Forwarding Table
Forwarding
Based on Exact Match on Fixed Length Label
MPLS

• Current network has multiple forwarding paradigms
• - classful longest prefix match (Class A,B,C
  boundaries)
• - classless longest prefix match (variable
  boundaries)
• - multicast (exact match on source and
  destination)
• - type-of-service (longest prefix. match on
  addr. exact match on ToS)
• As new routing methods change, new route look-up
  algorithms are required
• - like when CIDR was introduced
• Next generation routers will be based on hardware
  for route look-up
• - changes will require new hardware with new
  algorithm
• MPLS has a consistent algorithm for all types of
  forwarding partitions routing/fwding
• - minimizes impact of the introduction of new
  forwarding methods

MPLS introduces flexibility through a consistent
forwarding paradigm
93
Summary

• MPLS is an important emerging technology.
• MPLS/LDP/CR-LDP have been recommended by the ITU
  for IP transport on ATM in public networks.
• Basic functionality (Encapsulation and basic
  Label Distribution) has been defined by the IETF.
• Traffic Engineering based on MPLS/CR-LDP is just
  round the corner.