Multi-Protocol Label
Switch (MPLS)
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Outline
• Introduction
• MPLS Terminology
• MPLS Operation
– Label Encapsulation
• Label Distribution Protocol (LDP)
• Any Transport Service over MPLS
• MPLS Applications
– Traffic Engineering
– MPLS-based VPN
– MPLS and QoS
• Summary
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Why MPLS?
• Growth and evolution of the Internet
• The need for network convergence to support both voice and
data services on both carrier and enterprise networks
• The need for advanced and guaranteed service over the
Internet
– The need of virtual circuit but without the complexity of provisioning
and managing virtual circuits.
• PVC: too much provisioning and management work
• SVC: [signaling] too complex to support and trouble shoot
• The need for an efficient transport mechanism
– routing: flexibility
– forwarding: price/performance
– Can we forward IP packets? Answer: MPLS
Performance and service of Layer-2 and management of layer-3
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Motivation for Carriers
• Network convergence
– Single network to support voice and data traffic
• Ease of network management
– to provision new services
– to support various Service Level Agreements (SLA)
• Ease of Traffic Engineering
– To reroute during node failures or network congestion
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Motivation for Enterprises
• Network convergence
– Single network for voice and data
• A meshed topology (any-to-any) without the
nightmare of cost and management
– Confusion with too many Frame Relay PVCs
• Quality of Service (QoS) for intranet
– Ease of bandwidth management
– Flexibility of bandwidth provisioning
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MPLS History
• IP over ATM
• IP Switching by Ipsilon
• Cell Switching Router (CSR) by Toshiba
• Tag switching by Cisco
• Aggregate Route-based IP Switching (IBM)
• IETF – MPLS
– http://www.ietf.org/html.charters/mpls-charter.html
– RFC3031 – MPLS Architecture
– RFC2702 – Requirements for TE over MPLS
– RFC3036 – LDP Specification
– over 113 RFCs related to MPLS
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MPLS and OSI
(MPLS is a layer 2.5 protocol)
Applications
TCP UDP
IP
MPS
MPLS
PPP FR ATM Ethernet DWDM
Physical
When a layer is added, no modification is needed
on the existing layers.
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MPLS and OSI
(MPLS is a layer 2.5 protocol)
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Label Switching
(This is not new!)
• ATM: VPI/VCI
• Frame Relay: DLCI
• X.25: LCI (logical Channel Identifier)
• TDM: the time slot (Circuit Identification
Code)
• Ethernet switching: MAC Address
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Label Substitution (swapping)
Label-A1 Label-B1
Label-A2 Label-B2
Label-A3 Label-B3
Label-A4 Label-B4
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MPLS
• A protocol to establish an end-to-end path from source
to the destination
• A hop-by-hop forwarding mechanism
• Use labels to set up the path
– Require a protocol to set up the labels along the path
• Support multi-level label transport
• It builds a connection-oriented service on the IP
network
– Note: ATM and Frame Relay also support connection-
oriented services, but IP does not.
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Terminology
• LSR - Routers that support MPLS are called Label Switch
Router
• LER - LSR at the edge of the network is called Label Edge
Router (a.k.a Edge LSR)
– Ingress LER is responsible for adding labels to unlabeled IP packets.
– Egress LER is responsible for removing the labels.
• Label Switch Path (LSP) – the path defined by the labels
through LSRs between two LERs.
• Label Forwarding Information Base (LFIB) – a forwarding
table (mapping) between labels to outgoing interfaces.
• Forward Equivalent Class (FEC) – All IP packets follow the
same path on the MPLS network and receive the same
treatment at each node.
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How does it work?
Add label at the remove label at
ingress LER the egress LER
LSR LSR LER
LER
IP IP #L1 IP #L2 IP #L3 IP
IP Label Label IP
Routing Switching Switching Routing
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MPLS Operation
Label Path: R1 => R2 => R3 => R4
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Label Forwarding Information Base (LFIB)
Router Incoming Incoming Destination Outgoing Outgoing
Interface Network
Label Interface Label
(FEC)
R1 --- E0 172.16.1.0
S1 6
R2 6 S0 172.16.1.0
S2 11
R3 11 S0 172.16.1.0
S3 7
R4 7 S1 172.16.1.0
E0 --
Note: the label switch path is unidirectional.
Q: create LFIB for R4 => R3 => R2 => R1
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Label Encapsulation
Label information can be carried in a packet in a variety of ways:
• A small, shim label header inserted between the Layer 2 and
network layer headers.
• As part of the Layer 2 header, if the Layer 2 header provides
adequate semantics (such as ATM).
• As part of the network layer header (future, such as IPv6).
• In general, MPLS can be implemented over any media type,
including point-to-point, Ethernet, Frame Relay, and ATM
links. The label-forwarding component is independent of the
network layer protocol.
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Shim Header
The Label (Shim Header) is represented as a
sequence of Label stack entries
Each Label is 4 bytes (32 bits)
20 Bits is reserved for the Label Identifier
Label Identifier Exp S TTL
(20 bits) (3 bits) (1 bit) (8bits)
Label Identifier : Label value (0 to 15 are reserved)
Exp : Experimental Use
S: Bottom of Stack (set to 1 for the last entry in the label)
TTL : Time To Live
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MPLS and TTL
• TTL: Time to Live
• In IP, TTL is used to prevent packets to travel
indefinitely in the network.
• MPLS uses the same mechanism as IP.
• Why do we need TTL?
– MPLS may interwork with non-MPLS network.
• TTL is in the label header of PPP and Ethernet (shim
header)
• Not supported in ATM.
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Forward Equivalent Class
(FEC) Classification
When an unlabeled packet arrives at an ingress router, a
label has to be applied. A packet can be mapped to a
particular FEC based on the following criteria:
• destination IP address
• source IP address
• TCP/UDP port
• class of service (CoS) or type of service (ToS)
• application used
• any combination of the previous criteria.
Ingress Label FEC Egress Label
6 138.120.6.0/24 9
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Label Distribution Protocol (LDP)
• Labels are distributed between LERs and LSRs
using LDP
• LSRs regularly exchange label and reachability
information with each other using
standardized procedures
• Used to build a picture of the network that
can be used to forward packets
• Label Switch Paths are created by network
operators – similar to PVC and VPN
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MPLS over ATM/Frame
Relay/Ethernet
• A majority of MPLS examples are used to carry
IP traffic over Ethernet links
• But MPLS can also carry IP traffic over ATM
and frame relay links
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MPLS and ATM
Most carriers’ backbone are ATM-
based. What is the process of
migrating from ATM to MPLS-based
backbone?
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MPLS and ATM
• The label information of MPLS can be carried in the ATM VCI field. If two
levels of labeling are needed, then the ATM VPI field could be used. The
VCI field, however, is adequate for most applications.
• Implementing MPLS on an ATM switch would simplify integration of ATM
switches and routers. An ATM switch capable of MPLS would appear as a
router to an adjacent router. This approach avoids the issues of ATM
addressing, routing, and signaling schemes.
• Implementing MPLS on an ATM switch does not preclude the capability to
support a traditional ATM control plane (such as PNNI) on the same
switch.
• The two components, MPLS and the ATM control plane, would operate
independently with VPI/VCI space and other resources partitioned so that
the components would not interact.
• Two implementation schemes: VC-merging vs. non-merging
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ATM LSR - Non-Merging
(note: this is MPLS over ATM)
In/Intf In/Label Destination Out/Intf Out/Label
1 5 171.68 0 3
2 8 171.68 0 4
… … … …
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IP ATM 5
Packet cell
ATM
cell 1 0
4 3 4 3 4
8 ATM ATM ATM ATM ATM
IP
8
8
2 cell cell cell cell cell 171.68
Packet ATM
ATM cell
ATM cell
cell
Each ATM VC has its own MPLS label.
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ATM LSR - VC-Merging
In/Intf In/Label Destination Out/Intf Out/Label
1 5 171.68 0 3
2 8 171.68 0 3
… … … …
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IP ATM 5
Packet cell
0
ATM
cell 1
3 3 3 3 3 171.68
8
2
IP 8 ATM ATM ATM ATM ATM
Packet 8 cell cell cell cell cell
ATM
ATM cell
ATM cell
cell
Multiple AMT VCs with the same destination and QoS are
merged on the MPLS network with the same label.
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Frame Relay over MPLS Example
R2
R1 Ra Rb Rc
Lo: loopback interface. Why is it needed?
R1 and R2: legacy frame relay router
Ra and Rb: Label Switch Edge Router. Label path is between Ra and Rb.
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Frame Relay over MPLS (cont.)
192.168.34.1 192.168.34.5
192.168.34.2 192.168.34.6
IP IP IP
68.68.68.2
68.68.68.1
DLCI
DLCI
514
513
IP lo: 3.1.1.1 lo: 3.1.1.2 IP
RFC2427 RFC2427
Frame Frame
Relay FR RFC 4619 RFC 4619 FR
Relay
PHY PHY MPLS MPLS MPLS PHY
PHY
L2 L2 L2
PHY PHY PHY
Note: The IP addresses for MPLS configuration and
Frame Relay are on different IP subnets.
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MPLS Applications
• Traffic Engineering
• Virtual Private Network (VPN)
• Quality of Service (QoS)
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Traffic Engineering
• Traffic engineering allows a network administrator to select the path
between two notes and bypass the normal routed hop-by-hop paths. An
administrator may elect to explicitly define the path between nodes to
ensure QoS or have the traffic follow a specified path to avoid traffic
congestion at certain hops.
• The network administrator can reduce congestion by forcing the frame to
travel around the overloaded segments. Traffic engineering, then, enables
an administrator to define a policy for forwarding frames rather than
depending upon dynamic routing protocols.
• Traffic engineering is similar to source-routing in that an explicit path is
defined for the frame to travel. However, unlike source-routing, the hop-
by-hop definition is not carried with every frame. Rather, the hops are
configured in the LSRs ahead of time along with the appropriate label
values.
• The administrator could be a centrally located program.
• Traffic engineering is an important tool for network management. It is NOT
a customer service. (So you will not see it on a carrier’s web site.)
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MPLS – Traffic Engineering
Overload !!
LER 1 LER 4 IP
IP Overload !!
IP L
IP L
Forward to IP L
LSR 2
LSR 3
LSR 4 LSR 2 LSR 3
LSR X
End-to-End forwarding decision determined by
ingress node.
Enables Traffic Engineering
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MPLS-based VPN
• One of most popular MPLS applications is the
implementation of VPN.
• The basic concept is the same as ATM transparent
LAN.
• Using label (instead of IP address) to interconnect
multiple sites over a carrier’s network. Each site has
its own private IP address space.
• Different VPNs may use the same IP address space.
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MPLS VPN - Example
192.168.1.0 192.168.2.0
E1 E1
E3 E3
E1 E2 E2
E2
192.168.3.0 -- E1 10 E3 10 E1 30 E2 30 E3 -- E1 192.168.4.0
-- E2 20 E3 20 E1 40 E2 40 E3 -- E2
LSP
uni-direction
10 E3 -- E1 30 E2 10 E1 -- E1 30 E3
20 E3 -- E1 40 E2 20 E1 -- E2 40 E3
uni-direction LSP
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MPLS VPN Connection Model
MPLS MPLS
Edge Edge
VPN_A MPLS Core VPN_A
10.2.0.0 11.5.0.0
VPN_B VPN_A
10.2.0.0 10.1.0.0
VPN_A
11.6.0.0 VPN_B
10.3.0.0
VPN_B
10.1.0.0
VPN_A: 10.2.0.0/24, 11.5.0.0/24, 11.6.0.0/24, 10.1.0.0/24
VPN_B: 10.2.0.0/24, 10.1.0.0/24, 10.3.0.0/24
Q: For a meshed connection, how many label paths are needed?
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Case Study (I)
AT&T MPLS Private Transport Network Services
Features and Benefits
• Advanced Management options
• MPLS-based security
• Meshed topology for any-to-any
connectivity
• Traffic prioritization - 4 Classes
of Services (CoS)
• Service Level Agreements
(SLAs)
• Web-based reporting
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Case Study (II)
Verizon Private IP Service (MPLS)
History:
• MCI (Verizon) adopted MPLS on a large scale in 1998 as a traffic engineering
technology on its public Internet backbone
Features and Benefits:
• Exceptional Service. 24-hour monitoring customer service, and service level
agreements (SLAs).
• Any-to-Any Connectivity. Multiple locations are connected (meshed topology).
You no longer need PVCs to communicate between sites; rather
• Cost-Effective Solution. Private IP Service utilizes existing network infrastructure
without building and operating a private VPN.
• Intranets and Extranets. Private IP Service captures the enhanced networking
efficiencies associated with an IP-based WAN, bringing together all the elements
to support e-business applications within the company or between companies.
• MPLS Technology. Private IP Service provides varying Class of Services (CoS) and
flexible IP routing that optimize network’s performance.
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