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MPLS Architecture

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MPLS Network Model

MPLS

LSR = Label Switched RouterLER = Label Edge Router

LER

LER

LSR

LER

LSRLSR

IP

MPLS

IP

Internet

LSR

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MPLS Benefits

Comparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits:

• The performance characteristics of layer 2 networks

• The connectivity and network services of layer 3 networks

• Improves the price/performance of network layer routing

• Improved scalability

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MPLS Benefits (cont’d)

• Improves the possibilities for traffic engineering

• Supports the delivery of services with QoS guarantees

• Avoids need for coordination of IP and ATM address allocation and routing information

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Necessity of L3 Forwarding

• For security– To allow packet filtering at firewalls– Requires examination of packet contents,

including the IP header

• For forwarding at the initial router - used when hosts don’t support MPLS

• For Scaling– Forward on a finer granularity than the

labels can provide

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MPLS Architecture

• Down stream label assignment for unicast traffic– On demand– Unsolicited

• Path selection– Hop by hop– Explicit

• Ordered vs. independent control• Loop detection and prevention mechanisms

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Label Distribution Protocol (LDP)

• Set of procedures used by LSRs to establish LSPs

• Mapping between network-layer routing information directly to data-link layer switched paths

• LDP peers: – Two LSRs which use LDP to exchange label/stream

mapping – Information exchange known as “LDP Session”

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LDP Messages

• Discovery messages – Used to announce and maintain the presence of an LSR

• Session/Adjacency messages – Used to establish, maintain and terminate sessions

between LDP peers

• Advertisement messages– Used to create, change, and delete label mappings

• Notification messages– Used to provide advisory information and to signal error

information

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Forwarding Equivalence Class (FEC)

• Introduced to denote packet forwarding classes

• Comprises traffic – To a particular destination– To destination with distinct service

requirements

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LSP - FEC Mapping

• FEC specified as a set of two elements – IP Address Prefix - any length from 0 – 32– Host Address - 32 bit IP address

• A given packet matches a particular LSP if and only if IP Address Prefix FEC element matches packet’s IP destination address

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Label Spaces

• Useful for assignment and distribution of labels

• Two types of label spaces– Per interface label space: Interface-

specific labels used for interfaces that use interface resources for labels

– Per platform label space: Platform-wide incoming labels used for interfaces that can share the same label space

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LDP Discovery

• A mechanism that enables an LSR to discover potential LDP peers

• Avoids unnecessary explicit configuration of LSR label switching peers

• Two variants of the discovery mechanism– Basic discovery mechanism: used to discover LSR

neighbors that are directly connected at the link level

– Extended discovery mechanism: used to locate LSRs that are not directly connected at the link level

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LDP Discovery (Cont’d)

• Basic discovery mechanism– To engage - send LDP Hellos periodically– LDP Hellos sent as UDP packets for all routers on that

subnet

• Extended discovery mechanism– To engage - send LDP targeted Hellos periodically– Targeted Hellos are sent to a specific address– Targeted LSR decides whether to respond or to ignore

the targeted Hello

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Session Establishment

• Exchange of LDP discovery Hellos triggers session establishment

• Two step process– Transport connection establishment

• If LSR1 does not already have a LDP session for the exchange of label spaces LSR1:a and LSR2:b, it attempts to open a TCP connection with LSR2

• LSR1 determines the transport addresses at its end (A1) and LSR2’s end (A2) of the TCP connection

• If A1>A2, LSR1 plays the active role; otherwise it is passive– Session initialization

• Negotiate session parameters by exchanging LDP initialization messages

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Label Distribution and Management

• Two label distribution techniques– Downstream on demand label distribution:

An LSR can distribute a FEC label binding in response to an explicit request

– Downstream Unsolicited label distribution: Allows an LSR to distribute label bindings to LSRs that have not explicitly requested them

• Both can be used in the same network at the same time; however, each LSR must be aware of the distribution method used by its peer

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Label Distribution Control Mode

• Independent Label Distribution Control– Each LSR may advertise label mappings to its

neighbors at any time– Independent Downstream on Demand mode - LSR

answers without waiting for a label mapping from next hop

– Independent Downstream Unsolicited mode - LSR advertises label mapping for a FEC whenever it is prepared

– Consequence: upstream label can be advertised before a downstream label is received

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Distribution Control Mode (cont’d)

• Ordered Label Distribution Control– Initiates transmission of label mapping for a FEC only if it

has next FEC next hop or is the egress– If not, the LSR waits till it gets a label from downstream

LSR– LSR acts as an egress for a particular FEC, if

• Next hop router for FEC is outside of label switching network

• FEC elements are reachable by crossing a domain boundary

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Label Retention Mode

• Conservative Label Retention Mode– Advertised label mappings are retained only if they

are used for forwarding packets– Downstream on Demand Mode typically used with

Conservative Label Retention Mode– Advantage: only labels required are maintained– Disadvantage: a change in routing causes delay

• Liberal Retention Mode– All label mappings are retained regardless of whether

LSR is next hop or not– Faster reaction to routing changes

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Label Information Base

• LSR maintains learned labels in Label Information Base (LIB)

• Each entry of LIB associates an FEC with an (LDP Identifier, label) pair

• When next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB

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Domain #3

Domain #2

Domain #1

Hierarchical Routing in MPLS

C

12 3 4 5

6

D

EBA F

•External Routers A,B,C,D,E,F - Talk BGP

•Internal Routers 1,2,3,4,5,6 - Talk OSPF

Note: Internal routers in domains 1 and 3 not shown

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Hierarchical Routing (cont’d)

• When IP packet traverses domain #2, it will contain two labels, encoded as a “label stack”

• Higher level label used between routers C and D, which is encapsulated inside a lower level label used within Domain #2

• Operation at C– C needs to swap BGP label to put label that D expects– C also needs to add an OSPF label that 1 expects– C therefore pushes down the BGP label and adds a lower level

label

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Explicit Routing in MPLS

• Two options for route selection:– Hop by hop routing– Explicit routing

• Explicit Routing (Source Routing) is a very powerful technique– With pure datagram routing, overhead of carrying

complete explicit route is prohibitive– MPLS allows explicit route to be carried only at the

time the LSP is setup, and not with each packet– MPLS makes explicit routing practical

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Explicit Routing (Cont’d)

• In an explicitly routed LSP – LSP next hop is not chosen by the local

node– Selected by a single node, usually the

ingress

• The sequence of LSRs may be chosen by– Configuration (e.g., by an operator or by a

centralized server)

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Loops and Loop Handling

• Routing protocols used in conjunction with MPLS are based on distributed computation which may contain loops

• Loops handling - 3 categories– Loop Mitigation/Survival– Loop Detection– Loop Prevention

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Loop Mitigation

• Minimizes the impact of loops by limiting the amount of resources consumed by the loop

• Method– Based on use of TTL field which is

decremented at each hop– Use of dynamic routing protocol converging

rapidly to non-looping paths

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Loop Detection

• Loops may be setup but they are subsequently detected

• The detected loop is then broken by dropping label relationship

• Broken loops now necessitates packets to be forwarded using L3 forwarding

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Loop Detection (Cont’d)

• Method is based on transmitting a Loop Detection Control Packet (LDCP) whenever a route changes

• LDCP is forwarded towards the destination until– Last MPLS node along the path is reached– TTL of the LDCP expires– It returns to the node which originated it

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Loop Prevention

• Ensures that loops are never set up• Labels are not used until it is sure to be loop

free• Methods

– Labels are propagated starting at the egress switch– Use source routing to set up label bindings from the

egress switch to each ingress switch