Dipartimento di Informatica e Sistemisticabecchett/reti2/slide/17_MPLS.pdf · Perform forwarding of...

Dipartimento di Informatica e Sistemistica

Luca Becchetti

[email protected]

A.A. 2008/2009

RETI DI CALCOLATORI IIMulti Protocol Label Switching

(MPLS)

Luca BecchettiLuca Becchetti –– Computer NetworksComputer Networks II, A.A. 2008/2009 II, A.A. 2008/2009

Label Switching 1/4

Software-based forwarding

IP Router

Connectionless environment

Use of explicit addresses

Search in routing tables

Limited performance

Hardware-based forwarding

Label switching

Connection oriented environment

Flow identification --> use of labels

Direct access to routing tables

High performance


Label Switching 2/4

Forwarding Equivalent Class (FEC)

An aggregate of packets directed towards the same destination andwhich have to receive the same service in the network

FECs identify packet flows

different priorities

different services

different QoS level requirements

Label

identifies FEC to which a packet belongs

Constant and short length [hardware switching]

Local validity (limited to a subpart of packet path towards destination)


Label Switching 3/4

Switching (forwarding in IP jargon) occurs based only on label

A Label Switching Router

Inspects label associated to incoming packet

Finds output port

Changes old label to new value valid on outlink (label swapping)

Forwards packet to output port

Reduces routing table size

Number of entries in the order of number of different FECs onincoming links


Label Switching 4/4

Direct access to routing table

(port, label) pair determines function to apply to label

Pushing (assignment)

Swapping (label change)

Popping (label disposal)

(port, label) pair determines output port (port, label)


Label allocation 1/2

Binding

Assign label to a FEC

Necessary to notify assignment to all routers on the path followed bypackets belonging to FEC (Label Distribution Protocol)

Assignment normally based on destination prefix and required QoSlevel

Local binding

Assignment of a label to a FEC, autonomously done by a router

Remote binding

Assignment of a label to a FEC done by a remote router andcommunicated to this router using signalling



Control binding

FEC definition and label assignment done before packet sending ofpackets

Call set up phase necessary

Label distribution done by

signalling protocol (Label Distribution Protocol)

router configuration (static provisioning)

Data driven binding

FEC determined by data flow analysis

Label assignment starts at router that determines FEC

Label distribution signalling protocol (Label Distribution Protocol)

Dynamic environment


Label allocation:example


Label switching example


Multi Protocol Label Switching 1/2

Goals

Improve performance and scalability of IP routing

Separate control (routing) and transport (forwarding) functionalities soas to make them independent

One single transpost mechanism suitable to a multiplicity of differentprotocols

Make explicit routing and traffic engineering possible

Defined by IETF since 1997 (RFCs 3031,3032)


Multi Protocol Label Switching 2/2

Traffic Engineering – TE is the set of funcionalities aimed at optimizingnetwork performance (RFC 2702)

Tools used in traffic engineering

traffic measurement

traffic modelling

traffic characteristics

traffic control

Defined by IETF since 1997 (RFCs 3031,3032)


MPLS terminology 1/5

Label Switched Path (LSP)

Path followed by a given FEC when traversing an MPLS domain

Concatenation of a number of subpaths based on label switching

May be viewed as a tunnel used by IP datagrams


MPLS Terminology 2/5

Label Switching Router (LSR)

Perform forwarding of MPLS packets and implement full IPfunctionalities

Implement MPLS protocols (e.g., label allocation)



Ingress LSR

Router where an LSP starts

Processes traffic entering MPLS domain

Classifies incoming IP packets into suitable FEC

Generates labels and builds MPLS packets



Transit LSR

Intermediate router within an LSP

Process internal traffic within an MPLS domain

Performs MPLS packet forwarding and in particular label processing


Terminologia MPLS 5/5

Egress LSR

Last router along an LSP

Processes traffic leaving MPLS domain

Removes MPLS labels and extracts IP datagrams


Label Distribution Protocol (LDP)

Set of procedures through which an LSR communicates to another LSRthe binding between labels and FECs

Two LSR communicating by the Label Distribution Protocol are said LabelDistribution Peers

MPLS does not impose use of a specific Label Distribution Protocol

Protocol proposals:

BGP (extension)

RSVP (extension)

MPLS-LDP

CR-LDP



MPLS two label allocation policies

Independent Control

Ordered Control

Independent Control

Every LSR assigns and distributes labels independently of other LSRsin request to advertising of prefixes by IGP protocol

Assumes agreement between LSRs about FECs used in the network

Ordered Control

Label allocation and distribution occurs according to predefined orderamong LSRs along every LSP

Slower than independent control, but it allows higher control of theoperator over the network


Allocazione delle label 2/2

Downstream binding

Assignment of label to a FEC is done by downstream LSR accordingto traffic direction

Downstream On Demand

An LSR receives a request for the allocation of a label fromdownstream LSR

Unsolicited Downstream

Label distribution even without explicit request

Traffic direction


Label stack

An MPLS packet may carrydifferent labels

An LSR processesalways and only mostexternal label

Applications

Routing hierarchy

Aggregation ofindividual LSPs into“trunk” LSP

Virtual Private Networks(VPNs)


Label stack example


Forwarding table

One record in a forwarding table (Next Hop label Forwarding Entry -NHLFE) at an LSR contains

Output port to which packet should be forwarded (next hop)

Operation to perform on incoming label

Label swapping

o Conversion of valid external label on incoming link into validlable on outbound link

Label popping

o Removal of external label

Label pushing

o Insertion of a new label on top of label stack

Label swapping and label pushing operations may be performed onthe same packet by the same LSR


FEC - prefix association

It is possible to create

Distinct FECs for every destination prefix

Aggregate FECs for a set of destination prefixes (Aggregation)

Prefix aggregation in the same FEC reduces number of labels and amountof signalling traffic


Label merging

Packets with different entry labels to an LSR may be associated to thesame exit label


MPLS routing

Hop-by-hop routing

An LSR chooses next hop of LSP independently of other LSPs

Classical IP approach

Explicit routing

Strictly explicit routing

all LSRs traversed by LSP specified

Loosely explicit routing

only part of LSRs traversed by LSP specified

On-line: Constraint Based Routing (CBR)

Off-line: planning algorithms

Used to support traffic engineering


Label encoding

Label encoded as 32 bit word

Label value (20 bits)

Contains label value for a specific LSP

Experimental Use (EXP - 3 bits)

Reserved

Bottom of stack (S - 1 bit)

Indicates whether label is the most external one

Time To Live (TTL - 8 bits)

Same meaning as analogous field in IP datagram


Example A 1/2

LSP used to carry IP datagrams directed to a specific destination address

FEC identified by prefix “X” of destination network

LSR sequence in LSP computed by IGP protocol

An LSR belongs to LSP if its table contains a single record such that“X” is the longest prefix that determines routing towards destination

Every LSR

Associates a label to “X” prefix

Uses label distribution protocol to coomunicate association to its“peer” LSR

The role of egress LSR is played by the LSR for which prefix “X” is nolonger the “longest prefix” for the destination


Example A 2/2


Example B 1/2

Explicit Routed LSP

Route independent from the one determined by routing protocol

Route determined by

Configuration algorithms (Off-line)

Constraint-Based routing (On-line)

Used to support traffic engineering


Example B 2/2


Example C 1/4


Example C 2/4


Example C 3/4


Example C 4/4

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