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Virtual Circuit Switching and MPLS

Sandip Chakraborty

Department of Computer Science and Engineering,

INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

February 4, 2016

Sandip Chakraborty (IIT Kharagpur) CS 60008 February 4, 2016 0 / 18

Virtual Circuit Switching

Advantages and disadvantages for packet switching and circuitswitching

Virtual circuit switching - circuit switching over packet switchednetworks (PSN)


Virtual Circuit Switching - Advantages

Packets are delivered in order, since they all take the same route

The overhead in the packets is smaller, since there is no need for eachpacket to contain the full address;

The connection is more reliable, network resources are allocated atcall setup so that even during times of congestion, provided that acall has been setup, the subsequent packets should get through;

Traffic engineering - QoS implementation becomes easier - now youcan implement RSVP over the virtucal circuit switched path!


Virtual Circuit Switching - Connection Establishment

Virtual Circuit Identifier (VCI)

VCI directs the path for forwarding

I1

I2

I3

I4

VCI = 2

VCI IF_IN

2

IF_OUT

I2 I4


Multiprotocol Label Switching (MPLS)

In a traditional IP network

Each router performs an IP lookup (routing), determines a next-hopbased on its routing table, and forwards the packet to that next-hop

Rinse and repeat for every router, each making its own independentrouting decisions, until the final destination is reached


Multiprotocol Label Switching (MPLS)

MPLS follows “Label Switching”, where

The first device does a routing lookup, but instead of finding anext-hop, it finds the final destination router

It finds out a pre-determined path from itself to that final router.

The router applies a “label” (or “shim”) based on this information.(VCI is now the “label”)

Future routers use the label to route the traffic without needing toperform any additional IP lookups

At the final destination router the label is removed and the packet isdelivered via normal IP routing


Advantages of Label Switching

Originally MPLS was designed to reduce IP lookups. (IP lookupoverheads associated with CIDR, longest prefix match)

Label switching reduces IP lookups - the idea was to have only thefirst router do an IP lookup, then all future routes in the networkcould do exact match “switching” based on a label.

Modern circuits can executes hundreds of IP lookups very fast, sowhy do people still care about MPLS?


Advantages of Label Switching

Implementing Traffic-Engineering - The ability to control whereand how traffic is routed on your network, to manage capacity,prioritize different services, and prevent congestion

Implementing Multi-Service Networks - The ability to deliver datatransport services, as well as IP routing services, across the samepacket-switched network infrastructure

Improving network resiliency with MPLS “Fast Reroute”.


How MPLS Works?

MPLS Label Switched Path (“LSP”)One of the most important concepts for the actual use of MPLS.

Essentially a unidirectional tunnel between a pair of routers, routedacross an MPLS network


How MPLS Works?

MPLS routers -Label Edge Router (“LER”) or “ingress node” - The router whichfirst encapsulates a packet inside an MPLS LSP

Label Switching Router (“LSR”) or “transit node” - A routerwhich only does MPLS switching in the middle of an LSP

Egress Node - The final router at the end of an LSP, which removesthe label

LER

LSR

LSR

LSR

Egress


How MPLS Works?

Forward Equivalence Classes (FEC)A single flow or a group of traffic flows that follow same same LSP

Requires same level of QoS - can be grouped together

Associated with each FEC is a traffic characterization that defines QoSrequirements for that flow/group of flows.


How MPLS Works?

Source: MPLS by William Stallings, The Internet Protocol Journal


How MPLS Works?

Prior to routing and delivery of packets in a given FEC, the LSP mustbe defined. For this purpose, two protocols are used to exchangenecessary information among the routers;

An interior routing protocol, such as OSPF, is used to exchangereachability and routing information.

Labels must be assigned to the packets for a particular FEC - LabelDistribution Protocol (LDP)


How MPLS Works? - LDP


How MPLS Works?

Source: MPLS by William Stallings, The Internet Protocol Journal


Key Features of MPLS

An MPLS domain consists of a contiguous, or connected set of MPLSenabled routers - we can implement virtual private network (VPN)using MPLS.

The FEC for a packet can be determined by one or more parameters,like source and destination IP address, source and destination portnumbers, IP protocol ID, differentiated service code point, IPv6 flowlabel

Forwarding is achieved by doing a simple lookup in a predefined tablethat maps label values to next hop addresses.

A particular Per-Hop Behavior (PHB) can be defined at an LSR for agtiven FEC.

Packets sent between the same endpoints may belong to differentFECs. Example: VoIP packets and VoD packets.


Acknowledgements

Wiki documents

MPLS for Dummies - Richard A Steenbergen, nLayerCommunications, Inc, https:

//www.nanog.org/meetings/nanog49/presentations/Sunday/mpls-nanog49.pdf

Related documents and white-papers from the Internet.


Thank You


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