Virtual Circuit (VC) Switching

Virtual-Circuit Switching: ATM (Asynchronous Transmission Mode)

and MPLS (Multiprotocol Label Switching)

2007. 10

Virtual Circuit (VC) Switching Hybrid of packets and circuits

Circuits: establish and teardown along end-to-end path

Packets: divide the data into packets with identifiers

Packets carry a virtual-circuit identifier Associates each packet with the virtual

circuit Determines the next link along the path

Intermediate nodes maintain state VC Forwarding table entry Allocated resources

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Timing of Virtual-Circuit Packet Switching

Packet 1

Packet 2

Packet 3

Host 1 Host 2Node

1Node

2

propagation delay between Host 1 and Node 1VC

establishment

VCtermination

Datatransfer

Establishing the Circuit Signaling

Creating the entries in the forwarding tables Reserving resources for the virtual circuit, if

needed Two main approaches to signaling

Network administrator configures each node Source sends set-up message along the path

Set-up latency Time for the set-up message to traverse the

path … and return back to the source

Routing End-to-end path is selected during circuit set-

up

Virtual Circuit Identifier (VC ID) Virtual Circuit Identifier (VC ID)

Source set-up: establish path for the VC Switch: mapping VC ID to an outgoing

link Packet: fixed length label in the header

1

2

1: 72: 7

link 7 1: 142: 8

link 14link 8

Swapping the Label at Each Hop Problem: using VC ID along the whole

path Each virtual circuit consumes a unique

ID Starts to use up all of the ID space in

the network Label swapping

Map the VC ID to a new value at each hop

Table has old ID, and next link and new ID1

2

1: 7, 202: 7, 53 link 7

20: 14, 7853: 8, 42

link 14link 8

Virtual Circuits Similar to IP Datagrams Data divided in to packets

Sender divides the data into packets Packet has address (e.g., IP address or VC

ID) Store-and-forward transmission

Multiple packets may arrive at once Need buffer space for temporary storage

Multiplexing on a link No reservations: statistical multiplexing

•Packets are interleaved without a fixed pattern

Reservations: resources for group of packets•Guarantees to get a certain number of

“slots”

Virtual Circuits Differ from IP Datagrams Forwarding look-up

Virtual circuits: fixed-length connection id IP datagrams: destination IP address

Initiating data transmission Virtual circuits: must signal along the path IP datagrams: just start sending packets

Router state Virtual circuits: routers know about

connections IP datagrams: no state, easier failure

recovery Quality of service

Virtual circuits: resources and scheduling per VC

IP datagrams: difficult to provide QoS

Asynchronous Transfer Mode: ATM 1990’s/00 standard for high-speed (155Mbps

to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture

Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice,

video (versus Internet best-effort model) “next generation” telephony: technical

roots in telephone world packet-switching (fixed length packets,

called “cells”) using virtual circuits

ATM reference model

ATM architecture

adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer

ATM layer: “network” layer cell switching, routing

physical layer

ATM Physical Layer

Physical Medium Dependent (PMD) sublayer

SONET/SDH: transmission frame structure (like a container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 =

622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps

TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps

unstructured: just cells (busy/idle)

ATM Physical Layer (more)

Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS):

adapts ATM layer above to PMD sublayer below Physical Medium Dependent (PMD) : depends

on physical medium being used

TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer,

transmission of idle cells when no data cells to send

ATM Layer: Virtual Circuits analogous to IP network layer very different services than IP network layer VC transport: cells carried on VC from source

to dest call setup, teardown for each call before

data can flow each packet carries VC identifier (not

destination ID) every switch on source-dest path maintain

“state” for each passing connection link,switch resources (bandwidth, buffers)

may be allocated to VC: to get circuit-like perf.

ATM VCs

Advantages of ATM VC approach:QoS performance guarantee for

connection mapped to VC (bandwidth, delay, delay jitter)

Drawbacks of ATM VC approach:Inefficient support of datagram

trafficVC introduces call setup latency,

processing overhead for short lived connections

ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload

Why?: small payload -> short cell-creation delay for digitized voice

halfway between 32 and 64 (compromise!)

Cell header

Cell format

ATM cell header

VCI: virtual channel ID will change from link to link thru net

PT: Payload type (e.g. RM cell versus data cell)

CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be

discarded if congestion HEC: Header Error Checksum

cyclic redundancy check

ATM Service

very different services than IP network layer

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

ATM Adaptation Layer (AAL)

ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below

AAL present only in end systems, not in switches

AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells analogy: TCP segment in many IP packets

ATM Adaptation Layer (AAL) [more]Different versions of AAL layers, depending on

ATM service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit

emulation (phone) AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG

video AAL5: for data (eg, IP datagrams)

AAL PDU

ATM cell

User data

IP-Over-ATM

AALATMphyphy

Eth

IP

ATMphy

ATMphy

apptransport

IPAALATMphy

apptransport

IPEthphy

How far along are we?

Standardization bodies - ATM Forum, ITU-T We may never see end-to-end ATM (1997)

Backbone: - 1995 vBNS (ATM) - 1998 Abilene (SONET) - 2000 IP over DWDM

ATM - too complex - too expansive <IP> Internet technology + ATM philosophy

but ATM ideas continue to powerfully influence design of next-generation Internet

ex: MPLS, admission ctl., resource reservation, …...

Multiprotocol label switching Multiprotocol label switching (MPLS)(MPLS) initial goal: speed up IP forwarding by

using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit

(VC) but IP datagram still keeps IP

address!PPP or Ethernet header

IP header remainder of link-layer frameMPLS header

label Exp S TTL

20 3 1 5

Label SubstitutionLabel Substitution Have a friend go to B ahead of you using

one of the previous two techniques. 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.

Label Encapsulation

MPLS Encapsulation is specified over various media types. Top labels may use existing format, lower label(s) use a new “shim” label format.

MPLS Link Layers

MPLS -- run over multiple link layers Following link layers currently exist: • ATM: label -- in VCI/VPI field of ATM header • Frame Relay: label -- in DLCI field in FR header • PPP/LAN: uses ‘shim’ header inserted between L2 and L3 headers Translation between link layers types must be supported MPLS is between L2 and L3 It intended to be “multi-protocol” below and above

MPLS capable routers a.k.a. label-switched router forwards packets to outgoing interface based

only on label value (don’t inspect IP address) MPLS forwarding table distinct from IP

forwarding tables signaling protocol needed to set up forwarding

Hop-by-hop or source routing to establish labels

forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !!

use MPLS for traffic engineering RSVP-TE

must co-exist with IP-only routers

R1R2

D

R3R4R5

0

1

00

A

R6

in out outlabel label dest interface

6 - A 0

in out outlabel label dest interface

10 6 A 1

12 9 D 0

in out outlabel label dest interface

10 A 0

12 D 0

1

in out outlabel label dest interface

8 6 A 0

0

8 A 1

MPLS forwarding tables

Best of Both Worlds

MPLS + IP form a middle ground that combines the best of IP and the best of virtual circuit switching technologies

ATM and Frame Relay cannot easily come to the middle so IP has!

Multi-Protocol Label Switching Key ideas of MPLS

Label-switched path spans group of routers

Explicit path set-up, including backup paths

Flexible mapping of data traffic to paths

Motivating applicationsSmall routing tables and fast look-upsVirtual Private NetworksTraffic engineeringPath protection and fast reroute

Status of MPLS

Deployed in practice Small control and data plane overhead in

core Virtual Private Networks Traffic engineering and fast reroute

Challenges Protocol complexity Configuration complexity Difficulty of collecting measurement data

Continuing evolution Standards Operational practices and tools

Optical Networks 1 st Generation: optical fibers

substitute copper as physical layer Submarine Systems SONET (synchronous optical) in

TDM FDDI for LAN, Gbit Ethernet etc.

2 nd Generation: optical switching and multiplexing/ WDM

broadcast-and-select networks WDM rings wavelength routing networks

3 th Generation: optical packet switching???

Optical Switch 1-input 2-outoput illustration with four

wavelengths

1-D MEMS (micro-electromechanical system) with dispersive optics

Dispersive element separates the ’s from inputs

MEMS independently switches each Dispersive element recombines the

switched ’s into outputs

1-D MEMSMicro-mirror

Array

Digital MirrorControl

Electronics1011

Wavelength Dispersive Element

Input Fiber

Output Fiber 1

Output Fiber 2

Input & Output fiber array

All-Optical Switching Optical Cross-Connects (OXC)

Wavelength Routing Switches (WRS)route a channel from any I/P port to any O/P

port Natively switch s while they are still

multiplexed Eliminate redundant optical-electronic-optical

conversions

DWDMFibers

in

DWDMDemux

DWDMDemux

DWDMFibers

out

DWDMMux

DWDMMux

All-optical

OXC

MPS MPS = Multi-Protocol Lambda

Switching MPLS + OXC Combining MPLS traffic eng control with

OXC All packets with one label are sent on one

wavelength Next Hop Forwarding Label Entry (NHFLE)

<Input port, > to <output port, > mapping