Switching - EECS Instructional Support Group Home …inst.eecs.berkeley.edu/~ee122/sp07/switching...

Unit 18

Switching

Acknowledgments: These slides were originally developed by Prof. Jean Walrand for EE122.The past and current EE122 instructors including Profs. Kevin Fall, Abhay Parekh, Shyam Parekh,

and Adam Wolisz have contributed to their evolution.

TOC: Switching & Forwarding� Why?

� Switching Techniques

� Switch Characteristics

� Switch Examples

� Switch Architectures

� Summary

TOC – Switching

� Direct vs. Switched Networks:

� Direct Network Limitations:

� Shared medium

— Distance (coordination delay; propagation limitation)

— Number of hosts (collisions; shared bandwidth; address tables)

— Single link technology (cannot mix optical, wireless, …)

� Point-to-point links

— O(n2) links

� Internetworking: Externality gain at low cost

Why?

Single

link

n links

SwitchesDirect Switched

TOC – Switching – Why?

� Circuit-Switching (e.g., Telephone net.)

� Packet-Switching� Datagram (e.g., IP, Ethernet)

� Virtual Circuits (e.g., MPLS, ATM)

� Source Routing

� Comparison

Techniques

TOC – Switching – Techniques

� Mechanism:

Circuit-Switching Link divided into fixed,independent circuits

Circuit SwitchConnection list

Time scale: connection

(e.g., TDM, WDM)

• Packets not switched independently (establish circuit before sending data)

• Dedicated path and resources from source to destination• Setup time; low delays and guaranteed resources thereafter

� Features:

TOC – Switching – Techniques – Circuit

(Typical Implementation: Time-Space-Time Switch)

� Mechanism:

Packet-SwitchingWhole link shared by

all packets

Packet Switch

• Data separated into packets• Switching decision (output port) for each individual packet• Statistical multiplexing: Sum of peak rates may exceedlink bandwidth (as long as mean does not)

� Features:

TOC – Switching – Techniques – Packet

PS - Datagram

� General idea: no connection establishment, but each packet contains enough info to specify destination

� Switches contain forwarding tables (but no per-connection “state”)

� Forwarding tables contain info on which outgoing port to use for each destination

� Two types of addressing:� Layer 2 or Layer 3

TOC – Switching – Techniques – Packet - Datagram

Layer 2 (e.g., Ethernet)

� Flat address space (no structure)

� Forwarding table:Exact match of destination L2 address

a

b

c

d

e

f

12

34

12

34

a 1b 4c 3d 2e 2f 2

a 1b 1c 1d 2e 3f 4

e|b|

ToFrom

TOC – Switching – Techniques – Packet – Datagram – L2

Layer 3 (e.g., IP)� L3-network (e.g., IP)

� Topological structure – match prefix

� Either fixed prefix length or longest match

E.g. 1

E.g. 2

E.g. 3


Fixed

11010001

11001101

Longest

11001001

10100111

11010000

11001000

10100000

01000000

1101

Other A

B

C

DA’

C’

B’

11010101

110011

11100000

11001100

Layer 3 (e.g., IP) E.g. 1

11011001 matches 4 bits at A, 1 at B, 3 at C � A = LPM4 bits at A’ � A’ = EM


Fixed

11010001

11001101

Longest

11001001

10100111

11010000

11001000

10100000

01000000

1101

Other A

B

C

DA’

C’

B’

11010101

110011

11100000

11011001

11001100

Layer 3 (e.g., IP)E.g. 2

11001001 matches 5 bits at A, 1 at B, 7 at C � C = LPM


11001001Fixed

11010001

11001101

Longest

11001001

10100111

11010000

11001000

10100000

01000000

1101

OtherA

B

C

DA’

C’

B’

11010101110011

11100000

11001100

Layer 3 (e.g., IP)

E.g. 301100101 matches

0 bit at A’ � B’0 bit at B, 0 at C, 2 at D � D = LPM


Fixed

11010001

11001101

Longest

11001001

10100111

11010000

11001000

10100000

01000000

1101

Other A

B

C

DA’

C’

B’

11010101

110011

11100000

01100101

11001100

PS – Virtual CircuitConnection setup establishes a path through switches

� A virtual circuit ID (VCI) identifies path

� Uses packet switching, with packets containing VCI

� VCIs are often indices into per-switch connection tables; change at each hop

� Statistical multiplexing is typically used

1

2 3

4 1

2 3

4VC1

VC1

VC1

VC1

VC2

VC1

VC2

VC3

VC2

In, VC Out, VC1, 1 4, 11, 2 4, 32, 1 4, 2

….

TOC – Switching – Techniques – Packet – VC

Source Routing

1

2

3

4

1

2

3

4

1

2

3

4

1

2

3

4

1

2

3

4

1

2

3

4

4

source

3 4

4 3 4

4 3 4

Each packet specifies the sequence of routers (or of output ports) from source to destination

TOC – Switching – Techniques – Source Routing

Comparison

lowlowhighRobustness*

yesflexiblenoneResource

reservations

lowflexiblehighBandwidth

utilization

nonelowhighForwarding cost

Circuit

switching

Virtual circuit

switching

Datagram

*The idea is that in case of failure, circuit and VC are lost; datagram routing can adapt after routing update ….

TOC – Switching – Techniques – Comparison

Characteristics of Switch/Router

� Ports

� Fast Ethernet, OC-3, ATM, …

� Protocols

� ST, Link Agg., VLAN, OSPF, RIP, BGP, VPN, Load Balancing, WRED, WFQ

� Performance

� Throughput, Reliability, Power, …

TOC – Switching – Characteristics

Examples

� Juniper M160

� Cisco “GSR”

� Cisco “7600”

� Cisco “catalyst 6500”

� Extreme “Summit”

� Foundry “ServerIron”

TOC – Switching – Examples

Cisco GSR - 12416

� WAN Router – Large throughput; SONET links

� Up to 16 line cards at 10 Gbps each

� Crossbar Fabric

� Line Cards:

1-port OC-192c

4-port OC48c

Many others

(ATM, Ethernet, …)

Cisco GSR 12416

6ft

19”

2ft

TOC – Switching – Examples – GSR

Juniper M160

� WAN Router – Large throughput; SONET links

� Crossbar Fabric

� Line Cards:

1-port OC-192c

4-port OC48c

Many others

(ATM, Ethernet, …)

Juniper M160

3ft

2.5ft

19”

Capacity:

80Gb/s

Power: 2.6kW

TOC – Switching – Examples – M160

Cisco 7600

� MAN-WAN Router

� Up to 128 Gbps with Crossbar

Fabric

� 10Mbps – 10Gbps LAN

Interfaces

� OC-3 to OC-48 SONET

Interfaces

� MPLS, WFQ, LLQ, WRED,

Traffic Shaping

TOC – Switching – Examples – 7600

Cisco cat 6500� From LAN to Access

� 48 to 576 10/100 Ethernet Interfaces

� 10 GE, OC-3, OC-12, OC-48, ATM

� QoS, ACL

� Load Balancing; VPN

� Up to 128Gbps

(with crossbar)

� L4-7 Switching

� VLAN

� IP Telephony (E1, T1, inline-power Ethernet)

� SNMP, RMON

TOC – Switching – Examples – Cat6k

Extreme - Summit

� 48 10/100 ports

� 2 GE (SX, LX, or LX-70)

� 17.5Gbps non-blocking

� 10.1 Mpps

� Wire speed L2

� Wire speed L3 static or RIP

� OSPF, DVRMP, PIM, …

TOC – Switching – Examples – Summit

Foundry - ServerIron

� Server Load Balancing

� Transparent Cache Switching

� Firewall Load Balancing

� Global Server Load Balancing

� Extended Layer 4-7 functionality including URL-, Cookie-, and SSL Session ID-based switching

� Secure Network Address Translation (NAT) and Port address translation (PAT)

TOC – Switching – Examples – ServerIron

Architectures

� Generic Architecture

� First Generation

� Second Generation

� Third Generation

� Input Functions

� Output Functions

� Interconnection Designs� OUT� IN� VOB� Combined IN/OUT

TOC – Switching – Architectures

Generic

� Input and output interfaces

are connected through an

interconnect

� A interconnect can be

implemented by

� Shared memory

� low capacity routers (e.g.,

PC-based routers)

� Shared bus

� Medium capacity routers

� Point-to-point (switched)

bus

� High capacity routers

input interface output interface

Inter-

connect

TOC – Switching – Architectures – Generic

Route

TableCPU

Buffer

Memory

LineInterface

MAC

LineInterface

MAC

LineInterface

MAC

Typically < 0.5Gbps aggregate capacityLimited by rate of shared memory

Shared Backplane

Line Interface

CPU

Mem

ory

Slide by Nick McKeown

First Generation

TOC – Switching – Architectures – First

Route

TableCPU

LineCard

BufferMemory

LineCard

MAC

BufferMemory

LineCard

MAC

BufferMemory

FwdingCache

FwdingCache

FwdingCache

MAC

BufferMemory

Typically

< 5Gb/s aggregate capacityLimited by shared bus

Slide by Nick McKeown

Second Generation

TOC – Switching – Architectures – Second

LineCard

MAC

LocalBuffer

Memory

CPUCard

LineCard

MAC

LocalBuffer

Memory

Switched Backplane

Line Interface

CPU

Mem

ory FwdingTable

RoutingTable

FwdingTable

Typically

< 50Gbps aggregate capacity Slide by Nick McKeown

Third Generation

TOC – Switching – Architectures – Third

Input Functions

� Packet forwarding: decide to which output

interface to forward each packet based on the

information in packet header

�examine packet header

� lookup in forwarding table

�update packet header

TOC – Switching – Architectures – Input Functions

Output Functions� Buffer management: decide when and

which packet to drop

� Scheduler: decide when and which

packet to transmit

1

2

Scheduler

Buffer

TOC – Switching – Architectures – Output Functions

Output Functions (2)� Packet classification: map each packet to a

predefined flow/connection (for datagram forwarding)

� use to implement more sophisticated services (e.g.,

QoS)

� Flow: a subset of packets between any two endpoints in the network

1

2

Scheduler

flow 1

flow 2

flow n

Classifier

Buffer

management

TOC – Switching – Architectures – Output Functions

Output Queued � Only output interfaces

store packets

� Advantage

� Easy to design

algorithms: only one

congestion point

� Disadvantage

� Requires an output

speedup Ro/C = N, where

N is the number of

interfaces � not feasible

for large N


Backplane

CRO

TOC – Switching – Architectures – Output Queued

Input Queues

� Only input interfaces store packets

� Advantages� Easy to build� Simple algorithms

� Disadvantages� HOL Blocking

� In practice:Speedup of 2 suffices


Backplane

CRO

TOC – Switching – Architectures – Input Queued

Note: Head-of-line Blocking� The cell at the head of an input queue cannot be

transferred, thus blocking the following cells

Output 1

Output 2

Output 3

Input 1

Input 2

Input 3

Cannot be transferred because of output

contention

Being transferred

Cannot be transferred because of HOL blocking

TOC – Switching – Architectures – Input Queued: HOL

Virtual Output Buffers

� OUT buffers at each input port

� Complexity: Matching Problem

� Full throughput algorithm

� Good Heuristic

Note: Figure from Prof. Varaiya’s notes for EE228b

Crossbar

TOC – Switching – Architectures – VOB

VOB: Full Throughput

� Maximum Weighted Matching:

A = 14

B = 11

C = 15

D = 10

B + C > A + D => Serve (B, C)

TOC – Switching – Architectures – VOB: Full Throughput

VOB: Good Heuristic – i-SLIP� Inputs request permission to send from outputs

� Outputs grant permissions to inputs (round-robin)

� Inputs accept permissions (round-robin)

Request Grant

2

1

3

3

1

2

Last GrantLast Accept

Accept Iter

TOC – Switching – Architectures – VOB: iSLIP

Combined IN/OUT

� Both input and output interfaces store packets

� Advantages

� Easy to built

� Utilization 1 can be

achieved with limited

input/output speedup

(<= 2)

� Disadvantages

� Harder to design algorithms

� Two congestion points

� Need to design flow control


Backplane

CRO

TOC – Switching – Architectures – IN/OUT

� Switching needed for big networks

� Internetworking externality

� Circuit

� Packet – VC: QoS possible

� Packet – Datagram� L2: Limited by flat address space� L3:

� Exact Match: Easy lookup – less efficient

� Longest Prefix Match

� Switch functions: control and data

� Different Architectures: � cost vs. performance

Summary

TOC – Switching – Summary

Switching - EECS Instructional Support Group Home …inst.eecs.berkeley.edu/~ee122/sp07/switching...

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