1-1

EE 586 Com

munication and

Switching Networks (Fall 2010)

Lecture 24

Octob

er 25, 2010

Announcem

ents

�Midterm

1:

�Mean = 92.2

�Stdev

= 8

�Still grad

ing your program

s (sorry about

the delay)

Network Layer

4-2

1-3

Router Architecture Overview

Two key router functions:

�run routing algorithms/protocol

�forw

arding datagramsfrom

incoming to outgoing link

Linksys Broadband

Wireless Router

$59.99

Cisco 12816

Terab

it Router

$210,000

1-4

Three types of switching fabrics

1-5

Switching Via M

emory

First generation routers:

�traditional com

puters with switching under direct

control of CPU

�packet copied to system

’s mem

ory

�speed limited

by mem

ory bandwidth (2 bus

crossings per datagram)

Input

Port

Output

Port

Mem

ory

System Bus

1-6

Switching Via a Bus

�datagram from input port mem

ory

to output port mem

ory via a shared

bus

�bus contention:

switching speed

limited

by bus bandwidth

�32 Gbps bus, Cisco 5600: sufficient

speed for access and enterprise

routers

1-7

Switching Via An

Interconnection Network

�overcome bus bandwidth limitations

�Cross-bar consisting of 2n buses connected

n input ports to n output ports

�ad

vanced

design: fragm

enting datagram

into fixed

length cells, switch cells through

the fabric.

�Cisco 12000: sw

itches 60 Gbps through the

interconnection netw

ork

1-8

Output Ports

�Buffering

required

when datagramsarrive from

fabric faster than the transm

ission rate

�Expensive–input rate = switching rate >> line rate

�Sched

uling disciplinechooses am

ong queued

datagramsfor transm

ission

�First-In First-O

ut

�Weigh

ted Q

ueuing based

on flow

label (more later)

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Output port queueing

�buffering when arrival rate via sw

itch exceed

s output line speed

�queueing (de

lay) and

loss due to output port

buffer overflow!

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Input Port Functions

Dece

ntra

lized switch

ing:

�given datagram dest., lookup output port

using forw

arding table in input port

mem

ory

�goal: complete input port processing at

‘line speed

’�

queuing: if datagrams arrive faster than

forw

arding rate into sw

itch fabric

Physical layer:

bit-level reception

Data link layer:

e.g., Ethernet

see chapter 5

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Input Port Q

ueuing

�Fabric slow

er than input ports com

bined

-> queueing

may occur at input queues

�Head-of-the-Line (HOL) blocking:

queued

datagram

at front of queue prevents others in queue from

moving forw

ard -> loss of efficiency

�queueing

delay and loss due to input bu

ffer overflow!

Network Layer

4-12

The Internet Network layer

forw

arding

table

Host, router netw

ork layer functions:

Routing protocols

•path selection

•RIP, O

SPF

, BGP

IP protocol

•addressing conventions

•datagram format

•packet handling conventions

ICMP protocol

•error reporting

•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Network

layer

Network Layer

4-13

IPv4 datagram format

ver

length

32 bits

data

(variable length,

typically a TCP

or UDP segm

ent)

16-bit id

entifier

header

checksum

time to

live 32 bit source IP address

IP protocol version

number

header length

(bytes)

max num

ber

remaining hops

(decremented at

each router)

for

fragmentation/

reassembly

total datagram

length (bytes)

upper layer protocol

to deliver payload to

head.

len

type of

service

“type” of data

flgs

fragment

offset

upper

layer

32 bit destination IP address

Options (if any)

E.g. timestamp,

record route

taken, specify

list of routers

to visit.

how

much overhead

with TCP?

�20 bytes of TCP

�20 bytes of IP

�= 40 bytes + app

layer overhead

Network Layer

4-14

IP Fragm

entation & Reassem

bly

(IPv4 only)

�netw

ork links have MTU

(max.transfer size) -largest

possible link-level frame.

�different link types,

different M

TUs

�large IP datagram divided

(“fragmented”) within net

�one datagram becom

es

several d

atagrams

�“reassem

bled” only at final

destination

�IP header bits used

to

identify, order related

fragments

fragmentation:

in:one large datagram

out:3 smaller datagrams

reassembly

Network Layer

4-15

IP Fragm

entation and

Reassem

bly

ID =xoffset

=0fragflag

=0length

=4000

ID =xoffset

=0fragflag

=1length

=1500

ID =xoffset

=185

fragflag

=1length

=1500

ID =xoffset

=370

fragflag

=0length

=1040

One large datagram becom

esseveral smaller datagrams

Exam

ple

�4000 byte

datagram

�MTU = 1500 bytes

1480 bytes in

data field

offset =

1480/8

(always in 8-

byte chunk)

Network Layer

4-16

IP Addressing: introduction

�IP address:32-bit

identifier for host,

router interface

�interface:

connection

between host/router

and physical link

�router’s typically have

multiple interfaces

�host typically has one

interface

�IP addresses

associated

with each

interface

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3223.

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422

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27

223.

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1 =

110

1111

1 00

0000

01 0

0000

001

0000

0001

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1

Network Layer

4-17

Sub

nets

�IP address:

�subnet part (high

order bits)

�host part (low order

bits)

�What’s a subn

et ?

�device interfaces with

same subnet part of IP

address

�can ph

ysically reach

each other without

intervening router

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netw

ork consisting of 3 sub

nets

subnet

Network Layer

4-18

Sub

nets

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0/24

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0/24

223.

1.3.

0/24

Recipe

�To determine the

subnets, d

etach each

interface from

its

host or router,

creating islands of

isolated

networks.

Each isolated network

is called a sub

net.

Sub

net mask: /24

Network Layer

4-19

Sub

nets

How

many?

223.

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1

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3

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4

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1.2.

222

3.1.

2.1

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2

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0

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1

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2

Network Layer

4-20

IP addressing: CID

R

CID

R:Classless InterDom

ain Routing

�subnet portion of address of arbitrary length

�address format: a.b.c.d/x, where x is # bits in

subnet portion of address

1100

1000

000

1011

100

0100

00 0

0000

000

subnet

part

host

part

200.23.16.0/2

3

Network Layer

4-21

IP addresses: how

to get one?

Q:How

does a ho

stget IP address?

�hard-cod

ed by system

admin in a file

�Window

s: control-panel->netw

ork->configuration->tcp/ip-

>properties

�UNIX

: /etc/rc.config

�DHCP:

Dynam

ic Host Configuration Protocol: dynam

ically get address

from

as server

�“plug-and-play”

Q:How

does an ISP get block of addresses?

A:IC

ANN: Internet Corporation for Assigned

Nam

es and

Num

bers

�allocates addresses

�manages DNS

�assigns dom

ain names, resolves disputes

Network Layer

4-22

DHCP: Dynam

ic Host Configuration Protocol

Goal:allow host to dynam

ically obtain its IP address

from

network server when it joins netw

ork

�Can renew

its lease on address in use

�Allows reuse of addresses (only hold address while

connected an “on”)

�Support for mob

ile users who want to join netw

ork (m

ore

shortly)

Beside an IP address, the client may also request:

•address of first-hop router for client

•name and IP address of DNS sever

•netw

ork mask (ind

icating netw

ork versus host

portion of address)

Network Layer

4-23

DHCP client-server scenario

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422

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2223.

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222

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A B

E

DH

CP

se

rver

arriv

ing

DH

CP

cl

ient

need

sad

dres

s in

this

netw

ork

Network Layer

4-24

DHCP: exam

ple

�connecting laptop needs its

IP address, addr of first-

hop router, addr of DNS

server: use DHCP

router

(runs DHCP)

DH

CP

UD

PIP Eth

Phy

DH

CP

DH

CP

DH

CP

DH

CP

DH

CP

DH

CP

UD

PIP Eth

Phy

DH

CP

DH

CP

DH

CP

DH

CP

DH

CP

�DHCP request encapsulated

in UDP, encapsulated in IP,

encapsulated

in 802.1

Ethernet

�Ethernet fram

e broadcast

(dest: FFFFFFFFFFFF) on LAN,

received

at router running

DHCP server

�Ethernet dem

ux’ed to IP

dem

ux’ed, UDP dem

ux’ed to

DHCP

168.

1.1.

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