data communication and networks
Download
Skip this Video
Download Presentation
Data Communication and Networks

Loading in 2 Seconds...

play fullscreen
1 / 40

Data Communication and Networks - PowerPoint PPT Presentation


  • 115 Views
  • Uploaded on

Data Communication and Networks. Lecture 6 Networks: Part 1 Circuit Switching, Packet Switching, The Network Layer October 13, 2005. Switching Networks. Long distance transmission is typically done over a network of switched nodes Nodes not concerned with content of data

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Data Communication and Networks' - kolina


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
data communication and networks

Data Communication and Networks

Lecture 6

Networks: Part 1

Circuit Switching, Packet Switching, The Network Layer

October 13, 2005

Network Layer

switching networks
Switching Networks
  • Long distance transmission is typically done over a network of switched nodes
  • Nodes not concerned with content of data
  • End devices are stations
    • Computer, terminal, phone, etc.
  • A collection of nodes and connections is a communications network
  • Data routed by being switched from node to node

Network Layer

technology
Technology
  • Two different switching technologies
    • Circuit switching
    • Packet switching

Network Layer

circuit switching
Circuit Switching
  • Dedicated communication path between two stations (during conversation)
  • Three phases
    • Establish
    • Transfer
    • Disconnect
  • Must have switching capacity and channel capacity to establish connection
  • Must have intelligence to work out routing

Network Layer

circuit switching issues
Circuit Switching - Issues
  • Circuit switching is inefficient (designed for voice)
    • Resources dedicated to a particular call
    • Much of the time a data connection is idle
    • Data rate is fixed
      • Both ends must operate at the same rate
  • Set up (connection) takes time
  • Once connected, transfer is transparent

Network Layer

packet switching

Packet Switching

Network Layer

basic operation
Basic Operation
  • Data transmitted in small packets
    • Typically 1000 octets
    • Longer messages split into series of packets
    • Each packet contains a portion of user data plus some control info
  • Control info
    • Routing (addressing) info
  • Packets are received, stored briefly (buffered) and passed on to the next node
    • Store and forward

Network Layer

use of packets
Use of Packets

Network Layer

network layer
transport segment from sending to receiving host

on sending side encapsulates segments into datagrams

on rcving side, delivers segments to transport layer

network layer protocols in every host, router

Router examines header fields in all IP datagrams passing through it

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

network

data link

physical

application

transport

network

data link

physical

application

transport

network

data link

physical

Network layer

Network Layer

key network layer functions
Key Network-Layer Functions
  • forwarding: move packets from router’s input to appropriate router output
  • routing: determine route taken by packets from source to dest.
    • Routing algorithms

analogy:

  • routing: process of planning trip from source to dest
  • forwarding: process of getting through single interchange

Network Layer

slide12
routing algorithm

local forwarding table

header value

output link

0100

0101

0111

1001

3

2

2

1

value in arriving

packet’s header

1

0111

2

3

Interplay between routing and forwarding

Network Layer

connection setup
Connection setup
  • 3rd important function in some network architectures:
    • ATM, frame relay, X.25
  • Before datagrams flow, two hosts and intervening routers establish virtual connection
    • Routers get involved
  • Network and transport layer cnctn service:
    • Network: between two hosts
    • Transport: between two processes

Network Layer

network service model
Example services for individual datagrams:

guaranteed delivery

Guaranteed delivery with less than 40 msec delay

Example services for a flow of datagrams:

In-order datagram delivery

Guaranteed minimum bandwidth to flow

Restrictions on changes in inter-packet spacing

Network service model

Q: What service model for “channel” transporting datagrams from sender to rcvr?

Network Layer

network layer service models
Network layer service models:

Guarantees ?

Network

Architecture

Internet

ATM

ATM

ATM

ATM

Service

Model

best effort

CBR

VBR

ABR

UBR

Congestion

feedback

no (inferred

via loss)

no

congestion

no

congestion

yes

no

Bandwidth

none

constant

rate

guaranteed

rate

guaranteed

minimum

none

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Network Layer

network layer connection and connection less service
Network layer connection and connection-less service
  • Datagram network provides network-layer connectionless service
  • VC network provides network-layer connection service
  • Analogous to the transport-layer services, but:
    • Service: host-to-host
    • No choice: network provides one or the other
    • Implementation: in the core

Network Layer

virtual circuits
call setup, teardown for each call before data can flow

each packet carries VC identifier (not destination host address)

every router on source-dest path maintains “state” for each passing connection

link, router resources (bandwidth, buffers) may be allocated to VC

“source-to-dest path behaves much like telephone circuit”

performance-wise

network actions along source-to-dest path

Virtual circuits

Network Layer

vc implementation
VC implementation

A VC consists of:

    • Path from source to destination
    • VC numbers, one number for each link along path
    • Entries in forwarding tables in routers along path
  • Packet belonging to VC carries a VC number.
  • VC number must be changed on each link.
    • New VC number comes from forwarding table

Network Layer

forwarding table
VC number

22

32

12

3

1

2

interface

number

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 2 22

2 63 1 18

3 7 2 17

1 97 3 87

… … … …

Forwarding table

Forwarding table in

northwest router:

Routers maintain connection state information!

Network Layer

virtual circuits signaling protocols
used to setup, maintain teardown VC

used in ATM, frame-relay, X.25

not used in today’s Internet

application

transport

network

data link

physical

application

transport

network

data link

physical

Virtual circuits: signaling protocols

6. Receive data

5. Data flow begins

4. Call connected

3. Accept call

1. Initiate call

2. incoming call

Network Layer

datagram networks
no call setup at network layer

routers: no state about end-to-end connections

no network-level concept of “connection”

packets forwarded using destination host address

packets between same source-dest pair may take different paths

application

transport

network

data link

physical

application

transport

network

data link

physical

Datagram networks

1. Send data

2. Receive data

Network Layer

forwarding table1
Forwarding table

4 billion

possible entries

Destination Address RangeLink Interface

11001000 00010111 00010000 00000000

through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000

through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000

through 2

11001000 00010111 00011111 11111111

otherwise 3

Network Layer

longest prefix matching
Longest prefix matching

Prefix MatchLink Interface

11001000 00010111 00010 0

11001000 00010111 00011000 1

11001000 00010111 00011 2

otherwise 3

Examples

Which interface?

DA: 11001000 00010111 00010110 10100001

Which interface?

DA: 11001000 00010111 00011000 10101010

Network Layer

datagram or vc network why
Internet

data exchange among computers

“elastic” service, no strict timing req.

“smart” end systems (computers)

can adapt, perform control, error recovery

simple inside network, complexity at “edge”

many link types

different characteristics

uniform service difficult

ATM

evolved from telephony

human conversation:

strict timing, reliability requirements

need for guaranteed service

“dumb” end systems

telephones

complexity inside network

Datagram or VC network: why?

Network Layer

the internet network layer
Host, router network layer functions:
  • ICMP protocol
  • error reporting
  • router “signaling”
  • IP protocol
  • addressing conventions
  • datagram format
  • packet handling conventions
  • Routing protocols
  • path selection
  • RIP, OSPF, BGP

forwarding

table

The Internet Network layer

Transport layer: TCP, UDP

Network

layer

Link layer

physical layer

Network Layer

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 may have multiple interfaces

IP addresses associated with each interface

223.1.1.2

223.1.2.2

223.1.2.1

223.1.3.2

223.1.3.1

223.1.3.27

IP Addressing: introduction

223.1.1.1

223.1.2.9

223.1.1.4

223.1.1.3

223.1.1.1 = 11011111 00000001 00000001 00000001

223

1

1

1

Network Layer

subnets
IP address:

subnet part (high order bits)

host part (low order bits)

What’s a subnet ?

device interfaces with same subnet part of IP address

can physically reach each other without intervening router

Subnets

223.1.1.1

223.1.2.1

223.1.1.2

223.1.2.9

223.1.1.4

223.1.2.2

223.1.1.3

223.1.3.27

LAN

223.1.3.2

223.1.3.1

network consisting of 3 subnets

Network Layer

subnets1
Recipe

To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.

223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

Subnets

Subnet mask: /24

Network Layer

subnets2
How many?Subnets

223.1.1.2

223.1.1.1

223.1.1.4

223.1.1.3

223.1.7.0

223.1.9.2

223.1.9.1

223.1.7.1

223.1.8.1

223.1.8.0

223.1.2.6

223.1.3.27

223.1.2.1

223.1.2.2

223.1.3.1

223.1.3.2

Network Layer

ip addressing cidr
host

part

subnet

part

11001000 0001011100010000 00000000

200.23.16.0/23

IP addressing: CIDR

CIDR:Classless InterDomain Routing

  • subnet portion of address of arbitrary length
  • address format: a.b.c.d/x, where x is # bits in subnet portion of address

Network Layer

ip datagram format
IP protocol version

number

32 bits

total datagram

length (bytes)

header length

(bytes)

type of

service

head.

len

ver

length

for

fragmentation/

reassembly

fragment

offset

“type” of data

flgs

16-bit identifier

max number

remaining hops

(decremented at

each router)

upper

layer

time to

live

Internet

checksum

32 bit source IP address

32 bit destination IP address

upper layer protocol

to deliver payload to

E.g. timestamp,

record route

taken, specify

list of routers

to visit.

Options (if any)

data

(variable length,

typically a TCP

or UDP segment)

IP datagram format

how much overhead with TCP?

  • 20 bytes of TCP
  • 20 bytes of IP
  • = 40 bytes + app layer overhead

Network Layer

ip fragmentation reassembly
network links have MTU (max.transfer size) - largest possible link-level frame.

different link types, different MTUs

large IP datagram divided (“fragmented”) within net

one datagram becomes several datagrams

“reassembled” only at final destination

IP header bits used to identify, order related fragments

IP Fragmentation & Reassembly

fragmentation:

in: one large datagram

out: 3 smaller datagrams

reassembly

Network Layer

ip fragmentation and reassembly
length

=1500

length

=1040

length

=1500

length

=4000

ID

=x

ID

=x

ID

=x

ID

=x

fragflag

=0

fragflag

=0

fragflag

=1

fragflag

=1

offset

=0

offset

=185

offset

=0

offset

=370

One large datagram becomes

several smaller datagrams

IP Fragmentation and Reassembly

Example

  • 4000 byte datagram
  • MTU = 1500 bytes

1480 bytes in data field

offset =

1480/8

Network Layer

nat network address translation
NAT: Network Address Translation

rest of

Internet

local network

(e.g., home network)

10.0.0/24

10.0.0.1

10.0.0.4

10.0.0.2

138.76.29.7

10.0.0.3

Datagrams with source or

destination in this network

have 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving local

network have same single source NAT IP address: 138.76.29.7,

different source port numbers

Network Layer

nat network address translation1
NAT: Network Address Translation
  • Motivation: local network uses just one IP address as far as outside word is concerned:
    • no need to be allocated range of addresses from ISP: - just one IP address is used for all devices
    • can change addresses of devices in local network without notifying outside world
    • can change ISP without changing addresses of devices in local network
    • devices inside local net not explicitly addressable, visible by outside world (a security plus).

Network Layer

nat network address translation2
NAT: Network Address Translation

Implementation: NAT router must:

  • outgoing datagrams:replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

  • remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
  • incoming datagrams:replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer

nat network address translation3
2

4

1

3

S: 138.76.29.7, 5001

D: 128.119.40.186, 80

S: 10.0.0.1, 3345

D: 128.119.40.186, 80

1: host 10.0.0.1

sends datagram to

128.119.40, 80

2: NAT router

changes datagram

source addr from

10.0.0.1, 3345 to

138.76.29.7, 5001,

updates table

S: 128.119.40.186, 80

D: 10.0.0.1, 3345

S: 128.119.40.186, 80

D: 138.76.29.7, 5001

NAT: Network Address Translation

NAT translation table

WAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345

…… ……

10.0.0.1

10.0.0.4

10.0.0.2

138.76.29.7

10.0.0.3

4: NAT router

changes datagram

dest addr from

138.76.29.7, 5001 to 10.0.0.1, 3345

3: Reply arrives

dest. address:

138.76.29.7, 5001

Network Layer

nat network address translation4
NAT: Network Address Translation
  • 16-bit port-number field:
    • 60,000 simultaneous connections with a single LAN-side address!
  • NAT is controversial:
    • routers should only process up to layer 3
    • violates end-to-end argument
      • NAT possibility must be taken into account by app designers, eg, P2P applications
    • address shortage should instead be solved by IPv6

Network Layer

ad