Network Protocols: Understanding Layers, IP, TCP, UDP, and Application Protocols, Lecture notes of Computer science

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Basic Networking Concepts1. Introduction2. Protocols3. Protocol Layers4. Network Interconnection/Internet

1. Introduction -A network can be defined as a group of computers and other devices

connected in some ways so as to be able to exchange data. -Each of the devices on the network can be thought of as a node; each

node has a unique address.

-Addresses are numeric quantities that are easy for computers to work

with, but not for humans to remember. Example: 204.160.241.

-Some networks also provide names that humans can more easily

remember than numbers. Example: www.javasoft.com, corresponding to the above numericaddress.

NIC

addr

NIC

addrN

NIC

addr

…

Ports -An IP address identifies a host machine on the Internet.-An IP port will identify a specific application running on an Internet host

machine. -A port is identified by a number, the

port number

-The number of ports is not functionally limited, in contrast to serial

communications where only 4 ports are allowed.

-There are some port numbers which are dedicated for specific

applications.

79

Finger

23

Telnet

110

POP3 (e-mail)

25

SMTP (e-mail)

70

Gopher

20 and 21

FTP

80

HTTP

Port numbers

Applications

Data Transmission -In modern networks, data are transferred using

packet switching

-Messages are broken into units called

packets

, and sent from one

computer to the other. -At the destination, data are extracted from one or more packets and

used to reconstruct the original message. -Each packet has a maximum size, and consists of a header and a data

area. -The header contains the addresses of the source and destination

computers and sequencing information necessary to reassemblethe message at the destination.

1001….

00010000111…

packet

header

data

Sharedbus

(a) Ethernet LAN

Ring

(b) Token Ring LAN

Dual ring

(c) FDDI LAN

Network connectivity type

Speed

Transmission timefor 10 Mbytes

(Telephone) dial-up modem

14.4 Kbps

90 min

ISDN modem

56/128 Kbps

45/12min

T1 connection

1.54 Mbps

50s

Ethernet

10 Mbps

9s

Token ring

4/16 Mbps

Fast Ethernet

100 Mbps

FDDI

100 Mbps

Gigabit Ethernet

1 Gbps

ATM

25Mbps/2.4Gbs

Toronto branch

Ethernet LAN

Router

Vancouver branch

Token Ring LAN

Gateway

FrameRelayATM

T1 line

NY headquaters

Bridge/Route

Token Ring LAN

Router

Bridge

Bridge/Router

Ethernet

Network Topology Diagram The specification of the network topology diagram requires thedefinition of the characteristics and entities underlying the network:-Geographical locations of the different components or subnets

involved in the network.

-Description of the LAN topology-Description of the WAN topology-Description of the network connectors such as routers, bridges,

repeaters, and gateways.

S: MAIL FROM: [email protected]: 250 OKS: RCPT TO: [email protected]: 250 OKS: DATAR: 354 Beginning of mail; ending by <

CRLF>.

S: Blah blah blahS: …etc.S: .R: 250 OK

Request For Comments (RFC):

specifications of the protocols involved

in Internet Communications.-Example

sample of RFC 821 describing communications betweenSMTP server and client.

14

nth layer(n-1)th layer…

nth layer(n-1)th layer…

Network

-Protocols are designed based on a layered architecture such as the OSI

reference model. -Each entity at a layer

n

communicates only with entities at layer

n-

-The data exchanged, known as Protocol Data Unit (PDU), goes back

and forth through the layers, each layer adds or removes its own headerand vice-versa. Therefore a layer

n

PDU may become a layer

n-

data.

OSI Layers

Physical layer

(

defines the physical characteristics of the network)

Data-link layer

(provides safe

communication of data over the physical network)

Network layer

(handles connection to the

network by the higher layers)

Transport layer

(provides end-to-end errors detection and correction

)

Session layer

(manages sessions among applications

)

Presentation layer

(provides standard data representations for applications

)

Application layer

(applications connected to the network)

Physical layer:

ensures a safe and efficient travel of data; consists of

electronic circuits for data transmission etc. Data link layer:

in charge of data encapsulation under the form of

packets and their interpretation at the physical layer. Network layer:

in charge of packets transmission from a source A to a

destination B. Transport layer:

in charge of the delivery of packets from a source A

to a destination B Session layer:

in charge of the management of network access.

Presentation layer:

determines the format of the data transmitted to

applications, data compressing/decompressing, encrypting etc. Application layer:

contains the applications which are used by the

end-user, such as Java, Word etc.

Network layer -Provides the same functionality as the physical, the data link and

network layers in the OSI model.

-Mapping between IP addresses and network physical addresses.-Encapsulation of IP datagrams, e.g packets, in format understandable

by the network. Internet layer -Lies at the heart of TCP/IP.-Based on the Internet Protocol (IP), which provides the frame for

transmitting data from place

A

to place

B

Transport layer -Based on two main protocols: TCP (Transmission Control Protocol)

and UDP (User Datagram protocol)

Application layer -Combines the functions of the OSI application, presentation, and

session layers.

-Protocols involved in this layer: HTTP, FTP, SMTP etc.

4. Networks Interconnection/Internet Concept of Network Interconnection -First implemented in the Defense Advanced Research Project Agency

Network (Arpanet), in 1966 in USA.

-Consists of connecting several computer networks based on different

protocols -Requires the definition of a common interconnection protocol on top

the local protocols. -The

Internet Protocol (IP)

plays this role, by defining unique addresses

for a network and a host machine.

FTP

Telnet

SNMP

SMTP

TCP/UDP

IP

Ethernet

Arpanet

Token ring