CS2302-NOL

CS2302- COMPUTER NETWORKS

RAJALAKSHMI ENGINEERING COLLEGE

DEPARTMENT OF INFORMATION TECHNOLOGY

UNIT I

INTRODUCTION:

A computer network is a group of interconnected computers

A collection of computers and devices connected to each other.

Allows computers to communicate with each other and share resources and information.

Building a Network

To build a networkIdentify the set of constraints and requirements

based onApplication programmerNetwork designerNetwork provider

Requirements: Connectivity

point to point or multiple access Links - physical medium Nodes,clouds - computer

Switched Network Circuit Switched Packet Switched

Uses store and forward Establishes dedicated circuit More efficient in working

Routing Provides Systematic procedure for forwarding

messages Unicasting Multicasting

Cost effective Resources sharingHow system resource is shared effectively by multiple usersmultiplexing

Multiplexing methods

STDM- Synchronous time division multiplexing FDM - Frequency division multiplexing

Network Architecture Provides a general, effective, fair, and robust

connectivity of computers Provides a blueprint

Types

OSI Architecture Internet Architecture

OSI ARCHITECTURE

Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984

OSI model defines the communications process into Layers

Provides a standards for communication in thenetwork

Primary architectural model for inter-computing and Inter networking communications.

network communication protocols have a structure based on OSI Model

OSI Architecture

Internet Architecture TCP/IP Architecture Four Layer model TCP,UDP,FTP,HTTP,SMTP Protocols used Internet Protocol Graph

Direct Links: Outline Physical Layer

Link technologies Encoding

Link Layer Framing Error Detection Reliable Transmission (ARQ protocols) Medium Access Control:

Existing protocols: Ethernet, Token Rings, Wireless

Link Technologies

Cables: Cat 5 twisted pair, 10-100Mbps, 100m Thin-net coax, 10-100Mbps, 200m Thick-net coax, 10-100Mbps, 500m Fiber, 100Mbps-2.4Gbps, 2-40km

Leased Lines: Copper based: T1 (1.544Mbps), T3 (44.736Mbps) Optical fiber: STS-1 (51.84Mbps), STS-N (N*51.84Mbps)

Link Technologies

Last-Mile Links: POTS (56Kbps), ISDN (2*64Kbps) xDSL: ADSL (16-640Kbps, 1.554-8.448Mbps), VDSL

(12.96Mbps-55.2Mbps) CATV: 40Mbps downstream, 20Mbps upstream

Wireless Links: Cellular, Satellite, Wireless Local Loop

FRAMING

An efficient data transmission technique

It is a message forwarding system in which data packets, called frames, are passed from one or many start-points to one

Approaches Byte oriented Protocol(PPP)

BISYNCBinary Synchronous CommunicationDDCMPDigital Data Communication Message Protocol

Bit oriented Protocol(HDLC) Clock based Framing(SONET)

Byte oriented Protocol(PPP)

SYH SYH SOH Header STX Body ETX CRC

BISYNC FRAME FORMAT

Flag Address Control Protocol Payload Flag

PPP Frame Format

SYN SYN Class Count Header Body CRC

DDCMP Frame Format

Bit Oriented Protocol(HDLC)

Collection of Bits1.HDLC

High-Level Data Link Control

2.Closed Based Framing(SONET)Synchronous Optical Network

HDLC Frame FormatBeginning sequence

Header Body CRC Ending sequence

Bit Stufffing

After 5 consecutive 1s insert 0

Next bit is 0 – stuffed removed Next bit is 1 –end of frame or erorr

Closed Based Framing(SONET)

STS-1 Frame9 rows of 90 byte eachFirst 3 byte for overhead rest contains data

Payload bytes scrambled- exclusive OR Supports Multiplexing

90 columuns

Payloads

9 rows

ERROR DETECTION Detecting Errors In Transmission

Electrical Interference, thermal noise

ApproachesTwo Dimensional ParityInternet Checksum AlgorithmCyclic Redundancy Check

Two Dimensional Parity

7 bits of data 8 bits including parity

Number of 1s even odd

0000000 (0) 00000000 100000000

1010001 (3) 11010001 01010001

1101001 (4) 01101001 11101001

1111111 (7) 11111111 01111111

Transmission sent using even parity:

A wants to transmit: 1001

A computes parity bit value: 1^0^0^1 = 0

A adds parity bit and sends: 10010

B receives: 10010 B computes parity: 1^0^0^1^0 = 0

B reports correct transmission after observing expected even result.

Transmission sent using odd parity:

A wants to transmit: 1001 A computes parity bit value: ~(1^0^0^1) = 1 A adds parity bit and sends: 10011 B receives: 10011 B computes overall parity: 1^0^0^1^1 = 1 B reports correct transmission after observing

expected odd result.

Reliable Transmission

Deliver Frames Reliably

Accomplished by Acknowledgements and Timeouts

ARQ-Automatic Repeat Request

Mechanism:

Stop and Wait

Sliding Window

Concurrent Logical Channels

Stop And Wait ARQ The source station transmits a single frame and then

waits for an acknowledgement (ACK).

Data frames cannot be sent until the destination station’s reply arrives at the source station.

It discards the frame and sends a negative acknowledgement (NAK) back to the sender

causes the source to retransmit the damaged frame in case of error

Acknowledgements & Timeouts

Sender Receiver

Frame

ACK

Tim

eout

Tim

e

Sender Receiver

Frame

ACK

Tim

eout

Frame

ACKTim

eout

Sender Receiver

Frame

ACKTim

eout

Frame

ACKTim

eout

Sender Receiver

Frame

Tim

eout

Frame

ACKTim

eout

(a) (c)

(b) (d)

Stop & wait sequence numbers

Sender Receiver

Frame 0

ACK 0

Tim

eo

ut

Frame 0

ACK 0

Tim

eo

ut

Sender Receiver

Frame 0

ACK 0Tim

eo

ut

Frame 0

ACK 0Tim

eo

ut

(c) (d)

Sender Receiver

Frame 0

ACK 0

Frame 1

ACK 1

(e)

Frame 0

ACK 0

• Simple sequence numbers enable the client to discard duplicate copies of the same frame

• Stop & wait allows one outstanding frame, requires two distinct sequence numbers

Stop And Wait

Sliding Window bi-directional data transmission protocol used in the

data link layer (OSI model) as well as in TCP

It is used to keep a record of the frame sequences sent

respective acknowledgements received by both the users.

Sliding Window: Sender Assign sequence number to each frame (SeqNum) Maintain three state variables:

send window size (SWS) last acknowledgment received (LAR) last frame sent (LFS)

Maintain invariant: LFS - LAR <= SWS Advance LAR when ACK arrives Buffer up to SWS frames SWS

LAR LFS

… …

Sequence Number Space

SeqNum field is finite; sequence numbers wrap around Sequence number space must be larger then number of

outstanding frames SWS <= MaxSeqNum-1 is not sufficient

suppose 3-bit SeqNum field (0..7) SWS=RWS=7 sender transmit frames 0..6 arrive successfully, but ACKs lost sender retransmits 0..6 receiver expecting 7, 0..5, but receives the original incarnation

of 0..5 SWS < (MaxSeqNum+1)/2 is correct rule Intuitively, SeqNum “slides” between two halves of sequence

number space

Sliding Window: Receiver

Maintain three state variables receive window size (RWS) largest frame acceptable (LFA) last frame received (LFR)

Maintain invariant: LFA - LFR <= RWS

Frame SeqNum arrives: if LFR < SeqNum < = LFA accept if SeqNum < = LFR or SeqNum > LFA discarded

Send cumulative ACKs – send ACK for largest frame such that all frames less than this have been received

RWS

LFR LFA

… …

UNIT II LAN Technology LAN (Local Area Network) refers to a group of

computers interconnected into a network Objective: they are able to communicate, exchange information

and share resources (e.g. printers, application programs, database etc).

the same computer resources can be used by multiple users in the network, regardless of the physical location of the resources.

LAN Architecture Describes the way in which the components in a LocalArea Network are connectedLAN Topologies:

StarRingBusTree

Star All stations are connected by cable (or wireless) to a

central point, such as hub or a switch.

central node is operating in a broadcast fashion such as a Hub

transmission of a frame from one station to the node is retransmitted on all of the outgoing links.

Ring

All nodes on the LAN are connected in a loop and theirNetwork Interface Cards (NIC) are working as repeaters. No starting or ending point.

Each node will repeat any signal that is on the networkregardless its destination.

The destination station recognizes its address and copiesthe frame into a local buffer.The frame continues to circulate until it returns to thesource station, where it is removed.

Example:Token Ring (IEEE 802.5) FDDI (IEEE 802.6) another protocol used in the

Bus All nodes on the LAN are connected by one linear cable,

which is called the shared medium. Every node on this cable segment sees transmissions

from every other station on the same segment. At each end of the bus is a terminator, which absorbs

any signal, removing it from the bus. This medium cable apparently is the single point of

failure. Example:Ethernet (IEEE 802.3)

Tree Is a logical extension of the bus topology. The transmission medium is a branching cable no closed loops. The tree layout begins at a point called the head-end one or more cables start, and each of these may have

branches. The branches in turn may have additional branches to

allow quite complex layouts.

Topologies

Token Ring All stations are connected in a ring and each station can directly

hear transmissions only from its immediate neighbor. Permission to transmit is granted by a message (token) that

circulates around the ring. Token Ring as defined in IEEE 802.5 is originated from the IBM

Token Ring LAN technologies. Token-passing networks move a small frame, called a token Possession of the token grants the right to transmit. The information frame circulates the ring until it reaches the

intended destination station, which copies the information for further processing.

The information frame continues to circle the ring and is finally removed when it reaches the sending station.

The sending station can check the returning frame to see whether the frame was seen and subsequently copied by the destination.

Ehernet local-area network (LAN) covered by the

IEEE 802.3. two modes of operation:

half-duplex full-duplex modes. .

Three basic elements :1. the physical medium used to carry Ethernet signals between computers,

2. a set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel,

3. an Ethernet frame that consists of a standardized set of bits used to carry data over the system

IEEE 802.5 Format

Frame Format IEEE 802.5

IEEE 802.3 MAC Data Frame Format

Wireless

The process by which the radio waves are propagated through air and transmits data

Wireless technologies are differentiated by :

Protocol Connection type—Point-to-Point (P2P) Spectrum—Licensed or unlicensed

Types Infrared Wireless Transmission

Tranmission of data signals using infrared-light waves

Microwave Radio sends data over long distances (regions,

states, countries) at up to 2 megabits per second (AM/FM Radio)

Communications Satellites microwave relay stations in orbit around the earth.

UNIT III Packet Switching

Is a network communications method Groups all transmitted data, irrespective of content, type,

or structure into suitably-sized blocks, called packets. Optimize utilization of available link capacity Increase the robustness of communication. When traversing network adapters, switches and other

network nodes packets are buffered and queued, resulting in variable

delay and throughput, depending on the traffic

Types Connectionless

each packet is labeled with a connection ID rather than an address.

Example:Datagram packet switching

connection-oriented each packet is labeled with a destination

address Example:X.25 vs. Frame Relay

Star Topology

Source Routing

0

13

2

0

1 3

2

0

13

2

0

13

2

3 0 1 3 01

30 1

Switch 3

Host B

Switch 2

Host A

Switch 1

Virtual Circuit Switching Explicit connection setup (and tear-down)

phase Subsequence packets follow same circuit Sometimes called connection-oriented

model

0

13

2

01 3

2

0

13

25 11

4

7

Switch 3

Host B

Switch 2

Host A

Switch 1

Analogy: phone call

Each switch maintains a VC table

Datagram Switching No connection setup phase Each packet forwarded independently Sometimes called connectionless model

0

13

2

0

1 3

2

0

13

2

Switch 3Host B

Switch 2

Host A

Switch 1

Host C

Host D

Host EHost F

Host G

Host H

Analogy: postal system

Each switch maintains a forwarding (routing) table

Virtual Circuit Model Typically wait full RTT for connection setup before

sending first data packet.

While the connection request contains the full address for destination

each data packet contains only a small identifier, making the per-packet header overhead small.

If a switch or a link in a connection fails, the connection is broken and a new one needs to be established.

Connection setup provides an opportunity to reserve resources.

Datagram Model There is no round trip delay waiting for connection

setup; a host can send data as soon as it is ready.

Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up.

Since packets are treated independently, it is possible to route around link and node failures.

Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.

Bridges and Extended LANs

LANs have physical limitations (e.g., 2500m) Connect two or more LANs with a bridge

accept and forward strategy level 2 connection (does not add packet header)

Ethernet Switch = Bridge on Steroids

A

Bridge

B C

X Y Z

Port 1

Port 2

Spanning Tree Algorithm

Problem: loops

Bridges run a distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE 802.1 specification

B3

A

C

E

DB2

B5

B

B7 K

F

H

B4

J

B1

B6

G

I

Algorithm Details

Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be

root bridge distance (hops) from sending bridge to root

bridge Each bridge records current best configuration

message for each port Initially, each bridge believes it is the root

Algorithm Details

Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be

root bridge distance (hops) from sending bridge to root

bridge Each bridge records current best configuration

message for each port Initially, each bridge believes it is the root

Internetworking An internetwork is a collection of individual networks,

connected by intermediate networking devices, that functions as a single large network.

different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork

Types Local-area networks (LANs)enabled multiple users in a

relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers.

Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity.

technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others.

ETH

IPV4 Packet HeaderVersion HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

Destination Addr

Options(variable) Pad(variable)

Data

Datagram Delivery

Packet Format

IPV4 Packet header

Fragmentation and Reassembly

Fragmentation and Reassembly

Fragmentation and Reassembly

(RARP)Reverse Address Resolution Protocol

(RARP) is a Link layer networking protocol RARP is described in internet EngineeringTask ForceETF)

publication RFC 903 It has been rendered obsolete by the Bootstrap Protocol

(BOOTP) and the modern Dynamic Host Configuration Protocol(DHCP)

BOOTP configuration server assigns an IP address to each client from a pool of addresses.

BOOTP uses the User Datagram Protocol (UDP)

Routing

is the process of selecting paths in a network along which to send network traffic.

Routing is performed for many kinds of networks, including the telephone network electronic data networks (such as the Internet), and transportation networks.

Components determining optimal routing paths and transporting

information groups (typically called packets) through an internetwork.

In the context of the routing process, the latter of these is referred to as packet switching.

Although packet switching is relatively straightforward, path determination can be very complex.

Distance Vector:

Distance Vector routing protocols are based on Bellman and Ford algorithms.

Distance Vector routing protocols are less scalable such as RIP supports 16 hops and IGRP has a maximum of 100 hops.

Distance Vector are classful routing protocols which means that there is no support of Variable Length Subnet Mask (VLSM) and Classless Inter Domain Routing (CIDR).

Distance Vector routing protocols uses hop count and composite metric.

Distance Vector routing protocols support discontiguous subnets.

Link State:

Link State routing protocols are based on Dijkstra algorithms.

Link State routing protocols are very much scalable supports infinite hops.

Link State routing protocols are classless which means that they support VLSM and CIDR.

Cost is the metric of the Link State routing protocols. Link State routing protocols support contiguous

subnets.

UNIT IV Reliable Byte Stream

TCP Overview

End to end issues

Segment format

Connection establishment

TCP sliding window

Stream control Transmission Protocol

Simple demultiplexor

TCP Congestion Control Determines the network capacity Adjust the number of packets that can have safely in

transit Acks to pace the transmission of packets TCP is self clocking Avoids congestion Maxwindow=MIN(CongestionWindow,AdvertisedWindo

w) EffectiveWindow=MaxWindow-(LastByteSent-

LastByteAcked)

Caused By the shortage of buffer space. slow links. slow processors

Possible solutions End-to-end versus link-by-link control Rate-Based versus Credit-Based control The rate-based traffic-flow technique constantly Integrated congestion control

Integrated congestion control

Principles of Congestion Control

Congestion: informally: “too many sources sending too much

data too fast for network to handle” different from flow control! manifestations:

lost packets (buffer overflow at routers) long delays (queueing in router buffers)

a top-10 problem!

Scenario 1: Queuing Delays two senders, two

receivers one router,

infinite buffers no

retransmission

large delays when congested

maximum achievable throughput

unlimited shared output link buffers

Host Ain : original data

Host B

out

Scenario 2: Retransmits one router, finite buffers sender retransmission of lost packet

finite shared output link buffers

Host A in : original data

Host B

out

'in : original data, plus retransmitted data

Scenario 3: Congestion Near Receiver

four senders multihop paths timeout/retransmit

in

Q: what happens as and increase ?

in

finite shared output link buffers

Host Ain : original data

Host B

out

'in : original data, plus retransmitted data

Approaches towards congestion control

End-end congestion control:

no explicit feedback from network

congestion inferred from end-system observed loss, delay

approach taken by TCP

Network-assisted congestion control:

routers provide feedback to end systems single bit indicating

congestion (SNA, DECbit, TCP/IP ECN, ATM)

explicit rate sender should send at

Two broad approaches towards congestion control:

TCP Congestion Control end-end control (no network

assistance) sender limits transmission: LastByteSent-LastByteAcked

CongWin Roughly,

CongWin is dynamic, function of perceived network congestion

How does sender perceive congestion?

loss event = timeout or 3 duplicate acks

TCP sender reduces rate (CongWin) after loss event

three mechanisms: AIMD slow start conservative after

timeout events

rate = CongWin

RTT Bytes/sec

TCP AIMD

8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow

multiplicative decrease: cut CongWin in half after loss event

additive increase: increase CongWin by 1 MSS every RTT in the absence of loss events: probing

Long-lived TCP connection

TCP Slow Start When connection

begins, CongWin = 1 MSS Example: MSS = 500

bytes & RTT = 200 msec

initial rate = 20 kbps available bandwidth

may be >> MSS/RTT desirable to quickly

ramp up to respectable rate

When connection begins, increase rate exponentially fast until first loss event

TCP Slow Start (more) When connection begins,

increase rate exponentially until first loss event: double CongWin every

RTT done by incrementing CongWin for every ACK received

Summary: initial rate is slow but ramps up exponentially fast

Host A

one segment

RTT

Host B

time

two segments

four segments

Refinement (more)

Q: When should the exponential increase switch to linear?

A: When CongWin gets to 1/2 of its value before timeout.

Implementation: Variable Threshold At loss event, Threshold is

set to 1/2 of CongWin just before loss event

TCP sender congestion control

Event State TCP Sender Action Commentary

ACK receipt for previously unacked data

Slow Start (SS)

CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance”

Resulting in a doubling of CongWin every RTT

ACK receipt for previously unacked data

CongestionAvoidance (CA)

CongWin = CongWin+MSS * (MSS/CongWin)

Additive increase, resulting in increase of CongWin by 1 MSS every RTT

Loss event detected by triple duplicate ACK

SS or CA Threshold = CongWin/2, CongWin = Threshold,Set state to “Congestion Avoidance”

Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.

Timeout SS or CA Threshold = CongWin/2, CongWin = 1 MSS,Set state to “Slow Start”

Enter slow start

Duplicate ACK

SS or CA Increment duplicate ACK count for segment being acked

CongWin and Threshold not changed

Congestion Avoidance Mechanisms Helps to avoid congestion Additional functionality into the router to assist in

anticipation of congestion to  control  congestion  once  it  happens

to  repeatedly  increase  load  in  an  effort  to  find  the  point  at  which  congestion  occurs,  and  then  back  off

Mechanisms router-centric:  DECbit  and  RED  Gateways

host-centric:  TCP  Vegas

DECbit

DECbit

Add  binary  congestion  bit  to  each  packet  header Router

monitors  average  queue  length  over  last  busy+idle  cycle

set  congestion  bit  if  average  queue  length  greater  than  1  when  packet  arrives

attempts  to  balance  throughput  against  delay

DECbit

End  Hosts destination  echos  bit  back  to  source

source  records  how  many  packets  resulted  in  set  bit

if  less  than  50%  of  last  window's  worth  had  bit  set,  then  increase  CongestionWindow  by  1  packet

if  50%  or  more  of  last  window's  worth  had  bit  set,  then  decrease  CongestionWindow  by  0.875  times

Random Early Detection (RED)

Notification  is  implicit just  drop  the  packet  (TCP  will  timeout)

could  make  explicit  by  marking  the  packet

Early  random  drop rather  than  wait  for  queue  to  become  full, 

drop  each  arriving  packet  with  some  drop  probability  whenever  the  queue  length  exceeds  some  drop  level 

Random Early Detection (RED)

RED:  fills  in  the  details compute  average  queue  length

AvgLen=(1- Weight)*AvgLen+Weight*SampleLen                     

0  <  Weight  <  1  (usually  0.002)

SampleLen  is  queue  length  each  time  a  packet  arrives

Random Early Detection (RED

Random Early Detection (RED) two  queue  length  thresholds if  AvgLen  ?  MinThreshold  then

enqueue  the  packet

if  MinThreshold  <  AvgLen  <  MaxThreshold

calculate  probability  P

if  MaxThreshold  ?  AvgLen

drop  arriving  packet

UNIT V Domain Name Service is a hierarchical naming system for computers, services in the

Internet

is an IETF-standard name service.

enables client computers on your network to register and resolve DNS domain names.

names are used to find and access resources offered by other computers on your network or other networks, such as the Internet.

three main components of DNS:

Domain name space and associated resource records (RRs)

DNS Name Servers

DNS Resolvers

Domain name space for the Internet. Domain Names

Email Electronic mail abbreviated as e-mail or email

is method of creating, transmitting, or storing primarily text-based human communications with digital communications systems

based on a store-and-forward model in which e-mail computer server systems, accept, forward, or store messages on behalf of users

SMTP(Simple Mail Transfer Protocol) is an Internet standard for electronic mail transmission

is a TCP/IP protocol used in sending and receiving e-mail

to send and receive mail messages to send and receive mail messages

SMTP(Simple Mail Transfer Protocol)

SMTP(Simple Mail Transfer Protocol)

MIME Multipurpose Internet Mail Extensions SMTP is ASCII based allows multi part messages containing content of various

types combined into one message Types

GIF graphics files PostScript files MIME messages can contain

text, images, audio, video, and other application-specific data.

format of messages textual message bodies in character sets other than US-

ASCII, an extensible set of different formats for non-textual

message bodies, multi-part message bodies, and textual header information in character sets other than US-

ASCII.

HTTP is an application-level protocol for distributed,

collaborative, hypermedia information systems. It is a generic, stateless, protocol which can be

used for many tasks such as name servers and distributed object management systems, through extension of its request methods, error codes and headers [47].

typing and negotiation of data representation allows systems to be built independently of the

data being transferred.

SNMP to monitor network-attached devices for conditions

that warrant administrative attention

SNMP basic components Managed devices Agents Network-management stations (NMSs) Managed devices Agents Network-management stations (NMSs)

Email Features Email is Fast Email is Inexpensive Email is Easy to Filter Transmission is Secure and Reliable

1.Fast - Messages can be sent anywhere around the world in an instant 2.cheap - Transmission usually costs nothing, or at the most, very little 3.simple - Easy to use, after initial set-up 4.efficient - Sending to a group can be done in one step 5.versatile - Pictures, powerpoints or other files can be sent too

World Wide WebWorld Wide Web

Hypertext and Hypermedia

Browser Architecture

Static Document/HTML

Dynamic Document/CGI

Active Document/Java

Distributed services

Hypertext

Browser architecture

Categories of Web documents

Static document

Boldface tags

Effect of boldface tags

Beginning and ending tags

Common tags Common tags

BeginningTag

Ending Tag

Meaning

Skeletal Tags

<HTML> </HTML> Defines an HTML document

<HEAD> </HEAD> Defines the head of the document

<BODY> </BODY> Defines the body of the document

Title and Header Tags

<TITLE> </TITLE> Defines the title of the document

<Hn> </Hn> Defines the title of the document

Common tags (continued) Common tags (continued)

BeginningTag

Ending Tag

Meaning

Text Formatting Tags

<B> </B> Boldface

<I> </I> Italic

<U> </U> Underlined

<SUB> </SUB> Subscript

<SUP> </SUP> Superscript

Data Flow Tag

<CENTER> </CENTER> Centered

<BR> </BR> Line break

Common tags (continued) Common tags (continued)

BeginningTag

Ending Tag

Meaning

List Tags

<OL> </OL> Ordered list

<UL> </UL> Unordered list

<LI> </LI> An item in a list

Image Tag

<IMG> Defines an image

Hyperlink Tag

<A> </A> Defines an address (hyperlink)

Executable Contents

<APPLET> </APPLET> The document is an applet

Dynamic document

Active document

Skeleton of an applet

Instantiation of the object defined by an applet

Creation and compilation

HTML document carrying an applet

File Transfer File Transfer

Connections

Communication

File Transfer

User Interface

Anonymous

FTP uses the services of TCP. It needs two TCP connections. The well-known

port 21 is used for the control connection, and the well-known port 20 is used for the data connection.

NoteNote::

FTP

Using the control connection

Using the data connection

File transfer