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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.

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cs2302 computer networks

CS2302- COMPUTER NETWORKS

RAJALAKSHMI ENGINEERING COLLEGE

DEPARTMENT OF INFORMATION TECHNOLOGY

unit i
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
Building a Network
  • To build a network

Identify the set of constraints and requirements based on

Application programmer

Network designer

Network provider

slide4
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
slide5
Routing
    • Provides Systematic procedure for forwarding messages
      • Unicasting
      • Multicasting
  • Cost effective Resources sharing

How system resource is shared effectively by multiple users

multiplexing

slide6
Multiplexing methods
    • STDM - Synchronous time division multiplexing
    • FDM - Frequency division multiplexing
network architecture
Network Architecture
  • Provides a general, effective, fair, and robust connectivity of computers
  • Provides a blueprint
    • Types
      • OSI Architecture
      • Internet Architecture
osi 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 the

network

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

network communication protocols have a structure based on OSI Model

internet architecture
Internet Architecture
  • TCP/IP Architecture
  • Four Layer model
  • TCP,UDP,FTP,HTTP,SMTP Protocols used
  • Internet Protocol Graph
direct links outline
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
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 technologies13
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
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
Approaches
  • Byte oriented Protocol(PPP)

BISYNC

Binary Synchronous Communication

DDCMP

Digital Data Communication Message Protocol

  • Bit oriented Protocol(HDLC)
  • Clock based Framing(SONET)
byte oriented protocol ppp
Byte oriented Protocol(PPP)

BISYNC FRAME FORMAT

PPP Frame Format

bit oriented protocol hdlc
Bit Oriented Protocol(HDLC)
  • Collection of Bits

1.HDLC

High-Level Data Link Control

2.Closed Based Framing(SONET)

Synchronous Optical Network

hdlc frame format
HDLC Frame Format

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
Closed Based Framing(SONET)
  • STS-1 Frame

9 rows of 90 byte each

First 3 byte for overhead rest contains data

Payload bytes scrambled- exclusive OR

Supports Multiplexing

Payloads

9 rows

90 columuns

error detection
ERROR DETECTION
  • Detecting Errors In Transmission

Electrical Interference, thermal noise

Approaches

Two Dimensional Parity

Internet Checksum Algorithm

Cyclic Redundancy Check

transmission sent using even parity
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
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
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
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
stop wait sequence numbers
Stop & wait sequence numbers

Sender

Receiver

Sender

Receiver

Sender

Receiver

Frame 0

Frame 0

Frame 0

imeout

imeout

ACK 0

ACK 0

ACK 0

T

T

Frame 0

Frame 1

Frame 0

imeout

ACK 0

imeout

ACK 1

T

ACK 0

T

Frame 0

(c)

(d)

ACK 0

(e)

  • 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
sliding window
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

£

SWS

LAR

LFS

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
sequence number space
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

£

RWS

LFR

LFA

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
unit ii lan technology
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
LAN Architecture

Describes the way in which the components in a Local

Area Network are connected

LAN Topologies:

Star

Ring

Bus

Tree

slide37
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.
slide38
Ring

All nodes on the LAN are connected in a loop and their

Network Interface Cards (NIC) are working as repeaters.

No starting or ending point.

Each node will repeat any signal that is on the network

regardless its destination.

The destination station recognizes its address and copies

the frame into a local buffer.

The frame continues to circulate until it returns to the

source station, where it is removed.

Example:Token Ring (IEEE 802.5)

FDDI (IEEE 802.6) another protocol used in the

slide39
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)
slide40
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.
token ring
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
Ehernet
  • local-area network (LAN) covered by the IEEE 802.3.
  • two modes of operation:
    • half-duplex
    • full-duplex modes.

.

slide44
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

wireless
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
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
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
types52
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
virtual circuit switching

0

Switch 1

3

1

2

Switch 2

2

3

1

5

11

0

Host A

7

0

Switch 3

1

3

4

Host B

2

Virtual Circuit Switching
  • Explicit connection setup (and tear-down) phase
  • Subsequence packets follow same circuit
  • Sometimes called connection-oriented model
  • Analogy: phone call
  • Each switch maintains a VC table
datagram switching

Host D

Host E

0

Switch 1

Host F

3

1

Switch 2

2

Host C

2

3

1

0

Host A

0

Switch 3

Host B

Host G

1

3

2

Host H

Datagram Switching
  • No connection setup phase
  • Each packet forwarded independently
  • Sometimes called connectionless model
  • Analogy: postal system
  • Each switch maintains a forwarding (routing) table
virtual circuit model
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
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

A

B

C

Port 1

Bridge

Port 2

Z

X

Y

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
spanning tree algorithm

A

B

B3

C

B5

D

B7

K

B2

E

F

B1

G

H

B6

B4

I

J

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
algorithm details
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 details62
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
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
types65
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.
rarp reverse address resolution protocol
(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
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
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:
  • 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:
  • 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.
tcp congestion control
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,AdvertisedWindow)
  • EffectiveWindow=MaxWindow-(LastByteSent-LastByteAcked)
caused by
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!

Principles of Congestion Control
scenario 1 queuing delays
two senders, two receivers

one router, infinite buffers

no retransmission

large delays when congested

maximum achievable throughput

lout

lin : original data

unlimited shared output link buffers

Host A

Host B

Scenario 1: Queuing Delays
scenario 2 retransmits
one router, finite buffers

sender retransmission of lost packet

Scenario 2: Retransmits

Host A

lout

lin : original data

l\'in : original data, plus retransmitted data

Host B

finite shared output link buffers

scenario 3 congestion near receiver
four senders

multihop paths

timeout/retransmit

l

l

in

in

Host A

Host B

Scenario 3: Congestion Near Receiver

Q:what happens as and increase ?

lout

lin : original data

l\'in : original data, plus retransmitted data

finite shared output link buffers

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

Approaches towards congestion control

Two broad approaches towards congestion control:

tcp congestion control98
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

CongWin

rate =

Bytes/sec

RTT

TCP Congestion Control
tcp aimd
multiplicative decrease: cut CongWin in half after loss eventTCP AIMD

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

TCP Slow Start
  • 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

time

TCP Slow Start (more)

Host A

Host B

one segment

RTT

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

Refinement (more)
congestion avoidance mechanisms
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
Mechanisms
  • router-centric:  DECbit  and  RED  Gateways
  • host-centric:  TCP  Vegas
decbit107
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
decbit108
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
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 red110
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 red112
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
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.
slide114
three main components of DNS:
  • Domain name space and associated resource records (RRs)
  • DNS Name Servers
  • DNS Resolvers
email
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
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
slide120
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
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.
slide122
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.
slide124
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 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
slide126

World Wide Web

Hypertext and Hypermedia

Browser Architecture

Static Document/HTML

Dynamic Document/CGI

Active Document/Java

slide144

File Transfer

Connections

Communication

File Transfer

User Interface

Anonymous

slide145

Note:

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.

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