Computer Networks

1
Computer Networks

• Sadiq M. Sait, Ph.D.
• sadiq_at_ccse.kfupm.edu.sa
• Department of Computer Engineering
• King Fahd University of Petroleum and Minerals
• Dhahran, Saudi Arabia

February 17 - 21, 2001
2
Historical glimpses

• The past several decades have witnessed a
  phenomenal growth in the computer industry
• As computer proliferated, so did the need for
  data communication
• People became more and more interested in
  connecting several computers together.
• Computer Network
• Interconnected collection of autonomous computers
  and computer resources

3
Historical glimpses (contd.)

• Late 1960s -- ARPA (later became DARPA) began a
  partnership with 45 universities and research
  institutions to investigate Data Communication
  Technologies.
• 1969 -- ARPANET went into operation with 4 nodes.
• The experiment was a success and ARPANET grew
  into a network spanning the entire USA.
• 1974 -- Birth of the first LAN (Xerox)
• In early years of networking, each computer
  manufacturer developed its own communication
  solution
• Structured Network Architecture (SNA) of IBM
• DEC Network Architecture (DNA) of DEC
• ARPANET of ARPA
• etc.

4
Historical glimpses (contd.)

• 1977 -- ISO established a subcommittee to develop
  an architecture/structure that defines
  communication tasks and which would
• Serve as a reference model for international
  standards
• would facilitate efficient internetworking among
  systems from different technologies,
  manufacturers, administrations, nationalities,
  and enterprises.

5
Historical glimpses (contd.)

• 1978 -- Meeting of 40 experts in Washington, D.
  C. started work that yielded 6 years later the
  OSI Reference Model.
• Paper by Louis Pouzin and Hubert Zimmermann,
  Proc. Of the IEEE November 1978, pp. 1346 - 1370.
• 1975 -- ARPANET transitioned to Defense
  commercial agency.
• 1978-80 -- ARPANET protocol were upgraded with
  TCP/IP.
• Paper by Cerf and Khann, IEEE Trans. Comm., May
  1974.

6
Historical glimpses (contd.)

• February 1980 -- The IEEE started Project 802 to
  develop standards for the LAN market.
• 1981 -- A new host added to ARPANET every 20
  days.
• 1983 -- TCP/IP switchover complete.
• TCP/IP adopted as standard by DOD
• ARPANET had over 300 hosts.
• Over 1200 nodes by 1985.
• ARPANET split
• ARPANET Academic (Educational, Research)
• MILNET Military

7
Historical glimpses (contd.)

• 1984 -- The OSI-RM came out.
• Defines a strategic outline/vision
• Reduces degrees of freedom of standards
  developers
• Centered around the hierarchical decomposition of
  communication functions
• 1986 -- NSFnet backbone created.
• 1990 -- ARPANET put to rest
• 1987 -- over 25000 nodes
• 1989 -- 3000 networks for over 200000 users

8
Historical glimpses (contd.)

• 1991-- WWW invented Gopher introduced
• 1995
• Internet backbone privatized
• Over 7 million networks around the world
• 150000 users join the network every month
• July, 1998 -- over 36 million networks
• Jan, 1999 -- 157 million users
• Projected to be 327 million by year 2000

9
Historical glimpses (contd.)

• The Internet is an Information Highway
• Dedicated communication links (copper, fiber,
  satellite) functioning as the concrete/asphalt
• Usually T/E leased lines serve as the on-ramp
  connecting to regional networks
• Capacity of T1 highways is 1.544 Mbps
• that of T3 is 45 Mbps
• The Internet is becoming a platform for most
  computer needs.

10

• Network Evolution

11
1960s and 1970s Communications

• Centered around the host (mainframe).
• On a single computer, accessing resources,
  running programs, and copying files are
  relatively straightforward.

B
A
Unintelligent terminal
Unintelligent terminal
Low speed links
Value-added networks
12
1960s and 1970s Communications (contd.)

• Even on a system of only two computers,
  coordinating resources becomes much more complex.
  
• Transferring information requires, among other
  things, addressing, error detection, error
  correction, synchronization, and transmission
  coordination.

B
A
Unintelligent terminal
Unintelligent terminal
Low speed links
Value-added networks
13
1970s and 1980s Networks

• The introduction of PCs revolutionized computer
  communication and networking
• LANs evolved to share resources (Disks, Printers)
  
• Minicomputers and shared WANs evolved
• Facilitated the emergence of distributed
  processing
• Applications remained separate and independent,
  and different communication protocols were
  developed

Token Ring
14
1980s and 1990s Internetworks
Token -Ring
Private nets and Internet
FDDI
15
1980s and 1990s Internetworks

• Most of todays networks are a mixture of old
  and new technologies.
• The approach to computer communication in most
  organizations is changing rapidly in response to
  new technologies, evolving business requirements,
  and the need for more bandwidth and instant
  knowledge transfer.
• Internetworks tie LANs and WANs, computer
  systems, software, and related devices together
  to form the corporate communication
  infrastructure.

16
1990s Global Internetworking
17
1990s Global Internetworking

• Studies show that users increasingly require more
  bandwidth.
• Networks will have to meet these demands and
  provide low delay, bandwidth on demand, and other
  new services.
• Such networks are characterized by the following
• increasing use of graphics and imaging
• larger files and larger programs
• client/server computing
• bursty network traffic
• Global internetworking will provide an
  environment for emerging applications that will
  require even greater amounts of bandwidth.

18

• Basic Networking concepts

19
Simple Data Communication Model
001101
Analog/Digital
Digital
Transceiver
Transport System
Transceiver
Data Network
Digital
Public Telephone Network
001101
20
Terminology

• Networks are classified on the basis of
  geographic span.
• Local Area Networks (LANs)
• Metropolitan Area Networks (MANs)
• Wide Area Networks (WANs)
• The difference in geographical extent between
  WANs and LANs account for significant
  differences in their respective design issues.

21
LAN Characteristics

• LANs are designed to
• Operate within a limited geographic area
• Allow multi-access to high-bandwidth media
• Control the network privately under local
  administration
• Provide full-time connectivity to local services
• Connect physically adjacent devices

22
LAN Characteristics

• All nodes are connected by a single high speed
  shared channel.
• Data is packetized and packets are carried past
  all nodes in the network.
• Addressing is required but routing is not needed.
• Congestion control and network architecture are
  among design issues.
• Several topologies can be used but the choice of
  topology is not a major issue.

23
LAN Topologies

• Linear Bus Topology

24
LAN Topologies (Contd.)

• Star

25
LAN Topologies (Contd.)

• Ring.

26
LAN Topologies (Contd.)

• Hierarchical/Inverse Tree.
• Higher power at higher levels.

27
LAN Components

• A LAN has the following basic components
• Transmission Medium
• Cable or Cable-less. It connects the various
  stations. E.g. twisted pair, coaxial cable, CATV
  cable, fiber optics, radio waves.
• Stations
• Intelligent workstations which attach to the
  medium. E.g. PC or workstation.

28
LAN Components (Cont.)

• Non-intelligent which attach to a station. E.g.
  Printers, Modems, Hard disks, etc.
• File server
• The main unit in the network that offers various
  services to the network users.
• It refers to a computer, its hard disk, its
  network operating system, and the file server
  software that manages the network resources.

29
LAN Components (Cont.)

• Network Interface Card (NIC)
• Network adapter to send and receive messages. It
  is a circuit board with the components necessary
  for handling communication tasks
• The NIC is plugged onto one of the available
  slots on the PC expansion bus.
• Installed in each workstation and file server
  such as Ethernet NIC.

30
LAN Components (Cont.)

• Network Operating System (NOS)
• Installed on the hard disk of the file sever
  station. Its function is to control the access to
  the common shared resources, such as printers,
  hard disks, database applications, etc.
• Workstation Operating System
• Consists of a network shell installed on any one
  of the popular operating systems such as DOS,
  Unix, Linux, MAC-OS, etc.

31
Anatomy of a LAN node
Application level API
Application
Protocol API
Operating
Kernel level API
System
Network protocol
Driver Specification
Kernel level API
NIC Driver
Hardware Interface
Receive
Ethernet NIC
Transmit
Hub
32
LAN Characteristics

• What distinguishes one LAN from another
• Transmission Medium
• Twisted pair, Coax, CATV, Fiber Optic, or
  Wireless.
• Topology Star, Bus, Ring
• Transmission method Base/Broadband
• Medium Access Technique
• Random Access (CSMA/CD)
• Controlled Access (Token Passing)

33
LAN Characteristics (Cont.)

• Others
• Type (Peer-to-Peer or Server-based)
• Speed
• in bits per second (bps)
• Span
• distance between end stations
• Load
• number of stations.

34
Server-Based LANs

• Server-based A server-based network consists of
  a group of user-oriented PCs called clients that
  request and receive network services from
  specialized computers called servers.

35
Peer-to-Peer LANs

• Peer-to-peer A peer-to-peer network is a group
  of user-oriented PCs that basically operate as
  equals. Each PC is called a peer. The peers share
  resources, such as files and printers, but no
  specialized servers exists. Each peer is
  responsible for its own security, and, in a
  sense, each peer is both a client and a server.

36
Peer-to-Peer Networking (Workgroup)

• Resources are distributed throughout the network
  on computer systems that may act as both service
  requesters and service providers.
• The user of each PC is responsible for the
  administration and sharing of resources for his
  PC.
• Ideal for small organizations where security is
  not of concern.

37
Resources for Peer-to-Peer Network

• Personal computers.
• Network Operating System
• Ex Windows 98/ NT/ 2000, Linux, Unix.
• Network Interface Card (NIC) the driver of the
  NIC.
• Network cables.
• Hubs (in case twisted-pair cables are used).

38
Client Server Model

• Client-Server paradigm is the primary pattern of
  interactions among cooperating applications.
• This model constitutes the foundation on which
  distributed algorithms are built.

39
What is the Client-Server Paradigm?

• The paradigm divides communicating applications
  into 2 broad categories, depending on whether the
  application waits for communication or initiates
  it.
• An application that initiates a communication is
  called a client.
• End users usually invoke a client software when
  they use a network service.

40
Client Server Model (cont.)

• Server Any program that offers a service
  reachable over the network
• If a machines primary purpose is to support a
  particular server program, the term server is
  usually applied to both, the machine and the
  server program
• Client An executing program becomes a client
  when it sends a request to a server and waits for
  a response

41
Client Server Model (cont.)

• A server is any program that waits for incoming
  communication requests from a client.
• Each time a client application needs to contact a
  server, it sends a request and awaits a response.
• The server receives a clients request, performs
  the necessary computation, and returns the result
  to the client.
• When the response arrives at the client, the
  client continues processing.

42
Client Server Model (cont.)
Machine Running Client Application
Machine Running Server Application
Request
Server Program
Client Program
Reply
43
Client Server Model (cont.)

• A Misconception
• Technically, a server is a program and not a
  piece of hardware.
• However, computer users frequently (mis)apply the
  term to the computer responsible for running a
  particular server program.
• For example, Web Server, is usually a computer
  running the http server program.

44
WANs

• To make optimum use of expensive communication
  links, WANs are structured with irregular
  placement of the nodes. Store-and-Forward packet
  switching is used to deliver packets to their
  destination.

45
Wide-Area Networks and Devices

• WANs are designed to
• Allow access over serial interfaces operating at
  lower speeds
• Control the network subject to regulated public
  services
• Connect devices separated over wide, even global
  areas

46
WANs
S
D
Design Issues
Capacity assignment
Network topology
Routing algorithm
Congestion control
Network architecture
47
Enterprise Developments

• The enterprise is a corporation, agency, service,
  or other organization that will tie together its
  data, communication, computing, and storage
  resources.
• Developments on the enterprise network include
• LANs interconnected to provide client/server
  applications integrated with the traditional
  legacy applications from mainframe data centers
• End-user needs for higher bandwidth on the LAN,
  which can be consolidated at a switch and
  delivered on dedicated media
• Integration of formerly separate networks so that
  the non-bursty traffic from voice and video
  applications coexist on a single network
• Relaying technologies for WAN service, with very
  rapid growth in Frame Relay and cell relay (ATM)

48

• Network Architecture

49
Communication Protocols

• To provide error-free and maximally convenient
  information transfers, the network communication
  is regulated by a set of rules and conventions
  called network protocols.
• Protocols define connectors, cables, signals,
  data formats, error control techniques, and
  algorithms for message preparation, analysis and
  transfer.

50
Communication Protocols (Contd.)

• Network Protocol
• A set of rules defining the syntax (form) and
  semantics (meaning) in order to regulate
  communication between network nodes.
• Protocols can be implemented in either hardware
  or software
• The EIA-232-D is a physical layer protocol
  implemented in hardware.
• TCP/IP are implemented in software.

51
Protocol Data Units (PDU)

• Each PDU must contain two major parts
• Header
• Identifies how the following parts are to be
  handled and routed.
• Message
• This is the message body itself.
• This is where the protocol is determined to be
  character oriented or bit oriented.

52
Communication Standards

• The goal of the ISO subcommittee developing the
  OSI model was to provide a framework for network
  standards acceptable to all manufacturers

53
ISO OSI Reference Architecture

• The architecture is layered to reduce complexity.
• Each layer offers certain services to the layer
  immediately above it.
• Each layer shields the higher layer from the
  details of implementation of how the services are
  offered.
• Layer "n" on one station carries on a
  conversation with layer "n" on another network
  station.

54
OSI Reference Model

• The ISO OSI Layered Model
• Application File transfer, mail, rlogin, etc.
• Presentation Data formatting.
• Session Negotiation and connection.
• Transport End-to-end delivery.
• Network Routing of packets.
• Data link Transfer of frames.
• Physical Cabling system.

55
Why a Layered Model

• 7 Application
• 6 Presentation
• 5 Session
• 4 Transport
• 3 Network
• 2 Datalink
• 1 Physical
• Reduces complexity
• Standardizes interfaces
• Facilitates modular engineering
• Ensures interoperable technology
• Accelerates evolution
• Simplifies teaching and learning

56
Layer Functions

• 7 Application Network processes to
  applications
• 6 Presentation Data representation
• 5 Session Inter-host
  communication
• 4 Transport End-to-end connections
• 3 Network Addresses and best path
• 2 Datalink Access to media
• 1 Physical Binary transmission

57
Layer Functions

• Application Application
• Presentation Presentation
• Session Session
• Transport Transport
• Network Network
• Datalink Datalink
• Physical Physical

segments
packets
frames
bits
Host A Host B
58
Data Encapsulation

• Application Application
• Presentation Presentation
• Session Session
• Transport Transport
• Network Network
• Datalink Datalink
• Physical Physical

data
segment data header
network segment data header header
01001000111010

59
Data Encapsulation Example

data
Data Segment Packet Frame Bits
segment data header
network segment
data header header
Frame Network Segment Data Frame
header header header
trailer
01111111010101101000100100010110101
60

• Application,
• Presentation,
• and Session Layers

61
Application Layer

• Computer Applications
• Word Processing
• Presentation Graphics
• Spreadsheet
• Database
• Design/Manufacturing
• Project Planning
• Others

• Network Applications
• Electronic mail
• File Transfer
• Remote Access
• Client/Server Process
• Information Location
• Network Management
• WWW
• Video-Conferencing
• Others

62
Application Layer (cont.)

• Network Applications
• (For enterprise communication)
• Electronic mail
• File Transfer
• Remote Access
• Client/Server Process
• Information Location
• Network Management
• Others

• Internetwork Applications
• (Extend beyond the enterprise)
• Electronic Data Interchange
• World Wide Web
• E-mail Gateways
• Special-Interest Bulletin Boards
• Financial Transaction Services
• Internet Navigation Utilities
• Conferencing (Video, Voice, Data)

63
Presentation Layer

• Text
• Data
• ASCII, EBCDIC
• Encrypted
• Sound
• Video
• MIDI (Musical Instrument Digital Interface)
• MPEG (Motion Picture Experts Group)
• QuickTime

64
Presentation Layer

• Graphics
• Visual Images
• PICT(format to transfer QuickDraw graphics
  between Macintosh or PowerPC programs)
• TIFF (Tagged Image File Format)
• JPEG (Joint Photographic Experts Group)
• GIF
• Provides code formatting and conversion for
  applications

65
Session Layer

• Coordinates applications as they interact on
  different hosts

Service Request
Service Reply
66
Session Layer (contd.)

• Network File System (NFS)
• Allows trasnparent access to remote network
  resources
• Structured Query Language (SQL)
• Remote-Procedure Call (RPC)
• RPC procedures are built on clients and executed
  on servers
• X Window System
• Allows intelligent terminals to communicate with
  remote UNIX machines
• AppleTalk Session Protocol (ASP)
• Establishes and maintains sessions between an
  AppleTalk client and server
• DNA Session Control Protocol (SCP)

67

• Transport
• Layer

68
Transport Layer Overview

• Segments upper-layer applications
• Establishes an end-to-end connection
• Sends segments from one end host to another
• Ensures end-to-end data reliability

69
Segment Upper-Layer Applications

• Application Electronic
  File Terminal
• Presentation Mail Transfer
  Session
• Session
• Transport
• Transport segments share traffic stream

Application Data Application
Data port
port
70
Establishes Connection
receiver
sender
synchronize
Negotiate connection
synchronize
Connection established
Data transfer (send segments)
71
Establishes Connection
transmit
Buffer full process segments Buffer OK
not ready
ready
Resume Transmission
72
Reliability with Windowing

• In the most basic form of reliable
  connection-oriented transfer, data segments must
  be delivered to the recipient in the same
  sequence that they were transmitted.
• Windowing is a method to control the amount of
  information transferred end-to-end. Some
  protocols measure information in terms of number
  of packets

73
Reliability with Windowing

• send 1 window size 1 receive
  1
• Ack 2
• send 2
  receive 2
• 
  Ack 3
• send 1 window size 3 receive 1
• send 2
  receive 2
• send 3
  receive 3
• 
  Ack 4
• send 4

74
An Acknowledgement Technique

• Reliable delivery guarantees that a stream of
  data sent from one machine will be delivered
  through a functioning data link to another
  machine without duplication or data loss.
  Positive acknowledgement with retransmission is
  one technique that guarantees reliable delivery
  of data streams.
• The sender keeps the record of each segment it
  sends and waits for an acknowledgement.
• The sender also starts a timer when it sends a
  segment, and it retransmits a segment it the
  timer expires before an acknowledgement arrives.

75
An Acknowledgement Technique

• send 1
• send 2
• send 3
• Ack 4
• send 4
• send 5
• send 6
• Ack 5
• send 5
• Ack 7

X
76
Transport to Network Layer
Routed packets
77
Summary

• Presentation layer formats and converts network
  application data to represent text, graphics,
  images, video, and audio.
• Session-layer functions coordinate communication
  interactions between applications.
• Reliable transport-layer functions include
• Multiplexing
• Connection synchronization
• Flow control
• Error recovery
• Reliability through windowing

78

• Physical and
• Data Link Layers

79
Physical and Data-Link Standards

• The data link layer provides data transport
  across a physical link. To do so, the data link
  layer handles physical addressing, network
  topology, line discipline, error notification,
  orderly delivery of frames , and optional flow
  control.
• The physical layer specifies the electrical,
  mechanical, procedural, and functional
  requirements for activating, maintaining, and
  deactivating the physical link between end
  systems.
• These requirements and characteristics are
  codified into standards.

80
LAN Data-Link Sublayers

• Network LLC
• Data Link MAC
• Physical

Logical Link Control
Media Access Control
MAC Frame 802.2 LLC Packet or datagram
81
LAN Data-Link Sublayers

• LLC refers upward to higher-layer software
  functions.
• MAC refers downward to lower-layer hardware
  functions.
• LAN protocols occupy the bottom two layers of OSI
  reference model the physical layer and data link
  layer. The IEEE 802 committee subdivided the data
  link layer into two sublayers
• The logical link control (LLC) sublayer
• The media access control (MAC) sublayer

82
LAN Data-Link Sublayers

• The LLC sublayer provides for environments that
  need connectionless or connection-oriented
  services and the data link layer.
• The MAC sublayer provides access to the LAN
  medium in an orderly manner.

83
LLC Sublayer Functions

• Enable upper layers to gain independence over LAN
  media access.
• Allow service access points (SAPs) from interface
  sublayers to upper-layer functions.
• Provide optional connection, flow control, and
  sequencing services.

84
Summary

• Internetworking evolved to support current and
  future applications
• The OSI reference model organizes network
  functions into seven layers
• Data flows from upper-level user applications to
  lower-level bits transmitted over network media
• Peer-to-peer functions use encapsulation and
  de-encapsulation at layer interfaces
• Most network manager tasks configure the lower
  three layers

85

• Examples

86
HTTP and Web Browsing
Router
Request
Response
HTTP Client
HTTP Server
87
HTTP and Web Browsing

• Request http//www.commm.utoronto.ca/infocom/inde
  x.html
• Event Message content
• 1.User selects document.
• 2. Network Software of client locates the server
• host and establishes a two-way connection.
• 3. HTTP client sends message requesting GET/INFOC
  OM/INDEX.HTML http/1.0
• document.
• 4. HTTP daemon listening on TCP port 80
• interprets message.
• 5. HTTP daemon sends a result code and a
  http/1.1 200 OK
• description of the information that the
  client Server Apache/1.2.5 FrontPage 3.0.4
• will receive. Content-Length 414
• Content-Type text/html
• 6. HTTP daemon reads the file and sends the
  lthtmlgt lttitlegt
• requested file through the TCP port. lttitlegt
  IEEE Infocom pp The Future is now..
• 7. HTTP daemon disconnects the connection.
• 8. Text is displayed by client browser, which
• interprets the HTML format.

88
Domain Name System (DNS)

• The DNS is a distributed database that resides in
  multiple machines on the Internet and is used to
  convert between names and network addresses.
• The DNS protocol is used to manage the
  communication between DNS clients and servers
• The DNS clients are called resolvers.
• The programs that store the information about the
  domain name space are called name servers.

89
Name Servers

• The domain database is divided up into parts
  called zones, which are distributed among various
  name servers.
• The name server that handles a particular zone is
  said to have authority over that zone.

89
90
Resolvers

• Resolvers are clients that access name servers,
  and interface user programs to the DNS.
• Programs running on a host that need information
  from the domain name space use the resolver.
• The resolver is located on the same host as the
  program that requests the resolvers services.

90
91

• A name server can be authoritative over multiple
  zones as well.
• A zone contains the domain names and data that a
  domain contains, except for domain names and data
  that are delegated elsewhere.

91
92
kfupm zone
kfupm
ccse
itc
itc zone domain
ee
ri
ccse zone domain
kfupm domain
92
93
.
. name server
query for address of www.kfupm.edu.sa
Name server
referral to sa name server
sa name server
sa
edu.sa name server
edu
address of www.kfupm.edu.sa
resolver query
answer
kfupm.edu.sa name server
kfupm
Resolver
93
94
DNS Query and Response Example

• Steps to resolve the name www.commm.utoronto.ca
• Event Message content
• 1. Application requests name resolution.
• 2. Resolver composes query message. Header
  OPCODESQUERY
• Question QNAMEtesla.comm.toronto.edu.,
• QCLASSIN, QTYPEA
• 3. Resolver sends UDP datagram encapsulating
• the query message.
• 3. DNS server looks up address and
  prepares Header OPCODESQUERY, RESPONSE, AA
• a response message. Question
  QNAMEtesla.comm.toronto.edu.,
• QCLASSIN, QTYPEA
• Answer tesla.comm.toronto.edu., 86400
• IN A 128.100.11.56
• 4. DNS sends UDP datagram encapsulating the
  response message.

95
Simple Mail Transfer Protocol (SMTP)

• A mail client application interacts with a local
  SMTP sever to initiate the delivery of an email
  message.
• The user prepares an email message that includes
• Recipients email address,
• Subject line, and
• a body
• When the user clicks send, the mail application
• Prepares a file with above information and
  additional information specifying format (e.g.
  plain ASCII or MIME extensions to encode
  non-ASCII data)
• Resolve the name of the local SMTP server using
  DNS
• Then perform the following steps

96
Steps in sending e-mail using SMTP

• Event Message content
• 1. The mail application establishes a TCP
  connection
• (port 25) to its local SMTP server
• 2. SMTP daemon issues the following message
  to 220 tesla.com.utoronto.edu ESMTP
• the client indicating its readiness to
  receive mail. Sendmail 8.9.0/8.9.0 Thu, 2 Jul
  2000
• 050759 0400 (DT)
• 3. Client sends a HELO message and identifies
  itself. HELO bhaskara.com.utoronto.edu.ca
• 4. SMTP daemon issues a 250 message, indicating
  250 tesla.com.utoronto.edu Hello
• the client may proceed. bhaskara.com
  128.100.10.91,
• pleased to meet you
• 5. Client sends senders address MAIL FROM
  ltbanerjea_at_comm.utoronto.cagt
• 6. If successful, SMTP daemon replies with a
  250 ltbanerjea_at_comm.utoronto.cagt
• 250 message. Sender ok

97
Steps in sending e-mail using SMTP(contd.)

• Event Message content
• 7. Client sends recipients address. RCPT TO
  alg_at_nal.utoronto.ca
• 8. A 250 message is returned 250
  ltbanerjea_at_comm.utoronto.cagt
• Recipient ok
• 9. Client sends a DATA message requesting DATA
• permission to send the mail message.
• 10. Daemon sends a message giving the client 354
  Enter mail, end with . on a line
• permission to send. by itself
• 11. Client sends the actual text of the message
  Hi, please .
• 12. Daemon indicates that message is accepted
  250 FAA00803 Message accepted for delivery
• for delivery. A message ID is returned
• 13. Client indicates that the mail session is
  over. QUIT
• 14. Daemon confirms the end of the session 221
  tesla.comm.toronto.edu
• closing connection

98
Conceptual Model of a Mail System
Alias database
TCP connection
Alias expansion and forwarding
Outgoing mail spool area
User sends mail
Client (background transfer)
for outgoing mail
User Inter- face
User reads mail
TCP connection
Server (to accept mail)
Mailboxes for incoming mail
for incoming mail
99
Summary

• The examples clearly indicate that a protocol is
  solely concerned with the interaction between two
  peer processes, that is, the client and the
  server.
• Application layer protocols operate by using the
  communication services provided by the TCP and
  UDP protocols.
• Actually, both TCP and UDP operate by using the
  connectionless service of IP.