Client-Server%20Interaction

1
Client-Server Interaction

• Based on Chapter 28 in Computer Networks and
  Internets, Comer

2
Network Applications

• All of the lower layers of the network
  communication protocol stack exist so that
  applications on different computers can interact.
  
• Applications are high-level software used
  directly by users.
• The applications (e.g. Excel) sit on top of the
  application layer which provides various
  network-related services (e.g. FTP) to the
  applications.
• The basic paradigm for this application-to-applica
  tion interaction on an internet is the
  client-server model.

3
Application Software

• Applications and the application layer hide all
  of the details of the network from the user.
• Information hiding
• In most cases the user does not have to know the
  IP address (let alone the MAC address) of the
  computer he or she is communicating with.
• The application provides a user-friendly symbolic
  name, which is then translated into the
  corresponding number.
• E.g. a domain name service

4
Connection-Oriented

• While TCP is in part responsible for establishing
  a session between two computers (the three-way
  handshake), the TCP does not establish a
  connection on its own volition.
• The source application layer must direct its
  transport layer to do so.
• The destination application layer must direct its
  transport layer to listen for others trying to
  connect to it.
• In the phone analogy, the source must dial the
  number, the destination must hear the phone ring
  and answer it.

5
The client-server paradigm

• The approach in which the source initiates
  contact with a listening destination is said to
  adhere to the client-server paradigm.
• The initiating source is called the client.
• Clients are active.
• The listening destination is called the server.
• Servers are passive.

6
Two meanings
2

• In one sense, client-server is used to refer to
  the communication model
• Client Active requester
• Server Passive listener/responder
• In another sense, client-server is used to refer
  to machines which are designed to act as clients
  or as (dedicated) servers
• In this sense, a server is a machine capable of
  offering a such service to a number of clients.

7
What is a Server?

• A server can refer to a program that passively
  waits for communication.
• A server can refer to a computer dedicated to
  running server programs.
• Your book uses the terminology server class
  computer for the latter case.
• But most of the world uses the term server in
  both cases.
• Servers or server-class computers are typically
  powerful machines which run server software.

8
Not the only paradigm in town

• Client-server is not the only model for
  computer-to-computer communication.
• Peer-to-peer A type of network in which each
  workstation has equivalent capabilities and
  responsibilities. Peer-to-peer networks are
  generally simpler, but they usually do not offer
  the same performance under heavy loads.
• Master/slave master polls the slaves to see if
  they have anything to transmit.

9
Mainframe architecture

• Mainframes are an example of the master-slave
  model.
• Most of the data storage and computing power lies
  in the mainframe (a central computer).
• Users interact with the mainframe via terminals
  (dumb terminals) which is little more than a
  monitor and keyboard.
• In the early days of PCs, they were often
  disabled to work as dumb terminals in a
  mainframe.

10
Thin and fat clients

• There was an idea of returning to some extent to
  the notion of a dumb terminal.
• In client/server applications, a client designed
  to be especially small so that the bulk of the
  data processing occurs on the server is called a
  thin client.
• A client that performs the bulk of the data
  processing operations is called a fat client.
• Although the term usually refers to software, it
  can also apply to a relative abilities of a
  network computer.

11
Client/Server Operating Systems

• Often the version of the operating system one
  loads on a computer that mainly plays the client
  role will be different from the version of the
  operating system loaded on the server
• E.g. Windows 2000 is different from Windows 2000
  Server

12
The client

• Client software
• Requests service but also performs its own
  computations.
• Is invoked directly by a user and executes only
  for one session
• as opposed to starting automatically and running
  continuously
• Usually runs locally on a users PC
• Initiates contact with a server.
• May access multiple services, but only
  communicates with one server at a time.
• Does not require special hardware (beyond
  standard communications hardware) or Operating
  System.

13
The Server

• Server software
• Is a special-purpose, privileged program
  dedicated to providing one service.
• Can handle multiple remote clients at the same
  time.
• Is invoked automatically (typically when a system
  boots) and continues to execute through many
  sessions.
• Waits passively for contact from arbitrary remote
  clients.
• Accepts contact from arbitrary clients, but
  offers a single service.
• Requires powerful hardware and a sophisticated
  operating systems.

14
Application Software

• Applications (as opposed to services provided by
  the application layer) that communicate with
  other applications are often written with either
  the client role or server role specifically in
  mind.
• Thus one talks about client-side scripting and
  server-side scripting.

15
Scripting

• Client-side scripting is code written for the
  client end of a client-server system.
• For example, JavaScript scripts are client-side
  because they are executed by your browser (the
  client).
• Server-side scripting is code written for the
  server end of a client-server system.
• For example, CGI scripts are server-side because
  they run on the Web server.
• Java applets can be either server-side or
  client-side depending on which computer (the
  server or the client) executes them.

16
Client-Server Interaction

• Information between client-server passes in both
  directions.
• Clients request, servers respond.
• Sometimes the servers response is an ongoing
  communication based on one initial request.
• Client-server applications lead the protocol
  suite to establish the session and send/receive
  information.
• A computer needs all stacks of the software
  protocol suite to run as a client or server.

17
Client-Server
18
Multiple Services

• A powerful computer can run multiple clients and
  servers at the same time.
• You need sufficient hardware resources and an
  operating system which allows multiple
  application programs to execute concurrently
  (such as UNIX or WIN9x)

19
One connection/Many services

• A computer with multiple servers still requires
  only a single physical connection for
  communication.
• Different services will be associated with
  different ports.
• Since servers are often in passive modes, having
  one computer offer many services can help reduce
  administrative overhead and cost without a
  significant reduction in performance.
• However, if many clients are expected for a
  particular service it may be worthwhile having a
  machine dedicated to it.

20
Some Server Types

• A file server is a computer and storage device
  dedicated to storing files. Any user on the
  network can store files on the server.
• A print server is a computer that manages one or
  more printers.
• A network server is a computer that manages
  network traffic.
• A database server is a computer system that
  processes database queries.

21
Multiple services
22
A thread for each request

• In order to handle concurrency, that is, dealing
  with many clients at once, the server-class
  computer uses threads.
• We mentioned certain services being associated
  with well-known ports.
• Actually the main thread or listener (usually a
  daemon) is located at this port, but as soon as a
  request is made, a new thread is spun off
  (forked) for it.
• This allows the main listening thread to await
  the next client.

23
Thread

• A thread is the information needed to serve one
  individual user or a particular service request.
• If multiple users are using the program or
  concurrent requests from other programs occur, a
  thread is created and maintained for each of
  them.

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Thread

• The thread allows a program to know which client
  is being served as the program alternately gets
  re-entered on behalf of different users.
• One way thread information is kept is by storing
  it in a special data area and putting the address
  of that data area in a register.
• The operating system always saves the contents of
  the register when the program is interrupted and
  restores it when it gives the program control
  again.

25
Types of Transport Protocols

• The application services may be
  connection-oriented or connectionless.
• Connection-Oriented
• The applications must first establish the
  connection and then send the data across the
  connection. TCP requests the connection and once
  established the communication begins. Session
  must be terminated.

26
Types of Transport Protocols

• Connectionless Interface
• The application sends a message to the
  destination at any time. The sending application
  needs to specify the destination with each
  message sends. The UDP is the connectionless
  support mechanism in the TCP/IP protocol.
• For a client request using the connectionless
  interface, servers often require the request to
  be sent in a single message. The server responds
  in a single message.

27
Service Connections Choice

• Servers may provide a service through both
  connection methods.
• The choice of transport is dependent on the
  client.
• This choice requires the server to provide both
  transport implementations. This may be
  implemented by
• Setting up 2 servers for the same service, one
  for connectionless transport and one for
  connection-oriented transport.
• Setting up a single server that is able to
  interact with either type of transport protocol
  simultaneously.

28
Client-Server Interactions

• A client application is not restricted to
  accessing a single service. A single application
  can become the client of several servers over
  time.
• A client application is not restricted to
  accessing a single server for a given service. A
  client might send a request to multiple servers
  to improve performance.
• A server is not restricted from performing
  further client-server interactions . A server
  for one service can become a client of another.

29
Two-tier

• The term two-tier refers to client/server
  architectures in which the user interface runs on
  the client and the database is stored on the
  server.
• The actual application logic can run on either
  the client or the server.

30
Three tier

• A special type of client/server architecture
  consisting of three well-defined and separate
  processes, each running on a different platform
• The user interface, which runs on the user's
  computer (the client).
• The functional modules (business rules) that
  actually process data. This middle tier runs on a
  server and is often called the application
  server.
• A database management system (DBMS) that stores
  the data required by the middle tier. This tier
  runs on a second server called the database
  server.

31
Three tier (Cont.)

• The three-tier design has many advantages over
  traditional two-tier or single-tier designs,
  mainly
• The added modularity makes it easier to modify or
  replace one tier without affecting the other
  tiers. (Remember Layering and information
  hiding.)
• Separating the application functions from the
  database functions makes it easier to implement
  load balancing, i.e. spreading the processing out
  

32
Network Address Translation and Electronic Mail

• Based on Chapters 26 and 32 in Computer Networks
  and Internets, Comer

33
Network Address Translation

• NAT (Network Address Translation) is a scheme for
  a network to use one set of addresses internally
  but to have the outside world see a different set
  of addresses, possibly only one address.
• It can be used as a security measure since it
  hides internal addresses from the outside world.
• It can be used to expand ones address space.
  One can use non-routable addresses, and different
  networks can use the same internal addresses
  provided their external address is unique.

34
NAT Figure 26.4
35
NAT Tables

• The translator understands the local addresses
  and the external address. It maintains a table
  of what internal addresses (computers) are
  communicating with which external addresses.

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37
Dilemma

• But what if two internal addresses are
  communicating with the same external address?
• How would the translator know which internal
  address to deliver a message to?

38
Solution (NAPT)

• Recall that beyond IP addresses there is an
  additional part of the addressing scheme the
  ports. The translator can associate different
  internal addresses with its different ports.
  Then it can distinguish differ incoming messages
  by which port they come in on.
• This is known as Network Address and Port
  Translation (NAPT).

39
NAPT Table (Figure 26.4)
40
Another Dilemma

• Associating internal addresses with ports is fine
  for sessions initiated by the internal machines,
  but what if the the outside wants to initiate?
• The outside world will know the internal
  computers not by IP addresses but by domain
  names. Therefore this can be resolved by having
  the local DNS and NAT servers coordinate.

41
E-Mail

• E-mail was designed to serve as an electronic
  version of the traditional office memo.
• E-mail can be used to be used for one-to-one or
  one-to-many communication.
• It can also handle automatic response to
  information requests.

42
Mail box

• In order for a person to receive mail, they are
  assigned a mail box which is a location
  (typically a directory in storage) designated to
  hold incoming messages addressed to a particular
  user.
• The email server is given permission to write
  files in this directory.
• The user to whom the mail box is assigned is
  given permission to access these files.

43
Mail box

• A mailbox is often associated with an account,
  thus someone with multiple accounts may have
  multiple boxes.
• The mailbox is typically on the mail server and
  not on the users computer.
• While the user may have most of the permissions
  in regard to his or her mail box, the amount that
  can be stored there and so on is controlled by
  the administrator of the mail server.

44
Email Addresses

• Addresses have 2 parts, e.g. blum_at_lasalle.edu
• The first part blum_at_lasalle.edu denotes the mail
  box (and in turn the person to whom it is
  assigned)
• It allows the local administrators to assign
  mailbox identifiers independently (no central
  authority).
• The second part blum_at_lasalle.edu denotes the mail
  server at the domain, i.e. the computer to which
  the message should be sent.
• It allows users on arbitrary computer systems to
  exchange email messages (is assigned by central
  authority).

45
E-Mail Message Formats

• An email message consists of ASCII code.
• As with most information protocols we have
  studied, it has two parts
• A header containing information about delivery
  the senders and recipients address along with
  various other information.
• A body containing the actual message to be
  delivered.
• The two parts are separated by a blank line
  (carriage return character).

46
Header and body
header
body
47
Some header fields
48
SUBJECT, CC and BCC

• Most headers include a SUBJECT field
• Typically it conveys information about the body
  to the recipient.
• It can be used to send commands to certain
  automated processes, e.g. SUBJECT subscribe.
• The CC (carbon copy) field allows the message to
  be sent to users other than the primary
  recipients.
• BCC (Blind carbon copy) has a similar effect as
  CC except that a BCC recipients address does not
  appear in the message.
• The primary recipient does not know that a blind
  carbon copy was sent

49
User and Transfer Agents

• The Email system can be broken down into two
  parts
• One part is a user interface for composing and
  reading messages. It is then placed in a queue
• (Sometimes called mail user agent)
• Another part for delivering messages to the
  intended recipient on another computer (or
  indicating if delivery was unsuccessful). It
  takes messages from the queue and delivers them
  to the recipients mail box.
• (Sometimes called mail transfer agent)

50
Mail Transfer

• Delivery may be
• Local Sending a message to a user on the same
  computer simply requires that a copy of the
  message be placed in the recipients mailbox.
• Remote Sending the message to a user on a remote
  computer requires that senders transfer software
  become a client of the recipients transfer
  program. The remote computer must append the
  message to the recipients mailbox.

51
SMTP

• Simple Mail Transfer Protocol is a set of rules
  for transferring email messages using a TCP
  connection.
• SMTP allows the sender to identify itself,
  specify a recipient and send an e-mail message,
  once the connection has been established.
• SMTP requires reliable delivery of a message
  which means the sender keeps a copy of the
  message in storage (nonvolatile) until the
  receiver has stored the message.

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53
Two Stage Delivery

• Mail is not delivered directly to the typical end
  users computer
• Mail might arrive at any time and the mail server
  must be ready to receive it at that time (its a
  TCP session), many users computers are turned
  off when not in use.
• Plus, each computer would have to operate the
  mail server software.
• The e-mail is delivered to a mail server and then
  the user must retrieve it, so there is a second
  stage in the delivery process.

54
POP

• One choice for this second stage of delivery is
  POP (Post Office Protocol).
• The mail server (the computer with the mailboxes)
  also runs a POP server.
• The POP server allows a user to run e-mail
  software from his PC to access the mailbox
  remotely.
• The POP server can be accessed via a dial-up
  connection with the mailbox computer or via the
  internet.
• The POP server requires the client to
  authenticate itself (username and password).

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POP
56
IMAP

• Internet Message Access Protocol is another
  protocol for accessing e-mail from the mail
  server.
• The e-mail client views just the messages header
  and then decide whether or not to download it.
• IMAP also allows the client to create and
  manipulate folders or mailboxes on the server,
  delete messages, or search for certain parts or
  an entire note.

57
IMAP (Cont.)

• IMAP is more sophisticated than POP.
• IMAP requires continual access to the mail server
  while one is working with ones mail.
• With POP3, when one reads mail, it is immediately
  downloaded to the users computer and is no
  longer maintained on the server.
• IMAP can be thought of as a remote file server,
  whereas POP can be thought of as a
  "store-and-forward" service.

58
Multiple Recipients

• E-mail message can be effectively multicast, that
  is, there may be more than one recipient of the
  same message.
• The transfer programs do not send separate copies
  if more than one recipients mail box is on the
  same mail server.

59
Multiple Recipients

• One message is sent, then copies are made by the
  end mail server. This saves internet bandwidth
  and is more egalitarian.
• Recipients with mailboxes on the same mail server
  will all receive the message about the same time.
• If the internet fails, either all recipients or
  none will receive a copy.

60
Exploders, Lists and Forwarders

• A mail exploder, or forwarder, is a program that
  can forward copies of a message.
• A mailing list is a database entry which is a
  list of email addresses.
• If a message is sent to a list, the exploder
  forwards the message to each entry in the list.
• If an individual wishes to receive mail for a
  group, the individuals email address must be on
  the list.

61
List server

• A list server (mailing list server) is a program
  that handles subscription requests for a mailing
  list and distributes new messages, newsletters or
  other postings from the list's members to the
  entire list of subscribers as they occur or are
  scheduled.

62
E-Mail Application Programs

• When an email message arrives at a computer, the
  email software sends a copy of the message to the
  specified user. If the user is an application
  program, the email package sends the message to
  the application instead of the mailbox.
• This allows the user to make a request in an
  e-mail message and the message to be sent
  directly to the query system (application).
• The application will extract the request, look-up
  the answer and send an e-mail reply to the user.

63
Extending email

• Originally email was for sending messages, so it
  used a simple ASCII (text) format, and to
  exchange files, one used FTP. But
• Users were much more familiar with email and
  wanted to extend its capabilities rather than use
  an unfamiliar service.
• FTP (file transfer protocol) is essentially a
  pull technology while email is a push technology.
  

64
BinHex

• E-mail attachments are the way in which one uses
  the e-mail system meant to handle ACSII to
  deliver non-ASCII files.
• BinHex is one approach
• All files are in binary a group of four binary
  numbers can be though of as a number between 0
  and 15 or in hexadecimal notation a number
  between 0 and F the hexadecimal numbers can be
  replaced by the corresponding ASCII code for the
  digit.
• This replaces 4 bits with 7 or 8 (for ASCII) and
  while simple is inefficient.

65
uuencode

• A more efficient binary-to-ASCII algorithm is
  called uuencode.
• Originally, this was done as a separate step by
  the user before sending the file as an e-mail
  message. And the receiver had to uudecode the
  file.
• But now with MIME, many of these steps are done
  automatically and hidden from the user.

66
MIME

• Multipurpose Internet Mail Extension was invented
  to coordinate and unify the encoding schemes used
  for transfer. MIME does not dictate a single
  standard.
• The sender and receiver exchange a convenient
  encoding sequence. If a message has several
  parts, the encoding sequence for each part may be
  different.
• This allows transmission of graphics and plain
  text messages.
• MIME is flexible. MIME is compatible with older
  e-mail systems.

67
MIME (Cont.)

• In 1991, Nathan Borenstein of Bellcore proposed
  to the IETF that SMTP be extended so that
  Internet client and server could recognize and
  handle other kinds of data than ASCII text.
• As a result, new file types were added to "mail"
  as a supported Internet Protocol file type.

68
MIME (Cont.)

• Clients use a header to select an appropriate
  "player" application for the type of data the
  header indicates.
• Some of these players are built into the Web
  client or browser (for example, all browser come
  with GIF and JPEG image players as well as the
  ability to handle HTML files) other players may
  need to be downloaded.
• New MIME data types are registered with the
  Internet Assigned Numbers Authority (IANA).

69
S/MIME

• Secure Multi-Purpose Internet Mail Extensions is
  a secure method of sending e-mail attachments
  that uses the Rivest-Shamir-Adleman (RSA)
  encryption system.
• S/MIME is included in the latest versions of the
  Web browsers from Microsoft and Netscape and has
  also been endorsed by other vendors that make
  messaging products.
• An alternative to S/MIME is PGP/MIME.

70
finger

• A UNIX program that takes an e-mail address as
  input and returns information about the user who
  owns that e-mail address.
• On some systems, finger only reports whether the
  user is currently logged on. Other systems return
  additional information, such as the user's full
  name, address, and telephone number.
• Of course, the user (or system administrator)
  must first enter this information into the
  system. Many e-mail programs now have a finger
  utility built into them.

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finger
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Finger blocked
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Other References

• http//www.whatis.com
• http//www.webopedia.com