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Client-Server Interaction

Based on Chapter 28 in Computer Networks and

Internets, Comer

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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-application interaction on an internet is the client-server model.

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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

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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.

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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.”

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Two meanings

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.

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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.

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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.

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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.

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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.

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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

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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 user’s 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.

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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.

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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.”

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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.

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Client-Server Interaction

Information between client-server passes in both directions.

Clients request, servers respond. Sometimes the server’s 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.

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Client-Server

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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)

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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.

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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.

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Multiple services

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Three tier

A special type of client/server architecture consisting of three well-defined and separate processes, each running on a different platform:

1. The user interface, which runs on the user's computer (the client).

2. The functional modules (business rules) that actually process data. This middle tier runs on a server and is often called the application server.

3. 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.

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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

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Network Address Translation and Electronic Mail

Based on Chapters 26 and 32 in Computer Networks

and Internets, Comer

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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 one’s address space. One can use non-routable addresses, and different networks can use the same internal addresses provided their external address is unique.

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NAT: Figure 26.4

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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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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?

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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).

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NAPT Table (Figure 26.4)

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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.

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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.

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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.

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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 user’s 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.

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Email Addresses Addresses have 2 parts, e.g. blum@lasalle.edu The first part blum@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@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).

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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

sender’s and recipient’s 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).

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Header and body

header

body

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Some header fields

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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 recipient’s address does not appear in the message.

• The primary recipient does not know that a blind carbon copy was sent

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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 recipient’s mail box. • (Sometimes called mail transfer agent)

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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 recipient’s mailbox.

• Remote: Sending the message to a user on a remote computer requires that sender’s transfer software become a client of the recipient’s transfer program. The remote computer must append the message to the recipient’s mailbox.

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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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Two Stage Delivery

Mail is not delivered directly to the typical end user’s computer

• Mail might arrive at any time and the mail server must be ready to receive it at that time (it’s 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.

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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

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IMAP

Internet Message Access Protocol is another protocol for accessing e-mail from the mail server.

The e-mail client views just the message’s 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.

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IMAP (Cont.)

IMAP is more sophisticated than POP. IMAP requires continual access to the mail server

while one is working with one’s mail. With POP3, when one reads mail, it is

immediately downloaded to the user’s 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.

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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 recipient’s mail box is on the same mail server.

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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.

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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 individual’s email address must be on the list.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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