UNIT 5

APPLICATION LAYER
 Topics Covered:
 DNS-Domain Name System
 DNS in internet
 Electronic mail
 SMTP
 FTP
 WWW
 Architecture
 Web documents
 HTTP
 SNMP
Domain Name System

There are several applications in the application layer of the Internet model that follow the
client/server paradigm. The client/server programs can be divided into two categories:
those that can be directly used by the user, such as e-mail, and those that support other
application programs. The Domain Name System (DNS) is a supporting program that is
used by other programs such as e-mail.

1. Need for DNS

To identify an entity, TCP/IP Protocols use the IP address, which uniquely identifies the
connection of a host to an internet. In the case of ARPANET, a file named hosts.txt is used to
list all hosts and their IP addresses, this work suitable for small network but not for large
network due to heavy load and latency. Therefore, people prefer to use names instead of
addresses that is, we need a system that can map a name to an address and conversely an
address to a name. Thus, preferred system is called as Domain Name system.

2. DNS in the Internet:

DNS is a protocol that can be used in different platforms. In the Internet, the domain name
space is divided into three sections are

1. Generic domains

2. Country domains and

3. Inverse domain
1. Generic domain:

There are 14 generic domains, each specifying an organization type. The generic domain
defines registered hosts according to their generic behavior. Each node in the tree defines a
domain, which is an index to the domain mane space data base.

Looking at the tree, we see that the first level in the generic domain section allows seven
possible three-character labels. These labels describe the organization types as shown
below
Each country domain specifies a country. This section follows the same format as the
generic domains but uses two-character country abbreviations in place of three character
organizational abbreviations at the first level. Second level labels can be organizational, or
they can be more specific, national designations. The following figure 5.3 shows the
country domain section the address cs.Keio.ac.jp refers to computer science department of
Keio University in Japan. To create a new domain, permission is required of the domain in
which it will be included.

For example, if a new university is chartered, say the University of Chennai, it must ask the
manager of the edu domain to assign it unc.edu, in order to avoid conflicts and each
domain can keep track of all its sub domains

Once a new domain has been created and registered, it can create sub domains, such as
cs.unc.edu, without getting permission from anybody higher up the tree.
3. Inverse domain:

The inverse domain finds a domain name for a given IP address. This is called address-to-
name resolution. It is used to map an address to a name. This may happen, for example,
when a server lists only the IP address of the client. To determine if the client is on the
authorized list, it can be send a query to the DNS server and ask for a mapping of address
to name in figure 5.4

3. Types of Records:

There are two types of DNS records:

1. Question records

2. Resource records

Question Records:

The question records are used in the question section of the query and response messages.
It is used by the client to get information from a server.

Resource Records:

Every domain whether it is a single host or a top level domain, can have a set of resource
records associated with it. For a single host, the most common resource record is just its IP
address, but many other kinds also exist. When a resolver gives a domain name to DNS,
what it gets back are the resource records associated with that name. Thus, the primary
function of DNS is to map domain names onto resource records. The server database
consists of resource records.

This record is used in the answer, authoritative and additional information sections of the
response message.

4. Domain Name System:

DNS can be pictured as an inverted hierarchical tree structure with one root node at the
top and a maximum of 128 levels.

Labels:

Each node in the tree has a label, which is string with a maximum of 63 characters.

Domain Name:

Each node in the tree has a domain name. A full domain name is a sequence of labels
separated by dots (.).

Fully Qualified Domain Name (FQDN):

A FQDN is a domain name consisting of labels beginning with the host and going back
through each level to the root node. Exà Challenger.atc.fh.da.Edu

Partially Qualified Domain Name (PQDN):

In PQDN is a domain name that does not include all the levels between the host and the
root node. Exà Challenger.

5. Name Server:

In theory atleast, a single name server could contain the entire DNS database and respond
to all queries about it. In practice, this server would be so overloaded as to be useless. To
avoid problems associated with having only a single source of information, the DNS name
space is divided into non-overlapping zones. One possible way to divide the name space,
where the zone boundaries are placed within a zone is upto that zones administrator. This
decision is made in larger part based on how many name servers are desired. To improve
reliability, some servers for a zone can be located outside the zone.

The DNS client, called a resolver, maps a name to an address, or an address to a name.
When a resolver has a query about the domain name, it passes the query to one of the local
name servers. If the domain being sought falls under the jurisdiction of the name server,
such as ai.cs.yale.edu falling under cs.yale.edu, it returns the authoritative resource
records. An authoritative record is one that comes from the authority that manages the
record and it thus always correct. While, DNS helps in mapping names onto their IP
addresses. It does not help locate people, resources, services or objects in general. For
locating these things, another directory service has been defined, called LDAP (Light
Weight Directory Access protocol).

6. DNS Messages:

These are two types of DNS Messages queries and responses. Both types have the same
format. Queries Messages:

The query message consists of a header and question

Response Messages:

The response message consists of a header, question records, answer records,

authoritative records and additional records.

7. Header Format:

Both are have the same header format. The header is 12 bytes.

Identification

1. Number of questions records

2. Number of authoritative records. (All 0s in query message)

Flags

1. Number of answers records (All 0s in query message)

2. Number of additional records. (All 0s in query message)

The identification subfield is used by the client to match the response with the query.

The flag subfield is a collection of subfields that define the types of the message, the type
of answer requested, and the type of desired resolution and so on.

The Number of question records subfield contains the number of queries in the
question section of the message

The number of answer records subfield contains the number of answer records in the
answer section of the response message. Its value is zero in the query message.

The number of authoritative records subfield contains the number of authoritative

records in the authoritative section of a response message. Its value is zero in the query
section.

The number of additional records subfield contains the number of additional records in
the additional section of a response message. Its value is zero in the query message.

Electronic Mail

One of the most popular Internet services is electronic mail (e-mail). The designers of the
Internet probably never imagined the popularity of this application program. Its
architecture consists of several components. At the beginning of the Internet era, the
messages sent by electronic mail were short and consisted of text only; they let people
exchange quick memos.

1. Architecture

To explain the architecture of e-mail, we give four scenarios. We begin with the simplest
situation and add complexity as we proceed. The fourth scenario is the most common in
the exchange of email.

First Scenario

In the first scenario, the sender and the receiver of the e-mail are users (or application
programs) on the same system; they are directly connected to a shared system. The
administrator has created one mailbox for each user where the received messages are
stored. A mailbox is part of a local hard drive, a special file with permission restrictions.
Only the owner of the mailbox has access to it. When Alice, a user, needs to send a message
to Bob, another user, Alice runs a user agent (VA) program to prepare the message and
store it in Bob's mailbox. The message has the sender and recipient mailbox addresses
(names of files). Bob can retrieve and read the contents of his mailbox at his convenience,
using a user agent.

Second Scenario

In the second scenario, the sender and the receiver of the e-mail are users (or application
programs) on two different systems. The message needs to be sent over the Internet. Here
we need user agents (VAs) and message transfer agents (MTAs).

Third Scenario

In the third scenario, Bob, as in the second scenario, is directly connected to his system.
Alice, however, is separated from her system. Either Alice is connected to the system via a
point-to-point WAN, such as a dial-up modem, a DSL, or a cable modem; or she is
connected to a LAN in an organization that uses one mail server for handling e-mails-all
users need to send their messages to this mail server.
Fourth Scenario

In the fourth and most common scenario, Bob is also connected to his mail server by a
WAN or a LAN. After the message has arrived at Bob's mail server, Bob needs to retrieve it.
Here, we need another set of client/server agents, which we call message access agents
(MAAs). Bob uses an MAA client to retrieve his messages. The client sends a request to the
MAA server, which is running all the time, and requests the transfer of the messages.

2. User Agent

The first component of an electronic mail system is the user agent (VA). It provides service
to the user to make the process of sending and receiving a message easier.

Services Provided by a User Agent

A user agent is a software package (program) that composes reads, replies to, and
forwards messages. It also handles mailboxes.

Composing Messages A user agent helps the user compose the e-mail message to be sent
out.Most user agents provide a template on the screen to be filled in by the user. Some even
have a built-in editor that can do spell checking, grammar checking, and other tasks
expected from a sophisticated word processor. A user, of course, could alternatively use his
or her favourite text editor or word processor to create the message and import it, or cut
and paste it, into the user agent template.

Reading Messages The second duty of the user agent is to read the incoming messages.
When auser invokes a user agent, it first checks the mail in the incoming mailbox. Most
user agents show a one-line summary of each received mail. Each e-mail contains the
following fields.

1. A number field.

2. A flag field that shows the status of the mail such as new, already read but not replied to,
or read and replied to.

3. The size of the message.

4. The sender.

5. The optional subject field.

Replying to Messages After reading a message, a user can use the user agent to reply to
amessage. A user agent usually allows the user to reply to the original sender or to reply to
all recipients of the message. The reply message may contain the original message (for
quick reference) and the new message.

Forwarding Messages Replying is defined as sending a message to the sender a message

to the sender or recipients of the copy. Forwarding is defined as sending the message to a
third party. A user agent allows the receiver to forward the message, with or without extra
comments, to a third party.

3. User Agent Types

There are two types of user agents: command-driven and GUI-based.

Command-Driven

Command-driven user agents belong to the early days of electronic mail. They are still
present as the underlying user agents in servers. A command-driven user agent normally
accepts a one-character command from the keyboard to perform its task. For example, a
user can type the character r, at the command prompt, to reply to the sender of the
message, or type the character R to reply to the sender and all recipients. Some examples of
command-driven user agents are mail, pine, and elm.

GUI-Based Modem user agents are GUI-based. They contain graphical-user interface
(GUI)components that allow the user to interact with the software by using both the
keyboard and the mouse. They have graphical components such as icons, menu bars, and
windows that make the services easy to access. Some examples of GUI-based user agents
are Eudora, Microsoft's Outlook, and Netscape.

Some examples of GUI·based user agents are Eudora, Outlook, and Netscape.

4. Message Transfer Agent: SMTP

The actual mail transfer is done through message transfer agents. To send mail, a system
must have the client MTA, and to receive mail, a system must have a server MTA. The
formal protocol that defines the MTA client and server in the Internet is called the Simple
Mail Transfer Protocol (SMTP). As we said before, two pairs of MTA client/server
programs are used in the most common situation (fourth scenario).
SMTP is used two times, between the sender and the sender's mail server and between the
two mail servers. As we will see shortly, another protocol is needed between the mail
server and the receiver.

SMTP simply defines how commands and responses must be sent back and forth. Each
network is free to choose a software package for implementation.

Commands and Responses

SMTP uses commands and responses to transfer messages between an MTA client and an
MTA server. Each command or reply is terminated by a two-character (carriage return and
line feed) end-of-line token.

Commands: Commands are sent from the client to the server. It consists of a keyword
followedby zero or more arguments. SMTP defines 14 commands. The first five are
mandatory; every implementation must support these five commands. The next three are
often used and highly recommended. The last six are seldom used.

Responses: Responses are sent from the server to the client. A response is a three digit
code that may be followed by additional textual information.

5. Mail Transfer Phases

The process of transferring a mail message occurs in three phases: connection

establishment, mail transfer, and connection termination.

Currently two message access protocols are available: Post Office Protocol, version 3
(POP3) and Internet Mail Access Protocol, version 4 (IMAP4).
POP3

Post Office Protocol, version 3 (POP3) is simple and limited in functionality. The client
POP3 software is installed on the recipient computer; the server POP3 software is installed
on the mail server. Mail access starts with the client when the user needs to download e-
mail from the mailbox on the mail server. POP3 has two modes: the delete mode and the
keep mode. In the delete mode, the mail is deleted from the mailbox after each retrieval. In
the keep mode, the mail remains in the mailbox after retrieval. The delete mode is
normally used when the user is working at her permanent computer and can save and
organize the received mail after reading or replying. The keep mode is normally used when
the user accesses her mail away from her primary computer (e.g., a laptop). The mail is
read but kept in the system for later retrieval and organizing.

IMAP4

Another mail access protocol is Internet Mail Access Protocol, version 4 (IMAP4). IMAP4 is
similar to POP3, but it has more features; IMAP4 is more powerful and more complex

File Transfer Protocol (FTP):

File Transfer
Transferring files from one computer to another is one of the most common tasks
expected from a networking or internetworking environment. As a matter of fact, the
greatest volume of data exchange in the Internet today is due to file transfer.

File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a
file from one host to another. Although transferring files from one system to another seems
simple and straightforward, some problems must be dealt with first.

FTP differs from other client/server applications in that it establishes two connections
between the hosts. One connection is used for data transfer, the other for control
information (commands and responses). Separation of commands and data transfer makes
FTP more efficient. The control connection uses very simple rules of communication. We
need to transfer only a line of command or a line of response at a time. The data
connection, on the other hand, needs more complex rules due to the variety of data types
transferred. However, the difference in complexity is at the FTP level, not TCP. For TCP,
both connections are treated the same. FTP uses two well-known TCP ports: Port 21 is
used for the control connection, and port 20 is used for the data connection.

The client has three components: user interface, client control process, and the client data
transfer process. The server has two components: the server control process and the
server data transfer process. The control connection is made between the control
processes. The data connection is made between the data transfer processes.

The control connection remains connected during the entire interactive FTP session. The
data connection is opened and then closed for each file transferred. It opens each time
commands that involve transferring files are used, and it closes when the file is transferred.
In other words, when a user starts an FTP session, the control connection opens. While the
control connection is open, the data connection can be opened and closed multiple times if
several files are transferred.

1. Communication over Control Connection

FTP uses the same approach as SMTP to communicate across the control connection. It
uses the 7-bit ASCII character set. Communication is achieved through commands and
responses. This simple method is adequate for the control connection because we send one
command (or response) at a time. Each command or response is only one short line, so we
need not worry about file format or file structure. Each line is terminated with a two-
character (carriage return and line feed) end-of-line token.
2. Communication over Data Connection
The purpose of the data connection is different from that of the control connection. We
want to transfer files through the data connection. File transfer occurs over the data
connection under the control of the commands sent over the control connection. However,
we should remember that file transfer in FTP means one of three things:

· A file is to be copied from the server to the client. This is called retrieving aft/e. It is
done under the supervision of the RETR command.

· A file is to be copied from the client to the server. This is called storing aft/e. It is
done under the supervision of the STOR command.

· A list of directory or file names is to be sent from the server to the client. This is done
under the supervision of the LIST command. Note that FTP treats a list of directory or file
names as a file. It is sent over the data connection.

The client must define the type of file to be transferred, the structure of the data, and the
transmission mode. Before sending the file through the data connection, we prepare for
transmission through the control connection. The heterogeneity problem is resolved by
defining three attributes of communication: file type, data structure, and transmission
mode
File Type:
FTP can transfer one of the following file types across the data connection: an ASCII file,
EBCDIC file, or image file. The ASCII file is the default format for transferring text files. Each
character is encoded using 7-bit ASCII. The sender transforms the file from its own
representation into ASCII characters, and the receiver transforms the ASCII characters to
its own representation.

Data Structure
FTP can transfer a file across the data connection by using one of the following
interpretations about the structure of the data: file structure, record structure, and page
structure. In the file structure format, the file is a continuous stream of bytes. In the record
structure, the file is divided into records.

Transmission Mode
FTP can transfer a file across the data connection by using one of the following three
transmission modes: stream mode, block mode, and compressed mode. The stream mode
is the default mode. Data are delivered from FTP to TCP as a continuous stream of bytes.
TCP is responsible for chopping data into segments of appropriate size. If the data are
simply a stream of bytes (file structure), no end-of-file is needed. End-of-file in this case is
the closing of the data connection by the sender.
SMTP –SIMPLE MAIL TRANSFER PROTOCOL

Next we look at SMTP- the protocol used to transfer messages from one host to another. To
place SMTP in the right context, we need to identify the key players. First, users interact
with a mail reader when they compose ,file ,search, and read their email. There are
countless mail readers available ,just like there are many web browsers now include a mail
reader. Second ,there is a mail daemon running on each host. You can think of this process
as playing the role of a post office :mail readers give the daemon messages they want to
send to others users, the daemon uses SMTP running over TCP to transmit the message
into a daemon running on another machine, and the daemon puts incoming messages into
the user‟s mailbox. Since SMTP is a protocol that anyone could implement , in theory there
could be many different implementations of the mail daemon. It runs out, though that the
mail daemon running on most hosts is derived from the sendma il program originally
implemented on berkely u nix.

While it is certainly possible that the sendmail program on a s ender‟s machine

establishes an SMTP/TCP connection to the sendmail program on the recipient‟s machine,
in many cases the mail traverses o ne or more mail gateways on its route from the sender‟s
host to the receiver‟s host. Like the end hosts, these gateways also run a send-mail pr
ocess. It‟s not an accident that these intermediatee nodes are called “gateways” since their
job is to store and forward email messages.

Mail Reader:

The final step is for the user to actually receive her messages from th e mail box, read them
,reply to them, and possib ly save a copy for future reference .The user performs all the
actions by interacting with a ma il reader. In many cases ,this reader is just a pro gram
running on the same machine as the user‟s mailbox resides, in which case it simply reads
and writes the file that implements the mailbox .I n other cases ,the user accesses her
mailbox from a remote machine using yet another protocol, such as the Post Office
Protocol(POP ) or the Internet Message Access Control(IMAP).It is beyond the scope of this
book to discuss the user interface aspects of the mail reader but it is definitely within our
scope to talk about the access protocol. We consider IMAP, in particular.

IMAP is similar to S MTP in many ways .It is a client/server protocol running over TCP,
where the client (running on the user‟s desktop machine) issues commands in the form of
<CRLF> terminated ASCII text lines and the mail server(running on the machine that
maintains the user‟s mailbox) responds in-kind. The exchange begins with the client
authenticating herself, an d identifying the mailbox she wants to acc ess. This can be
represented by the simple state tr ansaction diagram shown in the figure. In this d iagram,
LOGIN, AUTHENTICATE, SELECT, EXAMINE, CLOSE and LOGOUT are exampl e commands
that the client can issue, while OK is one possible server response. Other common c
ommands include FETCH, STORE, DELETE, an d EXPUNGE, with the obvious meanings.
Additional server responds include NO (client d oes not have permission to perform that
operation) and BAD (command is ill-formed).

When the user asks to FETCH a message, the server returns it in MIM E format and
the mail reader decodes it. In addition to the message itself, IMAP also defines a set of
message attributes that are exchanged as part of other commands, independent of transfer
ring the message itself. Message attributes include information like the size of the
message, but more interestingly, various flags associated with a message, such as Seen,
Answered, Deleted and Recent. These flags are used to keep the client and server
synchronized, that is, when the user deletes a message in the mail reader, the client needs
to report this fact to the mail server. Later, should the user decide to expunge all deleted
messages, the client issues an EXPUNGE command to the server, which knows to actually
remove all earlier deleted messages from the mail box.

Finally, note that when the user replies to a message,or sends a new message, the
mail reader does not forward the message from the client to the mail server using IMAP
,but it instead uses SMTP .This means that the user‟s mail server is effectively the first mail
gateway traversed along the path from the desktop to the recipient‟s mail box.

TCP/IP protocol suite specifies a standard for the exchange of mail between
machines. It was derived from the (MTP) Mail Transfer Protocol. it deals with how the
underlying mail delivery system passes messages across a link from one machine to
another. The mail is enclosed in what is called an envelope. The envelope contains t he To
and From fields and these are followed by the mail . The mail consists of two parts namely
the Header and the Data. The Header has the To and From fields. If Headers are defined by
us they should start with X. The standard headers do not start with X. In SMTP data portion
can contain only printable ASCII characters The old method of sending a binary file was to
send it in uuencoded form but there was no way to distinguish between the many types of
binary files possible eg. .tar , .gz , .dvi etc.

WWW (World Wide Web)

The World Wide Web (WWW) is a repository of information linked together from points all
over the world. The WWW has a unique combination of flexibility, portability, and user-
friendly features that distinguish it from other services provided by the Internet. The
WWW project was initiated by CERN (European Laboratory for Particle Physics) to create
a system to handle distributed resources necessary for scientific research.

1. Architecture

The WWW today is a distributed client/server service, in which a client using a browser
can access a service using a server. However, the service provided is distributed over many
locations called sites.

Each site holds one or more documents, referred to as Web pages. Each Web page can
contain a link to other pages in the same site or at other sites. The request, among other
information, includes the address of the site and the Web page, called the URL, which we
will discuss shortly. The server at site A finds the document and sends it to the client. When
the user views the document, she finds some references to other documents, including a
Web page at site B. The reference has the URL for the new site. The user is also interested
in seeing this document. The client sends another request to the new site, and the new
page is retrieved.

Client (Browser)

A variety of vendors offer commercial browsers that interpret and display a Web
document, and all use nearly the same architecture. Each browser usually consists of three
parts: a controller, client protocol, and interpreters. The controller receives input from the
keyboard or the mouse and uses the client programs to access the document. After the
document has been accessed, the controller uses one of the interpreters to display the
document on the screen. The client protocol can be one of the protocols such as FTP or
HTTP.

Server

The Web page is stored at the server. Each time a client request arrives, the corresponding
document is sent to the client. To improve efficiency, servers normally store requested files
in a cache in memory; memory is faster to access than disk. A server can also become more
efficient through multithreading or multiprocessing. In this case, a server can answer more
than one request at a time.

Uniform Resource Locator

A client that wants to access a Web page needs the address. To facilitate the access of
documents distributed throughout the world, HTTP uses locators. The uniform resource
locator (URL) is a standard for specifying any kind of information on the Internet. The URL
defines four things: protocol, host computer, port, and path.

The protocol is the client/server program used to retrieve the document. Many different
protocols can retrieve a document; among them are FTP or HTTP. The most common today
is HTTP.

The host is the computer on which the information is located, although the name of the
computer can be an alias. Web pages are usually stored in computers, and computers are
given alias names that usually begin with the characters "www". This is not mandatory,
however, as the host can be any name given to the computer that hosts the Web page. The
URL can optionally contain the port number of the server. If the port is included, it is
inserted between the host and the path, and it is separated from the host by a colon.

Path is the pathname of the file where the information is located. Note that the path can
itself contain slashes that, in the UNIX operating system, separate the directories from the
subdirectories and files.

2. WEB DOCUMENTS

The documents in the WWW can be grouped into three broad categories: static, dynamic,
and active. The category is based on the time at which the contents of the document are
determined.

1. Static Documents
Static documents are fixed-content documents that are created and stored in a server. The
client can get only a copy of the document. In other words, the contents of the file are
determined when the file is created, not when it is used. Of course, the contents in the
server can be changed, but the user cannot change them. When a client accesses the
document, a copy of the document is sent. The user can then use a browsing program to
display the document.

1.1 HTML

Hypertext Markup Language (HTML) is a language for creating Web pages. The term
markup language comes from the book publishing industry. Before a book is typeset and
printed, a copy editor reads the manuscript and puts marks on it. These marks tell the
compositor how to format the text. For example, if the copy editor wants part of a line to be
printed in boldface, he or she draws a wavy line under that part. In the same way, data for a
Web page are formatted for interpretation by a browser

The two tags <B> and </B> are instructions for the browser. When the browser sees these
two marks, it knows that the text must be boldfaced. A markup language such as HTML
allows us to embed formatting instructions in the file itself. The instructions are included
with the text. In this way, any browser can read the instructions and format the text
according to the specific workstation.

A Web page is made up of two parts: the head and the body. The head is the first part of a
Web page. The head contains the title of the page and other parameters that the browser
will use. The actual contents of a page are in the body, which includes the text and the tags.
Whereas the text is the actual information contained in a page, the tags define the
appearance of the document. Every HTML tag is a name followed by an optional list of
attributes, all enclosed between less-than and greater-than symbols (< and >). An attribute,
if present, is followed by an equal’s sign and the value of the attribute. Some tags can be
used alone; others must be used in pairs. Those that are used in pairs are called beginning
and ending tags. The beginning tag can have attributes and values and starts with the name
of the tag. The ending tag cannot have attributes or values but must have a slash before the
name of the tag.

2.2 Dynamic Documents

A dynamic document is created by a Web server whenever a browser requests the

document. When a request arrives, the Web server runs an application program or a script
that creates the dynamic document. The server returns the output of the program or script
as a response to the browser that requested the document. Because a fresh document is
created for each request, the contents of a dynamic document can vary from one request to
another.

2.2.1 Common Gateway Interface (CGI)

The Common Gateway Interface (CGI) is a technology that creates and handles dynamic
documents. CGI is a set of standards that defines how a dynamic document is written, how
data are input to the program, and how the output result is used. The term common in CO1
indicates that the standard defines a set of rules that is common to any language or
platform. The term gateway here means that a COl program can be used to access other
resources such as databases, graphical packages, and so on. The term interface here means
that there is a set of predefined terms, variables, calls, and so on that can be used in any COl
program.

HTTP

The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the
World Wide Web. HTTP functions as a combination of FTP and SMTP. It is similar to FTP
because it transfers files and uses the services of TCP. However, it is much simpler than
FTP because it uses only one TCP connection. There is no separate control connection; only
data are transferred between the client and the server. HTTP is like SMTP because the data
transferred between the client and the server look like SMTP messages. In addition, the
format of the messages is controlled by MIME-like headers. Unlike SMTP, the HTTP
messages are not destined to be read by humans; they are read and interpreted by the
HTTP server and HTTP client (browser). SMTP messages are stored and forwarded, but
HTTP messages are delivered immediately. The commands from the client to the server are
embedded in a request message. The contents of the requested file or other information
are embedded in a response message. HTTP uses the services of TCP on well-known port
80.

1. HTTP Transaction

Although HTTP uses the services of TCP, HTTP itself is a stateless protocol. The client
initializes the transaction by sending a request message. The server replies by sending a
response.
Messages

The formats of the request and response messages are similar. A request message consists
of a request line, a header, and sometimes a body. A response message consists of a status
line, a header, and sometimes a body. Request and Status Lines The first line in a request
message is called a request line; the first line in the response message is called the status
line.

Request type: This field is used in the request message. In version1.1of HTTP, several
request types are defined.

Version: The most current version of HTTP is 1.1.

Status code: This field is used in the response message. The status code field is similar
tothose in the FTP and the SMTP protocols. It consists of three digits.

Status phrase: This field is used in the response message. It explains the status code
intext form.

Header: The header exchanges additional information between the client and the
server.For example, the client can request that the document be sent in a special format, or
the server can send extra information about the document. The header can consist of one
or more header lines. Each header line has a header name, a colon, a space, and a header
value.

Body: The body can be present in a request or response message. Usually, it contains the
document to be sent or received.

2. Persistent Versus Non persistent Connection

HTTP prior to version 1.1 specified a non persistent connection, while a persistent
connection is the default in version 1.1.

Non persistent Connection

In a non persistent connection, one TCP connection is made for each request/response.
The following lists the steps in this strategy:

1. The client opens a TCP connection and sends a request.

2. The server sends the response and closes the connection.

3. The client reads the data until it encounters an end-of-file marker; it then closes the
connection.

Persistent Connection

HTTP version 1.1 specifies a persistent connection by default. In a persistent connection,

the server leaves the connection open for more requests after sending a response. The
server can close the connection at the request of a client or if a time-out has been reached.
The sender usually sends the length of the data with each response. However, there are
some occasions when the sender does not know the length of the data. This is the case
when a document is created dynamically or actively.

Multimedia

Recent advances in technology have changed our use of audio and video. In the past, we
listened to an audio broadcast through a radio and watched a video program broadcast
through a TV. We used the telephone network to interactively communicate with another
party. But times have changed. People want to use the Internet not only for text and image
communications, but also for audio and video services.

We can divide audio and video services into three broad categories:

1. Streaming stored audio/video

2. Streaming live audio/video

3. Interactive audio/video

1. Streaming stored audio/video, the files are compressed and stored on a server. A
client downloads the files through the Internet.

2. Streaming live audio/video, a user listens to broadcast audio and video through the
Internet.
3. Interactive audio/video, people use the Internet to interactively communicate with
one another.

1. Digitizing Audio and Video

Before audio or video signals can be sent on the Internet, they need to be digitized.

Digitizing Audio

When sound is fed into a microphone, an electronic analog signal is generated which
represents the sound amplitude as a function of time. The signal is called an analog audio
signal. An analog signal, such as audio, can be digitized to produce a digital signal.
According to the Nyquist theorem, if the highest frequency of the signal is f, we need to
sample the signal 21 times per second. There are other methods for digitizing an audio
signal, but the principle is the same.

Digitizing Video

A video consists of a sequence of frames. If the frames are displayed on the screen fast
enough, we get an impression of motion. The reason is that our eyes cannot distinguish the
rapidly flashing frames as individual ones. There is no standard number of frames per
second; in North America 25 frames per second is common. However, to avoid a condition
known as flickering, a frame needs to be refreshed. The TV industry repaints each frame
twice. This means 50 frames need to be sent, or if there is memory at the sender site, 25
frames with each frame repainted from the memory.

2. Audio and Video Compression

To send audio or video over the Internet requires compression

Audio Compression

Audio compression can be used for speech or music. For speech, we need to compress a
64-kHz digitized signal; for music, we need to compress a 1.41 I-MHz signal. Two
categories of techniques are used for audio compression: predictive encoding and
perceptual encoding.

a. Predictive Encoding

In predictive encoding, the differences between the samples are encoded instead of
encoding all the sampled values. This type of compression is normally used for speech.
Several standards have been defined such as GSM (13 kbps), G.729 (8 kbps), and G.723.3
(6.4 or 5.3 kbps).

b. Perceptual Encoding: MP3

The most common compression technique that is used to create CD-quality audio is based
on the perceptual encoding technique. As we mentioned before, this type of audio needs at
least 1.411 Mbps; this cannot be sent over the Internet without compression. MP3 (MPEG
audio layer 3), a part of the MPEG standard (discussed in the video compression section),
uses this technique.

Video Compression

As we mentioned before, video is composed of multiple frames. Each frame is one image.
We can compress video by first compressing images. Two standards are prevalent in the
market. Joint Photographic Experts Group (JPEG) is used to compress images. Moving
Picture Experts Group (MPEG) is used to compress video.

a. Image Compression: JPEG

If the picture is not in color (gray scale) then each pixel can be represented by an 8-bit
integer (256 levels). If the picture is in color, each pixel can be represented by 24 bits (3 x 8
bits), with each 8 bits representing red, blue, or green (RBG). To simplify the discussion, we
concentrate on a gray scale picture.

b. Video Compression: MPEG

The Moving Picture Experts Group method is used to compress video. In principle, a
motion picture is a rapid flow of a set of frames, where each frame is an image. In other
words, a frame is a spatial combination of pixels, and a video is a temporal combination of
frames that are sent one after another. Compressing video, then, means spatially
compressing each frame and temporally compressing a set of frames.

3. Streaming Live Audio/video

Streaming live audio/video is similar to the broadcasting of audio and video by radio and
TV stations. Instead of broadcasting to the air, the stations broadcast through the Internet.
There are several similarities between streaming stored audio/video and streaming live
audio/video. They are both sensitive to delay; neither can accept retransmission. However,
there is a difference. In the first application, the communication is unicast and on-demand.
In the second, the communication is multicast and live. Live streaming is better suited to
the multicast services of IP and the use of protocols such as UDP and RTP.

4. Real-Time Interactive Audio/video

In real-time interactive audio/video, people communicate with one another in real time.
The Internet phone or voice over IP is an example of this type of application. Video
conferencing is another example that allows people to communicate visually and orally.

HTTP (HYPERTEXT TRANSFER PROTOCOL)

The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the
World Wide Web. The protocol transfer all data in the form of plain text, hypertext, audio,
video, and so on. However it is called the hypertext transfer protocol because its efficiency
allows its use in a hypertext environment where there are rapid jumps from one document
to another.

HTTP functions like a combination of FTP and SMTP. It is similar to FTP because it
transfers files and uses the services of TCP. However, it is much simpler than FTP because
it uses only data are transferred between the client and the server.

HTTP is like SMTP because the data transferred between the client and server look
like SMTP messages. In addition, the format of the messages is controlled by MIME-like
headers.

However, HTTP differs from SMTP in the way the messages are sent from the client to the
server and from the server to the client. Unlike SMTP, the HTTP messages are not destined
to be read by humans; they are read and interpreted by the HTTP server and HTTP client
(browser). SMTP messages are stored and forwarded, but HTTP messages are delivered
immediately.

The idea of HTTP is very simple. A client sends a request, which looks like mail, to the
server. The server sends the response, which looks like a mail reply, to the client. The
request and response messages carry data in the form of a letter with MIME-like format.

The commands from the client to the server are embedded in a letter like request message.
The contents of the requested file or other information are embedded in a letter like
response message.

HTTP Transaction

Figure illustrates the HTTP transaction between the client and server. The client initializes
the transaction by sending a request message. The server replies by sending a response.

Messages

There are two general types of HTTP messages, shown in figure request and response.
Both message types follow almost the same format.

Request Messages

A request message consists of a request line, headers, and sometimes a body.

Response Message

A response message consists of a status line, headers, and sometimes a body.

Uniform Resource Locator (URL)

A client that wants to access a document needs an address. To facilitate the access of
documents distributed throughout the world, HTTP uses the concept of locators. The
uniform resource locator (URL) is a standard for specifying any kind of information on the
Internet.

The URL defines four things:

Method

Host computer Port

Path
The method is the protocol used to retrieve the document, for example HTTP. The host is
the computer where the information is located, although the name can be an alias.

Web pages are usually stored in computers, and computers are given alias names that
usually begin with the characters “www”. This is not mandatory, however, as the host can
be any name given to the computer that hosts the web page.

The URL optionally can contain the port number of the server. If the port is included, it
should be inserted between the host and the path, and it should be separated from the host
by a colon.

Path is the pathname of the file where the information is located. Note that the path can
itself contain slashes that, in the UNIX operating system, separate the directories from
subdirectories and files.

SNMP(SIMPLE NETWORK MANAGEMENT PROTOCOL)

A large network can often get into various kinds of trouble due to routers (dropping too
many packets), hosts( going down) etc. One has to keep track of all these occurence and
adapt to such situations. A protocol has been defined. Under this scheme all entities in the
network belong to 4 classes:

1. Managed Nodes

2. Management Stations

3. Management Information (called Object)

4. A management protocol

The managed nodes can be hosts,routers,bridges,printers or any other device capable of

communicating status information to others. To be managed directly by SNMP, a node
must be capable of running am SNMP management process, called SNMP agent. Network
management is done by management stations by exchanging information with the nodes.
These are basically general purpose computers running special management software. The
management stations polls the stations periodically. Since SNMP uses unreliable service of
UDP the polling is essential to keep in touch with the nodes. Often the nodes send a trap
message indicating that it is going to go down. The management stations then periodically
checks (with an increased frequency) . This type of polling is called trap directed polling.
Often a group of nodes are represented by a single node which communicates with the
management stations. This type of node is called proxy agent. The proxy agent can also
server as a security arrangement. All the variables in these schemes are called Objects.
Each variable can be referenced by a specific addressing scheme adopted by this system.
The entire collection of all objects is called Management Information Base (MIB). The
addressing is hierarchical as seen in the picture.

Internet is addressed as 1.3.61. All the objects under this domain has this string at the
beginning. The information are exchanged in a standard and vendor-neutral way . All the
data are represented in Abstract Syntax Notation 1 (ASN.1). It is similar to XDR as in RPC
but it

have widely different representation scheme. A part of it actually adopted in SNMP and
modified to form Structure Of Information Base. The Protocol specifies various kinds of
messages that can be exchanged between the managed nodes and the management station.

Message : Description

1. Get_Request : Request the value for a variable

2. Get_Response : Returns the value of the variable asked for

3. Get_Next_Request : Request a variable next to the previous one

4. Set_Request : Set the value of an Object.

5. Trap : Agent to manager Trap report

6. Get_bulk_request : Request a set of variable of same type

7. Inform_Request : Exchange of MIB among Management stations

The last two options have been actually added in the SNMPv2. The fourth option need
some kind of authentication from the management station.

Addressing Example :

Following is an Example of the kind of address one can refer to when fetching a value in
the table :-

(20) IP-Addr-Table = Sequence of IPAddr-Entry (1)

IPAddrEntry = SEQUENCE

IPADDENTRYADDR : IPADDR (1)

Index : integer (2)

Netmask : IPAddr (3)

So when accessing the netmask of some IP-entity the variable name would be :
1.3.6.1.2.4.20 .1.3.key-value

Here since Ip-address the unique key to index any member of the array the address can be
like :- 1.3.6.1.2.4.20.1.3.128.10.2.3