Unit - 5

Application Layer

Q1) Write short note on DNS?

A1)

Domain name system (DNS)

DNS provides a protocol that allows client and servers to communicate with each other. To identify a computer on the internet, IP address is used by the TCP/IP protocol suit. However, it is difficult to remember so many IP addresses, for that people used to prefer names (such as www.puneatoz.net) instead of the numeric IP addresses. Therefore, we need a system that can map a name to an IP address or an IP address to a name. Domain Name System provides this facility.

Working of DNS

To map a URL name with IP address, the application program calls a library procedure called as ‘Resolver’. The URL name is passed to the Resolver as an argument. This Resolver sends a UDP packet to local DNS server which goes through its database, finds out the corresponding IP address associated with the URL name, and returns this IP address to the application program. After getting the actual IP address, the application program establishes the connection with the destination.

Q2) Explain header field in DNS?

A2)

The header fields are as follows

Identification: This is 16 bit field used by the client to match the response with the query.

Flags: This is 16 bit field, which is a collection of subfields that defines the type of the message, type of the answers requested, type of the desired resolution and so on.

Number of Question Records: It contains the number of queries in the question section of the message.

Number of Answer Records: It contains the number of answer records in the answer section of the response message. Its value is zero in the query message.

Number of Authoritative Records: It contains the number of authoritative records in the authoritative section of the response message. Its value is zero in the query message.

Number of Additional Records: It contains the number of additional records in the additional section of the response message. Its value is zero in the query message.

Question Section: This section consists of one or more question records. It is present on both query and response messages.

Answer Section: This is a section consisting of one or more resource records. It is present only in response message.

Authoritative Section: This section consists of one or more resource records. It is present only on response message. This section gives information about one or more authoritative servers for the query.

Additional Information Section: This is a section consisting of one or more resource records. It is present only on response message. This section provides additional information that may help the resolver.

Q3) Explain HTTP?

A3)

Hyper Text Transfer Protocol (HTTP)

Now a days Internet has become very essential for us. If we want to access the information on any topic, we just search the topic in World Wide Web. Now the public has become aware of the power of internet through WWW. HTTP protocol is used to access the data on World Wide Web. This protocol normally transfers the data in the form of plain text, hypertext, audio, video and so on. It is called as Hypertext Transfer Protocol because it is used in an environment where there are rapid jumps from one document to another. HTTP functions like a combination of FTP and SMTP. It is said to be similar to FTP because it transfers files.

It is said to be similar to SMTP because the data transferred between the client and server are similar to the SMTP messages. However, HTTP differs from SMTP in the way the messages are sent from client to server and from server to client. In case of SMTP, messages are only transferred from server to client.

How HTTP Works?

The working of HTTP is very simple.

A client sends a request and server sends a reply (response) to the client. See Fig. 6.10. HTTP uses the services of TCP on well-known port 80.

Fig: 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.

Q4) Write about stateless protocol in http?

A4)

HTTP is a Stateless Protocol In this protocol, the server does not store any information about the state of current transaction. So, when the client does a request for some files, server sends these files to client without storing any state information about the client. If same client asks for the same information again and again to the server, so the server would not understand that it has already transferred this information to the same client, so server resends this information back to the client again and again as and when the client requests for those files. As HTTP server does not maintain any information about the state of client, it is called as "stateless protocol".

Q5) Explain http connection?

A5)

HTTP Connections Types

HTTP connections are of two types

1. Persistent Connections
2. Non-Persistent Connections.

These are explained in detail as follows:

Persistent Connections

It is specified in HTTP version 1.1. In 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 timeout has been reached. The sender usually sends a length of the data with each response. However, there are certain situations where the sender does not know the length of the data. In such situation the document is created dynamically. In such cases, the server informs the client that the length of the data is not known and closes the connection after sending the data so the client knows that the end of the data has been reached. HTTP version 1.1 by default specifies a persistent connection.

Non-persistent Connection

In this type one TCP connection is made for each request/response by using the following steps: The client opens a TCP connection and sends a request. The server sends a response and closes the connection.

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

Q6) Define web caching?

A6)

Web server is a computer where the web content is stored. Basically, web server is used to host the web sites but there exist other web servers also such as gaming, storage, FTP, email etc.

Web Server Working Web server respond to the client request in either of the following two ways:

• Sending the file to the client associated with the requested URL.

• Generating response by invoking a script and communicating with database

Key Points

• When client sends request for a web page, the web server search for the requested page if requested page is found then it will send it to client with an HTTP response.

• If the requested web page is not found, web server will the send an HTTP response: Error 404 Not found.

• If client has requested for some other resources, then the web server will contact to the application server and data store to construct the HTTP response.

A Web cache (or HTTP cache) is an information technology for the temporary storage (caching) of Web documents, such as Web pages, images, and other types of Web multimedia, to reduce server lag.

A Web cache system stores copies of documents passing through it subsequent requests may be satisfied from the cache if certain conditions are met. A Web cache system can refer either to an appliance or to a computer program.

Q7) Explain SMTP?

A7)

Simple Mail Transfer Protocol E-mail system is implemented with the help of Message Transfer Agents (MTA). There are normally two MTAs in each mailing system One for sending e-mails and another for receiving e-mails. The formal protocol that defines the MTA client and server in the internet is called Simple Mail Transfer Protocol (SMTP).

Fig shows the range of SMTP protocol. By referring the above diagram, we can say that SMTP is used two times. That is between the sender and sender’s mail server and between the sender’s mail server and receiver’s mail server. Another protocol is used between the receiver’s mail server and receiver. SMTP simply defines how commands and responses must be sent back and forth.

SMTP is a simple ASCII protocol.

It establishes a TCP connection between a sender and port number 25 of the receiver. No checksums are generally required because TCP provides a reliable byte stream. After exchanging all the e-mail, the connection is released.

Commands and Responses

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

Fig. Commands and Responses

Commands

Client sends commands to the server. SMTP defines 14 commands.

Out of that first 5 commands are mandatory; every implementation must support these commands. The next three commands are often used and are highly recommended.

Q8) Write about MIME?

A8)

MIME Short for Multipurpose Internet Mail Extensions, a specification for formatting non-ASCII messages so that they can be sent over the Internet. Many e-mail clients now support MIME, which enables them to send and receive graphics, audio, and video files via the Internet mail system. In addition, MIME supports messages in character sets other than ASCII. There are many predefined MIME types, such as GIF graphics files and PostScript files. It is also possible to define your own MIME types.

In addition to e-mail applications, Web browsers also support various MIME types. This enables the browser to display or output files that are not in HTML format.

MIME-Version

The presence of this header indicates the message is MIME-formatted. The value is typically ”1.0" so this header appears as MIME-Version: 1.0

Content-Type This header indicates the media type of the message content, consisting of a type and subtype, for example

Content-Type: text/plain.

Through the use of the multipart type, MIME allows mail messages to have parts arranged in a tree structure where the leaf nodes are any non-multipart content type and the non-leaf nodes are any of a variety of multipart types. This mechanism supports: Simple text messages using text/plain (the default value for "Content-Type: ")Text plus attachments (multipart/mixed with a text/plain part and other non-text parts). A MIME message including an attached file generally indicates the file’s original name with the "Content-disposition:" header, so the type of file is indicated both by the MIME content-type and the (usually OS-specific) filename extension Reply with original attached (multipart/mixed with a text/plain part and the original message as a message∕rfc822 part) Alternative content, such as a message sent in both plain text and another format such as HTML (multipart/alterative with the same content in text/plain and text/html forms) Image, audio, video and application (for example, image/jpeg, audio∕mp3, video∕mp4, and application/msword and so on) Many other message constructs

Q9) Write about POP?

A9)

POP Stands for "Post Office Protocol." POP3, sometimes referred to as just "POP," is a simple, standardized method of delivering e-mail messages. A POP3 mail server receives e-mails and filters them into the appropriate user folders. When a user connects to the mail server to retrieve his mail, the messages are downloaded from mail server to the user's hard disk.

When you configure your e-mail client, such as Outlook (Windows) or Mail (Mac OS X), you will need to enter the type of mail server your e-mail account uses. This will typically be either a POP3 or IMAP server. IMAP mail servers are a bit more complex than POP3 servers and allow e-mail messages to be read and stored on the server. Many "webmail" interfaces use IMAP mail servers so that users can manage all their mail online. Still, most mail servers use the POP3 mail protocol because it is simple and well- supported. You may have to check with your ISP or whoever manages your mail account to find out what settings to use for configuring your mail program. If your e- mail account is on a POP3 mail server, you will need to enter the correct POP3 server address in your e-mail program settings. Typically, this is something like "mail.servemame.com" or "pop.servername.com." Of course, to successfully retrieve your mail, you will have to enter a valid username and password too.

POP, or Post Office Protocol, is a way of retrieving email information that dates back to a very different Internet than we use today. Computers only had limited, low bandwidth access to remote computers, so engineers created POP in an effort to create a dead simple way to download copies of emails for offline reading, then remove those mails from the remote server.

Mail Server Functionality

POP3 has become increasingly sophisticated so that some administrators can configure the protocol to "store" email on the server for a certain period of time, which would allow an individual to download it as many times as they wished within that given time frame. However, this method is not practical for the vast majority of email recipients. While mail servers can use alternate protocol retrieval programs, such as IMAP, POP3 is extremely common among most mail servers because of its simplicity and high rate of success. Although the newer version of POP offers more "features," at its basic level, POP3 is preferred because it does the job with a minimum of errors.

Working With Email Applications

Because POP3 is a basic method of storing and retrieving email, it can work with virtually any email program, as long as the email program is configured to host the protocol. Many popular email programs, including Eudora and Microsoft Outlook, are automatically designed to work with POP3. Each POP3 mail server has a different address, which is usually provided to an individual by their web hosting company.

This address must be entered into the email program in order for the program to connect effectively with the protocol. Generally, most email applications use the 110 port to connect to POP3. Those individuals who are configuring their email program to receive POP3 email will also need to input their username and password in order to successfully receive email. POP3 and IMAP are two different protocols (methods) used to access email. Both of the two, IMAP is the better option - and the recommended option - when you need to check your emails from multiple devices, such as a work laptop, a home computer, or a tablet, smart phone, or other mobile device. Tap into your synced (updated) account from any device with IMAP.

POP3 downloads email from a server to a single computer, then deletes it from the server. Because your messages get downloaded to a single computer or device and then deleted from the server, it can appear that mail is missing or disappearing from your Inbox if you try to check your mail from a different computer. Here are the differences between POP3 and IMAP.

Q10) Explain web mail and its importance?

A10)

Webmail

Webmail is a way of sending and receiving emails from a web browser, instead of from an email client. All email travels over the internet, and is stored on servers. Those servers can belong to email providers (like Gmail), internet service providers, or web hosting providers. This server is where email is collected and stored, until you delete it. To access your webmail provider's server, you connect to the internet, and log in to a site that connects to your email account. When you use webmail, you are directly accessing the email, from your provider's server. This lets you send and receive mail from anywhere in the world, from any device, as long as you have an internet- connected web browser. Your mail always remains on your provider's server, so if you do not have an internet connection, or the provider's servers are down, you will not be able to access your email using webmail. Also, webmail interfaces may not provide as much functionality as a more robust email client. Webmail accounts don't require you to install or use an email application software such as Microsoft Outlook on your computer. You can read, send, reply, forward, organize your email into folders and save attachments. The webmail service stores of all of your email on their computers and storage systems, and gives you a web page to use for accessing your email account. You login to your webmail account from the webmail service's web page using your email address and password. You might already have a webmail account, and common webmail providers include Google's Gmail, AOL Mail, Microsoft's Hotmail/Live/Outlook.com, Yahoo! Mail, and Apple's iCloud email. Many of the webmail services that are offered on the Internet are free but can include ads that display on the screen. An upgrade to a paid email plans will remove the ads. Your Internet service provider may also provide you with access to webmail when you subscribe to their service so your existing email account may already be webmail capable. You might have to check with them to see if your account can be used as a webmail account, or, you may be able to change your account to a webmail account and keep the same email address. One of the biggest advantages of webmail is that you can access it from anywhere with almost any device that can connect to the Internet. You don't need to carry your laptop with you just to check your email. Instead, you could check your webmail from your smart phone or even from an Internet cafe or public library. All your emails are stored on the providers' servers until you choose to delete them. For those who do not regularly check their email on the same computer, do not have a computer or who frequently travel, webmail is a good option to access their email on public computers. Most webmail systems so you only need you to go to the provider's website, and then enter your username and password to access your account. Often webmail accounts are referred to as IMAP accounts, and while most webmail accounts use IMAP, not all IMAP accounts can be used with a Web browser. IMAP stands for Internet Message Access Protocol, which is a type of email account service. Many webmail accounts use this type of email protocol, which keeps a master copy of all email on the email service's computers. Webmail is the ability to access any email account using a Web browser, regardless of the type of software running the email service. While webmail works with a Web browser, it's also possible to setup your email account with a smart phone using an app or on a computer using a local application. Why would you do this you might ask since the point of webmail is to use it on the Web, right?

Sometimes the apps and applications included with smart phones and computers are designed to make using email easier and faster, and sometimes they add additional capabilities. For example, the Mail app provided on Apple iPhones, iPads, and the Mail app provided on Android phones are all designed to work with touch gestures.

While you could use your webmail account using the Web browser on your device, you would need to navigate around the screen using mouse clicks and keyboard commands designed for when you use a desktop or laptop. When you use your email account with the built-in apps on a smart phone or tablet, your email experience is optimized for a touch-screen device. Whether it's a Microsoft Windows model or an Apple Mac, the built-in email application provides additional features and functions, and the built-in email applications usually work more like other applications on the computer, which can make them easier to use for many people. For both smart phones and computers, using the built-in email application on the device with your webmail account lets you store a local copy of your email, so you can read and write emails while you're not connected to the Internet. When you connect to the Internet again, your new email messages will arrive in your Inbox, and your pending messages will be sent to their recipient. One other advantage of using webmail with more than one device is that your email and folders stay up to date on all of your devices since a master copy is kept by the webmail service. Each device checks in and sends and receives updates whenever the device is connected to the Internet.

Q11) Write short note on FTP?

A11)

File Transfer Protocol (FTP)

The main purpose of computer networks is data and resource sharing.

File transfer is a very common operation on the computer network.

We require two types of protocols for transferring the files on the network: (1) FTP and (2) TFTP

FTP is a standard mechanism provided by TCP/IP for copying a file from one host to another. There are some problems associated with file transfer mechanism from one machine to another, which can be as follows: Two systems may have different ways to represent text and data. These systems may have different directory structure. It is necessary for the FTP to solve all such sort of problems for transferring a file. For transferring a file, FTP establishes two connections between the hosts. One connection is used for data transfer, and other for control information (commands and responses). Separation of commands and data transfer makes FTP more efficient. Data connection uses very complex rules for transmission of data (due to variety of data types transferred); on the other hand, control connection uses very simple rules of communication.

Points to be Remembered

FTP uses the service of TCP. It needs two TCP connections. Port 21 is used for control connection. Port 20 is used for data connection.

Following explains the basic architecture of FTP. The server has two major components and the client has two major components as given.

The control connection is made between the control processes at server and client side while the data connection is made between the data transfer processes. One more thing that is very important about the control and data connection is that, the control connection remains open during the entire FTP interactive session, while the data connection is opened when the user wants to transmit a file and then it is closed after the file transfer. In short, the data connection is opened and closed for each file transferred.

Fig. FTP

Q12) What is DHCP packet?

A12)

Q13) Describe DHCP header in detail?

A13)

Operation Code: This 8 bit field defines the type of BOOTP packet: 1) Request or 2) Reply.

Hardware Type: This 8 bit field defines the type of physical network. For Ethernet the value is 1.

Hardware Length: This 8 bit field defines the length of physical address in bytes. For example, for Ethernet the value is 6.

Hop Count: This 8 bit field defines the maximum number of hops the packet can travel.

Transaction ID: This is a 4 byte field carrying an integer. This is set by the client and used to match reply with the request.

Number of Seconds: This is a 16 bit field that indicates the number of seconds elapsed since the time the client started to boot.

Flag: Server uses this field to specify the client that it is a forced broadcast reply. For unicast reply to the client, the destination IP address of the IP packet is the address assigned to the client. If the client does not know its IP address, it discards the packet. But if the IP datagram is broadcast, every host will receive and process the broadcast message.

Client IP Address: This is a 4 byte field that contains the client IP address. If client does not have this information, this field has a value of 0.

Your IP Address: This is a 4 byte field that contains the client IP address. It is filled by the server at the request of the client.

Server IP Address: This is a 4 byte field that contains the server IP address. It is filled by the server in a reply message.

Gateway IP Address: This is a 4 byte field that contains the router IP address. It is filled by the server in a reply message.

Client Hardware Address: This is the physical address of the client.

Server Name: This is an optional 64 byte field, filled by the server in reply packet.

Boot Filename: This is an optional 128 byte field that can be filled by the server in a reply packet. It contains the full pathname of the boot file. The client can use this path to retrieve other booting information.

Options: Several options have been added to the list of options.

Q14) Describe SNMP?

A14)

SNMP stands for simple network management protocol. It is a way that servers can share information about their current state, and also a channel through which an administer can modify pre-defined values. While the protocol itself is very simple, the structure of programs that implement SNMP can be very complex. In this guide, we will introduce you to the basics of the SNMP protocol. We will go over its uses, the way that the protocol is typically used in a network, the differences in its protocol versions, and more.

Basic Concepts

SNMP is a protocol that is implemented on the application layer of the networking stack (click here to learn about networking layers). The protocol was created as a way of gathering information from very different systems in a consistent manner. Although it can be used in connection to a diverse array of systems, the method of querying information and the paths to the relevant information are standardized.

There are multiple versions of the SNMP protocol, and many networked hardware devices implement some form of SNMP access. The most widely used version is SNMP, but it is in many ways insecure. Its popularity largely stems from its ubiquity and long time in the wild. Unless you have a strong reason not to, we recommend you use SNMPv3, which provides more advanced security features.

In general, a network being profiled by SNMP will mainly consist of devices containing SNMP agents. An agent is a program that can gather information about a piece of hardware, organize it into predefined entries, and respond to queries using the SNMP protocol.

The component of this model that queries agents for information is called an SNMP manager. These machines generally have data about all of the SNMP-enabled devices in their network and can issue requests to gather information and set certain properties.

SNMP Managers

An SNMP manager is a computer that is configured to poll SNMP agent for information. The management component, when only discussing its core functionality, is actually a lot less complex than the client configuration, because the management component simply requests data. The manager can be any machine that can send query requests to SNMP agents with the correct credentials. Sometimes, this is implemented as part of a monitoring suite, while other times this is an administrator using some simple utilities to craft a quick request.

Almost all of the commands defined in the SNMP protocol (we will go over these in detail later) are designed to be sent by a manager component. These include GetRequest, GetNextRequest, GetBulkRequest, SetRequest, InfonnRequest, and Response. In addition to these, a manager is also designed to respond to Trap, and Response messages.

SNMP Agents

SNMP agents do the bulk of the work. They are responsible for gathering infoπnation about the local system and storing them in a format that can be queried. Updating a database called the "management information base", or MIB. The MIB is a hierarchical, pre-defined structure that stores information that can be queried or set. This is available to well-formed SNMP requests originating from a host that has authenticated with the correct credentials (an SNMP manager).

The agent computer configures which managers should have access to its information. It can also act as an intermediary to report information on devices it can connect to that are not configured for SNMP traffic. This provides a lot of flexibility in getting your components online and SNMP accessible.

SNMP agents respond to most of the commands defined by the protocol. These include GetRequest, GetNextRequest, GetBulkRequest, SetRequest and InfonnRequest. In addition, an agent is designed to send Trap messages.

Q15) Write various FTP commands?

A15)

Q16) Difference between FTP and TFTP?

A16)

Comparison between FTP and TFTP

Q17) Write short note on TELNET?

A17)

Where the TCP protocol makes it possible to connect the remote computers, the TELNET protocol makes it possible to use them. The TELNET protocol offers a user the possibility to connect and log on to any other hosts in the network from user’s own computer by offering a remote log on capability. Historically, TELNET was the first TCP/IP application and still is widely used as a terminal emulator. Today, while the applications are more and more equipped with the graphical user interface, the terminal-based applications are becoming minority among the applications. The TELNET has found its future as a tool kit lying below several client/server software. For example: FTP, SMTP, SNMP, NNTP and HTTP are more or less dependent on the TELNET protocol.

Q18) Write TELNET commands?

A18)

TELNET Commands

Q19) Write short note on DHCP?

A19)

BOOTP is not a dynamic configuration protocol.

When client requests its IP address, BOOTP server consults a table that matches the physical address of the client with its IP address.

This implies that the binding between the IP address and the physical address of the client already exists. The binding is predetermined.

As BOOTP is static configuration protocol, it cannot assign temporary IP address to the host; also it cannot handle the situation when the host moves from one physical network to the other. To remove the limitations of BOOTP, DHCP protocol comes into existence, where DHCP provides static and dynamic address allocation that can be manual or automatic.

DHCP is backward compatible with BOOTP, which means a host running the BOOTP client can request a static address from a DHCP server.

Q20) Explain various fields Of DHCP packet?

A20)

The fields are described below:

Operation Code: This 8 bit field defines the type of BOOTP packet: 1) Request or 2) Reply.

Hardware Type: This 8 bit field defines the type of physical network. For Ethernet the value is 1.

Hardware Length: This 8 bit field defines the length of physical address in bytes. For example, for Ethernet the value is 6.

Hop Count: This 8 bit field defines the maximum number of hops the packet can travel.

Transaction ID: This is a 4 byte field carrying an integer. This is set by the client and used to match reply with the request.

Number of Seconds: This is a 16 bit field that indicates the number of seconds elapsed since the time the client started to boot.

Flag: Server uses this field to specify the client that it is a forced broadcast reply. For unicast reply to the client, the destination IP address of the IP packet is the address assigned to the client. If the client does not know its IP address, it discards the packet. But if the IP datagram is broadcast, every host will receive and process the broadcast message.

Client IP Address: This is a 4 byte field that contains the client IP address. If client does not have this information, this field has a value of 0.

Your IP Address: This is a 4 byte field that contains the client IP address. It is filled by the server at the request of the client.

Server IP Address: This is a 4 byte field that contains the server IP address. It is filled by the server in a reply message.

Gateway IP Address: This is a 4 byte field that contains the router IP address. It is filled by the server in a reply message.

Client Hardware Address: This is the physical address of the client.

Server Name: This is an optional 64 byte field, filled by the server in reply packet.

Boot Filename: This is an optional 128 byte field that can be filled by the server in a reply packet. It contains the full pathname of the boot file. The client can use this path to retrieve other booting information.

Options: Several options have been added to the list of options. The option field in DHCP can be upto 312 bytes.