DNS, Email, FTP, TFTP
2. The resolver sends a UDP packet to the nearest DNS server (called as the local DNS server)
5.4 TRIVIAL FILE TRANSFER PROTOCOL (TFTP)
The Trivial File Transfer Protocol (TFTP) is a protocol used for transferring files between two computers, similar to what FTP is used for. However, TFTP is different from FTP is one major respect. Whereas FTP uses the reliable TCP as the underlying transport layer protocol, TFTP uses the unreliable UDP protocol for data transport. Other minor differences between FTP and TFTP are that while FTP allows changing directory of the remote computer or to obtain a list of files in the directory of the remote computer, TFTP does not allow this.
Also, there is no interactivity in TFTP. It is a protocol designed for purely transferring files.
There are situations where we need to simply copy a file without needing to use the sophisticated features provided by FTP. In such situations, TFTP is used. For example, when a diskless workstation or a router is booted, it is required to download the bootstrap and configuration files to that workstation or router. The device (workstation or router) in such a case simply needs to have the TFTP, UDP and IP software hard coded into its Read Only Memory (ROM). After receiving power, the device executes the code in ROM, which broadcasts a TFTP request across the network. A TFTP server on that network then sends the necessary files to
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the device, so that it can boot. Not much of error checking and authentication is required here. TFTP is a suitable candidate for such situations.
TFTP does not allow for user authentication unlike FTP. Therefore, TFTP must not be used on computers where sensitive/confidential information is stored.
TFTP transfers data in fixed-size blocks of 512 bytes each. The recipient must acknowledge each such data block before the sender sends the next block. Thus, the sender sends data packets in the form of blocks and expects acknowledgement packets, whereas the recipient receives data blocks and sends acknowledgement packets. Either of them must time out and retransmit if the expected data block or acknowledgement, as appropriate, does not arrive. Also, unlike FTP, there is no provision for resuming an aborted file transfer from its last point.
SUMMARY
l Computers work best with numbers but humans do not. Humans prefer names.
l Every computer on the Internet has a unique IP address. Since the IP addresses used on the Internet to identify computers are in the numerical form, it would be very difficult for humans to remember the IP addresses of even a few computers.
l This difference between the perceptions of humans and computers is resolved by assigning names to computers. Thus, the name of a computer is mapped to its IP address.
l The name given to a group of computers—that is, to a computer network, is called as its domain name.
l The Domain Name System (DNS) is a distributed database that contains the mappings between domain names and IP addresses.
l A client computer contacts its nearest DNS server to find out the IP address of the computer with which it wants to communicate. The DNS server consults its database to find a match. If it does not find a match, it relays the query on to another DNS server, which might relay it to a third DNS server, and so on, until either a match is found or it is detected that the domain name specified by the user is invalid.
l Electronic mail (email) was created to allow two individuals to communicate using computers.
l Email combines the best features of a telephone call (immediate delivery) and a letter delivered via post/courier (an immediate response is not compulsory, but is always possible).
l The underlying transport mechanism for email messages, like all Internet communications, is TCP/IP.
l An email mailbox is just a storage area on the disk of the email server computer. This area is used for storing received emails on behalf of the users similar to the way a postal mailbox stores postal mails.
l Simple Mail Transfer Protocol (SMTP) is actually responsible for transmitting an email message between the sender and the recipient.
l The Post Office Protocol (POP) is concerned with the retrieval of an email message stored on a server computer.
l Multipurpose Internet Mail Extensions (MIME) allow an email to not only contain text data, but also any binary file such as an image, audio, video, documents in different formats, etc.
l Two email privacy standards are available, Pretty Good Privacy (PGM) and Privacy Enhanced Mail (PEM).
l The File Transfer Protocol (FTP) is used to transfer files between the two computers.
l FTP presents the user with a prompt and allows entering various commands for accessing and downloading files that physically exist on a remote computer.
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l Unlike other applications at the application layer, FTP opens two connections between the client and the server computers. One connection (called as data transfer connection) is used for the actual file transfer, while the other connection (called as control connection) is used for exchanging control information.
l A simpler version of FTP, called as Trivial File Transfer Protocol (TFTP), also exists. Unlike in FTP, TFTP does not perform any validations or error control.
REVIEW QUESTIONS
Multiple-choice Questions
1. The hosts.txt file used to contain and .
(a) IP address, physical address (b) IP address, domain name (c) Domain name, IP address (d) Domain name, physical address 2. The com domain name refers to .
(a) common (b) commercial (c) computer (d) none of the above
3. is a storage area to store emails.
(a) Database (b) File (c) Mailbox (d) Server
4. The symbol is used to connect the user name and the domain name portions of an email id.
(a) & (b) @ (c) * (d) $
5. protocol is used to retrieve emails from a remote server.
(a) POP (b) IP (c) POP (d) SMTP
6. protocol is used for transferring mails over the Internet.
(a) POP (b) IP (c) POP (d) SMTP
7. allows non-text data to be sent along with an email message.
(a) PGP (b) MIME (c) PEM (d) MTA
8. For transferring big files over the Internet, the protocol is used.
(a) SMTP (b) POP (c) HTTP (d) FTP
9. uses TCP as the transport protocol.
(a) FTP (b) TFTP (c) all of the above (d) none of the above
10. FTP uses the control connection for transferring .
(a) data (b) control information
(c) data and control information (d) all of the above
Detailed Questions
1. What is the need for additional suffixes such as com, edu and gov?
2. What is DNS? Why is it required?
3. Explain the significance of a DNS server.
4. What is the purpose of an email server?
5. Why is email client required?
6. Discuss email architecture in brief along with its main components.
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7. Discuss SMTP.
8. What is the purpose of FTP?
9. Discuss the FTP connection mechanism between the client and the server.
10. What are the specific purposes of the control connection?
Exercises
1. Find out how the DNS server is configured in your organization. Find out how much money is required to register a new domain name, and what the procedure is for the same.
2. Try sending an attachment along with an email message through an email client like Outlook Express, and try the same using a Web-based email service (such as Yahoo or Hotmail). Note the differences.
3. Normally, when you download a file, HTTP and not FTP is used. Try downloading a file by using HTTP and then try the same using FTP. What are the differences according to you?
4. Investigate how you can send emails through programming languages/tools such as ASP.NET, JSP and Servlets.
5. Try to find out more information about the SMTP, POP, IMAP servers used in your organization, college or university.
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