15.3 Network Applications 15.3.1 Internet applications Instructor Note The purpose of this target indicator is to make a subtle distinction between computer applications particularly Internet applications -- and OSI application layer protocols. Computer applications -- Eudora for email, Netscape Navigator and Internet Explorer as browsers -- are classified "above" the 7 layers of the OSI model. It is only when such programs make network requests and require network services that they are dealing with OSI layer 7 protocols such as POP3, DNS, http, and ftp. Not everything fits within the OSI model. You select network applications based on the type of work you need to accomplish. A complete set of application layer programs is available to interface with the Internet. Each application program type is associated with its own application protocol. Although there are more programs and protocol types available, the following are the main focus of this chapter: The World Wide Web uses the HTTP protocol. Remote access programs use the Telnet protocol for directly connecting to remote resources. E-mail programs support the POP3 application layer protocol for electronic mail. File utility programs use the FTP protocol for copying and moving files between remote sites. Network data gathering and monitoring use the SNMP protocol. It is important to re-emphasize the fact that the application layer is just another protocol layer in the OSI or TCP/IP models. The programs interface with the application layer protocols.E-mail client applications (i.e. Eudora, Microsoft Mail, Pegasus, and Netscape Mail) work with the POP3 protocol. The same is true with Web browsers. The two most popular Web browsers are Microsoft Internet Explorer and Netscape Communicator. The appearance and operation of these two programs is very different, but they both work with the application layer HTTP protocol. 15.3 Network Applications 15.3.2 E-mail message Instructor Note This target indicator introduces the beginning of an e-mail process by comparing the e-mail server to a post office. Again, the postal analogy is useful at many layers within the OSI model. The purpose of this target indicator is to have the students do a simple activity they probably done many times -- send an email -- but to begin thinking about it in much greater detail. Electronic mail (e-mail) enables you to send messages between connected computers. The procedure for sending an e-mail document involves two separate processes. The first is to send the e-mail to the users post office, and the second is to deliver the e-mail from that post office to the users e-mail client (i.e. the recipient). The following steps will help you understand the process of sending an e-mail:Start your e-mail program. Type in a recipient's e-mail address. Type in a subject. Type a letter. Now, examine the e-mail address. This is an example of what it may look like: JJones@bigsky.com. It consists of two parts: the recipients name (located before the @ sign); and the recipients post office address (after the @ sign). The recipients name is only important after the message arrives at the post office address, which is a DNS entry that represents the IP address of the post office server. Web Links What is Email? Email Basics and Asynchronous Communication 15.3 Network Applications 15.3.3 DNS function Instructor Note The purpose of this target indicator is to relate email to a prior topic, DNS. DNS assists in the mail delivery process. Just as an e-mail message begins at a e-mail server (post office), so too much it end a an e-mail server (another post office). Then local delivery of the e-mail can be performed.Again, the common process of entering a password to retrieve your email is explained.E-mail is related to a prior topic, encapsulation, as it is a key theme of the entire semester and of the OSI model. Sending and receiving an e-mail involve encapsulation at all seven layers of the OSI model. This is an excellent opportunity to review the entire OSI model and what functions each layer performs.This target indicator describes in detail what a mail server does.This target indicator describes the file formats typically used with email. This is an excellent opportunity to review layer 6 topics. Whenever e-mail clients send letters, they request that a DNS connected to the network translate the domain names into their associated IP addresses. If the DNS is able to translate the names, it returns the IP addresses to the clients, thus enabling proper transport layer segmentation and encapsulation. If the DNS cannot translate the names, the requests are passed on until the names can be translated. The part of the e-mail address that contains the recipient's name becomes important at this point. The server extracts it from the e-mail message and checks to see if the he/she is a member of its post office. If the recipient is a member, it stores the message in his/her mailbox until someone retrieves it. If the recipient is not a member, the post office generates an error message and sends the e-mail back to the sender. The second part of the e-mailing process is the receiving process. E-mail message recipients must use the e-mail client software on their computers to establish requests to the e-mail post offices. When message recipients click the "Get Mail" or "Retrieve Mail" buttons on the e-mail client, they are usually prompted for a password. After they enter the password and click "OK", the e-mail software builds a request for the post office servers, which then extract the post office addresses from the configuration data that was entered when their e-mail software was configured. The process then uses another DNS search to find the IP addresses of the servers. Finally, the requests are segmented and sequenced by the transport layer.Data packets travel through the rest of the OSI model layers (i.e. network, data link, physical) and are then transmitted across the Internet to the destination e-mail post office. At this post office the packets are reassembled, in the proper sequence, and are checked for any data transmission errors. At the post office, requests are examined, and user names and passwords are verified. If everything is correct, the post office server transmits all e-mail messages to computers, where the messages are, again, segmented, sequenced, and encapsulated as data frames, to be sent to the client's or the e-mail recipient's computer. After e-mail messages arrive at a computer, you may open them and read them. If you click on the "Reply", or the "Forward" button, to send a response to a messages, the whole process starts over again. E-mail messages, themselves, are normally sent as ASCII text, but the attachments that are sometimes included with them, can be audio, video, graphic, or many other types of data. To correctly send and receive attachments, the encoding schemes must be the same on both the sending and the receiving computer. The two most common formats for e-mail attachments are the Multipurpose Internet Mail Extension (MIME) and UUencode (a Unix utility program).
