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<HTML><HEAD><TITLE>Chapter 5 -- How Intranet E-Mail Works</TITLE><META></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910"><H1><FONT SIZE=6 COLOR=#FF0000>Chapter 5</FONT></H1><H1><FONT SIZE=6 COLOR=#FF0000>How Intranet E-Mail Works</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#HowEMailIsDeliveredwithinanIntranet">How E-Mail Is Delivered within an Intranet</A><LI><A HREF="#HowEMailisDeliveredamongIntranets">How E-Mail is Delivered among Intranets</A></UL><HR><P>Intranets allow people to work together better by allowing themto communicate better. The most time-honored-and still the mostpopular-means of communication using computers is e-mail. Usinge-mail, people can send messages to anyone else on the intranet-and,in fact, to anyone connected to the Internet as well, or connectedto a computer network that has a connection to the Internet, suchas an online service.<P>Intranet e-mail uses the TCP/IP protocol. The TCP protocol breaksyour messages into packets, the IP protocol delivers the packetsto the proper location, and then TCP reassembles the message onthe receiving end so that it can be read.<P>You can also attach binary files, such as pictures, videos, sound,and executable files, to your e-mail messages. Since the Internetcan't directly handle binary files in e-mail, the file must firstbe <I>encoded</I> in one of a variety of encoding schemes. Popularschemes are Base64 and uuencode. The person who receives the attachedbinary file must <I>decode</I> the file with the same scheme thatwas used to encode the file. Many e-mail software packages dothis automatically.<P>When e-mail formats differ (and they often do, having developedfrom proprietary and incompatible products), <I>gateways</I> areused to translate the data into the appropriate format for therecipient. However, several standards have been developed thathave been adopted by most e-mail companies today that allow thevarious products to communicate with each other. The Simple MailTransfer Protocol (SMTP) and the X.400 MHS (message handling service)are the two most common protocols.<P>SMTP is based on a client/server model in which someone uses amail client to create mail and read mail, while servers do theactual processing and delivery of the mail.<P>To create e-mail, you use a mail client, which is called a <I>mailuser agent</I> or <I>user agent</I> (MUA or UA). There are multiplekinds and different agents for different types of computers. Whenmail is sent, the Message Transfer System (MTS) on a server usesa <I>mail transfer agent </I>(MTA) to examine the address of theperson to which the mail is being sent. If the person can be foundon the intranet, the mail is delivered to a <I>mail delivery agent</I>(MDA). The MDA then delivers the mail to the intended recipient.When you send e-mail to someone on another intranet or on theInternet, the message is instead sent by the MTA through the Internet.The message often has to travel through a series of networks beforeit reaches the recipient-networks that might use different e-mailformats. <P>Gateways are not attached to one particular machine or one combinationof hardware and software, nor are they restricted to e-mail processingfunctionality. They can fulfill a variety of roles in additionto protocol conversion. One example of this is actually translatingdata from one format to another, as they do for PC to mainframeconnectivity. Gateways differ from linking hardware such as bridgesand routers by operating at higher OSI levels, although routers,because of their protocol conversion functionality, are also commonlycalled gateways. <H2><A NAME="HowEMailIsDeliveredwithinanIntranet"><FONT SIZE=5 COLOR=#FF0000>How E-Mail Is Delivered within an Intranet</FONT></A></H2><P>Probably the most heavily used part of an intranet has nothingto do with corporate databases, flashy Web pages, or multimediacontent-it's the use of electronic mail. Corporate intranets canuse a number of different e-mail programs, such as cc:Mail, MicrosoftMail, or Lotus Notes, among others. But the most common architectureunderlying the use of intranet e-mail is the protocol called theSimple Mail Transfer Protocol, or SMTP. Illustrated here is howSMTP is used to deliver mail within an intranet.<OL><LI>As is true with many intranet and Internet applications, SMTPuses client/server architecture. When someone wants to createa mail message, they use a mail user agent or user agent (MUAor UA)-client software that runs on a computer-to create a pieceof electronic mail. This MUA can be one of any number of e-mailprograms, and can run on a variety of different computers, includingPCs, Macintoshes, and UNIX workstations. Popular MUAs includePine for UNIX computers; Pegasus, Eudora, cc:Mail, and MicrosoftMail for the PC; and Eudora for the Macintosh.<LI>After the message is completed, it is sent by the MUA to aprogram running on a server called a <I>mail transfer agent</I>(MTA). The MTA looks at the address of the intended recipientof the message. If the recipient of the message is on the intranet,the MTA sends the message to another server program on the intranetcalled a <I>mail delivery agent </I>(MDA). If, instead, the recipientis located on the Internet or another intranet, the file is sentover the Internet to the recipient. (See the next illustrationon how mail is transferred among intranets.) The MDA looks atthe address of the recipient, and sends the mail to the in boxof the proper person.<LI>Some mail systems use another e-mail protocol called the PostOffice Protocol (POP) in concert with SMTP. With POP, e-mail isnot delivered directly to your computer. Instead, the mail isdelivered to a mailbox on a server. To get the mail, someone logsonto the server using a password and user name, and retrievesmail with their mail agent. <LI>The recipient of the mail can now use a mail user agent toread the mail, file it, and respond to it.<LI>SMTP can only handle the e-mail transfer of plain ASCII textfiles. In order to send binary files such as spreadsheets, pictures,and word processing documents, they must first be converted intoan ASCII format by encoding them. The files can be encoded usinga variety of methods, including uuencoding and Base64. Some e-mailsoftware will automatically encode binary files. When an encodedfile is received by someone, they decode it and then can use orview the binary file. Again, many e-mail packages automaticallydecode encoded files.</OL><H2><A NAME="HowEMailisDeliveredamongIntranets"><FONT SIZE=5 COLOR=#FF0000>How E-Mail is Delivered among Intranets</FONT></A></H2><P>Often, e-mail created on an intranet will not be delivered toa computer on the intranet, but instead to someone on the Internet,to another intranet, or to an online service such as America Online,the Microsoft Network, or CompuServe. Here are the steps a typicalmessage might take when being delivered from an intranet to anothernetwork or intranet.<OL><LI>An e-mail message is created using SMTP, as described on theprevious page. As with all information sent across the Internet,the message is broken up by the Internet's TCP protocol into IPpackets. The address is examined by the intranet's mail transferagent. If the address is found on another network, the mail transferagent will send the mail across the intranet via routers to themail transfer agent on the receiving network.<LI>Before the mail can be sent out across the Internet, it firstmight have to pass through an intranet <I>firewall</I>-a computerthat shields the intranet so that intruders cannot break intothe network. The firewall keeps track of messages and data goinginto and out of the intranet. It keeps a record of traffic sothat any security breaches can be tracked down.<LI>The message leaves the intranet and is sent to an Internetrouter. The router examines the address and determines where themessage should be sent, and then sends the message on the way.<LI>The receiving network gets the e-mail message. A <I>gateway</I>there uses TCP to reconstruct the IP packets into a full message.The gateway then translates the message into the particular protocolthe target network uses (such as CompuServe's mail format), andsends it on its way. The message may be required to also passthrough a firewall on the receiving network.<LI>The receiving network examines the e-mail address and sendsthe message to the specific mailbox where the message was intendedto go, or uses the Post Office Protocol (POP) to deliver it toa mail server.<LI>Gateways can actually change data (if needed) for connectivity.For e-mail it may convert CompuServe protocol to SMTP. Gatewaysare also used to connect PC's to IBM mainframes for example, ASCIIto EBCDIC.</OL><HR><CENTER><P><A HREF="ch4.htm"><IMG SRC="PC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="#CONTENTS"><IMG SRC="CC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="contents.htm"><IMG SRC="HB.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><A HREF="ch6.htm"><IMG SRC="NC.GIF" BORDER=0 HEIGHT=88 WIDTH=140></A><HR WIDTH="100%"></P></CENTER></BODY></HTML>?? 快捷鍵說明
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