Free Windows Help Tutorials sponsored by inkjet printer cartridges dot org /title> <META HTTP-EQUIV="imagetoolbar" CONTENT="no"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; CHARSET=ISO 8859-1"> <META name="description" Content="Hundreds of downloadable Windows help tutorials, articles and how-to guides sponsored by inkjet printer cartridges dot org."><style type="text/css">  </style> </head> <body bgcolor=#999999 vlink=blue link=blue> <span name = "INTRA" class="pg" style="top:0px; left:5px; width:714px; height:1010px"></span> <span name = "INTRA" style='top:0px; left:5px; height:1010px; width:714px' class='img'> <img src="pg0001im.png" height = "1010 width = "714" border="0"> </span> <span class='nav' style='left: 10px; top: 10px'><pre></pre></span><span name = "INTRA" style='top:51px; left:570px' class="ft5"><pre><font face='Helvetica'>Connectivity:Overview</font></pre></span> <span name = "INTRA" style='top:107px; left:100px' class="ft4"><pre><font face='Arial'><b>Understanding</b> <b>TCP/IP</b></font></pre></span> <span name = "INTRA" style='top:213px; left:147px' class="ft2"><pre><font face='Arial'><i><b>We</b></i> <i><b>conclude</b></i> <i><b>our</b></i> <i><b>four part</b></i> <i><b>article</b></i> <i><b>looking</b></i> <i><b>in</b></i> <i><b>depth</b></i> <i><b>at</b></i> <i><b>the</b></i> <i><b>TCP/IP</b></i> <i><b>protocol.</b></i></font></pre></span> <span name = "INTRA" style='top:234px; left:135px' class="ft2"><pre><font face='Arial'><i><b>Here,</b></i> <i><b>we</b></i> <i><b>examine</b></i> <i><b>the</b></i> <i><b>difference</b></i> <i><b>between</b></i> <i><b>the</b></i> <i><b>SMTP</b></i> <i><b>and</b></i> <i><b>POP3</b></i> <i><b>email</b></i> <i><b>protocols.</b></i></font></pre></span> <span name = "INTRA" style='top:266px; left:576px' class="ft1"><pre><font face='Arial'>By Julian Moss</font></pre></span> <span name = "INTRA" style='top:321px; left:77px' class="ft0"><pre><font face='Arial'><b>I</b></font></pre></span> <span name = "INTRA" style='top:325px; left:96px' class="ft1"><pre><font face='Arial'>n this series of articles we have</font></pre></span> <span name = "INTRA" style='top:325px; left:272px' class="ft1"><pre><font face='Arial'>serves a group of users such as all those </font></pre></span> <span name = "INTRA" style='top:325px; left:466px' class="ft1"><pre><font face='Arial'>messages would make this impractical</font></pre></span> <span name = "INTRA" style='top:337px; left:96px' class="ft1"><pre><font face='Arial'>looked at the TCP/IP suite of pro </font></pre></span> <span name = "INTRA" style='top:337px; left:272px' class="ft1"><pre><font face='Arial'>int one domain. The server receives all</font></pre></span> <span name = "INTRA" style='top:337px; left:466px' class="ft1"><pre><font face='Arial'> so this is about the only way ISPs can</font></pre></span> <span name = "INTRA" style='top:349px; left:96px' class="ft1"><pre><font face='Arial'>tocols, beginning with the link</font></pre></span> <span name = "INTRA" style='top:349px; left:272px' class="ft1"><pre><font face='Arial'>mail intended for its users and then</font></pre></span> <span name = "INTRA" style='top:349px; left:466px' class="ft1"><pre><font face='Arial'>prevent non subscribers such as spam </font></pre></span> <span name = "INTRA" style='top:361px; left:77px' class="ft1"><pre><font face='Arial'>layer and progressing by stages to the</font></pre></span> <span name = "INTRA" style='top:361px; left:272px' class="ft1"><pre><font face='Arial'>allows them to collect it using POP3</font></pre></span> <span name = "INTRA" style='top:361px; left:466px' class="ft1"><pre><font face='Arial'>mers from using their mail servers to</font></pre></span> <span name = "INTRA" style='top:373px; left:77px' class="ft1"><pre><font face='Arial'>application layer. We have seen how</font></pre></span> <span name = "INTRA" style='top:373px; left:272px' class="ft1"><pre><font face='Arial'>(Post Office Protocol version 3) or some</font></pre></span> <span name = "INTRA" style='top:373px; left:466px' class="ft1"><pre><font face='Arial'>send out messages.</font></pre></span> <span name = "INTRA" style='top:385px; left:77px' class="ft1"><pre><font face='Arial'>each layer relies upon the layers below</font></pre></span> <span name = "INTRA" style='top:385px; left:272px' class="ft1"><pre><font face='Arial'>other mail protocol. Similarly, most</font></pre></span> <span name = "INTRA" style='top:385px; left:480px' class="ft1"><pre><font face='Arial'>Having received the correct ac </font></pre></span> <span name = "INTRA" style='top:397px; left:77px' class="ft1"><pre><font face='Arial'>it, so that network applications can be</font></pre></span> <span name = "INTRA" style='top:397px; left:272px' class="ft1"><pre><font face='Arial'>SMTP clients send messages to a single</font></pre></span> <span name = "INTRA" style='top:397px; left:466px' class="ft1"><pre><font face='Arial'>knowledgement the sender signs on to</font></pre></span> <span name = "INTRA" style='top:409px; left:77px' class="ft1"><pre><font face='Arial'>written without needing to take ac </font></pre></span> <span name = "INTRA" style='top:409px; left:272px' class="ft1"><pre><font face='Arial'>server, whose job it is to relay those mes </font></pre></span> <span name = "INTRA" style='top:409px; left:466px' class="ft1"><pre><font face='Arial'>the server by sending the string</font></pre></span> <span name = "INTRA" style='top:421px; left:77px' class="ft1"><pre><font face='Arial'>count of considerations such as how</font></pre></span> <span name = "INTRA" style='top:421px; left:272px' class="ft1"><pre><font face='Arial'>sages on to their eventual recipients.</font></pre></span> <span name = "INTRA" style='top:421px; left:466px' class="ft1"><pre><font face='Arial'> HELO hostname . HELO is the sign </font></pre></span> <span name = "INTRA" style='top:433px; left:77px' class="ft1"><pre><font face='Arial'>the network is constructed or what</font></pre></span> <span name = "INTRA" style='top:433px; left:286px' class="ft1"><pre><font face='Arial'>An SMTP transaction begins when</font></pre></span> <span name = "INTRA" style='top:433px; left:466px' class="ft1"><pre><font face='Arial'>on command and hostname is the</font></pre></span> <span name = "INTRA" style='top:445px; left:77px' class="ft1"><pre><font face='Arial'>type of hardware or cabling is used.</font></pre></span> <span name = "INTRA" style='top:445px; left:272px' class="ft1"><pre><font face='Arial'>the sender client opens a TCP connec </font></pre></span> <span name = "INTRA" style='top:445px; left:466px' class="ft1"><pre><font face='Arial'>name of the host. As we will see, the</font></pre></span> <span name = "INTRA" style='top:457px; left:91px' class="ft1"><pre><font face='Arial'>A striking point about many of the</font></pre></span> <span name = "INTRA" style='top:457px; left:272px' class="ft1"><pre><font face='Arial'>tion with the receiver using the well </font></pre></span> <span name = "INTRA" style='top:457px; left:466px' class="ft1"><pre><font face='Arial'>hostname is used in the Received:</font></pre></span> <span name = "INTRA" style='top:469px; left:77px' class="ft1"><pre><font face='Arial'>application layer protocols is how sim </font></pre></span> <span name = "INTRA" style='top:469px; left:272px' class="ft1"><pre><font face='Arial'>known port number 25. The server</font></pre></span> <span name = "INTRA" style='top:469px; left:466px' class="ft1"><pre><font face='Arial'>header which the server adds to the</font></pre></span> <span name = "INTRA" style='top:481px; left:77px' class="ft1"><pre><font face='Arial'>ple they are. The protocols based on</font></pre></span> <span name = "INTRA" style='top:481px; left:272px' class="ft1"><pre><font face='Arial'>acknowledges the connection by send </font></pre></span> <span name = "INTRA" style='top:481px; left:466px' class="ft1"><pre><font face='Arial'>message when it sends it on its way.</font></pre></span> <span name = "INTRA" style='top:493px; left:77px' class="ft1"><pre><font face='Arial'>TCP mostly use commands and re </font></pre></span> <span name = "INTRA" style='top:493px; left:272px' class="ft1"><pre><font face='Arial'>ing back a message of the form 220</font></pre></span> <span name = "INTRA" style='top:493px; left:466px' class="ft1"><pre><font face='Arial'>This information allows the recipient</font></pre></span> <span name = "INTRA" style='top:505px; left:77px' class="ft1"><pre><font face='Arial'>sponses in plain ASCII text, making</font></pre></span> <span name = "INTRA" style='top:505px; left:272px' class="ft1"><pre><font face='Arial'>SMTP Server Ready . SMTP uses a</font></pre></span> <span name = "INTRA" style='top:505px; left:466px' class="ft1"><pre><font face='Arial'>to trace the path taken by the message.</font></pre></span> <span name = "INTRA" style='top:517px; left:77px' class="ft1"><pre><font face='Arial'>them easier for a user to understand</font></pre></span> <span name = "INTRA" style='top:517px; left:272px' class="ft1"><pre><font face='Arial'>similar format of replies to ftp, which</font></pre></span> <span name = "INTRA" style='top:529px; left:77px' class="ft1"><pre><font face='Arial'>and for a programmer to implement.</font></pre></span> <span name = "INTRA" style='top:529px; left:272px' class="ft1"><pre><font face='Arial'>we looked at previously. The three </font></pre></span> <span name = "INTRA" style='top:531px; left:466px' class="ft2"><pre><font face='Arial'><i><b>Sending</b></i></font></pre></span> <span name = "INTRA" style='top:541px; left:77px' class="ft1"><pre><font face='Arial'>For further illustration we shall look at</font></pre></span> <span name = "INTRA" style='top:541px; left:272px' class="ft1"><pre><font face='Arial'>digit code is all the client software</font></pre></span> <span name = "INTRA" style='top:553px; left:77px' class="ft1"><pre><font face='Arial'>the two protocols that you may use</font></pre></span> <span name = "INTRA" style='top:553px; left:272px' class="ft1"><pre><font face='Arial'>needs to tell if everything is going OK.</font></pre></span> <span name = "INTRA" style='top:553px; left:480px' class="ft1"><pre><font face='Arial'>Once the sender gets a 250 OK </font></pre></span> <span name = "INTRA" style='top:565px; left:77px' class="ft1"><pre><font face='Arial'>every day to send and receive Internet</font></pre></span> <span name = "INTRA" style='top:565px; left:272px' class="ft1"><pre><font face='Arial'>The text is there to help the humans</font></pre></span> <span name = "INTRA" style='top:565px; left:466px' class="ft1"><pre><font face='Arial'>acknowledgement it can start sending</font></pre></span> <span name = "INTRA" style='top:577px; left:77px' class="ft1"><pre><font face='Arial'>email: SMTP and POP3.</font></pre></span> <span name = "INTRA" style='top:577px; left:272px' class="ft1"><pre><font face='Arial'>who might be troubleshooting a prob </font></pre></span> <span name = "INTRA" style='top:577px; left:466px' class="ft1"><pre><font face='Arial'>messages. The protocol is extremely</font></pre></span> <span name = "INTRA" style='top:589px; left:272px' class="ft1"><pre><font face='Arial'>lem by analysing a log of the transac </font></pre></span> <span name = "INTRA" style='top:589px; left:466px' class="ft1"><pre><font face='Arial'>simple. All the sender has to do is say</font></pre></span> <span name = "INTRA" style='top:603px; left:77px' class="ft2"><pre><font face='Arial'><i><b>SMTP</b></i></font></pre></span> <span name = "INTRA" style='top:601px; left:272px' class="ft1"><pre><font face='Arial'>tion. The box Application Protocol</font></pre></span> <span name = "INTRA" style='top:601px; left:466px' class="ft1"><pre><font face='Arial'>who the message is from, who it is to,</font></pre></span> <span name = "INTRA" style='top:613px; left:272px' class="ft1"><pre><font face='Arial'>Reply Codes provides more informa </font></pre></span> <span name = "INTRA" style='top:613px; left:466px' class="ft1"><pre><font face='Arial'>and supply the contents of the mes </font></pre></span> <span name = "INTRA" style='top:625px; left:91px' class="ft1"><pre><font face='Arial'>Simple Mail Transfer Protocol</font></pre></span> <span name = "INTRA" style='top:625px; left:272px' class="ft1"><pre><font face='Arial'>tion about message reply codes.</font></pre></span> <span name = "INTRA" style='top:625px; left:466px' class="ft1"><pre><font face='Arial'>sage.</font></pre></span> <span name = "INTRA" style='top:638px; left:77px' class="ft1"><pre><font face='Arial'>(SMTP) is one of the most venerable of</font></pre></span> <span name = "INTRA" style='top:637px; left:286px' class="ft1"><pre><font face='Arial'>An SMTP relay server might refuse</font></pre></span> <span name = "INTRA" style='top:638px; left:480px' class="ft1"><pre><font face='Arial'>Who a message is from is specified</font></pre></span> <span name = "INTRA" style='top:650px; left:77px' class="ft1"><pre><font face='Arial'>the Internet protocols. Designed in the</font></pre></span> <span name = "INTRA" style='top:650px; left:272px' class="ft1"><pre><font face='Arial'>a connection by sending back a mes </font></pre></span> <span name = "INTRA" style='top:650px; left:466px' class="ft1"><pre><font face='Arial'>with the command MAIL FROM: <ad </font></pre></span> <span name = "INTRA" style='top:662px; left:77px' class="ft1"><pre><font face='Arial'>early 1980s, its function is purely and</font></pre></span> <span name = "INTRA" style='top:662px; left:272px' class="ft1"><pre><font face='Arial'>sage with a 421 Service not available </font></pre></span> <span name = "INTRA" style='top:662px; left:466px' class="ft1"><pre><font face='Arial'>dress> . This command also tells the re </font></pre></span> <span name = "INTRA" style='top:674px; left:77px' class="ft1"><pre><font face='Arial'>simply to transfer electronic mail</font></pre></span> <span name = "INTRA" style='top:674px; left:272px' class="ft1"><pre><font face='Arial'>reply code. For example, an Internet</font></pre></span> <span name = "INTRA" style='top:674px; left:466px' class="ft1"><pre><font face='Arial'>ceiver that it is about to receive a new</font></pre></span> <span name = "INTRA" style='top:686px; left:77px' class="ft1"><pre><font face='Arial'>across and between networks and</font></pre></span> <span name = "INTRA" style='top:686px; left:272px' class="ft1"><pre><font face='Arial'>Service Provider's SMTP server pro </font></pre></span> <span name = "INTRA" style='top:686px; left:466px' class="ft1"><pre><font face='Arial'>message, so it knows to clear out its list</font></pre></span> <span name = "INTRA" style='top:698px; left:77px' class="ft1"><pre><font face='Arial'>other transport systems. As such, its</font></pre></span> <span name = "INTRA" style='top:698px; left:272px' class="ft1"><pre><font face='Arial'>vided for use by its subscribers to relay</font></pre></span> <span name = "INTRA" style='top:698px; left:466px' class="ft1"><pre><font face='Arial'>of recipients. The address in the angle</font></pre></span> <span name = "INTRA" style='top:710px; left:77px' class="ft1"><pre><font face='Arial'>use need not be restricted to systems</font></pre></span> <span name = "INTRA" style='top:710px; left:272px' class="ft1"><pre><font face='Arial'>outgoing mail might refuse a connec </font></pre></span> <span name = "INTRA" style='top:710px; left:466px' class="ft1"><pre><font face='Arial'>brackets (which are required) is the</font></pre></span> <span name = "INTRA" style='top:722px; left:77px' class="ft1"><pre><font face='Arial'>that use TCP/IP. Any communications</font></pre></span> <span name = "INTRA" style='top:722px; left:272px' class="ft1"><pre><font face='Arial'>tion from a host whose IP address in </font></pre></span> <span name = "INTRA" style='top:722px; left:466px' class="ft1"><pre><font face='Arial'>return path for the message. The return</font></pre></span> <span name = "INTRA" style='top:734px; left:77px' class="ft1"><pre><font face='Arial'>system capable of handling lines of up</font></pre></span> <span name = "INTRA" style='top:734px; left:272px' class="ft1"><pre><font face='Arial'>dicates that it is not a subscriber to that</font></pre></span> <span name = "INTRA" style='top:734px; left:466px' class="ft1"><pre><font face='Arial'>path is the address that any error re </font></pre></span> <span name = "INTRA" style='top:746px; left:77px' class="ft1"><pre><font face='Arial'>to 1,000 7 bit ASCII characters could be</font></pre></span> <span name = "INTRA" style='top:746px; left:272px' class="ft1"><pre><font face='Arial'>ISP. SMTP has no form of access con </font></pre></span> <span name = "INTRA" style='top:746px; left:466px' class="ft1"><pre><font face='Arial'>port such as would be generated if the</font></pre></span> <span name = "INTRA" style='top:758px; left:77px' class="ft1"><pre><font face='Arial'>used to carry messages using SMTP.</font></pre></span> <span name = "INTRA" style='top:758px; left:272px' class="ft1"><pre><font face='Arial'>trol the way it can be used to relay</font></pre></span> <span name = "INTRA" style='top:758px; left:466px' class="ft1"><pre><font face='Arial'>message is undeliverable is sent to.</font></pre></span> <span name = "INTRA" style='top:770px; left:77px' class="ft1"><pre><font face='Arial'>On a TCP/IP network, however, TCP</font></pre></span> <span name = "INTRA" style='top:782px; left:77px' class="ft1"><pre><font face='Arial'>provides the transport mechanism.</font></pre></span> <span name = "INTRA" style='top:794px; left:91px' class="ft1"><pre><font face='Arial'>In SMTP the sender is the client, but</font></pre></span> <span name = "INTRA" style='top:806px; left:77px' class="ft1"><pre><font face='Arial'>a client may communicate with many</font></pre></span> <span name = "INTRA" style='top:799px; left:318px' class="ft3"><pre><font face='Arial'><i> SMTP</i> <i>uses</i> <i>a</i> <i>similar</i> <i>format</i> <i>of</i></font></pre></span> <span name = "INTRA" style='top:818px; left:77px' class="ft1"><pre><font face='Arial'>different servers. Mail can be sent di </font></pre></span> <span name = "INTRA" style='top:830px; left:77px' class="ft1"><pre><font face='Arial'>rectly from the sending host to the re </font></pre></span> <span name = "INTRA" style='top:823px; left:316px' class="ft3"><pre><font face='Arial'><i>replies</i> <i>to</i> <i>ftp,</i> <i>which</i> <i>we</i> <i>looked</i> <i>at</i></font></pre></span> <span name = "INTRA" style='top:842px; left:77px' class="ft1"><pre><font face='Arial'>ceiving host, requiring a separate TCP</font></pre></span> <span name = "INTRA" style='top:854px; left:77px' class="ft1"><pre><font face='Arial'>connection to be made for each copy of</font></pre></span> <span name = "INTRA" style='top:847px; left:307px' class="ft3"><pre><font face='Arial'><i>previously.</i> <i>The</i> <i>three </i> <i>digit</i> <i>code</i> <i>is</i></font></pre></span> <span name = "INTRA" style='top:866px; left:77px' class="ft1"><pre><font face='Arial'>each message. However, few mail re </font></pre></span> <span name = "INTRA" style='top:878px; left:77px' class="ft1"><pre><font face='Arial'>cipients run their own SMTP servers.</font></pre></span> <span name = "INTRA" style='top:871px; left:332px' class="ft3"><pre><font face='Arial'><i>all</i> <i>the</i> <i>client</i> <i>software</i> <i>needs. </i></font></pre></span> <span name = "INTRA" style='top:890px; left:91px' class="ft1"><pre><font face='Arial'>It is more usual for the destination</font></pre></span> <span name = "INTRA" style='top:902px; left:77px' class="ft1"><pre><font face='Arial'>of an SMTP message to be a server that</font></pre></span> <span name = "INTRA" style='top:946px; left:52px' class="ft1"><pre><font face='Arial'>Issue 93 (March 1998) page 15</font></pre></span> <span name = "INTRA" style='top:946px; left:605px' class="ft1"><pre><font face='Arial'>File: C04100.13</font></pre></span> <span name = "INTRA" style='top:944px; left:261px' class="ft3"><pre><font face='Arial'><i>PC</i> <i>Network</i> <i>Advisor</i></font></pre></span> <span name = "INTRA" class='nav' style='left: 357px; top: 1010px'><pre>Next page <a href='page0002.htm'>></a></pre></span> <span name = "INTRA" style='top:1050px; left:50px' class="ft4"><pre><b><font color="#FF0000" size="3" face="Arial">New!</font><font color="#FF0000" size="3" face="Arial Narrow"> </font><font size="2" face="Arial">The best sites for quality <a href="http://www.inkjet-printer-cartridges.org">inkjet printer cartridges</a> and the best sites for <a href="http://www.cheap-inkjet-cartridges.com">cheap inkjet cartridges</a></font></b></pre></span></body> <span name = "INTRA" style='top:1100px; left:300px' class="ft4"><pre><b><font color="#FF0000" size="3" face="Arial"><a href="../index.html">Windows Help Desk Home</a> </font></b></pre></span></body></html>