2 000 LINES USA MAILACCESS.txt
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2 000 LINES USA MAILACCESS.txt
Wireless networking equipment was installed on three research vessels and at three shore stations during the 1998 Thin Layers Experiment in East Sound, Washington. This wireless network provided high-speed data communication between scientists on separate vessels and permitted rapid transfer of data from vessels and from moored instruments to a common file server at one of the shore stations. This server was connected, via wireless link, to a local Internet service provider, thus permitting continuous Internet access from each vessel and each shore station. The wireless network used 900-MHz and 2.4-GHz spread spectrum systems and provided throughput comparable to T1 lines. Omnidirectional antennas were used between vessels and shore stations, providing communications at ranges up to 12 km. Such systems provide the capability for rapid data exchange during coordinated field operations and give investigators on separate vessels the opportunity to adapt sampling protocols to rapidly evolving conditions observed a few kilometers away.
In practice, there are several problems with this idea. First, support for Internet development among government officials is neither as unanimous nor as enthusiastic as might appear at first glance. Some of the government's rules concerning the Internet industry seem to be at odds with its general promotion of network growth and e-commerce. Despite China's sanctioning of competition in the sector, for example, the country's privately owned Internet service providers must lease telephone lines at high cost from China Telecom. For this reason, many are forced to rely on outside funding or other government ties. China Telecom's Internet service provider, the country's largest, also charges local users per hour for dial-up Internet access, further limiting mass usage.
 American policy makers have frequently stated that democratic enlargement abroad is critical to the U.S. national interest. If they believe that the Internet will promote democracy in the developing countries, this would seem to be another reason why tackling the global digital divide should be a foreign policy priority. But while the Clinton administration has launched a few laudable small-scale initiatives in this arena, the Congress will not provide the resources for it to do anything more substantial on its own. As with many of the new issues raised by globalization, progress here will require coordinated efforts by the U.S. and other governments, international organizations, the global business community, and civil societal organizations. For some suggestions along these lines, see, William J. Drake, From the Global Digital Divide to the Global Digital Opportunity: Proposals Submitted to the G-8 Kyushu-Okinawa Summit 2000-Report of the World Economic Forum Task Force on the Global Digital Divide (Geneva: World Economic Forum, July 19, 2000), [ Home+-+Centres+-+Global+Digital+Divide+Initiative+-+Statement]. Return to text
It is important to observe that SMTP does not use intermediate mailservers for sending mail, even when the two mail servers are located atopposite ends of the world. If Alice's server is in Hong Kong and Bob'sserver is in Mobile, Alabama, the TCP "connection" is a direct connectionbetween the Hong Kong and Mobile servers. In particular, if Bob's mailserver is down, the message remains in Alice's mail server and waits fora new attempt -- the message does not get placed in some intermediate mailserver.Let's now take a closer look at how SMTP transfers a message from asending mail server to a receiving mail server. We will see thatthe SMTP protocol has many similarities with protocols that are used forface-to-face human interaction. First, the client SMTP(running on the sending mail server host) has TCP establish a connectionon port 25 to the server SMTP (running on the receiving mail server host).If the server is down, the client tries again later. Once this connectionis established, the server and client perform some application-layer handshaking.Just as humans often introduce themselves before transferring informationfrom one to another, SMTP clients and servers introduce themselves beforetransferring information. During this SMTP handshaking phase, the SMTPclient indicates the email address of the sender (the person who generatedthe message) and the email address of the recipient. Once the SMTP clientand server have introduced themselves to each other, the client sends themessage. SMTP can count on the reliable data transfer service of TCP toget the message to the server without errors. The client then repeats thisprocess over the same TCP connection if it has other messages to send tothe server; otherwise, it instructs TCP to close the connection.Let us take a look at an example transcript betweenclient (C) and server (S). The host name of the client is crepes.frand the host name of the server is hamburger.edu. The ASCII textprefaced with C: are exactly the lines the client sends into itsTCP socket; and the ASCII text prefaced with S: are exactly thelines the server sends into its TCP socket. The following transcript beginsas soon as the TCP connection is established: S: 220hamburger.eduC: HELOcrepes.frS: 250 Hello crepes.fr, pleased to meet youC: MAILFROM: S: firstname.lastname@example.org... Sender okC: RCPTTO: S: email@example.com ... Recipient okC: DATAS: 354Enter mail, end with "." on a line by itselfC: Doyou like ketchup?C: How about pickles?C: .S: 250Message accepted for deliveryC: QUITS: 221hamburger.edu closing connection
It is highly recommended that you use Telnet tocarry out a direct dialogue with an SMTP server. To do this, issue telnetserverName 25 . When you do this,you are simply establishing a TCP connection between your local host andthe mail server. After typing this line, you should immediately receivethe 220 reply from the server.Then issue the SMTP commands HELO, MAIL FROM, RCPT TO, DATA, andQUITat the appropriate times. If you Telnet into your friend's SMTP server,you should be able to send mail to your friend in this manner (i.e., withoutusing your mail user agent).Comparison with HTTPLet us now briefly compare SMTP to HTTP. Bothprotocols are used to transfer files from one host to another; HTTP transfersfiles (or objects) from Web server to Web user agent (i.e., the browser);SMTP transfers files (i.e., email messages) from one mail server to anothermail server. When transferring the files, both persistent HTTP and SMTPuse persistent connections, that is, they can send multiple files overthe same TCP connection. Thus the two protocols have common characteristics.However, there are important differences. First, HTTP is principally apullprotocol -- someone loads information on a Web server and users useHTTP to pull the information off the server at their convenience. In particular, the TCP connection is initiated by the machine that wants to receive thefile. On the other hand, SMTP is primarily a push protocol -- thesending mail server pushes the file to the receiving mail server. In particular, the TCP connection is initiated by the machine that wants to send the file.A second important difference, which we alludedto earlier, is that SMTP requires each message, including the body of eachmessage, to be in seven-bit ASCII format. Furthermore, the SMTP RFC requiresthe body of every message to end with a line consisting of only a period -- i.e., in ASCII jargon, the body of each message ends with "CRLF.CRLF",where CR and LF stand for carriage return and line feed, respectively.In this manner, while the SMTP server is receiving a series of messagesfrom an SMTP client over a persistent TCP connection, the server can delineatethe messages by searching for "CRLF.CRLF"in the byte stream. (This operation of searching through a character streamis referred to as "parsing".) Now suppose that the body of one ofthe messages is not ASCII text but instead binary data (for example, aJPEG image). It is possible that this binary data might accidentally havethe bit pattern associated with ASCII representation of "CR LF . CR LF"in the middle of the bit stream. This would cause the SMTP server to incorrectlyconclude that the message has terminated. To get around this and relatedproblems, binary data is first encoded to ASCII in such a way that certainASCII characters (including ".") are not used. Returning to our comparisonwith HTTP, we note that neither non-persistent nor persistent HTTP hasto bother with the ASCII conversion. For non-persistent HTTP, each TCPconnection transfers exactly one object; when the server closes the connection,the client knows it has received one entire response message. For persistentHTTP, each response message includes a Content-length:header line, enabling the client to delineate the end of each message.A third important difference concerns how a document consisting of textand images (along with possibly other media types) is handled. As we learnedin Section 2.3, HTTP encapsulates each object in its own HTTP responsemessage. Internet mail, as we shall discuss in greater detail below, placesall of the message's objects into one message.2.4.2 Mail Message Formats and MIMEWhen Alice sends an ordinary snail-mail letter toBob, she puts the letter into an envelope, on which there is all kindsof peripheral information such as Bob's address, Alice's return address,and the date (supplied by the postal service). Similarly, when an emailmessage is sent from one person to another, a header containing peripheralinformation proceeds the body of the message itself. This peripheral informationis contained in a series of header lines, which are defined in [RFC822]. The header lines and the body of message are separated by a blankline (i.e., by CRLF). RFC 822 specifiesthe exact format for mail header lines as well their semantic interpretations.As with HTTP, each header line contains readable text, consisting of akeyword followed by a colon followed by a value. Some of the keywords arerequired and others are optional. Every header must have a From:header line and a To: header line;a header may include a Subject:header line as well as other optional header lines. It is important tonote that these header lines are different from the SMTP commandswe studied in section 2.4.1 (even though they contain some common wordssuch as "from" and "to"). The commands in section 2.4.1 were part of theSMTP handshaking protocol; the header lines examined in this section arepart of the mail message itself. 041b061a72