The Illustrated Network- P59
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The Illustrated Network- P59:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices.
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and even as code (much to the chagrin of network Base64 encoding converts a binary data stream to a sequence of “text” This usually results in the size of the binary fi le growing by about 33% in terms of bytes. This is because 6 bits can indicate the numbers 0 through 63. But bytes are 8 bits, of course, at least where the Internet and TCP/IP are Example of a MIME a writer a short story to an editor as an email (been there, done that). What would the MIME headers that form the overall body of the email message look like? Well, they would resemble the boundary = "--- = is a message in MIME = _NextPart 21.3 MIME Content Types and plain, richtext, html, jpeg, gif, ief, tiff, g3fax, mpeg, basic, 32kadpcm, rtf, pdf, zip, msword, EDI-X12, EDIFACT, dec-dx, dca-rft, applefi le, slate, sgml, wita, ms-excel, mixed, digest, parallel, report, signed, rfc822, partial, news, http, 21.4 MIME Encoding Methods and Ordinary ASCII as used in the United States. Adds a few special and coding to ASCII Content is mapped into a “text” package (very Similar to 7bit, but can include 8-bit True binary encodings must have a name starting with 21 SMTP and Email take a look at the attached short story. = name = "new filename = "new of nonsense form the base64 = lines in bold are the MIME POP3 TO ACCESS EMAIL The original SMTP did not allow for limited messages to 1000 bytes, was a purely and never imagined a world of personal computers and email checking. STMP was built for immediate email delivery to a specifi c host, sort of what we think of as instant messaging (IM) today. Email today is often delivered to mailboxes on mail servers, not directly to the end user, that is, users who might only have dial-up Internet 21.8 A POP3 capture, how the email listing is sent to the PART IV POP3 3-way handshake and close POP3 User name accepted, password Mailbox open, 1 1 1108 �OK Mailbox scan listing 1108 Message 1 FIGURE 21.9 A POP3 used to fetch email, showing a more schematic view than the Internet users log in and access their mailbox with POP3 just called POP). POP3 does not send email: SMTP does that. But POP3 retrieves the email, and the IMAP4 protocol maintains and controls access to the mailbox uses TCP port 110, and users are by userID and password. POP3 then places a lock on the mailbox to avoid access confl icts. The POP3 server then enters mode for user access to messages. POP3 features include the abil- ity to view a list of email messages and their sizes and to retrieve or delete but many simply dump all waiting mail to the client. POP3 servers can be the same device as the SMTP mail server, but this is not a add POP3 to our network. We used the BSD hosts before, so let’s make lnxserver into our email server for the network. We can then compose a fairly long (1108 bytes) message and send it to user admin1. Figure 21.8 shows the sequence of packets used to retrieve the message from host lnxclient employs a +OK and not a code when normally to a client. The series of packets shown in Figure 21.8 is boiled down to its POP3 in Figure 21 SMTP and Email that the retrieval of the message (RETR) by the client and its deletion from the server (DELE) are separate steps. You don’t have to delete email as you read it, of course. The +OK Sayonara is also part of the POP3 protocol AND mentioned email headers already and supplied some details about MIME headers (header Email has its own proper set of headers as well, and an Internet email message is little more than a sequence of headers and their values, one after the other, from the start of the email message to the end. Table 21.5 outlines the basic email header fi eld names and groups by RFC 822. Now we have in place to examine the headers created when sending a short email message through our email server from a client host to another user. We’ll use the admin account on lnxclient to send a message to the admin user on Table 21.5 RFC 822 Email Header Fields and Group Field Name per Message Address Usually present 1 Primary recipient list Cc: Optional 1 Copy recipient Bcc: Optional 1 “Blind” cation Usually present 1 Unique code applied when Optional, normal for Provides method to Optional 1 Other documents or mes- sage Usually present 1 Topic of the Optional Unlimited Describe Optional Unlimited Useful search Mandatory 1 Date and time stamp for Mandatory 1 Source address of Optional 1 If different from Optional 1 If absent, reply goes to PART IV (these are not the same users: they just share a mailbox name). Then we’ll fetch the message from the email server mailbox using the admin account, showing that we can fetch our email almost anywhere, even from the sending host. We can use the same basic mail program as we did on the BSD hosts. This time, we’ll use the –s fl ag to create a subject for the message. The text is simple, and we end our message with a single dot as admin]$ mail –s "Here is another example" is we’ll use fetchmail to “fetch” the mail message with POP3 from the email server and bring it back to Note that when we run the program and have email we get a version of the familiar “you’ve got mail” admin]$ password for (not have new mail in our complete email would display the and the But there’s nothing magical about that. We can do the same with the command prompt, listing the mailbox content and the email message with normal Unix admin]$ ls –l 1 admin mail 3122 Jan 17 16:42 21.5 Group Field Name per Message Fields time message is resent, this block is and are all others used for an email message to Fields by email Used to trace the message through the email 21 SMTP and Email admin]$ cat Wed Jan 16 13:04:50 from localhost by with ESMTP id for Wed, 16 Jan 2008 13:04:50 from by localhost with POP3 for Wed, 16 Jan 2008 13:04:50 -0800 from by with ESMTP id for Wed, 16 Jan 2008 -0800 from by with ESMTP id for Wed, 16 Jan 2008 -0800 (from by id for Wed, 16 Jan 2008 -0800 Date: Wed, 16 Jan 2008 13:04:17 -0800 From: Here is another 8 Status: o X-UID: is important fi elds are Most of the other headers were added when the email was created, of course. Most useful is the series of Received: headers, which allows us to trace the message back to its origin. It might seem odd that there are fi ve receiver headers along the trace for a message that has gone from client to email server and then back to client. But the adds a localhost step at each end, at the sender and receiver (from to the message trace. The complete path of the message recorded in the headers (from “bottom to top”) is: 1. The mail receives the composed message from the local user. 2. The local mailbox receives the message using ESMTP. 3. The email server receives the message using PART IV The other client retrieves the message from the email server using POP3 The local host transfers the message to the local mailbox using ESMTP. 6. The use of these protocols is in the OFFICE end our email by showing that Windows uses the same protocols and headers to send and receive email over the Internet. This time, we’ll send a mes- sage from lnxclient on the Network to my home offi ce host (which uses all email have an option to view the complete headers. In it’s just “Message Header” in the singular, but the following is the result of viewing the message headers in Outlook. Only the headers are not the message text Mail Internet Headers Version from by with Microsoft Thu, 17 Jan 2008 07:37:14 from by over TLS secured channel with Microsoft Thu, 17 Jan 2008 07:37:13 from by with ESMTP id for Thu, 17 Jan 2008 07:37:13 -0700 (PDT) from by with ESMTP id for Thu, 17 Jan 2008 07:36:58 -0700 Received: (from by id for Thu, 17 Jan 2008 07:36:28 Thu, 17 Jan 2008 07:36:28 here is an email 17 Jan 2008 (UTC) 21 SMTP and Email page left FOR 21.10 shows some of the concepts discussed in this chapter and can be used to answer the following session Which port does POP3 use? 2. Which password is provided by the user? 3. Was the email message deleted after it was How long was the How many other messages are in the user’s the and Layers 1 TCP/IP Protocols and Devices Link 3 IPv4 and IPv6 Protocol 5 IPv4 and IPv6 Headers Control Message Protocol IP Packets 9 User Datagram Protocol Control Protocol and Sockets and Peering 13 IGPs: RIP, OSPF, and IS–IS Gateway Protocol 16 MPLS and IP Switching Host Conf guration Protocol 18 The Domain Name System 19 File Transfer Protocol 20 SMTP and Email Transfer Protocol Sockets with SSL Network Protocol Shell (Remote Access) Virtual Private Networks Address 28 IP Security 29 Voice over Internet Protocol 30 List of