TCP/IP Protocol Suite 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 23 Electronic Mail: SMTP, POP IMAP, and MIME

TCP/IP Protocol Suite 2 Chapter Outline 23.1 Architecture 架構 23.2 User Agent 使用者代理器 23.3 Message Transfer Agent 訊息傳送代理器 23.4 Message Access Agent 訊息存取代理器 23.5 MIME 23.6 Web-Based Mail 網頁式郵件

TCP/IP Protocol Suite ARCHITECTURE To explain the architecture of , we give four scenarios. We begin with the simplest situation and add complexity as we proceed. The fourth scenario is the most common in the exchange of . 為了解釋架構的電子郵件，我們給四種情況。 我們首先從最簡單的情況，並增加複雜性，我 們繼續。第四個方案是最常見的交換電子郵件 。

TCP/IP Protocol Suite 4 Topics Discussed in the Section First Scenario 第一種情況 Second Scenario 第二種情況 Third Scenario 第三種情況 Fourth Scenario 第四種情況

TCP/IP Protocol Suite 5 Figure 23.1 First scenario

TCP/IP Protocol Suite 6 When the sender and the receiver of an are on the same mail server, we need only two user agents. 當傳送端和接收端的電子郵件都在同一個郵 件服務器， 我們只需要兩個使用者代理器 Note

TCP/IP Protocol Suite 7 Figure 23.2 Second scenario

TCP/IP Protocol Suite 8 When the sender and the receiver of an are on different mail servers, we need two UAs and a pair of MTAs (client and server). 當傳送端和接收端的電子郵件在不同的郵件 服務器， 我們需要兩個 UA 和 MIA( 一對多邊貿易協議 ) （用戶端和伺服器）。 Note

TCP/IP Protocol Suite 9 Figure 23.3 Third scenario

TCP/IP Protocol Suite 10 When the sender is connected to the mail server via a LAN or a WAN, we need two UAs and two pairs of MTAs (client and server). 當傳送端是透過 LAN 或 WAN 來連接到郵件 伺服器，我們需要兩個 UA 和兩對 MIA( 多邊 貿易協議 ) （客戶端和服務器）。 Note

TCP/IP Protocol Suite 11 Figure 23.4 Fourth scenario

TCP/IP Protocol Suite 12 When both sender and receiver are connected to the mail server via a LAN or a WAN, we need two UAs, two pairs of MTAs (client and server), and a pair of MAAs (client and server). This is the most common situation today. 當傳送端和接收者都是透過 LAN 或 WAN 連 接到郵件伺服器，我們需要兩個 UA ，兩對 MIA( 多邊貿易協議 ) （用戶端和伺服器）， 以及一對 MAA 的（用戶端和伺服器）。這 是現今最常見的情況 。 Note

TCP/IP Protocol Suite 13 Figure 23.5 Push versus pull

TCP/IP Protocol Suite USER AGENT The first component of an electronic mail system is the user agent (UA). It provides service to the user to make the process of sending and receiving a message easier. 電子郵件系統的第一構成要素是使用者代理器 （ UA ）。它讓使用者可以更容易地收發郵件訊 息

TCP/IP Protocol Suite 15 Topics Discussed in the Section Services Provided by a User Agent User Agent Types Sending Mail Receiving Mail Addresses Mailing List or Group List

TCP/IP Protocol Suite 16 Some examples of command-driven user agents are mail, pine, and elm. 指令列模式的 UA 的一些相關例子有 mail, pine, and elm Note

TCP/IP Protocol Suite 17 Some examples of GUI-based user agents are Eudora, Outlook, And Netscape. CUI 模式 UA 的一些相關例子有微軟的 Outlook, 和 Netscape. Note

TCP/IP Protocol Suite 18 Figure 23.6 Format of an

TCP/IP Protocol Suite 19 Figure address

TCP/IP Protocol Suite MESSAGE TRANSFER AGENT The actual mail transfer is done through message transfer agents (MTAs). To send mail, a system must have the client MTA, and to receive mail, a system must have a server MTA. The formal protocol that defines the MTA client and server in the Internet is called Simple Mail Transfer Protocol (SMTP). As we said before, two pairs of MTA client-server programs are used in the most common situation (fourth scenario). Figure 23.8 shows the range of the SMTP protocol in this scenario. 實際的郵件傳輸是通過郵件傳輸代理（ MTA ）。要發送郵件，系統必須具有用戶端的 MTA ，並接收郵件，系統必須有一個伺服器上的 MTA 。正式協議，它定義了 MTA 用 戶端和伺服器的通訊協定稱為簡易郵件傳輸通訊協定（ SMTP ）。正如我們以前說過 ，兩對 MTA 的用戶端伺服器方案中使用了最常見的情況（第四種情況）。圖 23.8 說 明了一系列的 SMTP 通訊協定的涵蓋範圍。

TCP/IP Protocol Suite 21 Topics Discussed in the Section Commands and Responses 指令和回應 Mail Transfer Phases 郵件傳送階段

TCP/IP Protocol Suite 22 Figure 23.8 SMTP range

TCP/IP Protocol Suite 23 Figure 23.9 Commands and responses

TCP/IP Protocol Suite 24

TCP/IP Protocol Suite 25

TCP/IP Protocol Suite 26 Figure Connection establishment

TCP/IP Protocol Suite 27 Figure Message transfer

TCP/IP Protocol Suite 28 Figure Connection termination

TCP/IP Protocol Suite 29 Let us see how we can directly use SMTP to send an and simulate the commands and responses we described in this section. We use TELNET to log into port 25 (the well-known port for SMTP). We then use the commands directly to send an . In this example, is sending an to himself. The first few lines show TELNET trying to connect to the adelphia mail server. Example Example 23.1 After connection, we can type the SMTP commands and then receive the responses as shown below. We have shown the commands in black and the responses in color. Note that we have added for clarification some comment lines, designated by the “ = ” sign. These lines are not part of the procedure.

TCP/IP Protocol Suite 30 Example Example 19.1 Continued

TCP/IP Protocol Suite MESSAGE ACCESS AGENT The first and the second stages of mail delivery use SMTP. However, SMTP is not involved in the third stage because SMTP is a push protocol; it pushes the message from the client to the server. In other words, the direction of the bulk data (messages) is from the client to the server. On the other hand, the third stage needs a pull protocol; the client must pull messages from the server. The direction of the bulk data are from the server to the client. The third stage uses a message access agent.

TCP/IP Protocol Suite 32 Topics Discussed in the Section POP3 IMAP4

TCP/IP Protocol Suite 33 Figure Pop3 and IMAP4

TCP/IP Protocol Suite 34 Figure Pop3

TCP/IP Protocol Suite MIME Electronic mail has a simple structure. Its simplicity, however, comes with a price. It can send messages only in NVT 7-bit ASCII format. In other words, it has some limitations. Multipurpose Internet Mail Extensions (MIME) is a supplementary protocol that allows non- ASCII data to be sent through . MIME transforms non-ASCII data at the sender site to NVT ASCII data and delivers it to the client MTA to be sent through the Internet. The message at the receiving site is transformed back to the original data.

TCP/IP Protocol Suite 36 Topics Discussed in the Section MIME Headers

TCP/IP Protocol Suite 37 Figure MIME

TCP/IP Protocol Suite 38 Figure MIME header

TCP/IP Protocol Suite 39

TCP/IP Protocol Suite 40

TCP/IP Protocol Suite 41 Figure Base64

TCP/IP Protocol Suite 42

TCP/IP Protocol Suite 43 Figure Quoted printable

TCP/IP Protocol Suite WEB-BASED MAIL is such a common application that some websites today provide this service to anyone who accesses the site. Three common sites are Hotmail, Yahoo, and Google. The idea is very simple. Let us go through two cases:

TCP/IP Protocol Suite 45 Topics Discussed in the Section Case I Case II

TCP/IP Protocol Suite 46 Figure Web-based , case 1

TCP/IP Protocol Suite 47 Figure Web-based , case 2