McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Network Protocols Chapter 26 (Data Communication and Networking): Remote Logging, Electronic Mail, and File Transfer 1

McGraw-Hill©The McGraw-Hill Companies, Inc., REMOTE LOGGING It would be impossible to write a specific client/server program for each demand. The better solution is a general-purpose client/server program that lets a user access any application program on a remote computer. TELNET Topics discussed in this section: 2

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 TELNET is a general-purpose client/server application program. Note 3

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Telnet refers to TErminaL NETwrok (TELNET). TELNET enables the establishment of a connection to a remote system in such a way that the local terminal appears to be a terminal at the remote system. The server uses well known port 23 and the client uses ephemeral port. Telnet 4

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 26.1 Local and remote log-in 5

McGraw-Hill©The McGraw-Hill Companies, Inc., ELECTRONIC MAIL One of the most popular Internet services is electronic mail ( ). The designers of the Internet probably never imagined the popularity of this application program. Its architecture consists of several components that we will discuss in this chapter. Architecture User Agent Message Transfer Agent: SMTP Message Access Agent: POP and IMAP Web-Based Mail Topics discussed in this section: 6

McGraw-Hill©The McGraw-Hill Companies, Inc., Terms User Agent (UA): is a program that allows you to receive and send messages; it's usually just called an e- mail program. Message Transfer Agent (MTA): the program that is responsible for receiving incoming s and delivering the messages to individual users. (SMTP) The MTA transfers messages between computers. Hidden from the average user, it is responsible for routing messages to their proper destinations. MTAs receive messages from both MUAs and other MTAs Message Access Agent (MAA): A client-server program that pulls the stored messages. (POP and IMAP) 7

McGraw-Hill©The McGraw-Hill Companies, Inc., Architecture First Scenario 8

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 When the sender and the receiver of an are on the same system, we need only two user agents. Note 9

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Second Scenario 10

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 When the sender and the receiver of an are on different systems, we need two UAs and a pair of MTAs (client and server). Note 11

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Third Scenario 12

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 When the sender is connected to the mail server via LAN or a WAN, we need two UAs and two pairs of MTAs (client and server). Note 13

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Fourth Scenario 14

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 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 and a pair of MAAs. This is the most common situation today. Note 15

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 PUSH vs PULL in 16

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 It provides service to the user to make the process of sending and receiving a message easier Service provided by a user agent User agent is a software package (program) that composes, reads, replies to, and forward messages. It also handles mailboxes. USER AGENT 17

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 User Agent Types Command-Driven Normally accepts a one-character command from the keyboard to perform its task. For example, a user can type the character r, at the command prompt, to reply to the sender of the message GUI-Based Contain GUI components that allow the user to interact with the software by using both the keyboard and the mouse. Eudora, Microsoft’s Outlook, Gmail, Netscape, etc. 18

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 A Mail Transfer Agent or MTA (also called a mail server, or a mail exchange server) is a computer program or software agent which transfers electronic mail messages from one computer to anothercomputer programagentelectronic mail The MTA works behind the scenes, while the user usually interacts with another program, the Mail User Agent (MUA), which contacts a MTA for actual delivery of the mail.Mail User Agent Mail Transfer Agent 19

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Mail Transfer Agent 20

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Sending Mail 21

McGraw-Hill©The McGraw-Hill Companies, Inc., Address Local part: Name of a special file, called the user mailbox, where all the mail received for a user is stored for retrieval by the user agent. Domain Name: Organization usually selects one or more hosts to receive and send ; they are called mail servers/exchangers. Domain name assigned to each mail exchanger either comes from the DNS database or is a logical name (e.g., the name of the organization). 22

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 MIME Electronic mail can send messages only in ASCII format. In other words, it cannot be used for languages that are not supported by ASCII characters, such as French, German, Hebrew, Russian, Chinese, and Japanese. Electronic mail cannot be used to send binary files or video or audio data. Multipurpose Internet Mail Extensions (MIME) Supplementary protocol that allows non-ASCII data to be sent through an (SMTP). MIME transforms non-ASCII data at the sender side to ASCII data and delivers them to client SMTP to sent through the Internet. Reverse is done at the receiver side. We think of MIME as a set of software functions that transforms non-ASCII data to ASCII data and vice versa. 23

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 MIME Network Virtual Terminal (NVT) 24

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Message Transfer Agent: SMTP 25

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 SMTP The actual mail transfer is done through Message Transfer Agents (MTAs). To send mail, a system must have the client MTA. 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 the Simple Mail Transfer Protocol (SMTP). SMTP is used two times, between the sender and the sender's mail server and between the two mail servers. As we will see shortly, another protocol is needed between the mail server and the receiver. SMTP simply defines how commands and responses must be sent back and forth. 26

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Simple Mail Transfer Protocol – SMTP RANGE 27

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 26.9 Commands and responses SMTP uses commands and responses to transfer messages between a MTA client and a MTA server. Each command or reply is terminated by a two-character (carriage return and line feed). Commands Commands are sent from the client to the server. Consists of a keyword followed by zero and more arguments. SMTP defines 14 commands. Responses Responses are sent from server to client. Response is a three-digit code that may be followed by additional textual information. 28

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 SMTP COMMANDS 29

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Mail Transfer Phases Process of transferring a mail message occurs in three phases Connection establishment After a client has made a TCP connection to the well-known port 25, the SMTP server starts the connection phase. Message transfer After connection establishment, a single message between a sender and one or more recipients can be exchanged. Connection termination After the message is transferred successfully, the client terminates the connection. 30

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Mail Delivery Delivery of from sender to receiver consists of three stages 1 st stage goes from user agent to local server Mail does not go directly to remote server because the remote server may not be available at all times. Mail is stored in the local server until it can be sent. 2 nd stage is relayed by local server, which now acts as the SMTP client, to the remote server, which is the SMTP server. is delivered to remote server and not to the remote user agent; is stored in mailbox of the user for later retrieval.. server runs the SMTP server program. 3 rd stage Remote user agent uses a mail access protocol such as POP3 or IMAP4 to access the mailbox and obtain the mail. 31

McGraw-Hill©The McGraw-Hill Companies, Inc., Delivery 32

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Mail Access Protocols SMTP is a push protocol; it pushes the message from the sender to the receiver even if the receiver does not want it. 3 rd stage needs a pull protocol; the operation must start with the recipient. Mail must stay in the mail server mailbox until the recipient retrieves it. 33

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Post Office Protocol POP3 [Post Office Protocol] Client POP3 software is installed on the recipient computer. Server POP3 software is installed on the mail server. Client (user agent) opens a connection with the server on TCP port 110. It then sends its user name and password to access the mailbox. User can then list and receive mail messages, one by one. POP3 modes Delete mode: Mail is deleted from the mailbox after each retrieval. Keep mode: Mail remains in the mailbox after retrieval. 34

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure POP3 and IMAP4 35

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 POP3 36

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 POP3 assumes that each time a client accesses the server, the whole mailbox will be cleared out (transferred). POP3 does not allow user to organize mail on the server; user cannot have different folders on the server; POP3 does not allow the user to partially check the contents of the mail before downloading. User can check the header prior to downloading User can search the contents of for a specific string of characters prior to downloading User can partially download . User can create, delete, or rename mailboxes on mail server User can create a hierarchy of mailboxes in a folder for storage. Internet Mail Access Protocol (IMAP) 37

McGraw-Hill©The McGraw-Hill Companies, Inc., FILE TRANSFER PROTOCOL (FTP) Transferring files from one computer to another is one of the most common tasks expected from a networking or internetworking environment. As a matter of fact, the greatest volume of data exchange in the Internet today is due to file transfer. File Transfer Protocol (FTP) Anonymous FTP Topics discussed in this section: 38

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file from one host to another. Although transferring files from one system to another seems simple and straightforward, some problems must be dealt with first. For example, two systems may use different file name conventions. Two systems may have different ways to represent text and data. It establishes two connections between the hosts. One connection is used for data transfer, the other for control information (commands and responses). FTP can transfer one of the following file types across the data connection: an ASCII file, EBCDIC file, or image file. File Transfer Protocol 39

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 File Type: The ASCII file is the default format for transferring text files. Each character is encoded using 7-bit ASCII. The sender transforms the file from its own representation into ASCII characters, and the receiver transforms the ASCII characters to its own representation. If one or both ends of the connection use EBCDIC encoding (the file format used by IBM), the file can be transferred using EBCDIC encoding. The image file is the default format for transferring binary files. The file is sent as continuous streams of bits without any interpretation or encoding. File Transfer Protocol 40

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Data Structure: FTP can transfer a file across the data connection by using one of the following interpretations about the structure of the data: file structure, record structure, and page structure. In the file structure format, the file is a continuous stream of bytes. In the record structure, the file is divided into records. This can be used only with text files. In the page structure, the file is divided into pages, with each page having a page number and a page header. File Transfer Protocol 41

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 FTP uses the services of TCP. It needs two TCP connections. The well-known port 21 is used for the control connection and the well- known port 20 for the data connection. Note 42

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure FTP 43

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 To use FTP, a user needs an account to access the remote server. Some sites have a set of files available for public access. To access these files, a user does not need to have an account or password. Instead, the user can use anonymous as the user name and guest as the password. User access to the system is very limited. Anonymous FTP 44