Chapter 26 Remote Logging, Electronic Mail, and File Transfer
Chapter 26 of Data Communications and Networking by Behrouz A. Forouzan discusses Remote Logging, Electronic Mail, and File Transfer. Below are detailed notes:
26.1 Remote Logging
Remote logging allows users to access and run applications on a remote computer, then transfer the results back to their local machine. TELNET is a key protocol enabling remote logging, allowing users to connect to a remote computer and interact as if they were locally connected.
TELNET:
Client-Server Model: TELNET operates in a client/server mode where the local machine acts as a terminal to the remote server.
Timesharing Environment: Initially developed for systems where users shared resources on a large computer.
Remote Login: Users log in to a remote system by providing credentials (user ID and password) and gain access to the remote system's services.
26.2 Electronic Mail
Electronic mail (email) is one of the most widely used applications on the internet. Email consists of a set of protocols that manage sending, receiving, and storing messages between users.
Email Architecture:
User Agent (UA): A software tool that helps users compose, send, and read emails. Examples include Outlook or Gmail.
Message Transfer Agent (MTA): Manages the delivery of emails from the sender’s system to the recipient’s mail server using protocols like Simple Mail Transfer Protocol (SMTP).
Message Access Agent (MAA): Helps users retrieve emails from the mail server. Two key protocols used for this are Post Office Protocol Version 3 (POP3) and Internet Mail Access Protocol Version 4 (IMAP4).
SMTP (Simple Mail Transfer Protocol):
SMTP is a push protocol that transfers emails from the client to the server and between mail servers.
Commands like HELO, MAIL FROM, and RCPT TO are used to establish a connection, transfer mail, and terminate the connection.
POP3 and IMAP4:
POP3: Retrieves emails from the mail server. It operates in two modes:
Delete Mode: Removes mail from the server after downloading.
Keep Mode: Leaves mail on the server for later access.
IMAP4: Offers more flexibility, allowing users to preview, search, and organize emails on the server before downloading.
MIME (Multipurpose Internet Mail Extensions):
Extends the capability of email by supporting non-text content like audio, video, and images. It allows emails to contain multimedia content and attachments.
26.3 File Transfer
File Transfer Protocol (FTP) is one of the most common methods of transferring files over the Internet.
FTP Features:
Control and Data Connections: FTP uses two separate TCP connections for communication:
Control Connection: Manages commands and responses.
Data Connection: Handles the actual file transfer.
File Types and Structures: FTP supports different file types, including ASCII, EBCDIC, and binary files, and various data structures such as file structure and record structure.
Transmission Modes: FTP allows three modes of file transmission:
Stream Mode: Sends files as a continuous stream.
Block Mode: Sends files in blocks with headers.
Compressed Mode: Compresses files using techniques like run-length encoding.
Anonymous FTP:
Allows public access to files on a server without requiring an account. Users log in using “anonymous” as the username and “guest” as the password.
Key Concepts:
TELNET: Enables remote logins, making it seem like the user is directly connected to a remote machine.
Email Protocols: SMTP for sending emails, and POP3/IMAP4 for retrieving emails.
FTP: A reliable file transfer protocol that separates control and data connections, optimizing file transfers between hosts.
This chapter provides an in-depth understanding of core internet applications and the protocols that support remote access, email, and file transfers.
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