Chapter 5: Application Layer (Part2)

8.    SMTP

SMTP, or Simple Mail Transfer Protocol, is a standard protocol used for sending email messages between servers over a network. It's an application layer protocol within the TCP/IP suite that governs how email messages are transmitted and delivered.
SMTP plays a critical role in the delivery of email messages by facilitating communication between mail servers. While it is primarily responsible for sending messages, other protocols like POP3 and IMAP are used for retrieving messages from mail servers, and protocols like SPF and DKIM are used for email authentication and spam prevention.

9.  POP

POP, or Post Office Protocol, is a standard protocol used by email clients to retrieve email messages from a mail server. There are two main versions: POP3 (Post Office Protocol version 3) and POP2 (Post Office Protocol version 2). However, POP2 is rarely used today, with POP3 being the predominant version.

POP3 is widely supported by email servers and clients, its download-and-delete model and limited functionality compared to IMAP have led to its declining popularity in favor of IMAP for managing email messages across multiple devices and clients. However, POP3 is still commonly used, especially in situations where offline access to email is a priority.

10.  IMAP

IMAP, or Internet Message Access Protocol, is a standard protocol used by email clients to retrieve and manage email messages stored on a mail server. Unlike POP3, which primarily focuses on downloading messages to the client's device, IMAP allows users to access their email messages directly on the server, enabling them to manage their email across multiple devices seamlessly.

IMAP offers a more flexible and feature-rich email experience compared to POP3, making it the preferred choice for users who access their email from multiple devices or clients. IMAP's support for folder management, message flags, and offline access, combined with its ability to synchronize changes between the server and client, makes it well-suited for modern email workflows.

11.  DNS

DNS, or Domain Name System, is a decentralized hierarchical system used to translate human-readable domain names into numerical IP addresses. It serves as the "phone book" of the internet, allowing users to access websites, send emails, and perform other network activities using easy-to-remember domain names rather than complex IP addresses.

DNS plays a crucial role in enabling the internet to function smoothly by providing a scalable, distributed, and efficient mechanism for mapping domain names to IP addresses. Without DNS, accessing websites, sending emails, and performing other network activities would require users to memorize and use numerical IP addresses instead of user-friendly domain names.

12.  SNMP

SNMP, or Simple Network Management Protocol, is a standard protocol used for monitoring and managing network-connected devices, such as routers, switches, servers, printers, and more. It enables network administrators to collect and organize information about network devices, monitor their performance, and troubleshoot issues remotely.

SNMP plays a crucial role in network administration and monitoring by providing a standardized framework for managing network devices and collecting valuable information about their status and performance. Its flexibility, scalability, and extensibility make it a fundamental tool for maintaining and optimizing network infrastructure in organizations of all sizes.

Reference 
Andrew S. Tanenbaum, and David J. Wetherall, Computer Networks, 5th Edition, China Machine Press, 2011.