wenty-first century, FTP as a pro- tocol is still used for Internet connection measurements (e.g. bitrate measurements in downlink or uplink). 2.2 Fundamental Internet Network Architectures 2.1.5.3 Email Protocols Email communication is done with standardized IETF protocols [17]. The main email protocol is SMTP (Simple Mail Transfer Protocol), which is used for sending email from a standalone mail client (Thunderbird, Outlook, etc.) to a mail server and for commu- nication between email servers (e.g. for sending an email from the sending email server to the destination email server, of course after resolving the domain name of that email server via DNS). For access to the email messages received in the mailbox of the email server, there are standardized access email protocols, which include POP3 (Post Office Protocol version 3) and IMAP4 (Internet Message Access Protocol version 4). Popularization of free email accounts since the 2000s also increased the portion of Web-based access to email. In such cases the protocol used for access to email is HTTP. Fundamental Internet Network Architectures 2.2.1 Client-Server Internet Networking Most of the Internet applications use client-server communication, which means that there are two types of entities (e.g. hosts, network nodes) in each such communication: Client (e.g. a Web browser). It sends requests to the server. Server (e.g. Web server). It sends responses to the client's requests. The principle of the client-server communication is such that clients are the entities which request certain information or action from the server. The server is not intended to initiate any action by the client. This is native Internet network architecture, where clients are simpler machines which in average have lower processing power and lower memory capacity than servers because servers must handle multiple requests from mul- tiple clients in parallel. When there are many expected requests from many clients, a single server cannot serve them all, SO in such cases multiple servers a