1.Internet technologies (конспект)

Internet - a global computer network that covers the whole world. Today the Internet has about 15 million subscribers in more than 150 countries. Monthly network size increases by 7-10%. The Internet forms, as it were, the core, which provides the connection of various information networks belonging to different institutions all over the world, one on the other.

If previously the network was used exclusively as a medium for file transfer and e-mail messages, today more complex tasks of distributed access to resources are being solved. About two years ago, shells were created that supported the functions of network search and access to distributed information resources, electronic archives.

The topic I selected is quite relevant in the niche days. Almost everyone uses the Internet. Some are on the network every day, and someone occasionally finds information and checks the mail. Exchange of messages on the network also got a big spread. A huge number of people use e-mail, communicate in chat rooms and with the help of instant messaging programs being at a distance from each other

Connecting to the Internet

In 1969, the US Department of Defense established a network, which was the forerunner of the Internet, ARPANET. ARPANET was an experimental network. It was designed to study methods of building networks that would be resistant to partial damage, such as when bombarded, and capable of continuing normal operation under such conditions. In ARPANET, the connection between the source computer and the destination computer (destination station) is constantly present. The creators initially assumed that the network would be unreliable, i.e. any part of the network can disappear at any time. Network support in working order is assigned to the connecting computers. The basic principle of building a network: any computer communicates as an equal with an equal with any other computer.

The International Standards Organization (ISO) needed years to create the final standard for computer networks, but users did not wait, but acted. Active users began to install the software on all possible types of computers. In a few years, the Internet has become the only acceptable way to connect different platforms. Such a scheme suited the government and universities, which bought computers from various manufacturers. Everyone bought those computers that he liked, and had the right to expect that he could work on the network in conjunction with computers from other manufacturers.

The process of improving the network is ongoing. Most of these changes occur unnoticed by users. In fact, the Internet is not just a network - it's a structure that unites ordinary networks. The Internet is a "network of networks". The Internet includes: all networks working on the IP protocol and combined to form a unified network of their users. Initially, the Internet included various departmental networks, many regional networks, a network of educational institutions and some foreign (outside the US) network.

In the late seventies, the Internet attracted users of other networks (for example, BITNET, DECnet, Fidonet, etc.) that do not work over IP. They decided to provide Web services to their customers and developed methods of connecting these networks to the Internet. Initially, these connections, called gateways, served only for sending e-mail. However, some of them have developed ways of transferring and other services.

The World Wide Web - the World Wide Web (WWW) appeared much later, in 1991. Its author is Tim Berners Lee of the European Center for Nuclear Research (CERN), based in Geneva, Switzerland. Few known, appeared at the expense of enthusiasm, the technology has provided an avalanche-like growth of the popularity of the Internet and the appearance of that ocean of information that we see now. The year 1993 can be considered a milestone, when the number of connected servers exceeded one million. After that, the last doubts about the prospects of the network of networks were lost.

2. E-mail.ru

The history of e-mail is 32 years old. For the first time e-mail appeared in 1970. With the development of various computer networks, by combining them among themselves, there was a need for information exchange. More precisely, the impetus to the development and networking of networks was the need for information exchange.

Now registering your email address is not an issue. If it was still 5-7 years ago it was rather difficult to find a website that distributes free e-mail addresses, now only in Russia there are several dozens, or even hundreds. The most famous and popular ones are mail.ru, yandex.ru, rambler.ru … Also the majority of providers when connecting a user to the Internet, provides him with a free e-mail box. Well, we must add that when buying a hosting for your site, the hosting company gives the site owner the ability to create and register up to 10 or more e-mail addresses such as username @ site-name.

Email (e-mail)

E-mail is designed to exchange text messages between users of computers connected to the Internet.

Instead of an envelope, a message is used for the message that contains at least three required fields: To, From and Subject. The To and From fields contain the recipient / sender's email address. Because of the interaction in the Internet of different networks, the addresses can be recorded in different ways. You can not guess the address, so you should store useful addresses in the address book.

Most addresses have the following format:

username @ mail server_name

For example: info@elmech.mpei.ac.ru

The task Internet_services «e-mail» – to deliver a message to the mail server of the addressee. The user must independently «check the mailbox» and take the incoming mail from the server using a client mail program (it also «sends» outgoing mail messages). Windows includes the Exchange client program, and Outlook with Office. In addition, the mail client comes with the Netscape Communicator package. The Bat: The Bat: Forwarding files.

To e-mail messages, you can «attach» files and forward them without using the FTP protocol. But! Some providers limit the size of mail messages or charge an additional fee for each kilobyte of information. In addition, e-mail is designed for sending text messages, so when sending binary files, they are encoded / decoded. It is important that the sending and receiving parties use the same encoding standards (uuencode / uudecode, binhex). Now most systems support the MIME standard (Multi-purpose Internet Mail Extensions), which greatly facilitates «mutual understanding» when sending files. However, it is more reliable to send files packed in archives (the most popular one is ZIP). Inside the archive, no one will re-encode the file on the way.

Return mail.

If it is impossible to deliver the mail to the sender, a service message with a refusal comes. There are three main reasons for the refusal:

Host unknown – the domain name of the computer (mail server) can not be converted to the address – check the spelling of the computer name;

User unknown – user (mail server) is unknown – check the spelling of the user name;

Service unavailable or Can not send message for … days – the destination mail server does not work (no communication or the mail server software is disabled).

Mail retrieval protocols

After the mail reaches the destination server, it performs temporary or permanent storage of the received mail. There are two different models of working with mail: the concept of a mail storage (mailbox) and a mail terminal.

POP3

In the concept of mail storage, mail on the server is stored temporarily, in a limited amount (similar to the mailbox for paper mail), and the user periodically accesses the box and “picks up” letters (that is, the mail client downloads a copy of the letter to itself and deletes the original from the mailbox) . Based on this concept, the POP3 protocol operates.

IMAP

The concept of the postal terminal implies that all correspondence related to the mailbox (including copies of sent messages) is stored on the server, and the user accesses the repository (sometimes it is also traditionally called a “mailbox”) to view correspondence (both new and archive) and writing new letters (including replies to other letters). This principle is the protocol of IMAP and most of the web interfaces of free mail services. This storage of mail correspondence requires much higher capacity from mail servers, as a result, in many cases there is a separation between mail servers that forward mail and mail storage servers.

Differences

Based on the work of the protocols, they can be divided into two main criteria:

Server performance – in this case, IMAP is more resource-intensive than POP3, since all mail processing (such as searching) falls on the server’s shoulders, POP3 only transmits mail to the client;

Bandwidth of the channel – here IMAP in a prize; POP3 transmits the bodies of all letters entirely, whereas IMAP – can transmit separate parts of messages, for example, only text, and the rest – on request.

Under certain conditions, the mail storage server can be configured for behavior similar to the client: such a server accesses the mail server using the POP3 protocol and picks up the mail to itself. Such solutions are usually used in small organizations, in which there is no infrastructure for deploying full-fledged mail servers; In this case, a local server is used to store mail and the mail server of the provider that provides the service for receiving mail via POP3 (for example, using fetchmail). The main drawback of this solution is the delay in delivery (since the receiving software downloads to the server with some delay) – for example, a POP3 connector from Exchange 2003 Server in Windows SBS does not allow an interval of less than 15 minutes to be set by the configuration interface , since An excessive frequency of checks can cause problems with the load on the mail server. Some mail servers have the means to protect against this behavior.

Structure of the letter

When sending via SMTP, an e-mail consists of the following parts:

SMTP envelope data received by the server. Some of this data may be missing in the message itself. For example, in RCPT TO (envelope to) contains a list of recipients of the letter, while in the letter the recipient may not be specified. This information is sent out of the server only within the SMTP protocol, and changing the protocol when delivering mail (for example, at the destination node during internal routing) can lead to the loss of this information. In most cases, this information is not available to the final recipient, which uses non-SMTP protocols (POP3, IMAP) to access the mailbox. To be able to monitor the system’s performance, this information is usually stored in the mail server logs.

The message itself (in the terminology of the SMTP protocol – ‘DATA’), which in turn consists of the following parts, separated by an empty string:

Headers of the letter. In the header, the service information and the marks of the mail servers through which the letter passed, the priority notes, the address and the name of the sender and the recipient of the letter, the subject of the letter and other information are indicated in the header.

The body of the letter. In the body of the letter is, in fact, a letter message.

SMTP envelope data

The SMTP envelope data contains parameters that are specified by the same commands:

The parameter HELO / EHLO – contains the name (FQDN) of the sending node, or the address-literal of the sending node.

The MAIL FROM parameter contains the sender’s e-mail. The address can be arbitrary (including nonexistent, which is allowed by the SMTP protocol, however, RFC 5321 recommends using either Null Reverse-Path for special messages, or an existing e-mail [8]), but this value is used to generate error notifications (rather than the value from the From field of the message header). This address can also be checked during initial spam check [9] and in other cases [what?]. When sending a message, a regular mail client generates MAIL FROM from the contents of the From field.

The RCPT TO parameter is the most important content of the envelope for delivering mail, it contains the electronic address of one or several recipients. When creating an SMTP envelope, RCPT TO can be used several times. When sending a message, a regular mail client forms a list for RCPT TO from the contents of the To, Cc, and Bcc message fields.

Email headers

The headers of the letter are described in RFC standards:

RFC 2076 – Common Internet Message Headers, includes information from other RFCs: RFC 822, RFC 1036, RFC 1123, RFC 1327, RFC 1496, RFC 1521, RFC 1766, RFC 1806, RFC 1864, RFC 1911).

RFC 4021 – Registration of Mail and MIME Header Fields (registration of mail and MIME header fields).

Headers are separated from the body of the message by an empty line. Headers are used to log the passage of letters and service marks (sometimes they are called clads). In Microsoft Outlook, these headers are called “Internet headers”. The headers usually specify: the mail servers through which the email passed (each mail server adds information about who it received this email from), information about whether this email is similar to spam, information about scanning by antivirus, the level of urgency of the message (can change mail servers). Also in the header is usually written the program with which the letter was created. Since the headers are the service information, most often the mail clients hide them from the user during normal reading of letters, but also provide an opportunity to see these headers if there is a need for a more detailed analysis of the letter. In the event that a message from the SMTP format is converted to another format (for example, in Microsoft Exchange 2007, the letters are converted to MAPI), then the headers are saved separately for the possibility of diagnostics.

Headers are usually added from the bottom up (that is, every time a header is added to the message, it is appended to the first line, before all the previous ones).