Web protocols. Web protocols

Allows you to receive various resources, such as HTML documents. The HTTP protocol underlies data exchange on the Internet. HTTP is a client-server communication protocol, which means that requests to the server are initiated by the recipient himself, usually a web browser. The resulting final document will (may) consist of various sub-documents that are part of the final document: for example, separately received text, a description of the document structure, images, video files, scripts and much more.

Clients and servers communicate by exchanging single messages (rather than a stream of data). Messages sent by a client, usually a web browser, are called requests, and messages sent by the server are called answers.

Although HTTP was developed in the early 1990s, it has been continually improved due to its extensibility. HTTP is an application layer protocol that most often uses the capabilities of another protocol - TCP (or TLS - secure TCP) - to forward its messages, but any other reliable transport protocol can theoretically be used to deliver such messages. Due to its extensibility, it is used not only for the client to receive hypertext documents, images and videos, but also for transmitting content to servers, for example, using HTML forms. HTTP can also be used to retrieve only parts of a document for the purpose of updating a web page on demand (for example via an AJAX request).

Components of HTTP-based systems

HTTP is a client-server protocol, that is, requests are sent by one party - a participant in the exchange (user-agent) (or a proxy instead). Most often, the participant is a web browser, but it can be anyone, for example, a robot traveling the Web to replenish and update web page indexing data for search engines.

Each request request) is sent to the server, which processes it and returns a response (eng. response). Between these requests and responses there are typically numerous intermediaries called proxies, which perform various operations and act as gateways or caches, for example.

Typically, between the browser and the server there are many more different intermediary devices that play some role in processing the request: routers, modems, and so on. Due to the fact that the Network is built on the basis of a system of interaction levels (layers), these intermediaries are “hidden” at the network and transport levels. In this layering system, HTTP occupies the topmost layer, which is called the "application" layer (or "application layer"). Knowledge of the network layers, such as presentation, session, transport, network, link, and physical, is important for understanding network operation and diagnosing possible problems, but is not required for describing and understanding HTTP.

Client: exchange participant

A user agent is any tool or device that acts on behalf of a user. This task is primarily performed by the web browser; in some cases, the participants are programs that are used by engineers and web developers to debug their applications.

Browser Always is the entity that creates the request. The server usually does not do this, although over the years the network has developed ways to allow server-side requests to be fulfilled.

To display a web page, the browser sends an initial request to obtain the HTML document of that page. After this, the browser examines this document and requests additional files necessary to display the content of the web page (executable scripts, page layout information - CSS style sheets, additional resources in the form of images and video files), which are directly part of the source document, but located elsewhere on the network. Next, the browser connects all these resources to display them to the user in the form of a single document - a web page. Scripts executed by the browser itself can receive additional resources over the network at later stages of processing of the web page, and the browser updates the display of that page to the user accordingly.

A web page is a hypertext document. This means that some parts of the displayed text are links that can be activated (usually by clicking a mouse button) to retrieve and consequently display a new web page (following a link). This allows the user to “navigate” web pages (Internet). The browser converts these hyperlinks into HTTP requests and subsequently displays the received HTTP responses in a user-friendly form.

Web server

On the other side of the communication channel there is a server that serves (eng. serve) user, providing him with documents upon request. From the point of view of the end user, the server is always a single virtual machine that completely or partially generates a document, although in fact it can be a group of servers between which the load is balanced, that is, requests from different users are redistributed, or complex software that polls other computers (such such as caching servers, database servers, e-commerce application servers and others).

A server is not necessarily located on one machine, and vice versa - several servers can be located (hosted) on the same machine. According to HTTP/1.1 version and having a Host header, they can even share the same IP address.

Proxy

Between the web browser and the server there are a large number of network nodes transmitting HTTP messages. Due to their layered structure, most of them also operate at the transport network or physical layers, becoming transparent to the HTTP layer and potentially reducing performance. These application-level operations are called proxy . They may or may not be transparent (modifying requests will not pass through them), and can perform many functions:

• caching (cache can be public or private, like browser cache)
• filtering (like antivirus scanning, parental controls, ...)
• load balancing (allow multiple servers to serve different requests)
• authentication (control access to different resources)
• logging (permission to store transaction history)

Basic Aspects of HTTP

HTTP is simple

Even with the greater complexity introduced in HTTP/2 by encapsulating HTTP messages in frames, HTTP is generally simple and human-readable. HTTP messages can be read and understood by humans, providing easier testing for developers and reduced complexity for new users.

HTTP - extensible

The HTTP headers introduced in HTTP/1.0 made the protocol easy to extend and experiment with. New functionality can even be introduced by a simple agreement between client and server on the semantics of the new header.

HTTP is stateless but has a session

HTTP is stateless: there is no relationship between two requests that are executed sequentially over the same connection. This immediately implies the potential for problems for a user attempting to interact with a particular page sequentially, such as when using a shopping cart in an online store. But while HTTP core is stateless, cookies enable stateful sessions. Using header extensibility, cookies are added to the worker thread, allowing the session to share some context, or state, on each HTTP request.

HTTP and connections

The connection is managed at the transport layer, and therefore fundamentally goes beyond the boundaries of HTTP. Although HTTP does not require the underlying transport protocol to be connection-based, requiring only reliability, or no lost messages (i.e., at least an error representation). Among the two most common Internet transport protocols, TCP is reliable while UDP is not. HTTP subsequently relies on the TCP standard being connection-based, even though a connection is not always required.

HTTP/1.0 opened a TCP connection for each request/response exchange, with two important drawbacks: opening a connection requires multiple message exchanges and is therefore slow, although it becomes more efficient when sending multiple messages, or when sending messages regularly: warm connections are more effective than cold.

To mitigate these shortcomings, HTTP/1.1 introduced pipelining (which proved difficult to implement) and persistent connections: the underlying TCP connection can be partially controlled through the Connection header. HTTP/2 took the next step by adding multiplexing of messages across a simple connection, helping to keep the connection warm and more efficient.

Experiments are being conducted to develop a better transport protocol more suitable for HTTP. For example, Google is experimenting with QUIC, which is based on UDP, to provide a more reliable and efficient transport protocol.

What can be controlled via HTTP

The natural extensibility of HTTP has allowed greater control and functionality of the Web over time. Cache and authentication methods were early features in HTTP history. The ability to relax the original restrictions, on the other hand, was added in the 2010s.

The following are common functions managed with HTTP.

• 
  The server can instruct proxies and clients what to cache and for how long. The client can instruct intermediate cache proxies to ignore stored documents.
• Relaxing Source Constraints
  To prevent spyware and other privacy-violating intrusions, the web browser maintains strict separation between websites. Only pages from same source can access information on the web page. Although such restrictions are taxing on the server, HTTP headers can relax the strict separation on the server side, allowing the document to become part of information from different domains (for security reasons).
• Authentication
  Some pages are only available to special users. Basic authentication can be provided via HTTP, either through the use of the WWW-Authenticate and similar headers, or by setting up a special session using cookies.
• Proxy and tunneling
  Servers and/or clients are often located on an intranet, and hide their true IP addresses from others. HTTP requests go through a proxy to cross this network barrier. Not all proxies are HTTP proxies. The SOCKS protocol, for example, operates at a lower level. Others, such as ftp, can be handled by these proxies.
• Sessions
  Using an HTTP cookie allows you to associate a request with a state on the server. This creates a session, even though HTTP is a stateless protocol at its core. This is useful not only for shopping carts in online stores, but also for any sites that allow the user to customize the exit.

HTTP stream

When a client wants to communicate with a server, whether it is a final server or an intermediate proxy, it follows these steps:

1. Opening a TCP connection: A TCP connection will be used to send a request or requests and receive a response. The client can open a new connection, reuse an existing one, or open multiple TCP connections to the server.
2. Sending an HTTP message: HTTP messages (before HTTP/2) are human-readable. Since HTTP/2, simple messages are encapsulated in frames, making them impossible to read directly, but fundamentally remain the same. GET / HTTP/1.1 Host: site Accept-Language: fr
3. Reads response from server: HTTP/1.1 200 OK Date: Sat, 09 Oct 2010 14:28:02 GMT Server: Apache Last-Modified: Tue, 01 Dec 2009 20:18:22 GMT ETag: "51142bc1-7449-479b075b2891b" Accept-Ranges: bytes Content-Length: 29769 Content-Type: text/html
4. Closes or reuses the connection for further requests.

If the HTTP pipeline is enabled, multiple requests can be sent without waiting for the first response to be received in its entirety. The HTTP pipeline is difficult to integrate into existing networks, where old pieces of software coexist with modern versions. The HTTP pipeline was replaced in HTTP/2 with more reliable multiplexed requests in a frame.

HTTP messages

HTTP/1.1 and earlier HTTP messages are human-readable. In HTTP/2, these messages are embedded in a new binary structure, a frame, that allows optimizations such as header compression and multiplexing. Even if part of the original HTTP message is sent in this version of HTTP, the semantics of each message are not changed and the client recreates (virtually) the original HTTP request. It is also useful for understanding HTTP/2 messages in HTTP/1.1 format.

There are two types of HTTP messages, requests and responses, each in a different format.

Requests

Examples of HTTP requests:

• HTTP method, usually a verb like GET,
• Headers (optional) that provide additional information to the server.
• Or a body, for some methods such as POST, which contains the resource that was sent.

Answers

Example answers:

• HTTP protocol version.
• HTTP status code indicating the success of the request or the reason for failure.
• Status message -- a brief description of the status code.
• HTTP headers are similar to headers in requests.
• Optional: a body containing the resource being sent.

Conclusion

HTTP is an easy-to-use, extensible protocol. The client-server structure, coupled with the ability to easily add headers, allows HTTP to move forward with the expanding capabilities of the Web.

Although HTTP/2 adds some complexity by embedding HTTP messages in frames to improve performance, the basic message structure remains with HTTP/1.0. The session thread remains simple, allowing you to explore and debug with ease.

After the client has connected to the service on a specific port, he gains access to the service using the established protocol. A protocol is a pre-developed procedure for the exchange of information between the party wishing to use the service and the party providing the service. The “party” that needs the service can be a person, but most often it is a computer program, for example, a WEB browser. Protocols are often textual descriptions of the procedure for exchanging information between a client and a server.

UNIX

Perhaps the simplest protocol is for the daytime service. If you connect to port 13 of a machine that supports a daytime server, the server will respond with the current date and time information and then disconnect the connection. The protocol is as follows: “If a client establishes a connection with a daytime server, the date and time data is sent to it, after which the connection is disconnected.” Most UNIX machines support this server. You can contact him using the Telnet application. On UNIX, the session will look like this:

• %telnet web67.ntx.net 13
• Connection to web67.ntx.net.
• The cancel character "^]".
• Sunday 25 October 08:34:06 1998

Windows

On a Windows machine, you can access this server by entering “telnet web67.ntx.net 13” in the MSDOS window.

In this example, web67.ntx.net is the UNIX server machine, and 13 is the port number for the daytime service. The Telnet application connects to port 13 (telnet usually connects to port 23, but you can specify any other port to connect to), then the server sends the date and time data, and then closes the connection. Most versions of Telnet have the ability to specify a port number, and this feature can be used regardless of what version of Telnet is installed on the machine.

Most protocols are more complex than the daytime protocol and are defined in publicly available Requests for Comments (RFCs) (for a good archive of all RFCs see sunsite.auc.dk/RFC/). Every WEB server on the Internet complies with the requirements of the HTTP protocol, compiled in The Original HTTP document in 1991. The most important form of protocol that is understood by an HTTP server involves only one command: GET. If you establish a connection with a server running over the HTTP protocol and send a “GET file name” request, the server will respond by sending the contents of the specified file to the request source, and then disconnect the connection. A typical session looks like this:

• %telnet site 80
• Attempting to connect to 78.110.59.235…
• Connection with pcwork.ru.
• The cancel character "^]".
• The connection is disabled by the external host computer.

In the initial version of the HTTP protocol, only the actual file name needed to be sent, for example, [/] or The protocol was later changed to be able to handle full URLs. This allowed companies that deal with virtual domains, in conditions where many domains are located on one machine, to use one IP address for all such domains.

Summarize

After reading this article, you have learned a lot about. Specifically, you now know that when you type a URL into a browser, the following happens:

1. divided the URL into three parts:

• Protocol (“http”)
• Server name (“site”) - recently there has been a positive trend to shorten the first three letters www
• File name (“web server.htm”)

The browser contacts the name server to translate the server name into an IP address, which is used by that browser to connect to the corresponding server machine.

After receiving the IP address, the specified browser establishes a connection to the WEB server that has this IP address on port 80.

In accordance with the HTTP protocol, the browser sends a GET request to this server to receive the file (Note that along with the GET request, cookies can be sent from the browser to the server - details can be found in the article on how cookies work).

The server sends the HTML text of the requested WEB page to the browser. (Cookies can also be passed from the server to the browser in the page header.)

The browser reads the HTML tags and displays the corresponding page on the monitor screen.

Addendum: Security

From this description we can conclude that a WEB server can be a fairly simple program. It takes the file name sent with the GET command, finds the file it is looking for and sends it to the browser. Even with all the port management and port communication code, you can easily create a C program that acts as a simple WEB server in less than 500 lines of code. Of course, a full-featured corporate WEB server is more complex, but the basic principles of its operation still remain very simple.

Most servers add some level of security into the workflow. For this purpose, for example, password-protected pages are used. When you try to open such a page with your browser, a dialog box appears asking you to enter your username and password. The server provides the owner of the WEB page with the ability to use a list of names and passwords of people who are allowed access to this page; in this case, the server allows viewing the page only to those who have the appropriate password. Advanced servers have additional security features that encrypt information exchanged between the server and the browser, allowing sensitive information such as credit card numbers to be sent over the Internet.

These are practically all the functions that a server designed to send standard, static WEB pages can perform. Static pages are those WEB pages that do not change until they are edited by the developer.

Add-on: Dynamic Pages

What about dynamic WEB pages? For example:

• In any book of reviews you are allowed to leave messages in HTML form and the next time you view it, the new information entered is saved on this page.
• In the Network Solutions whois screen form, in response to entering a domain name, a WEB page is received, the appearance of which depends on the entered name.
• In any search engine, keywords are entered into an HTML form, after which the machine creates a page displaying the search result for these words.

In all of the above cases, the WEB server does not just search for the required file. It processes the received information and creates a WEB page that corresponds in a certain way to the received request. In almost all cases, the WEB server uses the so-called CGI procedure to solve this problem.

Each server machine exposes services from the Internet using numbered ports, one for each service available on the server. For example, if the server machine has a WEB server and an FTP server, usually the WEB server is accessed through port 80, and the FTP server is accessed through port 21. Clients connect to the service by selecting the appropriate address and connecting to the appropriate port.

Each popular service is assigned a specific port number. The following are the most commonly used port numbers:

• echo 7
• daytime 13
• qotd 17 (Quote of the day)
• ftp 21
• telnet 23
• smtp 25 (Email)
• time 37
• nameserver 53 (Name server)
• nicname 43 (Who's who)
• gopher 70
• finger 79
• WWW 80

Restrictions

If the server machine allows connection to a port from the Internet and this port is not protected, you can connect to it from anywhere on the Internet and use the corresponding service. It should be noted that there are no restrictions requiring, for example, the WEB server to connect on port 80. By commissioning your own machine and installing the WEB server software on it, you can, if desired, specify that the WEB server should work, for example, on port 918 , or on any other unoccupied port. Then, if the machine is named xxx.yyy.com, you can connect to this server over the Internet using the URL xxx.yyy.com:918. The ":918" part explicitly indicates the port number and must be added by anyone wishing to contact this server. If no port is specified, the browser defaults to trying to connect to the common port 80.

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Web service (service) is a program that organizes interaction between sites. Information from one portal is transferred to another.

For example, there is an airline. She has many flights, which means she has a lot of tickets. It transmits information through a web service to a travel aggregator site. A user who accesses the aggregator will be able to buy tickets for this airline directly there.

Another example of web services is a weather tracking site that contains information about weather conditions in a specific city or country as a whole. This information is also often used by third parties.

Information on the Internet is varied. Sites are managed by different systems. Different transmission and encryption protocols are used. Web services simplify the exchange of information between different sites.

Web services architecture and protocols

You can define 3 authorities that interact with each other: catalogue, contractor and customer. After creating the service, the contractor registers it in the catalog, and the customer finds the service there.

The data exchange mechanism is formed in the Web Services Description. This is a specification covering forwarding formats, content types, transport protocols that are used in the process of exchanging information between the customer and the service transporter.

Today, several technologies are most often used to implement various web services:

1. TCP/IP is a protocol that is understood by almost any network equipment, from mainframes to portable devices and PDAs.
2. HTML is a universal markup language used to display content on consumer devices.
3. XML is a universal tool for processing all types of data. Other information exchange protocols can work on its basis: SOAP and WSDL.
4. UDDI is a universal source of recognition, integration and description. It works, as a rule, in private networks and has not yet found sufficient distribution.

The versatility of the presented technologies is the basis for understanding web services. They operate on standard technologies that are independent of application providers and other network resources. Can be used in any operating systems, application servers, programming languages, etc.

Advantages

• Creating the necessary conditions for the interaction of software components, regardless of the platform.
• Web services are based on open standard protocols. Due to the introduction of XML, the creation and configuration of web services is simplified.
• The use of HTTP guarantees the interaction of systems through internetwork access.

Flaws

• Low performance and large volume of traffic, in comparison with RMI, CORBA, DCOM systems, due to the use of XML messages in the context of text.
• Security level. All modern web services must implement coding and require user authorization. Whether HTTPS is enough here or more reliable protocols are needed, such as XML Encryption, SAML, etc., are decided during development.

Web Services Tasks

Web services can be used in many areas.

B2B transactions

Integration of processes occurs immediately, without the participation of people. For example, updating the online store catalog with new products. They are brought to the warehouse, and the storekeeper notes the arrival in the database. The information is automatically transferred to the online store. And the buyer, instead of marking “Out of stock” on the product card, sees its quantity.

Integration of enterprise services

If the company uses corporate programs, then the web service will help set up their joint work.

Creating a client-server system

Services are used to configure the operation of the client and server. This provides benefits:

• You can sell not the software itself, but make paid access to the web service;
• It’s easier to solve problems using third-party software;
• it is easier to organize access to the content and materials of the server.

A web service is an application that simplifies the technical setup of resource interaction.

The World Wide Web is a ready-made platform for creating and using distributed machine-oriented systems based on web services. The web server acts as an application server that is accessed not by end users, but by third-party applications. This allows you to reuse functional elements, eliminate code duplication, and simplify the solution of application integration problems.

Web Service, web service(eng. web-service) is a network technology that provides inter-program interaction based on web standards. The W3C defines a web service as “a software system designed to support interoperable machine-to-machine communication over a network.”

Web Services: Concepts and Protocols

A web service is identified by a URI string. The web service has a software interface presented in a machine-processable format. Other systems interact with this web service by exchanging protocol messages. The HTTP protocol is used as a transport for messages. Descriptions of web services and their APIs can be found using . The conceptual diagram of the technology is shown in, and the relationship between the protocols is shown in Fig. 2.

Rice. 1. Web service concept

• SOAP(Simple Object Access Protocol) - message exchange protocol between the consumer and the web service provider;
• WSDL(Web Services Description Language) - language for describing external interfaces of a web service;
• UDDI(Universal Discovery, Description and Integration) is a universal recognition, description and integration interface used to create a catalog of web services and access it.

Rice. 2. Web services protocols

All specifications used in the technology are based on XML and, accordingly, inherit its advantages (structuredness, flexibility, etc.) and disadvantages (cumbersomeness, slowness).

SOAP

SOAP (originally Simple Object Access Protocol, and in version 1.2 there is no official decoding of the abbreviation) is a simple protocol for accessing objects (components of a distributed computing system), based on the exchange of structured messages. Like any text protocol, SOAP can be used with any application layer protocol: SMTP, FTP, HTTPS, etc., but most often SOAP is used over HTTP.

All SOAP messages are formatted in a structure called envelope(envelop), which includes the following elements:

• Message ID (local name).
• Optional Header element:
  • Zero or more properties available in this namespace.
• Required Body element (message body)
  • Zero or more references to used namespaces;
  • Message Body Child Elements

A detailed list of SOAP message elements is given in the data schema (for SOAP version 1.2).

Sample SOAP message:

Envelope xmlns:env=" http://www.w3.org/2003/05/soap-envelope"> Header> 1 2001-06-22T14:00:00-05:00 Header> Body> Get up at 6:30 AM Body> Envelope>

XML-RPC: Not a competitor, but an alternative to SOAP

XML-RPC is a very simple and efficient protocol for communicating web services. It is not designed to solve global problems like SOAP, but is widely used in many web developments.

XML-RPC Concept

Table 1. Basic elements of the WSDL protocol.

WSDL 1.1 element	WSDL 2.0 element	Short description
PortType	Interface	Represents a description of the web service interface (a list of operations and their parameters).
Service	Service	List of system functions
Binding	Binding	Specifies interfaces and sets binding parameters with the SOAP protocol: binding style (RPC/Document) and transport (SOAP). This section is also available for each of the operations
Operation	Operation	Defines the operation presented by the web server. A WSDL operation is an analogue to traditional functions and procedures.
Message	not used	A message associated with a specific operation. Contains information necessary to perform a given operation. Each message can consist of several logical parts describing data types and attribute names. In version 2.0 it was excluded because XML Schema support was introduced for all elements.
Types	Types	Description of data in accordance with XML Schema.

Rice. 3. WSDL protocol structure

The WSDL 1.1 specification defined 4 message exchange patterns (operation types):

• One-way operations: An operation can receive a message but will not return a response.
• Request-response: The operation can accept a request and must return a response.
• Question-answer (Solicit-response): the operation can send a request and will wait for a response to it.
• Notification: The operation can send a message, but will not wait for a response.

In WSDL 2.0, these templates are modified and expanded to support error messages (for example, the Robust-in-only template), but WSDL 1.1 types are supported for backward compatibility

UDDI relies on industry standards HTTP, XML, XML Schema (XSD), SOAP, and WSDL. The conceptual relationship between UDDI and other protocols in the web services stack is shown in .

Rice. 4. UDDI's Place in the Web Services Protocol Stack

The functionality of the UDDI registry is to represent data and metadata about web services. It can be used both on a public network and within the internal infrastructure of an organization. The UDDI registry provides a standards-based mechanism for classifying, cataloging, and managing web services so that web services can be used by other applications. This mechanism includes facilities for finding a service, invoking that service, and managing metadata about that service.

The key features of UDDI are publishing information about a service to a registry and having third-party applications look up that information. Along with these, functions typical for a directory service are also implemented: representation of the stored data model and infobase structure, relationships between registry objects, replication, security, etc. —All the main functions of the registry are presented as web services and are accessible through the UDDI API.

Web services: Pro et Contra

Advantages

• Ensure the interaction of software systems regardless of the platform.
• Based on open standards and protocols.
• Using HTTP allows applications to communicate across a firewall.

Flaws

• Lower performance and higher volume of network traffic compared to technologies such as CORBA or DCOM.

Permanent address of this page:

Protocol is a set of agreements that defines the exchange of data between different programs. Protocols define how messages are transmitted and errors handled in a network, and also allow the development of standards that are not tied to a specific hardware platform.

Network protocols prescribe rules for the operation of computers connected to the network. They are built on a multi-level principle. A protocol at some level defines one of the technical rules of communication. Currently, the OSI (Open System Interconnection) model is used for network protocols. The OSI model is a seven-layer logical model of network operation. The OSI model is implemented by a group of protocols and communication rules organized into several layers.

On physical level the physical (mechanical, electrical, optical) characteristics of communication lines are determined.

On link level the rules for using the physical layer by network nodes are determined.

- Network layer is responsible for addressing and delivering messages.

- Transport layer controls the order of transmission of message components.

Task session level- coordination of communication between two application programs running on different workstations.

- Presentation layer serves to convert data from the internal computer format to the transmission format.

- Application layer is the boundary between the application program and other levels. The application layer provides a convenient communication interface for user network programs.

TCP/IP protocol are two lower-level protocols that are the basis of Internet communications. TCP protocol(Transmission Control Protocol) breaks the transmitted information into portions and numbers all portions. By using IP protocol(Internet Protocol) all parts are transmitted to the recipient. Next, using the TCP protocol, it is checked whether all parts have been received. When receiving all the portions, TCP places them in the required order and assembles them into a single whole.

The most well-known protocols used on the Internet:

HTTP (Hyper Text Transfer Protocol) is a hypertext transfer protocol. The HTTP protocol is used to send Web pages from one computer to another.

FTP (File Transfer Protocol) is a protocol for transferring files from a special file server to the user’s computer. FTP allows the subscriber to exchange binary and text files with any computer on the network. Having established a connection with a remote computer, the user can copy a file from the remote computer to his own or copy a file from his computer to the remote one.

POP (Post Office Protocol) is a standard mail connection protocol. POP servers process incoming mail, and the POP protocol is designed to handle mail requests from client mail programs.

SMTP (Simple Mail Transfer Protocol) standard specifies a set of rules for sending mail. The SMTP server returns either an acknowledgment or an error message, or requests additional information.

UUCP (Unix to Unix Copy Protocol) is a now outdated but still used data transfer protocol, including for email. This protocol involves the use of a packet method of information transfer, in which a client-server connection is first established and a data packet is transmitted, and then it is independently processed, viewed or prepared.

TELNET is a remote access protocol. TELNET allows the subscriber to work on any computer on the Internet as if it were his own, that is, launch programs, change the operating mode, etc. In practice, the capabilities are limited by the access level set by the administrator of the remote machine.

DTN - a protocol designed to provide ultra-long-distance space communications.