TCP/IP socket HTTP)

The layer-7 network consists of the physical layer, data link layer, network layer, transmission layer, Session Layer, presentation layer, and application layer.
The physical layer, data link layer, and network layer are usually called media layers, which are the objects of network engineers;
The transport layer, Session Layer, presentation layer, and application layer are called the host layer, which are user-oriented and concerned content.

The HTTP protocol corresponds to the application layer.

The TCP protocol corresponds to the transport layer

The IP protocol corresponds to the network layer.
They are essentially incomparable. Moreover, HTTP is based on TCP connections.

TCP/IP is the transport layer protocol, which mainly solves how data is transmitted over the network. HTTP is the application layer protocol and mainly solves how to package data.
When transmitting data, we can only use the transport layer (TCP/IP). However, because there is no application layer, the data content cannot be identified. If you want to make the transmitted data meaningful, you must use the application layer protocol. There are many application layer protocols, including HTTP, FTP, and telnet. You can also define the application layer protocol by yourself. The Web uses HTTP as the transport layer protocol to encapsulate HTTP text information, and then uses TCP/IP as the transport layer protocol to send it to the network.

Socket is the encapsulation of TCP/IP protocol. socket itself is not a protocol, but an interface (API) called. Through socket, we can use TCP/IP protocol.

Differences between HTTP and socket connections

I believe that many new online mobile phone developers want to know what is the difference between HTTP and socket connections, and hope that they can help beginners through their own simple understanding.
1. TCP Connection
To understand the socket connection, you must first understand the TCP connection. The mobile phone can use the internet function because the mobile phone bottom layer implements the TCP/IP protocol, which enables the mobile terminal to establish a TCP connection through the wireless network. The TCP protocol can provide interfaces for upper-layer networks, so that data transmission over the upper-Layer Networks is established on a "non-differentiated" network.
To establish a TCP connection, you must go through the "three-way handshake ":
The first handshake: the client sends the SYN Packet (SYN = J) to the server and enters the syn_send status, waiting for confirmation from the server;
The second handshake: when the server receives the SYN packet, it must confirm the customer's Syn (ACK = J + 1) and send a SYN Packet (SYN = K), that is, the SYN + ACK packet, the server enters the syn_recv status;
The third handshake: the client receives the server's SYN + ACK package and sends the ACK (ACK = k + 1) Confirmation package to the server. After the package is sent, the client and server enter the established status, complete three handshakes.
The package transmitted during the handshake does not contain data. After three handshakes are completed, the client and the server formally start to transmit data. Ideally, once a TCP connection is established, the TCP connection will remain until either of the two parties closes the connection. When a TCP connection is disconnected, both the server and client can initiate a request to disconnect the TCP connection. The disconnection process must go through the "Four handshakes" (the process is not detailed, that is, the server interacts with the client, final confirmation of disconnection)

2. HTTP Connection
The HTTP protocol, Hypertext Transfer Protocol, is the foundation of Web networking and one of the commonly used protocols for mobile phone networking. HTTP is an application built on the TCP protocol.
The most notable characteristic of HTTP connection is that each request sent by the client requires a server to send a response. After the request ends, the connection is released. The process from establishing a connection to closing a connection is called "One connection ".
1) in HTTP 1.0, each client request requires a separate connection. After processing the request, the connection is automatically released.

2) in HTTP 1.1, multiple requests can be processed in a single connection, and multiple requests can be overlapped without waiting for a request to end before sending the next request.

The connection is automatically released after each HTTP request ends. Therefore, an HTTP connection is a "short connection" that requires the client program to be online, you must constantly initiate connection requests to the server. The common practice is to immediately get no data, and the client also keeps sending a "keep connection" request to the server at a fixed time, after receiving the request, the server replies to the client, indicating that the client is "online ". If the server cannot receive a request from the client for a long time, the client is considered to be "offline". If the client cannot receive a response from the server for a long time, the network is considered disconnected.

3. Socket Principle
3.1 socket Concept
Socket is the cornerstone of communication and the basic operation unit for network communication that supports TCP/IP protocol. It is an abstract representation of the endpoint in the network communication process. It contains five types of information required for network communication: the protocol used for connection, the IP address of the local host, and the protocol port of the local process, the IP address of the remote host and the protocol port of the remote process.
When the application layer communicates data through the transport layer, TCP may encounter concurrent services for multiple application processes at the same time. Multiple TCP connections or multiple application processes may need to transmit data through the same TCP port. To differentiate different application processes and connections, many computer operating systems provide socket interfaces for applications to interact with TCP/IP protocols. The application layer and the transport layer can use socket interfaces to distinguish between processes or network connections of different applications to implement concurrent data transmission services.

3.2 establish a socket connection
To establish a socket connection, you must have at least one socket. One of them runs on the client, which is called clientsocket, And the other runs on the server, which is called serversocket.
The connection process between sockets is divided into three steps: server listening, client requests, and connection confirmation.
Server listening: the server socket does not locate the specific client socket, but is waiting for connection. It monitors the network in real time and waits for client connection requests.
Client request: the client socket initiates a connection request, and the target is the server socket. Therefore, the client socket must first describe the socket of the server to be connected, point out the address and port number of the socket on the server, and then submit a connection request to the socket on the server.
Connection Confirmation: when the server socket monitors or receives a connection request from the client socket, it responds to the request from the client socket and creates a new thread, send the description of the server socket to the client. Once the client confirms the description, both parties establish a connection. The server socket continues to be in the listening status, and continues to receive connection requests from other client sockets.

4. socket connection and TCP Connection
When creating a socket connection, you can specify the transport layer protocol used. The socket can support different transport layer protocols (TCP or UDP). When using the TCP protocol for connection, the socket connection is a TCP connection.

5. socket connection and HTTP Connection
Generally, a socket connection is a TCP connection. Once a socket connection is established, both parties can send data to each other until the connection is closed. However, in actual network applications, communication between the client and the server often needs to traverse multiple intermediate nodes, such as routers, gateways, and firewalls, by default, most firewalls disable connections that are not active for a long time and cause the socket connection to be disconnected. Therefore, the network needs to be notified through polling that the connection is active.
The HTTP connection uses the "request-response" method. Not only do you need to establish a connection during the request, but you also need the client to send a request to the server before the server can reply to the data.
In many cases, the server needs to actively push data to the client to ensure real-time and synchronous data between the client and the server. If both parties establish a socket connection, the server can directly transmit data to the client. If both parties establish an HTTP connection, the server must wait until the client sends a request before sending the data back to the client. Therefore, the client periodically sends a connection request to the server, which can not only be kept online, at the same time, it is also "asking" whether the server has new data. If yes, it will send the data to the client.

What does HTTP connection mean?

HTTP is an object-oriented protocol at the application layer. It is applicable to distributed hypermedia information systems due to its simple and fast method. It proposed in 1990 that, after several years of use and development, it has been continuously improved and expanded. At present, the sixth version of HTTP/1.0 is used in WWW, the standardization of HTTP/1.1 is in progress, and the proposal of HTTP-NG (Next Generation of HTTP) has been put forward. (the Agreement is a global positioning !)
Core of WWW-http protocol

As we all know, the basic protocol of the Internet is TCP/IP. FTP and Archie gopher are widely used as application layer protocols based on TCP/IP, different protocols correspond to different applications. The main protocol used by the WWW server is the HTTP protocol, that is, the ultra-style transmission protocol. Because HTTP supports services not limited to www, but also other services, HTTP allows users to access different services using different protocols on a unified interface, such as FTP, Archie, SMTP, and nntp. In addition, HTTP can also be used for name server and distributed object management.

2.1 Introduction to HTTP
HTTP is an object-oriented protocol at the application layer. It is applicable to distributed hypermedia information systems due to its simple and fast method. It proposed in 1990 that, after several years of use and development, it has been continuously improved and expanded. Currently, the sixth version of HTTP/1.0 is used in WWW, standardization of HTTP/1.1 is in progress, and suggestions for HTTP-NG (Next Generation of HTTP) have been put forward.
The main features of HTTP are as follows:
1. Supports the customer/Server mode.
2. simple and fast: when a customer requests a service from the server, they only need to send the request method and path. Common Request methods include get, Head, and post. Each method specifies the type of contact between the customer and the server. Because the HTTP protocol is simple, the program size of the HTTP server is small, so the communication speed is fast.
3. Flexibility: HTTP allows transmission of any type of data objects. The type being transferred is marked by Content-Type.
4. No connection: No connection means that only one request is allowed for each connection. After the server processes the customer's request and receives the customer's response, the connection is disconnected. This method can save transmission time.
5. Stateless: HTTP is stateless. Stateless means that the Protocol has no memory for transaction processing. The lack of status means that if subsequent processing requires the previous information, it must be re-transmitted, which may increase the amount of data transmitted each connection. On the other hand, when the server does not need previous information, its response is faster.

2.2 important concepts of HTTP
1. connection: the actual circulation of a transport layer, which is established between two applications that communicate with each other.
2. Message: The basic unit of HTTP Communication, including a structured sequence of eight-element groups and transmitted through connections.
3. Request: The request information from the client to the server includes the method, resource identifier, and Protocol version number applied to the resource.
4. Response: The information returned by an slave server includes the HTTP version number, the Request status (for example, "successful" or "not found"), and the MIME type of the document.
5. Resource: network data object or service identified by Uri.
6. entity: A special representation of a data resource or service resource, which may be enclosed in a request or response. An object includes object header information and object content.
7. Client: an application that establishes a connection to send a request.
8. User Agent: the client that initializes a request. They are browsers, editors, or other user tools.
9. SERVER: an application that accepts connections and returns information for requests.
10. origin server: A server on which a given resource can reside or be created.
11. Proxy: An intermediate program that can act as a server or a client and create a request for other clients. Requests are transmitted to other servers through possible translation. A proxy must explain before sending the request information and rewrite it if possible.
A proxy is often used as a portal through a firewall client. A proxy can also be used as a help application to process requests that are not completed by a user proxy through a protocol.
12. Gateway: A server serving as the intermediate media of other servers. Different from the proxy, the gateway accepts the request as if it is the source server for the requested resource; the client sending the request does not realize that it is dealing with the gateway.
The gateway is often used as a portal for servers that use firewalls. The Gateway can also be used as a protocol translator to access resources stored in non-HTTP systems.
13. Tunnel: it is an intermediary program used as two connection relay. Once activated, the channel is considered not to belong to HTTP Communication, although the channel may be initialized by an HTTP request. When the two ends of the relay connection are closed, the channel disappears. The channel is frequently used when a portal must exist or intermediary cannot interpret the relay communication.
14. cache: Local Storage of response information.

2.3 How HTTP works
HTTP is based on the request/response paradigm. After a client establishes a connection with the server, it sends a request to the server in the format of Uniform Resource Identifier and Protocol version number, the mime information is followed by the request modifier, client information, and possible content. After receiving the request, the server sends a response in the format of a Protocol version number containing the information, a successful or wrong code in the status line, mime information is followed by server information, entity information, and possible content.
Many HTTP communications are initiated by a user proxy and include a request to request resources on the source server. The simplest case may be that a separate connection is established between the user proxy (UA) and the source server (O) (see Figure 2-1 ).

When one or more intermediaries appear in the request/response chain, the situation becomes more complex. There are three types of mediation: proxy, gateway, and tunnel ).
A proxy accepts the request according to the absolute format of the URI, overrides all or part of the message, and sends the formatted request to the server through the URI identifier.
The gateway is a receiving proxy and serves as the upper layer of some other servers. If necessary, you can translate the request to the lower layer server protocol.
A channel serves as a relay point between two connections that do not change messages. A channel is often used when communication requires an intermediary (such as a firewall) or an intermediary that cannot identify messages. Figure 2-2
Figure 2-2 above shows three intermediaries (A, B, and C) between the user proxy (UA) and the source server (o ). A request or response message through the entire chain must pass through four connection segments. This difference is important, because some HTTP Communication options may be applied to the nearest connection, without channel neighbors, to the end of the chain, or to all connections along the chain. Although Figure 2-2 is linear, each participant may engage in multiple concurrent communications. For example, B may receive requests from many clients instead of a, and/or send requests to a without C. At the same time, B may process requests from.
Any aggregation that is not used as a channel may enable an internal cache for processing requests. The cache effect is that the request/response chain is shortened, and the condition is that one of the participants along the chain has a cache response acting on that request. It indicates the result chain. The condition is that a request is not cached by UA or aplus, and B has a cache copy that passes through a previous response of C from O.
Figure 2-3
On the Internet, HTTP Communication usually occurs over TCP/IP connections. The default port is TCP 80, but other ports are also available. However, this does not indicate that the HTTP protocol can be completed over the Internet or other network protocols. HTTP only indicates a reliable transmission.
The above briefly introduces the macro operation mode of the HTTP protocol. The following describes the internal operation process of the HTTP protocol.
First, we will briefly introduce the information exchange process of the Client/Server mode based on the HTTP protocol, as shown in 2-4. It consists of four processes: establishing a connection, sending request information, sending response information, and closing a connection.
Figure 2-4
In WWW, "customer" is a relative concept with "server". It only exists in a specific connection period, that is, the customer in a connection may act as a server in another connection. When the WWW server is running, it keeps listening on the tcp80 port (the default WWW port), waiting for the connection to appear.
Next, we will discuss the implementation of information exchange in the Client/Server mode under the HTTP protocol.
1. Establish a connection by applying for a socket. The customer opens a socket and limits it to a port. If it succeeds, it is equivalent to creating a virtual file. Later, you can write data on the Virtual File and transmit it over the network.
2. Send a request
After a connection is opened, the client sends the request message to the server's Stop port to complete the request.
The format of the HTTP/1.0 request message is:
Request Message = request line (Common Information | Request Header | entity header) CRLF [entity content]
Request Line = method request url http Version Number CRLF
Method = GET | HEAD | post | Extension Method
U r l = protocol name + host name + directory and file name
The method description in the request line specifies the actions to be executed in the resource. Common methods include get, Head, and post.
The get results of different request objects are different, and their relations are as follows:
Object get result
File Content
Program Execution result of the program
Database query results
Head -- requires the server to find the metadata of an object, not the object itself.
Post: transmits data from a client to the server. The post method is used when the server and CGI are required for further processing. Post is mainly used to send Form Content in HTML text for CGI programs to process.
An example of a request is:
Get http://networking.zju.edu.cn/zju/index.htm HTTP/1.0
Header information is also known as metadata, that is, information. meta information can be used to implement conditional requests or responses.
Request Header-tells the server how to interpret the request, including the data type, compression method, and language that the user can accept.
Object Header: Object Information type, length, compression method, last modification time, data validity period, etc.
Entity-request or response object itself.
3. Send Response
The server sends a Response Message to the client after processing the customer's request.
The Response Message format for HTTP/1.0 is as follows:
Response Message = Status line (Common Information header | response header | entity header) CRLF [entity content 〕
Status line = HTTP Version Number status code reason description
Status Code indicates the response type
1 × Reserved
2×× indicates that the request is successfully received.
3×× further refine the request to complete the request
4×× customer Error
5×× Server Error
The response header information includes the service program name, the URL that notifies the customer of the request to be authenticated, and the requested resource when it can be used.
4. Close the connection
Both the client and server can end the TCP/IP conversation by closing the socket.