컴퓨터 네트워크 개념 - 당신이 필요로 하는 유일한 지침

1. URL parsing: The browser will extract the protocol and path of the requested resource.

2. Cache Validation: Then browser will verify whether such requested resource has been cached and still valid. If yes it will return the cached result immediately or else it will send a new request to the server.

3. DNS query: The browser checks the browser cache for a DNS record to find the corresponding IP address. If not found, it will proceed to the OS cache followed by router cache. Lastly, ISP(Internet Service Provider)’s DNS server initiates a DNS query to find the IP address.

4. Get the MAC address: After the browser gets the IP address, the data transmission also needs to know the destination host's MAC address.

5. The browser initiates a TCP connection with the server. It will require a three way handshake to establish the TCP connection between them.

6. The browser sends an HTTP request to the webserver. Once the TCP connection is established, the browser sends a HTTP request. Refer to the above section regarding HTTP request.

7. Server handle the request and sends out an HTTP response.
The server response contains the web page you requested as well as the status code, compression type (Content-Encoding), how to cache the page (Cache-Control), any cookies to set, privacy information, etc. Refer to the above section regarding HTTP response.

8. The browser parse and paint the HTML content The browser displays the HTML content in phases. First, it will render the DOM tree and CSSOM tree through the respective files and construct a render tree base on them. Once this is done the browser will layout the page through the render tree and use its api to paint the user interface on viewport.

9. TCP four way handshake to stop the connection Once the client thinks it has received everything it needs, it will start a four way handshake with the server to close the connection. Refer to the TCP section below for more information.

In HTTP1.0, by default is to create a new connection between the client and the server for each request/response, and disconnect immediately after completion, which is a short connection.

When the Keep-Alive mode is used, the Keep-Alive function keeps the client-to-server connection valid. When a subsequent request to the server occurs, the Keep-Alive function avoids establishing or re-establishing a connection, which is a long connection. The method of use is as follows:

• The client sends the request to the server while adding the Connection field to the header. The server sends the Connection: Keep-Alive field back to the client. The client receives the Connection field and then Keep-Alive connection established successfully.

• To close the connection, client side will send connection:closed inside of its request, server will respond and close the keep-alive connection.

Pros of long connection

1. less cpu and memory usage as there are less connections
2. allows pipelining (refer to the above session about pipelining)
3. let congestion control will be needed
4. less delay in the subsequent request as the client side do not need to perform the three way handshake on every request.

Cons of long connection
The long tcp connection may waste resources if it stays idle for long period.

Pros

1.Simple and fast: When a client requests a service from the server, it only needs to transmit the request method and path. Because the HTTP protocol is simple, the HTTP server request processing time will be very fast.

2.connectionless: No connection means it limits each connection to only process one request. After the server has processed the client's request and received the client's response, it will disconnect. This method can save transmission time.

3.Stateless: HTTP protocol is a stateless protocol, where the state refers to the context information of the communication process. The lack of state means that if the previous information is needed for subsequent processing, it must be retransmitted, which may result in an increase in the amount of data transmitted per connection. On the other hand, if the server does not need previous information, its response is faster.

4.Flexible: HTTP allows the transmission of any type of data object. The type being transmitted is marked by Content-Type.

Cons

Clear text transmission: The messages in the protocol are in text form, which is directly exposed to the outside world and is not safe. It may be manipulated by third party that's listening to the connection.

The WebSocket API is an advanced technology that makes it possible to open a two-way interactive communication session between the user's browser and a server. With this API, you can send messages to a server and receive event-driven responses without having to poll the server for a reply.

WebSocket mechanism: The client notifies the WebSocket server of an event with all recipient IDs, and the server immediately notifies all active clients after receiving it, only those client whose ID is in the recipient list will handle the event from websocket server.

The advantage of WebSocket are as follows:

• Support two-way communication, real-time data streaming

• We can send text or binary data

• Built on the TCP protocol, the implementation at the server side is relatively easy

• The data format is relatively lightweight, the resource required is small

• There is no same origin restriction, the client can communicate with any server

• It has good compatibility with HTTP protocol. The default ports used by websocket are also 80 and 443.

1. DNS is the abbreviation of Domain Name System, and it provides a host name to IP address conversion service. It is a distributed database composed of hierarchical DNS servers.

It is an application layer protocol that defines how the host queries this distributed database. DNS makes our life easier for not having to remember the IP address.

Function: Resolve the domain name into an IP address. The client sends a domain name query request to the DNS server (the DNS server has its own IP address), and the DNS server informs the client of the IP address of the Web server.

2. Does DNS use TCP and UDP at the same time?
DNS takes port 53 and uses TCP and UDP protocols at the same time.
(1) Use TCP protocol for zone transfer

The secondary domain name server will regularly (usually 3 hours) to query the main domain name server to find out whether the data has changed. If there is a change, a zone transfer will be performed for data synchronization. The zone transfer uses TCP instead of UDP, because the amount of data transferred synchronously is much more than the amount of data in a request response.
TCP is a reliable connection, which guarantees the accuracy of data.

(2) UDP packets are smaller in size. UDP packets can't be greater than 512 bytes. So any application needs data to be transferred greater than 512 bytes require TCP in place. For example, DNS uses both TCP and UDP for valid reasons described below. UDP messages aren't larger than 512 Bytes and are truncated when greater than this size. DNS uses TCP for Zone transfer and UDP for name, and queries either regular (primary) or reverse. UDP can be used to exchange small information whereas TCP must be used to exchange information larger than 512 bytes. If a client doesn't get response from DNS, it must retransmit the data using TCP after 3-5 seconds of interval.