Xiaoman network model & http1-http2 & browser cache

application layer

Is closest to the application OSI layer . Users use corresponding interfaces to implement their own services .

The agreement has ：HTTP FTP SMTP etc.

The presentation layer

Presentation of data 、 Security 、 Compress . It can ensure that the information sent by the application layer of one system is read by the application layer of another system . The format is : JPEG、ASCII、 Encryption format, etc .

In the presentation layer , The data is formatted according to a scheme that the network can understand . Manage data encryption and decryption .

The session layer

establish 、 management 、 Terminate the conversation , Corresponding host process , Refers to the ongoing session between the local host and the remote host . Be responsible for establishing between the two nodes of the network 、 Maintain and terminate Communications .

The functions of the conversation layer include ： Establish communication links , Keep the communication connection smooth during the conversation , Synchronize the conversation between two nodes , Decide whether the communication is sent by the terminal and where to resend when the communication is interrupted .

Some people will call the session layer network communication “ Traffic police ". When dialing to your ISP（ Internet service providers ） When you request to connect to the Internet ,ISP The session layer on the server communicates to you and your PC The session layer on the client negotiates the connection . If your telephone line falls off accidentally , The session layer will detect the connection interruption and restart the connection . The session layer sets the communication period by determining the priority of node communication and the length of communication time .

Transport layer

Define the protocol port number of data transmission , And flow control and error checking .

The agreement has ：TCP UDP etc. , Once the packet leaves the network card, it will enter the network transmission layer . Some protocols and port numbers for data transmission are defined , Such as ：TCP（ Transmission control protocol , Low transmission efficiency , High reliability , High requirements for transmission reliability , A large amount of data ）,UDP（ User datagram protocol , The requirements for transmission reliability are not high , A small amount of data ）. It mainly refers to the segmentation and transmission of data received from the lower layer , Regroup after arriving at the destination . This layer of data is often called segment .

OSI The final layer in the model . The transmission protocol performs browsing control at the same time or specifies an appropriate transmission rate based on the speed of the data that can be received by the receiver . besides , The transport layer compulsorily splits long packets according to the maximum size that the network can handle . for example , Ethernet cannot receive more than 1500 Byte packets . The transmission layer of the sender node divides the data into smaller pieces of data , At the same time, a serial number is arranged for each data slice , So that when the data reaches the transmission layer of the receiver node , Be able to regroup in the right order . This process is called sorting .

The network layer

Logical address addressing , Realize the path selection between different networks . The agreement has ：ICMP IGMP IP ARP RARP etc. .

Provides connectivity and path selection between two host systems in a network in different geographic locations .Internet The development of has greatly increased the number of users accessing information from sites around the world , The network layer formally manages the layer of this connection .

OSI The third layer of the model , Its main function is to translate the network address into the corresponding physical address , And decide how to route data from the sender to the receiver .

The network layer considers the priority of transmission through comprehensive consideration 、 The degree of network congestion 、 The quality of service and the cost of optional routing are determined from the nodes in a network A To another network node B The best way . Due to network layer processing , And intelligently guide data transmission , Routers connect all segments of the network , So routers belong to the network layer . In the network ,” route “ It's based on the addressing scheme 、 Use mode and accessibility to guide the sending of data .

The network layer is used locally LAN The computer system above the network segment establishes communication , The reason it can do this , Because it has its own routing address structure , This structure is separate from the second layer machine address 、 independent . This kind of protocol is called routing or routable protocol . Routing protocols include IP、Novell The company's IPX as well as AppleTalk agreement .

Data link layer

Establish logical connections 、 Address the hardware 、 Error checking and other functions . Combining bits into bytes and then into frames , use MAC Address access media , To discover but not correct .

The main function is how to reliably transmit data on unreliable physical lines . To ensure transmission , The data received from the network layer is divided into specific frames that can be transmitted by the physical layer . Frames are structural packets used to move data , It's not just raw data , It also includes physical addresses of sender and receiver as well as error detection and control information . The address determines where the frame will be sent , And the error correction and control information ensure that the frame is error free . If you're transmitting data , The receiver has detected an error in the transmitted data , We need to inform the sender to resend this frame .

The data link layer provides bit stream services on the basis of the physical layer , Establish data links between adjacent nodes , Error free transmission of data frames on the channel is provided through error control , And carry out the action series on each circuit . The main function is physical address addressing 、 Data framing 、 flow control 、 Error detection of data 、 Resend, etc .

The physical layer

establish 、 maintain 、 Disconnect the physical connection .

The main definition of physical equipment standards , Such as the interface type of network cable , Interface type of optical fiber , Transmission rate of various transmission media, etc . The main function is to transmit bit stream （ There is 1、0 It is transformed into power strength for transmission , After arriving at the destination, it is transformed into 1、0）.
 

The second section http1.1  http2

1. HTTP/2 Use binary format instead of text format
2. HTTP/2 It's fully multiplexed , Not ordered and blocked —— Only one connection is needed to achieve parallelism
3. Using header compression ,HTTP/2 Lower the cost
4. HTTP/2 Let the server take the initiative to respond “ push ” Into the client cache

1. Binary framing layer

On the binary framing layer ,HTTP 2.0 All transmitted information will be divided into smaller messages and frames , And they are encoded in binary format , among HTTP1.x The first message of will be encapsulated in Headers frame , And ours request body Then package it to Data Inside the frame .

2. Multiplexing

  As can be seen from the above figure ,HTTP The number of persistent connections can be reduced effectively TCP Number of connections established and disconnected , The advantage is that it reduces the extra burden on the server , And improve the whole HTTP Request time for

3. The head of compression

• Maintain the same static dictionary , Contains common header names , And common combinations of header names and values
• Maintain the same dynamic dictionary , You can add content dynamically
• By static Huffman Encoding encodes the header field of the transmission

demonstration demoHTTP/2: the Future of the Internet | Akamai

4. Client cache

Negotiate the cache

Negotiating cache is to judge whether the cache is available through the server

Last-Modify collocation If-Modify-Since： The first time a browser requests a resource , Server returned header I will add Last-Modify,Last-modify Is the last modification time of this resource ; When the browser requests the resource again ,request The request header of will contain If-Modify-Since, The value of the server header In return Last-Modify. Server received If-Modify-Since after , Determine whether to hit the cache according to the last modification time of the resource

Etag collocation If-None-Match：web When the server responds to a request , Will be in header To add a Etag It is used to tell the browser the unique identification of the current resource on the server （ The generation rules are determined by the server ）. Again to web Server request with header If-None-Match （Etag Value ）.web The server will If-None-Match And Etag compare , Decide whether to hit the negotiation cache ;

ETag and Last-Modified Function and usage of , Their differences ：

1.Etag Is better than Last-Modified.Last-Modified In seconds , If a file is in 1 I changed it many times in a second , So their Last-Modified It doesn't actually reflect the modification , however Etag It changes every time to ensure accuracy ;

2. In performance ,Etag To be worse than Last-Modified, After all Last-Modified Just record the time , and Etag You need the server to calculate one using an algorithm hash value ;

3. In priority , Server validation is preferred Etag.

If the request received by the server does not Etag value , Will If-Modified-Since Compare with the last modification time of the requested file , Consensus hits the negotiation cache , return 304; If not, a new last-modified And file and return 200

Strong cache

Expires： The value is absolute time , however Expires yes HTTP 1.0 Things that are , Now the default browser is used by default HTTP 1.1

Cache-Control： The value is relative time , If the Expires If it is set at the same time , Its priority is higher than Expires.

cache-control There are several commonly used settings ：

-max-age： Set the expiration time , The client is within this validity period , If the resource is requested again , Just read the cache directly

-no-cache： Do not use local cache . Cache negotiation required , First confirm with the server whether the returned response has been changed , If a previous response exists ETag, Then the request will be verified with the server , If the resource has not been changed , Then fetch data from the cache

-no-store： Directly prohibit the browser from caching data , Every time a user requests the resource , Will send a request to the server .

-public： Can be cached by all users , Including end users and CDN Wait for intermediate proxy server .

-private： Can only be cached by the end user's browser , Don't allow CDN Wait for the relay cache server to cache it .

The server is set by http in hdader Of Expires and cache-control Field tells the browser the validity of the swap . This method will have a fixed time , The problem is that if the server data is updated , But the expiration time of strong cache is not yet , The data cannot be updated