current position:Home>[final review] computer network layer section 1 (required for beginners)

[final review] computer network layer section 1 (required for beginners)

2022-01-27 00:43:10 @Li Fan

Blogger introduction :

I am a 了 all WeChat official account 【 The Milky way 】 Looking forward to your attention . In the future, let's cheer together ~


Preface

As we all know, computer network is divided into five layers ( From the bottom up ) Namely

  • The physical layer ( Bottom most )
  • Data link layer
  • The network layer
  • Transport layer
  • application layer ( Closest to the user )

What does the third layer of computer network do ?

Two problems remain in the data link layer

  • problem 1: The data link layer cannot achieve reliable transmission , Whether the network layer can do ?
  • problem 2: The data link layer cannot achieve the communication between users in different networks , Whether the network layer can do ?

4.1 Two services provided by network layer

First, let's follow the question in the preface , As the third layer of computer network , What is the purpose ? What two services does the network layer provide ?

The network layer should be transferred to the transportation layer ( The fourth layer of the network ) What kind of services are provided (“ Connection oriented ” still “ There is no connection ”), This has been in a state of debate . The issue at issue is : In computer communication , Who should be responsible for reliable delivery ? Is it a network or an end system ?

The first point is : If the network is responsible for reliable delivery

  • With the help of the successful experience of telecom network , Make the network responsible for reliable delivery , Computer network should imitate telecommunication network , Use connection oriented communication .
  • Build virtual circuits before you communicate , To ensure all network resources needed for communication between both parties .
  • If you use a reliable network protocol , Then the transmitted packets can arrive at the destination in order without error , No loss 、 No repetition .

Virtual circuit service ,H1 Send to H2 All packets of are transmitted along the same virtual circuit
Why is it called a virtual circuit ? Because it is logically connected .
 Insert picture description here

  • A virtual circuit means that this is just a Logical connection , Groups are connected along this logical line Transmit in store and forward mode , And not really establishing a physical connection .
  • Be careful , The telephone communication of circuit switching is to establish a Real connection .
  • So the virtual connection of packet switching and the connection of circuit switching are just similar , But it's not exactly the same .

Conclusion

At this time, we can see that the shortcomings are obvious , Establish a real connection , At this time, only this connection can achieve reliable transmission , If there are users who want to use the route to establish a connection, it cannot be used , This also causes a waste of resources , And the utilization rate is also very low . In this way, new ideas have emerged

The second view : The network provides datagram service

The pioneers of the Internet have put forward a new idea of network design .
The network layer only provides simple and flexible 、 No connection 、 Datagram service delivered as best as possible .
The network does not need to establish a connection before sending packets . Each group ( namely IP The datagram ) Send independently , It has nothing to do with the grouping before and after ( No numbering ).
The network layer does not provide a commitment to quality of service . In other words, the transmitted packet may be wrong 、 The loss of 、 Repetition and disorder ( Not in order to reach the end ), Of course, the time limit of packet transmission is not guaranteed .

Conclusion

So here we can see the way of reliable transmission using the network before , Feasible, but not applicable at present , So we still use the end-to-end system , This means that since the network can not achieve reliable transmission, the problem is given to both ends of the connection , So since we There is no obligation to make reliable transmission , Then you can allow the transmitted packets Possible error 、 The loss of 、 Problems such as repetition and disorder , The role of the network layer has made some eyebrows here , Only simple and flexible... Are provided to the upper level 、 No connection 、 Datagram service delivered as best as possible . Specifically How to do your best See what follow-up work has been done .

What efforts have been made

  • Because the transmission network does not provide reliable transmission services at the end , This makes it easier for routers in the network , And it's cheap ( Compared with the switch of telecommunication network ).
  • If the host ( End system ) Communication between processes in need to be reliable , Then the transport layer in the host of the network is responsible for reliable delivery ( Including error handling 、 Flow control, etc ).
  • The advantage of this design idea is : The cost of the network is greatly reduced , Flexible operation mode , Can adapt to a variety of applications .
  • The Internet can develop to today's scale , It fully proves the correctness of this design idea .

summary

Come here 【 problem 1】 I believe it's very clear , The network does not provide reliable transmission services . So the network does not need to guarantee reliability , Reliability should be guaranteed through two ends , The network only needs to provide a simple and flexible 、 No connection 、 Try your best to deliver datagram Services .

In the next section, we will continue to look at the best efforts made by the network layer

Datagram service
 Insert picture description here
Comparison between virtual circuit service and datagram service
 Insert picture description here


It's not easy to create , Give me some advice. !
If you need to see a collection later !
If you are interested in my article, please pay attention to !
If there are questions , You can pay attention to the official account. 【 The Milky way 】 Click to contact me for private chat .


copyright notice
author[@Li Fan],Please bring the original link to reprint, thank you.
https://en.cdmana.com/2022/01/202201270043074735.html

Random recommended