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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 ？

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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 .

• 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

Comparison between virtual circuit service and datagram service

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