What is the slice flag bit

If you look ahead MTU and MSS The article , You should know that if the data is too big , It is easy to lose in transmission , Therefore, the size of each data frame is limited on the network card , This is it. MTU, It means data sent from the network layer to the second layer （ip head +ipPayload）, Not greater than MTU, If it is big, it will be discarded , This requires that data packets should be divided into packets in the network layer MTU The size of the partition package . however , Some messages are for data integrity , I don't want to be in the network layer because MTU The relationship between , It would rather discard , I am not willing to send after slicing , What about this ？

Slice flag bit

What should I do about the above problems ？ It's simple , Since the message is fragmented at the network layer , I just add some information to the network layer , It is used to indicate whether slicing is allowed , This is the fragment flag bit field in the network layer protocol header

DF

Indicates whether fragmentation is allowed ,0 Can be said ,1 It means that it is not allowed to

So if you don't want the message to be sent over the network due to MTU The relationship between , It may be necessary to partition when passing through a router network card , You can send the message IP In the first part DF Value is set to 1, In this way, the router cannot be partitioned , Discard the message , At the same time ICMP To the source host

This brings out DF Another function of ：

Discovery path MTU,PMTUD,path mtu discovery, Through the DF Set to 1, When some network cards in the path MTU It is too small to be divided into pieces , It will send icmp To the source host , This icmp There is this network card in the message mtu The size of information , The source host can find the smallest on the path mtu, This is it. path mtu. Source host according to pmtu, You can resend smaller ip package

MF

It means that we should ip Is the bag the last piece ,0 It means the last piece ,1 It means not the last piece

What is the purpose of this flag bit ？

Data packets are sent after being partitioned in the network layer , At the receiving end , It also needs to be reorganized at the network layer , Wait for the partition package to complete the reorganization ？ Is to receive the last piece of data packet . If the last piece doesn't arrive , Recombined packets are incomplete , You can't send it to the top , You need to discard the whole package . Only complete packets can be sent to the upper layer

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