Network - data link layer

One 、 Data link layer functions

① Receive data in the form of bitstreams from the physical layer , And encapsulate it into frames , Send to the next level ; Again , The data frame of the upper layer , Data in the form of bitstream is forwarded to the physical layer

② Error detection function , Turn the physical channel with large errors into error free 、 A data link that can reliably transmit data frames ;

Two 、 Knowledge involved in the data link layer

Ethernet

It's not a specific network , It's a technical standard ; It includes the contents of the data link layer , It also contains some contents of the physical layer , for example ： Specifies the network topology 、 Access control mode 、 Transmission rate, etc .

Encapsulation of data link layer 、 Sub use is based on Ethernet data format （ Contains source mac、 Purpose mac)

contrast MAC and IP Address

IP The address describes the overall starting and ending point of the road ,MAC The address describes the beginning and end of each interval on the road ;

MTU

A regulation at the data link layer , Datagrams larger than a certain size , To slice （ to subcontract ） after , Send again , Different data link layers ,MTU The standards are different ;

MTU Yes IP The impact of the agreement

Because of the data link layer MTU The limitation of , For the larger IP Packets are to be sub contracted

① Will be bigger IP The package is divided into several small packages , And label each bag ,

② Every little bag of IP The head of the agreement 16 Bit mark （id） It's all the same ,

③ Every little bag of IP The head of the agreement 3 Bit flag field , The first 2 The position is 0, Indicates that fragmentation is allowed , The first 3 Bit indicates the end tag ,

④ Once any one of these packets is lost , The reorganization of the receiver will fail , but IP Layer will not retransmit ;

MTU Yes UDP The impact of the agreement

After slicing , Any one is lost , Whole UDP The datagram will lose packets ;

MTU Yes TCP The impact of the agreement

After slicing , Any one is lost , Whole TCP The datagram will lose packets , but TCP There is a timeout retransmission mechanism , Is reliable ;

ARP agreement

ARP Protocol is a protocol between data link layer and network layer

effect ： Host and router , There is one. ARP Cache table , Record IP and MAC The mapping relation of , Can pass IP look for MAC

How to determine the purpose MAC, First, you need to know which device to send data to ？

① Determine whether the destination is in the same network segment ,

② Not in the same segment , I don't know how to send , Give it to the router ,

③ The source host passes through the local ARP Cache table , Router based IP To find the corresponding MAC,

④ Purpose of datagram sent MAC： Router MAC;