A: OSI7 layer was list will be the number of network transmission
physical layer, data link layer, network layer, transport layer session layer denotes an application layer
1 the physical layer: the bottom in the OSI reference model, the physical layer is the main function of the data link layer provides a physical connection using the physical transmission medium.
2 Data Link Layer: This layer is connected on the sub-frame data, and the flow control, the physical layer of the shield, provides a physical link to the network layer, it is possible to go wrong in a physical connection, error-free data transport, and this layer is designated to provide hardware addressing topology
3 network layer: this layer is addressed by establishing a connection between two nodes, as the source of the transport layer packet sent, select the appropriate routing and switching node, the correct correctly transmitted to the destination address according to the transport layer, which comprises a relay and routing data through the Internet.
Transport Layer 4: General data delivery connectionless or connection-oriented, end provide a reliable and optimized data transmission and transparent mechanism for the service user session layer
5 session layer connection established between the two end nodes, for the application end of the system providing a dialogue procedure between the control mechanism
6 represents a layer for processing two communication systems to represent the information exchange, to solve the problem syntax for the upper user subscriber information, including data exchange format, the data encryption and decryption, data compression and recovery.
7 OSI application layer provides the highest level of means of access OSI environment for a particular type of network applications, provides remote operation and exchange of information needed for applications
FIN received a request for retransmission of the last ACK able to reply, so that delays in the network FIN / ACK data is lost in the network, will not affect the subsequent connection
of four waved a final time wait for the action
Two: the difference between hard and soft connection links:
hard link does not create a new inode, only to file one more source file name, a soft link to create a new inode, equivalent to re-create a file,
hard links can not span partitions, soft links across partitions. After a hard link to delete the source file, another file can be used. After the soft connection delete a file, the file can no longer connect.
Soft link file: ln -s file link file
hard-linked files: ln file link file