Summary of the computer network itself (fourth edition of Xie xiiren)

Computer network summary

1. p110 indicates the MAC frame format in V2 format. The data field (that is, IP packet) length ranges from 46 bytes to 1500 bytes, which is defined for Ethernet, "1500 bytes" is also called the Ethernet MTU (maximum transmission unit, that is, Ethernet limits the maximum length of IP datagram to 1500 bytes ). P185 indicates the IP datagram format. The "total length field" indicates the sum of the header and data. The "total length field" is 16 bits, so the maximum length of an IP datagram can be 65535 bytes (216), but this range obviously exceeds 1500 bytes. Therefore, although the maximum length of data packets over Ethernet can increase the transmission rate, due to the widespread application of Ethernet, the length of data packets cannot exceed 1500 bytes, sometimes the IP datagram length is limited to 576 bytes. When the length of a datagram exceeds the maximum transmission unit MTU (the MTU in Ethernet is 1500 bytes), it is necessary to partition the long datagram before transmission in the network. In this case, the "total length field" in the first part of the part datagram is the length of the IP datagram after the part.

2. in a MAC frame, the "type field" specifies the network protocol used on the upper layer of the link layer. For example, if the value is 0x0800, the upper layer uses the IP protocol. If the value is 0x1837, the upper layer uses the Novell IPX protocol.

3. Differences between IP addresses and hardware addresses (MAC) (p180 ):

The physical address is the address used by the data link layer and physical layer, while the IP address is used by the network layer and above.

As shown in Figure 6-11, the header of the datagram below the link layer is the MAC address, and the header of the IP layer and above is the IP address.

4. for actual network transmission, you still need to encapsulate it into a MAC frame and then transmit it in the physical layer. The MAC frame header is the MAC address notation, And the IP packet header is the IP address notation, therefore, when the IP packet is encapsulated into a MAC frame, you must refer to the ARP Address ing table, which is the source of ARP.

5. ARP is used to solve the problem that the IP address of the host or router corresponding to the MAC address on the same LAN. The "same LAN" is the so-called "Neighbor" in the Linux kernel ". The ARP ing table contains a ing between the IP address and MAC address of the vro in the local area network to connect to other networks from the local area network.

6. for the process of sending and receiving, see p183, it is worth noting that the receiver will extract the [Sender's IP address and MAC address] from the [ARP request] to [its ARP high-speed cache ], in this way, when the Receiver becomes the sender, it does not need to broadcast MAC any more.