Data link layer of network protocols

The data link layer has three main purposes:

* Sends and accepts IP data packets for the IP module;

* Send ARP requests to the ARP module and receive ARP responses;

* Send RARP requests and RARP responses to RARP.

The ARP protocol is mainly used to resolve the 32bit IP address to the MAC address (hardware address) corresponding to the 48bit (Ethernet ). The RARP protocol resolves the hardware address to an IP address. The two protocols are at the network layer, which is the same as IP datagram, all have their own Ethernet data frame types (that is, the corresponding MAC frame must be added to the incoming Ethernet ).

ARP attacks in LAN implement ARP spoofing by forging IP addresses and MAC addresses, which can generate a large amount of ARP traffic in the network and block the network.

TCP/IP supports a variety of different data link protocols, depending on the hardware used by the network, such as Ethernet, card ring network, FDDI, RS-232 serial line, etc, the main local network technology used by TCP/IP today is Ethernet.

Maximum Transmission Unit MTU:

The network in the data link layer has a limit on the length of the data frame. MTU values of different networks are different. The commonly used Ethernet is 1500 bytes. the MTU values of some other networks are shown in the following table:

MTU is mainly used to limit the maximum IP datagram value transmitted at a time. If the IP layer has a datagram to be transmitted and the data production is larger than the MTU value at the data link layer, you need to split the IP datagram so that each piece is smaller than MTU.

Path MYU:

When two hosts on the same network communicate with each other, the MTU of the network is very important. However, if the communication between the two hosts goes through multiple networks, the link layer of each network may have different MTU. In this case, the minimum MTU of each network in the two communication host paths should be considered as the MTU path.

The path MTU between the two hosts must be a constant. It depends on the route selected at that time, and the route selection is not necessarily symmetric, that is, the routes from A to B are different from those from B to A. Therefore, the MTU path is not necessarily the same in both directions.