TCP/IP detailed--Data link Layer & Ethernet frame format

The most common package on the link layer is the Ethernet and IEEE 802 packages, where Ethernet is used as the object of instruction.

First, the overview of the data link layer belongs to the bottom of the computer network, the use of the channel is mainly a point channel and broadcast channel two types.
In the TCP/IP protocol family, the data link layer has three main purposes: 1) Send and receive data for IP Module 2) Send ARP Request for ARP module and receive ARP reply 3) Send Rarp request and receive Rarp answer for RARP module            TCP/IP supports a number of different link layer protocols, depending on the hardware used by the network, such as Ethernet, Token Ring, FDDI (Fiber Distributed Data Interface), and RS-232 serial line.      Protocol data unit of the data Link layer-frame: The datagram of the IP layer (network layer) is added to the header and the trailer is encapsulated into frames. There are many kinds of data Link layer protocol, which will solve three basic problems, encapsulate frame, transparent transmission and error detection.

. Two, Ethernet and IEEE802 the package shown in the figure shows two different forms of package format. The most commonly used encapsulation format is the format defined by the RFC894. Both frame formats use the destination address and source address of 48bit (6 byte).

The Ethernet frame format only has 6 bytes of destination address and source address. The frame length field refers to the byte length of its subsequent data, but does not include the CRC check code. This length is the data payload provided to the upper layer protocol, that is, the upper-layer protocol encapsulates no longer than the data length of the Ethernet frame.         Minimum data frame tail 46, maximum data frame length is 1500. In the analysis of the length of the Ethernet frame, there are different statements, see http://blog.csdn.net/yusiguyuan/article/details/22490229.
Second, the Maximum Transmission unit MTU
The Maximum transmission unit (Maximum transmission UNIT,MTU) refers to the maximum packet size (in bytes) that can be passed on a layer of a communication protocol. If the IP layer is transmitting a datagram that is larger than the MTU of the link layer, then the IP layer will shard the datagram. A datagram is divided into slices, each with a size less than or equal to the MTU value of the link layer. When hosts on the same network communicate with each other, the MTU of the network is important to both sides of the communication. But when the host to communicate through many networks, the most important for both sides of communication is the minimum MTU in the communication path, because the link layer MTU of different networks on the communication path is different.      The smallest MTU in the communication path is called the Path MTU. The default values for some common link layer protocol MTU in the network are as follows: FDDI protocol: 4352 byte Ethernet (Ethernet) protocol: 1500 byte PPPoE (ADSL) protocol: 1492 byte X. Protocol (Dial up/modem): 576 bytes point-to-point:4470 byte Ethernet and 802.3 have a limit on the length of the data frame, with a maximum value of 1500 and 1492 bytes, respectively. This feature of the link layer is called the MTU, the largest transmission unit, and most of the different types of networks have an upper limit, the value is determined by the hardware.

If the IP layer has a datagram to pass, and the length of the data than the link layer MTU is also large, then the IP layer needs to be fragmented, the data into several pieces, so that each piece is less than the MTU.
Third, Path MTU
The path MTU is similar to the above, except that you need to know the minimum MTU of the path when transferring data.