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10: Ethernet - Ethernet II (version 2, or DIX) frame - length, type, and data fields part 12

Ethernet II (version 2, or DIX) frame - length, type, and data fields

Length/type :
This field occupies 2 bytes in the whole frame.  Its function is twofold : the first one, which is type, is to inform the receiving device which higher-layer protocol is encapsulated in the frame. For example, IP address or ARP. these protocols are represented in hexadecimal digits, such as 0x0800 for IP and 0x0806 for ARP. Take a look at this figure.
Ethernet II (version 2, or DIX) frame - length, type, and data fields
The second one, which is length, is responsible for indicating the exact length of DATA field of the frame. It is later utilized as part of the CRC (Cyclic redundacy Check) process to ensure that data was transmitted properly.

In early implementations of Ethernet, there was only  the Ethertype field without Length field, or rather each was in different frame versions. But in modern implementations, especially in 1997, both were combined in one field ; thus, the field that is used to describe the Ethertype is the same one that is used to describe the Length. To put it another way, knowing the fact that there was another Ethernet frame which is the original IEEE 802.3 in addition to  Ethernet II or (DIX), the former considered the two bytes of the field to represent the Length of the DATA, whereas the latter considered the two bytes to represent the Ethertype, and because of the fact that both were needed to be implemented at a time, another version that can combine the two functions, referred to as revised IEEE 802.3 (IEEE 802.3x-1997 unifying standard), was created. It required that EtherType values be greater than or equal to 1536.

This arises a very significant question. How can a driver know which frame format it is dealing with, if it configured to support both frames ? or, in other words, how can a given frame be recognized in a certain Ethernet segment ?

The answer is that if the field (Ethertype/ Length) has a value greater than or equal to 1536 bytes (= 0x0600 in hexadecimal), then it is considered to be Ethertype, that is, Ethernet II frame. that number was chosen because the maximum size of data field of an Ethernet 802.3 frame is 1500 bytes (= 0x05DC in hexadecimal). By contrast, if the field is less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field. The numbers between 1500 and 1536 are undefined.

By and large, for the purpose of allowing the coexistence of both standards on the same physical medium, the method of enabling software to determine whether a frame is an IEEE 802.3 frame or an Ethernet II frame is a viable solution.

Data :

This field is also known as payload. It is the one where data (the actual user data plus the upper-layer headers such as TCP/IP or IPX) is placed. This frame is assigned the two values 45 and 1500, which means that the data cannot be greater than 1500 bytes and less than 45 bytes. That is, 46 is minimum length and 1500 is the maximum length of data to be carried in a given frame.

However, sometimes some frames are produced with a data field less than 46 bytes. In this case, something called padding will be resorted to. Its function is to add a stream of zeros to the frame until it reaches the minimum allowed value, which is 46.

The maximum size of the whole frame is 1518 and the minimum size  is 64. It is calculated by adding 6 bytes of Destination MAC field, 6 bytes of Source MAC field, 2 bytes of Ethertype/Length field, 1500 bytes of Data field, and 4 bytes of FCS field, which aggregates 1518.

The next lesson is a sequel to this one 
10: Ethernet - Ethernet II (version 2, or DIX) frame - length, type, and data fields part 12 Reviewed by BOUFTIRA on 4:07:00 PM Rating: 5

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