Ethernet Protocols and Features

Ethernet Protocols and Features 

In this article you will learn about the Ethernet Protocols and Features. Ethernet is the predominant LAN technology in the world and works in the data link layer and in the physical layer. The Ethernet protocol standards define the format, size, timing and coding of frames in a network communication; It is therefore important to know the Ethernet standards that define Layer 2 protocols and Layer 1 technologies.

Ethernet has evolved since its creation in 1973. The first versions of Ethernet were relatively slow, with a speed of 10 Mbps, while the most recent ones run at 10 Gbps and even faster.

Some Key features of Ethernet are:

• Most used LAN technology.
• It works in the data link layer and in the physical layer.
• Family of network technologies defined in the IEEE 802.2 and 802.3 standards.
• Supports data bandwidths of 10, 100, 1000, 10,000, 40,000 and 100,000 Mbps (100 Gbps).

ETHERNET ENCAPSULATION

As I mentioned, Ethernet works in layer 1 and layer 2, and precisely, it operates in two separate sub-layers of the data link layer :

1. Logic Link Control (LLC) 
2. MAC 

SUBLAYER LLC

The LLC Sublayer (Logical Link Control or Logical Link Control) is implemented by software (its implementation is hardware independent) and is used to communicate with the upper layers of the protocol suite. The LLC takes the protocol data from the network, which is usually an IPv4 packet, and adds control information to help distribute the packet to the destination node. On a computer, the LLC can be considered the NIC driver software. The NIC controller is a program that interacts directly with the NIC hardware to transfer data between the MAC sublayer and physical media.

 MAC SUBLAYER

The MAC sublayer (Media Access Control or Media Access Control) constitutes the lower sublayer of the data link layer and is implemented by hardware, usually in the NIC of the PC.
It has two main responsibilities:

• Data encapsulation : Includes frame arming before transmission and frame disarming at the time of reception and provides three main functions:
• Frame delimitation : used to identify a group of bits that make up a frame. These delimiter bits provide synchronization between the transmission and reception nodes.
• Addressing : The encapsulation process contains the Layer 3 PDU and also provides data link layer addressing.
• Error detection : each frame contains a trailer used to detect transmission errors.

To assemble the frame, the MAC layer adds a header and a trailer to the PDU of the network layer.

• Media access control : It is responsible for placing the frames in the media and removing them from them (hence the name). This sublayer communicates directly with the physical layer.

The underlying logical topology of Ethernet is a multi-access bus in which all devices (nodes) of a single network segment share the medium. Ethernet is a contention network method, where any device can attempt to transmit data through the shared medium as long as it has data to send. To detect and resolve collisions, the process of multiple access by carrier detection with collision detection (CSMA / CD) in the mid-duplex Ethernet LANs is used.

ETHERNET FRAME FIELDS

The Ethernet II and IEEE 802.3 standards define that the minimum Ethernet frame size is 64 bytes, and the maximum is 1518 bytes and the following are taken into account:

• A length less than 64 bytes is considered a “collision fragment” or “runt frame”.
• All frames that are smaller than the minimum or greater than the maximum are discarded.
• It is possible that discarded frames originate in collisions or other unwanted signals and, therefore, are considered invalid.

Ethernet frame Fields table

FIELDS	DESCRIPTION
Preamble and Frame Start Delimiter	The Preamble (7 bytes) and Frame Start Delimiter (SFD) fields, also called “frame start” (1 byte), are used for synchronization between sending and receiving devices.
Destination MAC address	This 6-byte field is the identifier of the desired recipient.
Source MAC address	This 6-byte field identifies the NIC or the source interface of the frame.
EtherType	This 2-byte field identifies the upper layer protocol encapsulated in the Ethernet frame.
Data	This field (46 to 1500 bytes) contains the encapsulated data of an upper layer, which is a Layer 3 PDU (an IPv4 packet). All frames must be at least 64 bytes long.
FCS (Frame Verification Sequence Field)	(4 bytes) is used to detect errors in the frame. Use a cyclic redundancy check (CRC), if the calculations match the source and the receiver, no error occurred.

ETHERNET MAC ADDRESSES

An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits (4 bits per hexadecimal digit).

The decimal system is a base 10 number system, the hexadecimal system is a base 16 system, use the numbers from 0 to 9 and the letters from A to F

It's important to know:

• The hexadecimal system is represented in writing by means of the value preceded by "0x". Example: "0x0A" and "0x73"
• The hexadecimal value is used to represent Ethernet MAC addresses and IP addresses version 6 (IPv6).

MAC ADDRESS STRUCTURE

The MAC address value is the result of the standards implemented by the IEEE in order to guarantee unique addresses for each Ethernet device. The IEEE assigns the provider a 3-byte (24-bit) code, called “unique organization identifier (OUI)”.

The IEEE requires a provider to follow two simple rules:

• They must use the OUI assigned to the provider as the first 3 bytes.
• All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes.

FRAME PROCESSING

Often, the MAC address is known as " physical address (BIA) " because, historically, this address is physically recorded in the read-only memory (ROM) of the NIC. In other words, it is permanently encoded in the ROM chip.

This is how it works: When the computer starts up, the first thing the NIC does is copy the MAC address of the ROM to RAM. When a device forwards a message to an Ethernet network, it attaches the header information to the frame (which contains the source and destination MAC addresses).

MAC ADDRESS REPRESENTATIONS

MAC addresses have the following characteristics:

• They can be represented with two points, dashes or periods, and are not case sensitive.
• 00-60-2F-3A-07-BC, 00: 60: 2F: 3A: 07: BC, 0060.2F3A.07BC and 00-60-2f-3a-07-bc are valid representations of the same MAC address.

On a Windows host, the ipconfig / all command can be used to identify the MAC address of an Ethernet adapter

 UNICAST MAC ADDRESS

Unique address used when a frame is sent from a single transmitting device to a single receiving device. The source MAC address must always be unicast.

MAC BROADCAST ADDRESS

It is used when a frame is sent from a single transmitting device to all devices in the network.

Broadcast packets have a destination IPv4 address that contains only numbers one (1) in the host portion , meaning that all hosts on that local network (broadcast domain) will receive and process the packet. Many network protocols, such as DHCP and ARP, use broadcast.

Another feature is that when the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the FF-FF-FF-FF-FF-FF broadcast MAC address in hexadecimal (48 numbers one in binary).

MULTICAST MAC ADDRESS

Multicast addresses allow a source device to send a packet to a group of devices. There are some things to keep in mind:

• The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal (the remaining portion is created by converting the lower 23 bits of the IP address of the multicast group into six hexadecimal characters).
• The range of IPv4 multicast addresses ranges from 224.0.0.0 to 239.255.255.255 .
• The IPv6 multicast address range begins with FF00 :: / 8 .