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EthernetII帧结构

2012年2月13日 / 945次阅读
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文章《EthernetII帧结构》的特色图片

从共享Media(传输媒介)的LAN开始,不管传输技术和媒介如何变化,Ethernet一直保持着几乎相同的Frame结构,保持着简单,低价,可靠和对新技术的开放,这就是它巨大成功的原因。

EthernetII-frame,以太网帧结构图

Ethernet II Frame Structure (802.3 Frame). 与最早的Frame结构相比,就是增加了SFD(Start Frame Delimiter),复用了Length字段表示Type/Length。

最小帧长为64 byte。

Preamble and Start Frame Delimiter Fields

The Preamble (7 bytes) and Start Frame Delimiter (SFD) (1 byte) fields are used for synchronization between the sending and receiving devices. These first eight bytes of the frame are used to get the attention of the receiving nodes. Essentially, the first few bytes tell the receivers to get ready to receive a new frame.

Destination MAC Address Field
Source MAC Address Field
Length/Type Field

The Length/Type field (2 bytes) defines the exact length of the frame's data field. This is used later as part of the FCS to ensure that the message was received properly. Either a length or a type may be entered here. However, only one or the other may be used in a given implementation. If the purpose of the field is to designate a type, the Type field describes which protocol is implemented. The field labelled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version because both uses were common. The Ethernet II Type field is incorporated into the current 802.3 frame definition. Ethernet II is the Ethernet frame format that is used in TCP/IP networks. When a node receives a frame, it must examine the Length/Type field to determine which higher-layer protocol is present. If the two-octet value is equal to or greater than 0x0600 hexadecimal or 1536 decimal, then the contents of the Data Field are decoded according to the protocol indicated. (0x0800 is IPv4, 0x86dd is IPv6)

Data and Pad Fields

The Data and Pad fields (46 - 1500 bytes) contains the encapsulated data from a higher layer, which is a generic Layer 3 PDU, or more commonly, an IPv4 packet. All frames must be at least 64 bytes long. If a small packet is encapsulated, the Pad is used to increase the size of the frame to this minimum size.

Frame Check Sequence Field

The Frame Check Sequence (FCS) field (4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame. The receiving device receives the frame and generates a CRC to look for errors. If the calculations match, no error occurred. Calculations that do not match are an indication that the data has changed; therefore, the frame is dropped. A change in the data could be the result of a disruption of the electrical signals that represent the bits.

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