Ethernet Data Frame (802.3) maximum and minimum length

Ethernet Data Frame (802.3) maximum and minimum length
March 20, 2017 19:40:01 Farmwang read: 5370 more
Personal Category: TCP/IP
Ethernet (IEEE 802.3) frame format:

1, Preamble code: 7 byte 0x55, a string of 1, 0 interval, for signal synchronization
2, Frame starting delimiter: 1 bytes 0xd5 (10101011), indicating the beginning of a frame
3. DA (Destination mac): 6 bytes
4, SA (Source mac): 6 bytes
5, type/Length: 2 bytes, 0~1500 reserved as Length field value, 1536~65535 reserved as Type field value (0X0600~0XFFFF)
6. Data: 46~1500 bytes
7. Frame Check Sequence (FCS): 4 bytes, using CRC to calculate the checksum of the content from the destination Mac to the data field.

According to RFC894, the maximum length of IP packets in the Ethernet package is 1500 bytes, which means that the Ethernet maximum frame length should be the Ethernet header plus 1500, plus 7 bytes of preamble and 1 bytes of frame start delimiter, Specifically: 7-byte preamble synchronization +1-byte frame start delimiter + 6-byte destination mac+6 bytes of the source mac+2 byte of the frame type +1500+4 bytes of FCS.

According to the above, the maximum frame should be 1526 bytes, but in fact the maximum frame we get is 1514 bytes, why not 1526 bytes? The reason is that when the data frame arrives at the network card, in the physical layer of the Internet cards to first remove the leading synchronization code and frame start delimiter, and then the frame for CRC check, if the frame checksum is wrong, discard this frame. If the checksum is correct, it determines whether the destination hardware address of the frame conforms to its own receiving condition (the destination address is its physical hardware address, broadcast address, multicast hardware address to be received, etc.), and if so, the frame is delivered to the "device driver" for further processing. At this point, our capture software can catch the data, so the capture software caught is to remove the leading synchronization code, frame start delimiter, the data outside the FCS, the maximum value is 6+6+2+1500=1514.

Ethernet specifies that the Ethernet frame data domain portion is a minimum of 46 bytes, which means that the minimum Ethernet frame is 6+6+2+46+4=64. The 4-byte FCS is removed, so it is 60 bytes when the packet is caught. When the length of the data field is less than 46 bytes, the Mac child layer is populated after the data field to satisfy the data frame length of not less than 64 bytes. Because the padding data is the responsibility of the Mac child layer, which is the device driver.

A class of LANs with CSMA/CD as the MAC algorithm is called Ethernet. CSMA/CD Conflict Avoidance method: First listen to the post, edge-to-ear, random delay after the re-hair. Once a conflict occurs, each host must be detected. The minimum send gap and minimum frame length are also required to avoid collisions.

Consider the following scenario, the host sends a small frame, and the two conflicting hosts are far apart. B begins sending a frame when the frame transmitted by host a transmits to the first moment of B. Thus, when A's frame reaches B, b detects a conflict and sends a conflict signal. If A's frame has already been sent before the conflict signal in B is transmitted to a, then a will not detect the conflict and mistakenly think it has been sent successfully. Because the signal propagation is time-delay, it also takes a while to detect the conflict. This is also why there must be a minimum frame length limit.

by standard, 10Mbps Ethernet with repeaters, the maximum length of the connection is 2500 meters, up to 4 repeaters, so the minimum transmission time for a frame of 10Mbps Ethernet is 51.2 microseconds. The data that can be transmitted during this time is 512 bits, so it is also known as 512-bit time. This time is defined as an Ethernet timeslot, or a collision time slot. 512 bits = 64 bytes, which is the reason for the minimum 64 bytes of Ethernet frames.

512-bit is the time when the host captures the channel. If a host sends 64 bytes of a frame without conflict, there will be no further conflicts, which is called the host capturing the channel.

Because the channel is shared by all hosts, if the data frame is too long, the host will not be able to send the data for a long time, and some of the sending data may exceed the buffer size at the receiving end, causing buffer overflow. To avoid a single host occupying a channel for too long, the maximum frame length for an Ethernet frame is 1500.

When the time slot of the 100Mbps Ethernet is still 512 bits, the Ethernet specifies that the minimum transmit time for a frame must be 5.12μs.

The time slot of the 1000Mbps Ethernet is increased to 512 bytes, or 4,096 bits, 4.096μs.

Ethernet Data Frame (802.3) maximum and minimum length