Ethernet frames are the data containers that are transmitted across Ethernet media. These frames contain a payload, or data, as well as a header containing information about the frame.
The size of the Ethernet frame is dependent on the Ethernet standard that is being used. For example, if IEEE 802.3 10GBASE-TR is being used, then the maximum frame size is 2182 bytes.
Frames that are less than the maximum allowed frame size will not be discarded by the receiver. This is because receivers have a buffer to store incoming frames until they can be processed. If a frame does not fit in this buffer, then it is discarded.
Because of this requirement of a minimum sized frame to avoid discard at the receiver, determining what size your Ethernet frames should be can be important!
This article will discuss what the minimum sized acceptable Ethernet frames are and why changing this setting can help with network performance.
One byte minimum frame size
The minimum Ethernet frame size is one byte. A frame of one byte can be created, but it cannot include any information. This frame is called a null or empty frame.
Creating a null frame is useful for checking the integrity of the Ethernet connection. If the receiver discards the frame, then there is a problem with the connection between the devices. If the frame is accepted, then there is no problem with the connection.
Some devices may have internal bugs that cause them to discard null frames. Checking for these bugs via null frames can help detect and fix them. Some bug detectors ask you to send a null frame to check for this issue.
However, due to some internal complexities in Ethernet devices, sending only a one-byte frame may not correct some issues. In general, sending at least eight bytes is recommended to correct some issues on an Ethernet network.
Four byte minimum frame size
The minimum Ethernet frame size that will not be discarded by the receiver as a runt frame is four bytes. This size includes the frame header and data, and does not include any padding.
If you were to include padding, then the minimum frame size would be six bytes, including the five byte header and one byte of padding. The reason for the one byte of padding is because the last field in the header is a flag that tells the receiver whether or not to extract more data.
If there is no data to transmit then there is no need for this flag, so it is assumed not to exist. Therefore, one byte of padding must be added to make up for this missing piece of information.
Eight byte minimum frame size
When Ethernet frames are sent, the receiver will not accept frames that are less than eight bytes in size. This is due to the overhead added by the sender to create the frame.
The sender must add at least one byte of data, the protocol information (such as IP or Ethernet), and a couple of control bytes. The receiver must also add some information to check if the frame is valid.
Because of these requirements, frames that are less than eight bytes in size will be discarded. There is no minimum size for a valid frame, however. A one-byte frame may be valid depending on what information is provided in the frame header.
The confusion comes from the fact that there is a minimum size for a transmitted frame which will not be discarded by the receiver. This article explains how to determine what this size is for your network.
Sixteen byte minimum frame size
The minimum Ethernet frame size is sixteen bytes. This is the absolute smallest amount of data that can be sent over the network via the Ethernet protocol.
Any frames that are smaller than this will not be processed by the receiver. The receiver will process the frame, check its integrity, and then discard it because it does not contain enough information.
This is due to the fact that the header requires a certain amount of bytes. The data field can have a lower number, but it cannot be zero or less than what the header requires.
Therefore, if you send a one-byte frame, the receiver will check it and discard it because there is no data to process. If you send a seventeen-byte frame, then only those eighteen bytes will be processed, as the header requirement cannot be met for only seventeen bytes.
Thirty-two byte minimum frame size
The minimum frame size that will not be discarded by the receiver as a runt frame is thirty-two bytes. Any frame that is shorter than this minimum size will be considered a padding or overhead frame, and discarded.
The Ethernet protocol requires each frame to contain five fields of data, and each field must have a minimum number of bytes. The fields are the destination address, source address, type, data length, and checksum.
Because the data length field must contain a number of bytes equal to the length of the packet being transmitted, the total number of bytes in all fields except the checksum must be a multiple of eight. The checksum itself must be a multiple of two.
This makes it impossible to have a frame that is thirty-one or thirty-zero bytes in size.
Sixty-four byte minimum frame size
When the minimum Ethernet frame size is mentioned, it is usually in reference to the minimum transmission unit, or MTU. The MTU refers to the smallest amount of data that can be transmitted over the Ethernet network.
Most Ethernet networks have an MTU of 1500 bytes. This means that any frame that is received with less than 1500 bytes of data will be discarded as a runt frame.
Because each layer in the OSI model has its own overhead, the smallest possible frame size is 64 bytes. This includes all of the layers required to transmit a frame, including the Ethernet header and CRC field.
One hundred and twenty-eight byte minimum frame size
When the receiver detects a frame, it checks the CRC. If the CRC is good, then the receiver discards the frame if it is smaller than a minimum size of 128 bytes.
This is called the minimum Ethernet frame size that will not be discarded by the receiver as a runt frame. If a frame is smaller than this minimum size, then the receiver will discard it.
So, in order to have a chance to be received, a frame must be at least 128 bytes. The reason for this is because of overhead: there are two bytes for the destination network address and six bytes for the source network address.
That makes ten bytes out of 128 that are used for addressing and cannot hold data, so there has to be at least twelve additional bytes of data in order to make it an Ethernet frame.
Two hundred and fifty-six byte minimum frame size
The minimum Ethernet frame size that will not be discarded by the receiver as a runt frame is a 256-byte frame. Frames that are smaller than this are considered too small to carry valid data, so the receiver will discard them.
Even if you have very little data to transmit, you must include at least 256 bytes of data in each frame for the transmission to be valid. If you have only one byte of data to transmit, then you must include 257 bytes of data in the frame for the transmission to be valid.
The reason for this is because the Ethernet CRC field is eight bytes long. If your frame contains less than eight bytes of CRC then there would be no way to check whether the frame is corrupted or not. Hence, it must be at least 256 bytes in size so that there is enough room for the CRC field.
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