Ethernet frames are the data containers that hold your data and are sent across a wired Ethernet network. These frames contain the data being sent, along with a number of additional fields for the receiver to process and discard or to use for processing the received data.
One of these additional fields is the Frame Check Sequence field. The Frame Check Sequence field is a 32-bit value that is computed using other fields in the frame, such as the destination address and source address.
The receiver computes its own Frame Check Sequence value and compares this value to the one in the frame. If these values do not match, then the frame is discarded as an error.
There are situations where a receiver will discard a frame even if the Frame Check Sequence matches, though. If the frame size is less than 8 bytes, then it will be discarded as an error. 8 bytes is equal to 64 bits, which is equal to one full Frame Check Sequence field.
Minimum Ethernet frame size
The minimum Ethernet frame size is 64 bytes. This means that the smallest possible Ethernet frame contains 64 bytesof data, plus the 4-byte FCS field and a few reserved and padding bytes.
Theoretically, a frame could be composed of just the destination MAC address (6 bytes) and a transmitted bit (1 bit), but this would not convey any information. The receiver would have no idea what to do with just those two bits!
The minimum size for an Ethernet frame was once only 28 bytes, which is why this is called the “minimum” frame size. The addition of VLAN tagging increased the required size of the payload, hence the increase in minimum frame size.
Even though there is a minimum size for an Ethernet frame, it is possible for a transmitter to send a smaller packet than this. For example, if someone sends an IPv6 packet that is only 40 bytes long, then it cannot be placed into a standard-sized Ethernet frame.
Considerations for runt frames
The minimum size for an Ethernet frame depends on the data length, or how many bytes of data are being transferred. The minimum frame size is 46 bytes for a single transmission.
An Ethernet frame can be either a data frame or a control frame. A data frame contains data payload, and a control frame contains commands or responses to commands. Control frames do not have any data payload.
Both types of frames can have a minimum size of 46 bytes. This is because the header information takes up some space, and each section must be equal to at least 1 byte. All of the sections add up to at least 46 bytes, making this the minimum size for a valid frame.
Examples of runt frames
When the receiver detects a frame that is shorter than the minimum acceptable frame length, it will discard the frame as invalid. If there is data to be sent, this will result in additional frames being sent to re-transmit the data.
This can cause significant delays and re-transmissions, especially if the sender is using asynchronous communication protocols. Asynchronous protocols do not use acknowledgments to confirm receipt, which makes them much faster, but also more susceptible to errors.
The size of the minimum acceptable Ethernet frame size depends on the type of media being used. For instance, a 100 Mbps Ethernet network requires a minimum acceptable frame size of 64 bytes. A 1 Gbps Ethernet network requires a minimum acceptable frame size of 1,522 bytes.
Keep it short and sweet
The minimum Ethernet frame size is 64 bytes, or 48 data bits. Any shorter frames will be discarded by the receiver, so it is important to ensure your frames are at least 64 bytes.
If you need to send less than 64 bytes of data, then you must send at least two consecutive frames to accomplish this. The first frame must be at least 64 bytes and the second frame can be as small as needed.
When sending small packets, such as a single IP packet, it is important to keep in mind the protocol overhead. The IP protocol adds 44 bytes of information onto the frame, so the minimum IP packet that can be transmitted is 46 bytes long.
There are some situations where sending very short frames may be necessary. In those cases, it is important to check whether those frames are being discarded by the receiver.
Fortunately, Ethernet frames that are too small to contain data are not automatically accepted by the receiving device. Receivers have a built-in minimum frame size that they will accept as a valid frame.
All receivers have a minimum frame size of 64 bytes. This means that the receiver will not accept any frames that are less than 64 bytes in size. Any frames less than this minimum size are discarded, or more precisely, not processed.
Sending devices must ensure that their frames are at least the minimum frame size accepted by the receiver. The sending device does not determine if its frame is too small to contain data; it must check with the receiver first to see if it will accept the frame.
By knowing and abiding by the minimum frame size of the receiver, transmissions can be assured to not be discarded prior to being processed.
Using jumbo frames
An Ethernet frame is the unit of data transmission over a wired Ethernet network. The size of the frame is determined by the Ethernet standard used.
Ethernet frames can be of different sizes, called layers. The most common sizes are called standard or norm, and they are 5,000 octet (5000 byte) layers, and 9,000 octet (9000 byte) layers. These numbers refer to the number of bytes in the frame.
All Ethernet receivers have a buffer to store incoming frames. This buffer has a predetermined size based on the hardware manufacturer and model. If a frame is received that is larger than the buffer size, then it will be discarded.
You can set your network interface card (NIC) to transmit jumbo frames (larger than the standard frame size) to increase throughput (amount of data transmitted per session).
Adjusting MTU settings
Another way to deal with Ethernet frame sizes is to adjust the Maximum Transmission Unit, or MTU, setting. The MTU setting can be changed on the device, interface, or network level.
On a device level, the MTU setting can be adjusted for specific interfaces. This can be done by entering into the device configuration settings an MTU value that is smaller than the standard 1500 byte Ethernet frame size.
On the interface level, the MTU setting can be adjusted to any size above the minimum sized frames that will be discarded by the receiver. So if the receiver will discard 1000 byte frames, then your interface must have an MTU setting of at least 1000 bytes.
On a network level, the MTU setting can only be increased from its default value of 1500 bytes. This would help reduce congestion on the network, but it could lead to discards of larger frames.
Testing for runts
The Ethernet standard requires that all frames must be at least 64 bytes (512 bits) in size. Frames that are less than 64 bytes in size are considered runts, and they can create problems on the network.
Runt frames can be caused by several things. For example, when a computer sends out a frame, the network interface device must first collect enough data to fill a 64-byte frame.
If the computer is sending data very quickly, then it may take longer to collect enough data to fill a full frame. The device can then send out a partial frame, which will be classified as a runt frame by the receiver.
Another cause of runt frames is due to buffering on the network devices. When there is too much congestion on the network, devices will stop processing frames for a short period of time in order to prevent more congestion. This process is called buffering.