The Optimum MTU Size for the Ethernet

By Stephen Byron Cooper

The Maximum Transmission Unit, or MTU, is an important, but little known network setting. It dictates the maximum amount of data each computer can send across a network in a group. Ethernet is the most widely implemented set of standards in the world for the physical properties of Local Area Networks. The concept of MTU is easier to understand to those who have a background knowledge of network protocols.

Protocol Stacks

Network protocols are organized into layers in a stack. The protocols relating to the physical properties of networks are at the bottom of the stack and user facing protocols are at the top of the stack. From top to bottom, each successive layer is intended to manipulate the data passed by applications so that it can be transferred over a network. The further down the stack a protocol is, the more technical it is. The MTU affects the way each layer processes the data passed by the application.

Protocol Layers

Ethernet, at the bottom of the stack, lies at layers 1 and 2. These are the Physical Layer at Layer 1 and the Data Link Layer at Layer 2. The Internet Layer is usually referred to as Layer 3 and the Transport Layer is Layer 4. MTUs are influential at layers 2, 3 and 4.

Data Packets

The Transport Layer breaks data up into data packets, but the MTU is set at the Data Link Layer. MTU is usually a setting of the network adapter. Most networking software can either detect, or is shipped with, a default MTU value for each Data Link Layer type. The maximum MTU of a data packet traveling over an Ethernet Data Link Layer is 1500 bytes. This length does not include the header added on by Ethernet when it converts a data packet into a frame for transmission, but it does include the headers of the Transport Layer protocol and the Internet Layer protocol. The Internet Layer protocol is usually the Internet Protocol, which has a variable length header. Headers are pieces of information added onto the front of the data being transferred in order to aid their transmission.

Optimum Size

The larger the data packet, or Ethernet frame, the more efficient the transmission because the overhead added on by protocols in the form of headers is reduced in proportion to a larger packet. The Ethernet segment of a data packet's journey only lasts as far as the boundary of the sender's local area network. The biggest detraction with different MTUs is when a data packet travels to a different network with a smaller MTU and has to be broken up, which is called fragmentation. This increases the amount of overhead the headers add to the data packet by a factor of the number of smaller packets the packet is broken into. Therefore, optimization of MTUs relates more to traffic traveling outside a network, than to the requirements of Ethernet. The optimum size for a MTU over Ethernet is the maximum size allowed, which is 1500 bytes.