The Maximum Transmission Unit (MTU) is the largest possible frame length that can be sent over an existing physical transmission medium or over a LAN or WAN path without fragmentation. If longer frame lengths occur, they are fragmented according to the protocol rules used or the frame is discarded. Wide area networks (WAN) generally have smaller MTU sizes than local area networks (LAN).
The larger the MTU unit, the better the ratio of user data to header data. For a TCP header with 40 bytes and an MTU unit of 650 bytes, the ratio is 6.1 %; with a larger MTU unit of 1,500 bytes, the ratio drops to 2.66 %. However, since the frame length is directly included in the transmission time, the MTU unit plays a significant role for the delay time and the response time behavior, especially for real-time applications such as VoIP or online games.
With IPv6, the IP network must support data packets of at least 1,280 bytes. However, networks should be configured to carry 1,500 bytes or even more so that encapsulated Ethernet payloads can be transmitted without affecting fragmentation. In contrast, 802.15.4 provides a Maximum Transmission Unit (MTU) of only 127 bytes. This means that with 6LoWPAN, where IPv6 protocols are transmitted over low powerWPANs, an adjustment must be made through fragmentation.