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Standard Ethernet supports an MTU of 1500 bytes and Ethernet implementation supporting jumbo frames, allow for an MTU up to 9000 bytes. However, border protocols like PPPoE will reduce this. Path MTU Discovery exposes the difference between the MTU seen by Ethernet end-nodes and the Path MTU.
Super jumbo frames (SJFs) are frames that have a payload size over 9000 bytes. [21] As it has been a relatively difficult, and somewhat lengthy, process to increase the path MTU of high-performance national research and education networks from 1500 bytes to 9000 bytes or so, a subsequent increase, possibly to 64,000 bytes, is under consideration.
If it's less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field. Values between 1500 and 1536, exclusive, are undefined. [10] This convention allows software to determine whether a frame is an Ethernet II frame or an IEEE 802.3 frame, allowing the coexistence of both standards on the same physical medium.
This added overhead can mean that a reduced maximum length limit (so-called ‘MTU’ or ‘MRU’) of 1500 − 8 = 1492 bytes is imposed on (for example) IP packets sent or received, as opposed to the usual 1500-byte Ethernet frame payload length limit which applies to standard Ethernet networks.
That value was chosen because the maximum length of the data field of an Ethernet 802.3 frame is 1500 bytes and 1536 is equivalent to the number 600 in the hexadecimal numeral system. Thus, values of 1500 and below for this field indicate that the field is used as the size of the payload of the Ethernet frame while values of 1536 and above ...
For example, for data being transferred across Ethernet, the MTU is 1500 bytes, the minimum size of a TCP header is 20 bytes, and the minimum size of an IPv4 header is 20 bytes, so the maximum segment size is 1500−(20+20) bytes, or 1460 bytes.
The maximum segment size (MSS) is a parameter of the Options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not count the TCP header or the IP header (unlike, for example, the MTU for IP datagrams).
At 155 Mbit/s, a typical full-length 1,500 byte Ethernet frame would take 77.42 μs to transmit. On a lower-speed 1.544 Mbit/s T1 line , the same packet would take up to 7.8 milliseconds. A queuing delay induced by several such data packets might exceed the figure of 7.8 ms several times over.