MTU

MTU，Maximum Transmission Unit，指的是Ethernet Frame的Payload部分的长度限制，如下图：

1500这个数字则有一定的历史原因，

参考链接：

How 1500 bytes became the MTU of the internethttps://blog.benjojo.co.uk/post/why-is-ethernet-mtu-1500

The “Ethernet: Distributed Packet Switching for Local Computer Networks” paper from 1980is a early note of the efficiency cost analysis of larger packets on a network.

This being especially important to ethernet at the time, since ethernet networks would ether be sharing the same coax cable between all systems, or there would be ethernet hubs that would only allow one packet at a time to be transmitted around all members of the ethernet segment.

A number had to be picked that would mean that transmission latency on these shared (sometimes busy) segments would not be too high, but also that packet header overhead would not be too much. (see some of the tables on the paper linked above on page 15-16)

It would seem at best that the engineers at the time picked 1500 bytes, or around 12000 bits as the best “safe” value.

MTU可以被看作是3层报文的Size限制。