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Note that MTU (maximum transmission unit) is the size of the entire packet.
Each packet you send or receive is made up of two parts -- the "Header" section and the "Data Field" section. The Header section contains information such as the source and destination address, as well as the specific number of each packet. It is like the envelope around the actual data.
The Data Field contains the data -- it is the "contents" of your packet. This could include the text of your email, the HTML text of this web page, the data for your icon image or any number of things.
For a standard TCP/IP packet, there are two headers -- the IP Header and the TCP header. Each of these are 20 bytes in size. The maximum size Ethernet packet is 1500 bytes. Therefore, the maximum possible data field -- and thus the maximum possible MSS -- is 1460. If you use PPPoE, this adds a third header which is 8 bytes in size. Therefore, with PPPoE, the maximum MSS is 1452.
Since it is more efficient to send as large of a data field as possible, your transmission speed is generally helped by using the highest MSS you can. Your upload packet size is supposed to be the same as your download packet size. If there is a 40 bytes difference in size, something is not working correctly.