MTU and MSS Query

[punkh2113]

Below is my query:

MINIMUM MTU AND MSS WRITTEN AT SOME PLACES ARE 576 AND 536 MSS RESPECTIVELY.

AND THROUGH THE LOGIC OF MINIMUM 64 BYTE FRAME IS NECESSARY, MINIMUM MTU IS 46 BYTES WITH 6 BYTES MSS

AND WHILE CONFIGURING WE ARE ALLOWED TO ENTER MSS OF 68 ATLEAST, THAT MAKES MTU OF MINIMUM 108.

Can anyone please throw some light on it as why is this. I am sure I am missing something here, please help me to figure out what is it? Thanks

[Alexey Nesterkin]

Hello

MTU - maximum transmit union, MSS - maximum segment size. They don't limit minimum size, only maximum. Minimum packet size and minimum segment size not limited.

64 byte - minimum frame size, this size specific for Ethernet.

Frames must be at least 64 bytes long, not including the preamble, so, if the data field is shorter than 46 bytes, it must be compensated by the Pad field. The reason for specifying a minimum length lies with the collision-detect mechanism. In CSMA/CD a station must never be allowed to believe it has transmitted a frame successfully if that frame has, in fact, experienced a collision.

[Joseph W. Doherty]

I believe the MTU of 576 was an expectation (standard) that any IP media would at least support that. So, IP, if it doesn't know otherwise, it assumes a MTU of 576.

As for MSS being 536, that's assuming that the MTU is 576 and also assuming the IP and TCP headers are their minimal sizes of 20 bytes each this allow a TCP MSS of 536 (576 - 40).