Re: [lwip-users] STM32F4+lwIP: TCP MSS sets to 536 bytes on its own

[Sergio R. Caprile]

Well, besides the fact that one thing is TCP, layer-4, with its Maximum
_Segment_ Size and another is Ethernet, layer-2, with the number of
bytes in a frame, which is what you are seeing in wireshark;
You are on the right track. There are a number of algorithms to
gracefully handle the life of a transfer, including the start of it, and
if I'm correct lwIP starts sending 536 bytes and will then update
according to what happens during the transfer.

But, there is also your MTU, which will limit what can fit in an
Ethernet frame, and lwIP has code that adapts TCP segment size to that
too. The MTU is set by your driver in its low-level init function. this
is normally set higher, but you might check anyway.