While testing the tcp implementation on MSBA2 I noticed that
the value of `tcp_input_buffer_end` gets changed whenever acquiring or
releasing the mutex of the struct on the server side.
After deleting the packed attribute of the struct this problem was
resolved and the value stayed the same after acquiring and releasing.
This problem could maybe arise from badly placed cache lines due to
missing padding... I am not sure.
Anyway, I guess using the packed attribute is useless here and makes it
more error-prone.
The current implementation does not set the ack bit
for outgoing data segments and the fin segment.
However, RFC793 states that all segments
should have an ack bit set in order to present a valid
ack nr. in outgoing segments.
Currently, data segments and acknowledgement segments
are distinguished by the existence of their ack bit.
With the new assumption, that both of these types of
segments need an ack bit set, I had to change several
parts of the current implementation to make this
decision by inspecting the payload size.
destiny: added parens
