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
When accessing the length field of an ipv6_header a byte order switch (host -> network) is necessary.
Otherwise, it breaks calculations or the checksum and other tcp related computations.
Furthermore, when writing to ipv6_header->length it is important to switch this
from host byte order to network byte order.
