1) TCP takes place in the network layer of the OSI right? So does that mean it starts at the network layer or does the data flow from TCP still have to start at the physical layer? The book isnt very clear on this point.
2) In the line header length, Reserved, flags, sliding window..... what is the reserved area used for all the book says is its for later use and hardly ever used.
3) In the line Options/Padding is it possible for both of these to be 0 bits at the same time? And if so does this line get skipped completely? Are the special options worth knowing about?
4) Under Data what is meant by "The size of the data field depends on how much data needs to be transmitted(this half i understand its the next half which I don't), the constraints of the TCP segment size imposed by the network type, and the limitation that segment must fit within an IP datagram."