TCP Sliding Window

http://www.tcpipguide.com/free/t_TCPSlidingWindowAcknowledgmentSystemForDataTranspo-6.htm. If a transmission is not acknowledged after a period of time, it is retransmitted by its sender. This system is called positive acknowledgment with retransmission (PAR). 

At any point in time we can take a “snapshot” of the process. If we do, we can conceptually divide the bytes that the sending TCP has in its buffer into four categories, viewed as a timeline (Figure 206):

1. Bytes Sent And Acknowledged: The earliest bytes in the stream will have been sent and acknowledged. These are basically “accomplished” from the standpoint of the device sending data. For example, let's suppose that 31 bytes of data have already been send and acknowledged. These would fall into Category #1.
2. Bytes Sent But Not Yet Acknowledged: These are the bytes that the device has sent but for which it has not yet received an acknowledgment. The sender cannot consider these “accomplished” until they are acknowledged. Let's say there are 14 bytes here, in Category #2.
3. Bytes Not Yet Sent For Which Recipient Is Ready: These are bytes that have not yet been sent, but which the recipient has room for based on its most recent communication to the sender of how many bytes it is willing to handle at once. The sender will try to send these immediately (subject to certain algorithmic restrictions we'll explore later). Suppose there are 6 bytes in Category #3.
4. Bytes Not Yet Sent For Which Recipient Is Not Ready: These are the bytes further “down the stream” which the sender is not yet allowed to send because the receiver is not ready. There are 44 bytes in Category #4.