Receive (RCV) Pointers
The three receive categories are divided using two pointers:
Figure 220: TCP Receive Categories and Pointers
This diagram is the complement of Figure 219 , showing how the categories are set up for the receiving device. Categories #1 and #2 have been combined since there is no differentiation between received and unacknowledged and received and acknowledged. This example shows the state of the receiving device prior to receipt of the 14 bytes that in Figure 219 have already been sent.
Key Concept: A set of receive pointers are maintained by each device, which are the complement of the send pointers. A devices send pointers keep track of its outgoing data and its receive pointers the incoming data. The two receive pointers are RCV.NXT, which indicates the number of the next byte of data expected from the other device, and RCV.WND, which is the size of the receive window for that device. The RCV.WND of one device equals the SND.WND of the other device on the connection.
