Sliding Window Protocol - Protocol Operation

Protocol Operation

The transmitter and receiver each have a current sequence number n_t and n_r, respectively. They each also have a window size w_t and w_r. The window sizes may vary, but in simpler implementations they are fixed. The window size must be greater than zero for any progress to be made.

As typically implemented, n_t is the next packet to be transmitted, i.e. the sequence number of the first packet not yet transmitted. Likewise, n_r is the first packet not yet received. Both numbers are monotonically increasing with time; they only ever increase.

The receiver may also keep track of the highest sequence number not yet received; the variable n_s is one more than the sequence number of the highest sequence number received. For simple receivers that only accept packets in order (w_r = 1), this is the same as n_r, but can be greater if w_r > 1. Note the distinction: all packets below n_r have been received, no packets above n_s have been received, and between n_r and n_s, some packets have been received.

When the receiver receives a packet, it updates its variables appropriately and transmits an acknowledgment with the new n_r. The transmitter keeps track of the highest acknowledgment it has received n_a. The transmitter knows that all packets up to, but not including n_a have been received, but is uncertain about packets between n_a and n_s; i.e. n_a ≤ n_r ≤ n_s.

The sequence numbers always obey the rule that n_a ≤ n_r ≤ n_s ≤ n_t ≤ n_a + w_t. That is:

• n_a ≤ n_r: The highest acknowledgement received by the transmitter cannot be higher than the highest n_r acknowledged by the receiver.
• n_r ≤ n_s: The span of fully received packets cannot extend beyond the end of the partially received packets.
• n_s ≤ n_t: The highest packet received cannot be higher than the highest packet sent.
• n_t ≤ n_a + w_t: The highest packet sent is limited by the highest acknowledgement received and the transmit window size.