Introduction to Sliding Window Protocols

Sliding window is used by most connection oriented network protocols. In fact, TCP also uses sliding window. It assumes two -way communication (full duplex). It uses two types of frames:

• Data
• Ack (sequence number of last correctly received frame).

The basic data idea of sliding window protocol is that both sender and receiver keep a "window" of acknowledgment. The sender keeps the value of expected acknowledgment; while the receiver keeps the value of expected receiving frame. When it receives an acknowledgment from the receiver, the sender advances the window. When it receives the expected frame, the receiver advances the window.

The characteristics of sliding windows used at the sender and receiver usually involve

• error correction (by retransmission)
• flow control and
• message ordering by sender (FIFO).

The latter property can easily be incorporated in a sliding window protocol, but sometimes, it is preferred to be implemented as a separate protocol for easier maintenance.