Question

2. For a sliding window algorithm with SWS=4 frames and RWS=4 frames, the receiver uses cumulative ACKs. Use a timeout interval of about 2 x RTT. Assuming that the transmit time of a frame is equal to 0.25 RTT and the frames are processed instantaneously if they arrive in order.

a.What is the maximum number of packets that can be transmitted in one RTT?

b. Draw a timeline diagram (up to frame 6) for the situation when the Frame 1 is lost (the sequence number of frames start from 0).

*Note：On each data frame and ACK frame, you need to indicate the sequence number, what action is taken by the receiver when a frame is received, for example, processed, buffered, or discarded.*

Answer 1

In a sliding window algorithm with SWS and RWS both equal to 4, the maximum number of frames transmitted in one RTT is 4. For a timeline diagram with Frame 1 lost, Frames 2 and 3 are buffered until Frame 1 is retransmitted and received, then a cumulative ACK for Frame 3 is sent.

Step-by-step explanation:

Sliding Window Protocol and Maximum Transmission in RTT

In a sliding window algorithm with a Send Window Size (SWS) of 4 frames and a Receive Window Size (RWS) of 4 frames, where the transmit time for a frame is 0.25 RTT, the maximum number of frames that can be transmitted in one round-trip time (RTT) is determined by the smaller of SWS or RWS and the available time. Since the transmit time for one frame is 0.25 RTT, four frames (1 RTT / 0.25 RTT per frame) can be sent within one RTT, which is also the size of the SWS and RWS. Therefore, the maximum number of packets that can be transmitted in one RTT is 4.

Timeline Diagram for Lost Frame 1

The timeline will depict the sending of Frames 0 to 3 initially. Assuming Frame 1 is lost and never received:

• Frame 0 is sent and received; the receiver sends an ACK for Frame 0.
• Frames 1, 2, 3 are sent by the transmitter.
• Frame 1 is lost, Frames 2 and 3 arrive but cannot be processed as Frame 1 is missing. They might be buffered.
• The transmitter does not receive ACK for Frame 1 within the timeout period (assumed as 2 x RTT) and retransmits Frame 1.
• After Frame 1 is successfully received and processed, the receiver sends a cumulative ACK for Frame 3, since Frames 1, 2, and 3 have been processed.
• Frames 4, 5, and 6 are then sent following the sliding window protocol guidelines.

Frames that arrive out of order or after a lost frame are either buffered or discarded depending on the receiver's policy. For simplicity, we can assume in this case that they are buffered until the missing frame arrives.