How the sliding window protocol deals with duplicate frames?

In protocols where acknowledgments (ACKs) are used for reliable data transmission, such as Stop-and-Wait or sliding window protocols, dealing with duplicate frames due to lost ACKs is an important aspect of ensuring data integrity.

Here’s how these protocols typically handle duplicate frames when an ACK is lost:

Sequence Numbers:

Sender’s Perspective:

• When the sender transmits a data frame, it assigns a unique sequence number to each frame.
• After sending a frame, the sender waits for an ACK from the receiver within a timeout period.

Receiver’s Perspective:

• The receiver checks each incoming frame for errors using error detection mechanisms like CRC (Cyclic Redundancy Check).
• If a frame is error-free and the receiver has not yet processed it (i.e., it’s not a duplicate), the receiver sends an ACK back to the sender.
• The receiver maintains a buffer to store recently received and processed frames, identified by their sequence numbers.

Handling Duplicate Frames:

Sender’s Response to Lost ACK:

• If the sender does not receive an ACK within the timeout period, it assumes the ACK was lost in transit.
• To ensure reliability, the sender retransmits the same frame with the same sequence number.
• This retransmission is necessary because the sender cannot confirm whether the receiver actually received the frame without the ACK.

Receiver’s Perspective on Duplicate Frames:

• Upon receiving a duplicate frame (due to the lost ACK and subsequent retransmission), the receiver:

1. 1.  Checks the sequence number of the incoming frame against its buffer of recently received frames.
   2.  If the frame with the same sequence number has already been processed and an ACK has been sent, the receiver discards the duplicate frame to avoid processing it again.

Prevention of Duplicate Processing:

Sequence Number Management:

• Proper sequence number management at both sender and receiver ends helps prevent processing duplicate frames.
• The sender increments sequence numbers for each new frame sent, ensuring uniqueness.
• The receiver maintains a record of received sequence numbers to detect and discard duplicates efficiently.

Efficiency Considerations:

Impact on Efficiency:

• Dealing with duplicate frames due to lost ACKs can reduce transmission efficiency, especially in protocols like Stop-and-Wait where transmission halts until an ACK is received or a timeout occurs.
• Modern sliding window protocols allow for multiple frames to be in transit simultaneously, improving efficiency by reducing the impact of occasional lost ACKs on overall throughput.

Summary:

In summary, protocols that rely on acknowledgments for reliability must handle duplicate frames carefully when ACKs are lost. This involves retransmitting frames upon timeout, managing sequence numbers to detect duplicates.

It also ensures efficient processing to maintain data integrity without unnecessary reprocessing of duplicate data.

Proper implementation of these mechanisms helps balance reliability and efficiency in data transmission across networks.

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(Credit: This video content is owned by 5 Minutes Engineering YT channel.)