Question

In stop-and-wait transmission, suppose that both sender and receiver retransmit their last frame immediately on receipt of a duplicate ACK or data frame; such a strategy is superficially reasonable because receipt of such a duplicate is most likely to mean the other side has experienced a timeout. (a) Draw a timeline showing what will happen if the first data frame is somehow duplicated, but no frame is lost. How long will the duplications continue? This situation is known as the Sorcerer's Apprentice bug. (b) Suppose that, like data, ACKs are retransmitted if there is no response within the timeout period. Suppose also that both sides use the same timeout interval. Identify a reasonably likely scenario for triggering the Sorcerer's Apprentice bug.

Short answer

Duplication continues indefinitely if each side keeps retransmitting in response to timeouts. Synchronized timeouts are a likely scenario for this bug.

Step-by-step solution

- Understand the Stop-and-Wait Protocol

In the Stop-and-Wait protocol, the sender transmits a frame and waits for an acknowledgment (ACK) from the receiver before transmitting the next frame.

- Duplicate Data Frame Transmission

Consider what happens when the first data frame (Frame 0) is duplicated. Both the sender and receiver might interpret this duplicate differently. The receiver might send an ACK for each duplicate it receives, and the sender might believe the first ACK was lost and retransmit Frame 0.

- Timeline Construction

Draw a timeline where Frame 0 is sent by the sender at time t. The receiver receives and acknowledges Frame 0 at time t+d. At t+2d, if the sender has duplicated Frame 0, the receiver will send another ACK. This process continues indefinitely as long as each side resends frames/ACKs.

- Non-ending Duplications

The duplications will continue indefinitely (or until an external interruption) because each side believes the other did not receive the last frame or ACK, triggering continuous resends.

- Impact of Retransmitting ACKs

Now consider that ACKs are also retransmitted on timeouts. If the sender does not receive an ACK for Frame 0 in its timeout period, it resends the frame. The receiver then resends the ACK if it doesn’t receive a subsequent data frame in its timeout period, creating a feedback loop.

- Triggering Scenario

Both sides using the same timeout interval can lead to synchronized retransmissions. If the sender's Frame 0 doesn't get an ACK back in time, it retransmits Frame 0. The receiver, waiting for the next frame, resends the ACK for Frame 0 due to its timeout, thus creating a chain reaction.

Key concepts

Duplicate Data Frame

In networking, a Duplicate Data Frame happens when the same data frame is received more than once. This can occur due to network errors.
For example, if Frame 0 is sent by the sender and reaches the receiver, the receiver sends an acknowledgment (ACK).
If the sender does not receive the ACK in time, it will resend Frame 0. This causes the receiver to receive the same frame again, leading to a duplication.
Each duplicate frame causes the receiver to send another ACK, which can create confusion and inefficiency in data transmission.
Sometimes, both sender and receiver mistakenly think the previous frame or ACK was lost, causing continuous duplication.

Retransmission

Retransmission is the process of sending data again because the original transmission was not acknowledged. This is common in protocols like Stop-and-Wait.
In the Stop-and-Wait protocol, after sending a frame, the sender waits for an ACK. If the ACK is not received within a specified timeout period, the sender retransmits the frame.
This ensures that the data eventually gets through, even if the initial transmission fails.
However, retransmission can cause issues such as congestion and unnecessary traffic, especially when frames or ACKs are duplicated.

Sorcerer's Apprentice Bug

The Sorcerer's Apprentice Bug is a situation where both sender and receiver end up in an endless loop of retransmitting frames and ACKs. This usually happens when a duplicate frame triggers a misunderstanding between the sender and receiver.
Let's say the sender sends Frame 0 and the receiver sends back an ACK. If the sender doesn't get the ACK due to a network delay and retransmits Frame 0, the receiver will again send an ACK for the duplicate frame.
If both use the same timeout interval, they continue to retransmit, leading to an infinite loop.
This bug is named after a story where an apprentice's actions cause a simple task to spiral out of control, creating chaos.

Frame Timeout

Frame Timeout is the time period a sender waits for an ACK before deciding that the frame was not received and retransmitting it.
If the sender sends a frame and does not receive an ACK within the specified timeout period, it assumes the frame was lost and retransmits it.
This mechanism helps ensure data is reliably transmitted over the network.
However, setting the timeout period too short can cause unnecessary retransmissions, while setting it too long can slow down data transfer.

Feedback Loop

A Feedback Loop in networking is a situation where the actions of the sender and receiver continuously reinforce each other, often leading to repeated actions. In the context of the Stop-and-Wait protocol, a feedback loop can occur when retransmissions and ACKs turn into an endless cycle.
For example, if the sender sends Frame 0 and does not receive an ACK in time, it retransmits Frame 0. If the receiver had already sent an ACK and did not receive a new frame, it might resend the same ACK.
This repetition can create a loop where both sides keep sending the same frame and ACK indefinitely, believing the other side did not receive their previous message.
It is crucial for protocols to handle such feedback loops properly to maintain efficient and reliable data transmission.