In reliable transport protocols such as Transmission Control Protocol, what is the primary purpose of flow control between a sender and a receiver on a network connection?

Introduction / Context:
This question checks your knowledge of one of the key functions of Transport layer protocols such as TCP, namely flow control. While transport protocols also handle error recovery and sometimes congestion control, flow control specifically focuses on preventing a fast sender from overwhelming a slower receiver, which is essential for stable and efficient communication between endpoints.

Given Data / Assumptions:
- We are dealing with reliable Transport layer protocols such as TCP rather than simple datagram protocols.
- There is a sender and a receiver exchanging data over a network connection.
- The question refers specifically to the purpose of flow control, not error control or encryption.
- Mechanisms such as window sizes and acknowledgments are in play at this layer.

Concept / Approach:
Flow control is a mechanism that allows the receiver to advertise how much data it can buffer and process at any given time. In TCP this is typically implemented via the receive window field, which the receiver updates in acknowledgments to tell the sender how much additional data it may send without causing buffer overflow. The sender adjusts its sending rate accordingly. Flow control thus prevents packet loss and excessive retransmissions that would occur if the receiver were swamped with more data than it can handle.

Step-by-Step Solution:
Step 1: Separate the roles of various Transport layer functions: error control, flow control, and sometimes congestion control.Step 2: Recognize that error control deals with detecting and retransmitting lost or corrupted segments, while flow control deals with matching the sender rate to the receiver capacity.Step 3: Recall that TCP flow control uses a sliding window mechanism in which the receiver advertises a window size.Step 4: Understand that the sender uses this advertised window to determine how much unacknowledged data it may have in flight at any time.Step 5: Conclude that the primary purpose of flow control is to give the receiver the ability to govern how much data the sender transmits.

Verification / Alternative check:
You can verify this by thinking about what happens when the receiver application is slow or temporarily blocked. The TCP stack on the receiver will reduce the advertised window size, eventually to zero if necessary, which forces the sender to pause. When the application consumes data and buffer space becomes available, the receiver increases the window and the sender resumes transmission. This behavior is exactly what flow control is designed to provide.

Why Other Options Are Wrong:
Retransmitting data when acknowledgments are missing is part of error recovery and reliability, not specifically flow control. Reassembling segments in the correct sequence is also a Transport layer function but falls under ordered delivery rather than flow control. Regulating the size of each segment is related to maximum segment size negotiation and path MTU, again distinct from managing the sender rate. Encryption of segments is a security function and is not provided by flow control mechanisms.

Common Pitfalls:
Students often mix up flow control and congestion control. While both influence the sender rate, congestion control addresses conditions inside the network path such as queue buildup, whereas flow control protects the receiving endpoint. Mixing these concepts can lead to incorrect answers. Another pitfall is to assume that any reliability feature in TCP is part of flow control, when in fact reliability is a broader goal accomplished by several distinct mechanisms.

Final Answer:
The main purpose of flow control is to provide a means for the receiver to control how much data the sender transmits so that the receiver is not overwhelmed.