Flow Control

Introduction

Flow control regulates data transmission between sender and receiver to prevent buffer overflow or underflow. It ensures efficient, reliable communication by controlling the sender’s data rate according to receiver capacity and network conditions.

"Flow control is the mechanism that prevents a fast sender from overwhelming a slow receiver in data communication." -- Andrew S. Tanenbaum

Definition and Objectives

Definition

Flow control: process of managing data rate between two nodes to ensure receiver buffers are not exceeded and data integrity is maintained.

Primary Objectives

• Prevent receiver buffer overflow.
• Maintain data transmission synchronization.
• Maximize throughput without loss.
• Adapt to varying network conditions.

Secondary Goals

Reduce retransmissions, minimize latency, and improve overall connection stability.

Importance in Transport Layer

Transport Layer Role

Ensures end-to-end data delivery, reliability, and error control. Flow control is critical to guarantee receiver readiness.

Relationship to Reliability

Complements error control by controlling data volume; prevents data loss due to overflow.

Interaction with Other Layers

Works with network and data link layers for congestion control and error detection.

Flow Control Mechanisms

Stop-and-Wait

Sender transmits one frame, waits for acknowledgment before sending next. Simple, but inefficient at high delays.

Sliding Window

Allows multiple frames in transit. Sender maintains a window of frames permitted to send before receiving ACKs.

Credit-Based Control

Receiver grants credits indicating how many data units sender can transmit.

Rate-Based Control

Sender adjusts sending rate dynamically based on feedback.

Stop-and-Wait Protocol

Operation

Send one packet; wait for ACK; retransmit if timeout.

Advantages

Simplicity, easy implementation, reliable for low bandwidth-delay products.

Disadvantages

Poor utilization of link capacity in high latency networks; low throughput.

Use Cases

Simple control channels, error-prone links, or minimal hardware.

Sliding Window Protocol

Concept

Sender maintains a window of frames allowed for transmission; slides forward on ACK reception.

Types

• Go-Back-N: retransmit all frames after a lost frame.
• Selective Repeat: retransmit only erroneous frames.

Efficiency

Higher throughput, better link utilization, suitable for high bandwidth-delay product networks.

Window Management

Dynamic adjustment depending on network feedback and receiver buffer size.

Window Size and Sequence Numbers

Window Size

Defines number of unacknowledged frames allowed in transmission; impacts throughput and buffer usage.

Sequence Numbers

Unique identifiers for frames; enable ordering and loss detection.

Maximum Window Size

Limited by sequence number space to avoid ambiguity.

Wrap-Around Handling

Modulus arithmetic used to manage sequence number cycling.

Flow Control in TCP

TCP Sliding Window

Dynamic window size advertised by receiver via window field in TCP header.

Receiver Window

Indicates buffer space available; limits sender’s outstanding data.

Window Scaling

Extension to support windows larger than 65,535 bytes.

Zero Window

Receiver signals no buffer space; sender pauses transmission.

Window Update

Receiver sends window update when buffer frees space.

Buffer Management

Receiver Buffers

Store incoming data until application reads; size impacts flow control.

Sender Buffers

Hold sent but unacknowledged data for potential retransmission.

Buffer Overflow

Leads to data loss; flow control mechanisms prevent this.

Buffer Allocation Strategies

Static vs dynamic allocation; trade-off between memory use and performance.

Congestion Avoidance vs Flow Control

Flow Control

Prevents receiver overload; end-to-end control at transport layer.

Congestion Control

Prevents network overload; controls data rate globally using feedback.

Interaction

Independent but complementary; TCP integrates both.

Performance Considerations

Throughput

Window size directly impacts throughput; larger windows increase efficiency.

Latency

Stop-and-wait increases latency; sliding window reduces it.

Error Rates

High error rates require retransmissions; impact flow control effectiveness.

Buffer Sizes

Must be balanced against memory constraints and expected traffic volume.

Common Algorithms and Standards

Go-Back-N ARQ

Retransmits all frames after lost or erroneous one; simple but bandwidth-heavy.

Selective Repeat ARQ

Retransmits only erroneous frames; complex but efficient.

TCP Flow Control Algorithm

Uses advertised window and acknowledgments to adjust sending rate.

Standard Protocols

TCP, SCTP, and some data link layer protocols implement flow control variants.

Sliding Window Algorithm:1. Initialize send_base and next_seq_num to 0.2. While next_seq_num < send_base + window_size: Send packet[next_seq_num]. next_seq_num += 1.3. On ACK receipt for packet i: send_base = i + 1.4. If send_base == next_seq_num: Stop timer. Else: Restart timer.

References

• Peterson, L. L., & Davie, B. S. Computer Networks: A Systems Approach, 5th ed., Morgan Kaufmann, 2011, pp. 213-235.
• Tanenbaum, A. S., & Wetherall, D. J. Computer Networks, 5th ed., Pearson, 2010, pp. 245-270.
• Stevens, W. R. TCP/IP Illustrated, Volume 1: The Protocols, Addison-Wesley, 1994, pp. 180-210.
• Kurose, J. F., & Ross, K. W. Computer Networking: A Top-Down Approach, 7th ed., Pearson, 2017, pp. 260-290.
• RFC 793: Transmission Control Protocol, Postel, J., IETF, 1981, pp. 1-74.