Transmission Control Protocol (TCP) is the backbone of reliable communication on the internet. To ensure reliability, efficiency, and fairness, TCP uses sophisticated mechanisms known as flow control and congestion control.
This article explains TCP flow control in deep technical detail, covering window size, sliding window operation, sender and receiver coordination, and TCP congestion avoidance algorithms.
1. What Is TCP Flow Control?
Definition
TCP Flow Control is a mechanism that prevents a fast sender from overwhelming a slow receiver. It ensures that the sender transmits data only at a rate that the receiver can process.
Why Flow Control Is Needed
- Receivers have limited buffer memory
- Applications consume data at different speeds
- Uncontrolled sending leads to buffer overflow and data loss
Flow control focuses on the receiver’s capacity, not the network condition.
2. TCP Window Size – The Core Concept
What Is a TCP Window?
A TCP window defines how much data (in bytes) can be sent without waiting for an acknowledgment (ACK).
Types of TCP Windows
- Receiver Window (rwnd): Advertised by the receiver, based on buffer availability
- Congestion Window (cwnd): Maintained by the sender, based on network conditions
The actual sending limit is:
Effective Window = min(rwnd, cwnd)
3. Receiver Window (rwnd) – Receiver-Based Flow Control
What Is rwnd?
rwnd represents the amount of free buffer space available at the receiver. It is advertised in every TCP acknowledgment.
How rwnd Works
- Receiver buffers incoming data
- Receiver sends ACK with available buffer size
- Sender limits transmission based on rwnd
Why rwnd Is Important
- Prevents receiver buffer overflow
- Adapts dynamically as applications read data
4. TCP Sliding Window Mechanism
What Is Sliding Window?
The sliding window allows TCP to send multiple packets before receiving acknowledgments, improving throughput and efficiency.
Window Components (Sender Side)
- Sent & Acknowledged: Successfully delivered
- Sent but Not ACKed (In Flight): Awaiting acknowledgment
- Usable Window: Can be sent immediately
- Outside Window: Cannot be sent yet
Window Sliding Process
- Sender sends packets within window
- Receiver acknowledges received data
- Window slides forward
- New data becomes eligible for transmission
5. Sender vs Receiver Responsibilities
Sender Responsibilities
- Respect rwnd advertised by receiver
- Control cwnd based on congestion
- Retransmit lost packets
Receiver Responsibilities
- Advertise accurate buffer availability
- Acknowledge received data
- Maintain data ordering
6. TCP Congestion Control – Network Protection
What Is Congestion Control?
While flow control protects the receiver, congestion control protects the network itself from overload.
Key Variable: Congestion Window (cwnd)
cwnd limits how much data the sender can inject into the network.
7. Slow Start Algorithm
Purpose
Slow Start probes network capacity cautiously to avoid congestion collapse.
How Slow Start Works
- cwnd starts small (1 MSS)
- Doubles every RTT
- Exponential growth
Slow Start Threshold (ssthresh)
When cwnd reaches ssthresh, TCP exits slow start and enters congestion avoidance.
8. Congestion Avoidance Phase
Linear Growth
In congestion avoidance, cwnd increases slowly (≈1 MSS per RTT) to avoid overwhelming the network.
Why Linear Growth?
- Balances throughput and stability
- Reduces packet loss
9. Packet Loss & Congestion Events
How TCP Detects Congestion
- Timeouts
- Duplicate ACKs
Multiplicative Decrease
Upon detecting congestion:
- cwnd is reduced drastically
- ssthresh is updated
- TCP re-enters slow start or congestion avoidance
10. Combined Flow Control Logic
TCP transmission rate is governed by:
Send Window = min(rwnd, cwnd)
This ensures:
- Receiver is not overwhelmed
- Network is not congested
- Throughput is maximized safely
11. Real-World Example
- High-speed server sending data
- Mobile device with limited buffer
- Network congestion during peak hours
TCP dynamically adjusts rwnd and cwnd to maintain stable communication.
12. Why TCP Flow Control Matters in Cybersecurity
- Understanding DoS and DDoS behavior
- Traffic analysis during packet captures
- Performance tuning in secure systems
Conclusion
TCP flow control and congestion control are intelligent, adaptive mechanisms that allow the internet to function reliably at scale.
By combining sliding windows, receiver feedback, and congestion algorithms, TCP achieves a balance between speed, reliability, and fairness.
Mastering these concepts is essential for networking, cybersecurity, and system design.
