/siː-dʌbəl-juː-ɑːr/
n. “A TCP header flag used to indicate that congestion has been acknowledged and the sender can resume normal transmission.”
In the context of the Transmission Control Protocol (TCP), CWR (short for Congestion Window Reduced) works together with ECE as part of the Explicit Congestion Notification (ECN) mechanism. When a sender receives an ECE signal indicating network congestion, it reduces its transmission rate. After this reduction, the sender sets the CWR flag in the TCP header to notify the receiver that it has acknowledged the congestion and adjusted its window accordingly.
Key characteristics of CWR include:
- Congestion Acknowledgment: Confirms the sender has responded to congestion signals.
- Part of ECN: Works in tandem with ECE to manage network congestion efficiently.
- TCP Header Flag: One of the control bits in the TCP segment used for reliable signaling.
- Optional Use: Only relevant when both endpoints and network devices support ECN.
Conceptual example of CWR usage:
// TCP ECN workflow
Router marks packet for congestion (ECN)
Receiver sets ECE flag in acknowledgment
Sender receives ECE and reduces transmission rate
Sender sends segment with CWR flag set
Receiver acknowledges CWR
Normal transmission resumes with adjusted congestion windowConceptually, CWR is like a polite nod from the sender saying, “I saw your congestion warning and slowed down,” letting the network and receiver know it has handled the situation responsibly.