ECN bit values: what does the value 11 indicate?

ECN bit values: what does the value 11 indicate?

• A. 00: Not Protocol aware or negotiating
• B. 01/10: Both are Protocol aware
• C. 11: Congestion experienced and marked by router
• D. 10: Reserved for future use

Answer: (C)

ECN uses two bits in the IP header to signal congestion without dropping packets. These two bits have four possible values: 00, 01, 10, and 11. The value 11 corresponds to Congestion Experienced (CE): a router encountered congestion and marked the packet to tell the endpoints to slow down. This is why the correct answer describes congestion being experienced and the packet marked by the router. The other values represent not ECN-capable or ECN-capable-but-not-yet-congested states, or reserved for future use, so they don’t describe congestion signaling.

ECN uses two bits in the IP header to signal congestion without dropping packets. These two bits have four possible values: 00, 01, 10, and 11. The value 11 corresponds to Congestion Experienced (CE): a router encountered congestion and marked the packet to tell the endpoints to slow down. This is why the correct answer describes congestion being experienced and the packet marked by the router. The other values represent not ECN-capable or ECN-capable-but-not-yet-congested states, or reserved for future use, so they don’t describe congestion signaling.