Operation
ECN requires specific support at the Internet and Transport layers for the following reasons:
- In TCP/IP routers operate mostly on the Internet layer while transmission rate is handled by the endpoints at the Transport layer
- Congestion may be handled only by the transmitter but since it is known to have happened only after a packet was sent, there must be an echo of the congestion indication by the receiver to the transmitter.
Without ECN congestion indication echo is achieved indirectly by the detection of lost packets. With ECN, the congestion is indicated by setting the CE bits (see below) at an IP packet and is echoed back by the receiver to the transmitter by setting proper bits in the Transport Layer's protocol header. For example, when using TCP, the congestion indication is echoed back by setting the ECE bit (see below).
Read more about this topic: Explicit Congestion Notification
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