Dropping Networking packets by a router

Question

I have a quick question. If a TCP connection between two host is established and negotiated for ECN, then the sender will be notified of any network congestion by the receiver. In case if ECN is not negotiated and a packet is dropped by a router,does it send ICMP notification to sender about the dropping packet?

i am just wondering, Why cant cant an intermediate router send icmp message before dropping packets? because it has a way to communicate to sender right. why should sender wait till it receives acknowledgement from receiver?

Answer 1

No. The ECN bit is set and transmitted along with the packet as a mechanism to inform the sender that it should reduce its transmission rate. This should eliminate the dropping of packets.

However, the TCP protocol stack is resonsible for guaranteeing delivery, not the intervening routers. If a packet is dropped it is the responsibility of the receiver to detect this and request the packet again.