With TCP the receiver has to send an ACK back to the sender. If the receiver sees that the congestion bit is set on a packet it gets from the sender then it wil

With TCP the receiver has to send an ACK back to the sender. If the receiver sees that the congestion bit is set on a packet it gets from the sender then it will set the same bit on the ACK packet it sends back to the sender to acknowledge that the packet was received. This ACK is sent anyways, since it's part of how the sliding window is designed with TCP. There are built in ways for the TCP protocol to handle congestion, but it doesn't allow a router to signal congestion. The router just has to hope for the sender to detect the congestion fast enough.