recv() - this function is the main reception path for ACKs. Note that because only one direction of data flow is in use, this function should only ever be invoked with a pure ACK packet (i.e. no data). The function stores the timestamp from the ACK in ts_peer_, and checks for the presence of the ECN bit (reducing the send window if appropriate). If the ACK is a new ACK, it calls newack(), and otherwise checks to see if it is a duplicate of the last ACK seen. If so, it enters fast retransmit by closing the window, resetting the retransmission timer, and sending a packet by calling send_much.
newack() - this function processes a ``new'' ACK (one that contains an ACK number higher than any seen so far). The function sets a new retransmission timer by calling newtimer(), updates the RTT estimation by calling rtt_update, and updates the highest and last ACK variables.