Active open connection A TCP connection that a host has initiated i.e it sent the SYN
Passive open connection A TCP connection that is in state LISTENING
Pure ACK A TCP segment containing no data, just acknowledging sent data
Nagle algorithm The Nagle algorithm prevents the sending of a small segments if there are any outstanding unacknowledged data. This reduces the amount of tinygrams being sent. It is particularly elegant because on fast LAN ACKs will arrive quickly (such that the algorithm has minimal effect) whereas on a slow WAN the ACKs will arrive more slowly, so there will not be so many tinygrams. Users of high-speed networks, where capacity is not a significant issue but OWD is, may choose to disable the Nagle algorithm.