This article is based on a misconception. Please see RFC793. Also see Stevens 1 pg 248. Co-worker is rewriting former content as embryonic state article.
ATCP connections are referred to as '''half-open connection''' referswhen tothe astation [[Transmissionat Controlone end of the Protocol|TCP]] connection thathas iscrashed, partiallyor open,otherwise whichremoved isthe socket without formally innotifying the processother end of beingthe establishedTCP connection.
Due to the stateless nature of the Transfer Control Protocol, it is possible that if the end of the connection on which the socket still exists is not expecting a response, then the socket may persist indefinitely in this state, known as half-open.
The TCP protocol has a three state system for opening a connection. First, the originating site (A) sends a [[SYN (TCP)|SYN packet]] to the destination (B). A is now half-open, and awaiting a response. B now updates its kernel information to indicate the incoming connection from A, and sends out a request to open a channel back (the SYN/ACK packet).
Note: A common misconception is that the term TCP half-open applies to an partially completed TCP handshake state. A better term for this state is an embryonic state.
At this point, B is now "half-open" (it has sufficient information to receive packets, but not enough to send packets back). Note that B was put into this state by another machine, outside of B's control.
Under normal circumstances (see [[denial-of-service attack]] for deliberate failure cases), A will receive the SYN/ACK from B, update its tables (which now have enough information for A to both send and receive), and send a final ACK back to B.
Once B receives this final ACK, it also has sufficient information for two-way communication, and the connection is fully open.
