المدربA FIX session is the bidirectional conversation between two parties identified by their sender and target identifiers, from a logon to a logout, in which each side numbers the messages it sends 1, 2, 3, … and keeps them so that it can send them again. A heartbeat is a message sent when a side has sent nothing for an agreed interval, so that silence can be told from a dead connection. A resend request asks the other side to send again a range of its messages, after a gap in their sequence numbers has been detected. A gap fill is a sequence-reset message, sent in answer to a resend request, that stands for a run of administrative messages that will not be sent again, moving the receiver’s expected sequence number past them.
