The maximum number of sequentially numbered protocol data units that can be outstanding (unacknowledged) at any given time. The window size is usually a value that is determined by agreement between the same protocol layers of adjacent systems. On i5/OS, the term window size is used for different layers of communications; for example, X.25 high-level data link control (HDLC), X.25 packet level, and the X.400 reliable transfer server all use the concept of window size to control flow.