s its TCP to actually initiate a three-way handshake to connect to another ULP. Usually an active open is targeted to a passive open. This active/passive model supports server-oriented applications where a permanent resource, such as a database management process, can always be accessed by remote users. However, the three-way handshake also coordinates two simultaneous active opens to open a connection. Over an open connection, the ULP pair can exchange a continuous stream of data in both directions. Normally, TCP groups the data into TCP segments for transmission at its own convenience. However, a ULP can exercise a push service to force TCP to package and send data passed up to that point without waiting for additional data. This mechanism is intended to prevent possible deadlock situations 12.4 INTEGRATED SERVICES DIGITAL NETWORKS (ISDN) where a ULP waits for data internally buffered by TCP. For example, an interactive editor might wait forever for a single input line from a terminal. A push will force data through the TCPs to the awaiting process. A TCP also provides the means for a sending ULP to indicate to a receiving ULP that "urgent" data appear in the upcoming data stream. This urgent mechanism can support, for example, interrupts or breaks. When a data exchange is complete, the connection can be closed by either ULP to free TCP resources for other connections. Connection closing can happen in two ways. The first, called a graceful close, is based on the three-way handshake procedure to complete data exchange and coordinate closure between the TCPs. The second, called an abort, does not allow coordination and may result in the loss of unacknowledged data. NOTE: There is a certain military flavor in TCP/IP protocol family. There is good rea- son; their development (around 1975) was supported by the U.S. Department of Defense for ARPANET. Since then, this protocol family has become extremely popular in the commercial world (e.g., internet), both for LAN and WAN operations. 12.4 INTEGRATED SERVICES DIGITAL