Sequenced Packet Exchange (SPX) is the reliable transport protocol used by IPX when connection-oriented communication is required. Much like TCP, SPX uses sequence numbers and acknowledgments to be sure that data is reliably passed between hosts. Beginning with NetWare 4.0, a new (and backwards-compatible) version of SPX called SPX II was released, providing a sliding window flow-control mechanism. You should be aware that not all IPX network communication requires the use of SPX. IPX is also capable of interacting with some of the other upper-layer protocols directly.