Sequenced Packet Exchange (SPX) is a protocol in the IPX/SPX protocol stack that corresponds to a connection-oriented transport layer protocol in the OSI Model. Being reliable and connection-oriented, it is analogous to the Transmission Control Protocol (TCP) of TCP/Internet Protocol (IP), but it is a datagram protocol, rather than a stream protocol.
Source: https://en.wikipedia.org/wiki/Sequenced_Packet_Exchange