Although TCP and IP can be used separately, they were designed at the same time to work as part of a unified system, without duplicating the work done by each.
A computer connected to the Internet needs BOTH TCP and IP software. IP provides a way to transfer a packet from its source to its destination.
TCP handles problems like datagram loss or delivery out of order. Together they provide a reliable way to send data across the Internet."
Vendors aften sell a single package that includes software for TCP, IP and a few related communication protocols. Collectively, the set is known as TCP/IP software.