Reliability not achieved at lower layers shall be implemented at the higher layers and more importantly each Layer in TCP/IP protocol suite is responsible for some network operations. IP though unreliable it implements routing of network information across the nodes upto the destination. But TCP cannot do this it implements only the end-to-communication (i.e) the source and destination without taking care of the intermediate nodes across which the network information should pass through which is done by IP layer. Therefore IP is integrated with TCP.