actually end to end delivery of packets is done by network layer. Network layer is responsible for the packets to be delivered with reliability for every link i.e for every routers it come across its path.so packet is delivered with reliability from source to destination. where as transport layer is reaponsible for process to process reliability.it gets the packets from network layer demutiplex it to sen it to corresponding application (FTP,Email,...) depending upon the portnumber that is specified.