This quote explains that TCP/IP is indifferent to the content of packets, only the address on the envelope (datagram) matters. Network neutrality is built into the TCP/IP protocol.
In May 1974, they (Vint Cerf, Bob Kahn) complete their paper entitled, "A Protocol for Packet Network Intercommunication." They described a new protocol they called the transmission-control protocol (TCP). The main idea was to enclose packets in "datagrams." These datagrams were to act something like envelopes containing letters. The content and format of the letter is not important for its delivery. The information on the envelope is standardized to facilitate delivery. Gateway computers would simply read only the delivery information contained in the datagrams and deliver the contents to host computers. Only the host computers would actually "open" the envelope and read the actual contents of the packet. TCP allowed networks to be joined into a network of networks, or what we now call the Internet.
... In 1978, Cerf and several of his colleagues made a major refinement. They split TCP into two parts. They took the part of TCP that is responsible for routing packages and formed a separate protocol called the Internet Protocol (IP).TCP would remain responsible for dividing messages into datagrams, reassembling messages, detecting errors, putting packets in the right order, and resending lost packets. The new protocol was called TCP/IP. It went on to become the standard for all Internet communication.