1. A method of reliably constructing TCP/IP datagrams for transmission from a local node on one computer network to a remote node on another computer network comprising the steps of: generating a message, which forms a portion of a transport layer of a communications protocol; buffering said message in at least one of a plurality of buffers between said transport layer of said communications protocol and a network layer of said communications protocol; forwarding said message from said at least one of said plurality of buffers to a buffer in said network layer of said communications protocol executing on a network card; monitoring said forwarding step in order to calculate a TCP segment checksum; composing a TCP header including said TCP segment checksum, said composing step being executed on said network card; pre-pending said TCP header to said message thereby creating an IP datagram in a buffer on said network card; calculating an IP checksum; composing in protocol logic executing on said network card an IP header including said IP checksum; pre-pending said IP header to said IP datagram thereby creating datalink data in said buffer on said network card; forwarding said datalink data to a datalink layer protocol interface on said network card; composing a datalink routing header in said datalink layer protocol interface on said network card, said datalink routing header including an address of a next intermediate node in a network transmission path to a destination; pre-pending said datalink routing header to said datalink data to create a final datagram; and transmitting said final datagram over at least one physical medium to said remote node.