Other support protocols are defined at these two layers, and many application protocols run there too, as well as at the upper layers of the protocol stack. The core protocols of TCP/IP operate at layers three and four of the OSI model, corresponding to the Internet layer and Host-to-Host Transport layer of the TCP/IP architectural model. The TCP/IP protocol suite was generally designed to provide implementation of the networking stack from the network layer (layer three) and above. I then have a single topic that describes the operation of the rather simple Serial Line Internet Protocol (SLIP), and a comprehensive section on the much more capable Point-to-Point Protocol (PPP). I begin with a brief overview of the two protocols used to connect devices at layer two and how they fit into the TCP/IP protocol suite as a whole. In this section I describe the protocols used specifically to implement data link layer connectivity for TCP/IP internetworks. To enable TCP/IP to operate on these kinds of links, two special TCP/IP data link layer protocols have been created. However, certain types of connections exist that do not include a layer two protocol over which IP can run.

IP operates at layer three of the OSI Reference Model, and assumes that it will be layered on top of an existing layer two technology. The TCP/IP protocol suite is structured around the Internet Protocol (IP). With due acknowledgement to the free TCP/IP Guide from which this extract is taken

Tcp ip over serial ppp