An end system connection is a connection of the transport layer of the sending computer with the transport layer of the receiving computer( peer-to-peer). If routers are used on the path between the sending and receiving stations, the headers of the first three layers are removed in these routers and replaced by new ones.
These connections, sending computer - first router, last router - receiving computer, as well as the connections between the routers on the individual route sections are called secured system connections.
These are peer-to-peer connections on the third OSI layer, the network layer. Peer-to-peer connections on the second OSI layer, the link layer, are considered unsecured system connections, while traffic on the physical transmission medium under the physical layer is simply called a data connection.