The Relationship Between Hosts and the Subnet

To place it in perspective, the Network Layer software runs in dedicated devices termed routers - early editions of Tanenbaum introduced the term IMPs (Intermediate Message Processors), and the Transport Layer (coming soon) runs in the hosts.

Implications for Data Link Layer software

Except in the trivial 2-node case, some nodes will now have more than one physical link to manage.

When the Network Layer presents a packet (data or a control message) to the Data Link Layer, it must now indicate which physical link is involved.

Each link must have its own Data Link protocol - its own buffers for the sender's and receiver's windows and state variables such as ackexpected, nextframetosend, etc.