1a1. Logical link 0 will be a control link between any two HOSTs on the network
1a1a. Only one control link may exist between any two HOSTs on the network. Thus, if there are n HOSTs on the network, there are n-1 control links from each HOST.
1a2. It will be primarily used for communication between HOSTs for the purposes of:
1b1. A user at a given HOST may have exactly 1 primary link to each of the other HOSTs on the network.
1b1a. The primary link must be the first link established between a HOST user and another HOST.
1b1b. Primary links are global to a user, i.e. a user program may open a primary link, and that link remains open until it is specifically closed.
1b1c. The primary link is treated like a teletype connected over a normal data-phone or direct line by the remote HOST, i.e. the remote HOST considers a primary link to be a normal teletype user.
1b1d. The primary link is used for passing (user) control information to the remote HOST, e.g. it will be used for logging in to the remote host (using the remote hosts standard login procedure).