When a high-level protocol (or an application) opens a channel via some low-level protocol, it identifies the peer with which it wants to communicate by giving the peer's address. More precisely, as seen in Section 1, the high-level protocol passes a participant list---an argument of type Part---to xOpen. The participant list is simply an array of participant addresses where, by convention, the high-level protocol identifies itself in the first element of the array. Although in general the high-level protocol can list an arbitrary number of participants---a group of peers---that are to communicate via the newly established channel, in a unicast (one-to-one) communication, only a single other participant is identified.