Introduction Intermediate Systems ISs) are off-the-shelf elements that route Internet Protocol (IP) and ISO Connectionless Network Protocol (CLNP) datagrams between MD-ISs@ and between MD-ISs and Fixed End Systems (F-ESs) (Figure 1010-1). ISs also may be used to route mobile data link frames that are encapsulated in CLNP datagrams between Mobile Data Base Stations (MDBSs) and MD-ISs.