T-ROs hostIPA (PPP) nodeIPA (PPP) AddrPar LAN encapsulation, PPP encapsulation tunnelling, MPLS Serial layer elements links Dial-up network LAN, MAN and WAN T-RE(IP-user): REi->destIPA layer elements layer elements T-ROs(IP): destIPA-REi T-RE(IP): REi->(NWi,addrPar} Fig. 3.27 The IP switching node. IPA=IP address; mCP=management control point; PRN=proto- col number; PPP=point-to-point protocol As could be expected, names of configuration parameters are different from MTP L3: the APi is called protocol number (PRN), the NA is called IP address (IPA) and, in messages, the sender's address is referred to as a source address (instead of "origin" as in MTP L3) As in MTP L3, hosted nodes are of two classes: nodes that contain management functions (several types of route maintenance and service control protocols) and nodes of logical networks that use the IP datagram service. Different from SS7 is that management protocols are defined as separate layers, which considerably facilitates the understanding of IP management. The mode of operation in the IP layer is equal to MTP L3. A difference though is the large number of nodes that only terminate IP messages and therefore need no T-ROs. These are called hosts. In the Internet application of IP, many of these hosts 3 Node Structures connect only temporarily to the Internet over dial-up networks. For such hosts, a route between the host and an IP node must first be established (through some dial- up network), and a temporary hostIPA must be assigned. This is achieved by, e.g., the "point-to-point protocol" (PPP) stratum element of the model (see Fig. 3.27.) The PPP protocol specification does not describe a single layer/protocol, but a stra- tum of three layers: an encapsulating link layer (similar to HDLC); a link control layer (LCP) that defines and controls properties of the link, and a layer that assigns IPAs to hosts (IPCP). 7 APi values (i.e., "protocol numbers" in IP) are registered permanently to differ- ent types of layers by the Internet Assigned Numbers Authority