 (IANA). The pro- tocol number range in IP is 256, i.e., much larger than for MTP L3, but only a few values are still unassigned. Although the mode of operation of the IP and MTP L3 layers are similar, the routes they realize do not have equal properties. IP services are not guaranteed to be reliable since the IP layer can run on any type of supporting LAN, MAN, and WAN strata, and there is no correspondence to a link layer (similar to MTP L2) that checks user data for errors. Instead the Internet architecture defines a number of encapsulation and tunnelling layers, with the main purpose of framing IP-message data for transfer over different types of strata. The correspondence to signaling link supervision and maintenance in the MTP L3 layer exists, however, in terms of diverse routing layers and maintenance layers that run on specific protocol numbers (one of them, the "exterior gateway protocol" (EGP) runs on protocol number 8, as shown in the model). These are all layers that control the T-ROs and T-RE tables in the IP layer (as indicated by the management control point, mCP, in the model). 3.4.4 Socket Routes In general, switched routes in packet-switching networks do not offer an ideal serv- ice for application layers. There are at least four problems to consider: 1. The number of possible application layers may far exceed the range of the APi defined for the switching layer (16 in MTP L3, 256 in IP). For example, the Internet must be dimensioned for a very large number of applications. 2. In general, datagram networks do not guarantee in-sequence delivery of data. 3. To make the switching layer efficient, the size of message that can be sent is to be restricted, or even fixed (as in ATM). 4. The service may not check the correctness of user data (IP does not). PPP is intended for many types of dial-up access applications (e.g., Appletalk, Xerox, DECnet, Novell IPX). The encapsulation layer and the LCP are common for all PPP applications, while the IPCP is specific for IP networks. 3.4 Route Type Examples So