d Application TCP/IP protocol stack Services TCP/UDP Network Interface building block of the protocol, the IP packet, is designed to deal with all of these disparities, while TCP (and other related protocols, discussed later) take care of the QoS issues. Two network interface protocols are particularly important to TCP/IP. The Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) are used to provide data link layer services in situations in which no other data link protocol is present, such as in leased-line or older dial- up environments. Most TCP/IP software packages for desktop applica- tions include these two protocols, even though dial-up is rapidly fading into near-oblivion in the presence of growing levels of broadband access. With SLIP or PPP, a remote computer can attach directly to a host and connect to the Internet using IP rather than being limited to an asyn- chronous connection. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies The Point-to-Point Protocol (PPP) PPP, as its name implies, was created for the governance of point-to-point links. It has the ability to manage a variety of functions at the moment of connection and verification, including password verification, IP address resolution, compression (where required), and encryption for pri- vacy or security. It can also support multiple protocols over a single con- nection, an important capability for dial-up users who rely on IP or some other network layer protocol for routing and congestion control. It also supports inverse multiplexing and dynamic bandwidth allocation via the Multilink-PPP Protocol (ML-PPP), commonly used in ISDN environ- ments where bandwidth supersets are required over the connection. The PPP frame (see Figure 7-36) is similar to a typical HDLC frame, with delimiting flags, an address field,