Chapter 4. WAN Protocols and Technologies: Point-to-Point Protocol (PPP) Along with the explosion of the Internet came the explosion of the Internet's access protocol, the Point-to-Point Protocol (PPP). Most analog dialup connections today run PPP as their data-link protocol. This is primarily because of the Internet-friendly capabilities of PPP: Before PPP became the dominant Internet access protocol, it was often used ”and confused ”with the Serial Line Internet Protocol (SLIP). Many people commonly referred to point-to-point connections using SLIP/PPP. Unfortunately, SLIP supported only IP as its only network layer protocol and thereby fell short of the requirements needed for many network administrators running IP, IPX, and AppleTalk protocols. Cisco's implementation of PPP is modeled after RFC 1661. This RFC explains how PPP is used for encapsulating network layer protocol information over point-to-point links. PPP breaks down the data link layer into three sublayers . Each sublayer has a specific function: -
Network Control Protocol (NCP) ” Establishes and negotiates network layer protocols and addresses. -
Link Control Protocol (LCP) ” Establishes, authenticates, and optionally tests the link for quality. -
High-Level Data Link Control (HDLC) ” Encapsulates datagrams over the link. RFC 1662 describes this procedure. Table 4-1 outlines PPP and its sublayers. Table 4-1. PPP Sublayers of the OSI Mode | OSI Layer | Common Protocols | | Layer 3 | Network layer protocols, such as IP, IPX, and AppleTalk | | Layer 2 | Network Control Protocol (NCP) Link Control Protocol (LCP) High-Level Data Link Control (HDLC) | | Layer 1 | Physical layer EIA/TIA-232, V.24, V.35, ISDN, and so on | Other RFCs significant to PPP are listed here: -
RFC 1144 ” TCP/IP header compression. -
RFC 1220 ” Point-to-Point Protocol extensions for bridging. This RFC replaces RFC 1220. -
RFC 1334 ” PPP authentication protocols. -
RFC 1378 ” PPP AppleTalk Control Protocol (ATCP). -
RFC 1552 ” PPP Internetworking Packet Exchange Control Protocol (IPXCP). -
RFC 1570 ” PPP LCP extensions. -
RFC 1661 ” Point-to-Point Protocol (PPP). -
RFC 1662 ” PPP in HDLC-like framing. -
RFC 1990 ” PPP Multilink Protocol (MP). NOTE You can find all the RFCs online at www.isi.edu/in-notes/rfcxxxx.txt, where xxxx is the number of the RFC.
As referenced, PPP uses the HDLC protocol as a means by encapsulating datagrams over links. The frame structure and principles of PPP are outlined in the International Organization for Standardization (ISO) HDLC procedures 3309, amended by 1984/PDAD1 to allow use in asynchronous environments and the start/stop transmission. Many of PPP's extended capabilities, such as error correction and multiple network layer protocols, are controlled by its Link Control Protocol (LCP) and the Network Control Protocols (NCP) . The LCP frame is sent to configure and optionally test the data link. LCP operates in the following manner: - Step 1. Link establishment phase ” LCP first opens the connection and then negotiates communications parameters. These include the maximum receive unit, compression type, and link authentication protocol type. After link configuration is complete, a configuration-acknowledgment frame must be sent and received. This step is followed by an optional link-quality determination phase. Here, LCP determines whether the link quality is sufficient to bring up network layer protocols.
- Step 2. "Not so optional" authentication phase ” After the link is established and the authentication protocol is decided on, the peer goes through the authentication specified. Cisco offers the Challenge Handshake Authentication Protocol (CHAP) and the Password Authentication Protocol (PAP) for PPP authentication. The PPP standard does not require any authentication to take place; this is true for nondial connections and ISDN connections. For analog dial configurations running over asynchronous links, however, before a Cisco router can bind a route after Step 3, it requires a lower-level caller ID authentication or CHAP and PAP to take place. This might account for why PPP configs can work in one location and not in another. To protect yourself against this, the Cisco TAC recommends to always use CHAP authentication. RFC 1994 defines PPP CHAP, which obsoletes RFC 1334.
- Step 3. Network layer protocol phase ” At this point, LCP directs NCP to bring up the configured network layer protocols. After this phase is complete, packets from each network layer protocol can be sent over the link.
- Step 4. Link termination phase ” LCP can terminate the link upon user intervention or a physical event. LCP directs the NCP to close the Layer 3 protocols and take appropriate action.
LCP accomplishes these phases with three types of LCP frames : -
Link establishment frames ” Establishes the link -
Link termination frames ” Closes the link -
Link maintenance frames ” Used to manage and debug the link |
