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LCP Options
Several options can be negotiated by LCP as it establishes a link for use by one or more NCPs. We shall discuss two of the more common ones. The Asynchronous Control Character Map (ACCM) or simply “asyncmap” option defines which control characters (i.e., ASCII characters in the range 0x00–0x1F) need to be “escaped” as PPP operates. Escaping a character means that the true value of the character is not sent, but instead the PPP escape character (0x7D) is stuffed in front of a value formed by XORing the original control character with the value 0x20. For example, the XOFF character (0x13) would be sent as (0x7D33). ACCM is used in cases where control characters may affect the operation of the underlying hardware. For example, if software flow control using XON/XOFF characters is enabled and the XOFF character is passed through the link unescaped, the data transfer ceases until the hardware observes an XON character. The asyncmap option is generally specified as a 32-bit hexadecimal number where a 1 bit in the nth least significant bit position indicates that the control character with value n should be escaped. Thus, the asyncmap 0xffffffff would escape all control characters, 0x00000000 would escape none of them, and 0x000A0000 would escape XON (value 0x11) and XOFF (value 0x13). Although the value 0xffffffff is the specified default, many links today can operate safely with the asyncmap set to 0x00000000.
Multi link PPP (MP)
A special option to PPP called multilink PPP (MP) [RFC1990] can be used to aggregate multiple point-to-point links to act as one. This idea is similar to link aggregation, discussed earlier, and has been used for aggregating multiple circuit-switched channels together (e.g., ISDN B channels). MP includes a special LCP option to indicate multilink support as well as a negotiation protocol to fragment and recombine fragmented PPP frames across multiple links. An aggregated link, called a bundle, operates as a complete virtual link and can contain its own configuration information. The bundle comprises a number of member links. Each member link may also have its own set of options.
Compression Control Protocol (CCP)
Historically, PPP has been the protocol of choice when using relatively slow dialup modems. As a consequence, a number of methods have been developed to compress data sent over PPP links. This type of compression is distinct both from the types of compression supported in modem hardware (e.g., V.42bis, V.44) and also from protocol header compression, which we discuss later. Today, several compression options are available. To choose among them for each direction on a PPP link, LCP can negotiate an option to enable the Compression Control Protocol (CCP) [RFC1962]. CCP acts like an NCP (see Section 3.6.5) but handles the details of configuring compression once the compression option is indicated in the LCP link establishment exchange.
PPP Authentication
Before a PPP link becomes operational in the Network state, it is often necessary to establish the identity of the peer(s) of the link using some authentication (identity verification) mechanism. The basic PPP specification has a default of no authentication, so the authentication exchange of Figure 3-24 would not be used in such cases. More often, however, some form of authentication is required, and a number of protocols have evolved over the years to deal with this situation. In this chapter we discuss them only from a high-level point of view and leave the details for the chapter on security (Chapter 18). Other than no authentication, the simplest and least secure authentication scheme is called the Password Authentication Protocol (PAP). This protocol is very simple—one peer requests the other to send a password, and the password is so provided. As the password is sent unencrypted over the PPP link, any eavesdropper on the line can simply capture the password and use it later. Because of this significant vulnerability, PAP is not recommended for authentication. PAP packets are encoded as LCP packets with the Protocol field value set to 0xC023.
Network Control Protocols (NCPs)
Although many different NCPs can be used on a PPP link (even simultaneously), we shall focus on the NCPs supporting IPv4 and IPv6. For IPv4, the NCP is called the IP Control Protocol (IPCP) [RFC1332]. For IPv6, the NCP is IPV6CP [RFC5072]. Once LCP has completed its link establishment and authentication, each end of the link is in the Network state and may proceed to negotiate a network-layer association using zero or more NCPs (one, such as IPCP, is typical).
IPCP, the standard NCP for IPv4, can be used to establish IPv4 connectivity over a link and configure Van Jacobson header compression (VJ compression) [RFC1144]. IPCP packets may be exchanged after the PPP state machine has reached the Network state. IPCP packets use the same packet exchange mechanism and packet format as LCP, except the Protocol field is set to 0x8021, and the Code field is limited to the range 0–7. These values of the Code field correspond to the message types: vendor-specific (see [RFC2153]), configure-request, configure-ACK, configure-REJECT, terminate-request, terminate-ACK, and code-REJECT. IPCP can negotiate a number of options, including an IP compression protocol (2), the IPv4 address (3), and Mobile IPv4 [RFC2290] (4). Other options are available for learning the location of primary and secondary domain name servers.
Header Compression
PPP dial-up lines have historically been comparatively slow (54,000 bits/s or less), and many small packets are often used with TCP/IP (e.g., for TCP’s acknowledgments; see Chapter 15). Most of these packets contain a TCP and IP header that changes little from one packet to another on the same TCP connection. Other higher-layer protocols behave similarly. Thus, it is useful to have a way of compressing the headers of these higher-layer protocols (or eliminating them) so that fewer bytes need to be carried over relatively slow point-to-point links. The methods employed to compress or eliminate headers have evolved over time. We discuss them in chronological order, beginning with VJ compression, mentioned earlier.
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