The TCP/IP Guide - PPP General Control Protocol Frame Format and Option Format

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The TCP/IP Guide

The TCP/IP Guide
9 TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9 TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols
           9 TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)
                9 Point-to-Point Protocol (PPP)
                     9 PPP Protocol Frame Formats

PPP General Frame Format

PPP Link Control Protocol (LCP) Frame Formats

PPP General Control Protocol Frame Format and Option Format
(Page 3 of 4)

PPP Control Message Option Format

The various Configure- messages are used to negotiate configuration options in LCP and the other control protocols. In their Data fields they carry one or more options that are, again, specific to the protocol using them. For example, LCP uses one set of configuration options for the link as a whole; CCP uses options to negotiate a compression algorithm, MP uses it to set up multilink bundles and so on. Figure 35 shows how these options, which can vary in length, are placed in the Data field of a PPP control message (which is in turn nested inside the general PPP frame format).

**Figure 35: PPP Control Message Carrying Options**
This diagram shows a sample PPP control message carrying options in its *Data* field. Any number of options can be included, and/or mixed with other data, depending on the needs of the message.

Again, there is commonality here; while every option is different, they all use the same basic format, as described in Table 37 and illustrated in Figure 36:

**Table 37: PPP Control Message Option Format**
Field Name	Size (bytes)	Description
Type	1	*Type:* A type value that indicates the option type. The set of Type values is unique to each protocol. So, for example, LCP has one set of Type values corresponding to its configuration options, each NCP has a different set, CCP has its own set and so on.
Length	1	*Length:* Specifies the length of the option in bytes.
Data	Variable	*Data:* Contains the specific data for the configuration option.

The configuration options are described briefly in the individual protocol topics. I am not showing the specific contents of each option as there are just too many of them. These are in the RFCs.

**Figure 36: PPP Control Message Option Format**
Each option that appears in any of the many PPP control message types consists of this triplet of Type, Length and Data.

PPP General Frame Format

PPP Link Control Protocol (LCP) Frame Formats

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Version 3.0 - Version Date: September 20, 2005

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