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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
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.
Specifies the length of the option in bytes.
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.