LCP Configuration Options allow negotiation of modifications to the
default characteristics of a point-to-point link. If a Configuration
Option is not included in a Configure-Request packet, the default
value for that Configuration Option is assumed.
Some Configuration Options MAY be listed more than once. The effect
of this is Configuration Option specific, and is specified by each
such Configuration Option description. (None of the Configuration
Options in this specification can be listed more than once.)
The end of the list of Configuration Options is indicated by the
Length field of the LCP packet.
Unless otherwise specified, all Configuration Options apply in a
half-duplex fashion; typically, in the receive direction of the link
from the point of view of the Configure-Request sender.
Design Philosophy
The options indicate additional capabilities or requirements of
the implementation that is requesting the option. An
implementation which does not understand any option SHOULD
interoperate with one which implements every option.
A default is specified for each option which allows the link to
correctly function without negotiation of the option, although
perhaps with less than optimal performance.
Except where explicitly specified, acknowledgement of an option
does not require the peer to take any additional action other than
the default.
It is not necessary to send the default values for the options in
a Configure-Request.
A summary of the Configuration Option format is shown below. The
fields are transmitted from left to right.
The Type field is one octet, and indicates the type of
Configuration Option. Up-to-date values of the LCP Option Type
field are specified in the most recent "Assigned Numbers" RFC [2].
This document concerns the following values:
The Length field is one octet, and indicates the length of this
Configuration Option including the Type, Length and Data fields.
If a negotiable Configuration Option is received in a Configure-
Request, but with an invalid or unrecognized Length, a Configure-
Nak SHOULD be transmitted which includes the desired Configuration
Option with an appropriate Length and Data.
Data
The Data field is zero or more octets, and contains information
specific to the Configuration Option. The format and length of
the Data field is determined by the Type and Length fields.
When the Data field is indicated by the Length to extend beyond
the end of the Information field, the entire packet is silently
discarded without affecting the automaton.
