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
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.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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:
0 RESERVED
1 Maximum-Receive-Unit
3 Authentication-Protocol
4 Quality-Protocol
5 Magic-Number
7 Protocol-Field-Compression
8 Address-and-Control-Field-Compression
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.
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.