The TCP/IP Guide - PPP Link Control Protocol (LCP) Frame Formats

The previous topic described the general format used by the various protocols in PPP that exchange control messages. Of the many control protocols in PPP, the Link Control Protocol (LCP) is the most important, as it is responsible for basic PPP link setup and operation. It is also the protocol used as a “template” for many of the other control protocols.

Since it is so central to PPP, and since many of the other protocols use a similar messaging system, I want to make the general frame format seen in the previous page more “concrete” by showing the specific frame formats used for each of the LCP control frames. There are thirteen different ones, however, and since they have many fields in common, I didn't want to put here thirteen tables all of which look almost the same.

To save space and show how the messages differ, I have put them into a single large summary table instead of making thirteen smaller ones. Table 38 shows the contents and meaning for each of the fields in the thirteen LCP frame types:

**Table 38: PPP Link Control Protocol (LCP) Frame Types and Fields**
Frame Type	Code Field	Identifier Field	Length Field	Data Field
*Configure-Request*	1	New value generated for each frame.	4 + Length of all included configuration options.	Configuration options to be negotiated by the two peers on a link. The previous topic describes the general format of configuration options.
*Configure-Ack*	2	Copied from the Identifier field of the Configure-Request frame for which this Configure-Ack is a reply.	4 + Length of all included configuration options.	Configuration options being positively acknowledged (accepted during negotiation of the link).
*Configure-Nak*	3	Copied from the Identifier field of the Configure-Request frame for which this Configure-Nak is a reply.	4 + Length of all included configuration options.	Configuration options being negatively acknowledged (renegotiation requested).
*Configure-Reject*	4	Copied from the Identifier field of the Configure-Request frame for which this Configure-Reject is a reply.	4 + Length of all included configuration options.	Configuration options being rejected (since the device cannot negotiate them).
*Terminate-Request*	5	New value generated for each frame.	4 (or more if extra data is included).	Not required. See note below.
*Terminate-Ack*	6	Copied from the Identifier field of the matching Terminate-Request.	4 (or more if extra data is included).	Not required. See note below.
*Code-Reject*	7	New value generated for each frame.	4 + Length of rejected frame.	A copy of the LCP frame that was rejected. This is not the complete PPP frame, just the LCP control portion from its Information field.
*Protocol-Reject*	8	New value generated for each frame.	6 + Length of rejected frame.	The first two bytes contain the Protocol value of the frame rejected. The rest contains a copy of the Information field from the frame rejected.
*Echo-Request*	9	New value generated for each frame.	8 (or more if extra data is included).	Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero. May also contain additional “uninterpreted data”; see note below.
*Echo-Reply*	10	Copied from the Identifier field of the matching Echo-Request.	8 (or more if extra data is included).	Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero. May also contain additional “uninterpreted data”; see note below.
*Discard-Request*	11	New value generated for each frame.	8 (or more if extra data is included).	Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero. May also contain additional “uninterpreted data”; see note below.
*Identification*	12	New value generated for each frame.	8 (or more if extra data is included).	Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero. May also contain additional “uninterpreted data”; see note below.
*Time-Remaining*	13	New value generated for each frame.	12 (or more if extra data is included).	Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero. Also contains a 4-byte value indicating the number of seconds remaining in the current session. A value of all ones in this field is interpreted as “forever”, meaning the session will not expire. May also contain additional “uninterpreted data”; see note below.

All LCP control frames are encapsulated into a PPP frame by placing the frame structure into its Information field as discussed in the previous topic. The Protocol field is set to 0xC021 for LCP. For an explanation of how the frames are used, see the operational description of LCP.

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