LCP, Link Control Protocol

LCP, PPP Link Control Protocol

Protocol suite:	PPP.
Protocol type:	PPP link control protocol.
PPP protocol:	0xC021.
MIME subtype:
SNMP MIBs:
Working groups:
Links:	IANA: PPP Assigned numbers.

RFC 1661:

In order to be sufficiently versatile to be portable to a wide variety of environments, PPP provides a Link Control Protocol (LCP). The LCP is used to automatically agree upon the encapsulation format options, handle varying limits on sizes of packets, detect a looped-back link and other common misconfiguration errors, and terminate the link. Other optional facilities provided are authentication of the identity of its peer on the link, and determination when a link is functioning properly and when it is failing.

This protocol is used to establish, configure and test the data-link connection for a PPP link.

In order to establish communications over a point-to-point link, each end of the PPP link MUST first send LCP packets to configure and test the data link. After the link has been established, the peer MAY be authenticated.

PPP header

LCP header

Data :::

LCP header:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Code								Identifier								Length
Data :::

Code. 8 bits.
Specifies the function to be performed.

Code	Description	References
0	Vendor Specific.	RFC 2153
1	Configure-Request.
2	Configure-Ack.
3	Configure-Nak.
4	Configure-Reject.
5	Terminate-Request.
6	Terminate-Ack.
7	Code-Reject.
8	Protocol-Reject.
9	Echo-Request.
10	Echo-Reply.
11	Discard-Request.
12	Identification.	RFC 1570
13	Time-Remaining.	RFC 1570

Identifier. 8 bits.
Used to match requests and replies.

Length. 16 bits.
Size of the packet including the header.

Data. Variable length.
Zero or more bytes of data as indicated by the Length. This field may contain one or more LCP options.

LCP Configuration Options:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15
Type								Length
Data :::

Option Type. 8 bits.

Type	Length	Description	References
0		Vendor Specific.	RFC 2153
1	4	Maximum-Receive-Unit.	RFC 1661
3	>= 4	Authentication-Protocol.	RFC 1334, RFC 1661, RFC 1994
4	>= 4	Quality-Protocol.	RFC 1661
5	6	Magic-Number.	RFC 1661
7	2	Protocol-Field-Compression (Deprecated).	RFC 1661
8	2	Address-and-Control-Field-Compression.	RFC 1661
9	3	FCS-Alternatives.	RFC 1570
10	3	Self-Describing-Pad.	RFC 1570
11	>= 4	Numbered-Mode.	RFC 1663
12	>= 8	Identification.	RFC 1570
13	>= 3	Callback.	RFC 1570
14		Connect-Time (Deprecated).
15	2	Compound-Frames (Deprecated).	RFC 1570
16		Nominal-Data-Encapsulation (Deprecated).
17	4	Multilink Max-Receive-Reconstructed-Unit (MRRU).	RFC 1990
18	2	Multilink Short Sequence Number Header Format.	RFC 1990
19	variable	Multilink Endpoint Discriminator.	RFC 1990
20		Proprietary.
21		DCE-Identifier.
22	4	MP+ Procedure Option.	RFC 1934
23	4	Link Discriminator for BACP.	RFC 2125
24		LCP-Authentication-Option.
25		COBS, Consistent Overhead Byte Stuffing.
26	variable	Prefix Elision.	RFC 2686
27	4	Multilink header format.	RFC 2686
28	>= 7	Internationalization.	RFC 2484
29	2	Simple Data Link on SONET/SDH.	RFC 2823
30		Reserved until 14-Oct-2002.