LCP, PPP Link Control Protocol

Description Glossary RFCs Publications Obsolete RFCs


Protocol suite: PPP.
Protocol type:PPP link control protocol.
PPP protocol:0xC021.
MIME subtype:
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:

0001020304050607 0809101112131415 1617181920212223 2425262728293031
Code Identifier Length
Data :::

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

0Vendor Specific. RFC 2153
9 Echo-Request.  
10 Echo-Reply.  
11 Discard-Request.  
12Identification. RFC 1570
13Time-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:

0001020304050607 0809101112131415
Type Length
Data :::

Option Type. 8 bits.

0 Vendor Specific. RFC 2153
14Maximum-Receive-Unit. RFC 1661
3 >= 4Authentication-Protocol. RFC 1334, RFC 1661, RFC 1994
4>= 4Quality-Protocol. RFC 1661
56Magic-Number. RFC 1661
72Protocol-Field-Compression (Deprecated). RFC 1661
82Address-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
292Simple Data Link on SONET/SDH. RFC 2823
30 Reserved until 14-Oct-2002. 

Length. 8 bits.
Size of the option in bytes.

Data. Variable length.
Zero or more bytes of option specific data.



[RFC 1471] The Definitions of Managed Objects for the Link Control Protocol of the Point-to-Point Protocol.

[RFC 1570] PPP LCP Extensions.

[RFC 1661] The Point-to-Point Protocol (PPP).

[RFC 1663] PPP Reliable Transmission.

[RFC 1812] Requirements for IP Version 4 Routers.

[RFC 1934] Ascend's Multilink Protocol Plus (MP+).

[RFC 1989] PPP Link Quality Monitoring.

[RFC 1990] The PPP Multilink Protocol (MP).

[RFC 1994] PPP Challenge Handshake Authentication Protocol (CHAP).

[RFC 2125] The PPP Bandwidth Allocation Protocol (BAP) The PPP Bandwidth Allocation Control Protocol (BACP).

[RFC 2153] PPP Vendor Extensions.

[RFC 2484] PPP LCP Internationalization Configuration Option.

[RFC 2686] The Multi-Class Extension to Multi-Link PPP.

[RFC 2823] PPP over Simple Data Link (SDL) using SONET/SDH with ATM-like framing.


Obsolete RFCs:

[RFC 1333] PPP Link Quality Monitoring.

[RFC 1334] PPP Authentication Protocols.

[RFC 1548] The Point-to-Point Protocol (PPP).

[RFC 1716] Towards Requirements for IP Routers.

[RFC 1717] The PPP Multilink Protocol (MP).

Description Glossary RFCs Publications Obsolete RFCs