# Link Control Protocol

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In [computer networking](/source/Computer_network), the **Link Control Protocol** (LCP) forms part of the [Point-to-Point Protocol](/source/Point-to-Point_Protocol) (PPP), within the family of [Internet protocols](/source/Internet_protocols). In setting up PPP communications, both the sending and receiving devices send out LCP [packets](/source/Packet_(information_technology)) to determine the standards of the ensuing data transmission.

The [protocol](/source/Communications_protocol):

- checks the identity of the linked device and either accepts or rejects the device

- determines the acceptable packet size for transmission

- searches for errors in configuration

- can terminate the link if requirements exceed the parameters

LCP packets are sent during the *Link Establishment Phase* of PPP, after the [physical layer](/source/Physical_layer) has been made ready from the *Link Dead* state. No transmission of [network-layer protocols](/source/Network_layer) such as [IP](/source/Internet_Protocol) may be transmitted across PPP until after the *Link Establishment* or *Authentication* phases are complete. When a connection is requested by a peer to be terminated (or if *Authentication* fails), the *Link Termination Phase* is performed, and PPP returns to the *Link Dead* state.

## Structure

Offsets Octet 0 1 2 3 Octet Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Point-to-Point Protocol 0 0 Flag (0x7E) Address (0xFF) Control (0x03) Protocol (0xC0) 4 32 Protocol, cont. (0x21) Link Control Protocol 0 0 Code Identifier Length Link Configuration Code 1 Name Configure-Request Description Peer requesting connection with the specified options over link defaults. 2 Configure-Ack Peer requesting connection with the specified options over link defaults. 3 Configure-Nak Peer recognizes given options, but some are not acceptable. 4 Configure-Reject Peer does not recognize or is prohibited from accepting some given options. 4 32 List of option data, size variable Link Termination Code 5 Name Terminate-Request Description Peer requesting to close the connection. 6 Terminate-Ack Peer accepts connection closure, or requires re-negotiation. 4 32 Uninterpreted data by the sender, size variable Link Maintenance Code 7 Name Code-Reject Description Peer does not recognize a provided LCP code. 4 32 Faulting LCP Packet, size variable LCP-Specific Codes Code 8 Name Protocol-Reject Description Peer does not recognize a protocol used within the LCP Opened state. 4 32 Rejected PPP Protocol Faulting Packet, size variable Code 9 Name Echo-Request Description Peer is requesting an Echo-Reply. 10 Echo-Reply Peer is responding to an Echo-Request (only acceptable when in LCP Opened.) 11 Discard-Request Peer is sending data to be discarded ("sinked") by the remote peer. 4 32 Magic Number (0, if Configure-Request / Configure-Ack have not established one yet) 8 64 Uninterpreted data by the sender, size variable

### Link Configuration Option

Offsets Octet 0 1 2 3 Octet Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0 Type Length Type 0 Name Vendor Specific Description Vendor specific data not defined by the PPP specification. 2 16 Vendor's Organizationally Unique Identifier Kind (OUI Subtype) 6 48 Vendor defined data, size variable Type 1 Name Maximum-Receive-Unit Description Defines a peer's MRU, overriding the default of 1,500 octets. 2 16 The maximum-receive-unit for this peer's implementation Type 3 Name Authentication-Protocol Description Defines an authentication protocol for a remote peer to authenticate with before transitioning phases. 2 16 The requested authentication protocol, assigned by IANA[1] Additional data for this protocol, size variable Type 4 Name Quality-Protocol Description Defines a quality protocol for link quality monitoring. 2 16 The requested quality protocol Additional data for this protocol, size variable Type 5 Name Magic-Number Description Defines a unique identifier for a peer. 2 16 Magic number Type 7 Name Protocol-Field-Compression Description No data. Refer to RFC 1661 Section 6.5 for details. Type 8 Name Address-and-Control-Field-Compression Description No data. Refer to RFC 1661 Section 6.6 for details.

## References

1. **[^](#cite_ref-1)** ["Point-to-Point (PPP) Protocol Field Assignments"](https://www.iana.org/assignments/ppp-numbers/ppp-numbers.xhtml#ppp-numbers-9). *www.iana.org*. Retrieved 2025-03-18.

## External links

- RFC [1570](https://www.rfc-editor.org/rfc/rfc1570): PPP LCP Extensions

- RFC [1661](https://www.rfc-editor.org/rfc/rfc1661): The Point-to-Point Protocol (PPP)

- RFC [1663](https://www.rfc-editor.org/rfc/rfc1663): PPP Reliable Transmission

- Kozierok, Charles. ["PPP Link Control Protocol (LCP)"](http://www.tcpipguide.com/free/t_PPPLinkControlProtocolLCP.htm). *The TCP/IP Guide*. Retrieved 4 December 2014.

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Adapted from the Wikipedia article [Link Control Protocol](https://en.wikipedia.org/wiki/Link_Control_Protocol) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Link_Control_Protocol?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.