Connected: An Internet Encyclopedia Link Control Protocol (LCP) Options

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The PPP Link Control Protocol (LCP) offers a number of options that may be negotiated. These options include (among others) address and control field compression, protocol field compression, asynchronous character map, Maximum Receive Unit (MRU), Link Quality Monitoring (LQM), magic number (for loopback detection), Password Authentication Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP), and the 32-bit Frame Check Sequence (FCS).

A router MAY use address/control field compression on either synchronous or asynchronous links. A router MAY use protocol field compression on either synchronous or asynchronous links. A router that indicates that it can accept these compressions MUST be able to accept uncompressed PPP header information also.


These options control the appearance of the PPP header. Normally the PPP header consists of the address, the control field, and the protocol field. The address, on a point to point line, is 0xFF, indicating "broadcast". The control field is 0x03, indicating "Unnumbered Information." The Protocol Identifier is a two byte value indicating the contents of the data area of the frame. If a system negotiates address and control field compression it indicates to its peer that it will accept PPP frames that have or do not have these fields at the front of the header. It does not indicate that it will be sending frames with these fields removed.

Protocol field compression, when negotiated, indicates that the system is willing to receive protocol fields compressed to one byte when this is legal. There is no requirement that the sender do so.

Use of address/control field compression is inconsistent with the use of numbered mode (reliable) PPP.


Some hardware does not deal well with variable length header information. In those cases it makes most sense for the remote peer to send the full PPP header. Implementations may ensure this by not sending the address/control field and protocol field compression options to the remote peer. Even if the remote peer has indicated an ability to receive compressed headers there is no requirement for the local router to send compressed headers.

A router MUST negotiate the Asynchronous Control Character Map (ACCM) for asynchronous PPP links, but SHOULD NOT negotiate the ACCM for synchronous links. If a router receives an attempt to negotiate the ACCM over a synchronous link, it MUST ACKnowledge the option and then ignore it.


There are implementations that offer both synchronous and asynchronous modes of operation and may use the same code to implement the option negotiation. In this situation it is possible that one end or the other may send the ACCM option on a synchronous link.

A router SHOULD properly negotiate the maximum receive unit (MRU). Even if a system negotiates an MRU smaller than 1,500 bytes, it MUST be able to receive a 1,500 byte frame.

A router SHOULD negotiate and enable the link quality monitoring (LQM) option.


This memo does not specify a policy for deciding whether the link's quality is adequate. However, it is important (see Section [3.3.6]) that a router disable failed links.

A router SHOULD implement and negotiate the magic number option for loopback detection.

A router MAY support the authentication options (PAP - Password Authentication Protocol, and/or CHAP - Challenge Handshake Authentication Protocol).

A router MUST support 16-bit CRC frame check sequence (FCS) and MAY support the 32-bit CRC.

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Connected: An Internet Encyclopedia Link Control Protocol (LCP) Options