Link Configuration packets used to establish and configure a
link (Configure-Request, Configure-Ack, Configure-Nak and
Link Termination packets used to terminate a link (Terminate-
Request and Terminate-Ack).
Link Maintenance packets used to manage and debug a link
(Code-Reject, Protocol-Reject, Echo-Request, Echo-Reply, and
In the interest of simplicity, there is no version field in the LCP
packet. A correctly functioning LCP implementation will always
respond to unknown Protocols and Codes with an easily recognizable
LCP packet, thus providing a deterministic fallback mechanism for
implementations of other versions.
Regardless of which Configuration Options are enabled, all LCP Link
Configuration, Link Termination, and Code-Reject packets (codes 1
through 7) are always sent as if no Configuration Options were
negotiated. In particular, each Configuration Option specifies a
default value. This ensures that such LCP packets are always
recognizable, even when one end of the link mistakenly believes the
link to be open.
Exactly one LCP packet is encapsulated in the PPP Information field,
where the PPP Protocol field indicates type hex c021 (Link Control
A summary of the Link Control Protocol packet format is shown below.
The fields are transmitted from left to right.
The Identifier field is one octet, and aids in matching requests
and replies. When a packet is received with an invalid Identifier
field, the packet is silently discarded without affecting the
The Length field is two octets, and indicates the length of the
LCP packet, including the Code, Identifier, Length and Data
fields. The Length MUST NOT exceed the MRU of the link.
Octets outside the range of the Length field are treated as
padding and are ignored on reception. When a packet is received
with an invalid Length field, the packet is silently discarded
without affecting the automaton.
The Data field is zero or more octets, as indicated by the Length
field. The format of the Data field is determined by the Code