LCP Packets
LCP packets are used to establish and manage a PPP link and can be loosely
divided into three categories:
v

Link establishment packets

that exchange configuration information and establish
the link.
v

Link termination packets

that shut down the link or signal that a link is not
accepting connections at a particular time. They also can be used to signal that a
particular protocol is unrecognized (for example, during NCP negotiations).
v

Link maintenance packets

that monitor and debug a link.
Exactly one LCP packet is encapsulated in the information field of PPP Data Link
Layer frames. In the case of LCP packets, the protocol field reads “Link Control
Protocol” (C021 hexadecimal). Figure 28 illustrates the structure of the LCP packet
and is followed by a detailed description of each field.
Code The code field is one octet in length and identifies the type of LCP packet.
The codes in Table61 distinguish the packet types. They are described in
more detail in later sections.
Table61. LCP Packet Codes
Code Packet Type
1 Configure-Request (Link Establishment)
2 Configure-Ack (Link Establishment)
3 Configure-Nak (Link Establishment)
4 Configure-Reject (Link Establishment)
5 Terminate-Request(Link Termination)
6 Terminate-Ack(Link Termination)
7 Code-Reject (Link Establishment)
8 Protocol-Reject (Link Establishment)
9 Echo-Request (Link Maintenance)
10 Echo-Reply(Link Maintenance)
11 Discard-Request (Link Maintenance)
Identifier
The identifier field is one octet in length and is used to match packet
requests to replies.
Length The length field is two octets in length and indicates the total length (that is,
including all fields) of the LCP packet.
Data (Option)
The data field is zero or more octets as indicated by the length field. The
format of this field is determined by the code.
NCP packets are structured identically to LCP packets and are distinguished by
having different PPP “Protocol” values. Each LCP packet type (distinguished by the
code field) has the same meaning for each NCP,though an individual NCP may not
implement all possible LCP packet types. NCPs normally implement all of the link
┌─────┬────────────┬────────┬─────────────┐
│Code │ Identifier │ Length │ Data(option)│
└─────┴────────────┴────────┴─────────────┘
Figure 28. LCPFrame Structure (in PPP Information Field)
Using PPP
Chapter32. Using Point-to-Point Protocol Interfaces 453