IBM SC30-3681-08 manual LCP Packets, Identier, Length, Data Option, Code Packet Type

Using PPP

LCP Packets

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

vLink establishment packets that exchange con®guration information and establish the link.

vLink 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).

vLink maintenance packets that monitor and debug a link.

Exactly one LCP packet is encapsulated in the information ®eld of PPP Data Link Layer frames. In the case of LCP packets, the protocol ®eld reads ªLink Control Protocolº (C021 hexadecimal). Figure 28 illustrates the structure of the LCP packet and is followed by a detailed description of each ®eld.

~‡‡‡‡‡ƒ‡‡‡‡‡‡‡‡‡‡‡‡ƒ‡‡‡‡‡‡‡‡ƒ‡‡‡‡‡‡‡‡‡‡‡‡‡€ ˆCode ˆ Identifier ˆ Length ˆ Data(option)ˆ •‡‡‡‡‡„‡‡‡‡‡‡‡‡‡‡‡‡„‡‡‡‡‡‡‡‡„‡‡‡‡‡‡‡‡‡‡‡‡‡•

Figure 28. LCP Frame Structure (in PPP Information Field)

Code The code ®eld is one octet in length and identi®es the type of LCP packet. The codes in Table 61 distinguish the packet types. They are described in more detail in later sections.

Table 61. LCP Packet Codes

Code	Packet Type
1	Con®gure-Request (Link Establishment)
2	Con®gure-Ack (Link Establishment)
3	Con®gure-Nak (Link Establishment)
4	Con®gure-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)

Identi®er

The identi®er ®eld is one octet in length and is used to match packet requests to replies.

Length

The length ®eld is two octets in length and indicates the total length (that is, including all ®elds) of the LCP packet.

Data (Option)

The data ®eld is zero or more octets as indicated by the length ®eld. The format of this ®eld is determined by the code.

NCP packets are structured identically to LCP packets and are distinguished by having different PPP ªProtocolº values. Each LCP packet type (distinguished by the code ®eld) 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

Chapter 32. Using Point-to-Point Protocol Interfaces 453