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RFC1661 - часть 23

desired options are no longer appended. Max-Failure MUST be

configurable, but SHOULD default to five (5) transmissions.

Simpson [Page 25]

RFC 1661 Point-to-Point Protocol July 1994

5. LCP Packet Formats

There are three classes of LCP packets:

1. Link Configuration packets used to establish and configure a

link (Configure-Request, Configure-Ack, Configure-Nak and


2. Link Termination packets used to terminate a link (Terminate-

Request and Terminate-Ack).

3. 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.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1


| Code | Identifier | Length |


| Data ...



The Code field is one octet, and identifies the kind of LCP

Simpson [Page 26]

RFC 1661 Point-to-Point Protocol July 1994

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