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RFC1661 - часть 31

other functions.

Upon reception of an Echo-Request in the LCP Opened state, an

Echo-Reply MUST be transmitted.

Echo-Request and Echo-Reply packets MUST only be sent in the LCP

Opened state. Echo-Request and Echo-Reply packets received in any

state other than the LCP Opened state SHOULD be silently


A summary of the Echo-Request and Echo-Reply packet formats 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 |


| Magic-Number |


| Data ...



9 for Echo-Request;

10 for Echo-Reply.

Simpson [Page 36]

RFC 1661 Point-to-Point Protocol July 1994


On transmission, the Identifier field MUST be changed whenever the

content of the Data field changes, and whenever a valid reply has

been received for a previous request. For retransmissions, the

Identifier MAY remain unchanged.

On reception, the Identifier field of the Echo-Request is copied

into the Identifier field of the Echo-Reply packet.


The Magic-Number field is four octets, and aids in detecting links

which are in the looped-back condition. Until the Magic-Number

Configuration Option has been successfully negotiated, the Magic-

Number MUST be transmitted as zero. See the Magic-Number

Configuration Option for further explanation.


The Data field is zero or more octets, and contains uninterpreted

data for use by the sender. The data may consist of any binary

value. The end of the field is indicated by the Length.

5.9. Discard-Request


LCP includes a Discard-Request Code in order to provide a Data

Link Layer sink mechanism for use in exercising the local to

remote direction of the link. This is useful as an aid in

debugging, performance testing, and for numerous other functions.

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