LCP includes Echo-Request and Echo-Reply Codes in order to provide a Data Link Layer loopback mechanism for use in exercising both directions of the link. This is useful as an aid in debugging, link quality determination, performance testing, and for numerous 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 discarded.
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.
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.