LCP includes Terminate-Request and Terminate-Ack Codes in order to provide a mechanism for closing a connection.
An implementation wishing to close a connection SHOULD transmit a Terminate-Request. Terminate-Request packets SHOULD continue to be sent until Terminate-Ack is received, the lower layer indicates that it has gone down, or a sufficiently large number have been transmitted such that the peer is down with reasonable certainty.
Upon reception of a Terminate-Request, a Terminate-Ack MUST be transmitted.
Reception of an unelicited Terminate-Ack indicates that the peer is in the Closed or Stopped states, or is otherwise in need of re-negotiation.
A summary of the Terminate-Request and Terminate-Ack 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+
5 for Terminate-Request;
6 for Terminate-Ack.
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 Terminate-Request is copied into the Identifier field of the Terminate-Ack packet.
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.