Reception of a LCP packet with an unknown Code indicates that the peer is operating with a different version. This MUST be reported back to the sender of the unknown Code by transmitting a Code- Reject.
Upon reception of the Code-Reject of a code which is fundamental to this version of the protocol, the implementation SHOULD report the problem and drop the connection, since it is unlikely that the situation can be rectified automatically.
A summary of the Code-Reject 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rejected-Packet ...
+-+-+-+-+-+-+-+-+
7 for Code-Reject.
The Identifier field MUST be changed for each Code-Reject sent.
The Rejected-Packet field contains a copy of the LCP packet which is being rejected. It begins with the Information field, and does not include any Data Link Layer headers nor an FCS. The Rejected-Packet MUST be truncated to comply with the peer's established MRU.