Reception of a PPP packet with an unknown Protocol field indicates that the peer is attempting to use a protocol which is unsupported. This usually occurs when the peer attempts to configure a new protocol. If the LCP automaton is in the Opened state, then this MUST be reported back to the peer by transmitting a Protocol-Reject.
Upon reception of a Protocol-Reject, the implementation MUST stop sending packets of the indicated protocol at the earliest opportunity.
Protocol-Reject packets can only be sent in the LCP Opened state. Protocol-Reject packets received in any state other than the LCP Opened state SHOULD be silently discarded.
A summary of the Protocol-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-Protocol | Rejected-Information ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8 for Protocol-Reject.
The Identifier field MUST be changed for each Protocol-Reject sent.
The Rejected-Protocol field is two octets, and contains the PPP Protocol field of the packet which is being rejected.
The Rejected-Information field contains a copy of the 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-Information MUST be truncated to comply with the peer's established MRU.