If every instance of the received Configuration Options is recognizable, but some values are not acceptable, then the implementation MUST transmit a Configure-Nak. The Options field is filled with only the unacceptable Configuration Options from the Configure-Request. All acceptable Configuration Options are filtered out of the Configure-Nak, but otherwise the Configuration Options from the Configure-Request MUST NOT be reordered.
Options which have no value fields (boolean options) MUST use the Configure-Reject reply instead.
Each Configuration Option which is allowed only a single instance MUST be modified to a value acceptable to the Configure-Nak sender. The default value MAY be used, when this differs from the requested value.
When a particular type of Configuration Option can be listed more than once with different values, the Configure-Nak MUST include a list of all values for that option which are acceptable to the Configure-Nak sender. This includes acceptable values that were present in the Configure-Request.
Finally, an implementation may be configured to request the negotiation of a specific Configuration Option. If that option is not listed, then that option MAY be appended to the list of Nak'd Configuration Options, in order to prompt the peer to include that option in its next Configure-Request packet. Any value fields for the option MUST indicate values acceptable to the Configure-Nak sender.
On reception of a Configure-Nak, the Identifier field MUST match that of the last transmitted Configure-Request. Invalid packets are silently discarded.
Reception of a valid Configure-Nak indicates that when a new Configure-Request is sent, the Configuration Options MAY be modified as specified in the Configure-Nak. When multiple instances of a Configuration Option are present, the peer SHOULD select a single value to include in its next Configure-Request packet.
Some Configuration Options have a variable length. Since the Nak'd Option has been modified by the peer, the implementation MUST be able to handle an Option length which is different from the original Configure-Request.
A summary of the Configure-Nak 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+
3 for Configure-Nak.
The Identifier field is a copy of the Identifier field of the Configure-Request which caused this Configure-Nak.
The Options field is variable in length, and contains the list of zero or more Configuration Options that the sender is Nak'ing. All Configuration Options are always Nak'd simultaneously.