If the Peer-ID/Password pair received in an Authenticate-Request is both recognizable and acceptable, then the authenticator MUST transmit a PAP packet with the Code field set to 2 (Authenticate- Ack).
If the Peer-ID/Password pair received in a Authenticate-Request is not recognizable or acceptable, then the authenticator MUST transmit a PAP packet with the Code field set to 3 (Authenticate- Nak), and SHOULD take action to terminate the link.
A summary of the Authenticate-Ack and Authenticate-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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Msg-Length | Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
2 for Authenticate-Ack;
3 for Authenticate-Nak.
The Identifier field is one octet and aids in matching requests and replies. The Identifier field MUST be copied from the Identifier field of the Authenticate-Request which caused this reply.
The Msg-Length field is one octet and indicates the length of the Message field.
The Message field is zero or more octets, and its contents are implementation dependent. It is intended to be human readable, and MUST NOT affect operation of the protocol. It is recommended that the message contain displayable ASCII characters 32 through 126 decimal. Mechanisms for extension to other character sets are the topic of future research.