Password Authentication Protocol (PAP) Frames

PAP frames are exchanged during the peer authentication phase, when peer authentication based on the Password Authentication Protocol (PAP) is requested as one of the configuration options during the link establishment phase. They have the general form shown in Figure A-4.

Figure A-4 PAP Frame Format

Address Field

The address field is one octet in length, and is part of the HDLC-like framing for PPP. It is always set to 0xff.

Control Field

The control field is one octet in length, and is part of the HDLC-like framing for PPP. It is always set to 0x03.

Protocol Id

The protocol id identifies the type of information contained in the information field of the frame, and is always 0xc023 for PAP frames.

Code Field

The code field is one octet in length and identifies the type of PAP frame, based on the following codes:

0x01 Authenticate-request

0x02 Authenticate-ack

0x03 Authenticate-nak

Id Field

The id field is one octet in length, and carries an identifier that is used to match associated requests and replies.

Length Field

The length field is two octets in length, and indicates the total length of the PAP frame including the code, id, length, and data fields. The length must not exceed the maximum receive unit (MRU).

Data Field

The data field is zero or more octets in length, as indicated by the length field. It contains information associated with the authentication negotiation, in a format determined by the code field.

PAP Authenticate-request Frames

PAP Authenticate-request frames (code 0x01) are transmitted to start the authentication phase, and contain the PAP id and PAP password sent for authentication. Up to ten PAP Authenticate-request frames are transmitted without receiving a response before the authentication phase fails. PAP Authenticate-request frames have the format shown in Figure A-5:

Figure A-5 PAP Request Frame Format

The PAP id is zero or more octets in length, as indicated by the PAP id length field, and contains the character string specified by the send_pap_id parameter in the file ppp.conf.

The PAP password is zero or more octets in length, as indicated by the PAP password length field, and contains the character string specified by the send_pap_passwd parameter in the file ppp.conf.

PAP Authenticate-ack and Authenticate-nak Frames

A PAP Authenticate-ack frame (code 0x02) is transmitted by the authenticator when it receives a recognizable PAP Authenticate-request frame that contains an acceptable PAP id and PAP password.

A PAP Authenticate-nak frame (code 0x03) is transmitted by the authenticator when it receives a PAP Authenticate-request frame that is not recognizable, or that contains an unacceptable PAP id and PAP password pair. The link is always terminated.