A PAP packet has four fields carried one at a time in the PPP Information field: Code, Identifier,
Length, and Data.
The Code field is 8 bits long and can have one of three values:
Authenticate-Request
Authenticate-Ack
Authenticate-Nak
The Identifier field is also 8 bits long and is used for matching authentication requests and
replies. It changes every time an Authenticate-Request is sent.
Length is a 16-bit field indicating the packet’s length.
The Data field varies in length and format, depending on the packet type: Request, Ack, or Nak.