3.1. Configuration Option Format

A summary of the Authentication-Protocol Configuration Option format to negotiate the Challenge-Handshake Authentication Protocol 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Authentication-Protocol   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Algorithm   |
   +-+-+-+-+-+-+-+-+

Type

3

Length

5

Authentication-Protocol

c223 (hex) for Challenge-Handshake Authentication Protocol.

Algorithm

The Algorithm field is one octet and indicates the one-way hash method to be used. The most up-to-date values of the CHAP Algorithm field are specified in the most recent "Assigned Numbers" RFC [2]. Current values are assigned as follows:

         0-4     unused (reserved)
         5       MD5 [3]