The question might sound dumb, but you never learn if you do not ask.

When using PPP as a datalink layer, it offers you authentication by using username and password.

Where is the username and password stored in the frame? Is it part of the payload or is it part of the frame header? If it is stored in the payload, then we cant we use Ethernet and authenticate over it?

I am confused and would appreciate the help.

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    Commented Feb 19, 2018 at 5:29

1 Answer 1


The currently used authentication method for PPP is CHAP, and it doesn't send the actual password across the wire. Instead, it sends a challenge, and the respondent will hash the challenge with the password and return the result.

The challenge/response happens during the setup of the link, and it will also happen randomly during the life of the connection.

This happens at the datalink layer, and it is transparent to the layer-3 and above protocols.

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