MPPE stands for Microsoft Point to Point Encryption. MPPE is used to encrypt data traveling over a PPTP, or Point to Point Tunneling Protocol, connection.
When setting up a PPTP connection using DD-WRT firmware on your router there is a blank field to configure MPPE Encryption options in the VPN/PPTP setup. By default the text “mppe required” is typed into this field. You can add a couple other options which include no40, no56, and stateful or stateless. Descriptions of these options are listed below.
MPPE: Microsoft Point to Point Encryption
mppe required: requires MPPE encryption over the PPTP connection
no40: will not allow 40-bit encryption
no56: will not allow 56-bit encryption
stateful:makes the connection stateful
stateless: makes the connection stateless
So adding the following into the MPPE Encryption field: “mppe required,no40,no56,stateless” will provide you with a 128-bit encrypted stateless PPTP connection.
For more information check out RFC 3079.