Settings”
). The aXsGUARD Gatekeeper enforces 128 bit encryption by default,
as this is the most secure option.
Compression.
Compression reduces the amount of information necessary to
transmit data, hereby saving bandwidth and increasing the data transfer speed.
PPTP uses the Compression Control Protocol (CCP) used by the PPP protocol. PPP
negotiates MPPE (see above) with the aXsGUARD Gatekeeper PPTP server using
CCP.
Two hosts are involved in the deployment of PPTP:
• A PPTP Client with access to the Internet.
• A PPTP Server, such as the aXsGUARD Gatekeeper PPTP server.
A PPTP connection between the client and the server consists of two channels, a
control channel and a data channel. These are explained below.
PPTP Control Channel.
The PPTP control channel is the initial channel which is
negotiated between a PPTP client on the Internet and the aXsGUARD
Gatekeeper PPTP server. A TCP connection is therefore made to the PPTP server
on TCP port 1723, as shown in the illustration below. This control channel is
used to negotiate tunnel parameters, such as the encryption method and the
compression algorithm (see
Section 2.3.2, “Key Elements of PPTP Security”
).
The PPTP control channel also establishes, manages, and releases the PPTP data
channel.
PPTP Data Channel.
Once the PPTP control channel is up, a second channel is
negotiated to secure the data transfer within the tunnel. This is the PPTP data
channel. On this data channel, PPTP uses the GRE protocol (see
Section 2.3.2,
“Key Elements of PPTP Security”
) to encapsulate the PPP packets for secure
delivery to the aXsGUARD Gatekeeper PPTP server. In turn, the aXsGUARD
Gatekeeper PPTP server verifies and decapsulates these packets before delivery
to the destination host in the LAN.
2.4. Standard PPTP Deployment
Figure 2.3. PPTP Control and Data Channel
© VASCO Data Security 2011
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