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PPTP

(Point-to-Point Tunneling Protocol) A protocol from Microsoft that is used to create a virtual private network (VPN) over the Internet. Remote users can access their corporate networks via any ISP that supports PPTP on its servers.

PPTP encapsulates any type of network protocol (IP, IPX, etc.) and transports it over IP. Thus if IP is the original protocol, IP packets ride as encrypted messages inside PPTP packets running over IP. PPTP is based on the point-to-point protocol (PPP) protocol and the generic routing encapsulation (GRE) protocol. Encryption is performed by Microsoft's Point-to-Point Encryption (MPPE), which is based on RC4. See L2TP.



L2TP

(Layer 2 Tunneling Protocol) A protocol from the IETF that lets remote users access the corporate network. L2TP allows a PPP session to travel over multiple links and networks. PPP is used to encapsulate IP packets from the user's PC or mobile device to the ISP, and L2TP extends that session across the Internet. L2TP was derived from Microsoft's Point-to-Point Tunneling Protocol (PPTP) and Cisco's Layer 2 Forwarding (L2F) technology. See PPTP and L2F.

From Access Concentrator to Network Server
The "L2TP Access Concentrator" (LAC) encapsulates PPP frames with L2TP headers and sends them over the Internet as UDP packets (or over an ATM, frame relay or X.25 network). At the other end, the "L2TP Network Server" (LNS) terminates the PPP session and hands the IP packets to the LAN. L2TP software can also be run in the user's PC.

Carriers also use L2TP to offer remote points of presence (POPs) to smaller ISPs. Users in remote locations dial into the carrier's local modem pool, and the carrier's LAC forwards L2TP traffic to the ISP's LNS.

L2TP and IPsec
L2TP does not include encryption (as does PPTP), but is often used with IPsec in order to provide virtual private network (VPN) connections from remote users to the corporate LAN. See PPP, virtual private network and IPsec.




PPP and L2TP Traffic
PPP encapsulates IP packets from the user's PC to the ISP. L2TP tunnels those packets over multiple links.






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