I'm not sure I parse your query ... my (?) understanding is a bit different. I see PPTP and L2TP as both layer 2 protocols ... both encapsulate the frame in PPP and deliver it ... PPTP only over IP ... L2TP over any datagram service: IP, X.25, Frame Relay, or ATM.
I quote from:
microsoft.com
Tunneling Protocols
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For a tunnel to be established, both the tunnel client and the tunnel server must be using the same tunneling protocol.
Tunneling technology can be based on either a Layer 2 or a Layer 3 tunneling protocol. These layers correspond to the Open Systems Interconnection (OSI) Reference Model. Layer 2 protocols correspond to the data-link layer and use frames as their unit of exchange. PPTP and L2TP and Layer 2 Forwarding (L2F) are Layer 2 tunneling protocols; both encapsulate the payload in a PPP frame to be sent across an internetwork. Layer 3 protocols correspond to the Network layer, and use packets. IP-over-IP and IP Security (IPSec) Tunnel Mode are examples of Layer 3 tunneling protocols. These protocols encapsulate IP packets in an additional IP header before sending them across an IP internetwork.
How Tunneling Works
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For Layer 2 tunneling technologies, such as PPTP and L2TP, a tunnel is similar to a session; both of the tunnel endpoints must agree to the tunnel and must negotiate configuration variables, such as address assignment or encryption or compression parameters. In most cases, data transferred across the tunnel is sent using a datagram-based protocol. A tunnel maintenance protocol is used as the mechanism to manage the tunnel.
Layer 3 tunneling technologies generally assume that all of the configuration issues have been handled out of band, often by manual processes. For these protocols, there may be no tunnel maintenance phase. For Layer 2 protocols (PPTP and L2TP), however, a tunnel must be created, maintained, and then terminated.
Once the tunnel is established, tunneled data can be sent. The tunnel client or server uses a tunnel data transfer protocol to prepare the data for transfer. For example, when the tunnel client sends a payload to the tunnel server, the tunnel client first appends a tunnel data transfer protocol header to the payload. The client then sends the resulting encapsulated payload across the internetwork, which routes it to the tunnel server. The tunnel server accepts the packets, removes the tunnel data transfer protocol header, and forwards the payload to the target network. Information sent between the tunnel server and the tunnel client behaves similarly.
New tunneling technologies have been introduced in recent years. These newer technologies?which are the primary focus of this paper?include:
ú Point-to-Point Tunneling Protocol (PPTP). PPTP allows IP, IPX, or NetBEUI traffic to be encrypted, and then encapsulated in an IP header to be sent across a corporate IP internetwork or a public IP internetwork such as the Internet.
ú Layer 2 Tunneling Protocol (L2TP). L2TP allows IP, IPX, or NetBEUI traffic to be encrypted, and then sent over any medium that supports point-to-point datagram delivery, such as IP, X.25, Frame Relay, or ATM.
ú IP Security (IPSec) Tunnel Mode. IPSec Tunnel Mode allows IP payloads to be encrypted, and then encapsulated in an IP header to be sent across a corporate IP internetwork or a public IP internetwork such as the Internet.
PPTP Compared to L2TP
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Both PPTP and L2TP use PPP to provide an initial envelope for the data, and then append additional headers for transport through the internetwork. The two protocols are very similar. However, there are differences between PPTP and L2TP:
ú PPTP requires that the internetwork be an IP internetwork. L2TP requires only that the tunnel media provide packet-oriented point-to-point connectivity. L2TP can be used over IP (using UDP), Frame Relay permanent virtual circuits (PVCs), X.25 virtual circuits (VCs), or ATM VCs.
ú PPTP can support only a single tunnel between end points. L2TP allows for the use of multiple tunnels between end points. With L2TP, you can create different tunnels for different qualities of service.
ú L2TP provides for header compression. When header compression is enabled, L2TP operates with 4 bytes of overhead, as compared to 6 bytes for PPTP.
ú L2TP provides for tunnel authentication, while PPTP does not. However, when either protocol is used over IPSec, tunnel authentication is provided by IPSec so that Layer 2 tunnel authentication is not necessary. |
