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VPN Tunneling Protocols

Updated: February 13, 2009

Applies To: Windows 7, Windows Server 2008 R2

Tunneling enables the encapsulation of a packet from one type of protocol within the datagram of a different protocol. For example, VPN uses Point-to-Point Tunneling Protocol (PPTP) to encapsulate IP packets over a public network, such as the Internet. You can configure a VPN solution based on PPTP, Layer Two Tunneling Protocol (L2TP), Secure Socket Tunneling Protocol (SSTP), or Internet Protocol security (IPsec) using Internet Key Exchange version 2 (IKEv2).

PPTP, L2TP, and SSTP depend heavily on the features originally specified for Point-to-Point Protocol (PPP). PPP was designed to send data across dial-up or dedicated point-to-point connections. For IP, PPP encapsulates IP packets within PPP frames and then transmits the encapsulated PPP packets across a point-to-point link. PPP was originally defined as the protocol to use between a dial-up client and a network access server. Unlike the other tunnel types, IKEv2 does not run on top of PPP.

PPTP allows multiprotocol traffic to be encrypted and then encapsulated in an IP header to be sent across an IP network or a public IP network, such as the Internet. PPTP can be used for remote access and site-to-site VPN connections. When using the Internet as the public network for VPN, the PPTP server is a PPTP-enabled VPN server with one interface on the Internet and a second interface on the intranet.

PPTP encapsulates PPP frames in IP datagrams for transmission over the network. PPTP uses a TCP connection for tunnel management and a modified version of Generic Routing Encapsulation (GRE) to encapsulate PPP frames for tunneled data. The payloads of the encapsulated PPP frames can be encrypted, compressed, or both. The following figure shows the structure of a PPTP packet containing an IP datagram.

Structure of a PPTP packet containing an IP datagram

Structure of PPTP Packet Containing IP Datagram

The PPP frame is encrypted with Microsoft Point-to-Point Encryption (MPPE) by using encryption keys generated from the Microsoft Challenge Handshake Authentication Protocol version 2 (MS-CHAP v2) or Extensible Authentication Protocol-Transport Layer Security (EAP-TLS) authentication process. Virtual private networking clients must use the MS-CHAP v2 or EAP-TLS authentication protocols in order for the payloads of PPP frames to be encrypted. PPTP is taking advantage of the underlying PPP encryption and encapsulating a previously encrypted PPP frame. Only 128-bit RC4 encryption algorithm is supported. 40 and 56-bit RC4 support was removed starting with Windows Vista and Windows Server 2008, but can be added (not recommended) by changing a registry key.

L2TP/IPsec allows multiprotocol traffic to be encrypted and then sent over any medium that supports point-to-point datagram delivery, such as IP or Asynchronous Transfer Mode (ATM). L2TP is a combination of PPTP and Layer 2 Forwarding (L2F), a technology developed by Cisco Systems, Inc. L2TP represents the best features of PPTP and L2F.

Unlike PPTP, the Microsoft implementation of L2TP does not use MPPE to encrypt PPP datagrams. L2TP uses IPsec in Transport Mode for encryption services. The combination of L2TP and IPsec is known as L2TP/IPsec.

Both L2TP and IPsec must be supported by both the VPN client and the VPN server. Client support for L2TP is built in to the Windows remote access clients, and VPN server support for L2TP is built in to the Windows Server operating system.

L2TP/IPsec is installed with the TCP/IP protocol.

Encapsulation for L2TP/IPsec packets consists of two layers:

A PPP frame (an IP datagram) is wrapped with an L2TP header and a UDP header.

The following figure shows the structure of an L2TP packet containing an IP datagram.

Structure of an L2TP packet containing an IP datagram

Structure of L2TP Packet Containing an IP Datagram

The resulting L2TP message is then wrapped with an IPsec Encapsulating Security Payload (ESP) header and trailer, an IPsec Authentication trailer that provides message integrity and authentication, and a final IP header. In the IP header is the source and destination IP address that corresponds to the VPN client and VPN server.

The following illustration shows L2TP and IPsec encapsulation for a PPP datagram.

Encryption of L2TP traffic with IPsec ESP

Encryption of L2TP Traffic with IPsec ESP

The L2TP message is encrypted with one of the following protocols by using encryption keys generated from the IKE negotiation process: Advanced Encryption Standard (AES) 256, AES 192, AES 128, and 3DES encryption algorithms.

securitySecurity Note
Data Encryption Standard (DES) encryption algorithm with Message Digest 5 (MD5) integrity check support has been removed, but can be added (not recommended) by changing a registry key.

Secure Socket Tunneling Protocol (SSTP) is a tunneling protocol that uses the HTTPS protocol over TCP port 443 to pass traffic through firewalls and Web proxies that might block PPTP and L2TP/IPsec traffic. SSTP provides a mechanism to encapsulate PPP traffic over the Secure Sockets Layer (SSL) channel of the HTTPS protocol. The use of PPP allows support for strong authentication methods, such as EAP-TLS. SSL provides transport-level security with enhanced key negotiation, encryption, and integrity checking.

When a client tries to establish a SSTP-based VPN connection, SSTP first establishes a bidirectional HTTPS layer with the SSTP server. Over this HTTPS layer, the protocol packets flow as the data payload.

SSTP encapsulates PPP frames in IP datagrams for transmission over the network. SSTP uses a TCP connection (over port 443) for tunnel management as well as PPP data frames.

The SSTP message is encrypted with the SSL channel of the HTTPS protocol.

IKEv2 is a tunneling protocol that uses the IPsec Tunnel Mode protocol over UDP port 500. An IKEv2 VPN provides resilience to the VPN client when the client moves from one wireless hotspot to another or when it switches from a wireless to a wired connection. The use of IKEv2 and IPsec allows support for strong authentication and encryption methods.

IKEv2 encapsulates datagrams by using IPsec ESP or AH headers for transmission over the network.

The message is encrypted with one of the following protocols by using encryption keys generated from the IKEv2 negotiation process: Advanced Encryption Standard (AES) 256, AES 192, AES 128, and 3DES encryption algorithms.

When choosing between PPTP, L2TP/IPsec, SSTP, and IKEv2 remote access VPN solutions, consider the following:

  • PPTP can be used with a variety of Microsoft clients, including Microsoft Windows® 2000 and later versions of Windows. Unlike L2TP/IPsec and IKEv2, PPTP does not require the use of a public key infrastructure (PKI). By using encryption, PPTP-based VPN connections provide data confidentiality (captured packets cannot be interpreted without the encryption key). PPTP-based VPN connections, however, do not provide data integrity (proof that the data was not modified in transit) or data origin authentication (proof that the data was sent by the authorized user).

  • L2TP can be used with client computers running Windows 2000 and later versions of Windows. L2TP supports either computer certificates or a preshared key as the authentication method for IPsec. Computer certificate authentication, the recommended authentication method, requires a PKI to issue computer certificates to the VPN server computer and all VPN client computers. By using IPsec, L2TP/IPsec VPN connections provide data confidentiality, data integrity, and data authentication.

    Unlike PPTP and SSTP, L2TP/IPsec enables machine authentication at the IPsec layer and user level authentication at the PPP layer.

  • SSTP can only be used with client computers running Windows Vista Service Pack 1 (SP1), Windows Server 2008, and later versions of Windows. By using SSL, SSTP VPN connections provide data confidentiality, data integrity, and data authentication.

  • IKEv2 is supported only on computers running Windows 7 and Windows Server 2008 R2. By using IPsec, IKEv2 VPN connections provide data confidentiality, data integrity, and data authentication. IKEv2 supports the latest IPsec encryption algorithms. Because of its support for mobility (MOBIKE), it is much more resilient to changing network connectivity, making it a good choice for mobile users who move between access points and even switch between wired and wireless connections.

Each of the tunneling protocols is documented by an RFC maintained by the Internet Engineering Task Force (IETF) at IETF RFC Database (http://go.microsoft.com/fwlink/?linkid=3952).

  • PPTP is documented in RFC 2637.

  • L2TP is documented in RFC 2661.

  • L2TP/IPsec is documented in RFC 3193.

  • IKEv2 is documented in RFC 4306.

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