Layer 2 Tunneling Protocol

Layer 2 Tunneling Protocol (L2TP) is a network protocol, the frames of the link layer protocols (layer 2) of the OSI model between two network tunnels over the Internet, to create a virtual private network (VPN).

To this end, packed in a L2TP data message and sent over the Internet to an L2TP Network Server ( LNS) the network of cooperation in a local network frames of an L2TP Access Concentrator (LAC ). This unpacks the message and sends the content to the local network.

L2TP is a tunneling solution that combines the advantages of PPTP (Point- to-Point Tunneling Protocol) and L2F ( Layer 2 Forwarding ). Using a tunnel ID in the L2TP header multiple tunnels are next to each other as possible as the use of ( Network Address Translation ) NAT.

The authentication method L2TP offers the method (CHAP Challenge Handshake Authentication Protocol) and PAP (Password Authentication Protocol). Encryption is not included directly in L2TP. This must be signed by protocols of lower layers. L2TP is therefore often used in combination with IPSec. (RFC 3193 - "Securing L2TP using IPsec " )

  • 3.2.1 Attribute Value Pair for Version 3

L2TP Version 2

Layer 2 Tunneling Protocol Version 2 has been developed for the tunneling of PPP frames. Frames of other protocols can not be transferred. Tunneling via UDP or other packet- based networks ( Frame Relay, or ATM).

L2TPv2 is a proposed standard defined by IETF ( Internet Engineering Task Force) and is defined in RFC 2661 as RFC.

L2TP Version 3

Layer 2 Tunneling Protocol version 3 is a development of L2TP, which is an alternative to the MPLS protocol for encapsulation of different protocols on level 2 of the OSI model. It works like L2TPv2 over UDP or other PSN (Packet Switched Networks), but can also directly use IP. In addition, other layer protocols can be tunneled as PPP.

The specification is defined in RFC 3931.

L2TPv3 can be used as a stripped down version of MPLS are considered. A non -built feature sets such as the Traffic Engineering dar. These features, however, could easily be added in future products.

Header formats

Fields in the L2TP header:

  • T = Message Type. This field defines whether it is a control message ( value 1 ) or data ( value 0).
  • L = Length. Control messages must have set this bit.
  • S = Sequence. If this bit is set, the Ns and Nr fields are visible. Control messages must have set this bit.
  • O = Offset. If this bit is set, the Offset Size field is visible. Control messages must have this bit set to zero.
  • P = Priority. Control messages must have this bit set to zero.
  • Version. Describes which L2TP protocol is used (Version 2 = 2). Packets with an unknown value must be discarded.
  • Length ( length) is optional ( exists when L is set ). The total length of the message in bytes.
  • Tunnel ID.
  • Session ID.
  • Nr = sequence number expected ( exists when S is set ).
  • Offset Size ( offset size, exists when O set).
  • Offset pads (offset Edition) optionally have a variable length.
  • Data (variable length ).

Fields in the AVP (Attribute Value Pair):

  • M = Mandatory
  • H = Hidden
  • AVP Length ( values ​​6-1023 ). Specifies how many bytes includes the AVP.
  • AVP Vendor ID. The IANA divides the " SMI Network Management Private Enterprise Codes" values ​​(RFC 1700).
  • AVP type is 16 bits.

L2TPv2 header

Attribute Value Pair for Version 2

L2TPv3 header

Tunnel ID and session ID from Version 2 ( 16 bits each ) can be replaced by a 32 bits long session ID and Control Connection ID.

Attribute Value Pair for Version 3

This is determined by the set value 58 that the extended AVP Vendor ID is used.

Attribute Value Pair Type

RFCs for L2TP

  • RFC 2661 Layer Two Tunneling Protocol " L2TP ". Defines L2TP version 2
  • RFC 2809 L2TP Compulsory Tunneling via implementation of RADIUS.
  • RFC 2888 Secure Remote Access with L2TP.
  • RFC 3070 Layer Two Tunneling Protocol (L2TP ) over Frame Relay.
  • RFC 3145 L2TP Disconnect Cause Information.
  • RFC 3193 Securing L2TP using IPsec.
  • RFC 3301 Layer Two Tunnelling Protocol ( L2TP): ATM access network extensions.
  • RFC 3308 Layer Two Tunneling Protocol (L2TP ) Differentiated Services Extension.
  • RFC 3355 Layer Two Tunnelling Protocol ( L2TP) Over ATM Adaptation Layer 5 ( AAL5 ).
  • RFC 3371 Layer Two Tunneling Protocol " L2TP " Management Information Base.
  • RFC 3437 Layer Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation.
  • RFC 3438 Layer Two Tunneling Protocol (L2TP ) Internet Assigned Numbers Authority (IANA) Considerations Update.
  • RFC 3573 Signaling of Modem -On -Hold status in Layer 2 Tunneling Protocol ( L2TP).
  • RFC 3817 Layer 2 Tunneling Protocol ( L2TP) Active Discovery Relay for PPP over Ethernet ( PPPoE).
  • RFC 3931 Layer Two Tunneling Protocol - Version 3 ( L2TPv3 ). Defines L2TP version 3
  • RFC 4045 Extensions to Support Efficient Carrying of Multicast Traffic in Layer - 2 Tunneling Protocol (L2TP ).
Frame (networking) Encapsulation (networking) Point-to-Point Tunneling Protocol Point-to-Point Protocol Asynchronous Transfer Mode Teletraffic engineering Q.931
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