RFC Layer Two Tunneling Protocol “L2TP” (Q). request for comments publication. RFC; Layer Two Tunneling Protocol. Layer Two Tunneling Protocol “L2TP”, August Canonical URL: https://www. ; File formats: Plain Text PDF; Status: PROPOSED. In computer networking, Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to Published in as proposed standard RFC , L2TP has its origins primarily in two older tunneling protocols for point-to-point communication.

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One of the aforementioned difficulties is that it is not compatible with NAT. Views Read Edit View history. Tunnel ID in each message is that of the intended recipient, not the sender. L2TP tunnels are named by identifiers that have local significance only. Control messages MUST have this bit set.

Layer 2 Tunneling Protocol

Specifies the number of bytes past the L2TP header at which the payload data is expected to start. The value 1 is reserved to permit detection of L2F packets should they arrive intermixed with L2TP packets.

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If the M bit is set on an unrecognized AVP within a message associated with the overall tunnel, the entire tunnel and all sessions within MUST be terminated.

Because of the lack of confidentiality inherent in the L2TP protocol, it is often implemented along with IPsec. Indicates the sequence number expected in the next control message to be received.

The 26611 message must then continue to be processed as if the AVP had not been present. To be useful for networking, higher-level protocols are then run through the L2TP tunnel.


Indicates the L2TP protocol version. That is, the same session will be given different Session IDs by each end of the session. Also, it is not necessary to open UDP port on firewalls between the endpoints, since the inner packets are not acted upon until after IPsec data has been decrypted and stripped, which only takes place at the endpoints.

If the offset field is present, the L2TP header ends after the last byte of the offset padding. This capability can be used to avoid the passing of sensitive data, such as user passwords, as cleartext in an AVP.

Multiple AVPs make up 6261 Messages which are used in the establishment, maintenance, and teardown of tunnels. This feature is only for use rrc data messages. If the length is set to 6, then the AVP Value field is absent. L2TP sessions are named by identifiers that have local significance only. The number of bytes contained in this AVP.

Supported L2TP Standards – TechLibrary – Juniper Networks

From the end user, packets travel over a wholesale network service provider’s network to a server called a Broadband Remote Access Server BRASa protocol converter and router combined.

L2fp these configuration utilities are not without their difficulties, and unfortunately, there is very little documentation about both “netsh advfirewall” and the IPsec client in WFwAS. L2TP allows the creation of a virtual private dialup ltp VPDN [5] to connect a remote client to its corporate network by using a shared infrastructure, which could be the Internet or a service provider’s network.

Then using this tunnel and session id, data packets are exchanged with the compressed PPP frames as payload. Indicates the identifier for a session within a tunnel. This can be represented by four different tunneling models, namely:. Rather, it relies on an encryption protocol that it passes within the tunnel to provide privacy.


If the M bit 261 set on an unrecognized AVP within a message associated with a particular session, the session associated with this message MUST be terminated.

Supported L2TP Standards

Nr, sequence number expected. The packets exchanged within an L2TP tunnel are categorized as either control packets or data packets. Another problem is that servers must be specified only by IP address in the new Vista configuration utilities; the hostname of the server cannot be used, so if the IP address of the IPsec server changes, all clients will have to be informed of this new IP address which also rules out servers that addressed by utilities such as DynDNS.

IPsec is often used to secure L2TP packets by providing confidentiality, authentication and integrity. This page was last edited on 3 Octoberat If set, the Offset Size field is present. It does not provide any encryption or confidentiality by itself. L2TP provides reliability features for the control packets, but no reliability for data packets.

Windows Vista provides two new configuration utilities that attempt to make using L2TP without IPsec easier, both described in sections that follow below:.