Description

Glossary

RFCs

Publications

Obsolete RFCs

Description:

L2TP header, version 2:

T, Message Type. 1 bit.
Specifies if this is a data or control message.

L, Length present. 1 bit.
Control messages MUST have this bit set.

S, Sequence present. 1 bit.
If set, the Ns and Nr fields are present. Control messages MUST have this bit set.

O, Offset present. 1 bit.
If set, the Offset Size field is present. Control messages MUST have this bit cleared to zero.

P, Priority. 1 bit.
If set, this data message should receive preferential treatment in its local queuing and transmission. LCP echo requests used as a keepalive for the link, for instance, should generally be sent with this bit set. Without it, a temporary interval of local congestion could result in interference with keepalive messages and unnecessary loss of the link. This feature is only for use with data messages. Control messages MUST have this bit cleared to zero.

Version. 4 bits.
Indicates the L2TP protocol version. The value 1 is reserved to permit detection of L2F packets should they arrive intermixed with L2TP packets. Packets received with an unknown value MUST be discarded.

Length. 16 bits. Optional.
Total length of the message in bytes. This field exists only if the L bit is set.

Tunnel ID. 16 bits.
Indicates the identifier for the control connection. L2TP tunnels are named by identifiers that have local significance only. That is, the same tunnel will be given different Tunnel IDs by each end of the tunnel. Tunnel ID in each message is that of the intended recipient, not the sender. Tunnel IDs are selected and exchanged as Assigned Tunnel ID AVPs during the creation of a tunnel.

Session ID. 16 bits.
Indicates the identifier for a session within a tunnel. L2TP sessions are named by identifiers that have local significance only. That is, the same session will be given different Session IDs by each end of the session. Session ID in each message is that of the intended recipient, not the sender. Session IDs are selected and exchanged as Assigned Session ID AVPs during the creation of a session.

Ns, sequence number. 16 bits. Optional.
Indicates the sequence number for this data or control message, beginning at zero and incrementing by one (modulo 2**16) for each message sent.

Nr, sequence number expected. 16 bits. Optional.
Indicates the sequence number expected in the next control message to be received. Thus, Nr is set to the Ns of the last in-order message received plus one (modulo 2**16). In data messages, Nr is reserved and, if present (as indicated by the S bit), MUST be ignored upon receipt.

Offset Size. 16 bits. Optional.
Specifies the number of bytes past the L2TP header at which the payload data is expected to start. Actual data within the offset padding is undefined. If the offset field is present, the L2TP header ends after the last byte of the offset padding. This field exists if the O bit is set.

Offset Pad. Variable length. Optional.

Data. Variable length.

AVP, Attribute Value Pair.
(RFC 2661) The variable length concatenation of a unique Attribute (represented by an integer) and a Value containing the actual value identified by the attribute. Multiple AVPs make up Control Messages which are used in the establishment, maintenance, and teardown of tunnels.

M, Mandatory. 1 bit.
(RFC 2661) Controls the behavior required of an implementation which receives an AVP which it does not recognize. If the M bit is set on an unrecognized AVP within a message associated with a particular session, the session associated with this message MUST be terminated. 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. If the M bit is not set, an unrecognized AVP MUST be ignored. The control message must then continue to be processed as if the AVP had not been present.

H, Hidden. 1 bit.
(RFC 2661) Identifies the hiding of data in the Attribute Value field of an AVP. This capability can be used to avoid the passing of sensitive data, such as user passwords, as cleartext in an AVP.

AVP Length. 10 bits, 6 to 1023.
The number of bytes contained in this AVP. The length is calculated as 6 + the length of the Attribute Value field in bytes. If the length is set to 6, then the AVP Value field is absent.

AVP Vendor ID. 16 bits.
(RFC 2661) The IANA assigned "SMI Network Management Private Enterprise Codes" value. The value 0, corresponding to IETF adopted attribute values, is used for all AVPs defined within this document. Any vendor wishing to implement their own L2TP extensions can use their own Vendor ID along with private Attribute values, guaranteeing that they will not collide with any other vendor's extensions, nor with future IETF extensions.

AVP Type. 16 bits.

AVP Value. Variable length.

Glossary:

LAC, L2TP Access Concentrator.
(RFC 3070) A device attached to the switched network fabric (e.g., PSTN or ISDN) or co-located with a PPP end system capable of handling the L2TP protocol. The LAC need only implement the media over which L2TP is to operate to pass traffic to one or more LNS's. It may tunnel any protocol carried within PPP.

LNS, L2TP Network Server.
(RFC 3070) LNS operates on any platform capable of PPP termination. The LNS handles the server side of the L2TP protocol. L2TP is connection-oriented.

ZLB, Zero-Length Body Message.
(RFC 2661) A control packet with only an L2TP header. ZLB messages are used for explicitly acknowledging packets on the reliable control channel.

RFCs:

[RFC 2661] Layer Two Tunneling Protocol "L2TP".

• Category: Standards Track.
• Defines L2TP version 2.

[RFC 2809] Implementation of L2TP Compulsory Tunneling via RADIUS.

• Category: Informational.

[RFC 2888] Secure Remote Access with L2TP.

• Category: Informational.

[RFC 3070] Layer Two Tunneling Protocol (L2TP) over Frame Relay.

• Category: Standards Track.

[RFC 3145] L2TP Disconnect Cause Information.

• Category: Standards Track.

[RFC 3193] Securing L2TP using IPsec.

• Category: Standards Track.

[RFC 3301] Layer Two Tunnelling Protocol (L2TP): ATM access network extensions.

• Category: Standards Track.

[RFC 3308] Layer Two Tunneling Protocol (L2TP) Differentiated Services Extension.

• Category: Standards Track.
• Defines L2TP AVPs 47 (CCDS AVP) and 48 (SDS AVP).

[RFC 3355] Layer Two Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5 (AAL5).

• Category: Standards Track.

[RFC 3371] Layer Two Tunneling Protocol "L2TP" Management Information Base.

• Category: Standards Track.

[RFC 3437] Layer-Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation.

• Category: Standards Track.

[RFC 3438] Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update.

• BCP: 68.

[RFC 3573] Signaling of Modem-On-Hold status in Layer 2 Tunneling Protocol (L2TP).

• Category: Standards Track.
• Defines L2TP message type 17 (Modem Status).
• Defines L2TP AVP attributes 53 (Modem On-Hold Capable) and 54 (Modem On-Hold Status).

[RFC 3817] Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay for PPP over Ethernet (PPPoE).

• Category: Informational.
• Defines L2TP AVP attributes 55 (PPPoE Relay), 56 (PPPoE Relay Response Capability), 57(PPPoE Relay Forward Capability).

[RFC 3931] Layer Two Tunneling Protocol - Version 3 (L2TPv3).

• Category: Standards Track.
• Defines L2TP version 3.

[RFC 4045] Extensions to Support Efficient Carrying of Multicast Traffic in Layer-2 Tunneling Protocol (L2TP).

• Category: Experimental.
• Defines L2TP AVP attributes 80 (Multicast Capability), 81 (New Outgoing Sessions), 82 (New Outgoing Sessions Acknowledgement), 83 (Withdraw Outgoing Sessions),84 (Multicast Packets Priority).

[RFC 4349] High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3).

• Category: Standards Track.

[RFC 4454] Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3).

• Category: Standards Track.

Publications:

Obsolete RFCs:

Description

Glossary

RFCs

Publications

Obsolete RFCs