L2TP ( Layer Two Tunneling Protocol )

                                                         [Home LAN]

                 [LAC Client]----------+                     |

                                   ____|_____                +--[Host]

                                  |          |               |

                    [LAC]---------| Internet |-----[LNS]-----+

                      |           |__________|               |

                 _____|_____                                 :

                |           |

                |  PSTN     |

      [Remote]--|  Cloud    |

      [System]  |           |                            [Home LAN]

                |___________|                                |

                      |          ______________              +---[Host]

                      |         |              |             |

                    [LAC]-------| Frame Relay  |---[LNS]-----+

                                | or ATM Cloud |             |

                                |______________|             :

Figure 1

          +-------------------+

          | PPP Frames        |

          +-------------------+    +-----------------------+

          | L2TP Data Messages|    | L2TP Control Messages |

          +-------------------+    +-----------------------+

          | L2TP Data Channel |    | L2TP Control Channel  |

          | (unreliable)      |    | (reliable)            |

          +------------------------------------------------+

          |      Packet Transport (UDP, FR, ATM, etc.)     |

          +------------------------------------------------+



Figure 2 L2TP Protocol Structure

• T
  The T bit indicates the type of message. It is set to 0 for data messages and 1 for control messages.



• L
  When set, this indicates that the Length field is present, indicating the total length of the received packet. Must be set for control messages.



• X
  The X bits are reserved for future extensions. All reserved bits are set to 0 on outgoing messages and are ignored on incoming messages.



• S
  If the S bit is set, both the Nr and Ns fields are present. S must be set for control messages.



• O
  When set, this field indicates that the Offset Size field is present in payload messages. This bit is set to 0 for control messages.



• P
  If the Priority (P) bit is 1, this data message receives preferential treatment in its local queuing and transmission. LCP echo requests used as a keepalive for the link, for instance, are generally sent with this bit set to 1. 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. The P bit has a value of 0 for all control messages.



• Ver
  The value of the ver bit is always 002. This indicates a version 1 L2TP message.



• Length
  Overall length of the message, including header, message type AVP, plus any additional AVP's associated with a given control message type.



• Tunnel ID
  Identifies the tunnel to which a control message applies. If an Assigned Tunnel ID has not yet been received from the peer, Tunnel ID must be set to 0. Once an Assigned Tunnel ID is received, all further packets must be sent with Tunnel ID set to the indicated value.



• Call ID
  Identifies the user session within a tunnel to which a control message applies. If a control message does not apply to a single user session within the tunnel (for instance, a Stop-Control-Connection-Notification message), Call ID must be set to 0.



• Nr
  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.



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



• Offset size
  This field specifies the number of bytes past the L2TP header at which the payload data is expected to start. It is recommended that data thus skipped be initialized to 0s. If the offset size is 0, or the O bit is not set, the first byte following the last byte of the L2TP header is the first byte of payload data.



• AVP, Attribute Value Pair
  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.



6						16	32 bits
M	H	O	O	O	O	AVP length	AVP Vendor ID
AVP Type							AVP Value




• M, Mandatory
  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
  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
  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 6, then the AVP Value field is absent.



• AVP Vendor ID
  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



Type	Description	References
0	Message Type	Identifies the control message herein and defines the context in which the exact meaning of the AVPs.
1	Result Code	Indicates the reason for terminating the control channel or session.
2	Protocol Version	Indicates the L2TP protocol version of the sender.
3	Framing Capabilities	Provides the peer with an indication of the types of framing that will be accepted or requested by the sender.
4	Bearer Capabilities	Provides the peer with an indication of the bearer device types supported by the hardware interfaces of the sender for outgoing calls.
5	Tie Breaker	Indicates that the sender wishes a single tunnel to exist between the given LAC-LNS pair.
6	Firmware Revision	Indicates the firmware revision of the issuing device.
7	Host Name	Indicates the name of the issuing LAC or LNS.
8	Vendor Name	Contains a vendor specific (possibly human readable) string describing the type of LAC or LNS being used.
9	Assigned Tunnel ID	Encodes the ID being assigned to this tunnel by the sender.
10	Receive Window Size	Specifies the receive window size being offered to the remote peer.
11	Challenge	Indicates that the issuing peer wishes to authenticate the tunnel endpoints using a CHAP- style authentication mechanism.
12	Q.931 Cause Code	Q.931 Cause Code is used to give additional information in case of unsolicited call disconnection.
13	Response	Provides a response to a challenge received.
14	Assigned Session ID	Encodes the ID being assigned to this session by the sender.
15	Call Serial Number	Encodes an identifier assigned by the LAC or LNS to this call.
16	Minimum BPS	Encodes the lowest acceptable line speed for this call.
17	Maximum BPS	Encodes the highest acceptable line speed for this call.
18	Bearer Type	Encodes the bearer type for the incoming or outgoing call.
19	Framing Type	encodes the framing type for the incoming or outgoing call.
20
21	Called Number	Encodes the telephone number to be called for an OCRQ, and the Called number for an ICRQ.
22	Calling Number	Encodes the originating number for the incoming call.
23	Sub-Address	Encodes additional dialing information.
24	Tx Connect Speed BPS	Encodes the speed of the facility chosen for the connection attempt.
25	Physical Channel ID	Encodes the vendor specific physical channel number used for a call.
26	Initial Received LCP CONFREQ	Provides the LNS with the Initial CONFREQ received by the LAC from the PPP Peer.
27	Last Sent LCP CONFREQ	Provides the LNS with the Last CONFREQ sent by the LAC to the PPP Peer.
28	Last Received LCP CONFREQ	Provides the LNS with the Last CONFREQ received by the LAC from the PPP Peer.
29	Proxy Authen Type	Determines if proxy authentication should be used.
30	Proxy Authen Name	Specifies the name of the authenticating client when using proxy authentication.
31	Proxy Authen Challenge	Specifies the challenge sent by the LAC to the PPP Peer, when using proxy authentication.
32	Proxy Authen ID	Specifies the ID value of the PPP Authentication that was started between the LAC and the PPP Peer, when proxy authentication is being used.
33	Proxy Authen Response	Specifies the PPP Authentication response received by the LAC from the PPP Peer, when proxy authentication is used.
34	Call Errors	Call Errors is used by the LAC to send error information to the LNS.
35	ACCM	ACCM is used by the LNS to inform LAC of the ACCM negotiated with the PPP Peer by the LNS.
36	Random Vector	Random Vector is used to enable the hiding of the Attribute Value of arbitrary AVPs.
37	Private Group ID	Private Group ID is used by the LAC to indicate that this call is to be associated with a particular customer group.
38	Rx Connect Speed	Represents the speed of the connection from the perspective of the LAC (e.g. data flowing from the remote system to the LAC).
39	Sequencing Required	Indicates to the LNS that Sequence Numbers MUST always be present on the data channel.
40	Rx Minimum	Encodes the lowest acceptable line speed for this call in the receive direction, for these cases where asymmetric transmission is required.
41	Rx Maximum	Encodes the highest acceptable line speed for this call in the receive direction, for these cases where asymmetric transmission is required.
42	Service Category	Indicates optional extra information on the Quality of Service expected for the call establishment on the broadband bearer medium.
43	Service Name	Provides the peer with an textual name for referring to an ATM VC connection.
44	Calling Sub-Address	Encodes additional Calling Party subaddress information.
45	VPI/VCI Identifier	Encodes the VPI/VCI value used at the ATM interface at the LAC.
46	PPP Disconnect Cause Code	PPP Disconnect Cause Code is used to provide a human-readable disconnect reason to the user.
47	CCDS	The CCDS AVP is encoded as Vendor ID 0, and the Attribute Type is 47.
48	SDS	SDS AVP is encoded as Vendor ID 0, and the Attribute Value is 48.
49	LCP Want Options	LCP Want Options contains a list of options that the LAC wants to be negotiated by the LNS.
50	LCP Allow Options	LCP Allow Options contains a list of options that the LAC wants to be negotiated by the LNS.
51	LNS Last Sent LCP Confreq	LNS Last Sent LCP Confreq is used to report negotiated LCP parameters from the LNS to the LAC
52	LNS Last Received LCP Confreq	LNS Last Received LCP Confreq is used to report negotiated LCP parameters from the LNS to the LAC
53	Modem On-Hold Capable	Indicates that the sender (an LNS) is capable of receiving MDMST control messages.
54	Modem On-Hold Status	Indicates the current on-hold status of the client modem. This AVP MUST be present on the MDMST control message.
55	PPPoE Relay	Carries the entire PADI, PADO, PADR, PADS and PADT messages within, including Ethernet MAC source and destination addresses.
56	PPPoE Relay Response Capability	Indicates to an L2TP peer that the PPPoE Service Relay (SRRQ, SRRP) messages and the PPPoE Relay AVP will be processed and responded to when received.
57	PPPoE Relay Forward Capability	Indicates to an L2TP peer that PPPoE Service Relay (SRRQ, SRRP) messages and the PPPoE Relay AVP may be sent by this L2TP peer.
58	Extended Vendor ID	Encodes a vendor-specific AVP with a 32-bit Vendor ID space within the Attribute Value field.
59	Message Digest	Message Digest is used as an integrity and authentication check of the L2TP Control Message header and body.
60	Router ID	Router ID is an identifier used to identify an LCCE for control connection setup, tie breaking, and/or tunnel authentication.
61	Assigned Control Connection ID	Contains the ID being assigned to this control connection by the sender.
62	Pseudowire Capabilities List	Pseudowire Capabilities List indicates the L2 payload types the sender can support.
63	Local Session ID	Contains the identifier being assigned to this session by the sender.
64	Remote Session ID	Contains the identifier that was assigned to this session by the peer.
65	Assigned Cookie	Contains the Cookie value being assigned to this session by the sender.
66	Remote End ID	Contains an identifier used to bind L2TP sessions to a given circuit, interface, or bridging instance.
67	Application Code
68	Pseudowire Type	The payload type being carried within an L2TP session.
69	L2-Specific Sublayer	L2-Specific Sublayer is an intermediary layer between the L2TP session header and the start of the tunneled frame.
70	Data Sequencing	Indicates that the sender requires some or all of the data packets that it receives to be sequenced.
71	Circuit Status	Indicates the initial status of or a status change in the circuit to which the session is bound.
72	Preferred Language	Provides a method for an LCCE to indicate to the peer the language in which human - readable messages it sends SHOULD be composed.
73	Control Message Authentication Nonce	Control Message Authentication Nonce must contain a cryptographically random value. This value is used for Control Message Authentication.
74	Tx Connect Speed	Contains the speed of the facility chosen for the connection attempt.
75	Rx Connect Speed	Represents the speed of the connection from the perspective of the LAC (i.e., data flowing from the remote system to the LAC).
76	Failover Capability
77	Tunnel Recovery
78	Suggested Control Sequence
79	Failover Session State
80	Multicast Capability	In order to inform the LNS that an LAC has the ability to handle multicast sessions, the LAC sends a Multicast Capability AVP during the control connection establishment phase.
81	New Outgoing Sessions	New Outgoing Sessions piggybacks every Session ID to which the multicast traffic has to be forwarded.
82	New Outgoing Sessions Acknowledgement	New Outgoing Sessions Acknowledgement informs the LNS that the LAC is ready to replicate traffic for every Session ID listed in the AVP.
83	Withdraw Outgoing Sessions	Withdraw Outgoing Sessions is sent whenever there is one or more withdrawn subscriptions for the corresponding multicast flow.
84	Multicast Packets Priority	Multicast Packets Priority is an optional AVP intended to indicate to the LAC how to process multicast traffic against unicast traffic.
85	Frame-Relay Header Length
86	ATM Maximum Concatenated Cells
87	OAM Emulation Required
88	ATM Alarm Status
89	Attachment Group Identifier
90	Local End Identifier
91	Interface Maximum Transmission Unit

■ Parent layer
■ Child layer

TCP/UDP

L2TP