Network Working Group
Request for Comments: 4951
Category: Standards Track
V. Jain, Ed.
Riverstone Networks
August 2007

Fail Over Extensions for Layer 2 Tunneling Protocol (L2TP) "failover"

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The IETF Trust (2007).


Layer 2 Tunneling Protocol (L2TP) is a connection-oriented protocol that has a shared state between active endpoints. Some of this shared state is vital for operation, but may be volatile in nature, such as packet sequence numbers used on the L2TP Control Connection. When failure of one side of a control connection occurs, a new control connection is created and associated with the old connection by exchanging information about the old connection. Such a mechanism is not intended as a replacement for an active fail over with some mirrored connection states, but as an aid for those parameters that are particularly difficult to have immediately available. Protocol extensions to L2TP defined in this document are intended to facilitate state recovery, providing additional resiliency in an L2TP network, and improving a remote system's layer 2 connectivity.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
      1.2. Abbreviations ..............................................5
      1.3. Specification of Requirements ..............................5
   2. Overview ........................................................5
   3. Failover Protocol ...............................................7
      3.1. Failover Capability Negotiation ............................7
      3.2. Failover Recovery Procedure ................................7
           3.2.1. Recovery Tunnel Establishment .......................8
           3.2.2. Control Channel Reset ..............................10
           3.2.3. Data Channel Reset .................................10
      3.3. Session State Synchronization .............................11
   4. New Control Messages ...........................................12
      4.1. Failover Session Query ....................................13
      4.2. Failover Session Response .................................13
   5. New Attribute Value Pairs ......................................14
      5.1. Failover Capability AVP ...................................14
      5.2. Tunnel Recovery AVP .......................................15
      5.3. Suggested Control Sequence AVP ............................16
      5.4. Failover Session State AVP ................................17
   6. Configuration Parameters .......................................18
   7. IANA Considerations ............................................19
   8. Security Considerations ........................................19
   9. Acknowledgements ...............................................19
   10. Contributors ..................................................20
   11. References ....................................................20
      11.1. Normative References .....................................20
      11.2. Informative References ...................................20
   Appendix A ........................................................21
   Appendix B ........................................................23
   Appendix C ........................................................24

1. Introduction

The goal of this document is to aid the overall resiliency of an L2TP endpoint by introducing extensions to RFC 2661 [L2TPv2] and RFC 3931 [L2TPv3] that will minimize the recovery time of the L2TP layer after a failover, while minimizing the impact on its performance. Therefore, it is assumed that the endpoint's overall architecture is also supportive in the resiliency effort.

To ensure proper operation of an L2TP endpoint after a failover, the associated information of the control connection and sessions between them must be correct and consistent. This includes both the configured and dynamic information. The configured information is assumed to be correct and consistent after a failover, otherwise the tunnels and sessions would not have been setup in the first place.

The dynamic information, which is also referred to as stateful information, changes with the processing of the tunnel's control and data packets. Currently, the only such information that is essential to the tunnel's operation is its sequence numbers. For the tunnel control channel, the inconsistencies in its sequence numbers can result in the termination of the entire tunnel. For tunnel sessions, the inconsistency in its sequence numbers, when used, can cause significant data loss, which gives the perception of a "service loss" to the end user.

Thus, an optimal resilient architecture that aims to minimize "service loss" after a failover, must make provisions for the tunnel's essential stateful information, i.e., its sequence numbers. Currently, there are two options available: the first option is to ensure that the backup endpoint is completely synchronized with the active endpoint, with respect to the control and data sessions sequence numbers. The other option is to reestablish all the tunnels and their sessions after a failover. The drawback of the first option is that it adds significant performance and complexity impact to the endpoint's architecture, especially as tunnel and session aggregation increases. The drawback of the second option is that it increases the "service loss" time, especially as the architecture scales.

To alleviate the above-mentioned drawbacks of the current options, this document introduces a mechanism to bring the dynamic stateful information of a tunnel to a correct and consistent state after a failure. The proposed mechanism defines the recovery of tunnels and sessions that were in an established state prior to the failure.

1.1. Terminology

Endpoint: L2TP control connection endpoint, i.e., either LAC or LNS (also known as LCCE in [L2TPv3]).

Active Endpoint: An endpoint that is currently providing service.

Backup Endpoint: A redundant endpoint standing by for the active endpoint that has its database of active tunnels and sessions in sync with its active endpoint.

Failed Endpoint: The endpoint that was the active endpoint at the time of the failure.

Recovery Endpoint: The endpoint that initiates the failover protocol to recover from the failure of an active endpoint.

Remote Endpoint: The endpoint that peers with active endpoint before failure and with recovery endpoint after failure.

Failover: The action of a backup endpoint taking over the service of an active endpoint. This could be due to administrative action or failure of the active endpoint.

Old Tunnel: A control connection that existed before failure and is subjected to recovery upon failover.

Recovery Tunnel: A new control connection established only to recover an old tunnel.

Recovered Tunnel: After an old tunnel's control connection and sessions are restored using the mechanism described in this document, it is referred to as a Recovered Tunnel.

Control Channel Failure: Failure of the component responsible for establishing/maintaining tunnels and sessions at an endpoint.

Data Channel Failure: Failure of the component responsible for forwarding the L2TP encapsulated data.

1.2. Abbreviations

   LAC      L2TP Access Concentrator
   LNS      L2TP Network Server
   LCCE     L2TP Control Connection Endpoint
   AVP      Attribute Value Pair
   SCCRQ    Start-Control-Connection-Request
   SCCRP    Start-Control-Connection-Reply
   ZLB      Zero Length Body Message

1.3. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Overview

The following diagram depicts the redundancy architecture and pertaining entities used to describe the failover protocol:

                                           | L2TP active  |
   +----------+                        ----| endpoint (A) |
   |   L2TP   |                       /    +--------------+
   | endpoint |----------------------/
   |    (R)   |                      \     +--------------+
   +----------+                       \    | L2TP backup  |
                                       ----| endpoint (B) |

Active and backup endpoints may reside on the same device, however, they are not required to be that way. On other hand, some devices may not have a standby module altogether, in which case the failed endpoint, after reset, can become the recovery endpoint to recover from its prior failure.

Therefore, in the above diagram, upon A's (active endpoint's) failure:

  • Endpoint A would be called the failed endpoint.
  • If B is present, then it would become the recovery endpoint and also an active endpoint.
  • If B is not present, then A could become the recovery endpoint after it restarts, provided it saved the information about active tunnels/sessions in some persistent storage.
  • R does not initiate the failover protocol; rather it waits for a failure indication from recovery endpoint.

This document assumes that the actual detection of a failure in the backup endpoint is done in an implementation-specific way. It also assumes that failure detection by the backup endpoint is faster than the L2TP control channel timeout between the active and remote endpoints. Typically, active and backup endpoints reside on the same physical device, allowing fast and reliable failure detection without the need to communicate between these endpoints over the network.

Similarly, an active endpoint that acts also as a backup endpoint can easily start the recovery once it has rebooted. However, when the active and backup endpoints reside in separate devices, there is a need for communication between them in order to detect failures. As a solution for such situations, additional failure detection protocols, e.g., [BFD-MULTIHOP], may be needed.

A device could have three kinds of failures:

i) Control Channel Failure

ii) Data Channel Failure

iii) Control and Data Channel Failure

The protocol described in this document specifies the recovery in conditions i) and iii). It is perceived that not much (stateful information) could be recovered via a control protocol exchange in case of ii).

The failover protocol consists of three phases:

  1. Failover Capability Negotiation: An active endpoint and a remote endpoint exchange failover capabilities and attributes to be used during the recovery process.
  1. Failover Recovery: A recovery endpoint establishes a new L2TP control connection (called recovery tunnel) for every old tunnel that it wishes to recover. The recovery tunnel serves three purposes:
  • It identifies the old tunnel that is being recovered.
  • It provides a means of authentication and a three-way handshake to ensure both ends agree on the failover for the specified old tunnel.
  • It could exchange the Ns and Nr values to be used in the recovered tunnel.

Upon establishing the recovery tunnel, two endpoints reset the control and data channel(s) on the recovered tunnel using the procedures described in Section 3.2.2 and Section 3.2.3, respectively. The recovery tunnel could be torn down after that, and sessions that were established resume traffic.

  1. Session State Synchronization: The session state synchronization process occurs on the recovered or the old tunnel and allows the two endpoints to agree on the state of the various sessions in the tunnel after failover. The inconsistency, which could arise due to the failure, is handled in the following manner: first, the two endpoints silently clear the sessions that were not in the established state. Then, they utilize Failover Session Query (FSQ) and Failover Session Response (FSR) on the recovered tunnel to obtain the state of sessions as known to the peer endpoint and clear the sessions accordingly.

3. Failover Protocol

The protocol consists of three steps describing specifications during the life of a control connection - before and after failover.

3.1. Failover Capability Negotiation

The active and remote endpoints exchange the Failover Capability attribute-value pair (AVP) in Start-Control-Connection-Request (SCCRQ) and Start-Control-Connection-Reply (SCCRP) during control connection establishment as a part of the normal (before failover) operation. The Failover Capability AVP, defined in Section 5.1, allows an endpoint to specify if it is control and/or data channel failover capable and the time allowed for the recovery for the tunnel.

3.2. Failover Recovery Procedure

The Failover Recovery Procedure described in this section is performed only if there was a control channel failure. The selection of the tunnels to be recovered is implementation specific.

The Failover Recovery Procedure consists of following three steps, which are described in detail in the subsections below:

  • Recovery tunnel establishment
  • Control channel reset
  • Data channel reset

3.2.1. Recovery Tunnel Establishment

The recovery endpoint establishes a new control connection, called recovery tunnel, for every old tunnel it wishes to recover. The purpose of the recovery tunnel is solely to recover the corresponding old tunnel. There is a one to one relationship between recovery tunnel and recovered/old tunnel

Recovery tunnel establishment considerations:

  • An LCCE MUST follow the procedures described in [L2TPv2] or [L2TPv3] to establish the recovery tunnel.
  • The recovery tunnel MUST use the same L2TP version (and establishment procedures) that was used for the old tunnel.
  • The SCCRQ for Recovery tunnel MUST include the Tunnel Recovery AVP, defined in Section 5.2, to identify the old tunnel that is being recovered.
  • The recovery tunnel MUST NOT include the Failover Capability AVP in its SCCRQ or SCCRP messages.
  • An endpoint SHOULD NOT send any message other than the following on the recovery tunnel: SCCRQ, SCCRP, SCCCN, StopCCN, HELLO, ZLB, and ACK ([L2TPv3] only).
  • An endpoint MUST NOT use any old Tunnel ID for the recovery tunnel. The old tunnels MUST be valid until the recovery process concludes.
  • An endpoint MUST use the Tie Breaker AVP (Section 4.4.3 [L2TPv2]) or Control Connection Tie Breaker AVP (Section 5.4.3 [L2TPv3]) in the setup of the recovery tunnel to ensure that only a single recovery tunnel (when both endpoints have simultaneous failover) is established to recover an old tunnel. The tunnel that wins the tie is used to decide the suggested Ns and Nr values on the recovered tunnel. Therefore, the endpoint that loses the tie, should reset the Ns and Nr values (Section 3.2.2) as if it were a remote endpoint. Appendix B illustrates the double failover scenario.
  • Tie Breaker AVP processing: The scope of a tie breaker AVP's action for recovery and non recovery tunnels must be independent, and is defined as follows:
  • When Tie Breaker AVP is used in a non recovery tunnel, the scope of the tie breaker AVP's action MUST only be within non recovery tunnels. Therefore, losing a tie against a non recovery tunnel MUST NOT result in termination of any recovery tunnel.
  • When a Tie Breaker AVP is used in a recovery tunnel, the scope of tie breaker AVP's action is further restricted to the recovery tunnel(s) for a single tunnel to be recovered. Thus, an implementation MUST apply the tie breaker received in a recovery tunnel only to those tunnels that are a) recovery tunnels, and b) associated with the same tunnel to be recovered. It MUST NOT impact the operation of non- recovery tunnels and recovery tunnels associated with other old tunnels to be recovered.

Upon getting an SCCRQ with a Tunnel Recovery AVP, an endpoint validates the Recover Tunnel ID and the Recover Remote Tunnel ID and responds with an SCCRP. It MUST terminate the recovery tunnel if:

  • The Recover Tunnel ID or the Recover Remote Tunnel ID is unknown.
  • The active or remote endpoint (prior to failover) had not indicated that it was failover capable.
  • The L2TP version of recovery tunnel is different from the version used in the old tunnel.

If the remote endpoint accepts the SCCRQ, it SHOULD include the Suggested Control Sequence AVP, defined in Section 5.3, in the SCCRP message.

Authentication considerations:

  • To authenticate a peer endpoint during recovery tunnel establishment, an endpoint MUST follow the procedure described in either [L2TPv2] Section 5.1.1 or [L2TPv3] Section 4.3. It MUST use the same secret that was used to authenticate the old tunnel.
  • Not being able to authenticate could be a reason to terminate the recovery tunnel.
  • For L2TPv3 tunnels, a recovery tunnel MUST use the Control Message authentication (i.e., exchange the nonce values), as described in [L2TPv3] Section 4.3, if the old tunnel was configured to do control message authentication. An L2TPv3 recovered tunnel MUST reset its nonce values (both endpoints) to the nonce values exchanged in the recovery tunnel.

For any reason, if the recovery endpoint could not establish the recovery tunnel, then it MUST silently clear the old tunnel and sessions within, concluding that the recovery process has failed.

Any control packet received on the recovered tunnel before control channel reset (Section 3.2.2) MUST be silently discarded.

3.2.2. Control Channel Reset

Control channel reset allows new control messages to be sent and received over the recovered tunnel.

Control channel reset procedure:

  • An endpoint SHOULD flush the transmit/receive windows and reset the control channel sequence numbers (i.e., Ns and Nr values) on the recovered tunnel. The control channel on the recovery endpoint is reset upon getting a valid SCCRP on the recovery tunnel, whereas the control channel on the remote endpoint is reset upon getting a valid SCCCN on the recovery tunnel. If the recovery endpoint did not receive the Suggested Control Sequence (SCS) AVP in the SCCRP then it MUST reset the Ns and Nr values to zero. If the remote endpoint opted to not send the SCS AVP then it MUST reset the Ns and Nr values to zero. Either endpoint can tear down the recovery tunnel after the control channel reset procedure is complete.
  • An endpoint MUST prevent the establishment of new sessions until it has cleared (or marked for clearance) the sessions that were not in an established state, i.e., until after Step I, Section 3.3 is complete.

3.2.3. Data Channel Reset

A data channel reset procedure is applicable only for the sessions using sequence numbers. For L2TPv3 data channel, the terms Nr and Ns in this document are used to mean "expected sequence number" and "sequence number", respectively.

Data channel reset procedure:

  • The recovery endpoint sets the Ns value to zero.
  • The remote endpoint (recovery endpoint's peer) continues to use the Ns values it was using previously.
  • To reset Nr values during failover, if an endpoint receives 'n' out of order but in sequence packets, then it MUST set the Nr value based on the Ns value of the incoming packets, as suggested in Appendix C of [L2TPv3]. The value of 'n' SHOULD be configurable.
  • If one of the endpoints does not exhibit the capability (indicated in 'D' bit in the Failover Capability AVP) to reset the Nr value, then data channels using sequence numbers are considered non recoverable. Those sessions SHOULD be torn down by the recovery endpoint by sending a Call-Disconnect-Notify (CDN).
  • For data-channel-only failure, two endpoints MAY use the session state query/response mechanism on the control channel to synchronize the state of sessions as described in Section 3.3 below.

3.3. Session State Synchronization

If a control channel failure happens when a session was being established or torn down, then it is possible for an endpoint to consider a session in an established state while its peer considers the same session non existent. Two such situations occur when failure on an endpoint occurs immediately after sending:

  • A CDN message that never made it to the peer.
  • An ICCN message that never made it to the peer.

The following mechanism MUST be used to identify and clear the sessions that exists on an endpoint, but not on its peer:

Step I: For control channel failure, after the recovery tunnel is established, the sessions that were not in an established state MUST be silently cleared (i.e., without sending a CDN message) by each endpoint.

Step II: Both endpoints MAY identify the sessions that might have been in inconsistent states, perhaps based on data channel inactivity. FSQ and FSR messages have been introduced to synchronize session state at any given point during the life of a session between two endpoints. These messages are used when one endpoint determines or suspects in an implementation specific manner that its session state could be inconsistent with that of its peer's.

Step III: An endpoint sends a Failover Session Query (FSQ) message to query the state of sessions as known to its peer. An FSQ message contains one Failover Session State (FSS) AVP, defined in Section 5.4, for each session it wishes to query. Multiple FSS AVPs could be included in one FSQ message. An FSQ message MUST include at least one FSS AVP. An endpoint MAY send another FSQ message before getting a response for its previous FSQs.

An inconsistency about a session's existence during failover could result in an endpoint selecting the same Session ID for a new session. In such a situation, it would send an ICRQ for an already established session. Therefore, before all sessions are synchronized using an FSQ/FSR mechanism, if endpoint receives an ICRQ for a session in an established state, then it MUST respond to such an ICRQ with a CDN. The CDN message must set Assigned/Local Session ID AVP ([L2TPv2] Section 4.4.4, [L2TPv3] Section 5.4.4) to its local Session ID and clear the session that it considered established. Use of a least recently used Session ID for the new sessions could help reduce this symptom during failover.

When an endpoint receives an FSQ message, it MUST ensure that for each FSS AVP in the FSQ message, it includes an FSS AVP in the Failover Session Response (FSR) message. An endpoint could respond to multiple FSQs using one FSR message, or it could respond one FSQ with multiple FSRs. FSSs are not required to be responded in the same order in which they were received. For each FSS AVP received in FSQ messages, an endpoint MUST validate the Remote Session ID and determine if it is paired with the Session ID specified in the message. If an FSS AVP is not valid (i.e., session is non-existing or it is paired with different remote Session ID), then the Session ID field in the FSS AVP in the FSR MUST be set to zero. When session is discovered to be pairing with mismatching Session ID, the local session MUST not be cleared, but rather marked stale, to be queried later using an FSQ message. Appendix C presents an example dialogue between two endpoints with mismatching Session IDs.

When responding to an FSQ with an FSR message, the Remote Session ID in the FSS AVP of the FSR message is always set to the received value of the Session ID in the FSS AVP of the FSQ message.

When an endpoint receives an FSR message, for each FSS AVP it MUST use the Remote Session ID field to identify the local session and silently (without sending a CDN) clear the session if the Session ID in the AVP was zero. Otherwise, it MUST consider the session to be in an established state and recovered.

4. New Control Messages

This document introduces two new messages that could be sent over an established/recovered control connection.

4.1. Failover Session Query

The Failover Session Query (FSQ) control message is used by an endpoint during the recovery process to query the state of various sessions. It triggers a response from the peer, which contains the requested state of various sessions.

This control message is encoded as follows:

Vendor ID = 0 (IETF)
Attribute Type = 21

The following AVPs MUST be present in the FSQ control message:

Message Type
Failover Session State

The following AVPs MAY be present in the FSQ control message:

      Random Vector
      Message digest ([L2TPv3] tunnels only)

Other AVPs MUST NOT be sent in this control message and SHOULD be ignored on receipt.

The M-bit on the Message Type AVP for this control message MUST be set to 0.

4.2. Failover Session Response

The Failover Session Response (FSR) control message is used by an endpoint during the recovery process to respond with the local state of various sessions. It is sent as a response to an FSQ message. An endpoint MAY choose to respond to an FSQ message with multiple FSR messages.

This control message is encoded as follows:

Vendor ID = 0 (IETF)
Attribute Type = 22

The following AVPs MUST be present in the FSR control message:

Message Type
Failover Session State

The following AVPs MAY be present in the FSR control message:

      Random Vector
      Message digest ([L2TPv3] tunnels only)

Other AVPs MUST NOT be sent in this control message and SHOULD be ignored on receipt.

The M-bit on the Message Type AVP for this control message MUST be set to 0.

5. New Attribute Value Pairs

The following sections contain a list of new L2TP AVPs defined in this document.

5.1. Failover Capability AVP

The Failover Capability AVP, Attribute Type 76, indicates the capabilities of an endpoint required for the recovery process. The AVP format is defined as follows:

   Failover Capability AVP
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |M|H| rsvd  |      Length       |                0              |
   |         Attribute Type 76     |         Reserved          |D|C|
   |              Recovery Time (in milliseconds)                  |

The AVP MAY be hidden (the H-bit set to 0 or 1). The AVP is not mandatory (the M-bit MUST be set to 0).

The C bit governs the failover capability for the control channel. When the C bit is set, it indicates that the endpoint can recover from a control channel failure using the procedure described in Section 3.2.2.

When the C bit is not set, it indicates that the endpoint cannot recover from a control channel failover. In this case, the D bit MUST be set. Note that a control channel failover in this case would be fatal for the tunnel and all associated data channels.

The D bit governs the failover capability for data channels that use sequence numbers. Data channels that do not use sequence numbers do not need help to recover from a data channel failure.

When the D bit is set, it indicates that the endpoint is capable of resetting Nr value of data channels using the procedure described in Section 3.2.3 Data Channel reset procedure.

When the D bit is not set, it indicates that the endpoint cannot recover data channels that use sequence numbers. In the case of a failure, such data channels would be lost.

The Failover Capability AVP MUST NOT be sent with C bit and D bit cleared.

The Recovery Time, applicable only when the C bit is set, is the time in milliseconds an endpoint asks its peer to wait before assuming the recovery process has failed. This timer starts when an endpoint's control channel timeout ([L2TPv2] Section 5.8, [L2TPv3] Section 4.2) is started, and is not stopped (before expiry) until an endpoint successfully authenticates its peer during recovery. A value of zero does not mean that failover will not occur, it means no additional time is requested from the peer. The timer is also stopped if a control channel message is acknowledged by the peer in the situation when there was no failover, but the loss of the control channel message was a temporary phenomenon.

This AVP MUST NOT be included in any control message other than SCCRQ and SCCRP messages.

5.2. Tunnel Recovery AVP

The Tunnel Recovery AVP, Attribute Type 77, indicates that a sender would like to recover the tunnel identified in this AVP due to a failure. The AVP format is defined as follows:

Tunnel Recovery AVP for L2TPv3 tunnels:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |M|H| rsvd  |      Length       |                0              |
   |         Attribute Type 77     |           Reserved            |
   |                        Recover Tunnel ID                      |
   |                     Recover Remote Tunnel ID                  |

Tunnel Recovery AVP for L2TPv2 tunnels:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |M|H| rsvd  |      Length       |                0              |
   |         Attribute Type 77     |           Reserved            |
   |         Reserved              |     Recover Tunnel ID         |
   |         Reserved              |   Recover Remote Tunnel ID    |

This AVP MUST not be hidden (the H-bit is set to 0). The AVP is mandatory (the M-bit is set to 1).

The Recover Tunnel ID encodes the local Tunnel ID that an endpoint wants recovered. The Recover Remote Tunnel ID encodes the remote Tunnel ID corresponding to the old tunnel.

This AVP MUST NOT be included in any control message other than the SCCRQ message when establishing a Recovery Tunnel.

5.3. Suggested Control Sequence AVP

The Suggested Control Sequence (SCS) AVP, Attribute Type 78, specifies the Ns and Nr values to for the recovered tunnel. This AVP is included in an SCCRP message of a recovery tunnel by remote endpoint. The AVP format is defined as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |M|H| rsvd  |      Length       |                0              |
   |         Attribute Type 78     |            Reserved           |
   |        Suggested Ns           |         Suggested Nr          |

This AVP MAY be hidden (the H-bit set to 0 or 1). The AVP is not mandatory (the M-bit is set to 0).

This is an optional AVP, suggesting Ns and Nr values to be used by the recovery endpoint. If this AVP is present in an SCCRP message during recovery tunnel establishment, the recovery endpoint MUST set the Ns and Nr values of the recovered tunnel to the respective suggested values. When this AVP is not sent in an SCCRP or not present in an incoming SCCRP, the Ns and Nr values for the recovered tunnel are set to zero. Use of this AVP helps avoid the interference in the recovered tunnel's control channel with old control packets.

This AVP MUST NOT be included in any control message other than the SCCRP message when establishing a Recovery Tunnel.

5.4. Failover Session State AVP

The Failover Session State (FSS) AVP, Attribute Type 79, is used to query the state of a session from the peer end to clear the sessions that otherwise would remain in an undefined state after failover. The AVP format is defined as follows:

FSS AVP format for L2TPv3 sessions:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |M|H| rsvd  |      Length       |                0              |
   |      Attribute Type 79        |         Reserved              |
   |                          Session ID                           |
   |                       Remote Session ID                       |

FSS AVP format for L2TPv2 sessions:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |M|H| rsvd  |      Length       |                0              |
   |      Attribute Type 79        |         Reserved              |
   |            Reserved           |        Session ID             |
   |            Reserved           |      Remote Session ID        |

This AVP MAY be hidden (the H-bit set to 0 or 1). The AVP is mandatory (the M-bit is set to 1).

The Session ID identifies the local Session ID that the sender had assigned, for which it would like to query the state on its peer. A Remote Session Id is the remote Session ID for the same session.

An FSS AVP MUST NOT be used in any message other than FSQ and FSR messages.

6. Configuration Parameters

An L2TP endpoint MAY expose the following configuration parameters to be specified for control connections:

  • Control Channel Failover Capability: Failover Capability AVP (Section 5.1), C bit.
  • Data Channel Failover Capability: Failover Capability AVP (Section 5.1), D bit.
  • Recovery Time: Failover Capability AVP (Section 5.1).

The L2TP MIB defined in [L2TPv2-MIB] and [L2TPv3-MIB], defines a number of objects that may be used for monitoring the status L2TP nodes, but is seldom used for configuration purposes. It is expected that the above mentioned parameters will be configured by using a Command Line Interface (CLI) or other proprietary mechanism.

Asynchronous notifications for failover and recovery events may be sent by L2TP nodes to network management applications, but the specification of the protocol and format to be used for these notifications is out of the scope of this document.

7. IANA Considerations

This document defines the following values assigned by IANA.

  • Four Control Message Attribute Value Pairs (Section 10.1 [L2TPv3]):
           Failover Capability         : 76
           Tunnel Recovery             : 77
           Suggested Control Sequence  : 78
           Failover Session State      : 79
  • Two Message Type (Attribute Type 0) Values (Section 10.2 [L2TPv3]):
           Failover Session Query      : 21
           Failover Session Response   : 22

8. Security Considerations

A spoofed failover request (SCCRQ with Tunnel Recovery AVP) on behalf of an endpoint might cause a control channel termination if authentication measures mentioned in Section 3.2.1 are not used.

Even if the authentication measures (as described in Section 3.2.1) were used, it is still possible to learn an identity of an operational tunnel from an endpoint by issuing it spoofed failover requests that fail the authentication procedure. The probability of succeeding with a spoofed failover request is 1 in (2^16 - 1) for [L2TPv2] and 1 in (2^32 - 1) for [L2TPv3]. The discovered identity of an operational tunnel could then be misused to send control messages for a possible hindrance to the control connection. Typically, control messages that are outside the endpoint's receive window are discarded. However, if Suggested Control Sequence AVP (Section 5.3) is not used during the actual failover process, the sequence numbers might be reset to zero, thereby making the receive window predictable. To improve security under such circumstances, an endpoint may be configured with the possible set of recovery endpoints that could recover a tunnel, and use of Suggested Control Sequence AVP when recovering a tunnel.

9. Acknowledgements

Leo Huber provided suggestions to help define the failover concept. Mark Townsley, Carlos Pignataro, and Ignacio Goyret reviewed the document and provided valuable suggestions.

10. Contributors

   Paul Howard            Juniper Networks
   Vipin Jain             Riverstone Networks
   Sam Henderson          Cisco Systems
   Keyur Parikh           Harris Corporations

11. References

11.1. Normative References

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.
   [L2TPv2]       Townsley, W., Valencia, A., Rubens, A., Pall, G.,
                  Zorn, G., and B. Palter, "Layer Two Tunneling Protocol
                  "L2TP"", RFC 2661, August 1999.
   [L2TPv3]       Lau, J., Townsley, M., and I. Goyret, "Layer Two
                  Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
                  March 2005.

11.2. Informative References

   [L2TPv2-MIB]   Caves, E., Calhoun, P., and R. Wheeler, "Layer Two
                  Tunneling Protocol "L2TP" Management Information
                  Base", RFC 3371, August 2002.
   [L2TPv3-MIB]   Nadeau, T. and K. Koushik, "Layer Two Tunneling
                  Protocol (version 3) Management Information Base",
                  Work in Progress, August 2006.
   [BFD-MULTIHOP] Katz, D. and D. Ward, "BFD for Multihop Paths", Work
                  in Progress, March 2007.

Appendix A

Description below outlines the failover protocol operation for an example tunnel. The failover protocol does not preclude an endpoint from recovering multiple tunnels in parallel. It also allows an endpoint to send multiple FSQs, each including multiple FSS AVPs, to recover quickly.

Failover Capability Negotiation (Section 3.1):

  Endpoint                                             Peer
               (assigned tid = x, failover capable)
  SCCRQ       -------------------------------------->  validate SCCRQ
               (assigned tid = y, failover capable)
  validate    <--------------------------------------  send SCCRP
  SCCRP, etc.

.... <after tunnel gets created, sessions are established> ....

  < This Node fails >

The Recovery endpoint establishes the recovery tunnel (Section 3.2.1). Initiate recovery tunnel establishment for the old tunnel 'x':

  Recovery Endpoint                                     Peer
            (assigned tid = z, Recovery AVP)
  SCCRQ     ----------------------------------->  Detects failover
          (recover tid = x, recover remote tid = y)  validate SCCRQ
          (Suggested Control Sequence AVP, Suggested Ns/Nr = 3/100)
  validate <-----------------------------------   send SCCRP
  SCCRP    (recover tid = y, recover remote tid = x)
  reset Ns = 3, Nr = 100
  on the recovered tunnel
  SCCCN     ----------------------------------->  validate and reset
                                                  Ns = 100, Nr = 3 on
                                                  the recovered tunnel

Terminate the recovery tunnel

  tid = 'z'
  StopCCN  --------------------------------------> Cleanup 'w'

Session states are synchronized both endpoints may send FSQs and cleanup stale sessions (Section 3.3)

             (FSS AVP for sessions s1, s2, s3..)
  send FSQ  -------------------------------------> compute the state
                                                      of sessions in FSQ
                (FSS AVP for sessions s1, s2, s3...)
     deletes  <-------------------------------------- send FSR
     stale sessions, if any
                (FSS AVP for sessions s7, s8, s9...)
     compute  <-------------------------------------- send FSQ
     the sate of
     sessions in FSQ
                (FSS AVP for sessions s7, s8, s9...)
     send FSR --------------------------------------> delete stale
                                                      sessions, if any

Appendix B

This section shows an example dialogue to illustrate double failure recovery. The notable difference, as described in Section 3.2.1, in the procedure from single failover scenario is the use of a tie breaker by one of the recovery endpoints to use the recovery tunnel established by its peer (also a recovery endpoint) as a recovery tunnel.

      Recovery endpoint                     Recovery endpoint
      (assume old tid = A)                 (assume old tid = B)
                  Recovery AVP = (A, B)
      SCCRQ     -----------------------+
      (with tie  (recovery tunnel 'C') |
       breaker                         |
       AVP)                            |
                 Recovery AVP = (B, A) |
   +- valid    <--------------------------- Send SCCRQ
   |  SCCRQ      (recovery tunnel 'D') |    (with tie breaker AVP)
   |  This endpoint                    |
   |  loses tie;                       |
   |  Discards tunnel 'C'              +--> Valid SCCRQ
   |                                        This endpoint wins tie;
   |                                        Discards SCCRQ
   |              (may include SCS AVP)
   +->Send SCCRP -------------------------> Validate SCCRP
                                            Reset 'B';
                                            Set Ns, Nr values --+
      Validate SCCN <---------------------- Send SCCN    -------+
      Reset 'A';
      Set Ns, Nr values

FSQs and FSRs for the old tunnel (A, B) are exchanged on the recovered tunnel by both endpoints.

Appendix C

Session ID mismatch could not be a result of failure on one of the endpoints. However, failover session recovery procedure could exacerbate the situation, resulting into a permanent mismatch in Session IDs between two endpoints. The dialogue below outlines the behavior described in Section 3.3, Step III to handle such situations gracefully.

   Recovery endpoint                    Remote endpoint
   (assume a mismatch)                  (assume a mismatch)
   Sid = A, Remote Sid = B              Sid = B, Remote Sid = C
   Sid = C, Remote Sid = D
                  FSS AVP (A, B)
   send FSQ  -------------------------> No (B, A) pair exist;
                                        rather (B, C) exist.
                                        If it clears B then peer doesn't
                                        know if C is stale on other end.

Instead if it marks B stale and queries the session state via FSQ, C would be cleared on the other end.

                  FSS AVP (0, A)
   Clears A <-------------------------- send FSR

... some time later ...

                  FSS AVP (B, C)
   No (C,B) <-------------------------- send FSQ
   Mark C Stale
                  FSS AVP (0, B)
   Send FSR --------------------------> Clears B

Author Information

Vipin Jain
Riverstone Networks
5200 Great America Parkway
Santa Clara, CA 95054

Paul W. Howard
Juniper Networks
10 Technology Park Drive
Westford, MA 01886

Sam Henderson
Cisco Systems
7025 Kit Creek Rd.
PO Box 14987
Research Triangle Park, NC 27709

Keyur Parikh
Harris Corporation
4393 Digitalway
Mason, OH 45040

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