MPLS Working Group                                          Rajiv Asati
Internet Draft                                            Cisco Systems
Intended status: Standards Track
Expires: March 2009                                   Pradosh Mohapatra
                                                          Cisco Systems

                                                             Bob Thomas

                                                             Emily Chen
                                                    Huawei Technologies

                                                     September 15, 2008

                              LDP End-of-LIB

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

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   There are situations following Label Distribution Protocol (LDP)
   session establishment where it would be useful for an LDP speaker to
   know when its peer has advertised all of its labels.  The LDP
   specification provides no mechanism for an LDP speaker to notify a
   peer when it has completed its initial label advertisements to that
   peer.  This document specifies means for an LDP speaker to signal
   completion of its initial label advertisements following session

Table of Contents

   1. Introduction...................................................2
   2. Specification Language.........................................3
   3. Unrecognized Notification Capability...........................3
   4. Signaling Completion of Label Advertisement....................4
   5. Usage Guidelines...............................................5
      5.1. IGP-Sync..................................................5
      5.2. LDP Graceful Restart......................................6
      5.3. Wildcard Label Request....................................7
      5.4. Missing Expected End-of-LIB Notifications.................7
   6. Security Considerations........................................7
   7. IANA Considerations............................................7
   8. Acknowledgments................................................8
   9. References.....................................................9
      9.1. Normative References......................................9
      9.2. Informative References....................................9
   Author's Addresses...............................................10
   Intellectual Property Statement..................................10
   Disclaimer of Validity...........................................11

1. Introduction

   There are situations following LDP session establishment where it
   would be useful for an LDP speaker to know when its peer has
   advertised all of its labels.  For example, when an LDP speaker is
   using LDP-IGP synchronization procedures [LDPSync], it would be
   useful for the speaker to know when its peer has completed
   advertisement of its IP label bindings.  Similarly, after an LDP
   session is re-established when LDP Graceful Restart [RFC3478] is in

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   effect, it would be helpful for each peer to signal the other after
   it has advertised all its label bindings.

   The LDP specification [RFC5036] provides no mechanism for an LDP
   speaker to notify a peer when it has completed its initial label
   advertisements to that peer.

   This document specifies use of a Notification message with the "End-
   of-LIB" Status Code for an LDP speaker to signal completion of its
   label advertisements following session establishment.

   RFC5036 implicitly assumes that new Status Codes will be defined over
   the course of time.  However, it does not explicitly define the
   behavior of an LDP speaker which does not understand the Status Code
   in a Notification message.  To avoid backward compatibility issues
   this document specifies use of the LDP capability mechanism [LDPCap]
   at session establishment time for informing a peer that an LDP
   speaker is capable of handling a Notification message that carries an
   unrecognized Status Code.

2. Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3. Unrecognized Notification Capability

   An LDP speaker MAY include a Capability Parameter [LDPCap] in the
   Initialization message to inform a peer that it ignores Notification
   Messages that carry a Status TLV with a non-fatal Status Code unknown
   to it.

   The Capability Parameter for the Unrecognized Notification capability
   is a TLV with the following format:

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   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
   |U|F| Unrecog Notif (IANA)      |            Length             |
   |S| Reserved    |

           Figure 1 Unrecognized Notification Capability format


      U and F bits: Should be set 1 and 0 respectively as per section 4
   of LDP Capabilities [LDPCap].

      Unrecog Notif: TLV code point to be assigned by IANA.

      S-bit: Must be 1 (indicates that capability is being advertised).

   Upon receiving a Notification with an unrecognized Status Code an LDP
   speaker MAY generate a console or system log message for trouble
   shooting purposes.

4. Signaling Completion of Label Advertisement

   An LDP speaker MAY signal completion of its label advertisements to a
   peer by means of a Notification message, if its peer had advertised
   the Unrecognized Notification capability during session
   establishment. The LDP speaker MAY send the Notification message (per
   FEC Type) to a peer even if the LDP speaker had zero Label bindings
   to advertise to that peer.

   Such a Notification message MUST carry:

      - A status TLV with TLV E- and F-bits set to zero that carries an
        "End-of-LIB" Status Code.

      - A FEC TLV with the Typed Wildcard FEC Element [TypedWC] that
        identifies the FEC type for which initial label advertisements
        have been completed.  In terms of Section 3.5.1 of RFC5036,
        this TLV is an "Optional Parameter" of the Notification

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   An LDP speaker MUST NOT send a Notification which carries a Status
   TLV with the End-of-LIB Status Code to a peer unless the peer had
   advertised the Unrecognized Notification capability during session

   This applies to any LDP peers discovered via either basic discovery
   or extended discovery mechanism (per section 2.4 of [RFC5036]).

5. Usage Guidelines

   The FECs known to an LDP speaker and the labels the speaker has bound
   to those FECs may change over the course of time.  This makes
   determining when an LDP speaker has advertised "all" of its label
   bindings for a given FEC type an issue.  Ultimately, this
   determination is a judgement call the LDP speaker makes.  The
   following guidelines may be useful.

   An LDP speaker is assumed to "know" a set of FECs.  Depending on a
   variety of criteria, such as:

      - The label distribution control mode in use (Independent or

      - The set of FEC's to which the speaker has bound local labels;

      - Configuration settings which may constrain which label bindings
        the speaker may advertise to peers;

   the speaker can determine the set of bindings for a given FEC type
   that it is permitted to advertise to a given peer.

   LDP-IGP Sync, LDP Graceful Restart, and the response to a Wildcard
   Label Request [TypedWC] are situations that would benefit from End-
   of-LIB Notification.  In these situations, after an LDP speaker
   completes its label binding advertisements to a peer, sending an End-
   of-LIB Notification to the peer makes their outcome deterministic.
   The following subsections further explain each of these situations
   one by one.

5.1. LDP-IGP Sync

   The LDP-IGP Synchronization [LDPSync] specifies a mechanism by which
   directly connected LDP speakers may delay the use of the link between

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   them, for transit IP traffic forwarding until the labels required to
   support IP over MPLS traffic forwarding have been distributed and

   Without an End-of-LIB Notification, the speaker must rely on some
   heuristic to determine when it has received all of its peer's label
   bindings.  The heuristic chosen could cause LDP to signal the IGP too
   soon in which case the likelihood that traffic will be dropped
   increases, or too late in which case traffic is kept on sub-optimal
   paths longer than necessary.

   Following session establishment, with a directly connected peer that
   has advertised the Unrecognized Notification capability, an LDP
   speaker using LDP-IGP Sync may send the peer an End-of-LIB
   Notification after it completes advertisement of its IP label
   bindings to the peer. Similarly, the LDP speaker may use the End-of-
   LIB Notification received from a directly connected peer to determine
   when the peer has completed advertisement of its label bindings for
   IP prefixes.  After receiving the notification, the LDP speaker
   should consider LDP to be fully operational for the link and signal
   the IGP to start advertising the link with normal cost.

5.2. LDP Graceful Restart

   LDP Graceful Restart [RFC3478] helps to reduce the loss of MPLS
   traffic caused by the restart of a router's LDP component.  It
   defines procedures that allow routers capable of preserving MPLS
   forwarding state across the restart to continue forwarding MPLS
   traffic using forwarding state installed prior to the restart for a
   configured time period.

   The current behavior without End-of-LIB Notification is as follows:
   the restarting router and its peers consider the preserved forwarding
   state to be usable but stale until it is refreshed by receipt of new
   label advertisements following re-establishment of new LDP sessions
   or until the time period expires.  When the time period expires, any
   remaining stale forwarding state is removed by the router.

   Receiving End-of-LIB Notification from a peer in an LDP Graceful
   Restart scenario enables an LDP speaker to stop using stale
   forwarding information learned from that peer and to recover the
   resources it requires without having to wait until the time period
   expiry. The time period expiry can still be used if the End-of-LIB-
   Notification message is not received.

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5.3. Wildcard Label Request

   When an LDP speaker receives a Label Request message for a Typed
   Wildcard FEC (e.g. a particular FEC element type) from a peer it
   determines the set of bindings, it is permitted to advertise the peer
   for the FEC type specified by the request.  Assuming the peer had
   advertised the Unrecognized Notification capability at session
   initialization time, the speaker should send the peer an End-of-LIB
   Notification for the FEC type when it completes advertisement of the
   permitted bindings.

   As in the previous applications, receipt of the Notification
   eliminates uncertainty as to when the peer has completed its
   advertisements of label bindings for the requested Wildcard FEC
   Element Type.

5.4. Missing Expected End-of-LIB Notifications

   There is no guarantee that an LDP speaker will receive End-of-LIB
   Notifications from a peer even if the LDP speaker has signaled its
   capability. Therefore, an implementation SHOULD NOT depend on the
   receipt of such a Notification.

   To deal with the possibility of missing notifications, an LDP speaker
   may time out receipt of an expected End-of-LIB Notification, and if
   the timeout occurs, it may behave as if it had received the
   notification. If the End-of-LIB Notification message is received
   after the time-out occurs, then the message should be ignored.

6. Security Considerations

   No security considerations beyond those that apply to the base LDP
   specification and described in [RFC5036] apply to signaling the End-
   of-LIB condition as described in this document.

7. IANA Considerations

   This draft introduces a new LDP Status Code and a new LDP Capability
   both of which require IANA assignment.

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8. Acknowledgments

   The authors would like to thank Ina Minei, Alia Atlas, Yakov Rekhter,
   Loa Andersson and Luyuan Fang for their valuable feedback and

   This document was prepared using

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9. References

9.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
             Thomas, B., "LDP Specification", RFC 5036, January 2001.

   [LDPCap]  Thomas, B., Aggarwal, S., Aggarwal, R., Le Roux, J.L., "LDP
             Capabilities", draft-ietf-mpls-ldp-capabilities-02, Work in
             Progress, May 2007.

   [TypedWC] Thomas, B., Minei, I., "LDP Typed Wildcard FEC", draft-
             ietf-mpls-ldp-typed-wildcard-03, Work in Progress, March

9.2. Informative References

   [LDPSync] Jork, M., Atlas, A., Fang, L., "LDP IGP Synchronization",
             draft-ietf-mpls-ldp-igp-sync-02, Work in Progress, June

   [RFC3478] Leelanivas, M., Rekhter, Y., Aggarwal, R., "Graceful
             Restart Mechanism for Label Distribution Protocol",
             February 2003.

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Author's Addresses

   Rajiv Asati
   Cisco Systems,
   7025-6 Kit Creek Rd, RTP, NC, 27709-4987

   Pradosh Mohapatra
   Cisco Systems,
   3750 Cisco Way, San Jose, CA, 95134

   Bob Thomas

   Emily Chen
   Huawei Technologies
   No.5 Street, Shangdi Information, Haidian, Beijing, China

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Disclaimer of Validity

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