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rfc:rfc5919

Internet Engineering Task Force (IETF) R. Asati Request for Comments: 5919 P. Mohapatra Category: Standards Track Cisco Systems ISSN: 2070-1721 E. Chen

                                                   Huawei Technologies
                                                             B. Thomas
                                                           August 2010
            Signaling LDP Label Advertisement Completion

Abstract

 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
 establishment.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5919.

Asati, et al. Standards Track [Page 1] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................3
    1.1. Applicability - Label Advertisement Mode ...................3
 2. Specification Language ..........................................3
 3. Unrecognized Notification Capability ............................4
 4. Signaling Completion of Label Advertisement .....................4
    4.1. Missing Expected End-of-LIB Notifications ..................5
 5. Usage Guidelines ................................................6
    5.1. LDP-IGP Sync ...............................................6
    5.2. LDP Graceful Restart .......................................7
    5.3. Wildcard Label Request .....................................7
 6. Security Considerations .........................................8
 7. IANA Considerations .............................................8
 8. Acknowledgments .................................................8
 9. References ......................................................8
    9.1. Normative References .......................................8
    9.2. Informative References .....................................9

Asati, et al. Standards Track [Page 2] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

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 the labels from its Label Information Base (LIB).
 For example, when an LDP speaker is using LDP-IGP synchronization
 procedures [RFC5443], 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 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.
 RFC 5036 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 that does not understand the Status Code
 in a Notification message.  To avoid backward compatibility issues,
 this document specifies use of the LDP capability mechanism [RFC5561]
 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.

1.1. Applicability - Label Advertisement Mode

 The mechanisms specified in this document are deemed useful to LDP
 peering using the 'Downstream Unsolicited' label advertisement mode
 [RFC5036].  They are not deemed useful to any LDP peering using the
 'Downstream on Demand' label advertisement mode since the LDP speaker
 would request particular label binding(s) from the peer anyway and
 know when it has received them.

2. Specification Language

 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].

Asati, et al. Standards Track [Page 3] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

3. Unrecognized Notification Capability

 An LDP speaker MAY include a Capability Parameter [RFC5561] in the
 Initialization message to inform a peer that it ignores Notification
 Messages that carry a Status Type-Length-Value (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:
 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| Unrecognized Noti (0x0603)|            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |S| Reserved    |
 +-+-+-+-+-+-+-+-+
       Figure 1: Unrecognized Notification Capability Format
 Where:
    U- and F-bits: MUST be 1 and 0, respectively, as per Section 3 of
       LDP Capabilities [RFC5561].
    Unrecognized Notif: 0x0603
    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 that conforms to this specification SHOULD signal
 completion of its label advertisements to a peer by means of a
 Notification message, if its peer has advertised the Unrecognized
 Notification capability during session establishment.  The LDP
 speaker SHOULD send the Notification message (per Forwarding
 Equivalence Class (FEC) Type) to a peer even if the LDP speaker has
 zero Label bindings to advertise to that peer.
 Such a Notification message MUST carry:
  1. A status TLV (with TLV E- and F-bits set to zero) that carries

an End-of-LIB Status Code (0x0000002F).

Asati, et al. Standards Track [Page 4] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

  1. A FEC TLV with the Typed Wildcard FEC Element [RFC5918] that

identifies the FEC type for which initial label advertisements

      have been completed.  In terms of Section 3.5.1 of RFC 5036,
      this TLV is an "Optional Parameter" of the Notification message.
 An LDP speaker MUST NOT send a Notification that carries a Status TLV
 with the End-of-LIB Status Code to a peer unless the peer has
 advertised the Unrecognized Notification capability during session
 establishment.
 This applies to any LDP peers discovered via either basic discovery
 or extended discovery mechanisms (per Section 2.4 of [RFC5036]).

4.1. Missing Expected End-of-LIB Notifications

 There is no guarantee that an LDP speaker will receive (or send) an
 End-of-LIB Notification from (or to) a peer even if the LDP speaker
 has signaled the Unrecognized Notification capability (Section 3).
 Although it is expected that an LDP speaker supporting the
 Unrecognized Notification capability would support sending and
 receiving an End-of-LIB Notification, it is not mandatory by
 definition.
 Please note that this is not a concern since the LDP speaker would
 simply ignore the received Notification with an End-of-LIB status
 code (or any status code) that is not recognized or supported, by
 definition.
 To deal with the possibility of missing End-of-LIB Notifications
 after the LDP session establishment, an LDP speaker MAY time out
 receipt of an expected End-of-LIB Notification.  An LDP speaker
 SHOULD start a per-peer internal timer, called 'EOL Notification'
 timer (the default value of 60 seconds is RECOMMENDED, though the
 value of this timer SHOULD be configurable) immediately following the
 LDP session establishment.
 This timer is reset by the subsequent label advertisement, and
 stopped by the End-of-LIB Notification message.  Lacking any label
 advertisement from the peer, the timer would expire, causing the LDP
 speaker to behave as if it had received the End-of-LIB notification
 from the peer.
 If the End-of-LIB Notification message is received after the timer
 expires, then the message SHOULD be ignored.

Asati, et al. Standards Track [Page 5] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

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 it
 difficult to determine when an LDP speaker has advertised "all" of
 its label bindings for a given FEC type.  Ultimately, this
 determination is a judgment 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:
  1. the label distribution control mode in use (Independent or

Ordered);

  1. the set of FECs to which the speaker has bound local labels;
  1. configuration settings that 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 [RFC5918] 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 [RFC5443] specifies a mechanism by which
 directly connected LDP speakers may delay the use of the link
 (between them) for transit IP traffic forwarding until the labels
 required to support IP-over-MPLS traffic forwarding have been
 distributed and installed.
 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).

Asati, et al. Standards Track [Page 6] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

 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 should
 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.

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, the
 LDP speaker determines the set of bindings (as per any local
 filtering policy) to advertise to 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.

Asati, et al. Standards Track [Page 7] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

 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.

6. Security Considerations

 No security considerations beyond those that apply to the base LDP
 specification [RFC5036] and that are further described in [RFC5920]
 apply to signaling the End-of-LIB condition as described in this
 document.

7. IANA Considerations

 This document introduces a new LDP Status Code and a new LDP
 Capability.
    IANA has assigned the 'End-of-LIB' status code (0x0000002F) from
    the Status Code Name Space.  [RFC5036] partitions the Status Code
    Name Space into 3 regions: IETF Consensus region, First Come First
    Served region, and Private Use region.  The code point 0x0000002F
    is from the IETF Consensus range.
    IANA has assigned the 'Unrecognized Notification' capability
    (0x0603) from the TLV Type name space.  [RFC5036] partitions the
    TLV Type name space into 3 regions: IETF Consensus region, Vendor
    Private Use region, and Experimental Use region.  The code point
    0x0603 is from the IETF Consensus range.

8. Acknowledgments

    The authors would like to recognize Kamran Raza, who helped to
    formulate this draft.
    The authors would like to thank Ina Minei, Alia Atlas, Yakov
    Rekhter, Loa Andersson, and Luyuan Fang for their valuable
    feedback and contributions.

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., Ed., Minei, I., Ed., and B. Thomas, Ed.,
            "LDP Specification", RFC 5036, October 2007.

Asati, et al. Standards Track [Page 8] RFC 5919 Signaling LDP Label Advertisement Completion August 2010

 [RFC5561]  Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
            Le Roux, "LDP Capabilities", RFC 5561, July 2009.
 [RFC5918]  Asati, R., Minei, I., and B. Thomas, "Label Distribution
            Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
            (FEC)", RFC 5918, August 2010.

9.2. Informative References

 [RFC3478]  Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
            Restart Mechanism for Label Distribution Protocol", RFC
            3478, February 2003.
 [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
            Synchronization", RFC 5443, March 2009.
 [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
            Networks", RFC 5920, July 2010.

Authors' Addresses

 Rajiv Asati
 Cisco Systems
 7025-6 Kit Creek Rd.
 Research Triangle Park, NC  27709-4987
 EMail: rajiva@cisco.com
 Pradosh Mohapatra
 Cisco Systems
 3750 Cisco Way
 San Jose, CA  95134
 EMail: pmohapat@cisco.com
 Emily Chen
 Huawei Technologies
 No. 5 Street, Shangdi Information, Haidian
 Beijing, China
 EMail: chenying220@huawei.com
 Bob Thomas
 EMail: bobthomas@alum.mit.edu

Asati, et al. Standards Track [Page 9]

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