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 
                   draft-ietf-mpls-ldp-end-of-lib-01.txt 


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   Copyright (C) The IETF Trust (2008). 

 
 
 
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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. 

    

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", 
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 
   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 

    

   Where:  

      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 
        message. 

 
 
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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 
   establishment. 

   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 
        Ordered);  

      - 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 
   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. 

   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 
   contribution. 

   This document was prepared using 2-Word-v2.0.template.dot. 

    






































 
 
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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 
             2008. 

                                                             

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 
             2008. 

   [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 
   Email: rajiva@cisco.com 
    
   Pradosh Mohapatra 
   Cisco Systems, 
   3750 Cisco Way, San Jose, CA, 95134 
   Email: pmohapat@cisco.com 
    
   Bob Thomas 
   Email: bobthomas@alum.mit.edu 
    
   Emily Chen 
   Huawei Technologies 
   No.5 Street, Shangdi Information, Haidian, Beijing, China 
   Email: chenying220@huawei.com 
    
    

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   ietf-ipr@ietf.org. 


 
 
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Disclaimer of Validity 

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Copyright Statement 

   Copyright (C) The IETF Trust (2008). 

   This document is subject to the rights, licenses and restrictions 
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Acknowledgment 

   Funding for the RFC Editor function is currently provided by the 
   Internet Society. 

    























 
 
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