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MPLS Working Group                                         Santosh Esale
INTERNET-DRAFT                                           Raveendra Torvi
Intended Status: Proposed Standard                      Juniper Networks
Expires: September 14, 2017
                                                             Luyuan Fang
                                                               Microsoft

                                                              Luay Jalil
                                                                 Verizon

                                                          March 13, 2017


           Fast Reroute for Node Protection in LDP-based LSPs
                    draft-esale-mpls-ldp-node-frr-05


Abstract

   This document describes procedures to support node protection for
   unicast Label Switched Paths (LSPs) established by Label Distribution
   Protocol (LDP).  In order to protect a node N, the Point of Local
   Repair (PLR) of N must discover the Merge Points (MPs) of node N such
   that traffic can be redirected to them in case of node N failure.
   Redirecting the traffic around the failed node N depends on existing
   point-to-point LSPs originated from the PLR to the MPs while
   bypassing the protected node N.  The procedures described in this
   document are topology independent in a sense that they provide node
   protection in any topology so long as there is a alternate path in
   the network that avoids the protected node.


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at



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   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html


Copyright and License Notice

   Copyright (c) 2017 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
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   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.



Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1 Abbreviations  . . . . . . . . . . . . . . . . . . . . . . .  4
   3. Merge Point (MP) Discovery  . . . . . . . . . . . . . . . . . .  4
   4. Constructing Bypass LSPs  . . . . . . . . . . . . . . . . . . .  5
   5. Obtaining Label Mapping from MP . . . . . . . . . . . . . . . .  6
   6. Forwarding Considerations . . . . . . . . . . . . . . . . . . .  6
   7. Synergy with node protection in mLDP  . . . . . . . . . . . . .  7
   8. Security Considerations . . . . . . . . . . . . . . . . . . . .  7
   9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  7
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   11. Normative References . . . . . . . . . . . . . . . . . . . . .  7
   12. Informative References . . . . . . . . . . . . . . . . . . . .  7
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8



1. Introduction

   This document describes procedures to support node protection for
   unicast Label Switched Paths (LSPs) established by Label Distribution
   Protocol (LDP) [RFC5036]. In order to protect a node N, the Point of
   Local Repair (PLR) of N must discover the Merge Points (MPs) of node
   N such that traffic can be redirected to them in case of node N



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   failure.  Redirecting the traffic around the failed node N depends on
   existing explicit path Point-to-Point (P2P) LSPs originated from the
   PLR LSR to the MPs while bypassing node N. The procedures to setup
   these P2P LSPs are outside the scope of this document, but one option
   is to use RSVP-TE based techniques [RFC3209] to accomplish it.
   Finally, sending traffic from the PLR to the MPs requires the PLR to
   obtain FEC-label bindings from the MPs.  The procedures described in
   this document relies on Targeted LDP (tLDP) session [RFC5036] for the
   PLR to obtain such FEC-Label bindings.

   The procedure described in this document assumes the use of platform-
   wide label space. The procedures for node protection described in
   this document fall into the category of local protection. The
   procedures described in this document apply to LDP LSPs bound to
   either an IPv4 or IPv6 Prefix FEC element. The procedures described
   in this document are topology independent in a sense that they
   provide node protection in any topology so long as there is a
   alternate path in the network that avoids the protected node. Thus
   these procedures provide topology independent fast reroute.
































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1.1 Abbreviations

    PLR:  Point of Local Repair - the LSR that redirects the traffic to
          one or more Merge Point LSRs.

    MP:   Merge Point. Any LSR on the LDP-signaled (multi-point to
          point) LSP, provided that the path from that LSR to the
          egress of that LSP is not affected by the failure of the
          protected node.

    tLDP: A targeted LDP session is an LDP session between non-directly
          connected LSRs, established using the LDP extended discovery
          mechanism.

    FEC:  Forwarding equivalence class.

    IGP:  Interior Gateway Protocol.

    BR:   Border Router.


3. Merge Point (MP) Discovery

   For a given LSP that traverses the PLR, the protected node N, and a
   particular neighbor of the protected node, we'll refer to this
   neighbor as the "next next-hop". Note that from the PLR's perspective
   the protected node N is the next hop for the FEC associated with that
   LSP. Likewise, from the protected node's perspective the next next-
   hop is the next hop for that FEC.  If for a given <LSP, PLR, N>
   triplet the next next-hop is in the same routing subdomain (area) as
   the PLR, then that next next-hop acts as the MP for that triplet. For
   a given LSP traversing a PLR and the node protected by the PLR, the
   PLR discovers its next next-hops (MPs) that are in the same routing
   subdomain (IGP area) as the PLR from IGP shortest path first (SPF)
   calculations. The discovery of next next-hop, depending on an
   implementation, may not involve any additional SPF, above and beyond
   what will be needed by either ISIS or OSPF anyway, as the next next-
   hop, just like the next-hop, is a by-product of SPF computation.

   Also, the PLR may discover all possible MPs from either its traffic
   engineering database or link state database. Some implementations MAY
   need appropriate configuration to populate the traffic engineering
   database. The traffic engineering database is populated by routing
   protocols such as ISIS and OSPF or configured statically.

   If for a given <LSP, PLR, N> triplet the node protected by the PLR is
   an Border Router (BR), then the PLR and the next next-hop may end up
   in different routing subdomain. This could happen when an LSP



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   traversing the PLR and the protected node does not terminate in the
   same routing subdomain as the PLR.  In this situation the PLR may not
   be able to determine the next next-hop from shortest path first (SPF)
   calculations, and thus may not be able to use the next next-hop as
   the MP.  In this scenario the PLR uses an "alternative" BR as the MP,
   where an alternative BR is defined as follows. For a given LSP that
   traverses the PLR and the (protected) BR, an alternative BR is
   defined as any BR that advertises into PLR's own routing subdomain
   reachability to the FEC associated with the LSP.

   Note that even if a PLR protects an BR, for some of the LSPs
   traversing the PLR and the BR, the next next-hops may be in the same
   routing subdomain as the PLR, in which case these next next-hops act
   as MPs for these LSPs. Note that even if the protected node is not an
   BR, if an LSP traversing the PLR and the protected node does not
   terminate in the same routing subdomain as the PLR, then for this LSP
   the PLR MAY use an alternative BR (as defined earlier), rather than
   the next next-hop as the MP. When there are several candidate BRs for
   alternative BR, the LSR MUST select one BR. The algorithm used for
   the alternative BR selection is a local matter but one option is to
   select the BR per FEC based on shortest path from PLR to the BR.



4. Constructing Bypass LSPs

   As mentioned before, redirecting traffic around the failed node N
   depends on existing explicit path Point-to-Point (P2P) LSPs
   originated from the PLR to the MPs while bypassing node N. Let's
   refer to these LSPs as "bypass LSPs". While the procedures to signal
   these bypass LSPs are outside the scope of this document, this
   document assumes use of RSVP-TE LSPs [RFC3209] to accomplish it. Once
   a PLR that protects a given node N discovers the set of MPs
   associated with itself and the protected node, at the minimum the PLR
   MUST (automatically) establish bypass LSPs to all these MPs. The
   bypass LSPs MUST be established prior to the failure of the protected
   node.

   One could observe that if the protected node is not an BR and the PLR
   does not use alternative BR(s) as MP(s), then the set of all the IGP
   neighbors of the protected node forms a superset of the MPs. Thus it
   would be sufficient for the PLR to establish bypass LSPs with all the
   IGP neighbors of the protected node, even though some of these
   neighbors may not be MPs for any of the LSPs traversing the PLR and
   the protected node.

   The bypass LSPs MUST avoid traversing the protected node, which means
   that the bypass LSPs are explicitly routed LSPs. Of course, using



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   RSVP-TE to establish bypass LSPs allows these LSPs to be explicitly
   routed. As a given router may act as an MP for more than one LSP
   traversing the PLR, the protected node, and the MP, the same bypass
   LSP will be used to protect all those LSPs.

5. Obtaining Label Mapping from MP

   As mentioned before, sending traffic from the PLR to the MPs requires
   the PLR to obtain FEC-label bindings from the MPs. The solution
   described in this document relies on Targeted LDP (tLDP) session
   [RFC5036] for the PLR to obtain such mappings. Specifically, for a
   given PLR and the node protected by this PLR, at the minimum the PLR
   MUST (automatically) establish tLDP with all the MPs associated with
   this PLR and the protected node. These tLDP sessions MUST be
   established prior to the failure of the protected node. One could
   observe that if the protected node is not an BR and the PLR does not
   use alternative BR(s) as MP(s), then the set of all the IGP neighbors
   of the protected node forms a superset of the MPs. Thus it will be
   sufficient for the PLR to (automatically) establish tLDP session with
   all the IGP neighbors of the protected node - except the PLR - that
   are in the same area as the PLR, even though some of these neighbors
   may not be MPs for any of the LSPs traversing the PLR and the
   protected node.

   At the minimum for a given tLDP peer the PLR MUST obtain FEC-label
   mapping for the FEC(s) for which the peer acts as an MP. The PLR MUST
   obtain this mapping before the failure of the protected node. To
   obtain this mapping for only these FECs and no other FECs that the
   peer may maintain, the PLR SHOULD rely on the LDP Downstream on
   Demand (DoD) procedures [RFC5036]. Otherwise, without relying on the
   DoD procedures, the PLR may end up receiving from a given tLDP peer
   FEC-label mappings for all the FECs maintained by the peer, even if
   the peer does not act as an MP for some of these FECs. If the LDP DoD
   procedures are not used, then for the purpose of the procedures
   specified in this draft the only label mappings that SHOULD be
   exchanged are for the Prefix FEC elements whose PreLen value is
   either 32 (IPv4), or 128 (IPv6); label mappings for the Prefix FEC
   elements with any other PreLen value SHOULD NOT be exchanged.

   When a PLR has one or more BRs acting as MPs, the PLR MAY use the
   procedures specified in [draft-ietf-mpls-app-aware-tldp] to limit the
   set of FEC-label mappings received from non-BR MPs to only the
   mappings for the FECs associated with the LSPs that terminate in the
   PLR's own routing subdomain (area).

6. Forwarding Considerations

   When a PLR detects failure of the protected node then rather than



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   swapping an incoming label with a label that the PLR received from
   the protected node, the PLR swaps the incoming label with the label
   that the PLR receives from the MP, and then pushes the label
   associated with the bypass LSP to that MP.

   To minimize micro-loop during the IGP global convergence PLR may
   continue to use the bypass LSP during network convergence by adding
   small delay before switching to a new path.

7. Synergy with node protection in mLDP

   Both the bypass LSPs and tLDP sessions described in this document
   could also be used for the purpose of mLDP node protection, as
   described in  [draft-ietf-mpls-mldp-node-protection].

8. Security Considerations

   The same security considerations apply as those for the base LDP
   specification, as described in [RFC5036].

9. IANA Considerations

   This document introduces no new IANA Considerations.

10. Acknowledgements

   We are indebted to Yakov Rekhter for many discussions on this topic.
   We like to thank Hannes Gredler, Aman Kapoor, Minto Jeyananth, Eric
   Rosen, Vladimir Blazhkun and Loa Andersson for through review of this
   document.

11. Normative References

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

   [RFC3209] D. Awduche, et al., "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC3209, Decembet 2001.

   [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification",   RFC 5036, October 2007.

   [draft-ietf-mpls-app-aware-tldp] Esale, S., et al.,"Application-
              aware Targeted LDP", draft-esale-mpls-app-aware-tldp, work
              in progress.

12. Informative References




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   [RFC7715], IJ. Wijnands, et al., "Multipoint LDP (mLDP) Node
              Protection", RFC7715, January 2016.
Authors' Addresses

              Santosh Esale
              Juniper Networks
              EMail: sesale@juniper.net


              Raveendra Torvi
              Juniper Networks
              EMail: rtorvi@juniper.net


              Luyuan Fang
              Microsoft
              Email: lufang@microsoft.com


              Luay Jalil
              Verizon
              Email: luay.jalil@verizon.com





























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