Routing Area Working Group                                P. Sarkar, Ed.
Internet-Draft                                                H. Gredler
Intended status: Informational                                  S. Hegde
Expires: August 24, 2015                                       C. Bowers
                                                  Juniper Networks, Inc.
                                                             B. Decraene
                                                                  Orange
                                                       February 20, 2015


                 LFA selection for Multi-Homed Prefixes
              draft-psarkar-rtgwg-multihomed-prefix-lfa-01

Abstract

   This document shares experience gained from implementing algorithms
   to determine Loop-Free Alternates for multi-homed prefixes.  In
   particular, this document provides explicit inequalities that can be
   used to evaluate neighbors as a potential alternates for multi-homed
   prefixes.  It also provides detailed criteria for evaluating
   potential alternates for external prefixes advertised by OSPF ASBRs.

Requirements 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 [RFC2119].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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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."

   This Internet-Draft will expire on August 24, 2015.







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Copyright Notice

   Copyright (c) 2015 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
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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
   2.  LFA inequalities for MHPs . . . . . . . . . . . . . . . . . .   3
   3.  LFA selection for the multi-homed prefixes  . . . . . . . . .   4
   4.  LFA selection for the multi-homed external prefixes . . . . .   5
     4.1.  IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  OSPF  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Rules to select alternate ASBR  . . . . . . . . . . .   5
       4.2.2.  Multiple ASBRs belonging different area . . . . . . .   6
       4.2.3.  Type 1 and Type 2 costs . . . . . . . . . . . . . . .   7
       4.2.4.  RFC1583compatibility is set to enabled  . . . . . . .   7
       4.2.5.  Type 7 routes . . . . . . . . . . . . . . . . . . . .   7
       4.2.6.  Inequalities to be applied for alternate ASBR
               selection . . . . . . . . . . . . . . . . . . . . . .   7
         4.2.6.1.  Forwarding address set to non zero value  . . . .   7
         4.2.6.2.  ASBRs advertising type1 and type2 cost  . . . . .   8
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The use of Loop-Free Alternates (LFA) for IP Fast Reroute is
   specified in [RFC5286].  Section 6.1 of [RFC5286] describes a method
   to determine loop-free alternates for a multi-homed prefixes (MHPs).
   This document describes a procedure using explicit inequalities that
   can be used by a computing router to evaluate a neighbor as a
   potential alternate for a multi-homed prefix.  The results obtained



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   are equivalent to those obtained using the method described in
   Section 6.1 of [RFC5286].  However, some may find this formulation
   useful.

   Section 6.3 of [RFC5286] discusses complications associated with
   computing LFAs for multi-homed prefixes in OSPF.  This document
   provides detailed criteria for evaluating potential alternates for
   external prefixes advertised by OSPF ASBRs, as well as explicit
   inequalities.

2.  LFA inequalities for MHPs

   This document proposes the following set of LFA inequalities for
   selecting the most appropriate LFAs for multi-homed prefixes (MHPs).
   They can be derived from the inequalities in [RFC5286] combined with
   the observation that D_opt(N,P) = Min (D_opt(N,PO_i) + cost(PO_i,P))
   over all PO_i

   Link-Protection:
   D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,S) +
                                 D_opt(S,PO_best) + cost(PO_best,P)

   Link-Protection + Downstream-paths-only:
   D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(S,PO_best) + cost(PO_best,P)

   Node-Protection:
   D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
                                 D_opt(E,PO_best) + cost(PO_best,P)

   Where,
      S            - The computing router
      N            - The alternate router being evaluated
      E            - The primary next-hop on shortest path from S to
                     prefix P.
      PO_i         - The specific prefix-originating router being
                     evaluated.
      PO_best      - The prefix-originating router on the shortest path
                     from the computing router S to prefix P.
      Cost (X,P)   - Cost of reaching the prefix P from prefix
                    originating node X.
      D_opt(X,Y)   - Distance on the shortest path from node X to node
                     Y.

                    Figure 1: LFA inequalities for MHPs







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3.  LFA selection for the multi-homed prefixes

   To compute a valid LFA for a given multi-homed prefix P, through an
   alternate neighbor N a computing router S MUST follow one of the
   appropriate procedures below.

        Link-Protection :
        =================
        1. If alternate neighbor N is also prefix-originator of P,
           1.a. Select N as a LFA for prefix P (irrespective of
                the metric advertised by N for the prefix P).
        2. Else, evaluate the link-protecting LFA inequality for P with
           the N as the alternate neighbor.
           2.a. If LFA inequality condition is met,
                select N as a LFA for prefix P.
           2.b. Else, N is not a LFA for prefix P.

        Link-Protection + Downstream-paths-only :
        =========================================
        1. Evaluate the link-protecting + downstream-only LFA inequality
           for P with the N as the alternate neighbor.
           1.a. If LFA inequality condition is met,
                select N as a LFA for prefix P.
           1.b. Else, N is not a LFA for prefix P.

        Node-Protection :
        =================
        1. If alternate neighbor N is also prefix-originator of P,
           1.a. Select N as a LFA for prefix P (irrespective of
                the metric advertised by N for the prefix P).
        2. Else, evaluate the apporpriate node-protecting LFA inequality
           for P with the N as the alternate neighbor.
           2.a. If LFA inequality condition is met,
                select N as a LFA for prefix P.
           2.b. Else, N is not a LFA for prefix P.

                Figure 2: Rules for selecting LFA for MHPs

   In case an alternate neighbor N is also one of the prefix-originators
   of prefix P, N MAY be selected as a valid LFA for P.

   However if N is not a prefix-originator of P, the computing router
   SHOULD evaluate one of the corresponding LFA inequalities, as
   mentioned in Figure 1, once for each remote node that originated the
   prefix.  In case the inequality is satisfied by the neighbor N router
   S MUST choose neighbor N, as one of the valid LFAs for the prefix P.





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   When computing a dowstream-only LFA, in addition to being a prefix-
   originator of P, router N MUST also satisfy the downstream-only LFA
   inequality specified in Figure 1.

   For more specific rules please refer to the later sections of this
   document.

4.  LFA selection for the multi-homed external prefixes

   Redistribution of external routes into IGP is required in case of two
   different networks getting merged into one or during protocol
   migrations.  External routes could be distributed into an IGP domain
   via multiple nodes to avoid a single point of failure.

   During LFA calculation, alternate LFA next-hops to reach the best
   ASBR could be used as LFA for the routes redistributed via that ASBR.
   When there is no LFA available to the best ASBR, it may be desirable
   to consider the other ASBRs (referred to as alternate ASBR hereafter)
   redistributing the external routes for LFA selection as defined in
   [RFC5286] and leverage the advantage of having multiple re-
   distributing nodes in the network.

4.1.  IS-IS

   LFA evaluation for multi-homed external prefixes in IS-IS is similar
   to the multi-homed internal prefixes.  Inequalities described in sec
   2 would also apply to multi-homed external prefixes as well.

4.2.  OSPF

   Loop free Alternates [RFC 5286] describes mechanisms to apply
   inequalities to find the the loop free alternate neighbor.  For the
   selection of alternate ASBR for LFA consideration, additional rules
   have to be applied in selecting the alternate ASBR due to the
   external route calculation rules imposed by [RFC 2328].

   This document also defines the inequalities defined in RFC [5286]
   specifically for the alternate loop-free ASBR evaluation.

4.2.1.  Rules to select alternate ASBR

   The process to select an alternate ASBR is best explained using the
   rules below.  The below process is applied when primary ASBR for the
   concerned prefix is chosen and there is an alternate ASBR originating
   same prefix.






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   1. If RFC1583Compatibility is disabled

             1a. if primary ASBR and alternate ASBR are intra area
                     non-backbone path go to step 2.
             1b. If primary ASBR and alternate ASBR belong to
                     intra-area backbone and/or inter-area path go
                     to step 2.
             1c. for other paths, skip the alternate ASBR and
                     consider next ASBR.

   2. If cost type (type1/type2) advertised by alternate
      ASBR same as primary
           2a. If  not same skip alternate ASBR and consider next ASBR.

   3. If cost type is type1
             3a. If cost is same, program ECMP
             3b. else go to step 5.

   4  If cost type is type 2
             4a. If cost is different, skip alternate ASBR and
                     consider next ASBR
             4b. If type2 cost is same, compare type 1 cost.
             4c. If type1 cost is also same program ECMP.
             4d. If type 1 cost is different go to step 5.

   5. If route type (type 5/type 7)
              5a. If route type is same, check route p-bit,
                      forwarding address field for routes from both
                      ASBRs
                      match. If not skip alternate ASBR and consider
                      next ASBR.
              5b. If route type is not same, skip ASBR
                      and consider next ASBR.

    6. Apply inequality on the alternate ASBR.

           Figure 3: Rules for selecting alternate ASBR in OSPF

4.2.2.  Multiple ASBRs belonging different area

   When "RFC1583compatibility" is set to disabled, OSPF[RFC2328] defines
   certain rules of preference to choose the ASBRs.  While selecting
   alternate ASBR for loop evaluation for LFA, these rules should be
   applied and ensured that the alternate neighbor does not loop the
   traffic back.

   When there are multiple ASBRs belonging to different area advertising
   the same prefix, pruning rules as defined in RFC 2328 section 16.4.1



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   are applied.  The alternate ASBRs pruned using above rules are not
   considered for LFA evaluation.

4.2.3.  Type 1 and Type 2 costs

   If there are multiple ASBRs not pruned via rules defined in 3.2.2,
   the cost type advertised by the ASBRs is compared.  ASBRs advertising
   Type1 costs are preferred and the type2 costs are pruned.  If two
   ASBRs advertise same type2 cost, the alternate ASBRs are considered
   along with their type1 cost for evaluation.If the two ASBRs with same
   type2 as well as type1 cost, ECMP FRR is programmed.If there are two
   ASBRs with different type2 cost, the higher cost ASBR is pruned.The
   inequalities for evaluating alternate ASBR for type 1 and type 2
   costs are same, as the alternate ASBRs with different type2 costs are
   pruned and the evaluation is based on equal type 2 cost ASBRS.

4.2.4.  RFC1583compatibility is set to enabled

   When RFC1583Compatibility is set to enabled, multiple ASBRs belonging
   to different area advertising same prefix are chosen based on cost
   and hence are valid alternate ASBRs for the LFA evaluation.

4.2.5.  Type 7 routes

   Type 5 routes always get preference over Type 7 and the alternate
   ASBRs chosen for LFA calculation should belong to same type.Among
   Type 7 routes, routes with p-bit and forwarding address set have
   higher preference than routes without these attributes.  Alternate
   ASBRs selected for LFA comparison should have same p-bit and
   forwarding address attributes.

4.2.6.  Inequalities to be applied for alternate ASBR selection

   The alternate ASBRs selected using above mechanism described in
   3.2.1, are evaluated for Loop free criteria using below inequalities.

4.2.6.1.  Forwarding address set to non zero value














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  Link-Protection:
  F_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,S) +
                                F_opt(S,PO_best) + cost(PO_best,P)

  Link-Protection + Downstream-paths-only:
  F_opt(N,PO_i)+ cost(PO_i,P) < F_opt(S,PO_best) + cost(PO_best,P)

  Node-Protection:
  F_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
                                F_opt(E,PO_best) + cost(PO_best,P)

  Where,
  S            - The computing router
  N            - The alternate router being evaluated
  E            - The primary next-hop on shortest path from S to
                            prefix P.
  PO_i         - The specific prefix-originating router being
                            evaluated.
  PO_best      - The prefix-originating router on the shortest path
                            from the computing router S to prefix P.
  cost(X,Y)    - External cost for Y as advertised by X
  F_opt(X,Y)   - Distance on the shortest path from node X to Forwarding
                 address specified by ASBR Y.
  D_opt(X,Y)   - Distance on the shortest path from node X to node Y.


    Figure 4: LFA inequality definition when forwarding address in non-
                                   zero

4.2.6.2.  ASBRs advertising type1 and type2 cost





















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   Link-Protection:
   D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,S) +
                                 D_opt(S,PO_best) + cost(PO_best,P)

   Link-Protection + Downstream-paths-only:
   D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(S,PO_best) + cost(PO_best,P)

   Node-Protection:
   D_opt(N,PO_i)+ cost(PO_i,P) < D_opt(N,E) +
                                 D_opt(E,PO_best) + cost(PO_best,P)

   Where,
   S            - The computing router
   N            - The alternate router being evaluated
   E            - The primary next-hop on shortest path from S to
                             prefix P.
   PO_i         - The specific prefix-originating router being
                             evaluated.
   PO_best      - The prefix-originating router on the shortest path
                             from the computing router S to prefix P.
   cost(X,Y)    - External cost for Y as advertised by X.
   D_opt(X,Y)   - Distance on the shortest path from node X to node Y.


       Figure 5: LFA inequality definition for type1 and type 2 cost

5.  Acknowledgements

   Thanks to Alia Atlas and Salih K A for their useful feedback and
   inputs.

6.  IANA Considerations

   N/A. - No protocol changes are proposed in this document.

7.  Security Considerations

   This document does not introduce any change in any of the protocol
   specifications.  It simply proposes additional inequalities for
   selecting LFAs for multi-homed prefixes.

8.  References

8.1.  Normative References

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




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8.2.  Informative References

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

Authors' Addresses

   Pushpasis Sarkar (editor)
   Juniper Networks, Inc.
   Electra, Exora Business Park
   Bangalore, KA  560103
   India

   Email: psarkar@juniper.net


   Hannes Gredler
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: hannes@juniper.net


   Shraddha Hegde
   Juniper Networks, Inc.
   Electra, Exora Business Park
   Bangalore, KA  560103
   India

   Email: shraddha@juniper.net


   Chris Bowers
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: cbowers@juniper.net


   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com




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