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Versions: 00 01 02 03 draft-ietf-rtgwg-multihomed-prefix-lfa

Working Group                                                U. Chunduri
Internet-Draft                                               J. Tantsura
Intended status: Informational                             Ericsson Inc.
Expires: March 12, 2016                                        C. Bowers
                                                        Juniper Networks
                                                       September 9, 2015


    Extended procedures and considerations for evaluating Loop-Free
                               Alternates
            draft-chunduri-rtgwg-lfa-extended-procedures-03

Abstract

   This document provide few clarifications and extended procedures to
   IP Fast Reroute using Loop-Free Alternates as defined in RFC 5286.

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
   and may be updated, replaced, or obsoleted by other documents at any
   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 March 12, 2016.

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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  LFA Extended Procedures . . . . . . . . . . . . . . . . . . .   3
     2.1.  Multi Homed Prefixes  . . . . . . . . . . . . . . . . . .   3
       2.1.1.  IS-IS ATT Bit considerations  . . . . . . . . . . . .   5
     2.2.  Links with IGP MAX_METRIC . . . . . . . . . . . . . . . .   5
     2.3.  Multi Topology Considerations . . . . . . . . . . . . . .   6
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Loop Free Alternatives (LFAs) as defined in [RFC5286] have been
   widely deployed, and the operational and manageability considerations
   are described in great detail in [I-D.ietf-rtgwg-lfa-manageability].

   This document intends to provide clarifications, additional
   considerations to [RFC5286], to address a few coverage and
   operational observations.  These observations are in the area of
   handling Muti-homed prefixes (MHPs), IS-IS attach (ATT) bit in L1
   area, links provisioned with MAX_METRIC for traffic engineering (TE)
   purposes and in the area of Multi Topology (MT) IGP deployments.  All
   these are elaborated in detail in Section 2.

1.1.  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 RFC 2119 [RFC2119].

1.2.  Acronyms

   AF      -  Address Family

   ATT     -  IS-IS Attach Bit

   ECMP    -  Equal Cost Multi Path

   IGP     -  Interior Gateway Protocol




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   IS-IS   -  Intermediate System to Intermediate System

   OSPF    -  Open Shortest Path First

   MHP     -  Multi-homed Prefix

   MT      -  Multi Topology

   SPF     -  Shortest Path First PDU

2.  LFA Extended Procedures

   This section explains the additional considerations in various
   aspects as listed below to the base LFA specification [RFC5286].

2.1.  Multi Homed Prefixes

   LFA base specification [RFC5286] Section 6.1 recommends that a router
   compute the alternate next-hop for an IGP multi-homed prefix by
   considering alternate paths via all routers that have announced that
   prefix.  However, it also allows for the router to simplify the
   multi-homed prefix calculation by assuming that the MHP is solely
   attached to the router that was its pre-failure optimal point of
   attachment, at the expense of potentially lower coverage.  If an
   implementation chooses to simplify the multi-homed prefix calculation
   by assuming that the MHP is solely attached to the router that was
   its pre-failure optimal point of attachment, the procedure described
   in this memo can potentially improve coverage for equal cost multi
   path (ECMP) MHPs without incurring extra computational cost.

   The approach as specified in [RFC5286] Section 6.1 last paragraph, is
   to simplify the MHP is solely attached to the router that was its
   pre-failure optimal point of attachment.  While this is very scalable
   approach and simplifies computation, as [RFC5286] notes this may
   result in little less coverage.

   This memo improves the above approach to provide loop-free
   alternatives without any additional cost for equal cost multi path
   MHPs as described through the below example network.  The approach
   specified here MAY also applicable for handling default routes as
   explained in Section 2.1.1.










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                         5   +---+  8   +---+  5  +---+
                       +-----| S |------| A |-----| B |
                       |     +---+      +---+     +---+
                       |       |                    |
                       |     5 |                  5 |
                       |       |                    |
                     +---+ 5 +---+   4 +---+  1    +---+
                     | C |---| E |-----| M |-------| F |
                     +---+   +---+     +---+       +---+
                               |   10           5    |
                               +-----------p---------+

                   Figure 1: MHP with same ECMP Next-hop

   In the above network a prefix p, is advertised from both Node E and
   Node F.  With simplified approach taken as specified in [RFC5286]
   Section 6.1, prefix p will get only link protection LFA through the
   neighbor C while a node protection path is available through neighbor
   A.  In this scenario, E and F both are pre-failure optimal points of
   attachment and share the same primary next-hop.  Hence, an
   implementation MAY compare the kind of protection A provides to F
   (link-and-node protection) with the kind of protection C provides to
   E (link protection) and inherit the better alternative to prefix p
   and here it is A.

   However, in the below network prefix p has an ECMP through both node
   E and node F with cost 20.  Though it has 2 pre-failure optimal
   points of attachment, the primary next-hop to each pre-failure
   optimal point of attachment is different.  In this case, prefix p
   shall inherit corresponding LFA to each primary next-hop calculated
   for the router advertising the same respectively (node E's and node
   F's LFA).


                             +---+         3      +---+
                             | S |----------------| B |
                             +---+                +---+
                               |                    |
                            10 |                  1 |
                               |                    |
                             +---+        6        +---+
                             | E |-----------------| F |
                             +---+                 +---+
                               |   10          16    |
                               +-----------p---------+

                Figure 2: MHP with different ECMP Next-hops




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   In summary, if there are multiple pre-failure points of attachment
   for a MHP and primary next-hop of a MHP is same as that of the
   primary next-hop of the router that was pre-failure optimal point of
   attachment, an implementation MAY provide the better protection to
   MHP without incurring any additional computation cost.

2.1.1.  IS-IS ATT Bit considerations

   Per [RFC1195] a default route needs to be added in Level1 (L1) router
   to the closest reachable Level1/Level2 (L1/L2) router in the network
   advertising ATT (attach) bit in its LSP-0 fragment.  All L1 routers
   in the area would do this during the decision process with the next-
   hop of the default route set to the adjacent router through which the
   closest L1/L2 router is reachable.  The base LFA specification
   [RFC5286] does not specify any procedure for computing LFA for a
   default route in IS-IS L1 area.  Potentially one MAY consider a
   default route is being advertised from the boarder L1/L2 router where
   ATT bit is set and can do LFA computation for the default route.
   But, when multiple ECMP L1/L2 routers are reachable in an L1 area
   corresponding best LFAs SHOULD be given for each primary next-hop
   associated with default route.  Considerations as specified in
   Section 2.1 are applicable for default routes, if the default route
   is considered as ECMP MHP.

2.2.  Links with IGP MAX_METRIC

   Section 3.5 and 3.6 of [RFC5286] describes procedures for excluding
   nodes and links from use in alternate paths based on the maximum link
   metric (as defined in for IS-IS in [RFC5305] or as defined in
   [RFC3137] for OSPF).  If these procedures are strictly followed,
   there are situations, as described below, where the only potential
   alternate available which satisfies the basic loop-free condition
   will not be considered as alternative.


















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                             +---+  10  +---+  10 +---+
                             | S |------|N1 |-----|D1 |
                             +---+      +---+     +---+
                               |                    |
                            10 |                 10 |
                               |MAX_MET(N2 to S)    |
                               |                    |
                               |       +---+        |
                               +-------|N2 |--------+
                                       +---+
                                     10  |
                                       +---+
                                       |D2 |
                                       +---+


                    Figure 3: Link with IGP MAX_METRIC

   In the simple example network, all the link costs have a cost of 10
   in both directions, except for the link between S and N2.  The S-N2
   link has a cost of 10 in the direction from S to N2, and a cost of
   MAX_METRIC in the direction from N2 to S (0xffffff /2^24 - 1 for IS-
   IS and 0xffff for OSPF) for a specific end to end Traffic Engineering
   (TE) requirement of the operator.  At node S, D1 is reachable through
   N1 with cost 20, and D2 is reachable through N2 with cost 20.  Even
   though neighbor N2 satisfies basic loop-free condition (inequality 1
   of [RFC5286]) for D1 this could be excluded as potential alternative
   because of the current exclusions as specified in section 3.5 and 3.6
   procedure of [RFC5286].  But, as the primary traffic destined to D2
   is continue to use the link and hence irrespective of the reverse
   metric in this case, the same link MAY be used as a potential LFA for
   D1.

   Alternatively, reverse metric of the link MAY be configured with
   MAX_METRIC-1, so that the link can be used as an alternative while
   meeting the TE requirements.

2.3.  Multi Topology Considerations

   Section 6.2 and 6.3.2 of [RFC5286] state that multi-topology OSPF and
   ISIS are out of scope for that specification.  This memo clarifies
   and describes the applicability.

   In Multi Topology (MT) IGP deployments, for each MT ID, a separate
   shortest path tree (SPT) is built with topology specific adjacencies,
   the LFA principles laid out in [RFC5286] are actually applicable for
   MT IS-IS [RFC5120] LFA SPF.  The primary difference in this case is,
   identifying the eligible-set of neighbors for each LFA computation



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   which is done per MT ID.  The eligible-set for each MT ID is
   determined by the presence of IGP adjacency from Source to the
   neighboring node on that MT-ID apart from the administrative
   restrictions and other checks laid out in [RFC5286].  The same is
   also applicable for OSPF [RFC4915] [MT-OSPF] or different AFs in
   multi instance OSPFv3 [RFC5838].

   However for MT IS-IS, if a default topology is used with MT-ID 0
   [RFC5286] and both IPv4 [RFC5305] and IPv6 routes/AFs [RFC5308]  are
   present, then the condition of network congruency is applicable for
   LFA computation as well.  Network congruency here refers to, having
   same address families provisioned on all the links and all the nodes
   of the network with MT-ID 0.  Here with single decision process both
   IPv4 and IPv6 next-hops are computed for all the prefixes in the
   network and similarly with one LFA computation from all eligible
   neighbors per [RFC5286], all potential alternatives can be computed.

3.  IANA Considerations

   This document defines no new namespaces and no actions for IANA.

4.  Security Considerations

   This document does not introduce any new security issues or any
   change in security considerations as noted in the LFA base
   specification [RFC5286].

5.  Acknowledgements

   Authors would like to thank Alia Atlas for detailed review of initial
   document and providing valuable suggestions.  We also thank Bruno
   Decreane, Stephane Litkowski for their initial review and feedback on
   the document.

6.  References

6.1.  Normative References

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <http://www.rfc-editor.org/info/rfc1195>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.





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   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <http://www.rfc-editor.org/info/rfc5286>.

6.2.  Informative References

   [I-D.ietf-rtgwg-lfa-manageability]
              Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar, "Operational management of
              Loop Free Alternates", draft-ietf-rtgwg-lfa-
              manageability-11 (work in progress), June 2015.

   [RFC3137]  Retana, A., Nguyen, L., White, R., Zinin, A., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 3137,
              DOI 10.17487/RFC3137, June 2001,
              <http://www.rfc-editor.org/info/rfc3137>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <http://www.rfc-editor.org/info/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <http://www.rfc-editor.org/info/rfc5120>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <http://www.rfc-editor.org/info/rfc5305>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <http://www.rfc-editor.org/info/rfc5308>.

   [RFC5838]  Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
              R. Aggarwal, "Support of Address Families in OSPFv3",
              RFC 5838, DOI 10.17487/RFC5838, April 2010,
              <http://www.rfc-editor.org/info/rfc5838>.

Authors' Addresses








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   Uma Chunduri
   Ericsson Inc.
   300 Holger Way,
   San Jose, California  95134
   USA

   Phone: 408 750-5678
   Email: uma.chunduri@ericsson.com


   Jeff Tantsura
   Ericsson Inc.
   300 Holger Way,
   San Jose, California  95134
   USA

   Email: jeff.tantsura@ericsson.com


   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, California  94089
   USA

   Email: cbowers@juniper.net

























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