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Versions: (draft-litkowski-rtgwg-uloop-delay) 00 01 02 03 04

Routing Area Working Group                                  S. Litkowski
Internet-Draft                                               B. Decraene
Intended status: Standards Track                                  Orange
Expires: October 8, 2017                                     C. Filsfils
                                                             P. Francois
                                                           Cisco Systems
                                                           April 6, 2017


     Micro-loop prevention by introducing a local convergence delay
                    draft-ietf-rtgwg-uloop-delay-04

Abstract

   This document describes a mechanism for link-state routing protocols
   to prevent local transient forwarding loops in case of link failure.
   This mechanism proposes a two-steps convergence by introducing a
   delay between the convergence of the node adjacent to the topology
   change and the network wide convergence.

   As this mechanism delays the IGP convergence it may only be used for
   planned maintenance or when fast reroute protects the traffic between
   the link failure time and the IGP convergence.

   The proposed mechanism will be limited to the link down event in
   order to keep simplicity.

   Simulations using real network topologies have been performed and
   show that local loops are a significant portion (>50%) of the total
   forwarding loops.

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].

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/.





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   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
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   This Internet-Draft will expire on October 8, 2017.

Copyright 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
   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
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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Transient forwarding loops side effects . . . . . . . . . . .   3
     2.1.  Fast reroute inefficiency . . . . . . . . . . . . . . . .   4
     2.2.  Network congestion  . . . . . . . . . . . . . . . . . . .   6
   3.  Overview of the solution  . . . . . . . . . . . . . . . . . .   7
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Current IGP reactions . . . . . . . . . . . . . . . . . .   8
     4.3.  Local events  . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Local delay for link down . . . . . . . . . . . . . . . .   9
   5.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Applicable case: local loops  . . . . . . . . . . . . . .   9
     5.2.  Non applicable case: remote loops . . . . . . . . . . . .  10
   6.  Simulations . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Deployment considerations . . . . . . . . . . . . . . . . . .  11
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Local link down . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  Local and remote event  . . . . . . . . . . . . . . . . .  15
     8.3.  Aborting local delay  . . . . . . . . . . . . . . . . . .  17
   9.  Comparison with other solutions . . . . . . . . . . . . . . .  19
     9.1.  PLSN  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.2.  OFIB  . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. Existing implementations  . . . . . . . . . . . . . . . . . .  20
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  21



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   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     14.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Micro-forwarding loops and some potential solutions are well
   described in [RFC5715].  This document describes a simple targeted
   mechanism that solves micro-loops that are local to the failure;
   based on network analysis, these are a significant portion of the
   micro-forwarding loops.  A simple and easily deployable solution for
   these local micro-loops is critical because these local loops cause
   some traffic loss after a fast-reroute alternate has been used (see
   Section 2.1).

   Consider the case in Figure 1 where S does not have an LFA to protect
   its traffic to D.  That means that all non-D neighbors of S on the
   topology will send to S any traffic destined to D if a neighbor did
   not, then that neighbor would be loop-free.  Regardless of the
   advanced fast-reroute (FRR) technique used, when S converges to the
   new topology, it will send its traffic to a neighbor that was not
   loop-free and thus cause a local micro-loop.  The deployment of
   advanced fast-reroute techniques motivates this simple router-local
   mechanism to solve this targeted problem.  This solution can be work
   with the various techniques described in [RFC5715].

             1
        D ------ C
        |        |
      1 |        | 5
        |        |
        S ------ B
             1
        Figure 1

   When S-D fails, a transient forwarding loop may appear between S and
   B if S updates its forwarding entry to D before B.

2.  Transient forwarding loops side effects

   Even if they are very limited in duration, transient forwarding loops
   may cause high damages for a network.






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2.1.  Fast reroute inefficiency

           D
         1 |
           |    1
           A ------ B
           |        |    ^
        10 |        | 5  | T
           |        |    |
           E--------C
           |    1
         1 |
           S

        Figure 2 - RSVP-TE FRR case

   In the Figure 2, an RSVP-TE tunnel T, provisioned on C and
   terminating on B, is used to protect against C-B link failure (IGP
   shortcut is activated on C).  The primary path of T is C->B and FRR
   is activated on T providing an FRR bypass or detour using path
   C->E->A->B.  On the router C, the nexthop to D is the tunnel T thanks
   to the IGP shortcut.  When C-B link fails:

   1.  C detects the failure, and updates the tunnel path using
       preprogrammed FRR path, the traffic path from S to D becomes:
       S->E->C->E->A->B->A->D.

   2.  In parallel, on router C, both the IGP convergence and the TE
       tunnel convergence (tunnel path recomputation) are occurring:

       *  The Tunnel T path is recomputed and now uses C->E->A->B.

       *  The IGP path to D is recomputed and now uses C->E->A->D.

   3.  On C, the tail-end of the TE tunnel (router B) is no more on the
       shortest-path tree (SPT) to D, so C does not encapsulate anymore
       the traffic to D using the tunnel T and updates its forwarding
       entry to D using the nexthop E.

   If C updates its forwarding entry to D before router E, there would
   be a transient forwarding loop between C and E until E has converged.

   +-----------+------------+------------------+-----------------------+
   |  Network  |    Time    | Router C events  |    Router E events    |
   | condition |            |                  |                       |
   +-----------+------------+------------------+-----------------------+
   |    S->D   |            |                  |                       |
   |  Traffic  |            |                  |                       |



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   |     OK    |            |                  |                       |
   |           |            |                  |                       |
   |    S->D   |     t0     |  Link B-C fails  |     Link B-C fails    |
   |  Traffic  |            |                  |                       |
   |    lost   |            |                  |                       |
   |           |            |                  |                       |
   |           | t0+20msec  |  C detects the   |                       |
   |           |            |     failure      |                       |
   |           |            |                  |                       |
   |    S->D   | t0+40msec  | C activates FRR  |                       |
   |  Traffic  |            |                  |                       |
   |     OK    |            |                  |                       |
   |           |            |                  |                       |
   |           | t0+50msec  |  C updates its   |                       |
   |           |            |  local LSP/LSA   |                       |
   |           |            |                  |                       |
   |           | t0+60msec  | C schedules SPF  |                       |
   |           |            |     (100ms)      |                       |
   |           |            |                  |                       |
   |           | t0+70msec  |   C floods its   |                       |
   |           |            |  local updated   |                       |
   |           |            |     LSP/LSA      |                       |
   |           |            |                  |                       |
   |           | t0+87msec  |                  |   E receives LSP/LSA  |
   |           |            |                  |  from C and schedules |
   |           |            |                  |      SPF (100ms)      |
   |           |            |                  |                       |
   |           | t0+117msec |                  | E floods LSP/LSA from |
   |           |            |                  |           C           |
   |           |            |                  |                       |
   |           | t0+160msec |  C computes SPF  |                       |
   |           |            |                  |                       |
   |           | t0+165msec |     C starts     |                       |
   |           |            |   updating its   |                       |
   |           |            |     RIB/FIB      |                       |
   |           |            |                  |                       |
   |           | t0+193msec |                  |     E computes SPF    |
   |           |            |                  |                       |
   |           | t0+199msec |                  | E starts updating its |
   |           |            |                  |        RIB/FIB        |
   |           |            |                  |                       |
   |    S->D   | t0+255msec |  C updates its   |                       |
   |  Traffic  |            |  RIB/FIB for D   |                       |
   |    lost   |            |                  |                       |
   |           |            |                  |                       |
   |           | t0+340msec |  C convergence   |                       |
   |           |            |       ends       |                       |
   |           |            |                  |                       |



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   |    S->D   | t0+443msec |                  | E updates its RIB/FIB |
   |  Traffic  |            |                  |         for D         |
   |     OK    |            |                  |                       |
   |           |            |                  |                       |
   |           | t0+470msec |                  |   E convergence ends  |
   +-----------+------------+------------------+-----------------------+

                    Route computation event time scale

   The issue described here is completely independent of the fast-
   reroute mechanism involved (TE FRR, LFA/rLFA, MRT ...).  The
   protection enabled by fast-reroute is working perfectly, but ensures
   a protection, by definition, only until the PLR has converged.  When
   implementing FRR, a service provider wants to guarantee a very
   limited loss of connectivity time.  The previous example shows that
   the benefit of FRR may be completely lost due to a transient
   forwarding loop appearing when PLR has converged.  Delaying FIB
   updates after the IGP convergence may allow to keep the fast-reroute
   path until the neighbors have converged and preserves the customer
   traffic.

2.2.  Network congestion

             1
        D ------ C
        |        |
      1 |        | 5
        |        |
   A -- S ------ B
      / |    1
     F  E

   In the figure above, as presented in Section 1, when the link S-D
   fails, a transient forwarding loop may appear between S and B for
   destination D.  The traffic on the S-B link will constantly increase
   due to the looping traffic to D.  Depending on the TTL of the
   packets, the traffic rate destinated to D and the bandwidth of the
   link, the S-B link may be congested in few hundreds of milliseconds
   and will stay overloaded until the loop is solved.

   The congestion introduced by transient forwarding loops is
   problematic as it is impacting traffic that is not directly concerned
   by the failing network component.  In our example, the congestion of
   the S-B link will impact some customer traffic that is not directly
   concerned by the failure: e.g.  A to B, F to B, E to B.  Some class
   of services may be implemented to mitigate the congestion, but some
   traffic not directly concerned by the failure would still be dropped




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   as a router is not able to identify the looping traffic from the
   normally forwarded traffic.

3.  Overview of the solution

   This document defines a two-step convergence initiated by the router
   detecting the failure and advertising the topological changes in the
   IGP.  This introduces a delay between the convergence of the local
   router and the network wide convergence.

   The proposed solution is kept limited to local link down events for
   simplicity reason.

   This ordered convergence, is similar to the ordered FIB proposed
   defined in [RFC6976], but limited to only a "one hop" distance.  As a
   consequence, it is simpler and becomes a local only feature not
   requiring interoperability; at the cost of only covering the
   transient forwarding loops involving this local router.  The proposed
   mechanism also reuses some concept described in
   [I-D.ietf-rtgwg-microloop-analysis] with some limitations.

4.  Specification

4.1.  Definitions

   This document will refer to the following existing IGP timers:

   o  LSP_GEN_TIMER: The delay used to batch multiple local events in
      one single local LSP/LSA update.  It is often associated with a
      damping mechanism to slow down reactions by incrementing the timer
      when multiple consecutive events are detected.

   o  SPF_DELAY: The delay between the first IGP event triggering a new
      routing table computation and the start of that routing table
      computation.  It is often associated with a damping mechanism to
      slow down reactions by incrementing the timer when the IGP becomes
      unstable.  As an example, [I-D.ietf-rtgwg-backoff-algo] defines a
      standard SPF delay algorithm.

   This document introduces the following new timer:

   o  ULOOP_DELAY_DOWN_TIMER: used to slow down the local node
      convergence in case of link down events.








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4.2.  Current IGP reactions

   Upon a change of the status of an adjacency/link, the existing
   behavior of the router advertising the event is the following:

   1.  The Up/Down event is notified to the IGP.

   2.  The IGP processes the notification and postpones the reaction in
       LSP_GEN_TIMER msec.

   3.  Upon LSP_GEN_TIMER expiration, the IGP updates its LSP/LSA and
       floods it.

   4.  The SPF computation is scheduled in SPF_DELAY msec.

   5.  Upon SPF_DELAY expiration, the SPF is computed, then the RIB and
       FIB are updated.

4.3.  Local events

   The mechanism described in this document assumes that there has been
   a single link failure as seen by the IGP area/level.  If this
   assumption is violated (e.g. multiple links or nodes failed), then
   standard IP convergence MUST be applied (as described in
   Section 4.2).

   To determine if the mechanism can be applicable or not, an
   implementation SHOULD implement a logic to correlate the protocol
   messages (LSP/LSA) received during the SPF scheduling period in order
   to determine the topology changes that occured.  This is necessary as
   multiple protocol messages may describe the same topology change and
   a single protocol message may describe multiple topology changes.  As
   a consequence, determining a particular topology change MUST be
   independent of the order of reception of those protocol messages.
   How the logic works is let to implementation details.

   Using this logic, if an implementation determines that the associated
   topology change is a single local link failure, then the router MAY
   use the mechanism described in this document, otherwise the standard
   IP convergence MUST be used.

   Example:

          +--- E ----+--------+
          |          |        |
   A ---- B -------- C ------ D





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   Let router B be the computing router when the link B-C fails.  B
   updates its local LSP/LSA describing the link B->C as down, C does
   the same, and both start flooding their updated LSP/LSAs.  During the
   SPF_DELAY period, B and C learn all the LSPs/LSAs to consider.  B
   sees that C is flooding as down a link where B is the other end and
   that B and C are describing the same single event.  Since B receives
   no other changes, B can determine that this is a local link failure
   and may decide to activate the mechanism described in this document.

4.4.  Local delay for link down

   Upon an adjacency/link down event, this document introduces a change
   in step 5 (Section 4.2) in order to delay the local convergence
   compared to the network wide convergence: the node SHOULD delay the
   forwarding entry updates by ULOOP_DELAY_DOWN_TIMER.  Such delay
   SHOULD only be introduced if all the LSDB modifications processed are
   only reporting a single local link down event (Section 4.3).  If a
   subsequent LSP/LSA is received/updated and a new SPF computation is
   triggered before the expiration of ULOOP_DELAY_DOWN_TIMER, then the
   same evaluation SHOULD be performed.

   As a result of this addition, routers local to the failure will
   converge slower than remote routers.  Hence it SHOULD only be done
   for a non-urgent convergence, such as for administrative de-
   activation (maintenance) or when the traffic is protected by fast-
   reroute.

5.  Applicability

   As previously stated, the mechanism only avoids the forwarding loops
   on the links between the node local to the failure and its neighbor.
   Forwarding loops may still occur on other links.

5.1.  Applicable case: local loops

        A ------ B ----- E
        |              / |
        |             /  |
    G---D------------C   F        All the links have a metric of 1

        Figure 2

   Let us consider the traffic from G to F.  The primary path is
   G->D->C->E->F.  When link C-E fails, if C updates its forwarding
   entry for F before D, a transient loop occurs.  This is sub-optimal
   as C has FRR enabled and it breaks the FRR forwarding while all
   upstream routers are still forwarding the traffic to itself.




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   By implementing the mechanism defined in this document on C, when the
   C-E link fails, C delays the update of its forwarding entry to F, in
   order to let some time for D to converge.  FRR keeps protecting the
   traffic during this period.  When the timer expires on C, its
   forwarding entry to F is updated.  There is no transient forwarding
   loop on the link C-D.

5.2.  Non applicable case: remote loops

        A ------ B ----- E --- H
        |                      |
        |                      |
    G---D--------C ------F --- J ---- K

    All the links have a metric of 1 except BE=15

        Figure 3

   Let us consider the traffic from G to K.  The primary path is
   G->D->C->F->J->K.  When the C-F link fails, if C updates its
   forwarding entry to K before D, a transient loop occurs between C and
   D.

   By implementing the mechanism defined in this document on C, when the
   link C-F fails, C delays the update of its forwarding entry to K,
   letting time for D to converge.  When the timer expires on C, its
   forwarding entry to F is updated.  There is no transient forwarding
   loop between C and D.  However, a transient forwarding loop may still
   occur between D and A.  In this scenario, this mechanism is not
   enough to address all the possible forwarding loops.  However, it
   does not create additional traffic loss.  Besides, in some cases
   -such as when the nodes update their FIB in the following order C, A,
   D, for example because the router A is quicker than D to converge-
   the mechanism may still avoid the forwarding loop that was occurring.

6.  Simulations

   Simulations have been run on multiple service provider topologies.













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                            +----------+------+
                            | Topology | Gain |
                            +----------+------+
                            |    T1    | 71%  |
                            |    T2    | 81%  |
                            |    T3    | 62%  |
                            |    T4    | 50%  |
                            |    T5    | 70%  |
                            |    T6    | 70%  |
                            |    T7    | 59%  |
                            |    T8    | 77%  |
                            +----------+------+

                Table 1: Number of Repair/Dst that may loop

   We evaluated the efficiency of the mechanism on eight different
   service provider topologies (different network size, design).  The
   benefit is displayed in the table above.  The benefit is evaluated as
   follows:

   o  We consider a tuple (link A-B, destination D, PLR S, backup
      nexthop N) as a loop if upon link A-B failure, the flow from a
      router S upstream from A (A could be considered as PLR also) to D
      may loop due to convergence time difference between S and one of
      his neighbor N.

   o  We evaluate the number of potential loop tuples in normal
      conditions.

   o  We evaluate the number of potential loop tuples using the same
      topological input but taking into account that S converges after
      N.

   o  The gain is how much loops (remote and local) we succeed to
      suppress.

   On topology 1, 71% of the transient forwarding loops created by the
   failure of any link are prevented by implementing the local delay.
   The analysis shows that all local loops are obviously solved and only
   remote loops are remaining.

7.  Deployment considerations

   Transient forwarding loops have the following drawbacks:

   o  They limit FRR efficiency: even if FRR is activated in 50msec, as
      soon as PLR has converged, the traffic may be affected by a
      transient loop.



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   o  They may impact traffic not directly concerned by the failure (due
      to link congestion).

   This local delay proposal is a transient forwarding loop avoidance
   mechanism (like OFIB).  Even if it only addresses local transient
   loops, the efficiency versus complexity comparison of the mechanism
   makes it a good solution.  It is also incrementally deployable with
   incremental benefits, which makes it an attractive option for both
   vendors to implement and Service Providers to deploy.  Delaying the
   convergence time is not an issue if we consider that the traffic is
   protected during the convergence.

8.  Examples

   We will consider the following figure for the associated examples :

           D
         1 |        F----X
           |    1   |
           A ------ B
           |        |    ^
        10 |        | 5  | T
           |        |    |
           E--------C
           |    1
         1 |
           S

   The network above is considered to have a convergence time about 1
   second, so ULOOP_DELAY_DOWN_TIMER will be adjusted to this value.  We
   also consider that FRR is running on each node.

8.1.  Local link down

   The table below describes the events and associating timing that
   happens on router C and E when link B-C goes down.  As C detects a
   single local event corresponding to a link down (its LSP + LSP from B
   received), it decides to apply the local delay down behavior and no
   microloop is formed.

   +-----------+-------------+------------------+----------------------+
   |  Network  |     Time    | Router C events  |   Router E events    |
   | condition |             |                  |                      |
   +-----------+-------------+------------------+----------------------+
   |    S->D   |             |                  |                      |
   |  Traffic  |             |                  |                      |
   |     OK    |             |                  |                      |
   |           |             |                  |                      |



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   |    S->D   |      t0     |  Link B-C fails  |    Link B-C fails    |
   |  Traffic  |             |                  |                      |
   |    lost   |             |                  |                      |
   |           |             |                  |                      |
   |           |  t0+20msec  |  C detects the   |                      |
   |           |             |     failure      |                      |
   |           |             |                  |                      |
   |    S->D   |  t0+40msec  | C activates FRR  |                      |
   |  Traffic  |             |                  |                      |
   |     OK    |             |                  |                      |
   |           |             |                  |                      |
   |           |  t0+50msec  |  C updates its   |                      |
   |           |             |  local LSP/LSA   |                      |
   |           |             |                  |                      |
   |           |  t0+60msec  | C schedules SPF  |                      |
   |           |             |     (100ms)      |                      |
   |           |             |                  |                      |
   |           |  t0+67msec  |    C receives    |                      |
   |           |             |  LSP/LSA from B  |                      |
   |           |             |                  |                      |
   |           |  t0+70msec  |   C floods its   |                      |
   |           |             |  local updated   |                      |
   |           |             |     LSP/LSA      |                      |
   |           |             |                  |                      |
   |           |  t0+87msec  |                  |  E receives LSP/LSA  |
   |           |             |                  | from C and schedules |
   |           |             |                  |     SPF (100ms)      |
   |           |             |                  |                      |
   |           |  t0+117msec |                  |   E floods LSP/LSA   |
   |           |             |                  |        from C        |
   |           |             |                  |                      |
   |           |  t0+160msec |  C computes SPF  |                      |
   |           |             |                  |                      |
   |           |  t0+165msec |   C delays its   |                      |
   |           |             |  RIB/FIB update  |                      |
   |           |             |     (1 sec)      |                      |
   |           |             |                  |                      |
   |           |  t0+193msec |                  |    E computes SPF    |
   |           |             |                  |                      |
   |           |  t0+199msec |                  |  E starts updating   |
   |           |             |                  |     its RIB/FIB      |
   |           |             |                  |                      |
   |           |  t0+443msec |                  |    E updates its     |
   |           |             |                  |    RIB/FIB for D     |
   |           |             |                  |                      |
   |           |  t0+470msec |                  |  E convergence ends  |
   |           |             |                  |                      |
   |           | t0+1165msec |     C starts     |                      |



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   |           |             |   updating its   |                      |
   |           |             |     RIB/FIB      |                      |
   |           |             |                  |                      |
   |           | t0+1255msec |  C updates its   |                      |
   |           |             |  RIB/FIB for D   |                      |
   |           |             |                  |                      |
   |           | t0+1340msec |  C convergence   |                      |
   |           |             |       ends       |                      |
   +-----------+-------------+------------------+----------------------+

                    Route computation event time scale

   Similarly, upon B-C link down event, if LSP/LSA from B is received
   before C detects the link failure, C will apply the route update
   delay if the local detection is part of the same SPF run.

   +-----------+-------------+------------------+----------------------+
   |  Network  |     Time    | Router C events  |   Router E events    |
   | condition |             |                  |                      |
   +-----------+-------------+------------------+----------------------+
   |    S->D   |             |                  |                      |
   |  Traffic  |             |                  |                      |
   |     OK    |             |                  |                      |
   |           |             |                  |                      |
   |    S->D   |      t0     |  Link B-C fails  |    Link B-C fails    |
   |  Traffic  |             |                  |                      |
   |    lost   |             |                  |                      |
   |           |             |                  |                      |
   |           |  t0+32msec  |    C receives    |                      |
   |           |             |  LSP/LSA from B  |                      |
   |           |             |                  |                      |
   |           |  t0+33msec  | C schedules SPF  |                      |
   |           |             |     (100ms)      |                      |
   |           |             |                  |                      |
   |           |  t0+50msec  |  C detects the   |                      |
   |           |             |     failure      |                      |
   |           |             |                  |                      |
   |    S->D   |  t0+55msec  | C activates FRR  |                      |
   |  Traffic  |             |                  |                      |
   |     OK    |             |                  |                      |
   |           |             |                  |                      |
   |           |  t0+55msec  |  C updates its   |                      |
   |           |             |  local LSP/LSA   |                      |
   |           |             |                  |                      |
   |           |  t0+70msec  |   C floods its   |                      |
   |           |             |  local updated   |                      |
   |           |             |     LSP/LSA      |                      |
   |           |             |                  |                      |



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   |           |  t0+87msec  |                  |  E receives LSP/LSA  |
   |           |             |                  | from C and schedules |
   |           |             |                  |     SPF (100ms)      |
   |           |             |                  |                      |
   |           |  t0+117msec |                  |   E floods LSP/LSA   |
   |           |             |                  |        from C        |
   |           |             |                  |                      |
   |           |  t0+160msec |  C computes SPF  |                      |
   |           |             |                  |                      |
   |           |  t0+165msec |   C delays its   |                      |
   |           |             |  RIB/FIB update  |                      |
   |           |             |     (1 sec)      |                      |
   |           |             |                  |                      |
   |           |  t0+193msec |                  |    E computes SPF    |
   |           |             |                  |                      |
   |           |  t0+199msec |                  |  E starts updating   |
   |           |             |                  |     its RIB/FIB      |
   |           |             |                  |                      |
   |           |  t0+443msec |                  |    E updates its     |
   |           |             |                  |    RIB/FIB for D     |
   |           |             |                  |                      |
   |           |  t0+470msec |                  |  E convergence ends  |
   |           |             |                  |                      |
   |           | t0+1165msec |     C starts     |                      |
   |           |             |   updating its   |                      |
   |           |             |     RIB/FIB      |                      |
   |           |             |                  |                      |
   |           | t0+1255msec |  C updates its   |                      |
   |           |             |  RIB/FIB for D   |                      |
   |           |             |                  |                      |
   |           | t0+1340msec |  C convergence   |                      |
   |           |             |       ends       |                      |
   +-----------+-------------+------------------+----------------------+

                    Route computation event time scale

8.2.  Local and remote event

   The table below describes the events and associating timing that
   happens on router C and E when link B-C goes down, in addition F-X
   link will fail in the same time window.  C will not apply the local
   delay because a non local topology change is also received.

   +-----------+------------+-----------------+------------------------+
   |  Network  |    Time    | Router C events |    Router E events     |
   | condition |            |                 |                        |
   +-----------+------------+-----------------+------------------------+
   |    S->D   |            |                 |                        |



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   |  Traffic  |            |                 |                        |
   |     OK    |            |                 |                        |
   |           |            |                 |                        |
   |    S->D   |     t0     |  Link B-C fails |     Link B-C fails     |
   |  Traffic  |            |                 |                        |
   |    lost   |            |                 |                        |
   |           |            |                 |                        |
   |           | t0+20msec  |  C detects the  |                        |
   |           |            |     failure     |                        |
   |           |            |                 |                        |
   |           | t0+36msec  |  Link F-X fails |     Link F-X fails     |
   |           |            |                 |                        |
   |    S->D   | t0+40msec  | C activates FRR |                        |
   |  Traffic  |            |                 |                        |
   |     OK    |            |                 |                        |
   |           |            |                 |                        |
   |           | t0+50msec  |  C updates its  |                        |
   |           |            |  local LSP/LSA  |                        |
   |           |            |                 |                        |
   |           | t0+54msec  |    C receives   |                        |
   |           |            |  LSP/LSA from F |                        |
   |           |            |  and floods it  |                        |
   |           |            |                 |                        |
   |           | t0+60msec  | C schedules SPF |                        |
   |           |            |     (100ms)     |                        |
   |           |            |                 |                        |
   |           | t0+67msec  |    C receives   |                        |
   |           |            |  LSP/LSA from B |                        |
   |           |            |                 |                        |
   |           | t0+69msec  |                 |   E receives LSP/LSA   |
   |           |            |                 | from F, floods it and  |
   |           |            |                 | schedules SPF (100ms)  |
   |           |            |                 |                        |
   |           | t0+70msec  |   C floods its  |                        |
   |           |            |  local updated  |                        |
   |           |            |     LSP/LSA     |                        |
   |           |            |                 |                        |
   |           | t0+87msec  |                 |   E receives LSP/LSA   |
   |           |            |                 |         from C         |
   |           |            |                 |                        |
   |           | t0+117msec |                 | E floods LSP/LSA from  |
   |           |            |                 |           C            |
   |           |            |                 |                        |
   |           | t0+160msec |  C computes SPF |                        |
   |           |            |                 |                        |
   |           | t0+165msec |     C starts    |                        |
   |           |            |   updating its  |                        |
   |           |            |   RIB/FIB (NO   |                        |



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   |           |            |      DELAY)     |                        |
   |           |            |                 |                        |
   |           | t0+170msec |                 |     E computes SPF     |
   |           |            |                 |                        |
   |           | t0+173msec |                 | E starts updating its  |
   |           |            |                 |        RIB/FIB         |
   |           |            |                 |                        |
   |    S->D   | t0+365msec |  C updates its  |                        |
   |  Traffic  |            |  RIB/FIB for D  |                        |
   |    lost   |            |                 |                        |
   |           |            |                 |                        |
   |    S->D   | t0+443msec |                 | E updates its RIB/FIB  |
   |  Traffic  |            |                 |         for D          |
   |     OK    |            |                 |                        |
   |           |            |                 |                        |
   |           | t0+450msec |  C convergence  |                        |
   |           |            |       ends      |                        |
   |           |            |                 |                        |
   |           | t0+470msec |                 |   E convergence ends   |
   |           |            |                 |                        |
   +-----------+------------+-----------------+------------------------+

                    Route computation event time scale

8.3.  Aborting local delay

   The table below describes the events and associating timing that
   happens on router C and E when link B-C goes down, in addition F-X
   link will fail during local delay run.  C will first apply local
   delay, but when the new event happens, it will fall back to the
   standard convergence mechanism without delaying route insertion
   anymore.  In this example, we consider a ULOOP_DELAY_DOWN_TIMER
   configured to 2 seconds.

   +-----------+------------+-------------------+----------------------+
   |  Network  |    Time    |  Router C events  |   Router E events    |
   | condition |            |                   |                      |
   +-----------+------------+-------------------+----------------------+
   |    S->D   |            |                   |                      |
   |  Traffic  |            |                   |                      |
   |     OK    |            |                   |                      |
   |           |            |                   |                      |
   |    S->D   |     t0     |   Link B-C fails  |    Link B-C fails    |
   |  Traffic  |            |                   |                      |
   |    lost   |            |                   |                      |
   |           |            |                   |                      |
   |           | t0+20msec  |   C detects the   |                      |
   |           |            |      failure      |                      |



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   |           |            |                   |                      |
   |    S->D   | t0+40msec  |  C activates FRR  |                      |
   |  Traffic  |            |                   |                      |
   |     OK    |            |                   |                      |
   |           |            |                   |                      |
   |           | t0+50msec  |   C updates its   |                      |
   |           |            |   local LSP/LSA   |                      |
   |           |            |                   |                      |
   |           | t0+60msec  |  C schedules SPF  |                      |
   |           |            |      (100ms)      |                      |
   |           |            |                   |                      |
   |           | t0+67msec  |     C receives    |                      |
   |           |            |   LSP/LSA from B  |                      |
   |           |            |                   |                      |
   |           | t0+70msec  |    C floods its   |                      |
   |           |            |   local updated   |                      |
   |           |            |      LSP/LSA      |                      |
   |           |            |                   |                      |
   |           | t0+87msec  |                   |  E receives LSP/LSA  |
   |           |            |                   | from C and schedules |
   |           |            |                   |     SPF (100ms)      |
   |           |            |                   |                      |
   |           | t0+117msec |                   |   E floods LSP/LSA   |
   |           |            |                   |        from C        |
   |           |            |                   |                      |
   |           | t0+160msec |   C computes SPF  |                      |
   |           |            |                   |                      |
   |           | t0+165msec |    C delays its   |                      |
   |           |            | RIB/FIB update (2 |                      |
   |           |            |        sec)       |                      |
   |           |            |                   |                      |
   |           | t0+193msec |                   |    E computes SPF    |
   |           |            |                   |                      |
   |           | t0+199msec |                   |  E starts updating   |
   |           |            |                   |     its RIB/FIB      |
   |           |            |                   |                      |
   |           | t0+254msec |   Link F-X fails  |    Link F-X fails    |
   |           |            |                   |                      |
   |           | t0+300msec |     C receives    |                      |
   |           |            |   LSP/LSA from F  |                      |
   |           |            |   and floods it   |                      |
   |           |            |                   |                      |
   |           | t0+303msec |  C schedules SPF  |                      |
   |           |            |      (200ms)      |                      |
   |           |            |                   |                      |
   |           | t0+312msec |     E receives    |                      |
   |           |            |   LSP/LSA from F  |                      |
   |           |            |   and floods it   |                      |



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   |           |            |                   |                      |
   |           | t0+313msec |  E schedules SPF  |                      |
   |           |            |      (200ms)      |                      |
   |           |            |                   |                      |
   |           | t0+502msec |   C computes SPF  |                      |
   |           |            |                   |                      |
   |           | t0+505msec | C starts updating |                      |
   |           |            |  its RIB/FIB (NO  |                      |
   |           |            |       DELAY)      |                      |
   |           |            |                   |                      |
   |           | t0+514msec |                   |    E computes SPF    |
   |           |            |                   |                      |
   |           | t0+519msec |                   |  E starts updating   |
   |           |            |                   |     its RIB/FIB      |
   |           |            |                   |                      |
   |    S->D   | t0+659msec |   C updates its   |                      |
   |  Traffic  |            |   RIB/FIB for D   |                      |
   |    lost   |            |                   |                      |
   |           |            |                   |                      |
   |    S->D   | t0+778msec |                   |    E updates its     |
   |  Traffic  |            |                   |    RIB/FIB for D     |
   |     OK    |            |                   |                      |
   |           |            |                   |                      |
   |           | t0+781msec |   C convergence   |                      |
   |           |            |        ends       |                      |
   |           |            |                   |                      |
   |           | t0+810msec |                   |  E convergence ends  |
   +-----------+------------+-------------------+----------------------+

                    Route computation event time scale

9.  Comparison with other solutions

   As stated in Section 3, our solution reuses some concepts already
   introduced by other IETF proposals but tries to find a tradeoff
   between efficiency and simplicity.  This section tries to compare
   behaviors of the solutions.

9.1.  PLSN

   PLSN ([I-D.ietf-rtgwg-microloop-analysis]) describes a mechanism
   where each node in the network tries to avoid transient forwarding
   loops upon a topology change by always keeping traffic on a loop-free
   path for a defined duration (locked path to a safe neighbor).  The
   locked path may be the new primary nexthop, another neighbor, or the
   old primary nexthop depending how the safety condition is satisfied.





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   PLSN does not solve all transient forwarding loops (see
   [I-D.ietf-rtgwg-microloop-analysis] Section 4 for more details).

   Our solution reuses some concept of PLSN but in a more simple
   fashion:

   o  PLSN has three different behaviors: keep using old nexthop, use
      new primary nexthop if it is safe, or use another safe nexthop,
      while our solution only have one: keep using the current nexthop
      (old primary, or already activated FRR path).

   o  PLSN may cause some damage while using a safe nexthop which is not
      the new primary nexthop in case the new safe nexthop does not
      enough provide enough bandwidth (see [RFC7916]).  Our solution may
      not experience this issue as the service provider may have control
      on the FRR path being used preventing network congestion.

   o  PLSN applies to all nodes in a network (remote or local changes),
      while our mechanism applies only on the nodes connected to the
      topology change.

9.2.  OFIB

   OFIB ([RFC6976]) describes a mechanism where the convergence of the
   network upon a topology change is made ordered to prevent transient
   forwarding loops.  Each router in the network must deduce the failure
   type from the LSA/LSP received and computes/applies a specific FIB
   update timer based on the failure type and its rank in the network
   considering the failure point as root.

   This mechanism allows to solve all the transient forwarding loop in a
   network at the price of introducing complexity in the convergence
   process that may require a strong monitoring by the service provider.

   Our solution reuses the OFIB concept but limits it to the first hop
   that experiences the topology change.  As demonstrated, our proposal
   allows to solve all the local transient forwarding loops that
   represents an high percentage of all the loops.  Moreover limiting
   the mechanism to one hop allows to keep the network-wide convergence
   behavior.

10.  Existing implementations

   At this time, there are three different implementations of this
   mechanism: CISCO IOS-XR, CISCO IOS-XE and Juniper JUNOS.  The three
   implementations have been tested in labs and demonstrated a good
   behavior in term of local micro-loop avoidance.  The feature has also




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   been deployed in some live networks.  No side effects have been
   found.

11.  Security Considerations

   This document does not introduce any change in term of IGP security.
   The operation is internal to the router.  The local delay does not
   increase the attack vector as an attacker could only trigger this
   mechanism if he already has be ability to disable or enable an IGP
   link.  The local delay does not increase the negative consequences as
   if an attacker has the ability to disable or enable an IGP link, it
   can already harm the network by creating instability and harm the
   traffic by creating forwarding packet loss and forwarding loss for
   the traffic crossing that link.

12.  Acknowledgements

   We would like to thanks the authors of [RFC6976] for introducing the
   concept of ordered convergence: Mike Shand, Stewart Bryant, Stefano
   Previdi, and Olivier Bonaventure.

13.  IANA Considerations

   This document has no actions for IANA.

14.  References

14.1.  Normative References

   [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>.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, January
              2010, <http://www.rfc-editor.org/info/rfc5715>.

14.2.  Informative References

   [I-D.ietf-rtgwg-backoff-algo]
              Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
              Francois, P., and C. Bowers, "SPF Back-off algorithm for
              link state IGPs", draft-ietf-rtgwg-backoff-algo-04 (work
              in progress), January 2017.






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   [I-D.ietf-rtgwg-microloop-analysis]
              Zinin, A., "Analysis and Minimization of Microloops in
              Link-state Routing Protocols", draft-ietf-rtgwg-microloop-
              analysis-01 (work in progress), October 2005.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <http://www.rfc-editor.org/info/rfc3630>.

   [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
              B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
              <http://www.rfc-editor.org/info/rfc6571>.

   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
              2013, <http://www.rfc-editor.org/info/rfc6976>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <http://www.rfc-editor.org/info/rfc7490>.

   [RFC7916]  Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar, "Operational Management of
              Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916,
              July 2016, <http://www.rfc-editor.org/info/rfc7916>.

Authors' Addresses

   Stephane Litkowski
   Orange

   Email: stephane.litkowski@orange.com


   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com







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   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Pierre Francois
   Cisco Systems

   Email: pifranco@cisco.com









































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