





Internet Engineering Task Force (IETF)                      S. Litkowski
Request for Comments: 8333                                   B. Decraene
Category: Standards Track                                         Orange
ISSN: 2070-1721                                              C. Filsfils
                                                           Cisco Systems
                                                             P. Francois
                                                  Individual Contributor
                                                              March 2018


     Micro-loop Prevention by Introducing a Local Convergence Delay

Abstract

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

   Because this mechanism delays the IGP convergence, it may only be
   used for planned maintenance or when Fast Reroute (FRR) protects the
   traffic during the time between the link failure and the IGP
   convergence.

   The mechanism is limited to the link-down event in order to keep the
   mechanism simple.

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8333.






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Copyright Notice

   Copyright (c) 2018 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
   (https://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.





































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Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................4
      2.1. Acronyms ...................................................4
      2.2. Requirements Language ......................................5
   3. Side Effects of Transient Forwarding Loops ......................5
      3.1. FRR Inefficiency ...........................................5
      3.2. Network Congestion .........................................8
   4. Overview of the Solution ........................................9
   5. Specification ...................................................9
      5.1. Definitions ................................................9
      5.2. Regular IGP Reaction ......................................10
      5.3. Local Events ..............................................10
      5.4. Local Delay for Link-Down Events ..........................11
   6. Applicability ..................................................11
      6.1. Applicable Case: Local Loops ..............................12
      6.2. Non-applicable Case: Remote Loops .........................12
   7. Simulations ....................................................13
   8. Deployment Considerations ......................................14
   9. Examples .......................................................15
      9.1. Local Link-Down Event .....................................15
      9.2. Local and Remote Event ....................................19
      9.3. Aborting Local Delay ......................................21
   10. Comparison with Other Solutions ...............................23
      10.1. PLSN .....................................................23
      10.2. oFIB .....................................................24
   11. IANA Considerations ...........................................24
   12. Security Considerations .......................................24
   13. References ....................................................25
      13.1. Normative References .....................................25
      13.2. Informative References ...................................25
   Acknowledgements ..................................................26
   Authors' Addresses ................................................26

















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RFC 8333          Micro-loop Prevention by Local Delay        March 2018


1.  Introduction

   Micro-loops and some potential solutions are described in [RFC5715].
   This document describes a simple targeted mechanism that prevents
   micro-loops that are local to the failure.  Based on network
   analysis, local micro-loops make up a significant portion of the
   micro-loops.  A simple and easily deployable solution for these local
   micro-loops is critical because these local loops cause some traffic
   loss after an FRR alternate has been used (see Section 3.1).

   Consider the case in Figure 1 where S does not have an LFA (Loop-Free
   Alternate) to protect its traffic to D when the S-D link fails.  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 FRR technique used,
   when S converges to the new topology, it will send its traffic to a
   neighbor that is not loop-free and will thus cause a local micro-
   loop.  The deployment of advanced FRR techniques motivates this
   simple router-local mechanism to solve this targeted problem.  This
   solution can work with the various techniques described in [RFC5715].

                                  D ------ C
                                  |        |
                                  |        | 5
                                  |        |
                                  S ------ B

                                 Figure 1

   In Figure 1, all links have a metric of 1 except the B-C link, which
   has a metric of 5.  When the S-D link fails, a transient forwarding
   loop may appear between S and B if S updates its forwarding entry to
   D before B does.

2.  Terminology

2.1.  Acronyms

   FIB: Forwarding Information Base

   FRR: Fast Reroute

   IGP: Interior Gateway Protocol

   LFA: Loop-Free Alternate

   LSA: Link State Advertisement




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   LSP: Link State Packet

   MRT: Maximally Redundant Tree

   oFIB: Ordered FIB

   PLR: Point of Local Repair

   PLSN: Path Locking via Safe Neighbors

   RIB: Routing Information Base

   RLFA: Remote Loop-Free Alternate

   SPF: Shortest Path First

   TTL: Time to Live

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Side Effects of Transient Forwarding Loops

   Even if they are very limited in duration, transient forwarding loops
   may cause significant network damage.

3.1.  FRR Inefficiency

   In Figure 2, we consider an IP/LDP routed network.

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

                                 Figure 2



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   An RSVP-TE tunnel T, provisioned on C and terminating on B, is used
   to protect the traffic against C-B link failure (the IGP shortcut
   feature, defined in [RFC3906], 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 router C, the next hop to D is the
   tunnel T, thanks to the IGP shortcut.  When the C-B link fails:

   1.  C detects the failure and updates the tunnel path using a
       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 longer on
       the shortest-path tree (SPT) to D, so C does not continue to
       encapsulate the traffic to D using the tunnel T and updates its
       forwarding entry to D using the next-hop 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.

   Table 1 describes a theoretical sequence of events happening when the
   B-C link fails.  This theoretical sequence of events should only be
   read as an example.

   +------------+--------+---------------------+-----------------------+
   |  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+20  |    C detects the    |                       |
   |            |   ms   |       failure       |                       |
   |            |        |                     |                       |







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   |    S->D    | t0+40  |   C activates FRR   |                       |
   | Traffic OK |   ms   |                     |                       |
   |            |        |                     |                       |
   |            | t0+50  | C updates its local |                       |
   |            |   ms   |       LSP/LSA       |                       |
   |            |        |                     |                       |
   |            | t0+60  |  C floods its local |                       |
   |            |   ms   |   updated LSP/LSA   |                       |
   |            |        |                     |                       |
   |            | t0+62  |   C schedules SPF   |                       |
   |            |   ms   |       (100 ms)      |                       |
   |            |        |                     |                       |
   |            | t0+87  |                     |   E receives LSP/LSA  |
   |            |   ms   |                     |  from C and floods it |
   |            |        |                     |                       |
   |            | t0+92  |                     |  E schedules SPF (100 |
   |            |   ms   |                     |          ms)          |
   |            |        |                     |                       |
   |            | t0+163 |    C computes SPF   |                       |
   |            |   ms   |                     |                       |
   |            |        |                     |                       |
   |            | t0+165 |  C starts updating  |                       |
   |            |   ms   |     its RIB/FIB     |                       |
   |            |        |                     |                       |
   |            | t0+193 |                     |     E computes SPF    |
   |            |   ms   |                     |                       |
   |            |        |                     |                       |
   |            | t0+199 |                     | E starts updating its |
   |            |   ms   |                     |        RIB/FIB        |
   |            |        |                     |                       |
   |    S->D    | t0+255 |    C updates its    |                       |
   |  Traffic   |   ms   |    RIB/FIB for D    |                       |
   |    lost    |        |                     |                       |
   |            |        |                     |                       |
   |            | t0+340 |  C convergence ends |                       |
   |            |   ms   |                     |                       |
   |            |        |                     |                       |
   |    S->D    | t0+443 |                     | E updates its RIB/FIB |
   | Traffic OK |   ms   |                     |         for D         |
   |            |        |                     |                       |
   |            | t0+470 |                     |   E convergence ends  |
   |            |   ms   |                     |                       |
   +------------+--------+---------------------+-----------------------+

                                  Table 1






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   The issue described here is completely independent of the FRR
   mechanism involved (e.g., TE FRR, LFA/RLFA, MRT, etc.) when the
   primary path uses hop-by-hop routing.  The protection enabled by FRR
   works perfectly but only ensures protection until the PLR has
   converged (as soon as the PLR has converged, it replaces its FRR path
   with a new primary path).  When implementing FRR, a service provider
   wants to guarantee a very limited loss of connectivity time.  The
   example described in this section 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
   (1) may allow the FRR path to be kept until the neighbors have
   converged and (2) preserves the customer traffic.

3.2.  Network Congestion

   In Figure 3, when the S-D link 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 destined to D,
   and the bandwidth of the link, the S-B link may become congested in a
   few hundreds of milliseconds and will stay congested until the loop
   is eliminated.

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

                                 Figure 3

   The congestion introduced by transient forwarding loops is
   problematic as it can affect traffic that is not directly affected by
   the failing network component.  In Figure 3, the congestion of the
   S-B link will impact some customer traffic that is not directly
   affected by the failure, e.g., traffic from A to B, F to B, and E to
   B.  Class of service may mitigate the congestion for some traffic.
   However, some traffic not directly affected by the failure will still
   be dropped as a router is not able to distinguish the looping traffic
   from the normally forwarded traffic.








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4.  Overview of the Solution

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

   The solution described in this document is limited to local link-down
   events in order to keep the solution simple.

   This ordered convergence is similar to the ordered FIB (oFIB)
   approach defined in [RFC6976], but it is limited to only a "one-hop"
   distance.  As a consequence, it is more simple and becomes a local-
   only feature that does not require interoperability.  This benefit
   comes with the limitation of eliminating transient forwarding loops
   involving the local router only.  The mechanism also reuses some
   concepts described in [PLSN].

5.  Specification

5.1.  Definitions

   This document refers to the following existing IGP timers.  These
   timers may be standardized or implemented as a vendor-specific local
   feature.

   o  LSP_GEN_TIMER: The delay between the consecutive generation of two
      local LSPs/LSAs.  From an operational point of view, this delay is
      usually tuned to batch multiple local events in a single local
      LSP/LSA update.  In IS-IS, this timer is defined as
      minimumLSPGenerationInterval [ISO10589].  In OSPF version 2, this
      timer is defined as MinLSInterval [RFC2328].  It is often
      associated with a vendor-specific 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, [BACKOFF] defines a standard SPF delay
      algorithm.









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   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.

5.2.  Regular IGP Reaction

   When the status of an adjacency or link changes, the regular IGP
   convergence behavior of the router advertising the event involves the
   following main steps:

   1.  IGP is notified of the up/down event.

   2.  The IGP processes the notification and postpones the reaction for
       LSP_GEN_TIMER ms.

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

   4.  The SPF computation is scheduled in SPF_DELAY ms.

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

5.3.  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
   regular IP convergence must be applied (as described in Section 5.2).

   To determine if the mechanism is applicable or not, an implementation
   SHOULD implement logic to correlate the protocol messages (LSP/LSA)
   received during the SPF scheduling period in order to determine the
   topology changes that occurred.  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 left to the implementation.

   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 regular
   IP convergence MUST be used.






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   In Figure 4, 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 LSPs/LSAs.
   During the SPF_DELAY period, B and C learn all the LSPs/LSAs to
   consider.  B sees that C is flooding an advertisement that indicates
   that a link is down, and B is the other end of that link.  B
   determines 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.

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

                                 Figure 4

5.4.  Local Delay for Link-Down Events

   This document introduces a change in step 5 (see list in Section 5.2)
   so that, upon an adjacency or link-down event, the local convergence
   is delayed compared to the network-wide convergence.  The new step 5
   is described below:

   5.  Upon SPF_DELAY timer expiration, the SPF is computed.  If the
       condition of a single local link-down event has been met, then an
       update of the RIB and the FIB MUST be delayed for
       ULOOP_DELAY_DOWN_TIMER ms.  Otherwise, the RIB and FIB SHOULD be
       updated immediately.

   If a new convergence occurs while ULOOP_DELAY_DOWN_TIMER is running,
   ULOOP_DELAY_DOWN_TIMER is stopped, and the RIB/FIB SHOULD be updated
   as part of the new convergence event.

   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 administrative deactivation
   (maintenance) or when the traffic is protected by FRR.

6.  Applicability

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







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6.1.  Applicable Case: Local Loops

   In Figure 5, let us consider the traffic from G to F.  The primary
   path is G->D->C->E->F.  When the link C-E fails, if C updates its
   forwarding entry for F before D, a transient loop occurs.  This is
   sub-optimal as it breaks C's FRR forwarding even though upstream
   routers are still forwarding the traffic to C.

                          A ------ B ----- E
                          |              / |
                          |             /  |
                      G---D------------C   F

                      All the links have a metric of 1

                                 Figure 5

   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 allow some time for D to converge.  FRR on C keeps
   protecting the traffic during this period.  When
   ULOOP_DELAY_DOWN_TIMER expires on C, its forwarding entry to F is
   updated.  There is no transient forwarding loop on the link C-D.

6.2.  Non-applicable Case: Remote Loops

   In Figure 6, 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.

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

               All the links have a metric of 1 except B-E=15

                                 Figure 6

   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,
   allowing time for D to converge.  When ULOOP_DELAY_DOWN_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



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   some cases -- such as when the nodes update their FIB in the order C,
   A, D because the router A is quicker than D to converge -- the
   mechanism may still avoid the forwarding loop that would have
   otherwise occurred.

7.  Simulations

   Simulations have been run on multiple service-provider topologies.
   We evaluated the efficiency of the mechanism on eight different
   service-provider topologies (different network size and design).
   Table 2 displays the gain for each topology.

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

                                  Table 2

   We evaluated the gain as follows:

   o  We considered a tuple (link A-B, destination D, PLR S, backup
      next-hop 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
      its neighbors N.

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

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

   o  The gain is the relative number of loops (both remote and local)
      we succeed in suppressing.







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

8.  Deployment Considerations

   Transient forwarding loops have the following drawbacks:

   o  They limit FRR efficiency.  Even if FRR is activated within 50 ms,
      as soon as the PLR has converged, the traffic may be affected by a
      transient loop.

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

   The local delay mechanism 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.

   The ULOOP_DELAY_DOWN_TIMER value should be set according to the
   maximum IGP convergence time observed in the network (usually
   observed in the slowest node).

   This mechanism is limited to link-down events.  When a link goes
   down, it eventually goes back up.  As a consequence, with this
   mechanism deployed, only the link-down event will be protected
   against transient forwarding loops while the link-up event will not.
   If the operator wants to limit the impact of transient forwarding
   loops during the link-up event, it should make sure to use specific
   procedures to bring the link back online.  As examples, the operator
   can decide to put the link back online outside of business hours, or
   it can use some incremental metric changes to prevent loops (as
   proposed in [RFC5715]).












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9.  Examples

   We consider the following figure for the examples in this section:

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

                                 Figure 7

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

9.1.  Local Link-Down Event

   Table 3 describes the events and their timing on routers C and E when
   the link B-C goes down.  It is based on a theoretical sequence of
   events that should only been read as an example.  As C detects a
   single local event corresponding to a link-down event (its LSP + LSP
   from B received), it applies the local delay down behavior, and no
   micro-loop is formed.





















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   +------------+---------+---------------------+----------------------+
   |  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+20  |    C detects the    |                      |
   |            |    ms   |       failure       |                      |
   |            |         |                     |                      |
   |    S->D    |  t0+40  |   C activates FRR   |                      |
   | Traffic OK |    ms   |                     |                      |
   |            |         |                     |                      |
   |            |  t0+50  | C updates its local |                      |
   |            |    ms   |       LSP/LSA       |                      |
   |            |         |                     |                      |
   |            |  t0+53  |  C floods its local |                      |
   |            |    ms   |   updated LSP/LSA   |                      |
   |            |         |                     |                      |
   |            |  t0+60  |   C schedules SPF   |                      |
   |            |    ms   |       (100 ms)      |                      |
   |            |         |                     |                      |
   |            |  t0+67  |  C receives LSP/LSA |                      |
   |            |    ms   |  from B and floods  |                      |
   |            |         |          it         |                      |
   |            |         |                     |                      |
   |            |  t0+87  |                     |  E receives LSP/LSA  |
   |            |    ms   |                     | from C and floods it |
   |            |         |                     |                      |
   |            |  t0+90  |                     | E schedules SPF (100 |
   |            |    ms   |                     |         ms)          |
   |            |         |                     |                      |
   |            |  t0+161 |    C computes SPF   |                      |
   |            |    ms   |                     |                      |
   |            |         |                     |                      |
   |            |  t0+165 |     C delays its    |                      |
   |            |    ms   |  RIB/FIB update (1  |                      |
   |            |         |         sec)        |                      |
   |            |         |                     |                      |
   |            |  t0+193 |                     |    E computes SPF    |
   |            |    ms   |                     |                      |
   |            |         |                     |                      |
   |            |  t0+199 |                     |  E starts updating   |
   |            |    ms   |                     |     its RIB/FIB      |



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RFC 8333          Micro-loop Prevention by Local Delay        March 2018


   |            |         |                     |                      |
   |            |  t0+443 |                     |    E updates its     |
   |            |    ms   |                     |    RIB/FIB for D     |
   |            |         |                     |                      |
   |            |  t0+470 |                     |  E convergence ends  |
   |            |    ms   |                     |                      |
   |            |         |                     |                      |
   |            | t0+1165 |  C starts updating  |                      |
   |            |    ms   |     its RIB/FIB     |                      |
   |            |         |                     |                      |
   |            | t0+1255 |    C updates its    |                      |
   |            |    ms   |    RIB/FIB for D    |                      |
   |            |         |                     |                      |
   |            | t0+1340 |  C convergence ends |                      |
   |            |    ms   |                     |                      |
   +------------+---------+---------------------+----------------------+

                                  Table 3

   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.  Table 4
   describes the associated theoretical sequence of events.  It should
   only been read as an example.

   +------------+---------+---------------------+----------------------+
   |  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+32  |  C receives LSP/LSA |                      |
   |            |    ms   |  from B and floods  |                      |
   |            |         |          it         |                      |
   |            |         |                     |                      |
   |            |  t0+33  |   C schedules SPF   |                      |
   |            |    ms   |       (100 ms)      |                      |
   |            |         |                     |                      |
   |            |  t0+50  |    C detects the    |                      |
   |            |    ms   |       failure       |                      |
   |            |         |                     |                      |





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   |    S->D    |  t0+55  |   C activates FRR   |                      |
   | Traffic OK |    ms   |                     |                      |
   |            |         |                     |                      |
   |            |  t0+55  | C updates its local |                      |
   |            |    ms   |       LSP/LSA       |                      |
   |            |         |                     |                      |
   |            |  t0+70  |  C floods its local |                      |
   |            |    ms   |   updated LSP/LSA   |                      |
   |            |         |                     |                      |
   |            |  t0+87  |                     |  E receives LSP/LSA  |
   |            |    ms   |                     | from C and floods it |
   |            |         |                     |                      |
   |            |  t0+90  |                     | E schedules SPF (100 |
   |            |    ms   |                     |         ms)          |
   |            |         |                     |                      |
   |            |  t0+135 |    C computes SPF   |                      |
   |            |    ms   |                     |                      |
   |            |         |                     |                      |
   |            |  t0+140 |     C delays its    |                      |
   |            |    ms   |  RIB/FIB update (1  |                      |
   |            |         |         sec)        |                      |
   |            |         |                     |                      |
   |            |  t0+193 |                     |    E computes SPF    |
   |            |    ms   |                     |                      |
   |            |         |                     |                      |
   |            |  t0+199 |                     |  E starts updating   |
   |            |    ms   |                     |     its RIB/FIB      |
   |            |         |                     |                      |
   |            |  t0+443 |                     |    E updates its     |
   |            |    ms   |                     |    RIB/FIB for D     |
   |            |         |                     |                      |
   |            |  t0+470 |                     |  E convergence ends  |
   |            |    ms   |                     |                      |
   |            |         |                     |                      |
   |            | t0+1145 |  C starts updating  |                      |
   |            |    ms   |     its RIB/FIB     |                      |
   |            |         |                     |                      |
   |            | t0+1255 |    C updates its    |                      |
   |            |    ms   |    RIB/FIB for D    |                      |
   |            |         |                     |                      |
   |            | t0+1340 |  C convergence ends |                      |
   |            |    ms   |                     |                      |
   +------------+---------+---------------------+----------------------+

                                  Table 4






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9.2.  Local and Remote Event

   Table 5 describes the events and their timing on router C and E when
   the link B-C goes down and when the link F-X fails in the same time
   window.  C will not apply the local delay because a non-local
   topology change is also received.  Table 5 is based on a theoretical
   sequence of events that should only been read as an example.

   +-----------+--------+-------------------+--------------------------+
   |  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+20  |   C detects the   |                          |
   |           |   ms   |      failure      |                          |
   |           |        |                   |                          |
   |           | t0+36  |   Link F-X fails  |      Link F-X fails      |
   |           |   ms   |                   |                          |
   |           |        |                   |                          |
   |    S->D   | t0+40  |  C activates FRR  |                          |
   |  Traffic  |   ms   |                   |                          |
   |     OK    |        |                   |                          |
   |           |        |                   |                          |
   |           | t0+50  |   C updates its   |                          |
   |           |   ms   |   local LSP/LSA   |                          |
   |           |        |                   |                          |
   |           | t0+54  |     C receives    |                          |
   |           |   ms   |   LSP/LSA from F  |                          |
   |           |        |   and floods it   |                          |
   |           |        |                   |                          |
   |           | t0+60  |  C schedules SPF  |                          |
   |           |   ms   |      (100 ms)     |                          |
   |           |        |                   |                          |
   |           | t0+67  |     C receives    |                          |
   |           |   ms   |   LSP/LSA from B  |                          |
   |           |        |   and floods it   |                          |
   |           |        |                   |                          |
   |           | t0+69  |                   | E receives LSP/LSA from  |
   |           |   ms   |                   |     F, floods it and     |
   |           |        |                   |  schedules SPF (100 ms)  |
   |           |        |                   |                          |



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   |           | t0+70  |    C floods its   |                          |
   |           |   ms   |   local updated   |                          |
   |           |        |      LSP/LSA      |                          |
   |           |        |                   |                          |
   |           | t0+87  |                   | E receives LSP/LSA from  |
   |           |   ms   |                   |            C             |
   |           |        |                   |                          |
   |           | t0+117 |                   | E floods LSP/LSA from C  |
   |           |   ms   |                   |                          |
   |           |        |                   |                          |
   |           | t0+160 |   C computes SPF  |                          |
   |           |   ms   |                   |                          |
   |           |        |                   |                          |
   |           | t0+165 | C starts updating |                          |
   |           |   ms   |  its RIB/FIB (NO  |                          |
   |           |        |       DELAY)      |                          |
   |           |        |                   |                          |
   |           | t0+170 |                   |      E computes SPF      |
   |           |   ms   |                   |                          |
   |           |        |                   |                          |
   |           | t0+173 |                   |  E starts updating its   |
   |           |   ms   |                   |         RIB/FIB          |
   |           |        |                   |                          |
   |    S->D   | t0+365 |   C updates its   |                          |
   |  Traffic  |   ms   |   RIB/FIB for D   |                          |
   |    lost   |        |                   |                          |
   |           |        |                   |                          |
   |    S->D   | t0+443 |                   |  E updates its RIB/FIB   |
   |  Traffic  |   ms   |                   |          for D           |
   |     OK    |        |                   |                          |
   |           |        |                   |                          |
   |           | t0+450 |   C convergence   |                          |
   |           |   ms   |        ends       |                          |
   |           |        |                   |                          |
   |           | t0+470 |                   |    E convergence ends    |
   |           |   ms   |                   |                          |
   |           |        |                   |                          |
   +-----------+--------+-------------------+--------------------------+

                                  Table 5











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9.3.  Aborting Local Delay

   Table 6 describes the events and their timing on routers C and E when
   the link B-C goes down.  In addition, we consider what happens when
   the F-X link fails during local delay of the FIB update.  C will
   first apply the local delay, but when the new event happens, it will
   fall back to the standard convergence mechanism without further
   delaying route insertion.  In this example, we consider a
   ULOOP_DELAY_DOWN_TIMER configured to 2 seconds.  Table 6 is based on
   a theoretical sequence of events that should only been read as an
   example.

   +------------+--------+----------------------+----------------------+
   |  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+20  |    C detects the     |                      |
   |            |   ms   |       failure        |                      |
   |            |        |                      |                      |
   |    S->D    | t0+40  |   C activates FRR    |                      |
   | Traffic OK |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+50  | C updates its local  |                      |
   |            |   ms   |       LSP/LSA        |                      |
   |            |        |                      |                      |
   |            | t0+55  |  C floods its local  |                      |
   |            |   ms   |   updated LSP/LSA    |                      |
   |            |        |                      |                      |
   |            | t0+57  | C schedules SPF (100 |                      |
   |            |   ms   |         ms)          |                      |
   |            |        |                      |                      |
   |            | t0+67  |  C receives LSP/LSA  |                      |
   |            |   ms   | from B and floods it |                      |
   |            |        |                      |                      |
   |            | t0+87  |                      |  E receives LSP/LSA  |
   |            |   ms   |                      | from C and floods it |
   |            |        |                      |                      |
   |            | t0+90  |                      | E schedules SPF (100 |
   |            |   ms   |                      |         ms)          |
   |            |        |                      |                      |




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   |            | t0+160 |    C computes SPF    |                      |
   |            |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+165 | C delays its RIB/FIB |                      |
   |            |   ms   |    update (2 sec)    |                      |
   |            |        |                      |                      |
   |            | t0+193 |                      |    E computes SPF    |
   |            |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+199 |                      |  E starts updating   |
   |            |   ms   |                      |     its RIB/FIB      |
   |            |        |                      |                      |
   |            | t0+254 |    Link F-X fails    |    Link F-X fails    |
   |            |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+300 |  C receives LSP/LSA  |                      |
   |            |   ms   | from F and floods it |                      |
   |            |        |                      |                      |
   |            | t0+303 | C schedules SPF (200 |                      |
   |            |   ms   |         ms)          |                      |
   |            |        |                      |                      |
   |            | t0+312 |  E receives LSP/LSA  |                      |
   |            |   ms   | from F and floods it |                      |
   |            |        |                      |                      |
   |            | t0+313 | E schedules SPF (200 |                      |
   |            |   ms   |         ms)          |                      |
   |            |        |                      |                      |
   |            | t0+502 |    C computes SPF    |                      |
   |            |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+505 |  C starts updating   |                      |
   |            |   ms   |   its RIB/FIB (NO    |                      |
   |            |        |        DELAY)        |                      |
   |            |        |                      |                      |
   |            | t0+514 |                      |    E computes SPF    |
   |            |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+519 |                      |  E starts updating   |
   |            |   ms   |                      |     its RIB/FIB      |
   |            |        |                      |                      |
   |    S->D    | t0+659 |    C updates its     |                      |
   |  Traffic   |   ms   |    RIB/FIB for D     |                      |
   |    lost    |        |                      |                      |
   |            |        |                      |                      |







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   |    S->D    | t0+778 |                      |    E updates its     |
   | Traffic OK |   ms   |                      |    RIB/FIB for D     |
   |            |        |                      |                      |
   |            | t0+781 |  C convergence ends  |                      |
   |            |   ms   |                      |                      |
   |            |        |                      |                      |
   |            | t0+810 |                      |  E convergence ends  |
   |            |   ms   |                      |                      |
   +------------+--------+----------------------+----------------------+

                                  Table 6

10.  Comparison with Other Solutions

   As stated in Section 4, the local delay solution reuses some concepts
   already introduced by other IETF proposals but tries to find a trade-
   off between efficiency and simplicity.  This section tries to compare
   behaviors of the solutions.

10.1.  PLSN

   PLSN [PLSN] 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 next hop, another neighbor, or the old primary next hop
   depending on how the safety condition is satisfied.

   PLSN does not solve all transient forwarding loops (see Section 4 of
   [PLSN] for more details).

   The solution defined in this document reuses some concepts of PLSN
   but in a more simple fashion:

   o  PLSN has three different behaviors: (1) keep using the old next
      hop, (2) use the new primary next hop if it is safe, or (3) use
      another safe next hop.  The local delay solution, however, only
      has one: keep using the current next hop (i.e., the old primary
      next hop or an already-activated FRR path).

   o  PLSN may cause some damage while using a safe next hop that is not
      the new primary next hop if the new safe next hop does not provide
      enough bandwidth (see [RFC7916]).  The solution defined in this
      document may not experience this issue as the service provider may
      have control on the FRR path being used, preventing network
      congestion.





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

10.2.  oFIB

   oFIB [RFC6976] describes a mechanism where the convergence of the
   network upon a topology change is ordered in order to prevent
   transient forwarding loops.  Each router in the network deduces 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.

   The oFIB mechanism solves all the transient forwarding loops in a
   network at the price of introducing complexity in the convergence
   process that may require careful monitoring by the service provider.

   The solution defined in this document reuses the oFIB concept but
   limits it to the first hop that experiences the topology change.  As
   demonstrated, the mechanism defined in this document allows all the
   local transient forwarding loops to be solved; these represent a high
   percentage of all the loops.  Moreover, limiting to one hop allows
   network-wide convergence behavior to be kept.

11.  IANA Considerations

   This document has no IANA actions.

12.  Security Considerations

   This document does not introduce any change in terms of IGP security.
   The operation is internal to the router.  The local delay does not
   increase the number of attack vectors as an attacker could only
   trigger this mechanism if it already has the ability to disable or
   enable an IGP link.  The local delay does not increase the negative
   consequences.  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.












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13.  References

13.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
              November 2002.

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

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

13.2.  Informative References

   [BACKOFF]  Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
              Francois, P., and C. Bowers, "SPF Back-off Delay algorithm
              for link state IGPs", Work in Progress, draft-ietf-rtgwg-
              backoff-algo-10, March 2018.

   [PLSN]     Zinin, A., "Analysis and Minimization of Microloops in
              Link-state Routing Protocols", Work in Progress,
              draft-ietf-rtgwg-microloop-analysis-01, October 2005.

   [RFC3906]  Shen, N. and H. Smit, "Calculating Interior Gateway
              Protocol (IGP) Routes Over Traffic Engineering Tunnels",
              RFC 3906, DOI 10.17487/RFC3906, October 2004,
              <https://www.rfc-editor.org/info/rfc3906>.

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






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   [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, <https://www.rfc-editor.org/info/rfc6976>.

   [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, <https://www.rfc-editor.org/info/rfc7916>.

Acknowledgements

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

Authors' Addresses

   Stephane Litkowski
   Orange

   Email: stephane.litkowski@orange.com


   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com


   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Pierre Francois
   Individual Contributor

   Email: pfrpfr@gmail.com










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