Routing Area Working Group                                  S. Litkowski
Internet-Draft                                               B. Decraene
Intended status: Standards Track                                  Orange
Expires: April 16, 2016                                      C. Filsfils
                                                           Cisco Systems
                                                             P. Francois
                                                          IMDEA Networks
                                                        October 14, 2015

     Microloop prevention by introducing a local convergence delay


   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 and the IGP convergence.

   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",
   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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 16, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Transient forwarding loops side effects . . . . . . . . . . .   3
     2.1.  Fast reroute unefficiency . . . . . . . . . . . . . . . .   3
     2.2.  Network congestion  . . . . . . . . . . . . . . . . . . .   5
   3.  Overview of the solution  . . . . . . . . . . . . . . . . . .   6
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Current IGP reactions . . . . . . . . . . . . . . . . . .   7
     4.3.  Local events  . . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Local delay . . . . . . . . . . . . . . . . . . . . . . .   8
       4.4.1.  Link down event . . . . . . . . . . . . . . . . . . .   8
       4.4.2.  Link up event . . . . . . . . . . . . . . . . . . . .   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.  Comparison with other solutions . . . . . . . . . . . . . . .  12
     8.1.  PLSN  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  OFIB  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  14

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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 local to the failure; based on
   network analysis, these are a significant portion of the micro-
   forwarding loops.  A simple and easily deployable solution to these
   local micro-loops is critical because these local loops cause traffic
   loss after an advanced 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 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].

        D ------ C
        |        |
      1 |        | 5
        |        |
        S ------ B
        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 damage for the network.

2.1.  Fast reroute unefficiency

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         1 |
           |    1
           A ------ B
           |        |    ^
        10 |        | 5  | T
           |        |    |
           |    1
         1 |

        Figure 2 - RSVPTE FRR case

   In figure 2, a RSVP-TE tunnel T, provisionned on C and terminating on
   B, is used to protect against C-B link failure (IGP shortcut
   activated on C).  Primary path of T is C->B and FRR is activated on T
   providing a FRR bypass or detour using path C->E->A->B.  On C,
   nexthop to D is tunnel T thanks to IGP shortcut.  When C-B link fails

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

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

       *  T path is recomputed : C->E->A->B

       *  IGP path to D is recomputed : C->E->A->D

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

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          Router C timeline                Router E timeline

  --- + ---- t0 C-B link fails
 LoC  | ---- t1 C detects failure
  --- + ---- t2 C activates FRR
 T    | ---- t3 C updates local LSA/LSP
 R    |
 A    | ---- t4 C floods local LSA/LSP
 F    |
 F    | ---- t5 C computes SPF             --- t0 E receives LSA/LSP
 I    |
 C    | ---- t6 C updates RIB/FIB          --- t1 E floods LSA/LSP
 O    |                                    --- t2 E computes SPF
 K    |
  --- + * (t6' C updates FIB for D)        --- t3 E updates RIB/FIB
 LoC  | ---- t7 Convergence ended on C
  --- + * (Traffic restored to D)          * (t3' E updates FIB for D)
      |                                    --- t4 Convergence ended on E

   The issue described here is completely independent of the fast-
   reroute mechanism involved (TE FRR, LFA/rLFA, MRT ...).  Fast-reroute
   is working perfectly but ensures 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 IGP convergence may permit to
   keep fast-reroute path until neighbor has converged and preserve
   customer traffic.

2.2.  Network congestion

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

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

   Congestion introduced by transient forwarding loops are problematic
   as they are impacting traffic that is not directly concerned by the
   failing network component.  In our example, the congestion of S-B
   link will impact customer traffic that is not directly concerned by
   the failure : e.g.  A to B, F to B, E to B.  Class of services may be
   implemented to mitigate the congestion but some traffic not directly
   concerned by the failure would still be dropped as a router is not
   able to identify looped traffic from normal 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.  This delay is positive in
   case of "down" events and negative in case of "up" events.

   This ordered convergence, is similar to the ordered FIB proposed
   defined in [RFC6976], but limited to only 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 limitation.

4.  Specification

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4.1.  Definitions

   This document will refer to the following existing IGP timers:

   o  LSP_GEN_TIMER: to batch multiple local events in one single local
      LSP update.  It is often associated with damping mechanism to
      slowdown reactions by incrementing the timer when multiple
      consecutive events are detected.

   o  SPF_TIMER: to batch multiple events in one single computation.  It
      is often associated with damping mechanism to slowdown reactions
      by incrementing the timer when the IGP is instable.

   o  IGP_LDP_SYNC_TIMER: defined in [RFC5443] to give LDP some time to
      establish the session and learn the MPLS labels before the link is

   This document introduces the following two new timers :

   o  ULOOP_DELAY_DOWN_TIMER: slowdown the local node convergence in
      case of link down events.

   o  ULOOP_DELAY_UP_TIMER: slowdown the network wide IGP convergence in
      case of link up events.

4.2.  Current IGP reactions

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

   1.  UP/Down event is notified to IGP.

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

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

   4.  SPF is scheduled in SPF_TIMER msec.

   5.  Upon SPF_TIMER expiration, SPF is computed and RIB/FIB are

4.3.  Local events

   The mechanisms described in this document assume that there has been
   a single failure as seen by the IGP area/level.  If this assumption
   is violated (e.g. multiple links or nodes failed), then standard IP

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   convergence MUST be applied.  There are three types of single
   failures: local link, local node, and remote failure.

   Example :

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

   Let 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_TIMER
   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.

   [Editor s Note: Detection of a failed broadcast link involves
   additional complexity and will be described in a future version.]

   If a router determines that the event is local link failure, then the
   router may use the mechanism described in this document.

   Distinguishing local node failure from remote or multiple link
   failure requires additional logic which is future work to fully
   describe.  To give a sense of the work necessary, if node C is
   failing, routers B,E and D are updating and flooding updated LSPs/
   LSAs.  B would need to determine the changes in the LSPs/LSAs from E
   and D and see that they all relate to node C which is also the far-
   end of the locally failed link.  Once this detection is accurately
   done, the same mechanism of delaying local convergence can be

4.4.  Local delay

4.4.1.  Link down event

   Upon an adjacency/link down event, this document introduces a change
   in step 5 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
   down local events .  Note that determining that all topological
   change are only local down events requires analyzing all modified
   LSP/LSA as a local link or node failure will typically be notified by
   multiple nodes.  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.

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

4.4.2.  Link up event

   Upon an adjacency/link up event, this document introduces the
   following change in step 3 where the node SHOULD:

   o  Firstly build a LSP/LSA with the new adjacency but setting the
      metric to MAX_METRIC .  It SHOULD flood it but not compute the SPF
      at this time.  This step is required to ensure the two way
      connectivity check on all nodes when computing SPF.

   o  Then build the LSP/LSA with the target metric but SHOULD delay the
      flooding of this LSP/LSA by SPF_TIMER + ULOOP_DELAY_UP_TIMER.
      MAX_METRIC is equal to MaxLinkMetric (0xFFFF) for OSPF and 2^24-2
      (0xFFFFFE) for IS-IS.

   o  Then continue with next steps (SPF computation) without waiting
      for the expiration of the above timer.  In other word, only the
      flooding of the LSA/LSP is delayed, not the local SPF computation.

   As as result of this addition, routers local to the failure will
   converge faster than remote routers.

   If this mechanism is used in cooperation with "LDP IGP
   Synchronization" as defined in [RFC5443] then the mechanism defined
   in RFC 5443 is applied first, followed by the mechanism defined in
   this document.  More precisely, the procedure defined in this
   document is applied once the LDP session is considered "fully
   operational" as per [RFC5443].

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

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   Let us consider the traffic from G to F.  The primary path is
   G->D->C->E->F.  When link CE 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.

   By implementing the mechanism defined in this document on C, when the
   CE link fails, C delays the update of his 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, forwarding
   entry to F is updated.  There is no transient forwarding loop on the
   link CD.

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 CF link fails, if C updates its
   forwarding entry to K before D, a transient loop occurs between C and

   By implementing the mechanism defined in this document on C, when the
   link CF fails, C delays the update of his forwarding entry to K,
   letting time for D to converge.  When the timer expires on C,
   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 occuring.

6.  Simulations

   Simulations have been run on multiple service provider topologies.
   So far, only link down event have been tested.

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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

   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

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

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

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   o  It 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 address 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
   convergence time is not an issue if we consider that the traffic is
   protected during the convergence.

8.  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.

8.1.  PLSN

   PLSN ([I-D.ietf-rtgwg-microloop-analysis]) describes a mechanism
   where each node in the network tries a 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 satisified.

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

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

   o  PLSN has 3 different behavior : keep using old nexthop, use new
      primary nexthop if 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
      [I-D.ietf-rtgwg-lfa-manageability]).  Our solution 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 our mechanism applies only on the nodes connected to the
      topology change.

8.2.  OFIB

   OFIB ([RFC6976]) describes a mechanism where 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 compute/apply a specific FIB
   update timer based on the failure type and its rank in the network
   considering the failure point as root.

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

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

9.  Security Considerations

   This document does not introduce 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.

10.  Acknowledgements

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

11.  IANA Considerations

   This document has no actions for IANA.

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12.  References

12.1.  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,

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
              2009, <>.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, January
              2010, <>.

12.2.  Informative References

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

              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,

   [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,

   [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, <>.

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Internet-Draft                 uloop-delay                  October 2015

   [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,

Authors' Addresses

   Stephane Litkowski


   Bruno Decraene


   Clarence Filsfils
   Cisco Systems


   Pierre Francois
   IMDEA Networks


Litkowski, et al.        Expires April 16, 2016                [Page 15]