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Network Working Group                                            H. Chen
Internet-Draft                                                  D. Cheng
Intended status: Standards Track                     Huawei Technologies
Expires: September 12, 2019                                       M. Toy
                                                                 Verizon
                                                                 Y. Yang
                                                                     IBM
                                                                 A. Wang
                                                           China Telecom
                                                                  X. Liu
                                                          Volta Networks
                                                                  Y. Fan
                                                            Casa Systems
                                                                  L. Liu
                                                          March 11, 2019


                   LS Distributed Flooding Reduction
                   draft-cc-lsr-flooding-reduction-03

Abstract

   This document proposes an approach to flood link states on a topology
   that is a subgraph of the complete topology per underline physical
   network, so that the amount of flooding traffic in the network is
   greatly reduced, and it would reduce convergence time with a more
   stable and optimized routing environment.  The approach can be
   applied to any network topology in a single area.  In this approach,
   every node in the area automatically calculates a flooding topology
   by using a same algorithm concurrently.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 https://datatracker.ietf.org/drafts/current/.





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   Internet-Drafts are draft documents valid for a maximum of six months
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Copyright Notice

   Copyright (c) 2019 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Flooding Topology . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Flooding Topology Construction  . . . . . . . . . . . . .   4
     3.2.  Scheduling for Flooding Topology Computation  . . . . . .   5
       3.2.1.  Scheduler with Exponential Delay  . . . . . . . . . .   6
       3.2.2.  Scheduler with Constant Delay . . . . . . . . . . . .   6
     3.3.  Flooding Topology Consistency . . . . . . . . . . . . . .   7
     3.4.  Flooding Topology Protection  . . . . . . . . . . . . . .   7
   4.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Extensions for Operations . . . . . . . . . . . . . . . .   8
       4.1.1.  Extensions to OSPF  . . . . . . . . . . . . . . . . .   8
       4.1.2.  Extensions to IS-IS . . . . . . . . . . . . . . . . .  10
     4.2.  Extensions for Consistency  . . . . . . . . . . . . . . .  11
       4.2.1.  Extensions to OSPF  . . . . . . . . . . . . . . . . .  11
       4.2.2.  Extensions to IS-IS . . . . . . . . . . . . . . . . .  12
   5.  Flooding Behavior . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Nodes Perform Flooding Reduction without Failure  . . . .  13
       5.1.1.  Receiving an LS . . . . . . . . . . . . . . . . . . .  13
       5.1.2.  Originating an LS . . . . . . . . . . . . . . . . . .  13
       5.1.3.  Establishing Adjacencies  . . . . . . . . . . . . . .  13
     5.2.  An Exception Case . . . . . . . . . . . . . . . . . . . .  14
       5.2.1.  Multiple Failures . . . . . . . . . . . . . . . . . .  14
       5.2.2.  Changes on Flooding Topology  . . . . . . . . . . . .  14



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   6.  Operations on Flooding Reduction  . . . . . . . . . . . . . .  15
     6.1.  Configuring Flooding Reduction  . . . . . . . . . . . . .  15
       6.1.1.  Configurations for Distributed Flooding Reduction . .  15
     6.2.  Migration to Flooding Reduction . . . . . . . . . . . . .  15
       6.2.1.  Migration to Distributed Flooding Reduction . . . . .  15
     6.3.  Roll Back to Normal Flooding  . . . . . . . . . . . . . .  15
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  OSPF  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.2.  IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Algorithms to Build Flooding Topology  . . . . . . .  19
     A.1.  Algorithms to Build Tree without Considering Others . . .  19
     A.2.  Algorithms to Build Tree Considering Others . . . . . . .  20
     A.3.  Connecting Leaves . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   For some networks such as dense Data Center (DC) networks, the
   existing Link State (LS) flooding mechanism is not efficient and may
   have some issues.  The extra LS flooding consumes network bandwidth.
   Processing the extra LS flooding, including receiving, buffering and
   decoding the extra LSs, wastes memory space and processor time.  This
   may cause scalability issues and affect the network convergence
   negatively.

   This document proposes an approach to minimize the amount of flooding
   traffic in the network.  Thus the workload for processing the extra
   LS flooding is decreased significantly.  This would improve the
   scalability, speed up the network convergence, stable and optimize
   the routing environment.

   In this approach, every node in the network automatically calculates
   a flooding topology by using a same algorithm concurrently at almost
   the same time.  It floods a link state on the flooding topology.
   There may be multiple algorithms for computing a flooding topology.
   Users can select one they prefer, and smoothly switch from one to
   another.  The approach is applicable to any network topology in a
   single area.  It is backward compatible.







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2.  Terminology

   LSA:  A Link State Advertisement in OSPF.

   LSP:  A Link State Protocol Data Unit (PDU) in IS-IS.

   LS: A Link Sate, which is an LSA or LSP.

   FTC:  Flooding Topology Computation.

3.  Flooding Topology

   For a given network topology, a flooding topology is a sub-graph or
   sub-network of the given network topology that has the same
   reachability to every node as the given network topology.  Thus all
   the nodes in the given network topology MUST be in the flooding
   topology.  All the nodes MUST be inter-connected directly or
   indirectly.  As a result, LS flooding will in most cases occur only
   on the flooding topology, that includes all nodes but a subset of
   links.  Note even though the flooding topology is a sub-graph of the
   original topology, any single LS MUST still be disseminated in the
   entire network.

3.1.  Flooding Topology Construction

   Many different flooding topologies can be constructed for a given
   network topology.  A chain connecting all the nodes in the given
   network topology is a flooding topology.  A circle connecting all the
   nodes is another flooding topology.  A tree connecting all the nodes
   is a flooding topology.  In addition, the tree plus the connections
   between some leaves of the tree and branch nodes of the tree is a
   flooding topology.

   The following parameters need to be considered for constructing a
   flooding topology:

   o  Number of links: The number of links on the flooding topology is a
      key factor for reducing the amount of LS flooding.  In general,
      the smaller the number of links, the less the amount of LS
      flooding.

   o  Diameter: The shortest distance between the two most distant nodes
      on the flooding topology (i.e., the diameter of the flooding
      topology) is a key factor for reducing the network convergence
      time.  The smaller the diameter, the less the convergence time.

   o  Redundancy: The redundancy of the flooding topology means a
      tolerance to the failures of some links and nodes on the flooding



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      topology.  If the flooding topology is split by some failures, it
      is not tolerant to these failures.  In general, the larger the
      number of links on the flooding topology is, the more tolerant the
      flooding topology to failures.

   There are three different ways to construct a flooding topology for a
   given network topology: centralized, distributed and static mode.
   This document focuses on the distributed mode, in which each node in
   the network automatically calculates a flooding topology by using a
   same algorithm concurrently at almost the same time.

   Note that the flooding topology constructed by a node is dynamic in
   nature, that means when the base topology (the entire topology graph)
   changes, the flooding topology (the sub-graph) MUST be re-computed/
   re-constructed to ensure that any node that is reachable on the base
   topology MUST also be reachable on the flooding topology.

   For reference purpose, some algorithms that allow nodes to
   automatically compute flooding topology are elaborated in Appendix A.
   However, this document does not attempt to standardize how a flooding
   topology is established.

3.2.  Scheduling for Flooding Topology Computation

   In a network consisting of routers from multiple vendors, there are
   different schedulers for SPF.  Using different schedulers is in favor
   of creating more micro routing loops during the convergence process
   due to discrepancies of schedulers than using a same scheduler.  More
   micro routing loops will lead to more traffic lose.  Service
   providers are already aware to use similar timers (values and
   behavior), but sometimes it is not possible due to limitations of
   schedulers [I-D.ietf-rtgwg-spf-uloop-pb-statement].  In order to let
   every node run a flooding topology computation (FTC) at almost the
   same time, we need to have a same scheduler for FTC to be implemented
   by multiple vendors.

   Two schedulers are described below.  One uses a constant delay such
   as 200ms for each of multiple consecutive FTCs.  For example, for
   four consecutive FTCs, the second FTC will be triggered 200ms after
   the first FTC; the third FTC will be triggered 200ms after the second
   FTC; and the forth FTC will be triggered 200ms after the third FTC.

   The other uses an exponential delay starting from a given hold time
   such as 100ms for consecutive FTCs.  For example, for four
   consecutive FTCs, the second FTC will be triggered 2 x 100ms = 200ms
   after the first FTC; the third FTC will be triggered 2 x 200ms =
   400ms after the second FTC; and the forth FTC will be triggered 2 x
   400ms = 800ms after the second FTC.



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   If these two schedulers are used in a network, it is almost
   impossible to let every node in the network run FTC at almost same
   time for multiple consecutive FTCs.

3.2.1.  Scheduler with Exponential Delay

   There are three parameters for the scheduler: initial-delay, minimum-
   hold-time and maximum-wait-time.  The initial-delay is the delay in
   milliseconds from detecting a topology change to triggering FTC.  Its
   default value is 50ms.

   The minimum-hold-time is the minimum hold time in milliseconds
   between two consecutive FTCs.  Its default value is 100ms.

   The maximum-wait-time is the maximum wait time in milliseconds for
   triggering FTC.  Its default value is 2000ms.

   The behavior of the scheduler with these parameters is described as
   follows:

     1. When FTC is to be called first time, initial-delay for FTC.
     2. When FTC is to be called n-th (n > 1) time consecutively,
        delay T = minimum-hold-time x 2^(n-2) for FTC if T is less
        than maximum-wait-time
     3. When T = hold-time x 2^(n-2) reaches maximum-wait-time,
        delay maximum-wait-time for FTC. Then repeats step 1
        (i.e., the next FTC call is considered as first time again).

                     Scheduler with Exponential Delay


3.2.2.  Scheduler with Constant Delay

   There are three parameters for the scheduler: constant-delay, number-
   of-runs and maximum-wait-time.  The constant-delay is the constant
   time to delay in milliseconds from detecting a topology change to
   triggering FTC.  Its default value is 200ms.

   The number-of-runs is the maximum number of times that FTC can run
   consecutively.  Its default value is 5.

   The maximum-wait-time is the maximum wait time in milliseconds for
   triggering FTC.  Its default value is 2000ms.

   The behavior of the scheduler with these parameters is described as
   follows:





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     1. When FTC is to be called first time, constant-delay for FTC.
     2. When FTC is to be called n-th (n > 1) time consecutively,
        delay constant-delay for FTC if n <= number-of-runs
     3. If n == number-of-runs,
        delay maximum-wait-time, and then repeats step 1
        (i.e., the next FTC call is considered as first time again).

                     Scheduler with Constant Delay


3.3.  Flooding Topology Consistency

   The flooding topology computed by one node needs to be the same as
   the one computed by another node.  When two flooding topologies
   computed by two nodes are different, this inconsistency needs to be
   detected and handled accordingly.

3.4.  Flooding Topology Protection

   It is hard to construct a flooding topology that reduces the amount
   of LS flooding greatly and is tolerant to multiple failures.  Without
   any protection against a flooding topology split when multiple
   failures on the flooding topology happen, we may have a slow
   convergence.  It is better to have some simple and fast methods for
   protecting the flooding topology split.  Thus the convergence is not
   slowed down.

   In one way, when two or more failures on the current flooding
   topology occur almost in the same time, each of the nodes within a
   given distance (such as 3 hops) to a failure point, floods the link
   state (LS) that it receives to all the links (except for the one from
   which the LS is received) until a new flooding topology is built.

   In other words, when the failures happen, each of the nodes within a
   given distance to a failure point, adds all its local links to the
   flooding topology temporarily until a new flooding topology is built.

   In another way, each node computes and maintains a small number of
   backup paths.  For a backup path for a link L on the flooding
   topology, a node N computes and maintains it only if the backup path
   goes through node N.  Node N stores the links (e.g., local link L1
   and L2) attached to it and on the backup path.  When link L fails and
   there are one or more failures on the flooding topology (and
   additionally the number of nodes collected through traversing the
   flooding topology is less than the number of live nodes in the area),
   node N adds the links (e.g., L1 and L2) to the flooding topology
   temporarily until a new flooding topology is built.  Note that




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   checking the additional condition will slow down the convergence when
   the flooding topology is split.  It is optional.

   Suppose that the two end nodes of link L is A and B, and A's ID is
   smaller than B's.  Node N computes a path from A to B with minimum
   hops and whose links are not on the flooding topology.  This path is
   a backup path for link L.  A backup path can be computed before link
   L fails or computed after link L fails and there is a need for it.
   Using the former will make the convergence time shorter.  For the
   former, when the pre-computed backup path is broken because of
   failures, a new backup path needs to be computed.

   Similarly, for a backup path for a connection crossing a node M on
   the flooding topology, a node N computes and maintains it only if the
   backup path goes through node N.  Node N stores the links (e.g.,
   local link La and Lb) attached to it and on the backup path for node
   M.

4.  Protocol Extensions

   The extensions comprises two parts: one part is for operations on
   flooding reduction, the other is for flooding topology consistency.

4.1.  Extensions for Operations

4.1.1.  Extensions to OSPF

   The OSPF Dynamic Flooding sub-TLV and area leader sub-TLV are defined
   in [I-D.ietf-lsr-dynamic-flooding].  The former may contains a number
   of algorithms.  The latter contains instructions about flooding
   reduction.

   Every node supporting the distributed flooding reduction MUST
   indicate its algorithms for flooding topology computation in a OSPF
   Dynamic Flooding sub-TLV.  This sub-TLV in a RI LSA will be
   advertised to the area leader.

   When the distributed flooding reduction is selected, every node MUST
   receive the OSPF area leader sub-TLV in a RI LSA from the area
   leader, which indicates the distributed mode and an algorithm to be
   used.  It SHOULD receive the parameters needed for the algorithm and
   the distributed mode.

   The parameters for the distributed mode include those three
   parameters configured for the scheduler for flooding topology
   computation.  Through configuring these parameters on one place such
   as the area leader and automatically advertising them to every node
   in the network, we simplify the operation on flooding reduction and



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   reduce the errors on configurations (comparing to manually
   configuring these parameters on every node).

   A new sub-TLV, called OSPF Scheduler Parameters sub-TLV, is defined
   for advertising the three parameters configured for the scheduler.
   Its format is illustrated below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Type (TBD1)          |          Length (8)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         initial-delay         |       minimum-hold-time       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      maximum-wait-time        |    Reserved (MUST be zero)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       OSPF Scheduler Parameters sub-TLV


   Type:  TBD1 for Scheduler Parameters is to be assigned by IANA.

   Length:  8.

   initial-delay:  2 octets' field representing the initial delay in
      milliseconds.

   minimum-hold-time:  2 octets' field representing the minimum hold
      time in milliseconds.

   maximum-wait-time:  2 octets' field representing the maximum wait
      time in milliseconds.

   Reserved:  MUST be set to 0 while sending and ignored on receipt.

   In the case where the distributed flooding reduction is selected and
   an algorithm for flooding topology computation is given already,
   there are some operational changes.  These changes include:

   1  the algorithm given is changed to another algorithm;

   2  the distributed flooding reduction is rolled back to normal
      flooding; and

   3  the distributed flooding reduction is changed to centralized
      flooding reduction.





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   For the first change, every node MUST receive the OSPF area leader
   sub-TLV in a RI LSA from the leader, which indicates that another
   algorithm is to be used.  After receiving the sub-TLV, it uses the
   new algorithm to compute a new flooding topology, and floods link
   states over both the flooding topology computed by the old algorithm
   and the new flooding topology for a given time.  And then it will
   floods link states over the flooding topology computed by the new
   algorithm.

   For the second change, every node MUST receive the OSPF area leader
   sub-TLV in a RI LSA from the leader, which indicates the current
   flooding reduction is to be rolled back to normal flooding.  After
   receiving the sub-TLV, it stops computing flooding topology and
   flooding link states over a flooding topology.  It floods link states
   using all its local links instead of the local links on the flooding
   topology.

   Note that the OSPF area leader sub-TLV defined in
   [I-D.ietf-lsr-dynamic-flooding] needs to be extended to allow users
   to roll back to normal flooding.  The Flooding Reduction Instruction
   sub-TLV defined in version 01 of this draft supports this.

4.1.2.  Extensions to IS-IS

   Similar to OSPF, the IS-IS Dynamic Flooding sub-TLV and area leader
   sub-TLV are also defined in [I-D.ietf-lsr-dynamic-flooding].

   Every node supporting the distributed flooding reduction MUST
   indicate its algorithms for flooding topology computation in an IS-IS
   Dynamic Flooding sub-TLV in an LSP to be advertised to the leader.

   When the distributed flooding reduction is selected, every node MUST
   receive the IS-IS area leader sub-TLV in an LSP, which indicates the
   distributed mode and an algorithm to be used.  It SHOULD receive the
   parameters needed for the algorithm and the distributed mode.

   A new sub-TLV, called IS-IS Scheduler Parameters sub-TLV, is defined
   for advertising the three parameters configured for the scheduler.
   Its format is illustrated below.












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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Type (TBD2) |   Length (8)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         initial-delay         |       minimum-hold-time       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      maximum-wait-time        |    Reserved (MUST be zero)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       IS-IS Scheduler Parameters sub-TLV


   Type:  TBD1 for Scheduler Parameters is to be assigned by IANA.

   Length:  8.

   initial-delay:  2 octets' field representing the initial delay in
      milliseconds.

   minimum-hold-time:  2 octets' field representing the minimum hold
      time in milliseconds.

   maximum-wait-time:  2 octets' field representing the maximum wait
      time in milliseconds.

   Reserved:  MUST be set to 0 while sending and ignored on receipt.

4.2.  Extensions for Consistency

4.2.1.  Extensions to OSPF

   RFC 5613 defines a TLV called Extended Options and Flag (EOF) TLV.  A
   OSPF Hello may contain this TLV in link-local signaling (LLS) data
   block.  A new flag bit (bit 30 suggested), called link on flooding
   topology (FT-bit for short), is defined in EOF TLV.

   When a node B receives a Hello from its adjacent node A over a link,
   FT-bit set to one in the Hello indicates that the link is on the
   flooding topology (FT) from node A's point of view.

   For a link between node A and node B, not on the current FT, after
   node A computes a new FT and the link is on the new FT, it sends a
   Hello with FT-bit set to one to node B.  Similarly, after node B
   computes a new FT and the link is on the new FT, it sends a Hello
   with FT-bit set to one to node A.  Note that Hello may include FT-bit
   after the state of the adjacency between A and B is FULL.




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   For a link between node A and node B, on the current FT, after node A
   computes a new FT and the link is not on the new FT, it sends a Hello
   with FT-bit set to zero to node B.  Similarly, after node B computes
   a new FT and the link is not on the new FT, it sends a Hello with FT-
   bit set to zero to node A.

   If the Hellos from the two nodes have the same FT-bit value, then the
   FT for the link between the two nodes is consistent; otherwise, it is
   not consistent.

   If one of the two nodes receives the Hellos with FT-bit set to one
   from the other, but sends the Hellos with FT-bit set to zero for a
   number of Hellos such as 5 Hellos or a given time, then the FT for
   the link between the two nodes is not consistent.

   When an inconsistency on the FT for a link is detected, a warning is
   issued or logged, and the node receiving the Hellos with FT-bit set
   to one from the other node assumes that the link is on the FT
   temporarily and floods the link states over the link.

4.2.2.  Extensions to IS-IS

   A new TLV, called Extended Options and Flag (EOF) TLV, is defined.
   It may be included in an IS-IS Hello.  Its format is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type (TBD)   |  Length (4)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Extended Options and Flags                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         EOF-TLV in IS-IS Hello

   Similar to OSPF, a new flag bit (bit 31 suggested), called link on
   flooding topology (FT-bit for short), is defined in EOF TLV.  When a
   node B receives a Hello from its adjacent node A over a link, FT-bit
   set to one in the Hello indicates that the link is on the FT from
   node A's point of view.

5.  Flooding Behavior

   This section describes the revised flooding behavior for a node.  The
   revised flooding procedure MUST flood an LS to every node in the
   network in any case, as the standard flooding procedure does.





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5.1.  Nodes Perform Flooding Reduction without Failure

5.1.1.  Receiving an LS

   When a node receives a newer LS that is not originated by itself from
   one of its interfaces, it floods the LS only to all the other
   interfaces that are on the flooding topology.

   When the LS is received from an interface on the flooding topology,
   it is flooded only to all the other interfaces that are on the
   flooding topology.  When the LS is received on an interface that is
   not on the flooding topology, it is also flooded only to all the
   other interfaces that are on the flooding topology.

   In any case, the LS must not be transmitted back to the receiving
   interface.

   Note before forwarding a received LS, the node would do the normal
   processing as usual.

5.1.2.  Originating an LS

   When a node originates an LS, it floods the LS to its interfaces on
   the flooding topology if the LS is a refresh LS (i.e., there is no
   significant change in the LS comparing to the previous LS); otherwise
   (i.e., there are significant changes such as link down in the LS), it
   floods the LS to all its interfaces.  Choosing flooding the LS with
   significant changes to all the interfaces instead of limiting to the
   interfaces on the flooding topology would speed up the distribution
   of the significant link state changes.

5.1.3.  Establishing Adjacencies

   Adjacencies being established can be classified into two categories:
   adjacencies to new nodes and adjacencies to existing nodes.

5.1.3.1.  Adjacency to New Node

   An adjacency to a new node is an adjacency between an existing node
   (say node E) on the flooding topology and the new node (say node N)
   which is not on the flooding topology.  There is not any adjacency
   between node N and a node in the network area.  The procedure for
   establishing the adjacency between E and N is the existing normal
   procedure unchanged.

   When the adjacency between N and E is established, node E adds node N
   and the link between N and E to the flooding topology temporarily
   until a new flooding topology is built.  New node N adds node N and



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   the link between N and E to the flooding topology temporarily until a
   new flooding topology is built.

5.1.3.2.  Adjacency to Existing Node

   An adjacency to an existing node is an adjacency between two nodes
   (say nodes E and X) on the flooding topology.  The procedure for
   establishing the adjacency between E and X is the existing normal
   procedure unchanged.

   Both node E and node X assume that the link between E and X is not on
   the flooding topology until a new flooding topology is built.  After
   the adjacency between E and X is established, node E does not send
   node X any new or updated LS that it receives or originates, and node
   X does not send node E any new or updated LS that it receives or
   originates until a new flooding topology is built.

5.2.  An Exception Case

5.2.1.  Multiple Failures

   When a node detects that two or more failures on the current flooding
   topology occur before a new flooding topology is built, it enables
   one flooding topology protection method in section 3.4.

5.2.2.  Changes on Flooding Topology

   After one or more failures split the current (old) flooding topology,
   some link states may be out of synchronization among some nodes.
   This can be resolved as follows.

   After a node N computes a new flooding topology, for a local link L
   attached to node N, if 1) link L is not on the current (old) flooding
   topology and is on the new flooding topology, and 2) there is a
   failure after the current (old) flooding topology is built, then node
   N sends a delta of the link states that it received or originated to
   its adjacent node over link L.

   For node N, the delta of the link states is the link states with
   changes that node N received or originated during the period of time
   in which the current (old) flooding topology is split.

   Suppose that Max_Split_Period is a number (in seconds), which is the
   maximum period of time in which a flooding topology is split.  Tc is
   the time at which the current (old) flooding topology is built, Tn is
   the time at which the new flooding topology is built, and Ts is the
   bigger one between Tc and (Tn - Max_Split_Period).  Node N sends its




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   adjacent node over link L the link states with changes that it
   received or originated from Ts to Tn.

6.  Operations on Flooding Reduction

6.1.  Configuring Flooding Reduction

   This section describes configurations for distributed flooding
   reduction (i.e., flooding reduction in distributed mode).

6.1.1.  Configurations for Distributed Flooding Reduction

   For distributed flooding reduction, an algorithm for computing a
   flooding topology needs to be configured.  The algorithm and
   distributed mode are configured on a node such as the area leader,
   which tells the other nodes in the area the algorithm and the mode
   via advertising the number of the algorithm and the mode.  Every node
   participating in the distributed flooding reduction uses this same
   algorithm.

6.2.  Migration to Flooding Reduction

   Migrating a OSPF or IS-IS area from normal flooding to flooding
   reduction smoothly takes a few steps or stages.  This section
   describes the steps for migrating an area to distributed flooding
   reduction from normal flooding.

6.2.1.  Migration to Distributed Flooding Reduction

   At first, a user selects the distributed mode on a node such as the
   area leader node, which tells the other nodes in the area to use
   distributed flooding reduction.

   After a node knows that the distributed mode is used, it advertises
   the algorithms it supports.  A user may check whether every node
   advertises its supporting algorithms through showing the link state
   containing the algorithms.

   And then, a user selects an algorithm and activates the flooding
   reduction through using configurations such as perform flooding
   Reduction, which tells all the nodes in the area to use the given
   algorithm and start the distributed flooding reduction.

6.3.  Roll Back to Normal Flooding

   For rolling back from flooding reduction to normal flooding, a user
   de-activates the flooding reduction through configuring roll back to




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   normal flooding on a node, which tells all the nodes in the area to
   roll back to normal flooding.

   After receiving a configuration to roll back to normal flooding, the
   node floods link states using all its local links instead of the
   local links on the flooding topology.  It also advertises the roll
   back to Normal flooding to all the other nodes in the area.  When
   each of the other nodes receives the advertisement, it rolls back to
   normal flooding (i.e., floods link states using all its local links
   instead of the local links on the flooding topology).

   In distributed mode, every node in the area will not compute or build
   flooding topology.

7.  Manageability Considerations

   Section 6 "Operations on Flooding Reduction" outlines the
   configuration process and deployment scenarios for link state
   flooding reduction.  The flooding reduction function may be
   controlled by a policy module and assigned a suitable user privilege
   level to enable.  A suitable model may be required to verify the
   flooding reduction status on routers participating in the flooding
   reduction, including their role as a normal node advertising link
   states using flooding topology.  The mechanisms defined in this
   document do not imply any new liveness detection and monitoring
   requirements in addition to those indicated in [RFC2328] and
   [RFC1195].

8.  Security Considerations

   A notable beneficial security aspect of link state flooding reduction
   is that a link state is not advertised over every link, but over the
   links on the flooding topology.  The malicious node could inject a
   link state with a different algorithm, which could trigger the
   flooding topology computation using the algorithm.  Good security
   practice might reuse the IS-IS authentication in [RFC5304] as well as
   [RFC5310], and the OSPF authentication and other security mechanisms
   described in [RFC2328], [RFC4552] and [RFC7474] to mitigate this type
   of risk.

9.  IANA Considerations

9.1.  OSPF

   Under Registry Name: OSPF Router Information (RI) TLVs [RFC7770],
   IANA is requested to assign one new TLV value for OSPF distributed
   flooding reduction as follows:




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     +===========+===========================+===============+
     | TLV Value |         TLV Name          | reference     |
     +===========+===========================+===============+
     |    11     | Scheduler Parameters TLV  | This document |
     +-----------+---------------------------+---------------+

9.2.  IS-IS

   Under Registry Name: IS-IS TLV Codepoints, IANA is requested to
   assign new TLV values for IS-IS distributed flooding reduction as
   follows:

    +===========+==============================+===============+
    | TLV Value |           TLV Name           | reference     |
    +===========+==============================+===============+
    |    151    |Scheduler Parameters TLV      | This document |
    +-----------+------------------------------+---------------+
    |    152    |Extended Options and Flag TLV | This document |
    +-----------+------------------------------+---------------+

10.  Acknowledgements

   The authors would like to thank Acee Lindem, Zhibo Hu, Robin Li,
   Stephane Litkowski and Alvaro Retana for their valuable suggestions
   and comments on this draft.

11.  References

11.1.  Normative References

   [I-D.ietf-lsr-dynamic-flooding]
              Li, T., Psenak, P., Ginsberg, L., Przygienda, T., Cooper,
              D., Jalil, L., and S. Dontula, "Dynamic Flooding on Dense
              Graphs", draft-ietf-lsr-dynamic-flooding-00 (work in
              progress), February 2019.

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <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>.



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   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC5250]  Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
              OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
              July 2008, <https://www.rfc-editor.org/info/rfc5250>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

11.2.  Informative References

   [I-D.ietf-rtgwg-spf-uloop-pb-statement]
              Litkowski, S., Decraene, B., and M. Horneffer, "Link State
              protocols SPF trigger and delay algorithm impact on IGP
              micro-loops", draft-ietf-rtgwg-spf-uloop-pb-statement-10
              (work in progress), January 2019.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.








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Appendix A.  Algorithms to Build Flooding Topology

   There are many algorithms to build a flooding topology.  A simple and
   efficient one is briefed below.

   o  Select a node R according to a rule such as the node with the
      biggest/smallest node ID;

   o  Build a tree using R as root of the tree (details below); and then

   o  Connect k (k>=0) leaves to the tree to have a flooding topology
      (details follow).

A.1.  Algorithms to Build Tree without Considering Others

   An algorithm for building a tree from node R as root starts with a
   candidate queue Cq containing R and an empty flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If Cq is empty, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in Ft and X1, X2, ..., Xn are in a special order.  For
       example, X1, X2, ..., Xn are ordered by the cost of the link
       between A and Xi.  The cost of the link between A and Xi is less
       than the cost of the link between A and Xj (j = i + 1).  If two
       costs are the same, Xi's ID is less than Xj's ID.  In another
       example, X1, X2, ..., Xn are ordered by their IDs.  If they are
       not ordered, then make them in the order.

   4.  Add Xi (i = 1, 2, ..., n) into the end of Cq, goto step 1.

   Another algorithm for building a tree from node R as root starts with
   a candidate queue Cq containing R and an empty flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If Cq is empty, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in Ft and X1, X2, ..., Xn are in a special order.  For
       example, X1, X2, ..., Xn are ordered by the cost of the link
       between A and Xi.  The cost of the link between A and Xi is less
       than the cost of the link between A and Xj (j = i + 1).  If two
       costs are the same, Xi's ID is less than Xj's ID.  In another
       example, X1, X2, ..., Xn are ordered by their IDs.  If they are
       not ordered, then make them in the order.



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   4.  Add Xi (i = 1, 2, ..., n) into the front of Cq and goto step 1.

   A third algorithm for building a tree from node R as root starts with
   a candidate list Cq containing R associated with cost 0 and an empty
   flooding topology Ft:

   1.  Remove the first node A from Cq and add A into Ft

   2.  If all the nodes are on Ft, then return with Ft

   3.  Suppose that node A is associated with a cost Ca which is the
       cost from root R to node A, node Xi (i = 1, 2, ..., n) is
       connected to node A and not in Ft and the cost of the link
       between A and Xi is LCi (i=1, 2, ..., n).  Compute Ci = Ca + LCi,
       check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci.  If Xi
       is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq,
       then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in
       Cq; If Cxi == Ci then add Xi with cost Ci into Cq.

   4.  Make sure Cq is in a special order.  Suppose that Ai (i=1, 2,
       ..., m) are the nodes in Cq, Cai is the cost associated with Ai,
       and IDi is the ID of Ai.  One order is that for any k = 1, 2,
       ..., m-1, Cak < Caj (j = k+1) or Cak = Caj and IDk < IDj.  Goto
       step 1.

A.2.  Algorithms to Build Tree Considering Others

   An algorithm for building a tree from node R as root with
   consideration of others's support for flooding reduction starts with
   a candidate queue Cq containing R associated with previous hop PH=0
   and an empty flooding topology Ft:

   1.  Remove the first node A that supports flooding reduction from the
       candidate queue Cq if there is such a node A; otherwise (i.e., if
       there is not such node A in Cq), then remove the first node A
       from Cq.  Add A into the flooding topology Ft.

   2.  If Cq is empty or all nodes are on Ft, then return with Ft

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in the flooding topology Ft and X1, X2, ..., Xn are in a
       special order considering whether some of them that support
       flooding reduction (.  For example, X1, X2, ..., Xn are ordered
       by the cost of the link between A and Xi.  The cost of the link
       between A and Xi is less than that of the link between A and Xj
       (j = i + 1).  If two costs are the same, Xi's ID is less than
       Xj's ID.  The cost of a link is redefined such that 1) the cost
       of a link between A and Xi both support flooding reduction is



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       much less than the cost of any link between A and Xk where Xk
       with F=0; 2) the real metric of a link between A and Xi and the
       real metric of a link between A and Xk are used as their costs
       for determining the order of Xi and Xk if they all (i.e., A, Xi
       and Xk) support flooding reduction or none of Xi and Xk support
       flooding reduction.

   4.  Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into
       the end of the candidate queue Cq, and goto step 1.

   Another algorithm for building a tree from node R as root with
   consideration of others' support for flooding reduction starts with a
   candidate queue Cq containing R associated with previous hop PH=0 and
   an empty flooding topology Ft:

   1.  Remove the first node A that supports flooding reduction from the
       candidate queue Cq if there is such a node A; otherwise (i.e., if
       there is not such node A in Cq), then remove the first node A
       from Cq.  Add A into the flooding topology Ft.

   2.  If Cq is empty or all nodes are on Ft, then return with Ft.

   3.  Suppose that node Xi (i = 1, 2, ..., n) is connected to node A
       and not in the flooding topology Ft and X1, X2, ..., Xn are in a
       special order considering whether some of them support flooding
       reduction.  For example, X1, X2, ..., Xn are ordered by the cost
       of the link between A and Xi.  The cost of the link between A and
       Xi is less than the cost of the link between A and Xj (j = i +
       1).  If two costs are the same, Xi's ID is less than Xj's ID.
       The cost of a link is redefined such that 1) the cost of a link
       between A and Xi both support flooding reduction is much less
       than the cost of any link between A and Xk where Xk does not
       support flooding reduction; 2) the real metric of a link between
       A and Xi and the real metric of a link between A and Xk are used
       as their costs for determining the order of Xi and Xk if they all
       (i.e., A, Xi and Xk) support flooding reduction or none of Xi and
       Xk supports flooding reduction.

   4.  Add Xi (i = 1, 2, ..., n) associated with previous hop PH=A into
       the front of the candidate queue Cq, and goto step 1.

   A third algorithm for building a tree from node R as root with
   consideration of others' support for flooding reduction (using flag F
   = 1 for support, and F = 0 for not support in the following) starts
   with a candidate list Cq containing R associated with low order cost
   Lc=0, high order cost Hc=0 and previous hop ID PH=0, and an empty
   flooding topology Ft:




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   1.  Remove the first node A from Cq and add A into Ft.

   2.  If all the nodes are on Ft, then return with Ft

   3.  Suppose that node A is associated with a cost Ca which is the
       cost from root R to node A, node Xi (i = 1, 2, ..., n) is
       connected to node A and not in Ft and the cost of the link
       between A and Xi is LCi (i=1, 2, ..., n).  Compute Ci = Ca + LCi,
       check if Xi is in Cq and if Cxi (cost from R to Xi) < Ci.  If Xi
       is not in Cq, then add Xi with cost Ci into Cq; If Xi is in Cq,
       then If Cxi > Ci then replace Xi with cost Cxi by Xi with Ci in
       Cq; If Cxi == Ci then add Xi with cost Ci into Cq.

   4.  Suppose that node A is associated with a low order cost LCa which
       is the low order cost from root R to node A and a high order cost
       HCa which is the high order cost from R to A, node Xi (i = 1, 2,
       ..., n) is connected to node A and not in the flooding topology
       Ft and the real cost of the link between A and Xi is Ci (i=1, 2,
       ..., n).  Compute LCxi and HCxi: LCxi = LCa + Ci if both A and Xi
       have flag F set to one, otherwise LCxi = LCa HCxi = HCa + Ci if A
       or Xi does not have flag F set to one, otherwise HCxi = HCa If Xi
       is not in Cq, then add Xi associated with LCxi, HCxi and PH = A
       into Cq; If Xi associated with LCxi' and HCxi' and PHxi' is in
       Cq, then If HCxi' > HCxi then replace Xi with HCxi', LCxi' and
       PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise (i.e.,
       HCxi' == HCxi) if LCxi' > LCxi , then replace Xi with HCxi',
       LCxi' and PHxi' by Xi with HCxi, LCxi and PH=A in Cq; otherwise
       (i.e., HCxi' == HCxi and LCxi' == LCxi) if PHxi' > PH, then
       replace Xi with HCxi', LCxi' and PHxi' by Xi with HCxi, LCxi and
       PH=A in Cq.

   5.  Make sure Cq is in a special order.  Suppose that Ai (i=1, 2,
       ..., m) are the nodes in Cq, HCai and LCai are low order cost and
       high order cost associated with Ai, and IDi is the ID of Ai.  One
       order is that for any k = 1, 2, ..., m-1, HCak < HCaj (j = k+1)
       or HCak = HCaj and LCak < LCaj or HCak = HCaj and LCak = LCaj and
       IDk < IDj.  Goto step 1.

A.3.  Connecting Leaves

   Suppose that we have a flooding topology Ft built by one of the
   algorithms described above.  Ft is like a tree.  We may connect k (k
   >=0) leaves to the tree to have a enhanced flooding topology with
   more connectivity.

   Suppose that there are m (0 < m) leaves directly connected to a node
   X on the flooding topology Ft.  Select k (k <= m) leaves through
   using a deterministic algorithm or rule.  One algorithm or rule is to



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   select k leaves that have smaller or larger IDs (i.e., the IDs of
   these k leaves are smaller/bigger than the IDs of the other leaves
   directly connected to node X).  Since every node has a unique ID,
   selecting k leaves with smaller or larger IDs is deterministic.

   If k = 1, the leaf selected has the smallest/largest node ID among
   the IDs of all the leaves directly connected to node X.

   For a selected leaf L directly connected to a node N in the flooding
   topology Ft, select a connection/adjacency to another node from node
   L in Ft through using a deterministic algorithm or rule.

   Suppose that leaf node L is directly connected to nodes Ni (i =
   1,2,...,s) in the flooding topology Ft via adjacencies and node Ni is
   not node N, IDi is the ID of node Ni, and Hi (i = 1,2,...,s) is the
   number of hops from node L to node Ni in the flooding topology Ft.

   One Algorithm or rule is to select the connection to node Nj (1 <= j
   <= s) such that Hj is the largest among H1, H2, ..., Hs.  If there is
   another node Na ( 1 <= a <= s) and Hj = Ha, then select the one with
   smaller (or larger) node ID.  That is that if Hj == Ha and IDj < IDa
   then select the connection to Nj for selecting the one with smaller
   node ID (or if Hj == Ha and IDj < IDa then select the connection to
   Na for selecting the one with larger node ID).

   Suppose that the number of connections in total between leaves
   selected and the nodes in the flooding topology Ft to be added is
   NLc.  We may have a limit to NLc.

Authors' Addresses

   Huaimo Chen
   Huawei Technologies
   Boston
   USA

   Email: huaimo.chen@huawei.com


   Dean Cheng
   Huawei Technologies
   Santa Clara
   USA

   Email: dean.cheng@huawei.com






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   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com


   Yi Yang
   IBM
   Cary, NC
   United States of America

   Email: yyietf@gmail.com


   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209
   China

   Email: wangaj.bri@chinatelecom.cn


   Xufeng Liu
   Volta Networks
   McLean, VA
   USA

   Email: xufeng.liu.ietf@gmail.com


   Yanhe Fan
   Casa Systems
   USA

   Email: yfan@casa-systems.com


   Lei Liu
   USA

   Email: liulei.kddi@gmail.com








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