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Versions: (draft-li-dynamic-flooding-isis) 00 01 02 03 04 05 draft-li-lsr-dynamic-flooding

Internet Engineering Task Force                                    T. Li
Internet-Draft                                           Arista Networks
Intended status: Informational                                 P. Psenak
Expires: December 30, 2018                           Cisco Systems, Inc.
                                                           June 28, 2018


                    Dynamic Flooding on Dense Graphs
                      draft-li-dynamic-flooding-05

Abstract

   Routing with link state protocols in dense network topologies can
   result in sub-optimal convergence times due to the overhead
   associated with flooding.  This can be addressed by decreasing the
   flooding topology so that it is less dense.

   This document discusses the problem in some depth and an
   architectural solution.  Specific protocol changes for IS-IS, OSPFv2,
   and OSPFv3 are described in this document.

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
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   This Internet-Draft will expire on December 30, 2018.

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
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Solution Requirements . . . . . . . . . . . . . . . . . . . .   4
   4.  Dynamic Flooding  . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Leader election . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Computing the Flooding Topology . . . . . . . . . . . . .   7
     4.4.  Topologies on Complete Bipartite Graphs . . . . . . . . .   8
       4.4.1.  A Minimal Flooding Topology . . . . . . . . . . . . .   8
       4.4.2.  Xia Topologies  . . . . . . . . . . . . . . . . . . .   9
       4.4.3.  Optimization  . . . . . . . . . . . . . . . . . . . .  10
     4.5.  Encoding the Flooding Topology  . . . . . . . . . . . . .  10
     4.6.  Analysis of Topology Changes  . . . . . . . . . . . . . .  10
       4.6.1.  Link Addition . . . . . . . . . . . . . . . . . . . .  10
       4.6.2.  Node Addition . . . . . . . . . . . . . . . . . . . .  11
       4.6.3.  Link Failures Off the Flooding Topology . . . . . . .  11
       4.6.4.  Failure of the Area Leader  . . . . . . . . . . . . .  11
       4.6.5.  Failures on the Flooding Topology . . . . . . . . . .  11
       4.6.6.  Recovery from Multiple Failures . . . . . . . . . . .  12
   5.  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  IS-IS TLVs  . . . . . . . . . . . . . . . . . . . . . . .  12
       5.1.1.  IS-IS Area Leader Sub-TLV . . . . . . . . . . . . . .  13
       5.1.2.  IS-IS Area System IDs TLV . . . . . . . . . . . . . .  14
       5.1.3.  IS-IS Flooding Path TLV . . . . . . . . . . . . . . .  15
     5.2.  OSPF LSAs and TLVs  . . . . . . . . . . . . . . . . . . .  16
       5.2.1.  OSPF Area Leader Sub-TLV  . . . . . . . . . . . . . .  16
       5.2.2.  OSPFv2 Dynamic Flooding Opaque LSA  . . . . . . . . .  16
       5.2.3.  OSPFv3 Dynamic Flooding LSA . . . . . . . . . . . . .  18
       5.2.4.  OSPF Area Router IDs TLV  . . . . . . . . . . . . . .  18
       5.2.5.  OSPF Flooding Path TLV  . . . . . . . . . . . . . . .  19
   6.  Behavioral Specification  . . . . . . . . . . . . . . . . . .  20
     6.1.  Leader Election . . . . . . . . . . . . . . . . . . . . .  21
     6.2.  Area Leader Responsibilities  . . . . . . . . . . . . . .  21
     6.3.  Distributed Flooding Topology Calculation . . . . . . . .  21
     6.4.  Flooding Behavior . . . . . . . . . . . . . . . . . . . .  22
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     7.2.  OSPF  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
       7.2.1.  OSPF Dynamic Flooding LSA TLVs Registry . . . . . . .  24
     7.3.  IGP . . . . . . . . . . . . . . . . . . . . . . . . . . .  24



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   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     10.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   In recent years, there has been increased focused on how to address
   the dynamic routing of networks that have a bipartite (a.k.a. spine-
   leaf or leaf-spine), Clos [Clos], or Fat Tree [Leiserson] topology.
   Conventional Interior Gateway Protocols (IGPs, i.e., IS-IS
   [ISO10589], OSPFv2 [RFC2328], and OSPFv3 [RFC5340]) under-perform,
   redundantly flooding information throughout the dense topology,
   leading to overloaded control plane inputs and thereby creating
   operational issues.  For practical considerations, network architects
   have resorted to applying unconventional techniques to address the
   problem, applying BGP in the data center [RFC7938].  However it is
   very clear that using an Exterior Gateway Protocol as an IGP is sub-
   optimal, if only due to the configuration overhead.

   The primary issue that is demonstrated when conventional mechanisms
   are applied is the poor reaction of the network to topology changes.
   Normal link state routing protocols rely on a flooding algorithm for
   state distribution.  In a dense topology, this flooding algorithm is
   highly redundant, resulting in unnecessary overhead.  Each node in
   the topology receives each link state update multiple times.
   Ultimately, all of the redundant copies will be discarded, but only
   after they have reached the control plane and been processed.  This
   creates issues because significant link state database updates can
   become queued behind many redundant copies of another update.  This
   delays convergence as the link state database does not stabilize
   promptly.

   In a real world implementation, the packet queues leading to the
   control plane are necessarily of finite size, so if the flooding rate
   exceeds the update processing rate for long enough, the control plane
   will be obligated to drop incoming updates.  If these lost updates
   are of significance, this will further delay stabilization of the
   link state database and the convergence of the network.

   This is not a new problem.  Historically, when routing protocols have
   been deployed in networks where the underlying topology is a complete
   graph, there have been similar issues.  This was more common when the
   underlying link layer fabric presented the network layer with a full
   mesh of virtual connections.  This was addressed by reducing the




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   flooding topology through IS-IS Mesh Groups [RFC2973], but this
   approach requires careful configuration of the flooding topology.

   Thus, the root problem is not limited to massively scalable data
   centers.  It exists with any dense topology at scale.

   This problem is not entirely surprising.  Link state routing
   protocols were conceived when links were very expensive and
   topologies were sparse.  The fact that those same designs are sub-
   optimal in a dense topology should not come as a huge surprise.  The
   fundamental premise that was addressed by the original designs was an
   environment of extreme cost and scarcity.  Technology has progressed
   to the point where links are cheap and common.  This represents a
   complete reversal in the economic fundamentals of network
   engineering.  The original designs are to be commended for continuing
   to provide correct operation to this point, and optimizations for
   operation in today's environment are to be expected.

1.1.  Requirements Language

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

2.  Problem Statement

   In a dense topology, the flooding algorithm that is the heart of
   conventional link state routing protocols causes a great deal of
   redundant messaging.  This is exacerbated by scale.  While the
   protocol can survive this combination, the redundant messaging is
   unnecessary overhead and delays convergence.  Thus, the problem is to
   provide routing in dense, scalable topologies with rapid convergence.

3.  Solution Requirements

   A solution to this problem must then meet the following requirements:

   Requirement 1    Provide a dynamic routing solution.  Reachability
       must be restored after any topology change.

   Requirement 2    Provide a significant improvement in convergence.

   Requirement 3    The solution should address a variety of dense
       topologies.  Just addressing a complete bipartite topology such
       as K5,8 is insufficient.  Multi-stage Clos topologies must also
       be addressed, as well as topologies that are slight variants.
       Addressing complete graphs is a good demonstration of generality.




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   Requirement 4    There must be no single point of failure.  The loss
       of any link or node should not unduly hinder convergence.

   Requirement 5    Dense topologies are subgraphs of much larger
       topologies.  Operational efficiency requires that the dense
       subgraph not operate in a radically different manner than the
       remainder of the topology.  While some operational differences
       are permissible, they should be minimized.  Changes to nodes
       outside of the dense subgraph are not acceptable.  These
       situations occur when massively scaled data centers are part of
       an overall larger wide-area network.  Having a second protocol
       operating just on this subgraph would add much more complexity at
       the edge of the subgraph where the two protocols would have to
       inter-operate.

4.  Dynamic Flooding

   We have observed that the combination of the dense topology and
   flooding on the physical topology in a scalable network is sub-
   optimal.  However, if we decouple the flooding topology from the
   physical topology and only flood on a greatly reduced portion of that
   topology, we can have efficient flooding and retain all of the
   resilience of existing protocols.

   In this idea, the flooding topology is computed either centrally on
   an elected node or in a distributed manner on all nodes that are
   supporting Dynamic Flooding.  If the flooding topology is computed
   centrally, it is encoded into and distributed as part of the normal
   link state database.  We call this the centralized mode of operation.
   If the flooding topology is computed in a distributed fashion, we
   call this the distributed mode of operation.  Nodes within the dense
   topology would only flood on the flooding topology.  On links outside
   of the normal flooding topology, normal database synchronization
   mechanisms (i.e., OSPF database exchange, IS-IS CSNPs) would apply,
   but flooding would not.  New link state information that arrives from
   outside of the flooding topology suggests that the sender has a
   different or no flooding topology information and that the link state
   update should be flooded on the flooding topology as well.

   Since the flooding topology is computed prior to topology changes, it
   does not factor into the convergence time and can be done when the
   topology is stable.  The speed of the computation and its
   distribution, in the case of a centralized mode, is not a significant
   issue.

   If a node does not have any flooding topology information when it
   receives new link state information, it should flood according to




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   legacy flooding rules.  This situation will occur when the dense
   topology is first established, but is unlikely to recur.

   When centralized mode is used and if, during a transient, there are
   multiple flooding topologies being advertised, then nodes should
   flood link state updates on all of the flooding topologies.  Each
   node should locally evaluate the election of the lead node for the
   dense subgraph and first flood on the topology of the lead node.  The
   rationale behind this is straightforward: if there is a transient and
   there has been a recent change in the elected node, then propagating
   topology information promptly along the most likely flooding topology
   should be the priority.

   During transients, it is possible that loops will form in the
   flooding topology.  This is not problematic, as the legacy flooding
   rules would cause duplicate updates to be ignored.  Similarly, during
   transients, it is possible that the forwarding topology may become
   disconnected.  To address this, nodes can perform a database
   synchronization check anytime a link is added to or removed from the
   flooding topology.

4.1.  Applicability

   In a complete graph, this approach is appealing because it
   drastically decreases the flooding topology without the manual
   configuration of mesh groups.  By controlling the diameter of the
   flooding topology, as well as the maximum degree node in the flooding
   topology, convergence time goals can be met and the stability of the
   control plane can be assured.

   Similarly, in a massively scaled data center, where there are many
   opportunities for redundant flooding, this mechanism ensures that
   flooding is redundant, with each leaf and spine well connected, while
   ensuring that no update need make too many hops and that no node
   shares an undue portion of the flooding effort.

   In a network where only a portion of the nodes support Dynamic
   Flooding, the remaining nodes will continue to perform universal
   flooding.  This is not an issue for correctness, as no node can
   become isolated.

   Flooding that is initiated within the flooding topology will remain
   within that flooding topology until it reaches a legacy node, which
   will resume legacy flooding.  Legacy flooding will be bounded by the
   flooding topology, which can help limit the propagation of
   unnecessary flooding.  Whether or not the network can remain stable
   in this condition is unknown and may be very dependent on the number
   and location of the nodes that support Dynamic Flooding.



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4.2.  Leader election

   A single node within the dense topology is elected as an Area Leader.

   A generalization of the mechanisms used in existing Designated Router
   (OSPF) or Designated Intermediate-System (IS-IS) elections suffices.
   The elected node is known as the Area Leader.

   In the case of centralized mode, the Area Leader is responsible for
   computing and distributing the flooding topology.  When a new node is
   elected and has distributed new flooding topology information, then
   the old node should withdraw its flooding topology information from
   the link state database.  If the old node does not return to the
   topology in a timely manner, the new node may remove the old node's
   information from the link state database.

   In the case of distributed mode, the distributed algorithm advertised
   by the Area Leader MUST be used by all routers that participate in
   Dynamic Flooding.

   Not every router needs to be a candidate to be Area Leader within an
   area, as a single candidate is sufficient for correct operation.  For
   redundancy, however, it is strongly RECOMMENDED that there be
   multiple candidates.

4.3.  Computing the Flooding Topology

   There is a great deal of flexibility in how the flooding topology may
   be computed.  For resilience, it needs to at least contain a cycle of
   all nodes in the dense subgraph.  However, additional links could be
   added to decrease the convergence time.  The trade-off between the
   density of the flooding topology and the convergence time is a matter
   for further study.  The exact algorithm for computing the flooding
   topology in the case of the centralized computation need not be
   standardized, as it is not an interoperability issue.  Only the
   encoding of the result needs to be documented.  In the case of
   distributed mode, all nodes in the IGP area need to use the same
   algorithm to compute the flooding topology.  It is possible to use
   private algorithms to compute flooding topology, so long as all nodes
   in the IGP area use the same algorithm.

   While the flooding topology should be a covering cycle, it need not
   be a Hamiltonian cycle where each node appears only once.  In fact,
   in many relevant topologies this will not be possible e.g., K5,8.
   This is fortunate, as computing a Hamiltonian cycle is known to be
   NP-complete.





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   A simple algorithm to compute the topology for a complete bipartite
   graph is to simply select unvisited nodes on each side of the graph
   until both sides are completely visited.  If the number of nodes on
   each side of the graph are unequal, then revisiting nodes on the less
   populated side of the graph will be inevitable.  This algorithm can
   run in O(N) time, so is quite efficient.

   While a simple cycle is adequate for correctness and resiliency, it
   may not be optimal for convergence.  At scale, a cycle may have a
   diameter that is half the number of nodes in the graph.  This could
   cause an undue delay in link state update propagation.  Therefore it
   may be useful to have a bound on the diameter of the flooding
   topology.  Introducing more links into the flooding topology would
   reduce the diameter, but at the trade-off of possibly adding
   redundant messaging.  The optimal trade-off between convergence time
   and graph diameter is for further study.

   Similarly, if additional redundancy is added to the flooding
   topology, specific nodes in that topology may end up with a very high
   degree.  This could result in overloading the control plane of those
   nodes, resulting in poor convergence.  Thus, it may be optimal to
   have an upper bound on the degree of nodes in the flooding topology.
   Again, the optimal trade-off between graph diameter, node degree, and
   convergence time, and topology computation time is for further study.

   If the leader chooses to include a multi-node broadcast LAN segment
   as part of the flooding topology, all of the connectivity to that LAN
   segment should be included as well.  Once updates are flooded onto
   the LAN, they will be received by every attached node.

4.4.  Topologies on Complete Bipartite Graphs

   Complete bipartite graph topologies have become popular for data
   center applications and are commonly called leaf-spine or spine-leaf
   topologies.  In this section, we discuss some flooding topologies
   that are of particular interest in these networks.

4.4.1.  A Minimal Flooding Topology

   We define a Minimal Flooding Topology on a complete bipartite graph
   as one in which the topology is connected and each node has at least
   degree two.  This is of interest because it guarantees that the
   flooding topology has no single points of failure.

   In practice, this implies that every leaf node in the flooding
   topology will have a degree of two.  As there are usually more leaves
   than spines, the degree of the spines will be higher, but the load on
   the individual spines can be evenly distributed.



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   This type of flooding topology is also of interest because it scales
   well.  As the number of leaves increases, we can construct flooding
   topologies that perform well.  Specifically, for n spines and m
   leaves, if m >= n(n/2-1), then there is a flooding topology that has
   a diameter of four.

4.4.2.  Xia Topologies

   We define a Xia Topology on a complete bipartite graph as one in
   which all spine nodes are bi-connected through leaves with degree
   two, but the remaining leaves all have degree one and are evenly
   distributed across the spines.

   Constructively, we can create a Xia topology by iterating through the
   spines.  Each spine can be connected to the next spine by selecting
   any unused leaf.  Since leaves are connected to all spines, all
   leaves will have a connection to both the first and second spine and
   we can therefore choose any leaf without loss of generality.
   Continuing this iteration across all of the spines, selecting a new
   leaf at each iteration, will result in a path that connects all
   spines.  Adding one more leaf between the last and first spine will
   produce a cycle of n spines and n leaves.

   At this point, m-n leaves remain unconnected.  These can be
   distributed evenly across the remaining spines, connected by a single
   link.

   Xia topologies represent a compromise that trades off increased risk
   and decreased performance for lower flooding amplification.  Xia
   topologies will have a larger diameter.  For m spines, the diameter
   will be m + 2.

   In a Xia topology, some leaves are singly connected.  This represents
   a risk in that in some failures, convergence may be delayed.
   However, there may be some alternate behaviors that can be employed
   to mitigate these risks.  If a leaf node sees that its single link on
   the flooding topology has failed, it can compensate by performing a
   database synchronization check with a different spine.  Similarly, if
   a leaf determines that its connected spine on the flooding topology
   has failed, it can compensate by performing a database
   synchronization check with a different spine.  In both of these
   cases, the synchronization check is intended to ameliorate any delays
   in link state propagation due to the fragmentation of the flooding
   topology.

   The benefit of this topology is that flooding load is easily
   understood.  Each node in the spine cycle will never receive an




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   update more than twice.  For n leaves and m spines, a spine never
   transmits more than m/n updates.

4.4.3.  Optimization

   If two systems have multiple links between them, only one of the
   links should be part of the flooding topology.  Moreover, symmetric
   selection of the link to use for flooding is not required.

4.5.  Encoding the Flooding Topology

   There are a variety of ways that the flooding topology could be
   encoded efficiently.  If the topology was only a cycle, a simple list
   of the nodes in the topology would suffice.  However, this is
   insufficiently flexible as it would require a slightly different
   encoding scheme as soon as a single additional link is added.
   Instead, we choose to encode the flooding topology as a set of
   intersecting paths, where each path is a set of connected edges.

   Other encodings are certainly possible.  We have attempted to make a
   useful trade off between simplicity, generality, and space.

4.6.  Analysis of Topology Changes

   In this section, we explicitly consider a variety of different
   topological failures in the network and how dynamic flooding should
   address them.

4.6.1.  Link Addition

   If a link is added to the topology, the protocol will form a normal
   adjacency on the link and update the appropriate link state
   advertisements for the routers on either end of the link.  These link
   state updates will be flooded on the flooding topology.

   In centralized mode, the Area Leader, upon receiving these updates,
   may choose to retain the existing flooding topology or may choose to
   modify the flooding topology.  If it elects to change the flooding
   topology, it will update the flooding topology in the link state
   database and flood it using the new flooding topology.

   In distributed mode, any change in the topology, including the link
   addition, should trigger the flooding topology recalculation.  This
   is done to ensure that all nodes converge on the same flooding
   topology, regardless of the time of the calculation.






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4.6.2.  Node Addition

   In centralized mode, if a node is added to the topology, then at
   least one link is also added to the topology.  The paragraph above
   applies and the Area Leader will necessarily need to add the new node
   to the flooding topology.

   In distributed mode, the addition of a node should trigger flooding
   topology recalculation.

   Until the new node is incorporated into the flooding topology at
   least a single link towards the new node MUST be added to the
   flooding topology locally on all of its neighbors.

4.6.3.  Link Failures Off the Flooding Topology

   If a link that is not part of the flooding topology fails, then the
   adjoining routers will update their link state advertisements and
   flood them on the flooding topology.  There is no need for changes to
   the flooding topology.

4.6.4.  Failure of the Area Leader

   The failure of the Area Leader can be detected by observing that it
   is disconnected from the area topology.  In this case, the Area
   Leader election process is repeated and a new Area Leader is elected.

   In the centralized mode, the new Area Leader will compute a new
   flooding topology and flood it using the new flooding topology.

   As an optimization, applicable to centralized mode, the new Area
   Leader MAY compute a new flooding topology that has as much in common
   as possible with the old flooding topology.  This will minimize the
   risk of over-flooding.

4.6.5.  Failures on the Flooding Topology

   If there is a failure on the flooding topology, the adjoining routers
   will update their link state advertisements and flood them.  If the
   original flooding topology is bi-connected, the flooding topology
   should still be connected despite a single failure.

   In centralized mode, the Area Leader will notice the change in the
   flooding topology, recompute the flooding topology, and flood it
   using the new flooding topology.






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   In distributed mode, all routers supporting dynamic flooding will
   notice the change in the flooding topology and recompute the new
   flooding topology.

4.6.6.  Recovery from Multiple Failures

   In the unlikely event of multiple failures on the flooding topology,
   it may become disconnected.  The nodes that remain active on the
   edges of the flooding topology will recognize this, update their own
   link state advertisements and flood them on the remainder of the
   flooding topology.  At this point, nodes will be able to compute that
   the flooding topology is partitioned.

   Note that this is very different from partitioning the area itself.
   The area may remain connected and forwarding may still be effective.

   When this condition is detected, the flooding topology can no longer
   be expected to deliver link state updates in a prompt manner.  Nodes
   on the edges of the flooding topology should perform database
   synchronization on all links not on the flooding topology.  Updates
   received from off of the flooding topology should be flooded on the
   remaining flooding topology.  Any links that provide updates or
   require updates that are not part of the flooding topology should
   temporarily be added to the flooding topology.  This should repair
   the current flooding topology, albeit in a sub-optimal manner.

   In centrailzed mode, the Area Leader will also detect this condition,
   compute a new flooding topology, and flood it using the new flooding
   topology.

   In distributed mode, all routers that actively participate in Dynamic
   Flooding will compute the new flooding topology.

5.  Protocol Elements

5.1.  IS-IS TLVs

   The following TLVs are added to IS-IS:

   1.  A TLV that an IS may inject into its LSP to indicate its
       preference for becoming Area Leader.

   2.  A TLV to carry the list of system IDs that compromise the
       flooding topology for the area.

   3.  A TLV to carry the adjacency matrix for the flooding topology for
       the area.




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5.1.1.  IS-IS Area Leader Sub-TLV

   The Area Leader Sub-TLV allows a system to:

   1.  Indicate its eligibility and priority for becoming Area Leader.

   2.  Indicate whether centralized or distributed mode is to be used to
       compute the flooding topology in the area.

   3.  Indicate the algorithm identifier for the algorithm that is used
       to compute the flooding topology in distributed mode.

   Intermediate Systems (routers) that are not advertising this Sub-TLV
   are not eligible to become Area Leader.

   The Area Leader is the router with the numerically highest Area
   Leader priority in the area.  In the event of ties, the router with
   the numerically highest system ID is the Area Leader.  Due to
   transients during database flooding, different routers may not agree
   on the Area Leader.

   The Area Leader Sub-TLV is advertised as a Sub-TLV of the IS-IS
   Router Capability TLV-242 that is defined in [RFC7981] and has the
   following format:

            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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           | TLV Type      | TLV Length    | Priority      |   Algorithm   |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TLV Type: TBD1

      TLV Length: 2

      Priority: 0-255, unsigned integer

      Algorithm - a numeric identifier in the range 0-255 that
      identifies the algorithm used to calculate the flooding topology.
      The following values are defined:

         0: Centralized computation by the Area Leader.

         1-127: Standardized distributed algorithms.  Individual values
         area assigned and managed by IANA.  Before any assignments can
         be made, there MUST be an IETF specification that specifies
         IANA allocation for any value from this range (see
         Section 7.3).



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         128-254: Private distributed algorithms.  Values from this
         range will not be registered with IANA and MUST NOT be
         mentioned by RFCs.

         255: Reserved

5.1.2.  IS-IS Area System IDs TLV

   IS-IS Area System IDs TLV is only used in centralized mode.

   The Area System IDs TLV is used by the Area Leader to enumerate the
   system IDs that it has used in computing the flooding topology.
   Conceptually, the Area Leader creates a list of system IDs for all
   routers in the area, assigning indices to each system, starting with
   index 0.

   Because the space in a single TLV is small, more than one TLV may be
   required to encode all of the system IDs in the area.  This TLV may
   be present in multiple LSPs.

   The format of the Area System IDs TLV is:

            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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           | TLV Type      | TLV Length    | Starting Index                |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |L| Reserved    | System IDs ...
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           System IDs continued ....
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TLV Type: TBD2

      TLV Length: 3 + (System ID length * (number of System IDs))

      Starting index: The index of the first system ID that appears in
      this TLV.

      L (Last): This bit is set if the index of the last system ID that
      appears in this TLV is equal to the last index in the full list of
      system IDs for the area.

      System IDs: A concatenated list of system IDs for the area.

   If there are multiple IS-IS Area System IDs TLVs with the L bit set
   advertised by the same router, the TLV which specifies the smaller
   maximum index is used and the other TLV(s) with L bit set are



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   ignored.  TLVs which specify system IDs with indices greater than
   that specified by the TLV with the L bit set are also ignored.

5.1.3.  IS-IS Flooding Path TLV

   IS-IS Flooding Path TLV is only used in centralized mode.

   The Flooding Path TLV is used to denote a path in the flooding
   topology.  The goal is an efficient encoding of the links of the
   topology.  A single link is a simple case of a path that only covers
   two nodes.  A connected path may be described as a sequence of
   indices: (I1, I2, I3, ...), denoting a link from the system with
   index 1 to the system with index 2, a link from the system with index
   2 to the system with index 3, and so on.

   If a path exceeds the size that can be stored in a single TLV, then
   the path may be distributed across multiple TLVs by the replication
   of a single system index.

   Complex topologies that are not a single path can be described using
   multiple TLVs.

   The Flooding Path TLV contains a list of system indices relative to
   the systems advertised through the Area System IDs TLV.  At least 2
   indices must be included in the TLV.  Due to the length restriction
   of TLVs, this TLV can contain at most 126 system indices.

   The Flooding Path TLV has the format:

            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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           | TLV Type      | TLV Length    | Starting Index                |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           | Index 2                       | Additional indices ...
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TLV Type: TBD3

      TLV Length: 2 * (number of indices in the path)

      Starting index: The index of the first system in the path.

      Index 2: The index of the next system in the path.

      Additional indices (optional): A sequence of additional indices to
      systems along the path.




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5.2.  OSPF LSAs and TLVs

   This section defines new LSAs and TLVs for both OSPFv2 and OSPFv3.

5.2.1.  OSPF Area Leader Sub-TLV

   The usage of the OSPF Area Leader Sub-TLV is identical to IS-IS and
   is described in Section 5.1.1.

   The OSPF Area Leader Sub-TLV is used by both OSPFv2 and OSPFv3.

   The OSPF Area Leader Sub-TLV is advertised as a top-level TLV of the
   RI LSA that is defined in [RFC7770] and has the following format:


       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             |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Priority   |   Algorithm   |            Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: TBD4

      Length: 4 octets

      Priority: 0-255, unsigned integer

      Algorithm: as defined in Section 5.1.1.

5.2.2.  OSPFv2 Dynamic Flooding Opaque LSA

   The OSPFv2 Dynamic Flooding Opaque LSA is only used in centralized
   mode.

   The OSPFv2 Dynamic Flooding Opaque LSA is used to advertise
   additional data related to the dynamic flooding in OSPFv2.  OSPFv2
   Opaque LSAs are described in [RFC5250].

   Multiple OSPFv2 Dynamic Flooding Opaque LSAs can be advertised by an
   OSPFv2 router.  The flooding scope of the OSPFv2 Dynamic Flooding
   Opaque LSA is area-local.

   The format of the OSPFv2 Dynamic Flooding Opaque LSA is as follows:






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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   |   LS Type     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      TBD5     |                 Opaque ID                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Advertising Router                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     LS sequence number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-                            TLVs                             -+
      |                             ...                               |

                    OSPFv2 Dynamic Flooding Opaque LSA

   The opaque type used by OSPFv2 Dynamic Flooding Opaque LSA is TBD.
   The opaque type is used to differentiate the various type of OSPFv2
   Opaque LSAs and is described in section 3 of [RFC5250].  The LS Type
   is 10.  The LSA Length field [RFC2328] represents the total length
   (in octets) of the Opaque LSA including the LSA header and all TLVs
   (including padding).

   The Opaque ID field is an arbitrary value used to maintain multiple
   Dynamic Flooding Opaque LSAs.  For OSPFv2 Dynamic Flooding Opaque
   LSAs, the Opaque ID has no semantic significance other than to
   differentiate Dynamic Flooding Opaque LSAs originated by the same
   OSPFv2 router.

   The format of the TLVs within the body of the OSPFv2 Dynamic Flooding
   Opaque LSA is the same as the format used by the Traffic Engineering
   Extensions to OSPF [RFC3630].

   The Length field defines the length of the value portion in octets
   (thus a TLV with no value portion would have a length of 0).  The TLV
   is padded to 4-octet alignment; padding is not included in the length
   field (so a 3-octet value would have a length of 3, but the total
   size of the TLV would be 8 octets).  Nested TLVs are also 32-bit
   aligned.  For example, a 1-octet value would have the length field
   set to 1, and 3 octets of padding would be added to the end of the
   value portion of the TLV.  The padding is composed of zeros.







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5.2.3.  OSPFv3 Dynamic Flooding LSA

   The OSPFv3 Dynamic Flooding Opaque LSA is only used in centralized
   mode.

   The OSPFv3 Dynamic Flooding LSA is used to advertise additional data
   related to the dynamic flooding in OSPFv3.

   The OSPFv3 Dynamic Flooding LSA has a function code of TBD.  The
   flooding scope of the OSPFv3 Dynamic Flooding LSA is area-local.  The
   U bit will be set indicating that the OSPFv3 Dynamic Flooding LSA
   should be flooded even if it is not understood.  The Link State ID
   (LSID) value for this LSA is the Instance ID.  OSPFv3 routers MAY
   advertise multiple Dynamic Flooding Opaque LSAs in each area.

   The format of the OSPFv3 Dynamic Flooding LSA is as follows:


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |1|0|1|          TBD6           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Link State ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Advertising Router                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    LS sequence number                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        LS checksum            |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                            TLVs                             -+
       |                             ...                               |


                        OSPFv3 Dynamic Flooding LSA

5.2.4.  OSPF Area Router IDs TLV

   The OSPF Area Router IDs TLV is a top level TLV of the OSPFv2 Dynamic
   Flooding Opaque LSA and OSPFv3 Dynamic Flooding LSA.

   The OSPF Area Router IDs TLV is used by the Area Leader to enumerate
   the Router IDs that it has used in computing the flooding topology.
   Conceptually, the Area Leader creates a list of Router IDs for all
   routers in the area, assigning indices to each router, starting with
   index 0.



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   Because the space in a single OSPF Area Router IDs TLV is limited,
   more than one TLV may ve required to encode all of the Router IDs in
   the area.  This TLV may also recur in multiple OSPFv2 Dynamic
   Flooding Opaque LSAs or OSPFv3 Dynamic Flooding LSA, so that all
   Router IDs can be advertised.

   The format of the Area Router IDs TLV is:


        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             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Starting Index             |L| Flags       |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                        Router IDs                           -+
       |                           ...                                 |

                         OSPF Area Router IDs TLV

      TLV Type: 1

      TLV Length: 4 + (Router ID length * (number of Router IDs))

      Starting index: The index of the first Router ID that appears in
      this TLV.

      L (Last): This bit is set if the index of the last system ID that
      appears in this TLV is equal to the last index in the full list of
      Rourer IDs for the area.

      Router IDs: A concatenated list of Router IDs for the area.

   If there are multiple OSPF Area Router IDs TLVs with the L bit set
   advertised by the same router, the TLV which specifies the smaller
   maximum index is used and the other TLV(s) with L bit set are
   ignored.  TLVs which specify Router IDs with indices greater than
   that specified by the TLV with the L bit set are also ignored.

5.2.5.  OSPF Flooding Path TLV

   The OSPF Flooding Path TLV is a top level TLV of the OSPFv2 Dynamic
   Flooding Opaque LSAs and OSPFv3 Dynamic Flooding LSA.

   The usage of the OSPF Flooding Path TLV is identical to IS-IS and is
   described in Section 5.1.3.



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   The OSPF Flooding Path TLV contains a list of Router ID indices
   relative to the Router IDs advertised through the OSPF Area Router
   IDs TLV.  At least 2 indices must be included in the TLV.

   Multiple OSPF Flooding Path TLVs can be advertised in a single OSPFv2
   Dynamic Flooding Opaque LSA or OSPFv3 Dynamic Flooding LSA.  OSPF
   Flooding Path TLVs can also be advertised in multiple OSPFv2 Dynamic
   Flooding Opaque LSAs or OSPFv3 Dynamic Flooding LSA, if they all can
   not fit in a single LSA.

   The Flooding Path TLV has the format:


        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             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Starting Index             |       Index 2                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +-                        Additional Indices                   -+
       |                           ...                                 |



                          OSPF Flooding Path TLV

      TLV Type: 2

      TLV Length: 2 * (number of indices in the path)

      Starting index: The index of the first Router ID in the path.

      Index 2: The index of the next Router ID in the path.

      Additional indices (optional): A sequence of additional indices to
      Router IDs along the path.

6.  Behavioral Specification

   In this section, we specify the detailed behaviors of the nodes
   participating in the IGP.








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6.1.  Leader Election

   Any node that is capable MAY advertise its eligibility to become Area
   Leader.

   Nodes that are not reachable are not eligible as Area Leader.  Nodes
   that do not advertise their eligibility to become Area Leader are not
   eligible.  Amongst the eligible nodes, the node with the numerically
   highest priority is the Area Leader.  If multiple nodes all have the
   highest priority, then the node with the numerically highest system
   identifier in the case of IS-IS, or Router-ID in the case of OSPFv2
   and OSPFv3 is the Area Leader.

6.2.  Area Leader Responsibilities

   If the Area Leader operates in centralized mode, it MUST advertise
   algorithm 0 in its Area Leader Sub-TLV.  It also MUST compute and
   advertise a flooding topology for the area.  The Area Leader MAY
   update the flooding topology at any time, however, it should not
   destabilize the network with undue or overly frequent topology
   changes.

   The flooding topology MUST include all reachable nodes in the area.
   If nodes become unreachable on the flooding topology, the flooding
   topology MUST be recalculated.  In centralized mode, the Area Leader
   MUST advertise a new flooding topology.

   The flooding topology MAY be bi-connected.  This is strongly
   RECOMMENDED but not required.

6.3.  Distributed Flooding Topology Calculation

   If the Area Leader advertises a non-zero algorithm in its Area Leader
   Sub-TLV, all routers in the area that support Dynamic Flooding and
   the value of algorithm advertised by the Area Leader MUST compute the
   flooding topology based on the Area Leader's advertised algorithm.
   Routers that do not support the value of algorithm advertised by the
   Area Leader MUST continue to use legacy flooding mechanism as defined
   by the protocol.

   If the value of the algorithm advertised by the Area Leader is from
   the range 128-254 (Private distributed algorithms), it is the
   responsibility of the network operator to guarantee that all nodes in
   the area have a common understanding of what the given algorithm
   value represents.






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6.4.  Flooding Behavior

   Nodes that support Dynamic Flooding MUST use the flooding topology
   for flooding.  The flooding topology is calculated locally in the
   case of distributed mode.  In centralized mode the flooding topology
   is advertised in the area link state database.  Link state updates
   received on one link in the flooding topology MUST be flooded on all
   other links in the flooding topology other than the link on which the
   update has been received.  Link state updates received on a link not
   in the flooding topology MUST be flooded on all links in the flooding
   topology.

   In centralized mode, if multiple flooding topologies are present in
   the area link state database, the node SHOULD flood on the union of
   the topologies.

   When the flooding topology changes on a node, either as a result of
   the local computation in distributed mode or as a result of the
   advertisement from the Area Leader in centralized mode, the node MUST
   continue to flood on the union of the old and new flooding topology
   for a limited amount of time.  This is required to provide all nodes
   sufficient time to migrate to the new flooding topology.

   When failures occur, nodes will learn about them from link state
   updates and can compare those to the existing flooding topology.  If
   the flooding topology becomes disconnected, then the nodes at the
   edges of the flooding topology should perform a database
   synchronization on all links.  While the flooding topology is
   disconnected, if a new link state update is received on a link not in
   the flooding topology, then the node SHOULD temporarily consider the
   link as part of the flooding topology.  When a new flooding topology
   is received or locally calculated, this MUST be discontinued.

7.  IANA Considerations

7.1.  IS-IS

   This document requests the following code point from the "sub-TLVs
   for TLV 242" registry (IS-IS Router CAPABILITY TLV).

      Type: TBD1

      Description: IS-IS Area Leader Sub-TLV

      Reference: This document (Section 5.1.1)






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   This document requests that IANA allocate and assign two code points
   from the "IS-IS TLV Codepoints" registry.  One for each of the
   following TLVs:

      Type: TBD2

      Description: IS-IS Area System IDs TLV

      Reference: This document (Section 5.1.2)

      Type: TBD3

      Description: IS-IS Flooding Path TLV

      Reference: This document (Section 5.1.3)

7.2.  OSPF

   This document requests the following code point from the "OSPF Router
   Information (RI) TLVs" registry:

      Type: TBD4

      Description: OSPF Area Leader Sub-TLV

      Reference: This document (Section 5.2.1)

   This document requests the following code point from the "Opaque
   Link-State Advertisements (LSA) Option Types" registry:

      Type: TBD5

      Description: OSPFv2 Dynamic Flooding Opaque LSA

      Reference: This document (Section 5.2.2)

   This document requests the following code point from the "OSPFv3 LSA
   Function Codes" registry:

      Type: TBD6

      Description: OSPFv3 Dynamic Flooding LSA

      Reference: This document (Section 5.2.3)







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7.2.1.  OSPF Dynamic Flooding LSA TLVs Registry

   This specification also requests one new registry - "OSPF Dynamic
   Flooding LSA TLVs".  New values can be allocated via IETF Review or
   IESG Approval

   The "OSPF Dynamic Flooding LSA TLVs" registry will define top-level
   TLVs for the OSPFv2 Dynamic Flooding Opaque LSA and OSPFv3 Dynamic
   Flooding LSAs.  It should be added to the "Open Shortest Path First
   (OSPF) Parameters" registries group.

   The following initial values are allocated:

      Type: 0

      Description: Reserved

      Reference: This document

      Type: 1

      Description: OSPF Area Router IDs TLV

      Reference: This document (Section 5.2.4)

      Type: 2

      Description: OSPF Flooding Path TLV

      Reference: This document (Section 5.2.5)

   Types in the range 32768-33023 are for experimental use; these will
   not be registered with IANA, and MUST NOT be mentioned by RFCs.

   Types in the range 33024-65535 are not to be assigned at this time.
   Before any assignments can be made in the 33024-65535 range, there
   MUST be an IETF specification that specifies IANA Considerations that
   covers the range being assigned.

7.3.  IGP

   IANA is requested to set up a registry called "IGP Algorithm Type For
   Computing Flooding Topology" under an existing "Interior Gateway
   Protocol (IGP) Parameters" IANA registries.  The registration policy
   for this registry is "Standards Action" ([RFC8126] and [RFC7120]).

   Values in this registry come from the range 0-255.




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   The initial values in the IGP Algorithm Type For Computing Flooding
   Topology registry are:

      0: Reserved for centralized mode.

      1-127: Available for standards action.

      128-254: Reserved for private use.

      255: Reserved.

8.  Security Considerations

   This document introduces no new security issues.  Security of routing
   within a domain is already addressed as part of the routing protocols
   themselves.  This document proposes no changes to those security
   architectures.

   It is possible that an attacker could become Area Leader and
   introduce a flawed flooding algorithm into the network thus
   compromising the operation of the protocol.  Authentication methods
   as describe in [RFC5304] and [RFC5310] for IS-IS, [RFC2328] and
   [RFC7474] for OSPFv2 and [RFC5340] and [RFC4552] for OSPFv3 SHOULD be
   used to prevent such attack.

9.  Acknowledgements

   The authors would like to thank Les Ginsberg, Zeqing (Fred) Xia,
   Naiming Shen, Adam Sweeney and Olufemi Komolafe for their helpful
   comments.

   The authors would like to thank Tom Edsall for initially introducing
   them to the problem.

10.  References

10.1.  Normative References

   [ISO10589]
              International Organization for Standardization,
              "Intermediate System to Intermediate System Intra-Domain
              Routing Exchange Protocol for use in Conjunction with the
              Protocol for Providing the Connectionless-mode Network
              Service (ISO 8473)", ISO/IEC 10589:2002, Nov. 2002.







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   [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>.

   [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>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/info/rfc7120>.

   [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>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,
              <https://www.rfc-editor.org/info/rfc7981>.



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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

10.2.  Informative References

   [Clos]     Clos, C., "A Study of Non-Blocking Switching Networks",
              The Bell System Technical Journal Vol. 32(2), DOI
              10.1002/j.1538-7305.1953.tb01433.x, March 1953,
              <http://dx.doi.org/10.1002/j.1538-7305.1953.tb01433.x>.

   [Leiserson]
              Leiserson, C., "Fat-Trees: Universal Networks for
              Hardware-Efficient Supercomputing", IEEE Transactions on
              Computers 34(10):892-901, 1985.

   [RFC2973]  Balay, R., Katz, D., and J. Parker, "IS-IS Mesh Groups",
              RFC 2973, DOI 10.17487/RFC2973, October 2000,
              <https://www.rfc-editor.org/info/rfc2973>.

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

   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,
              <https://www.rfc-editor.org/info/rfc7938>.

Authors' Addresses

   Tony Li
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California  95054
   USA

   Email: tony.li@tony.li











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   Peter Psenak
   Cisco Systems, Inc.
   Eurovea Centre, Central 3
   Pribinova Street 10
   Bratislava  81109
   Slovakia

   Email: ppsenak@cisco.com











































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