





Network Working Group                                           R. Ogier
Request for Comments: 5614                             SRI International
Category: Experimental                                       P. Spagnolo
                                                                  Boeing
                                                             August 2009


            Mobile Ad Hoc Network (MANET) Extension of OSPF
             Using Connected Dominating Set (CDS) Flooding

Abstract

   This document specifies an extension of OSPFv3 to support mobile ad
   hoc networks (MANETs).  The extension, called OSPF-MDR, is designed
   as a new OSPF interface type for MANETs.  OSPF-MDR is based on the
   selection of a subset of MANET routers, consisting of MANET
   Designated Routers (MDRs) and Backup MDRs.  The MDRs form a connected
   dominating set (CDS), and the MDRs and Backup MDRs together form a
   biconnected CDS for robustness.  This CDS is exploited in two ways.
   First, to reduce flooding overhead, an optimized flooding procedure
   is used in which only (Backup) MDRs flood new link state
   advertisements (LSAs) back out the receiving interface; reliable
   flooding is ensured by retransmitting LSAs along adjacencies.
   Second, adjacencies are formed only between (Backup) MDRs and a
   subset of their neighbors, allowing for much better scaling in dense
   networks.  The CDS is constructed using 2-hop neighbor information
   provided in a Hello protocol extension.  The Hello protocol is
   further optimized by allowing differential Hellos that report only
   changes in neighbor states.  Options are specified for originating
   router-LSAs that provide full or partial topology information,
   allowing overhead to be reduced by advertising less topology
   information.

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.












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Copyright Notice

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   document authors.  All rights reserved.

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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................5
   2. Overview ........................................................7
      2.1. Selection of MDRs, BMDRs, Parents, and Adjacencies .........8
      2.2. Flooding Procedure .........................................9
      2.3. Link State Acknowledgments ................................10
      2.4. Routable Neighbors ........................................10
      2.5. Partial and Full Topology LSAs ............................11
      2.6. Hello Protocol ............................................12
   3. Interface and Neighbor Data Structures .........................12
      3.1. Changes to Interface Data Structure .......................12
      3.2. New Configurable Interface Parameters .....................13
      3.3. Changes to Neighbor Data Structure ........................15
   4. Hello Protocol .................................................17
      4.1. Sending Hello Packets .....................................17
      4.2. Receiving Hello Packets ...................................20
      4.3. Neighbor Acceptance Condition .............................24
   5. MDR Selection Algorithm ........................................25
      5.1. Phase 1: Creating the Neighbor Connectivity Matrix ........27
      5.2. Phase 2: MDR Selection ....................................27
      5.3. Phase 3: Backup MDR Selection .............................29
      5.4. Phase 4: Parent Selection .................................29
      5.5. Phase 5: Optional Selection of Non-Flooding MDRs ..........30



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   6. Interface State Machine ........................................31
      6.1. Interface States ..........................................31
      6.2. Events that Cause Interface State Changes .................31
      6.3. Changes to Interface State Machine ........................32
   7. Adjacency Maintenance ..........................................32
      7.1. Changes to Neighbor State Machine .........................33
      7.2. Whether to Become Adjacent ................................34
      7.3. Whether to Eliminate an Adjacency .........................35
      7.4. Sending Database Description Packets ......................35
      7.5. Receiving Database Description Packets ....................36
   8. Flooding Procedure .............................................37
      8.1. LSA Forwarding Procedure ..................................38
      8.2. Sending Link State Acknowledgments ........................41
      8.3. Retransmitting LSAs .......................................42
      8.4. Receiving Link State Acknowledgments ......................42
   9. Router-LSAs ....................................................43
      9.1. Routable Neighbors ........................................44
      9.2. Backbone Neighbors ........................................45
      9.3. Selected Advertised Neighbors .............................45
      9.4. Originating Router-LSAs ...................................46
   10. Calculating the Routing Table .................................47
   11. Security Considerations .......................................49
   12. IANA Considerations ...........................................50
   13. Acknowledgments ...............................................51
   14. Normative References ..........................................51
   15. Informative References ........................................51
   Appendix A.  Packet Formats .......................................52
      A.1.  Options Field ............................................52
      A.2.  Link-Local Signaling .....................................52
      A.3.  Hello Packet DR and Backup DR Fields .....................57
      A.4.  LSA Formats and Examples .................................57
   Appendix B.  Detailed Algorithms for MDR/BMDR Selection ...........62
      B.1.  Detailed Algorithm for Step 2.4 (MDR Selection) ..........62
      B.2.  Detailed Algorithm for Step 3.2 (BMDR Selection) .........63
   Appendix C.  Min-Cost LSA Algorithm ...............................65
   Appendix D.  Non-Ackable LSAs for Periodic Flooding ...............68
   Appendix E.  Simulation Results ...................................69














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1.  Introduction

   This document specifies an extension of OSPFv3 [RFC5340] to support a
   new interface type for mobile ad hoc networks (MANETs), i.e., for
   broadcast-capable, multihop wireless networks in which routers and
   hosts can be mobile.  Note that OSPFv3 is specified by describing the
   modifications to OSPFv2 [RFC2328].  This MANET extension of OSPFv3 is
   also applicable to non-mobile mesh networks using layer-3 routing.
   This extension does not preclude the use of any existing OSPF
   interface types, and is fully compatible with legacy OSPFv3
   implementations.

   Existing OSPF interface types do not perform adequately in MANETs,
   due to scaling issues regarding the flooding protocol operation,
   inability of the Designated Router election protocol to converge in
   all scenarios, and large numbers of adjacencies when using a point-
   to-multipoint interface type.

   The approach taken is to generalize the concept of an OSPF Designated
   Router (DR) and Backup DR to multihop wireless networks, in order to
   reduce overhead by reducing the number of routers that must flood new
   LSAs and reducing the number of adjacencies.  The generalized
   (Backup) Designated Routers are called (Backup) MANET Designated
   Routers (MDRs).  The MDRs form a connected dominating set (CDS), and
   the MDRs and Backup MDRs together form a biconnected CDS for
   robustness (if the network itself is biconnected).  By definition,
   each router in the MANET either belongs to the CDS or is one hop away
   from it.  A distributed algorithm is used to select and dynamically
   maintain the biconnected CDS.  Adjacencies are established only
   between (Backup) MDRs and a subset of their neighbors, thus resulting
   in a dramatic reduction in the number of adjacencies in dense
   networks, compared to the approach of forming adjacencies between all
   neighbor pairs.  The OSPF extension is called OSPF-MDR.

   Hello packets are modified, using OSPF link-local signaling (LLS; see
   [RFC5613]), for two purposes: to provide neighbors with 2-hop
   neighbor information that is required by the MDR selection algorithm,
   and to allow differential Hellos that report only changes in neighbor
   states.  Differential Hellos can be sent more frequently without a
   significant increase in overhead, in order to respond more quickly to
   topology changes.

   Each MANET router advertises a subset of its MANET neighbors as
   point-to-point links in its router-LSA.  The choice of which
   neighbors to advertise is flexible, allowing overhead to be reduced
   by advertising less topology information.  Options are specified for
   originating router-LSAs that provide full or partial topology
   information.



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   This document is organized as follows.  Section 2 presents an
   overview of OSPF-MDR, Section 3 presents the new interface and
   neighbor data items that are required for the extension, Section 4
   describes the Hello protocol, including procedures for maintaining
   the 2-hop neighbor information, Section 5 describes the MDR selection
   algorithm, Section 6 describes changes to the Interface state
   machine, Section 7 describes the procedures for forming adjacencies
   and deciding which neighbors should become adjacent, Section 8
   describes the flooding procedure, Section 9 specifies the
   requirements and options for the contents of router-LSAs, and Section
   10 describes changes in the calculation of the routing table.

   The appendices specify packet formats, detailed algorithms for the
   MDR selection algorithm, an algorithm for the selection of a subset
   of neighbors to advertise in the router-LSA to provide shortest-path
   routing, a proposed option that uses non-ackable LSAs to provide
   periodic flooding without the overhead of Link State Acknowledgments,
   and simulation results that predict the performance of OSPF-MDR in
   mobile networks with up to 200 nodes.  Additional information and
   resources for OSPF-MDR can be found at http://www.manet-routing.org.

1.1.  Terminology

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

   In addition, this document uses the following terms:

   MANET Interface
      A MANET Interface is a new OSPF interface type that supports
      broadcast-capable, multihop wireless networks.  Two neighboring
      routers on a MANET interface may not be able to communicate
      directly with each other.  A neighboring router on a MANET
      interface is called a MANET neighbor.  MANET neighbors are
      discovered dynamically using a modification of OSPF's Hello
      protocol.

   MANET Router
      A MANET Router is an OSPF router that has at least one MANET
      interface.

   Differential Hello
      A Differential Hello is a Hello packet that reduces the overhead
      of sending full Hellos, by including only the Router IDs of
      neighbors whose state changed recently.





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   2-Hop Neighbor Information
      This information specifies the bidirectional neighbors of each
      neighbor.  The modified Hello protocol provides each MANET router
      with 2-hop neighbor information, which is used for selecting MDRs
      and Backup MDRs.

   MANET Designated Router (MDR)
      A MANET Designated Router is one of a set of routers responsible
      for flooding new LSAs, and for determining the set of adjacencies
      that must be formed.  The set of MDRs forms a connected dominating
      set and is a generalization of the DR found in broadcast networks.
      Each router runs the MDR selection algorithm for each MANET
      interface, to decide whether the router is an MDR, Backup MDR, or
      neither for that interface.

   Backup MANET Designated Router (Backup MDR or BMDR)
      A Backup MANET Designated Router is one of a set of routers
      responsible for providing backup flooding when neighboring MDRs
      fail.  The set of MDRs and Backup MDRs forms a biconnected
      dominating set.  The Backup MDR is a generalization of the Backup
      DR found in broadcast networks.

   MDR Other
      A router is an MDR Other for a particular MANET interface if it is
      neither an MDR nor a Backup MDR for that interface.

   Parent
      Each router selects a Parent for each MANET interface.  The Parent
      of a non-MDR router will be a neighboring MDR if one exists.  The
      Parent of an MDR is always the router itself.  Each non-MDR router
      becomes adjacent with its Parent.  The Router ID of the Parent is
      advertised in the DR field of each Hello sent on the interface.

   Backup Parent
      If the option of biconnected adjacencies is chosen, then each MDR
      Other selects a Backup Parent, which will be a neighboring MDR or
      BMDR if one exists that is not the Parent.  The Backup Parent of a
      BMDR is always the router itself.  Each MDR Other becomes adjacent
      with its Backup Parent if it exists.  The Router ID of the Backup
      Parent is advertised in the Backup DR field of each Hello sent on
      the interface.

   Bidirectional Neighbor
      A bidirectional neighbor is a neighboring router whose neighbor
      state is 2-Way or greater.






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   Routable Neighbor
      A bidirectional MANET neighbor becomes routable if the SPF
      calculation has produced a route to the neighbor and the neighbor
      satisfies a quality condition.  Once a neighbor becomes routable,
      it remains routable as long as it remains bidirectional.  Only
      routable and Full neighbors can be used as next hops in the SPF
      calculation, and can be included in the router-LSA originated by
      the router.

   Non-Flooding MDR
      A non-flooding MDR is an MDR that does not automatically flood
      received LSAs back out the receiving interface, but performs
      backup flooding like a BMDR.  Some MDRs may declare themselves
      non-flooding in order to reduce flooding overhead.

2.  Overview

   This section provides an overview of OSPF-MDR, including motivation
   and rationale for some of the design choices.

   OSPF-MDR was motivated by the desire to extend OSPF to support
   MANETs, while keeping the same design philosophy as OSPF and using
   techniques that are similar to those of OSPF.  For example, OSPF
   reduces overhead in a broadcast network by electing a Designated
   Router (DR) and Backup DR, and by having two neighboring routers form
   an adjacency only if one of them is the DR or Backup DR.  This idea
   can be generalized to a multihop wireless network by forming a
   spanning tree, with the edges of the tree being the adjacencies and
   the interior (non-leaf) nodes of the tree being the generalized DRs,
   called MANET Designated Routers (MDRs).

   To provide better robustness and fast response to topology changes,
   it was decided that a router should decide whether it is an MDR based
   only on local information that can be obtained from neighbors'
   Hellos.  The resulting set of adjacencies therefore does not always
   form a tree globally, but appears to be a tree locally.  Similarly,
   the Backup DR can be generalized to Backup MDRs (BMDRs), to provide
   robustness through biconnected redundancy.  The set of MDRs forms a
   connected dominating set (CDS), and the set of MDRs and BMDRs forms a
   biconnected dominating set (if the network itself is biconnected).

   The following subsections provide an overview of each of the main
   features of OSPF-MDR, starting with a summary of how MDRs, BMDRs, and
   adjacencies are selected.







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2.1.  Selection of MDRs, BMDRs, Parents, and Adjacencies

   The MDR selection algorithm is distributed; each router selects
   itself as an MDR, BMDR, or other router (called an "MDR Other") based
   on information about its one-hop neighborhood, which is obtained from
   Hello packets received from neighbors.  Routers are ordered
   lexicographically based on the tuple (RtrPri, MDR Level, RID), where
   RtrPri is the Router Priority, MDR Level represents the current state
   of the router (2 for an MDR, 1 for a BMDR, and 0 for an MDR Other),
   and RID is the Router ID.  Routers with lexicographically larger
   values of (RtrPri, MDR Level, RID) are given preference for becoming
   MDRs.

   The MDR selection algorithm can be summarized as follows.  If the
   router itself has a larger value of (RtrPri, MDR Level, RID) than all
   of its neighbors, it selects itself as an MDR.  Otherwise, let Rmax
   denote the neighbor with the largest value of (RtrPri, MDR Level,
   RID).  The router then selects itself as an MDR unless each neighbor
   can be reached from Rmax in at most k hops via neighbors that have a
   larger value of (RtrPri, MDR Level, RID) than the router itself,
   where k is the parameter MDRConstraint, whose default value is 3.

   This parameter serves to control the density of the MDR set, since
   the MDR set need not be strictly minimal.

   Similarly, a router that does not select itself as an MDR will select
   itself as a BMDR unless each neighbor can be reached from Rmax via
   two node-disjoint paths, using as intermediate hops only neighbors
   that have a larger value of (RtrPri, MDR Level, RID) than the router
   itself.

   When a router selects itself as an MDR, it also decides which MDR
   neighbors it should become adjacent with, to ensure that the set of
   MDRs and the adjacencies between them form a connected backbone.
   Each non-MDR router selects and becomes adjacent with an MDR neighbor
   called its Parent, thus ensuring that all routers are connected to
   the MDR backbone.

   If the option of biconnected adjacencies is chosen (AdjConnectivity =
   2), then additional adjacencies are selected to ensure that the set
   of MDRs and BMDRs, and the adjacencies between them, form a
   biconnected backbone.  In this case, each MDR Other selects and
   becomes adjacent with an MDR/BMDR neighbor called its Backup Parent,
   in addition to its Parent.







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   OSPF-MDR also provides the option of full-topology adjacencies
   (AdjConnectivity = 0).  If this option is selected, then each router
   forms an adjacency with each bidirectional neighbor.  Although BMDR
   selection is optional if AdjConnectivity is 0 or 1, it is recommended
   since BMDRs improve robustness by providing backup flooding.

   Prioritizing routers according to (RtrPri, MDR Level, RID) allows
   neighboring routers to agree on which routers should become an MDR,
   and gives higher priority to existing MDRs, which increases the
   lifetime of MDRs and the adjacencies between them.  In addition,
   Parents are selected to be existing adjacent neighbors whenever
   possible, to avoid forming new adjacencies unless necessary.  Once a
   neighbor becomes adjacent, it remains adjacent as long as the
   neighbor is bidirectional and either the neighbor or the router
   itself is an MDR or BMDR (similar to OSPF).  The above rules reduce
   the rate at which new adjacencies are formed, which is important
   since database exchange must be performed whenever a new adjacency is
   formed.

2.2.  Flooding Procedure

   When an MDR receives a new link state advertisement (LSA) on a MANET
   interface, it floods the LSA back out the receiving interface unless
   it can be determined that such flooding is unnecessary (as specified
   in Section 8.1).  The router MAY delay the flooding of the LSA by a
   small random amount of time (e.g., less than 100 ms).  The delayed
   flooding is useful for coalescing multiple LSAs in the same Link
   State Update packet, and it can reduce the possibility of a collision
   in case multiple MDRs received the same LSA at the same time.
   However, such collisions are usually avoided with wireless MAC
   protocols.

   When a Backup MDR receives a new LSA on a MANET interface, it waits a
   short interval (BackupWaitInterval), and then floods the LSA only if
   it has a neighbor that did not flood or acknowledge the LSA and is
   not known to be a neighbor of another neighbor (of the Backup MDR)
   that flooded the LSA.

   MDR Other routers never flood LSAs back out the receiving interface.
   To exploit the broadcast nature of MANETs, a new LSA is processed
   (and possibly forwarded) if it is received from any neighbor in state
   2-Way or greater.  The flooding procedure also avoids redundant
   forwarding of LSAs when multiple interfaces exist.








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2.3.  Link State Acknowledgments

   All Link State Acknowledgment packets are multicast.  An LSA is
   acknowledged if it is a new LSA, or if it is a duplicate LSA received
   as a unicast.  (A duplicate LSA received as multicast is not
   acknowledged.)  An LSA that is flooded back out the same interface is
   treated as an implicit acknowledgment.  Link State Acknowledgments
   may be delayed to allow coalescing multiple acknowledgments in the
   same packet.  The only exception is that (Backup) MDRs send a
   multicast Link State Acknowledgment immediately when a duplicate LSA
   is received as a unicast, in order to prevent additional
   retransmissions.  Only Link State Acknowledgments from adjacent
   neighbors are processed, and retransmitted LSAs are sent (via
   unicast) only to adjacent neighbors.

2.4.  Routable Neighbors

   In OSPF, a neighbor must typically be fully adjacent (in state Full)
   for it to be used in the SPF calculation.  An exception exists for an
   OSPF broadcast network, to avoid requiring all pairs of routers in
   such a network to form adjacencies, which would generate a large
   amount of overhead.  In such a network, a router can use a non-
   adjacent neighbor as a next hop as long as both routers are fully
   adjacent with the Designated Router.  We define this neighbor
   relationship as a "routable neighbor" and extend its usage to the
   MANET interface type.

   A MANET neighbor becomes routable if it is bidirectional and the SPF
   calculation has produced a route to the neighbor.  (A flexible
   quality condition may also be required.)  Only routable and Full
   neighbors can be used as next hops in the SPF calculation, and can be
   included in the router-LSA originated by the router.  The idea is
   that if the SPF calculation has produced a route to the neighbor,
   then it makes sense to take a "shortcut" and forward packets directly
   to the neighbor.

   The routability condition is a generalization of the way that
   neighbors on broadcast networks are treated in the SPF calculation.
   The network-LSA of an OSPF broadcast network implies that a router
   can use a non-adjacent neighbor as a next hop.  But a network-LSA
   cannot describe the general topology of a MANET, making it necessary
   to explicitly include non-adjacent neighbors in the router-LSA.
   Allowing only adjacent neighbors in LSAs would either result in
   suboptimal routes or require a large number of adjacencies.







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2.5.  Partial and Full Topology LSAs

   OSPF-MDR allows routers to originate both full-topology LSAs, which
   advertise links to all routable and Full neighbors, and partial-
   topology LSAs, which advertise only a subset of such links.  In a
   dense network, partial-topology LSAs are typically much smaller than
   full-topology LSAs, thus achieving better scalability.

   Each router advertises a subset of its neighbors as point-to-point
   links in its router-LSA.  The choice of which neighbors to advertise
   is flexible.  As a minimum requirement, each router must advertise a
   minimum set of "backbone" neighbors in its router-LSA.  An LSA that
   includes only this minimum set of neighbors is called a minimal LSA
   and corresponds to LSAFullness = 0.  This choice results in the
   minimum amount of LSA flooding overhead, but does not ensure routing
   along shortest paths.  However, it is useful for achieving
   scalability to networks with a large number of nodes.

   At the other extreme, if LSAFullness = 4, then the router originates
   a full-topology LSA, which includes all routable and Full neighbors.

   Setting LSAFullness to 1 results in min-cost LSAs, which provide
   routing along shortest (minimum-cost) paths.  Each router decides
   which neighbors to include in its router-LSA based on 2-hop neighbor
   information obtained from its neighbors' Hellos.  Each router
   includes in its LSA the minimum set of neighbors necessary to provide
   a shortest path between each pair of its neighbors.

   Setting LSAFullness to 2 also provides shortest-path routing, but
   allows the router to advertise additional neighbors to provide
   redundant routes.

   Setting LSAFullness to 3 results in MDR full LSAs, causing each MDR
   to originate a full-topology LSA while other routers originate
   minimal LSAs.  This choice does not provide routing along shortest
   paths, but simulations have shown that it provides routing along
   nearly shortest paths with relatively low overhead.

   The above LSA options are interoperable with each other, because they
   all require the router-LSA to include a minimum set of neighbors, and
   because the construction of the router-LSA (described in Section 9.4)
   ensures that the router-LSAs originated by different routers are
   consistent.  Routing along shortest paths is provided if and only if
   every router selects LSAFullness to be 1, 2, or 4.







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2.6.  Hello Protocol

   OSPF-MDR uses the same Hello format as OSPFv3, but appends additional
   information to Hello packets using link-local signaling (LLS), in
   order to indicate the set of bidirectional neighbors and other
   information that is used by the MDR selection algorithm and the min-
   cost LSA algorithm.  In addition to full Hellos, which include the
   same set of neighbor IDs as OSPFv3 Hellos, OSPF-MDR allows the use of
   differential Hellos, which include only the IDs of neighbors whose
   state (or other information) has recently changed (within the last
   HelloRepeatCount Hellos).

   Hellos are sent every HelloInterval seconds.  Full Hellos are sent
   every 2HopRefresh Hellos, and differential Hellos are sent at all
   other times.  For example, if 2HopRefresh is equal to 3, then every
   third Hello is a full Hello.  The default value of 2HopRefresh is 1;
   i.e., the default is to send only full Hellos.  The default value for
   HelloInterval is 2 seconds.  Differential Hellos are used to reduce
   overhead and to allow Hellos to be sent more frequently, for faster
   reaction to topology changes.

3.  Interface and Neighbor Data Structures

3.1.  Changes to Interface Data Structure

   The following modified or new data items are required for the
   Interface Data Structure of a MANET interface:

   Type
      A router that implements this extension can have one or more
      interfaces of type MANET, in addition to the OSPF interface types
      defined in [RFC2328].

   State
      The possible states for a MANET interface are the same as for a
      broadcast interface.  However, the DR and Backup states now imply
      that the router is an MDR or Backup MDR, respectively.

   MDR Level
      The MDR Level is equal to MDR (value 2) if the router is an MDR,
      Backup MDR (value 1) if the router is a Backup MDR, and MDR Other
      (value 0) otherwise.  The MDR Level is used by the MDR selection
      algorithm.

   Parent
      The Parent replaces the Designated Router (DR) data item of OSPF.
      Each router selects a Parent as described in Section 5.4.  The
      Parent of an MDR is the router itself, and the Parent of a non-MDR



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      router will be a neighboring MDR, if one exists.  The Parent is
      initialized to 0.0.0.0, indicating the lack of a Parent.  Each
      router advertises the Router ID of its Parent in the DR field of
      each Hello sent on the interface.

   Backup Parent
      The Backup Parent replaces the Backup Designated Router data item
      of OSPF.  The Backup Parent of a BMDR is the router itself.  If
      the option of biconnected adjacencies is chosen, then each MDR
      Other selects a Backup Parent, which will be a neighboring
      MDR/BMDR if one exists that is not the Parent.  The Backup Parent
      is initialized to 0.0.0.0, indicating the lack of a Backup Parent.
      Each router advertises the Router ID of its Backup Parent in the
      Backup DR field of each Hello sent on the interface.

   Router Priority
      An 8-bit unsigned integer.  A router with a larger Router Priority
      is more likely to be selected as an MDR.  The Router Priority for
      a MANET interface can be changed dynamically based on any
      criteria, including bandwidth capacity, willingness to be a relay
      (which can depend on battery life, for example), number of
      neighbors (degree), and neighbor stability.  A router that has
      been a (Backup) MDR for a certain amount of time can reduce its
      Router Priority so that the burden of being a (Backup) MDR can be
      shared among all routers.  If the Router Priority for a MANET
      interface is changed, then the interface variable
      MDRNeighborChange must be set.

   Hello Sequence Number (HSN)
      The 16-bit sequence number carried by the MDR-Hello TLV.  The HSN
      is incremented by 1 (modulo 2^16) every time a Hello packet is
      sent on the interface.

   MDRNeighborChange
      A single-bit variable set to 1 if a neighbor change has occurred
      that requires the MDR selection algorithm to be executed.

3.2.  New Configurable Interface Parameters

   The following new configurable interface parameters are required for
   a MANET interface.  The default values for HelloInterval,
   RouterDeadInterval, and RxmtInterval for a MANET interface are 2, 6,
   and 7 seconds, respectively.

   The default configuration for OSPF-MDR uses uniconnected adjacencies
   (AdjConnectivity = 1) and partial-topology LSAs that provide
   shortest-path routing (LSAFullness = 1).  This is the most scalable
   configuration that provides shortest-path routing.  Other



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   configurations may be preferable in special circumstances.  For
   example, setting LSAFullness to 4 provides full-topology LSAs, and
   setting LSAFullness to 0 provides minimal LSAs that minimize overhead
   but do not ensure shortest-path routing.  Setting AdjConnectivity to
   2 may improve robustness by providing a biconnected adjacency
   subgraph, and setting AdjConnectivity to 0 results in full-topology
   adjacencies.

   All possible configurations of the new interface parameters are
   functional, except that if AdjConnectivity is 0 (full-topology
   adjacencies), then LSAFullness must be 1, 2, or 4 (see Section 9.3).

   Differential Hellos should be used to reduce the size of Hello
   packets when the average number of neighbors is large (e.g., greater
   than 50).  Differential Hellos are obtained by setting the parameter
   2HopRefresh to an integer greater than 1, with the recommended value
   being 3.  Good performance in simulated mobile networks with up to
   160 nodes has been obtained using the default configuration with
   differential Hellos.  Good performance in simulated mobile networks
   with up to 200 nodes has been obtained using the same configuration
   except with minimal LSAs (LSAFullness = 0).  Simulation results are
   presented in Appendix E.

   Although all routers should preferably choose the same values for the
   new configurable interface parameters, this is not required.  OSPF-
   MDR was carefully designed so that correct interoperation is achieved
   even if each router sets these parameters independently of the other
   routers.

   AdjConnectivity
      If equal to the default value of 1, then the set of adjacencies
      forms a (uni)connected graph.  If equal to the optional value of
      2, then the set of adjacencies forms a biconnected graph.  If
      AdjConnectivity is 0, then adjacency reduction is not used; i.e.,
      the router becomes adjacent with all of its neighbors.

   MDRConstraint
      A parameter of the MDR selection algorithm, which affects the
      number of MDRs selected and must be an integer greater than or
      equal to 2.  The default value of 3 results in nearly the minimum
      number of MDRs.  Values larger than 3 result in slightly fewer
      MDRs, and the value 2 results in a larger number of MDRs.

   BackupWaitInterval
      The number of seconds that a Backup MDR must wait after receiving
      a new LSA before it decides whether to flood the LSA.  The default
      value is 0.5 second.




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   AckInterval
      The interval between Link State Acknowledgment packets when only
      delayed acknowledgments need to be sent.  AckInterval MUST be less
      than RxmtInterval, and SHOULD NOT be larger than 1 second.  The
      default value is 1 second.

   LSAFullness
      Determines which neighbors a router should advertise in its
      router-LSA.  The value 0 results in minimal LSAs that include only
      "backbone" neighbors.  The values 1 and 2 result in partial-
      topology LSAs that provide shortest-path routing, with the value 2
      providing redundant routes.  The value 3 results in MDRs
      originating full-topology LSAs and other routers originating
      minimal LSAs.  The value 4 results in all routers originating
      full-topology LSAs.  The default value is 1.

   2HopRefresh
      One out of every 2HopRefresh Hellos sent on the interface must be
      a full Hello.  All other Hellos are differential.  The default
      value is 1; i.e., the default is to send only full Hellos.  If
      differential Hellos are used, the recommended value of 2HopRefresh
      is 3.

   HelloRepeatCount
      The number of consecutive Hellos in which a neighbor must be
      included when its state changes, if differential Hellos are used.
      This parameter must be set to 3.

3.3.  Changes to Neighbor Data Structure

   The neighbor states are the same as for OSPF.  However, the data for
   a MANET neighbor that has transitioned to the Down state must be
   maintained for at least HelloInterval * HelloRepeatCount seconds, to
   allow the state change to be reported in differential Hellos.  The
   following new data items are required for the Neighbor Data Structure
   of a neighbor on a MANET interface.

   Neighbor Hello Sequence Number (NHSN)
      The Hello sequence number contained in the last Hello received
      from the neighbor.

   A-bit
      The A-bit copied from the MDR-Hello TLV of the last Hello received
      from the neighbor.  This bit is 1 if the neighbor is using full-
      topology adjacencies, i.e., is not using adjacency reduction.






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   FullHelloRcvd
      A single-bit variable equal to 1 if a full Hello has been received
      from the neighbor.

   Neighbor's MDR Level
      The MDR Level of the neighbor, based on the DR and Backup DR
      fields of the last Hello packet received from the neighbor or from
      the MDR-DD TLV in a Database Description (DD) packet received from
      the neighbor.

   Neighbor's Parent
      The neighbor's choice for Parent, obtained from the DR field of
      the last Hello packet received from the neighbor or from the MDR-
      DD TLV in a DD packet received from the neighbor.

   Neighbor's Backup Parent
      The neighbor's choice for Backup Parent, obtained from the Backup
      DR field of the last Hello packet received from the neighbor or
      from the MDR-DD TLV in a DD packet received from the neighbor.

   Child
      A single-bit variable equal to 1 if the neighbor is a child, i.e.,
      if the neighbor has selected the router as a (Backup) Parent.

   Dependent Neighbor
      A single-bit variable equal to 1 if the neighbor is a Dependent
      Neighbor, which is decided by the MDR selection algorithm.  Each
      MDR/BMDR router becomes adjacent with its Dependent Neighbors
      (which are also MDR/BMDR routers) to form a connected backbone.
      The set of all Dependent Neighbors on a MANET interface is called
      the Dependent Neighbor Set (DNS) for the interface.

   Dependent Selector
      A single-bit variable equal to 1 if the neighbor has selected the
      router to be dependent.

   Selected Advertised Neighbor (SAN)
      A single-bit variable equal to 1 if the neighbor is a Selected
      Advertised Neighbor.  Selected Advertised Neighbors are neighbors
      that the router has selected to be included in the router-LSA,
      along with other neighbors that are required to be included.  The
      set of all Selected Advertised Neighbors on a MANET interface is
      called the Selected Advertised Neighbor Set (SANS) for the
      interface.

   Routable
      A single-bit variable equal to 1 if the neighbor is routable.




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   Neighbor's Bidirectional Neighbor Set (BNS)
      The neighbor's set of bidirectional neighbors, which is updated
      when a Hello is received from the neighbor.

   Neighbor's Dependent Neighbor Set (DNS)
      The neighbor's set of Dependent Neighbors, which is updated when a
      Hello is received from the neighbor.

   Neighbor's Selected Advertised Neighbor Set (SANS)
      The neighbor's set of Selected Advertised Neighbors, which is
      updated when a Hello is received from the neighbor.

   Neighbor's Link Metrics
      The link metric for each of the neighbor's bidirectional
      neighbors, obtained from the Metric TLV appended to Hello packets.

4.  Hello Protocol

   The MANET interface utilizes Hellos for neighbor discovery and for
   enabling neighbors to learn 2-hop neighbor information.  The protocol
   is flexible because it allows the use of full or differential Hellos.
   Full Hellos list all neighbors on the interface that are in state
   Init or greater, as in OSPFv3, whereas differential Hellos list only
   neighbors whose status as a bidirectional neighbor, Dependent
   Neighbor, or Selected Advertised Neighbor has recently changed.
   Differential Hellos are used to reduce overhead, and they allow
   Hellos to be sent more frequently (for faster reaction to topology
   changes).  If differential Hellos are used, full Hellos are sent less
   frequently to ensure that all neighbors have current 2-hop neighbor
   information.

4.1.  Sending Hello Packets

   Hello packets are sent according to [RFC5340], Section 4.2.1.1, and
   [RFC2328], Section 9.5, with the following MANET-specific
   specifications beginning after paragraph 3 of Section 9.5.  The Hello
   packet format is defined in [RFC5340], Section A.3.2, except for the
   ordering of the Neighbor IDs and the meaning of the DR and Backup DR
   fields as described below.

   Similar to [RFC2328], the DR and Backup DR fields indicate whether
   the router is an MDR or Backup MDR.  If the router is an MDR, then
   the DR field is the router's own Router ID, and if the router is a
   Backup MDR, then the Backup DR field is the router's own Router ID.
   These fields are also used to advertise the router's Parent and
   Backup Parent, as specified in Section A.3 and Section 5.4.





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   Hellos are sent every HelloInterval seconds.  Full Hellos are sent
   every 2HopRefresh Hellos, and differential Hellos are sent at all
   other times.  For example, if 2HopRefresh is equal to 3, then every
   third Hello is a full Hello.  If 2HopRefresh is set to 1, then all
   Hellos are full (the default).

   The neighbor IDs included in the body of each Hello are divided into
   the following five disjoint lists of neighbors (some of which may be
   empty), and must appear in the following order:

   List 1. Neighbors whose state recently changed to Down (included only
           in differential Hellos).

   List 2. Neighbors in state Init.

   List 3. Dependent Neighbors.

   List 4. Selected Advertised Neighbors.

   List 5. Unselected bidirectional neighbors, defined as bidirectional
           neighbors that are neither Dependent nor Selected Advertised
           Neighbors.

   Note that all neighbors in Lists 3 through 5 are bidirectional
   neighbors.  These lists are used to update the neighbor's
   Bidirectional Neighbor Set (BNS), Dependent Neighbor Set (DNS), and
   Selected Advertised Neighbor Set (SANS) when a Hello is received.

   Note that the above five lists are disjoint, so each neighbor can
   appear in at most one list.  Also note that some or all of the five
   lists can be empty.

   Link-local signaling (LLS) is used to append up to two TLVs to each
   MANET Hello packet.  The format for LLS is given in Section A.2.  The
   MDR-Hello TLV is appended to each (full or differential) MANET Hello
   packet.  It indicates whether the Hello is full or differential, and
   gives the Hello Sequence Number (HSN) and the number of neighbor IDs
   in each of Lists 1 through 4 defined above.  The size of List 5 is
   then implied by the packet length field of the Hello.  The format of
   the MDR-Hello TLV is given in Section A.2.3.

   In both full and differential Hellos, the appended MDR-Hello TLV is
   built as follows.

   o  The Sequence Number field is set to the current HSN for the
      interface; the HSN is then incremented (modulo 2^16).





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   o  The D-bit of the MDR-Hello TLV is set to 1 for a differential
      Hello and 0 for a full Hello.

   o  The A-bit of the MDR-Hello TLV is set to 1 if AdjConnectivity is 0
      (the router is using full-topology adjacencies); otherwise, it is
      set to 0.

   o  The N1, N2, N3, and N4 fields are set to the number of neighbor
      IDs in the body of the Hello that are in List 1, List 2, List 3,
      and List 4, respectively.  (N1 is always zero in a full Hello.)

   The MDR-Metric TLV (or Metric TLV) advertises the link cost to each
   bidirectional neighbor on the interface, to allow the selection of
   neighbors to include in partial-topology LSAs.  If LSAFullness is 1
   or 2, a Metric TLV must be appended to each MANET Hello packet unless
   all link costs are 1.  The format of the Metric TLV is given in
   Section A.2.5.  The I bit of the Metric TLV can be set to 0 or 1.  If
   the I bit is set to 0, then the Metric TLV does not contain neighbor
   IDs, and contains the metric for each bidirectional neighbor listed
   in the (full or differential) Hello, in the same order.  If the I bit
   is set to 1, then the Metric TLV includes the neighbor ID and metric
   for each bidirectional neighbor listed in the Hello whose metric is
   not equal to the Default Metric field of the TLV.

   The I bit should be chosen to minimize the size of the Metric TLV.
   This can be achieved by choosing the I bit to be 1 if and only if the
   number of bidirectional neighbors listed in the Hello whose metric
   differs from the Default Metric field is less than 1/3 of the total
   number of bidirectional neighbors listed in the Hello.

   For example, if all neighbors have the same metric, then the I bit
   should be set to 1, with the Default Metric equal to this metric,
   avoiding the need to include neighbor IDs and corresponding metrics
   in the TLV.  At the other extreme, if all neighbors have different
   metrics, then the I bit should be set to 0 to avoid listing the same
   neighbor IDs in both the body of the Hello and the Metric TLV.

   In both full and differential Hello packets, the L bit is set in the
   Hello's option field to indicate LLS.

4.1.1.  Full Hello Packet

   In a full Hello, the neighbor ID list includes all neighbors on the
   interface that are in state Init or greater, in the order described
   above.  The MDR-Hello TLV is built as described above.  If a Metric
   TLV is appended, it is built as specified in Section A.2.5.





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4.1.2.  Differential Hello Packet

   In a differential Hello, the five neighbor ID lists defined in
   Section 4.1 are populated as follows:

   List 1 includes each neighbor in state Down that has not yet been
   included in HelloRepeatCount Hellos since transitioning to this
   state.

   List 2 includes each neighbor in state Init that has not yet been
   included in HelloRepeatCount Hellos since transitioning to this
   state.

   List 3 includes each Dependent Neighbor that has not yet been
   included in HelloRepeatCount Hellos since becoming a Dependent
   Neighbor.

   List 4 includes each Selected Advertised Neighbor that has not yet
   been included in HelloRepeatCount Hellos since becoming a Selected
   Advertised Neighbor.

   List 5 includes each unselected bidirectional neighbor (defined in
   Section 4.1) that has not yet been included in HelloRepeatCount
   Hellos since becoming an unselected bidirectional neighbor.

   In addition, a bidirectional neighbor must be included (in the
   appropriate list) if the neighbor's BNS does not include the router
   (indicating that the neighbor does not consider the router to be
   bidirectional).

   If a Metric TLV is appended to the Hello, then a bidirectional
   neighbor must be included (in the appropriate list) if it has not yet
   been included in HelloRepeatCount Hellos since its metric last
   changed.

4.2.  Receiving Hello Packets

   A Hello packet received on a MANET interface is processed as
   described in [RFC5340], Section 4.2.2.1, and the first two paragraphs
   of [RFC2328], Section 10.5, followed by the processing specified
   below.

   The source of a received Hello packet is identified by the Router ID
   found in the Hello's OSPF packet header.  If a matching neighbor
   cannot be found in the interface's data structure, one is created






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   with the Neighbor ID set to the Router ID found in the OSPF packet
   header, the state initialized to Down, all MANET-specific neighbor
   variables (specified in Section 3.3) initialized to zero, and the
   neighbor's DNS, SANS, and BNS initialized to empty sets.

   The neighbor structure's Router Priority is set to the value of the
   corresponding field in the received Hello packet.  The Neighbor's
   Parent is set to the value of the DR field, and the Neighbor's Backup
   Parent is set to the value of the Backup DR field.

   Now the rest of the Hello Packet is examined, generating events to be
   given to the neighbor and interface state machines.  These state
   machines are specified to be either executed or scheduled (see
   [RFC2328], Section 4.4, "Tasking support").  For example, by
   specifying below that the neighbor state machine be executed in line,
   several neighbor state transitions may be affected by a single
   received Hello.

   o  If the L bit in the options field is not set, then an error has
      occurred and the Hello is discarded.

   o  If the LLS contains an MDR-Hello TLV, the neighbor state machine
      is executed with the event HelloReceived.  Otherwise, an error has
      occurred and the Hello is discarded.

   o  The Hello Sequence Number and the A-bit in the MDR-Hello TLV are
      copied to the neighbor's data structure.

   o  The DR and Backup DR fields are processed as follows.

      (1) If the DR field is equal to the neighbor's Router ID, set the
          neighbor's MDR Level to MDR.

      (2) Else if the Backup DR field is equal to the neighbor's Router
          ID, set the neighbor's MDR Level to Backup MDR.

      (3) Else, set the neighbor's MDR Level to MDR Other and set the
          neighbor's Dependent Neighbor variable to 0.  (Only MDR/BMDR
          neighbors can be Dependent.)

      (4) If the DR or Backup DR field is equal to the router's own
          Router ID, set the neighbor's Child variable to 1; otherwise,
          set it to 0.

   The neighbor ID list of the Hello is divided as follows into the five
   lists defined in Section 4.1, where N1, N2, N3, and N4 are obtained
   from the corresponding fields of the MDR-Hello TLV.  List 1 is
   defined to be the first N1 neighbor IDs, List 2 is defined to be the



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   next N2 neighbor IDs, List 3 is defined to be the next N3 neighbor
   IDs, List 4 is defined to be the next N4 neighbor IDs, and List 5 is
   defined to be the remaining neighbor IDs in the Hello.

   Further processing of the Hello depends on whether it is full or
   differential, which is indicated by the value of the D-bit of the
   MDR-Hello TLV.

4.2.1.  Full Hello Packet

   If the received Hello is full (the D-bit of the MDR-Hello TLV is 0),
   the following steps are performed:

   o  If the N1 field of the MDR-Hello TLV is not zero, then an error
      has occurred and the Hello is discarded.  Otherwise, set
      FullHelloRcvd to 1.

   o  In the neighbor structure, modify the neighbor's DNS to equal the
      set of neighbor IDs in the Hello's List 3, modify the neighbor's
      SANS to equal the set of neighbor IDs in the Hello's List 4, and
      modify the neighbor's BNS to equal the set of neighbor IDs in the
      union of Lists 3, 4, and 5.

   o  If the router itself appears in the Hello's neighbor ID list, the
      neighbor state machine is executed with the event 2-WayReceived
      after the Hello is processed.  Otherwise, the neighbor state
      machine is executed with the event 1-WayReceived after the Hello
      is processed.

4.2.2.  Differential Hello Packet

   If the received Hello is differential (the D-bit of the MDR-Hello TLV
   is 1), the following steps are performed:

   (1) For each neighbor ID in List 1 or List 2 of the Hello:

       o  Remove the neighbor ID from the neighbor's DNS, SANS, and BNS,
          if it belongs to the neighbor set.

   (2) For each neighbor ID in List 3 of the Hello:

       o  Add the neighbor ID to the neighbor's DNS and BNS, if it does
          not belong to the neighbor set.

       o  Remove the neighbor ID from the neighbor's SANS, if it belongs
          to the neighbor set.

   (3) For each neighbor ID in List 4 of the Hello:



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       o  Add the neighbor ID to the neighbor's SANS and BNS, if it does
          not belong to the neighbor set.

       o  Remove the neighbor ID from the neighbor's DNS, if it belongs
          to the neighbor set.

   (4) For each neighbor ID in List 5 of the Hello:

       o  Add the neighbor ID to the neighbor's BNS, if it does not
          belong to the neighbor set.

       o  Remove the neighbor ID from the neighbor's DNS and SANS, if it
          belongs to the neighbor set.

   (5) If the router's own RID appears in List 1, execute the neighbor
       state machine with the event 1-WayReceived after the Hello is
       processed.

   (6) If the router's own RID appears in List 2, 3, 4, or 5, execute
       the neighbor state machine with the event 2-WayReceived after the
       Hello is processed.

   (7) If the router's own RID does not appear in the Hello's neighbor
       ID list, and the neighbor state is 2-Way or greater, and the
       Hello Sequence Number is less than or equal to the previous
       sequence number plus HelloRepeatCount, then the neighbor state
       machine is executed with the event 2-WayReceived after the Hello
       is processed (the state does not change).

   (8) If 2-WayReceived is not executed, then 1-WayReceived is executed
       after the Hello is processed.

4.2.3.  Additional Processing for Both Hello Types

   The following applies to both full and differential Hellos.

   If the router itself belongs to the neighbor's DNS, the neighbor's
   Dependent Selector variable is set to 1; otherwise, it is set to 0.

   The receiving interface's MDRNeighborChange variable is set to 1 if
   any of the following changes occurred as a result of processing the
   Hello:

   o  The neighbor's state changed from less than 2-Way to 2-Way or
      greater, or vice versa.






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   o  The neighbor is bidirectional and any of the following neighbor
      variables has changed: MDR Level, Router Priority, FullHelloRcvd,
      and Bidirectional Neighbor Set (BNS).

   The neighbor state machine is scheduled with the event AdjOK?  if any
   of the following changes occurred as a result of processing the
   Hello:

   o  The neighbor's state changed from less than 2-Way to 2-Way or
      greater.

   o  The neighbor is bidirectional and its MDR Level has changed, or
      its Child variable or Dependent Selector variable has changed from
      0 to 1.

   If the LLS contains a Metric TLV, it is processed by updating the
   neighbor's link metrics according to the format of the Metric TLV
   specified in Section A.2.5.  If the LLS does not contain a Metric TLV
   and LSAFullness is 1 or 2, the metric for each of the neighbor's
   links is set to 1.

4.3.  Neighbor Acceptance Condition

   In wireless networks, a single Hello can be received from a neighbor
   with which a poor connection exists, e.g., because the neighbor is
   almost out of range.  To avoid accepting poor-quality neighbors, and
   to employ hysteresis, a router may require that a stricter condition
   be satisfied before changing the state of a MANET neighbor from Down
   to Init or greater.  This condition is called the "neighbor
   acceptance condition", which by default is the reception of a single
   Hello or DD packet.  For example, the neighbor acceptance condition
   may require that 2 consecutive Hellos be received from a neighbor
   before changing the neighbor's state from Down to Init.  Other
   possible conditions include the reception of 3 consecutive Hellos, or
   the reception of 2 of the last 3 Hellos.  The neighbor acceptance
   condition may also impose thresholds on other measurements such as
   received signal strength.

   The neighbor state transition for state Down and event HelloReceived
   is thus modified (see Section 7.1) to depend on the neighbor
   acceptance condition.










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5.  MDR Selection Algorithm

   This section describes the MDR selection algorithm, which is run for
   each MANET interface to determine whether the router is an MDR,
   Backup MDR, or MDR Other for that interface.  The algorithm also
   selects the Dependent Neighbors and the (Backup) Parent, which are
   used to decide which neighbors should become adjacent (see Section
   7.2).

   The MDR selection algorithm must be executed just before sending a
   Hello if the MDRNeighborChange bit is set for the interface.  The
   algorithm SHOULD also be executed whenever a bidirectional neighbor
   transitions to less than 2-Way, and MAY be executed at other times
   when the MDRNeighborChange bit is set.  The bit is cleared after the
   algorithm is executed.

   To simplify the implementation, the MDR selection algorithm MAY be
   executed periodically just before sending each Hello, to avoid having
   to determine when the MDRNeighborChange bit should be set.  After
   running the MDR selection algorithm, the AdjOK? event may be invoked
   for some or all neighbors as specified in Section 7.

   The purpose of the MDRs is to provide a minimal set of relays for
   flooding LSAs, and the purpose of the Backup MDRs is to provide
   backup relays to flood LSAs when flooding by MDRs does not succeed.
   The set of MDRs forms a CDS, and the set of MDRs and Backup MDRs
   forms a biconnected CDS (if the network itself is biconnected).

   Each MDR selects and becomes adjacent with a subset of its MDR
   neighbors, called Dependent Neighbors, forming a connected backbone.
   Each non-MDR router connects to this backbone by selecting and
   becoming adjacent with an MDR neighbor called its Parent.  Each MDR
   selects itself as Parent, to inform neighbors that it is an MDR.

   If AdjConnectivity = 2, then each (Backup) MDR selects and becomes
   adjacent with additional (Backup) MDR neighbors to form a biconnected
   backbone, and each MDR Other selects and becomes adjacent with a
   second (Backup) MDR neighbor called its Backup Parent, thus becoming
   connected to the backbone via two adjacencies.  Each BMDR selects
   itself as Backup Parent, to inform neighbors that it is a BMDR.

   The MDR selection algorithm is a distributed CDS algorithm that uses
   2-hop neighbor information obtained from Hellos.  More specifically,
   it uses as inputs the set of bidirectional neighbors (in state 2-Way
   or greater), the triplet (Router Priority, MDR Level, Router ID) for
   each such neighbor and for the router itself, and the neighbor





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   variables Bidirectional Neighbor Set (BNS) and FullHelloRcvd for each
   such neighbor.  The MDR selection algorithm can be implemented in
   O(d^2) time, where d is the number of neighbors.

   The above triplet will be abbreviated as (RtrPri, MDR Level, RID).
   The triplet (RtrPri, MDR Level, RID) is said to be larger for Router
   A than for Router B if the triplet for Router A is lexicographically
   greater than the triplet for Router B.  Routers that have larger
   values of this triplet are preferred for selection as an MDR.  The
   algorithm therefore prefers routers that are already MDRs, resulting
   in a longer average MDR lifetime.

   The MDR selection algorithm consists of five phases, the last of
   which is optional.  Phase 1 creates the neighbor connectivity matrix
   for the interface, which determines which pairs of neighbors are
   neighbors of each other.  Phase 2 decides whether the calculating
   router is an MDR, and which MDR neighbors are Dependent.  Phase 3
   decides whether the calculating router is a Backup MDR and, if
   AdjConnectivity = 2, which additional MDR/BMDR neighbors are
   Dependent.  Phase 4 selects the Parent and Backup Parent.

   The algorithm simplifies considerably if AdjConnectivity is 0 (full-
   topology adjacencies).  In this case, the set of Dependent Neighbors
   is empty and MDR Other routers need not select Parents.  Also, Phase
   3 (BMDR selection) is not required if AdjConnectivity is 0 or 1.
   However, Phase 3 MUST be executed if AdjConnectivity is 2, and SHOULD
   be executed if AdjConnectivity is 0 or 1, since BMDRs improve
   robustness by providing backup flooding.

   A router that has selected itself as an MDR in Phase 2 MAY execute
   Phase 5 to possibly declare itself a non-flooding MDR.  A non-
   flooding MDR is the same as a flooding MDR except that it does not
   automatically flood received LSAs back out the receiving interface,
   because it has determined that neighboring MDRs are sufficient to
   flood the LSA to all neighbors.  Instead, a non-flooding MDR performs
   backup flooding just like a BMDR.  A non-flooding MDR maintains its
   MDR level (rather than being demoted to a BMDR) in order to maximize
   the stability of adjacencies.  (The decision to form an adjacency
   does not depend on whether an MDR is non-flooding.)  By having MDRs
   declare themselves to be non-flooding when possible, flooding
   overhead is reduced.  The resulting reduction in flooding overhead
   can be dramatic for certain regular topologies, but has been found to
   be less than 15% for random topologies.

   The following subsections describe the MDR selection algorithm, which
   is applied independently to each MANET interface.  For convenience,
   the term "bi-neighbor" will be used as an abbreviation for
   "bidirectional neighbor".



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5.1.  Phase 1: Creating the Neighbor Connectivity Matrix

   Phase 1 creates the neighbor connectivity matrix (NCM) for the
   interface.  The NCM is a symmetric matrix that defines a topology
   graph for the set of bi-neighbors on the interface.  The NCM assigns
   a value of 0 or 1 for each pair of bi-neighbors; a value of 1
   indicates that the neighbors are assumed to be bi-neighbors of each
   other in the MDR selection algorithm.  Letting i denote the router
   itself, NCM(i,j) and NCM(j,i) are set to 1 for each bi-neighbor j.
   The value of the matrix is set as follows for each pair of bi-
   neighbors j and k on the interface.

   (1.1) If FullHelloRcvd is 1 for both neighbors j and k: NCM(j,k) =
         NCM(k,j) is 1 only if j belongs to the BNS of neighbor k and k
         belongs to the BNS of neighbor j.

   (1.2) If FullHelloRcvd is 1 for neighbor j and is 0 for neighbor k:
         NCM(j,k) = NCM(k,j) is 1 only if k belongs to the BNS of
         neighbor j.

   (1.3) If FullHelloRcvd is 0 for both neighbors j and k: NCM(j,k) =
         NCM(k,j) = 0.

   In Step 1.1 above, two neighbors are considered to be bi-neighbors of
   each other only if they both agree that the other router is a bi-
   neighbor.  This provides faster response to the failure of a link
   between two neighbors, since it is likely that one router will detect
   the failure before the other router.  In Step 1.2 above, only
   neighbor j has reported its full BNS, so neighbor j is believed in
   deciding whether j and k are bi-neighbors of each other.  As Step 1.3
   indicates, two neighbors are assumed not to be bi-neighbors of each
   other if neither neighbor has reported its full BNS.

5.2.  Phase 2: MDR Selection

   Phase 2 depends on the parameter MDRConstraint, which affects the
   number of MDRs selected.  The default value of 3 results in nearly
   the minimum number of MDRs, while the value 2 results in a larger
   number of MDRs.  If AdjConnectivity = 0 (full-topology adjacencies),
   then the following steps are modified in that Dependent Neighbors are
   not selected.

   (2.1) The set of Dependent Neighbors is initialized to be empty.








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   (2.2) If the router has a larger value of (RtrPri, MDR Level, RID)
         than all of its bi-neighbors, the router selects itself as an
         MDR; selects all of its MDR bi-neighbors as Dependent
         Neighbors; if AdjConnectivity = 2, selects all of its BMDR bi-
         neighbors as Dependent Neighbors; then proceeds to Phase 4.

   (2.3) Let Rmax be the bi-neighbor with the largest value of (RtrPri,
         MDR Level, RID).

   (2.4) Using NCM to determine the connectivity of bi-neighbors,
         compute the minimum number of hops, denoted hops(u), from Rmax
         to each other bi-neighbor u, using only intermediate nodes that
         are bi-neighbors with a larger value of (RtrPri, MDR Level,
         RID) than the router itself.  If no such path from Rmax to u
         exists, then hops(u) equals infinity. (See Appendix B for a
         detailed algorithm using breadth-first search.)

   (2.5) If hops(u) is at most MDRConstraint for each bi-neighbor u, the
         router selects no Dependent Neighbors, and sets its MDR Level
         as follows: If the MDR Level is currently MDR, then it is
         changed to BMDR if Phase 3 will be executed and to MDR Other if
         Phase 3 will not be executed.  Otherwise, the MDR Level is not
         changed.

   (2.6) Else, the router sets its MDR Level to MDR and selects the
         following neighbors as Dependent Neighbors: Rmax if it is an
         MDR or BMDR; each MDR bi-neighbor u such that hops(u) is
         greater than MDRConstraint; and if AdjConnectivity = 2, each
         BMDR bi-neighbor u such that hops(u) is greater than
         MDRConstraint.

   (2.7) If steps 2.1 through 2.6 resulted in the MDR Level changing to
         BMDR, or to MDR with AdjConnectivity equal to 1 or 2, then
         execute steps 2.1 through 2.6 again.  (This is necessary
         because the change in MDR Level can cause the set of Dependent
         Neighbors and the BFS tree to change.)  This step is not
         required if the MDR selection algorithm is executed
         periodically.

   Step 2.4 can be implemented using a breadth-first search (BFS)
   algorithm to compute min-hop paths from Rmax to all other bi-
   neighbors, modified to allow a bi-neighbor to be an intermediate node
   only if its value of (RtrPri, MDR Level, RID) is larger than that of
   the router itself.  A detailed description of this algorithm, which
   runs in O(d^2) time, is given in Appendix B.






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5.3.  Phase 3: Backup MDR Selection

   (3.1) If the MDR Level is MDR (after running Phase 2) and
         AdjConnectivity is not 2, then proceed to Phase 4.  (If the MDR
         Level is MDR and AdjConnectivity = 2, then Phase 3 may select
         additional Dependent Neighbors to create a biconnected
         backbone.)

   (3.2) Using NCM to determine the connectivity of bi-neighbors,
         determine whether or not there exist two node-disjoint paths
         from Rmax to each other bi-neighbor u, using only intermediate
         nodes that are bi-neighbors with a larger value of (RtrPri, MDR
         Level, RID) than the router itself.  (See Appendix B for a
         detailed algorithm.)

   (3.3) If there exist two such node-disjoint paths from Rmax to each
         other bi-neighbor u, then the router selects no additional
         Dependent Neighbors and sets its MDR Level to MDR Other.

   (3.4) Else, the router sets its MDR Level to Backup MDR unless it
         already selected itself as an MDR in Phase 2, and if
         AdjConnectivity = 2, adds each of the following neighbors to
         the set of Dependent Neighbors: Rmax if it is an MDR or BMDR,
         and each MDR/BMDR bi-neighbor u such that Step 3.2 did not find
         two node-disjoint paths from Rmax to u.

   (3.5) If steps 3.1 through 3.4 resulted in the MDR Level changing
         from MDR Other to BMDR, then run Phases 2 and 3 again.  (This
         is necessary because running Phase 2 again can cause the MDR
         Level to change to MDR.)  This step is not required if the MDR
         selection algorithm is executed periodically.

   Step 3.2 can be implemented in O(d^2) time using the algorithm given
   in Appendix B.  A simplified version of the algorithm is also
   specified, which results in a larger number of BMDRs.

5.4.  Phase 4: Parent Selection

   Each router selects a Parent for each MANET interface.  The Parent of
   a non-MDR router will be a neighboring MDR if one exists.  If the
   option of biconnected adjacencies is chosen, then each MDR Other
   selects a Backup Parent, which will be a neighboring MDR/BMDR if one
   exists that is not the Parent.  The Parent of an MDR is always the
   router itself, and the Backup Parent of a BMDR is always the router
   itself.






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   The (Backup) Parent is advertised in the (Backup) DR field of each
   Hello sent on the interface.  As specified in Section 7.2, each
   router forms an adjacency with its Parent and Backup Parent if it
   exists and is a neighboring MDR/BMDR.

   For a given MANET interface, let Rmax denote the router with the
   largest value of (RtrPri, MDR Level, RID) among all bidirectional
   neighbors, if such a neighbor exists that has a larger value of
   (RtrPri, MDR Level, RID) than the router itself.  Otherwise, Rmax is
   null.

   If the calculating router has selected itself as an MDR, then the
   Parent is equal to the router itself, and the Backup Parent is Rmax.
   (The latter design choice was made because it results in slightly
   better performance than choosing no Backup Parent.)  If the router
   has selected itself as a BMDR, then the Backup Parent is equal to the
   router itself.

   If the calculating router is a BMDR or MDR Other, the Parent is
   selected to be any adjacent neighbor that is an MDR, if such a
   neighbor exists.  If no adjacent MDR neighbor exists, then the Parent
   is selected to be Rmax.  By giving preference to neighbors that are
   already adjacent, the formation of a new adjacency is avoided when
   possible.  Note that the Parent can be a non-MDR neighbor temporarily
   when no MDR neighbor exists.  (This design choice was also made for
   performance reasons.)

   If AdjConnectivity = 2 and the calculating router is an MDR Other,
   then the Backup Parent is selected to be any adjacent neighbor that
   is an MDR or BMDR, other than the Parent selected in the previous
   paragraph, if such a neighbor exists.  If no such adjacent neighbor
   exists, then the Backup Parent is selected to be the bidirectional
   neighbor, excluding the selected Parent, with the largest value of
   (RtrPri, MDR Level, RID), if such a neighbor exists.  Otherwise, the
   Backup Parent is null.

5.5.  Phase 5: Optional Selection of Non-Flooding MDRs

   A router that has selected itself as an MDR MAY execute the following
   steps to possibly declare itself a non-flooding MDR.  An MDR that
   does not execute the following steps is by default a flooding MDR.

   (5.1) If the router has a larger value of (RtrPri, MDR Level, RID)
         than all of its bi-neighbors, the router is a flooding MDR.
         Else, proceed to Step 5.2.

   (5.2) Let Rmax be the bi-neighbor that has the largest value of
         (RtrPri, MDR Level, RID).



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   (5.3) Using NCM to determine the connectivity of bi-neighbors,
         compute the minimum number of hops, denoted hops(u), from Rmax
         to each other bi-neighbor u, using only intermediate nodes that
         are MDR bi-neighbors with a smaller value of (RtrPri, RID) than
         the router itself. (This can be done using BFS as in Step 2.4).

   (5.4) If hops(u) is at most MDRConstraint for each bi-neighbor u,
         then the router is a non-flooding MDR.  Else, it is a flooding
         MDR.

6.  Interface State Machine

6.1.  Interface States

   No new states are defined for a MANET interface.  However, the DR and
   Backup states now imply that the router is an MDR or Backup MDR,
   respectively.  The following modified definitions apply to MANET
   interfaces:

   Waiting
      In this state, the router learns neighbor information from the
      Hello packets it receives, but is not allowed to run the MDR
      selection algorithm until it transitions out of the Waiting state
      (when the Wait Timer expires).  This prevents unnecessary changes
      in the MDR selection resulting from incomplete neighbor
      information.  The length of the Wait Timer is 2HopRefresh *
      HelloInterval seconds (the interval between full Hellos).

   DR Other
      The router has run the MDR selection algorithm and determined that
      it is not an MDR or a Backup MDR.

   Backup
      The router has selected itself as a Backup MDR.

   DR
      The router has selected itself as an MDR.

6.2.  Events that Cause Interface State Changes

   All interface events defined in [RFC2328], Section 9.2, apply to
   MANET interfaces, except for BackupSeen and NeighborChange.
   BackupSeen is never invoked for a MANET interface (since seeing a
   Backup MDR does not imply that the router itself cannot also be an
   MDR or Backup MDR).






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   The event NeighborChange is replaced with the new interface variable
   MDRNeighborChange, which indicates that the MDR selection algorithm
   must be executed due to a change in neighbor information (see Section
   4.2.3).

6.3.  Changes to Interface State Machine

   This section describes the changes to the interface state machine for
   a MANET interface.  The two state transitions specified below are for
   state-event pairs that are described in [RFC2328], but have modified
   action descriptions because MDRs are selected instead of DRs.  The
   state transition in [RFC2328] for the event NeighborChange is
   omitted; instead, the new interface variable MDRNeighborChange is
   used to indicate when the MDR selection algorithm needs to be
   executed.  The state transition for the event BackupSeen does not
   apply to MANET interfaces, since this event is never invoked for a
   MANET interface.  The interface state transitions for the events
   Loopback and UnloopInd are unchanged from [RFC2328].

       State:  Down
       Event:  InterfaceUp
   New state:  Depends on action routine.

      Action:  Start the interval Hello Timer, enabling the periodic
               sending of Hello packets out the interface.  The state
               transitions to Waiting and the single shot Wait Timer
               is started.


       State:  Waiting
       Event:  WaitTimer
   New state:  Depends on action routine.

      Action:  Run the MDR selection algorithm, which may result in a
               change to the router's MDR Level, Dependent Neighbors,
               and (Backup) Parent.  As a result of this calculation,
               the new interface state will be DR Other, Backup, or DR.

               As a result of these changes, the AdjOK? neighbor event
               may be invoked for some or all neighbors.  (See
               Section 7.)

7.  Adjacency Maintenance

   Adjacency forming and eliminating on non-MANET interfaces remain
   unchanged.  Adjacency maintenance on a MANET interface requires
   changes to transitions in the neighbor state machine ([RFC2328],
   Section 10.3), to deciding whether to become adjacent ([RFC2328],



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   Section 10.4), sending of DD packets ([RFC2328], Section 10.8), and
   receiving of DD packets ([RFC2328], Section 10.6).  The specification
   below relates to the MANET interface only.

   If full-topology adjacencies are used (AdjConnectivity = 0), the
   router forms an adjacency with each bidirectional neighbor.  If
   adjacency reduction is used (AdjConnectivity is 1 or 2), the router
   forms adjacencies with a subset of its neighbors, according to the
   rules specified in Section 7.2.

   An adjacency maintenance decision is made when any of the following
   four events occur between a router and its neighbor.  The decision is
   made by executing the neighbor event AdjOK?.

      (1) The neighbor state changes from Init to 2-Way.

      (2) The MDR Level changes for the neighbor or for the router
          itself.

      (3) The neighbor is selected to be the (Backup) Parent.

      (4) The neighbor selects the router to be its (Backup) Parent.

7.1.  Changes to Neighbor State Machine

   The following specifies new transitions in the neighbor state
   machine.

    State(s):  Down
       Event:  HelloReceived
   New state:  Depends on action routine.

      Action:  If the neighbor acceptance condition is satisfied (see
               Section 4.3), the neighbor state transitions to Init and
               the Inactivity Timer is started.  Otherwise, the neighbor
               remains in the Down state.


    State(s):  Init
       Event:  2-WayReceived
   New state:  2-Way

      Action:  Transition to neighbor state 2-Way.

    State(s):  2-Way
       Event:  AdjOK?
   New state:  Depends on action routine.




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      Action:  Determine whether an adjacency should be formed with the
               neighboring router (see Section 7.2).  If not, the
               neighbor state remains at 2-Way and no further action is
               taken.

               Otherwise, the neighbor state changes to ExStart, and the
               following actions are performed.  If the neighbor has a
               larger Router ID than the router's own ID, and the
               received packet is a DD packet with the initialize (I),
               more (M), and master (MS) bits set, then execute the
               event NegotiationDone, which causes the state to
               transition to Exchange.

               Otherwise (negotiation is not complete), the router
               increments the DD sequence number in the neighbor data
               structure.  If this is the first time that an adjacency
               has been attempted, the DD sequence number should be
               assigned a unique value (like the time of day clock).  It
               then declares itself master (sets the master/slave bit to
               master), and starts sending Database Description packets,
               with the initialize (I), more (M), and master (MS) bits
               set, the MDR-DD TLV included in an LLS, and the L bit
               set.  This Database Description packet should be
               otherwise empty.  This Database Description packet should
               be retransmitted at intervals of RxmtInterval until the
               next state is entered (see [RFC2328], Section 10.8).


    State(s):  ExStart or greater
       Event:  AdjOK?
   New state:  Depends on action routine.

      Action:  Determine whether the neighboring router should still be
               adjacent (see Section 7.3).  If yes, there is no state
               change and no further action is necessary.  Otherwise,
               the (possibly partially formed) adjacency must be
               destroyed.  The neighbor state transitions to 2-Way.  The
               Link state retransmission list, Database summary list,
               and Link state request list are cleared of LSAs.

7.2.  Whether to Become Adjacent

   The following defines the method to determine if an adjacency should
   be formed between neighbors in state 2-Way.  The following procedure
   does not depend on whether AdjConnectivity is 1 or 2, but the
   selection of Dependent Neighbors (by the MDR selection algorithm)
   depends on AdjConnectivity.




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   If adjacency reduction is not used (AdjConnectivity = 0), then an
   adjacency is formed with each neighbor in state 2-Way.  Otherwise, an
   adjacency is formed with a neighbor in state 2-Way if any of the
   following conditions is true:

   (1) The router is a (Backup) MDR and the neighbor is a (Backup) MDR
       and is either a Dependent Neighbor or a Dependent Selector.

   (2) The neighbor is a (Backup) MDR and is the router's (Backup)
       Parent.

   (3) The router is a (Backup) MDR and the neighbor is a child.

   (4) The neighbor's A-bit is 1, indicating that the neighbor is using
       full-topology adjacencies.

   Otherwise, an adjacency is not established and the neighbor remains
   in state 2-Way.

7.3.  Whether to Eliminate an Adjacency

   The following defines the method to determine if an existing
   adjacency should be eliminated.  An existing adjacency is maintained
   if any of the following is true:

   (1) The router is an MDR or Backup MDR.

   (2) The neighbor is an MDR or Backup MDR.

   (3) The neighbor's A-bit is 1, indicating that the neighbor is using
       full-topology adjacencies.

   Otherwise, the adjacency MAY be eliminated.

7.4.  Sending Database Description Packets

   Sending a DD packet on a MANET interface is the same as [RFC5340],
   Section 4.2.1.2, and [RFC2328], Section 10.8, with the following
   additions to paragraph 3 of Section 10.8.

   If the neighbor state is ExStart, the standard initialization packet
   is sent with an MDR-DD TLV appended using LLS, and the L bit is set
   in the DD packet's option field.  The format for the MDR-DD TLV is
   specified in Section A.2.4.  The DR and Backup DR fields of the MDR-
   DD TLV are set exactly the same as the DR and Backup DR fields of a
   Hello sent on the same interface.





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7.5.  Receiving Database Description Packets

   Processing a DD packet received on a MANET interface is the same as
   [RFC2328], Section 10.6, except for the changes described in this
   section.  The following additional steps are performed before
   processing the packet based on neighbor state in paragraph 3 of
   Section 10.6.

   o  If the DD packet's L bit is set in the options field and an MDR-DD
      TLV is appended, then the MDR-DD TLV is processed as follows.

      (1) If the DR field is equal to the neighbor's Router ID:

          (a) Set the MDR Level of the neighbor to MDR.

          (b) Set the neighbor's Dependent Selector variable to 1.

      (2) Else if the Backup DR field is equal to the neighbor's Router
          ID:

          (a) Set the MDR Level of the neighbor to Backup MDR.

          (b) Set the neighbor's Dependent Selector variable to 1.

      (3) Else:

          (a) Set the MDR Level of the neighbor to MDR Other.

          (b) Set the neighbor's Dependent Neighbor variable to 0.

      (4) If the DR or Backup DR field is equal to the router's own
          Router ID, set the neighbor's Child variable to 1; otherwise,
          set it to 0.

   o  If the neighbor state is Init, the neighbor event 2-WayReceived is
      executed.

   o  If the MDR Level of the neighbor changed, the neighbor state
      machine is scheduled with the event AdjOK?.

   o  If the neighbor's Child status has changed from 0 to 1, the
      neighbor state machine is scheduled with the event AdjOK?.

   o  If the neighbor's neighbor state changed from less than 2-Way to
      2-Way or greater, the neighbor state machine is scheduled with the
      event AdjOK?.





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   In addition, the Database Exchange optimization described in
   [RFC5243] SHOULD be performed as follows.  If the router accepts a
   received DD packet as the next in sequence, the following additional
   step should be performed for each LSA listed in the DD packet
   (whether the router is master or slave).  If the Database summary
   list contains an instance of the LSA that is the same as or less
   recent than the listed LSA, the LSA is removed from the Database
   summary list.  This avoids listing the LSA in a DD packet sent to the
   neighbor, when the neighbor already has an instance of the LSA that
   is the same or more recent.  This optimization reduces overhead due
   to DD packets by approximately 50% in large networks.

8.  Flooding Procedure

   This section specifies the changes to [RFC2328], Section 13, for
   routers that support OSPF-MDR.  The first part of Section 13 (before
   Section 13.1) is the same except for the following three changes.

   o  To exploit the broadcast nature of MANETs, if the Link State
      Update (LSU) packet was received on a MANET interface, then the
      packet is dropped without further processing only if the sending
      neighbor is in a lesser state than 2-Way.  Otherwise, the LSU
      packet is processed as described in this section.

   o  If the received LSA is the same instance as the database copy, the
      following actions are performed in addition to Step 7.  For each
      MANET interface for which a BackupWait Neighbor List exists for
      the LSA (see Section 8.1):

      (a) Remove the sending neighbor from the BackupWait Neighbor List
          if it belongs to the list.

      (b) For each neighbor on the receiving interface that belongs to
          the BNS for the sending neighbor, remove the neighbor from the
          BackupWait Neighbor List if it belongs to the list.

   o  Step 8, which handles the case in which the database copy of the
      LSA is more recent than the received LSA, is modified as follows.
      If the sending neighbor is in a lesser state than Exchange, then
      the router does not send the LSA back to the sending neighbor.

   There are no changes to Sections 13.1, 13.2, or 13.4.  The following
   subsections describe the changes to Sections 13.3 (Next step in the
   flooding procedure), 13.5 (Sending Link State Acknowledgments), 13.6
   (Retransmitting LSAs), and 13.7 (Receiving Link State
   Acknowledgments) of [RFC2328].





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8.1.  LSA Forwarding Procedure

   When a new LSA is received, Steps 1 through 5 of [RFC2328], Section
   13.3, are performed without modification for each eligible (outgoing)
   interface that is not of type MANET.  This section specifies the
   modified steps that must be performed for each eligible MANET
   interface.  The eligible interfaces depend on the LSA's flooding
   scope as described in [RFC5340], Section 4.5.2.  Whenever an LSA is
   flooded out a MANET interface, it is included in an LSU packet that
   is sent to the multicast address AllSPFRouters.  (Retransmitted LSAs
   are always unicast, as specified in Section 8.3.)

   Step 1 of [RFC2328], Section 13.3, is performed for each eligible
   MANET interface with the following modification, so that the new LSA
   is placed on the Link State retransmission list for each appropriate
   adjacent neighbor.  Step 1c is replaced with the following action, so
   that the LSA is not placed on the retransmission list for a neighbor
   that has already acknowledged the LSA.

   o  If the new LSA was received from this neighbor, or a Link State
      Acknowledgment (LS Ack) for the new LSA has already been received
      from this neighbor, examine the next neighbor.

   To determine whether an Ack for the new LSA has been received from
   the neighbor, the router maintains an Acked LSA List for each
   adjacent neighbor, as described in Section 8.4.  When a new LSA is
   received, the Acked LSA List for each neighbor, on each MANET
   interface, should be updated by removing any LS Ack that is for an
   older instance of the LSA than the one received.

   The following description will use the notion of a "covered"
   neighbor.  A neighbor k is defined to be covered if the LSA was sent
   as a multicast by a MANET neighbor j, and neighbor k belongs to the
   Bidirectional Neighbor Set (BNS) for neighbor j.  A neighbor k is
   also defined to be covered if the LSA was sent to the multicast
   address AllSPFRouters by a neighbor j on a broadcast interface on
   which both j and k are neighbors.  (Note that j must be the DR or
   Backup DR for the broadcast network, since only these routers may
   send LSAs to AllSPFRouters on a broadcast network.)

   The following steps must be performed for each eligible MANET
   interface, to determine whether the new LSA should be forwarded on
   the interface.

   (2) If every bidirectional neighbor on the interface satisfies at
       least one of the following three conditions, examine the next
       interface (the LSA is not flooded out this interface).




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      (a) The LSA was received from the neighbor.

      (b) The LSA was received on a MANET or broadcast interface and the
          neighbor is covered (defined above).

      (c) An Ack for the LSA has been received from the neighbor.

          Condition (c) MAY be omitted (thus ignoring Acks) in order to
          simplify this step.  Note that the above conditions do not
          assume the outgoing interface is the same as the receiving
          interface.

   (3) If the LSA was received on this interface, and the router is an
       MDR Other for this interface, examine the next interface (the LSA
       is not flooded out this interface).

   (4) If the LSA was received on this interface, and the router is a
       Backup MDR or a non-flooding MDR for this interface, then the
       router waits BackupWaitInterval before deciding whether to flood
       the LSA.  To accomplish this, the router creates a BackupWait
       Neighbor List for the LSA, which initially includes every
       bidirectional neighbor on this interface that does not satisfy
       any of the conditions in Step 2.  A single-shot BackupWait Timer
       associated with the LSA is started, which is set to expire after
       BackupWaitInterval seconds plus a small amount of random jitter.
       (The actions performed when the BackupWait Timer expires are
       described below in Section 8.1.2.)  Examine the next interface
       (the LSA is not yet flooded out this interface).

   (5) If the router is a flooding MDR for this interface, or if the LSA
       was originated by the router itself, then the LSA is flooded out
       the interface (whether or not the LSA was received on this
       interface) and the next interface is examined.

   (6) If the LSA was received on a MANET or broadcast interface that is
       different from this (outgoing) interface, then the following two
       steps SHOULD be performed to avoid redundant flooding.

      (a) If the router has a larger value of (RtrPri, MDR Level, RID)
          on the outgoing interface than every covered neighbor (defined
          above) that is a neighbor on BOTH the receiving and outgoing
          interfaces (or if no such neighbor exists), then the LSA is
          flooded out the interface and the next interface is examined.

      (b) Else, the router waits BackupWaitInterval before deciding
          whether to flood the LSA on the interface, by performing the
          actions in Step 4 for a Backup MDR (whether or not the router
          is a Backup MDR on this interface).  A separate BackupWait



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          Neighbor List is created for each MANET interface, but only
          one BackupWait Timer is associated with the LSA.  Examine the
          next interface (the LSA is not yet flooded out this
          interface).

   (7) If this step is reached, the LSA is flooded out the interface.

8.1.1.  Note on Step 6 of LSA Forwarding Procedure

   Performing the optional Step 6 can greatly reduce flooding overhead
   if the LSA was received on a MANET or broadcast interface.  For
   example, assume that the LSA was received from the DR of a broadcast
   network that includes 100 routers, and 50 of the routers (not
   including the DR) are also attached to a MANET.  Assume that these 50
   routers are neighbors of each other in the MANET and that each has a
   neighbor in the MANET that is not attached to the broadcast network
   (and is therefore not covered).  Then by performing Step 6 of the LSA
   forwarding procedure, the number of routers that forward the LSA from
   the broadcast network to the MANET is reduced from 50 to just 1
   (assuming that at most 1 of the 50 routers is an MDR).

8.1.2.  BackupWait Timer Expiration

   If the BackupWait Timer for an LSA expires, then the following steps
   are performed for each (MANET) interface for which a BackupWait
   Neighbor List exists for the LSA.

   (1) If the BackupWait Neighbor List for the interface contains at
       least one router that is currently a bidirectional neighbor, the
       following actions are performed.

      (a) The LSA is flooded out the interface.

      (b) If the LSA is on the Ack List for the interface (i.e., is
          scheduled to be included in a delayed Link State
          Acknowledgment packet), then the router SHOULD remove the LSA
          from the Ack List, since the flooded LSA will be treated as an
          implicit Ack.

      (c) If the LSA is on the Link State retransmission list for any
          neighbor, the retransmission SHOULD be rescheduled to occur
          after RxmtInterval seconds.

   (2) The BackupWait Neighbor List is then deleted (whether or not the
       LSA is flooded).






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8.2.  Sending Link State Acknowledgments

   This section describes the procedure for sending Link State
   Acknowledgments (LS Acks) on MANET interfaces.  Section 13.5 of
   [RFC2328] remains unchanged for non-MANET interfaces, but does not
   apply to MANET interfaces.  To minimize overhead due to LS Acks, and
   to take advantage of the broadcast nature of MANETs, all LS Ack
   packets sent on a MANET interface are multicast using the IP address
   AllSPFRouters.  In addition, duplicate LSAs received as a multicast
   are not acknowledged.

   When a router receives an LSA, it must decide whether to send a
   delayed Ack, an immediate Ack, or no Ack.  The interface parameter
   AckInterval is the interval between LS Ack packets when only delayed
   Acks need to be sent.  A delayed Ack SHOULD be delayed by at least
   (RxmtInterval - AckInterval - 0.5) seconds and at most (RxmtInterval
   - 0.5) seconds after the LSA instance being acknowledged was first
   received.  If AckInterval and RxmtInterval are equal to their default
   values of 1 and 7 seconds, respectively, this reduces Ack traffic by
   increasing the chance that a new instance of the LSA will be received
   before the delayed Ack is sent.  An immediate Ack is sent immediately
   in a multicast LS Ack packet, which may also include delayed Acks
   that were scheduled to be sent.

   The decision whether to send a delayed or immediate Ack depends on
   whether the received LSA is new (i.e., is more recent than the
   database copy) or a duplicate (the same instance as the database
   copy), and on whether the LSA was received as a multicast or a
   unicast (which indicates a retransmitted LSA).  The following rules
   are used to make this decision.

   (1) If the received LSA is new, a delayed Ack is sent on each MANET
       interface associated with the area, unless the LSA is flooded out
       the interface.

   (2) If the LSA is a duplicate and was received as a multicast, the
       LSA is not acknowledged.

   (3) If the LSA is a duplicate and was received as a unicast:

       (a) If the router is an MDR, or AdjConnectivity = 2 and the
           router is a Backup MDR, or AdjConnectivity = 0, then an
           immediate Ack is sent out the receiving interface.

       (b) Otherwise, a delayed Ack is sent out the receiving interface.






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   The reason that (Backup) MDRs send an immediate Ack when a
   retransmitted LSA is received is to try to prevent other adjacent
   neighbors from retransmitting the LSA, since (Backup) MDRs usually
   have a large number of adjacent neighbors.  MDR Other routers do not
   send an immediate Ack (unless AdjConnectivity = 0) because they have
   fewer adjacent neighbors, and so the potential benefit does not
   justify the additional overhead resulting from sending immediate
   Acks.

8.3.  Retransmitting LSAs

   LSAs are retransmitted according to Section 13.6 of [RFC2328].  Thus,
   LSAs are retransmitted only to adjacent routers.  Therefore, since
   OSPF-MDR does not allow an adjacency to be formed between two MDR
   Other routers, an MDR Other never retransmits an LSA to another MDR
   Other, only to its Parents, which are (Backup) MDRs.

   Retransmitted LSAs are included in LSU packets that are unicast
   directly to an adjacent neighbor that did not acknowledge the LSA
   (explicitly or implicitly).  The length of time between
   retransmissions is given by the configurable interface parameter
   RxmtInterval, whose default is 7 seconds for a MANET interface.  To
   reduce overhead, several retransmitted LSAs should be included in a
   single LSU packet whenever possible.

8.4.  Receiving Link State Acknowledgments

   A Link State Acknowledgment (LS Ack) packet that is received from an
   adjacent neighbor (in state Exchange or greater) is processed as
   described in Section 13.7 of [RFC2328], with the additional steps
   described in this section.  An LS Ack packet that is received from a
   neighbor in a lesser state than Exchange is discarded.

   Each router maintains an Acked LSA List for each adjacent neighbor,
   to keep track of any LSA instances the neighbor has acknowledged but
   that the router itself has NOT yet received.  This is necessary
   because (unlike [RFC2328]) each router acknowledges an LSA only the
   first time it is received as a multicast.

   If the neighbor from which the LS Ack packet was received is in state
   Exchange or greater, then the following steps are performed for each
   LS Ack in the received LS Ack packet:

   (1) If the router does not have a database copy of the LSA being
       acknowledged, or has a database copy that is less recent than the
       one being acknowledged, the LS Ack is added to the Acked LSA List
       for the sending neighbor.




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   (2) If the router has a database copy of the LSA being acknowledged,
       which is the same as the instance being acknowledged, then the
       following action is performed.  For each MANET interface for
       which a BackupWait Neighbor List exists for the LSA (see Section
       8.1), remove the sending neighbor from the BackupWait Neighbor
       List if it belongs to the list.

9.  Router-LSAs

   Unlike the DR of an OSPF broadcast network, an MDR does not originate
   a network-LSA, since a network-LSA cannot be used to describe the
   general topology of a MANET.  Instead, each router advertises a
   subset of its MANET neighbors as point-to-point links in its router-
   LSA.  The choice of which MANET neighbors to include in the router-
   LSA is flexible.  Whether or not adjacency reduction is used, the
   router can originate either partial-topology or full-topology LSAs.

   If adjacency reduction is used (AdjConnectivity is 1 or 2), then as a
   minimum requirement each router must advertise a minimum set of
   "backbone" neighbors in its router-LSA.  This minimum choice
   corresponds to LSAFullness = 0, and results in the minimum amount of
   LSA flooding overhead, but does not provide routing along shortest
   paths.

   Therefore, to allow routers to calculate shortest paths, without
   requiring every pair of neighboring routers along the shortest paths
   to be adjacent (which would be inefficient due to requiring a large
   number of adjacencies), a router-LSA may also advertise non-adjacent
   neighbors that satisfy a synchronization condition described below.

   To motivate this, we note that OSPF already allows a non-adjacent
   neighbor to be a next hop, if both the router and the neighbor belong
   to the same broadcast network (and are both adjacent to the DR).  A
   network-LSA for a broadcast network (which includes all routers
   attached to the network) implies that any router attached to the
   network can forward packets directly to any other router attached to
   the network (which is why the distance from the network to all
   attached routers is zero in the graph representing the link-state
   database).

   Since a network-LSA cannot be used to describe the general topology
   of a MANET, the only way to advertise non-adjacent neighbors that can
   be used as next hops is to include them in the router-LSA.  However,
   to ensure that such neighbors are sufficiently synchronized, only
   "routable" neighbors are allowed to be included in LSAs, and to be
   used as next hops in the SPF calculation.





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9.1.  Routable Neighbors

   If adjacency reduction is used, a bidirectional MANET neighbor
   becomes routable if the SPF calculation has found a route to the
   neighbor and the neighbor satisfies the routable neighbor quality
   condition (defined below).  Since only routable and Full neighbors
   are advertised in router-LSAs, and since adjacencies are selected to
   form a connected spanning subgraph, this definition implies that
   there exists, or recently existed, a path of full adjacencies from
   the router to the routable neighbor.  The idea is that, since a
   routable neighbor can be reached through an acceptable path, it makes
   sense to take a "shortcut" and forward packets directly to the
   routable neighbor.

   This requirement does not guarantee perfect synchronization, but
   simulations have shown that it performs well in mobile networks.
   This requirement avoids, for example, forwarding packets to a new
   neighbor that is poorly synchronized because it was not reachable
   before it became a neighbor.

   To avoid selecting poor-quality neighbors as routable neighbors, a
   neighbor that is selected as a routable neighbor must satisfy the
   routable neighbor quality condition.  By default, this condition is
   that the neighbor's BNS must include the router itself (indicating
   that the neighbor agrees the connection is bidirectional).
   Optionally, a router may impose a stricter condition.  For example, a
   router may require that two Hellos have been received from the
   neighbor that (explicitly or implicitly) indicate that the neighbor's
   BNS includes the router itself.

   The single-bit neighbor variable Routable indicates whether the
   neighbor is routable, and is initially set to 0.  If adjacency
   reduction is used, Routable is updated as follows when the state of
   the neighbor changes, or the SPF calculation finds a route to the
   neighbor, or a Hello is received that affects the routable neighbor
   quality condition.

   (1) If Routable is 0 for the neighbor, the state of the neighbor is
       2-Way or greater, there exists a route to the neighbor, and the
       routable neighbor quality condition (defined above) is satisfied,
       then Routable is set to 1 for the neighbor.

   (2) If Routable is 1 for the neighbor and the state of the neighbor
       is less than 2-Way, Routable is set to 0 for the neighbor.

   If adjacency reduction is not used (AdjConnectivity = 0), then
   routable neighbors are not computed and the set of routable neighbors
   remains empty.



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9.2.  Backbone Neighbors

   The flexible choice for the router-LSA is made possible by defining
   two types of neighbors that are included in the router-LSA: backbone
   neighbors and Selected Advertised Neighbors.

   If adjacency reduction is used, a bidirectional neighbor is defined
   to be a backbone neighbor if and only if it satisfies the condition
   for becoming adjacent (see Section 7.2).  If adjacency reduction is
   not used (AdjConnectivity = 0), a bidirectional neighbor is a
   backbone neighbor if and only if the neighbor's A-bit is 0
   (indicating that the neighbor is using adjacency reduction).  This
   definition allows the interoperation between routers that use
   adjacency reduction and routers that do not.

   If adjacency reduction is used, then a router MUST include in its
   router-LSA all Full neighbors and all routable backbone neighbors.  A
   minimal LSA, corresponding to LSAFullness = 0, includes only these
   neighbors.  This choice guarantees connectivity, but does not ensure
   shortest paths.  However, this choice is useful in large networks to
   achieve maximum scalability.

9.3.  Selected Advertised Neighbors

   To allow flexibility while ensuring that router-LSAs are symmetric
   (i.e., router i advertises a link to router j if and only if router j
   advertises a link to router i), each router maintains a Selected
   Advertised Neighbor set (SANS), which consists of MANET neighbors
   that the router has selected to advertise in its router-LSA, not
   including backbone neighbors.  Since the SANS does not include
   backbone neighbors (and thus Dependent Neighbors), the SANS and DNS
   are disjoint.  Both of these neighbor sets are advertised in Hellos.

   If LSAFullness is 0 (minimal LSAs), then the SANS is empty.  At the
   other extreme, if LSAFullness is 4 (full-topology LSAs), the SANS
   includes all bidirectional MANET neighbors except backbone neighbors.
   In between these two extremes, a router that is using adjacency
   reduction may select any subset of bidirectional non-backbone
   neighbors as its SANS.  The resulting router-LSA is constructed as
   specified in Section 9.4.

   Since a router that is not using adjacency reduction typically has no
   backbone neighbors (unless it has neighbors that are using adjacency
   reduction), to ensure connectivity, such a router must choose its
   SANS to contain the SANS corresponding to LSAFullness = 1.  Thus, if
   AdjConnectivity is 0 (no adjacency reduction), then LSAFullness must
   be 1, 2, or 4.




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   If LSAFullness is 1, the router originates min-cost LSAs, which are
   partial-topology LSAs that (when flooded) provide each router with
   sufficient information to calculate a shortest (minimum-cost) path to
   each destination.  Appendix C describes the algorithm for selecting
   the neighbors to include in the SANS that results in min-cost LSAs.
   The input to this algorithm includes information obtained from Hellos
   received from each MANET neighbor, including the neighbor's
   Bidirectional Neighbor Set (BNS), Dependent Neighbor Set (DNS),
   Selected Advertised Neighbor Set (SANS), and the Metric TLV.  The
   Metric TLV, specified in Section A.2.5, is appended to each Hello
   (unless all link costs are 1) to advertise the link cost to each
   bidirectional neighbor.

   If LSAFullness is 2, the SANS must be selected to be a superset of
   the SANS corresponding to LSAFullness = 1.  This choice provides
   shortest-path routing while allowing the router to advertise
   additional neighbors to provide redundant routes.

   If LSAFullness is 3, each MDR originates a full-topology LSA (which
   includes all Full and routable neighbors), while each non-MDR router
   originates a minimal LSA.  If the router has multiple MANET
   interfaces, the router-LSA includes all Full and routable neighbors
   on each interface for which it is an MDR, and advertises only Full
   neighbors and routable backbone neighbors on its other interfaces.
   This choice provides routing along nearly shortest paths with
   relatively low overhead.

   Although this document specifies a few choices of the SANS, which
   correspond to different values of LSAFullness, it is important to
   note that other choices are possible.  In addition, it is not
   necessary for different routers to choose the same value of
   LSAFullness.  The different choices are interoperable because they
   all require the router-LSA to include a minimum set of neighbors, and
   because the construction of the router-LSA (described below) ensures
   that the router-LSAs originated by different routers are consistent.

9.4.  Originating Router-LSAs

   When a new router-LSA is originated, it includes a point-to-point
   (type 1) link for each MANET neighbor that is advertised.  The set of
   neighbors to be advertised is determined as follows.  If adjacency
   reduction is used, the router advertises all Full neighbors, and
   advertises each routable neighbor j that satisfies any of the
   following three conditions.  If adjacency reduction is not used
   (AdjConnectivity = 0), the router advertises each Full neighbor j
   that satisfies any of the following three conditions.

   (1) The router's SANS (for any interface) includes j.



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   (2) Neighbor j's SANS includes the router (to ensure symmetry).

   (3) Neighbor j is a backbone neighbor.

   Note that backbone neighbors and neighbors in the SANS need not be
   routable or Full, but only routable and Full neighbors may be
   included in the router-LSA.  This is done so that the SANS, which is
   advertised in Hellos, does not depend on routability.

   The events that cause a new router-LSA to be originated are the same
   as in [RFC2328] and [RFC5340] except that a MANET neighbor changing
   to/from the Full state does not always cause a new router-LSA to be
   originated.  Instead, a new router-LSA is originated whenever a
   change occurs that causes any of the following three conditions to
   occur:

   o  There exists a MANET neighbor j that satisfies the above
      conditions for inclusion in the router-LSA, but is not included in
      the current router-LSA.

   o  The current router-LSA includes a MANET neighbor that is no longer
      bidirectional.

   o  The link metric has changed for a MANET neighbor that is included
      in the current router-LSA.

   The above conditions may be checked periodically just before sending
   each Hello, instead of checking them every time one of the neighbor
   sets changes.  The following implementation was found to work well.
   Just before sending each Hello, and whenever a bidirectional neighbor
   transitions to less than 2-Way, the router runs the MDR selection
   algorithm; updates its adjacencies, routable neighbors, and Selected
   Advertised Neighbors; then checks the above conditions to see if a
   new router-LSA should be originated.  In addition, if a neighbor
   becomes bidirectional or Full, the router updates its routable
   neighbors and checks the above conditions.

10.  Calculating the Routing Table

   The routing table calculation is the same as specified in [RFC2328],
   except for the following changes to Section 16.1 (Calculating the
   shortest-path tree for an area).  If full-topology adjacencies and
   full-topology LSAs are used (AdjConnectivity = 0 and LSAFullness =
   4), there is no change to Section 16.1.

   If adjacency reduction is used (AdjConnectivity is 1 or 2), then
   Section 16.1 is modified as follows.  Recall from Section 9 that a
   router can use any routable neighbor as a next hop to a destination,



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   whether or not the neighbor is advertised in the router-LSA.  This is
   accomplished by modifying Step 2 so that the router-LSA associated
   with the root vertex is replaced with a dummy router-LSA that
   includes links to all Full neighbors and all routable MANET
   neighbors.  In addition, Step 2b (checking for a link from W back to
   V) MUST be skipped when V is the root vertex and W is a routable
   MANET neighbor.  However, Step 2b must still be executed when V is
   not the root vertex, to ensure compatibility with OSPFv3.

   If LSAFullness is 0 (minimal LSAs), then the calculated paths need
   not be shortest paths.  In this case, the path actually taken by a
   packet can be shorter than the calculated path, since intermediate
   routers may have routable neighbors that are not advertised in any
   router-LSA.

   If full-topology adjacencies and partial-topology LSAs are used, then
   Section 16.1 is modified as follows.  Step 2 is modified so that the
   router-LSA associated with the root vertex is replaced with a dummy
   router-LSA that includes links to all Full neighbors.  In addition,
   Step 2b MUST be skipped when V is the root vertex and W is a Full
   MANET neighbor.  (This is necessary since the neighbor's router-LSA
   need not contain a link back to the router.)

   If adjacency reduction is used with partial-topology LSAs, then the
   set of routable neighbors can change without causing the contents of
   the router-LSA to change.  This could happen, for example, if a
   routable neighbor that was not included in the router-LSA transitions
   to the Down or Init state.  Therefore, if the set of routable
   neighbors changes, the shortest-path tree must be recalculated, even
   if the router-LSA does not change.

   After the shortest-path tree and routing table are calculated, the
   set of routable neighbors must be updated, since a route to a non-
   routable neighbor may have been discovered.  If the set of routable
   neighbors changes, then the shortest-path tree and routing table must
   be calculated a second time.  The second calculation will not change
   the set of routable neighbors again, so two calculations are
   sufficient.  If the set of routable neighbors is updated periodically
   every HelloInterval seconds, then it is not necessary to update the
   set of routable neighbors immediately after the routing table is
   updated.










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11.  Security Considerations

   As with OSPFv3 [RFC5340], OSPF-MDR can use the IPv6 Authentication
   Header (AH) [RFC4302] and/or the IPv6 Encapsulation Security Payload
   (ESP) [RFC4303] to provide authentication, integrity, and/or
   confidentiality.  The use of AH and ESP for OSPFv3 is described in
   [RFC4552].

   Generic threats to routing protocols are described and categorized in
   [RFC4593].  The mechanisms described in [RFC4552] provide protection
   against many of these threats, but not all of them.  In particular,
   as mentioned in [RFC5340], these mechanisms do not provide protection
   against compromised, malfunctioning, or misconfigured routers (also
   called Byzantine routers); this is true for both OSPFv3 and OSPF-MDR.

   The extension of OSPFv3 to include MANET routers does not introduce
   any new security threats.  However, the use of a wireless medium and
   lack of infrastructure, inherent with MANET routers, may render some
   of the attacks described in [RFC4593] easier to mount.  Depending on
   the network context, these increased vulnerabilities may increase the
   need to provide authentication, integrity, and/or confidentiality, as
   well as anti-replay service.

   For example, sniffing of routing information and traffic analysis are
   easier tasks with wireless routers than with wired routers, since the
   attacker only needs to be within the radio range of a router.  The
   use of confidentiality (encryption) provides protection against
   sniffing but not traffic analysis.

   Similarly, interference attacks are also easier to mount against
   MANET routers due to their wireless nature.  Such attacks can be
   mounted even if OSPF packets are protected by authentication and
   confidentiality, e.g., by transmitting noise or replaying outdated
   OSPF packets.  As discussed below, an anti-replay service (provided
   by both ESP and AH) can be used to protect against the latter attack.

   The following threat actions are also easier with MANET routers:
   spoofing (assuming the identify of a legitimate router),
   falsification (sending false routing information), and overloading
   (sending or triggering an excessive amount of routing updates).
   These attacks are only possible if authentication is not used, or the
   attacker takes control of a router or is able to forge legitimacy
   (e.g., by discovering the cryptographic key).

   [RFC4552] mandates the use of manual keying when current IPsec
   protocols are used with OSPFv3.  Routers are required to use manually
   configured keys with the same security association (SA) parameters
   for both inbound and outbound traffic.  For MANET routers, this



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   implies that all routers attached to the same MANET must use the same
   key for multicasting packets.  This is required in order to achieve
   scalability and feasibility, as explained in [RFC4552].  Future
   specifications can explore the use of automated key management
   protocols that may be suitable for MANETs.

   As discussed in [RFC4552], the use of manual keys can increase
   vulnerability.  For example, manual keys are usually long lived, thus
   giving an attacker more time to discover the keys.  In addition, the
   use of the same key on all routers attached to the same MANET leaves
   all routers insecure against impersonation attacks if any one of the
   routers is compromised.

   Although [RFC4302] and [RFC4303] state that implementations of AH and
   ESP SHOULD NOT provide anti-replay service in conjunction with SAs
   that are manually keyed, it is important to note that such service is
   allowed if the sequence number counter at the sender is correctly
   maintained across local reboots until the key is replaced.
   Therefore, it may be possible for MANET routers to make use of the
   anti-replay service provided by AH and ESP.

   When an OSPF routing domain includes both MANET networks and fixed
   networks, the frequency of OSPF updates either due to actual topology
   changes or malfeasance could result in instability in the fixed
   networks.  In situations where this is a concern, it is recommended
   that the border routers segregate the MANET networks from the fixed
   networks with either separate OSPF areas or, in cases where legacy
   routers are very sensitive to OSPF update frequency, separate OSPF
   instances.  With separate OSPF areas, the 5-second MinLSInterval will
   dampen the frequency of changes originated in the MANET networks.
   Additionally, OSPF ranges can be configured to aggregate prefixes for
   the areas supporting MANET networks.  With separate OSPF instances,
   more conservative local policies can be employed to limit the volume
   of updates emanating from the MANET networks.

12.  IANA Considerations

   This document defines three new LLS TLV types: MDR-Hello TLV (14),
   MDR-Metric TLV (16), and MDR-DD TLV (15) (see Section A.2).












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13.  Acknowledgments

   Thanks to Aniket Desai for helpful discussions and comments,
   including the suggestion that Router Priority should come before MDR
   Level in the lexicographical comparison of (RtrPri, MDR Level, RID)
   when selecting MDRs and BMDRs, and that the MDR calculation should be
   repeated if it causes the MDR Level to change.  Thanks also to Tom
   Henderson, Acee Lindem, and Emmanuel Baccelli for helpful discussions
   and comments.

14.  Normative References

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

   [RFC2328]   Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC4302]   Kent, S., "IP Authentication Header", RFC 4302, December
               2005.

   [RFC4303]   Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
               4303, December 2005.

   [RFC4552]   Gupta, M. and N. Melam, "Authentication/Confidentiality
               for OSPFv3", RFC 4552, June 2006.

   [RFC5243]   Ogier, R., "OSPF Database Exchange Summary List
               Optimization", RFC 5243, May 2008.

   [RFC5340]   Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
               for IPv6", RFC 5340, July 2008.

   [RFC5613]   Zinin, A., Roy, A.,  Nguyen, L., Friedman, B., and D.
               Yeung, "OSPF Link-Local Signaling", RFC 5613, August
               2009.

15.  Informative References

   [Lawler]    Lawler, E., "Combinatorial Optimization: Networks and
               Matroids", Holt, Rinehart, and Winston, New York, 1976.

   [Suurballe] Suurballe, J.W. and R.E. Tarjan, "A Quick Method for
               Finding Shortest Pairs of Disjoint Paths", Networks, Vol.
               14, pp. 325-336, 1984.

   [RFC4593]   Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
               Routing Protocols", RFC 4593, October 2006.




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Appendix A.  Packet Formats

A.1.  Options Field

   The L bit of the OSPF options field is used for link-local signaling,
   as described in [RFC5613].  Routers set the L bit in Hello and DD
   packets to indicate that the packet contains an LLS data block.
   Routers set the L bit in a self-originated router-LSA to indicate
   that the LSA is non-ackable.

A.2.  Link-Local Signaling

   OSPF-MDR uses link-local signaling [RFC5613] to append the MDR-Hello
   TLV and MDR-Metric TLV to Hello packets, and to append the MDR-DD TLV
   to Database Description packets.  Link-local signaling is an
   extension of OSPFv2 and OSPFv3 that allows the exchange of arbitrary
   data using existing OSPF packet types.  Here we use LLS for OSPFv3,
   which is accomplished by adding an LLS data block at the end of the
   OSPFv3 packet.  The OSPF packet length field does not include the
   length of the LLS data block, but the IPv6 packet length does include
   this length.

A.2.1.  LLS Data Block

   The data block used for link-local signaling is formatted as
   described below in Figure A.1.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Checksum           |       LLS Data Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                           LLS TLVs                            |
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure A.1: Format of LLS Data Block

   The Checksum field contains the standard IP checksum of the entire
   contents of the LLS block.

   The 16-bit LLS Data Length field contains the length (in 32-bit
   words) of the LLS block including the header and payload.
   Implementations should not use the Length field in the IPv6 packet
   header to determine the length of the LLS data block.



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   The rest of the block contains a set of Type/Length/Value (TLV)
   triplets as described in the following section.  All TLVs must be
   32-bit aligned (with padding if necessary).

A.2.2.  LLS TLV Format

   The contents of the LLS data block are constructed using TLVs.  See
   Figure A.2 for the TLV format.

   The Type field contains the TLV ID, which is unique for each type of
   TLV.  The Length field contains the length of the Value field (in
   bytes) that is variable and contains arbitrary data.

        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              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       .                                                               .
       .                             Value                             .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure A.2: Format of LLS TLVs

   Note that TLVs are always padded to a 32-bit boundary, but padding
   bytes are not included in the TLV Length field (though they are
   included in the LLS Data Length field of the LLS block header).  All
   unknown TLVs MUST be silently ignored.

A.2.3.  MDR-Hello TLV

   The MDR-Hello TLV is appended to each MANET Hello using LLS.  It
   includes the current Hello sequence number (HSN) for the transmitting
   interface and the number of neighbors of each type that are listed in
   the body of the Hello (see Section 4.1).  It also indicates whether
   the Hello is differential (via the D-bit), and whether the router is
   using full-topology adjacencies (via the A-bit).












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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               |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Hello Sequence Number      |          Reserved         |A|D|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      N1       |      N2       |      N3       |      N4       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Type: Set to 14.

   o  Length: Set to 8.

   o  Hello Sequence Number: A circular two-octet unsigned integer
      indicating the current HSN for the transmitting interface.  The
      HSN for the interface is incremented by 1 (modulo 2^16) every time
      a (differential or full) Hello is sent on the interface.

   o  Reserved: Set to 0.  Reserved for future use.

   o  A (1 bit): Set to 1 if AdjConnectivity is 0; otherwise, set to 0.

   o  D (1 bit): Set to 1 for a differential Hello and 0 for a full
      Hello.

   o  N1 (8 bits): The number of neighbors listed in the Hello that are
      in state Down.  N1 is zero if the Hello is not differential.

   o  N2 (8 bits): The number of neighbors listed in the Hello that are
      in state Init.

   o  N3 (8 bits): The number of neighbors listed in the Hello that are
      Dependent.

   o  N4 (8 bits): The number of neighbors listed in the Hello that are
      Selected Advertised Neighbors.

A.2.4.  MDR-DD TLV

   When a Database Description packet is sent to a neighbor in state
   ExStart, an MDR-DD TLV is appended to the packet using LLS.  It
   includes the same two Router IDs that are included in the DR and
   Backup DR fields of a Hello sent by the router, and is used to
   indicate the router's MDR Level and Parent(s).






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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               |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               DR                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Backup DR                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Type: Set to 15.

   o  Length: Set to 8.

   o  DR: The same Router ID that is included in the DR field of a Hello
      sent by the router (see Section A.3).

   o  Backup DR: The same Router ID that is included in the Backup DR
      field of a Hello sent by the router (see Section A.3).

A.2.5.  MDR-Metric TLV

   If LSAFullness is 1 or 2, an MDR-Metric TLV must be appended to each
   MANET Hello packet using LLS, unless all link metrics are 1.  This
   TLV advertises the link metric for each bidirectional neighbor listed
   in the body of the Hello.  At a minimum, this TLV advertises a single
   default metric.  If the I bit is set, the Router ID and link metric
   are included for each bidirectional neighbor listed in the body of
   the Hello whose link metric is not equal to the default metric.  This
   option reduces overhead when all neighbors have the same link metric,
   or only a few neighbors have a link metric that differs from the
   default metric.  If the I bit is zero, the link metric is included
   for each bidirectional neighbor that is listed in the body of the
   Hello and the neighbor RIDs are omitted from the TLV.

















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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               |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Default Metric           |        Reserved             |I|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Neighbor ID (1)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Neighbor ID (2)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             ...                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Metric (1)            |        Metric (2)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Type: Set to 16.

   o  Length: Set to 4 + 6*N if the I bit is 1, and to 4 + 2*N if the I
      bit is 0, where N is the number of neighbors included in the TLV.

   o  Default Metric: If the I bit is 1, this is the link metric that
      applies to every bidirectional neighbor listed in the body of the
      Hello whose RID is not listed in the Metric TLV.

   o  Neighbor ID: If the I bit is 1, the RID is listed for each
      bidirectional neighbor (Lists 3 through 5 as defined in Section
      4.1) in the body of the Hello whose link metric is not equal to
      the default metric.  Omitted if the I bit is 0.

   o  Metric: Link metric for each bidirectional neighbor, listed in the
      same order as the Neighbor IDs in the TLV if the I bit is 1, and
      in the same order as the Neighbor IDs of bidirectional neighbors
      (Lists 3 through 5 as defined in Section 4.1) in the body of the
      Hello if the I bit is 0.














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A.3.  Hello Packet DR and Backup DR Fields

   The Designated Router (DR) and Backup DR fields of a Hello packet are
   set as follows:

   o  DR:  This field is the router's Parent, or is 0.0.0.0 if the
      Parent is null.  The Parent of an MDR is always the router's own
      RID.

   o  Backup DR:  This field is the router's Backup Parent, or is
      0.0.0.0 if the Backup Parent is null.  The Backup Parent of a BMDR
      is always the router's own RID.

A.4.  LSA Formats and Examples

   LSA formats are specified in [RFC5340], Section 4.4.  Figure A.3
   below gives an example network map for a MANET in a single area.

   o  Four MANET routers RT1, RT2, RT3, and RT4 are in area 1.

   o  RT1's MANET interface has links to RT2 and RT3's MANET interfaces.

   o  RT2's MANET interface has links to RT1 and RT3's MANET interfaces.

   o  RT3's MANET interface has links to RT1, RT2, and RT3's MANET
      interfaces.

   o  RT4's MANET interface has a link to RT3's MANET interface.

   o  RT1 and RT2 have stub networks attached on broadcast interfaces.

   o  RT3 has a transit network attached on a broadcast interface.



















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       ..........................................
       .                                  Area 1.
       .     +                                  .
       .     |                                  .
       .     |  2+---+1                      1+---+
       .  N1 |---|RT1|----+               +---|RT4|----
       .     |   +---+    |\             /    +---+
       .     |            | \           /       .
       .     +            |  \   N3    /        .
       .                  |   \       /         .
       .     +            |    \     /          .
       .     |            |     \   /           .
       .     |  2+---+1   |      \ /            .
       .  N2 |---|RT2|----+-------+             .
       .     |   +---+            |1            .
       .     |                  +---+           .
       .     |                  |RT3|----------------
       .     +                  +---+           .
       .                          |2            .
       .                   +------------+       .
       .                      |1   N4           .
       .                    +---+               .
       .                    |RT5|               .
       .                    +---+               .
       ..........................................

       Figure A.3: Area 1 with IP Addresses Shown


      Network   IPv6 prefix
      -----------------------------------
      N1        5f00:0000:c001:0200::/56
      N2        5f00:0000:c001:0300::/56
      N4        5f00:0000:c001:0400::/56

      Table 1: IPv6 link prefixes for sample network















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      Router     interface   Interface ID  IPv6 global unicast prefix
      -----------------------------------------------------------
      RT1      LOOPBACK      0             5f00:0001::/64
               to N3         1             n/a
               to N1         2             5f00:0000:c001:0200::RT1/56
      RT2      LOOPBACK      0             5f00:0002::/64
               to N3         1             n/a
               to N2         2             5f00:0000:c001:0300::RT2/56
      RT3      LOOPBACK      0             5f00:0003::/64
               to N3         1             n/a
               to N4         2             5f00:0000:c001:0400::RT3/56
      RT4      LOOPBACK      0             5f00:0004::/64
               to N3         1             n/a
      RT5      to N4         1             5f00:0000:c001:0400::RT5/56

      Table 2: IPv6 link prefixes for sample network


      Router   interface   Interface ID   link-local address
      -------------------------------------------------------
      RT1      LOOPBACK    0              n/a
               to N1       1              fe80:0001::RT1
               to N3       2              fe80:0002::RT1
      RT2      LOOPBACK    0              n/a
               to N2       1              fe80:0001::RT2
               to N3       2              fe80:0002::RT2
      RT3      LOOPBACK    0              n/a
               to N3       1              fe80:0001::RT3
               to N4       2              fe80:0002::RT3
      RT4      LOOPBACK    0              n/a
               to N3       1              fe80:0001::RT4
      RT5      to N4       1              fe80:0002::RT5

      Table 3: OSPF interface IDs and link-local addresses

















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A.4.1.  Router-LSAs

   As an example, consider the router-LSA that node RT3 would originate.
   The node consists of one MANET, one broadcast, and one loopback
   interface.

   RT3's router-LSA

   LS age = DoNotAge+0              ;newly originated
   LS type = 0x2001                 ;router-LSA
   Link State ID = 0                ;first fragment
   Advertising Router = 192.1.1.3   ;RT3's Router ID
   bit E = 0                        ;not an AS boundary router
   bit B = 1                        ;area border router
   Options = (V6-bit|E-bit|R-bit)
     Type = 1                        ;p2p link to RT1
     Metric = 1                      ;cost to RT1
     Interface ID = 1                ;Interface ID
     Neighbor Interface ID = 1       ;Interface ID
     Neighbor Router ID = 192.1.1.1  ;RT1's Router ID
     Type = 1                        ;p2p link to RT2
     Metric = 1                      ;cost to RT2
     Interface ID = 1                ;Interface ID
     Neighbor Interface ID = 1       ;Interface ID
     Neighbor Router ID = 192.1.1.2  ;RT2's Router ID
     Type = 1                        ;p2p link to RT4
     Metric = 1                      ;cost to RT4
     Interface ID = 1                ;Interface ID
     Neighbor Interface ID = 1       ;Interface ID
     Neighbor Router ID = 192.1.1.4  ;RT4's Router ID
     Type = 2                        ;connects to N4
     Metric = 1                      ;cost to N4
     Interface ID = 2                ;RT3's Interface ID
     Neighbor Interface ID = 1       ;RT5's Interface ID (elected DR)
     Neighbor Router ID = 192.1.1.5  ;RT5's Router ID  (elected DR)
















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A.4.2.  Link-LSAs

   Consider the link-LSA that RT3 would originate for its MANET
   interface.

   RT3's link-LSA for its MANET interface

   LS age = DoNotAge+0              ;newly originated
   LS type = 0x0008                 ;Link-LSA
   Link State ID = 1                ;Interface ID
   Advertising Router = 192.1.1.3   ;RT3's Router ID
   RtrPri = 1                       ;default priority
   Options = (V6-bit|E-bit|R-bit)
   Link-local Interface Address = fe80:0001::RT3
   # prefixes = 0                   ;no global unicast address

A.4.3.  Intra-Area-Prefix-LSAs

   A MANET node originates an intra-area-prefix-LSA to advertise its own
   prefixes, and those of its attached networks or stub links.  As an
   example, consider the intra-area-prefix-LSA that RT3 will build.

   RT2's intra-area-prefix-LSA for its own prefixes

   LS age = DoNotAge+0              ;newly originated
   LS type = 0x2009                 ;intra-area-prefix-LSA
   Link State ID = 177              ;or something
   Advertising Router = 192.1.1.3   ;RT3's Router ID
   # prefixes = 2
   Referenced LS type = 0x2001      ;router-LSA reference
   Referenced Link State ID = 0     ;always 0 for router-LSA reference
   Referenced Advertising Router = 192.1.1.3 ;RT2's Router ID
     PrefixLength = 64               ;prefix on RT3's LOOPBACK
     PrefixOptions = 0
     Metric = 0                      ;cost of RT3's LOOPBACK
     Address Prefix = 5f00:0003::/64
     PrefixLength = 56               ;prefix on RT3's interface 2
     PrefixOptions = 0
     Metric = 1                      ;cost of RT3's interface 2
     Address Prefix = 5f00:0000:c001:0400::RT3/56    ;pad











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Appendix B.  Detailed Algorithms for MDR/BMDR Selection

   This section provides detailed algorithms for Step 2.4 of Phase 2
   (MDR selection) and Step 3.2 of Phase 3 (BMDR selection) of the MDR
   selection algorithm described in Section 5.  Step 2.4 uses a breadth-
   first search (BFS) algorithm, and Step 3.2 uses an efficient
   algorithm for finding pairs of node-disjoint paths from Rmax to all
   other neighbors.  Both algorithms run in O(d^2) time, where d is the
   number of neighbors.

   For convenience, in the following description, the term "bi-neighbor"
   will be used as an abbreviation for "bidirectional neighbor".  Also,
   node i denotes the router performing the calculation.

B.1.  Detailed Algorithm for Step 2.4 (MDR Selection)

   The following algorithm performs Step 2.4 of the MDR selection
   algorithm, and assumes that Phase 1 and Steps 2.1 through 2.3 have
   been performed, so that the neighbor connectivity matrix NCM has been
   computed and Rmax is the bi-neighbor with the (lexicographically)
   largest value of (RtrPri, MDR Level, RID).  The BFS algorithm uses a
   FIFO queue so that all nodes 1 hop from node Rmax are processed
   first, then 2 hops, etc.  When the BFS algorithm terminates, hops(u),
   for each bi-neighbor node u of node i, will be equal to the minimum
   number of hops from node Rmax to node u, using only intermediate
   nodes that are bi-neighbors of node i and that have a larger value of
   (RtrPri, MDR Level, RID) than node i.  The algorithm also computes,
   for each node u, the tree parent p(u) and the second node r(u) on the
   tree path from Rmax to u, which will be used in Step 3.2.

   (a)  Compute a matrix of link costs c(u,v) for each pair of bi-
        neighbors u and v as follows: If node u has a larger value of
        (RtrPri, MDR Level, RID) than node i, and NCM(u,v) = 1, then set
        c(u,v) to 1.  Otherwise, set c(u,v) to infinity.  (Note that the
        matrix NCM(u,v) is symmetric, but the matrix c(u,v) is not.)

   (b)  Set hops(u) = infinity for all bi-neighbors u other than Rmax,
        and set hops(Rmax) = 0.  Initially, p(u) is undefined for each
        neighbor u.  For each bi-neighbor u such that c(Rmax,u) = 1, set
        r(u) = u; for all other u, r(u) is initially undefined.  Add
        node Rmax to the FIFO queue.

   (c)  While the FIFO queue is nonempty:  Remove the node at the head
        of the queue; call it node u.  For each bi-neighbor v of node i
        such that c(u,v) = 1:
          If hops(v) > hops(u) + 1, then set hops(v) = hops(u) + 1, set
          p(v) = u, set r(v) = r(u) if hops(v) > 1, and add node v to
          the tail of the queue.



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B.2.  Detailed Algorithm for Step 3.2 (BMDR Selection)

   Step 3.2 of the MDR selection algorithm requires the router to
   determine whether there exist two node-disjoint paths from Rmax to
   each other bi-neighbor u, via bi-neighbors that have a larger value
   of (RtrPri, MDR Level, RID) than the router itself.  This information
   is needed to determine whether the router should select itself as a
   BMDR.

   It is possible to determine separately for each bi-neighbor u whether
   there exist two node-disjoint paths from Rmax to u, using the well-
   known augmenting path algorithm [Lawler] that runs in O(n^2) time,
   but this must be done for all bi-neighbors u, thus requiring a total
   run time of O(n^3).  The algorithm described below makes the same
   determination simultaneously for all bi-neighbors u, achieving a much
   faster total run time of O(n^2).  The algorithm is a simplified
   variation of the Suurballe-Tarjan algorithm [Suurballe] for finding
   pairs of disjoint paths.

   The algorithm described below uses the following output of Phase 2:
   the tree parent p(u) of each node (which defines the BFS tree
   computed in Phase 2), and the second node r(u) on the tree path from
   Rmax to u.

   The algorithm uses the following concepts.  For any node u on the BFS
   tree other than Rmax, we define g(u) to be the first labeled node on
   the reverse tree path from u to Rmax, if such a labeled node exists
   other than Rmax.  (The reverse tree path consists of u, p(u),
   p(p(u)), ..., Rmax.)  If no such labeled node exists, then g(u) is
   defined to be r(u).  In particular, if u is labeled then g(u) = u.
   Note that g(u) either must be labeled or must be a neighbor of Rmax.

   For any node k that either is labeled or is a neighbor of Rmax, we
   define the unlabeled subtree rooted at k, denoted S(k), to be the set
   of nodes u such that g(u) = k.  Thus, S(k) includes node k itself and
   the set of unlabeled nodes downstream of k on the BFS tree that can
   be reached without going through any labeled nodes.  This set can be
   obtained in linear time using a depth-first search starting at node
   k, and using labeled nodes to indicate the boundaries of the search.
   Note that g(u) and S(k) are not maintained as variables in the
   algorithm given below, but simply refer to the definitions given
   above.

   The BMDR algorithm maintains a set B, which is initially empty.  A
   node u is added to B when it is known that two node-disjoint paths
   exist from Rmax to u via nodes that have a larger value of (RtrPri,
   MDR Level, RID) than the router itself.  When the algorithm
   terminates, B consists of all nodes that have this property.



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   The algorithm consists of the following two steps.

   (a) Mark Rmax as labeled.  For each pair of nodes u, v on the BFS
       tree other than Rmax such that r(u) is not equal to r(v) (i.e., u
       and v have different second nodes), NCM(u,v) = 1, and node u has
       a greater value of (RtrPri, MDR level, RID) than the router
       itself, add v to B.  (Clearly there are two disjoint paths from
       Rmax to v.)

   (b) While there exists a node in B that is not labeled, do the
       following.  Choose any node k in B that is not labeled, and let j
       = g(k).  Now mark k as labeled. (This creates a new unlabeled
       subtree S(k), and makes S(j) smaller by removing S(k) from it.)
       For each pair of nodes u, v such that u is in S(k), v is in S(j),
       and NCM(u,v) = 1:

       o  If u has a larger value of (RtrPri, MDR level, RID) than the
          router itself, and v is not in B, then add v to B.

       o  If v has a larger value of (RtrPri, MDR level, RID) than the
          router itself, and u is not in B, then add u to B.

   A simplified version of the algorithm MAY be performed by omitting
   step (b).  However, the simplified algorithm will result in more
   BMDRs, and is not recommended if AdjConnectivity = 2 since it will
   result in more adjacencies.

   The above algorithm can be executed in O(n^2) time, where n is the
   number of neighbors.  Step (a) clearly requires O(n^2) time since it
   considers all pairs of nodes u and v.  Step (b) also requires O(n^2)
   time because each pair of nodes is considered at most once.  This is
   because labeling nodes divides unlabeled subtrees into smaller
   unlabeled subtrees, and a given pair u, v is considered only the
   first time u and v belong to different unlabeled subtrees.

















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Appendix C.  Min-Cost LSA Algorithm

   This section describes the algorithm for determining which MANET
   neighbors to include in the router-LSA when LSAFullness is 1.  The
   min-cost LSA algorithm ensures that the link-state database provides
   sufficient information to calculate at least one shortest (minimum-
   cost) path to each destination.  The algorithm assumes that a router
   may have multiple interfaces, at least one of which is a MANET
   interface.  The algorithm becomes significantly simpler if the router
   has only a single (MANET) interface.

   The input to this algorithm includes information obtained from Hellos
   received from each neighbor on each MANET interface, including the
   neighbor's Bidirectional Neighbor Set (BNS), Dependent Neighbor Set
   (DNS), Selected Advertised Neighbor Set (SANS), and link metrics.
   The input also includes the link-state database if the router has a
   non-MANET interface.

   The output of the algorithm is the router's SANS for each MANET
   interface.  The SANS is used to construct the router-LSA as described
   in Section 9.4.  The min-cost LSA algorithm must be run to update the
   SANS (and possibly originate a new router-LSA) either periodically
   just before sending each Hello, or whenever any of the following
   events occurs:

   o  The state or routability of a neighbor changes.

   o  A Hello received from a neighbor indicates a change in its MDR
      Level, Router Priority, FullHelloRcvd, BNS, DNS, SANS, Parent(s),
      or link metrics.

   o  An LSA originated by a non-MANET neighbor is received.

   Although the algorithm described below runs in O(d^3) time, where d
   is the number of neighbors, an incremental version for a single
   topology change runs in O(d^2) time, as discussed following the
   algorithm description.

   For convenience, in the following description, the term "bi-neighbor"
   will be used as an abbreviation for "bidirectional neighbor".  Also,
   router i will denote the router doing the calculation.  To perform
   the min-cost LSA algorithm, the following steps are performed.

   (1) Create the neighbor connectivity matrix (NCM) for each MANET
       interface, as described in Section 5.1.  Create the multiple-
       interface neighbor connectivity matrix MNCM as follows.  For each
       bi-neighbor j, set MNCM(i,j) = MNCM(j,i) = 1.  For each pair j, k
       of MANET bi-neighbors, set MNCM(j,k) = 1 if NCM(j,k) equals 1 for



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       any MANET interface.  For each pair j, k of non-MANET bi-
       neighbors, set MNCM(j,k) = 1 if the link-state database indicates
       that a direct link exists between j and k.  Otherwise, set
       MNCM(j,k) = 0.  (Note that a given router can be a neighbor on
       both a MANET interface and a non-MANET interface.)

   (2) Create the inter-neighbor cost matrix (COST) as follows.  For
       each pair j, k of routers such that each of j and k is a bi-
       neighbor or router i itself:

       (a) If MNCM(j,k) = 1, set COST(j,k) to the metric of the link
           from j to k obtained from j's Hellos (for a MANET interface),
           or from the link-state database (for a non-MANET interface).
           If there are multiple links from j to k (via multiple
           interfaces), COST(j,k) is set to the minimum cost of these
           links.

       (b) Otherwise, set COST(j,k) to LSInfinity.

   (3) Create the backbone neighbor matrix (BNM) as follows.  BNM
       indicates which pairs of MANET bi-neighbors are backbone
       neighbors of each other, as defined in Section 9.2.1.  If
       adjacency reduction is not used (AdjConnectivity = 0), set all
       entries of BNM to zero and proceed to Step 4.

       In the following, if a link exists from router j to router k on
       more than one interface, we consider only interfaces for which
       the cost from j to k equals COST(j,k); such interfaces will be
       called "candidate" interfaces.

       For each pair j, k of MANET bi-neighbors, BNM(j,k) is set to 1 if
       j and k are backbone neighbors of each other on a candidate MANET
       interface.  That is, BNM(j,k) is set to 1 if, for any candidate
       MANET interface, NCM(j,k) = 1 and either of the following
       conditions is satisfied:

       (a) Router k is included in j's DNS or router j is included in
           k's DNS.

       (b) Router j is the (Backup) Parent of router k or router k is
           the (Backup) Parent of router j.

       Otherwise, BNM(j,k) is set to 0.

   (4) Create the Selected Advertised Neighbor Matrix (SANM) as follows.
       For each pair j, k of routers such that each of j and k is a bi-
       neighbor or router i itself, SANM(j,k) is set to 1 if, for any




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       candidate MANET interface, NCM(j,k) = 1 and k is included in j's
       SANS.  Otherwise, SANM(j,k) is set to 0.  Note that SANM(i,k) is
       set to 1 if k is currently a Selected Advertised Neighbor.

   (5) Compute the new set of Selected Advertised Neighbors as follows.
       For each MANET bi-neighbor j, initialize the bit variable
       new_sel_adv(j) to 0. (This bit will be set to 1 if j is
       selected.)  For each MANET bi-neighbor j:

       (a) If j is a bi-neighbor on more than one interface, consider
           only candidate interfaces (for which the cost to j is
           minimum).  If one of the candidate interfaces is a non-MANET
           interface, examine the next neighbor (j is not selected since
           it will be advertised anyway).

       (b) If adjacency reduction is used, and one of the candidate
           interfaces is a MANET interface on which j is a backbone
           neighbor (see Section 9.2), examine the next neighbor (j is
           not selected since it will be advertised anyway).

       (c) Otherwise, if there is more than one candidate MANET
           interface, select the "preferred" interface by using the
           following preference rules in the given order: an interface
           is preferred if (1) router i's SANS for that interface
           already includes j, (2) router i's Router Priority is larger
           on that interface, and (3) router i's MDR Level is larger on
           that interface.

       (d) For each bi-neighbor k (on any interface) such that COST(k,j)
           > COST(k,i) + COST(i,j), determine whether there exists
           another bi-neighbor u such that either COST(k,u) + COST(u,j)
           < COST(k,i) + COST(i,j), or COST(k,u) + COST(u,j) = COST(k,i)
           + COST(i,j) and either of the following conditions is true:

           o  BNM(u,j) = 1, or

           o  (SANM(j,u), SANM(u,j), RtrPri(u), RID(u)) is
              lexicographically greater than (SANM(j,i), SANM(i,j),
              RtrPri(i), RID(i)).

       If for some such bi-neighbor k, there does not exist such a bi-
       neighbor u, then set new_sel_adv(j) = 1.

   (6) For each MANET interface I, update the SANS to equal the set of
       all bi-neighbors j such that new_sel_adv(j) = 1 and I is the
       preferred interface for j.





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   (7) With the SANS updated, a new router-LSA may need to be originated
       as described in Section 9.4.

   The lexicographical comparison of Step 5d gives preference to links
   that are already advertised, in order to improve LSA stability.

   The above algorithm can be run in O(d^2) time if a single link change
   occurs.  For example, if link (x,y) fails where x and y are neighbors
   of router i, and either SANS(x,y) = 1 or BNM(x,y) = 1, then Step 5
   need only be performed for pairs j, k such that either j or k is
   equal to x or y.

Appendix D.  Non-Ackable LSAs for Periodic Flooding

   In a highly mobile network, it is possible that a router almost
   always originates a new router-LSA every MinLSInterval seconds.  In
   this case, it should not be necessary to send Acks for such an LSA,
   or to retransmit such an LSA as a unicast, or to describe such an LSA
   in a DD packet.  In this case, the originator of an LSA MAY indicate
   that the router-LSA is "non-ackable" by setting the L bit in the
   options field of the LSA (see Section A.1).  For example, a router
   can originate non-ackable LSAs if it determines (e.g., based on an
   exponential moving average) that a new LSA is originated every
   MinLSInterval seconds at least 90 percent of the time. (Simulations
   can be used to determine the best threshold.)

   A non-ackable LSA is never acknowledged, nor is it ever retransmitted
   as a unicast or described in a DD packet, thus saving substantial
   overhead.  However, the originating router must periodically
   retransmit the current instance of its router-LSA as a multicast
   (until it originates a new LSA, which will usually happen before the
   previous instance is retransmitted), and each MDR must periodically
   retransmit each non-ackable LSA as a multicast (until it receives a
   new instance of the LSA, which will usually happen before the
   previous instance is retransmitted).  For this option to work,
   RxmtInterval must be larger than MinLSInterval so that a new instance
   of the LSA is usually received before the previous one is
   retransmitted.  Note that the reception of a retransmitted
   (duplicate) LSA does not result in immediate forwarding of the LSA;
   only a new LSA (with a larger sequence number) may be forwarded
   immediately, according to the flooding procedure of Section 8.










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Appendix E.  Simulation Results

   This section presents simulation results that predict the performance
   of OSPF-MDR for up to 160 nodes with min-cost LSAs and up to 200
   nodes with minimal LSAs.  The results were obtained using the GTNetS
   simulator with OSPF-MDR version 1.01, available at
   http://hipserver.mct.phantomworks.org/ietf/ospf.

   The following scenario parameter values were used: radio range = 200
   m and 250 m, grid length = 500 m, wireless alpha = 0.5, (maximum)
   velocity = 10 m/s, pause time = 0, packet rate = 10 pkts/s, packet
   size = 40 bytes, random seed = 8, start time (for gathering
   statistics) = 1800 s.  The stop time was 3600 s for up to 80 nodes
   and 2700 s for more than 80 nodes.  The source and destination are
   selected randomly for each generated UDP packet.  The simulated MAC
   protocol is 802.11b.

   Tables 4 and 6 show the results for the default configuration of
   OSPF-MDR, except that differential Hellos were used (2HopRefresh = 3)
   since they are recommended when the number of neighbors is large.
   Tables 5 and 7 show the results for the same configuration except
   that minimal LSAs were used instead of min-cost LSAs.  The tables
   show the results for total OSPF overhead in kb/s, the total number of
   OSPF packets per second, the delivery ratio for UDP packets, and the
   average number of hops traveled by UDP packets that reach their
   destination.

   Tables 5 and 7 for minimal LSAs also show the following statistics:
   the average number of bidirectional neighbors per node, the average
   number of fully adjacent neighbors per node, the number of changes in
   the set of bidirectional neighbors per node per second, and the
   number of changes in the set of fully adjacent neighbors per node per
   second.  These statistics do not change significantly when min-cost
   LSAs are used instead of minimal LSAs.

   The results show that OSPF-MDR achieves good performance for up to at
   least 160 nodes when min-cost LSAs are used, and up to at least 200
   nodes when minimal LSAs are used.  Also, the results for the number
   of hops show that the routes obtained with minimal LSAs are only 2.3%
   to 4.5% longer than with min-cost LSAs when the range is 250 m, and
   3.5% to 7.4% longer when the range is 200 m.

   The results also show that the number of adjacencies per node and the
   number of adjacency changes per node per second do not increase as
   the number of nodes increases, and are dramatically smaller than the
   number of neighbors per node and the number of neighbor changes per
   node per second, respectively.  These factors contribute to the low
   overhead achieved by OSPF-MDR.  For example, the results in Table 5



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   imply that with 200 nodes and range 250 m, there are 2.136/.039 = 55
   times as many adjacency formations with full-topology adjacencies as
   with uniconnected adjacencies.  Additional simulation results for
   OSPF-MDR can be found at http://www.manet-routing.org.

                                      Number of nodes
                        20     40     60     80    100    120    160
   ------------------------------------------------------------------
   OSPF kb/s           27.1   74.2  175.3  248.6  354.6  479.2  795.7
   OSPF pkts/s         29.9   69.2  122.9  163.7  210.3  257.2  357.7
   Delivery ratio      .970   .968   .954   .958   .957   .956   .953
   Avg no. hops       1.433  1.348  1.389  1.368  1.411  1.361  1.386

   Table 4: Results for range 250 m with min-cost LSAs


                                      Number of nodes
                        20     40     60     80    120    160    200
   ------------------------------------------------------------------
   OSPF kb/s           15.5   41.6   91.0  132.9  246.3  419.0  637.4
   OSPF pkts/sec       18.8   42.5   78.6  102.8  166.8  245.6  321.0
   Delivery ratio      .968   .968   .951   .953   .962   .956   .951
   Avg no. hops       1.466  1.387  1.433  1.412  1.407  1.430  1.411
   Avg no. nbrs/node  11.38  25.82  36.30  50.13  75.87  98.65 125.59
   Avg no. adjs/node   2.60   2.32   2.38   2.26   2.25   2.32   2.13
   Nbr changes/node/s  .173   .372   .575   .752  1.223  1.654  2.136
   Adj changes/node/s  .035   .036   .046   .040   .032   .035   .039

   Table 5: Results for range 250 m with minimal LSAs


                                      Number of nodes
                        20     40     60     80    100    120    160
   ------------------------------------------------------------------
   OSPF kb/s           40.5  123.4  286.5  415.7  597.5  788.9 1309.8
   OSPF pkts/s         37.6   83.9  135.1  168.6  205.4  247.7  352.3
   Delivery ratio      .926   .919   .897   .900   .898   .895   .892
   Avg no. hops       1.790  1.628  1.666  1.632  1.683  1.608  1.641

   Table 6: Results for range 200 m with min-cost LSAs











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                                      Number of nodes
                        20     40     60     80    120    160    200
   ------------------------------------------------------------------
   OSPF kb/s           24.0   63.6  140.6  195.2  346.9  573.2  824.6
   OSPF pkts/sec       26.4   58.8  108.3  138.8  215.2  311.3  401.3
   Delivery ratio      .930   .927   .897   .907   .907   .904   .902
   Avg no. hops       1.853  1.714  1.771  1.743  1.727  1.758  1.747
   Avg no. nbrs/node   7.64  18.12  25.27  35.29  52.99  68.13  86.74
   Avg no. adjs/node   2.78   2.60   2.70   2.50   2.39   2.36   2.24
   Nbr changes/node/s  .199   .482   .702   .959  1.525  2.017  2.611
   Adj changes/node/s  .068   .069   .081   .068   .055   .058   .057

   Table 7: Results for range 200 m with minimal LSAs

Authors' Addresses

   Richard G. Ogier
   SRI International

   EMail: rich.ogier@earthlink.net or rich.ogier@gmail.com


   Phil Spagnolo
   Boeing Phantom Works

   EMail: phillipspagnolo@gmail.com

























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