RFC 5820

Internet Engineering Task Force (IETF)                       A. Roy, Ed.
Request for Comments: 5820                                 Cisco Systems
Category: Experimental                                   M. Chandra, Ed.
ISSN: 2070-1721                                               March 2010

         Extensions to OSPF to Support Mobile Ad Hoc Networking


   This document describes extensions to OSPF to support mobile ad hoc
   networks (MANETs).  The extensions, called OSPF-OR (OSPF-Overlapping
   Relay), include mechanisms for link-local signaling (LLS), an OSPF-
   MANET interface, a simple technique to reduce the size of Hello
   packets by only transmitting incremental state changes, and a method
   for optimized flooding of routing updates.  OSPF-OR also provides a
   means to reduce unnecessary adjacencies to support larger MANETs.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

Copyright Notice

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   document authors.  All rights reserved.

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   Contributions published or made publicly available before November
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

Table of Contents

   1. Introduction ....................................................4
      1.1. Problem Statement ..........................................4
      1.2. Motivation for Extending OSPF to Support MANETs ............5
   2. Requirements Notation ...........................................5
   3. Proposed Enhancements ...........................................5
      3.1. OSPF-MANET Interface .......................................7
           3.1.1. Interface Operation .................................8
           3.1.2. LSA Formats and Examples ............................8
      3.2. Incremental OSPF-MANET Hellos .............................12
           3.2.1. The I Option Bit ...................................12
           3.2.2. State Check Sequence TLV (SCS TLV) .................12
           3.2.3. Neighbor Drop TLV ..................................13
           3.2.4. Request From TLV (RF TLV) ..........................14
           3.2.5. Full State For TLV (FSF TLV) .......................14
           3.2.6. Neighbor Adjacencies ...............................15
           3.2.7. Sending Hellos .....................................16
           3.2.8. Receiving Hellos ...................................17
           3.2.9. Interoperability ...................................19
           3.2.10. Support for OSPF Graceful Restart .................19
      3.3. Optimized Flooding (Overlapping Relays) ...................20
           3.3.1. Operation Overview .................................20
           3.3.2. Determination of Overlapping Relays ................21
           3.3.3. Terminology ........................................21
           3.3.4. Overlapping Relay Discovery Process ................22
           3.3.5. The F Option Bit ...................................23
           3.3.6. Active Overlapping Relay TLV (AOR TLV) .............23
           3.3.7. Willingness TLV ....................................24
           3.3.8. Flooding and Relay Decisions .......................25
           3.3.9. Intelligent Transmission of Link State
                  Acknowledgments ....................................26
           3.3.10. Important Timers ..................................27
           3.3.11. Miscellaneous Protocol Considerations .............28
           3.3.12. Interoperability ..................................28
      3.4. New Bits in LLS Type 1 Extended Options and Flags .........29
      3.5. Smart Peering .............................................29
           3.5.1. Rationale for Smart Peering ........................29
           3.5.2. Previous Related Work ..............................30
           3.5.3. Smart Peering Solution .............................30
           3.5.4. Advertising 2-Way Links in Router-LSAs .............33
   4. Security Considerations ........................................36
   5. IANA Considerations ............................................38
   6. Contributors ...................................................39
   7. Acknowledgments ................................................39
   8. References .....................................................39
      8.1. Normative References ......................................39
      8.2. Informative References ....................................40

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

1.  Introduction

   Mobile ad hoc networks (MANETs) have been an area of study for some
   time within various working groups and areas within the IETF, various
   military branches, and various government agencies.  Recently,
   networks with mobile ad hoc requirements have been proposed and are
   being seriously considered for deployment in the near term, which
   means the concepts and research now need to be applied to deployed
   networks.  Towards that end, this document applies many of the
   principles and concepts learned through prior work to [OSPFv3], along
   with new concepts based on current requirements.

1.1.  Problem Statement

   MANETs are synonymous with packet radio networks, which have been
   around since the 1960s in a limited military capacity.  With the boom
   in mobile devices and wireless communications, MANETs are finding
   scope in commercial and military environments.  The aim of these
   networks is to support robust and efficient communication in a mobile
   wireless network by incorporating routing functionality into mobile

   A MANET is an autonomous set of nodes distributed over a wide
   geographical area that communicate over bandwidth-constrained
   wireless links.  Each node may represent a transmitter, receiver, or
   relay station with varying physical capabilities.  Packets may
   traverse through several intermediate (relay) nodes before reaching
   their destination.  These networks typically lack infrastructure:
   nodes are mobile, and there is no central hub or controller; thus,
   there is no fixed network topology.  Moreover, MANETs must contend
   with a difficult and variable communication environment.  Packet
   transmissions are plagued by the usual problems of radio
   communication, which include propagation path loss, signal multipath
   and fading, and thermal noise.  These effects vary with terminal
   movement, which also induces Doppler spreading in the frequency of
   the transmitted signal.  Finally, transmissions from neighboring
   terminals, known as multi-access interference, hostile jammers, and
   impulsive interference, e.g., ignition systems, generators, and other
   non-similar in-band communications, may contribute additional

   Given this nature of MANETs, the existence of a communication link
   between a pair of nodes is a function of their variable link quality,
   including signal strength and bandwidth.  Thus, routing paths vary,
   based on environment and the resulting network topology.  In such
   networks, the topology may be stable for periods of time and then
   suddenly become unpredictable.  Since MANETs are typically
   decentralized systems, there are no central controllers or specially

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   designated routers to determine the routing paths as the topology
   changes.  All of the routing decisions and forwarding (relaying) of
   packets must be done by the nodes themselves, and communication is on
   a peer-to-peer basis.

1.2.  Motivation for Extending OSPF to Support MANETs

   The motivation to extend a standard protocol, OSPF (described in
   [OSPF] and [OSPFv3]), to operate on MANETs is twofold.  The primary
   reason is for interoperability -- MANET devices need to be able to
   work when plugged into a wireline network in as many cases as
   possible.  The junction point between a MANET and wire-line network
   should also be as fluid as possible, allowing a MANET to "plug in" to
   just about any location within a wire-line network, and also find
   connectivity, etc., as needed.

   While routes could be redistributed between two routing protocols,
   one designed just for wire-line networks, and the other just for
   MANETs, this adds complexity and overhead to the MANET/wireline
   interface, increases the odds of an error being introduced between
   the two domains, and decreases flexibility.

   The second motivation is that OSPF is a well-understood and widely
   deployed routing protocol.  This provides a strong basis of
   experience and skills from which to work.  A protocol that is known
   to work can be extended, rather than developing a new protocol that
   must then be completely troubleshot, tested, and modified over a
   number of years.  Working with a well-known protocol allows
   development effort to be placed in a narrowly focused area, rather
   than rebuilding, from scratch, many things that are already known to

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [KEY].

3.  Proposed Enhancements

   This document proposes modifications to [OSPFv3] to support mobile ad
   hoc networks (MANETs).  Note that it is possible to use the
   mechanisms defined in Sections 3.2 and 3.3 independently of one

   The challenges with deploying standard [OSPFv3] in a MANET
   environment fit into two categories.  First, traditional link-state
   routing protocols are designed for a statically configured

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   environment.  As a result, most of the configuration is done manually
   when a new router is placed in the network.  Thus, OSPF will not
   function in an environment where routers interconnect and disconnect
   in somewhat random topologies and combinations.  There are
   modifications that must be made in order for routers running the same
   protocol to communicate in a heterogeneous and dynamic environment.

   Currently there is no defined interface type that describes a
   wireless network.  Wireless links have characteristics of both multi-
   access and point-to-multipoint links.  Treating wireless links as
   multi-access does not take into account that not all nodes on the
   same Layer 2 link have bi-directional connectivity.  However, any
   transmission on a link will reach nodes that are within transmission
   range.  In this way, the link is multi-access due to the fact that
   two simultaneous transmissions may collide.  A new interface type
   needs to be defined in order to accurately describe this behavior.

   The second category of challenges involves scalability.  A MANET must
   transmit more state information to maintain reachability.  Therefore,
   OSPF will need scalability enhancements to support MANETs.  While
   some flooding optimizations are present in OSPF, such as designated
   router (DR) election, many of these were built under the assumption
   of a true multi-access network.  Wireless networks are not true
   multi-access networks, because it cannot be assumed that there is
   2-way connectivity between everyone on the same Layer 2 link.
   Therefore, optimizations such as DR election will not perform
   correctly in MANET networks.  Without any further optimizations in
   link-state flooding, current OSPF would not be able to operate in a
   highly dynamic environment in which links are constantly being formed
   and broken.  The amount of information that would need to be flooded
   would overload the network.

   Another scalability issue is the periodic transmission of Hello
   messages.  Currently, even if there are no changes in a router's
   neighbor list, the Hello messages still list all the neighbors on a
   particular link.  For a MANET router, where saving bandwidth and
   transmission power is a critical issue, the transmission of
   potentially large Hello messages is particularly wasteful.

   Finally, current routing protocols will form a neighbor relationship
   with any router on a Layer 2 link that is correctly configured.  For
   MANET routers in a wireless network, this may lead to an excessive
   number of parallel links between two routers if communication is
   achieved via multiple interfaces.  In a statically configured
   network, this is not a problem, since the physical topology can be
   built to prevent excessive redundancy.  However, in a dynamic
   network, there must exist additional mechanisms to prevent too many
   redundant links.  (Note that links between two nodes on different

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

   radio types, different antennae, different channels, etc., are
   considered different links and not redundant links.)  In scalability
   tests, it has been demonstrated that the presence of too many
   redundant links will both increase the size of routing updates and
   cause extra flooding, resulting in even relatively small networks not

3.1.  OSPF-MANET Interface

   Interfaces are defined as the connection between a router and one of
   its attached networks [OSPF].  Four types of interfaces have been
   defined and supported in [OSPF] and [OSPFv3]: broadcast, Non-
   Broadcast Multi-Access (NBMA), point-to-point, and point-to-

   The point-to-multipoint model has been chosen to represent MANET
   interfaces.  (The features designed in this document MAY be included
   on other interface types as appropriate.)  The MANET interface allows
   the following:

   o  OSPF treats all router-to-router connections over the MANET
      interface as if they were point-to-point links.

   o  Link metric can be set on a per-neighbor basis.

   o  Broadcast and multicast can be accomplished through Layer 2
      broadcast or Layer 2 pseudo-broadcast.

      *  The MANET interface supports Layer 2 broadcast if it is able to
         address a single physical message to all of the attached
         neighbors.  One such example is 802.11.

      *  The MANET interface supports Layer 2 pseudo-broadcast if it is
         able to pick up a packet from the broadcast queue, replicate
         the packet, and send a copy over each point-to-point link.  One
         such example is Frame Relay.

   o  An API must be provided for Layer 3 to determine the Layer 2
      broadcast capability.  Based on the return of the API, OSPF
      classifies the MANET interfaces into the following three types:
      MANET broadcast, MANET pseudo-broadcast, and MANET non-broadcast.

   o  Multicast SHOULD be used for OSPF packets.  When the MANET
      interface supports Layer 2 broadcast or pseudo-broadcast, the
      multicast process is transparent to OSPF.  Otherwise, OSPF MUST
      replicate multicast packets by itself.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

3.1.1.  Interface Operation

   A MANET node has at least one MANET interface.  MANET nodes can
   communicate with each other through MANET interfaces.  MANET nodes
   can communicate with non-MANET routers only through normal
   interfaces, such as Ethernet, ATM, etc.

   For scalability reasons, it is not required to configure IPv6 global
   unicast addresses on MANET interfaces.  Instead, a management
   loopback interface with an IPv6 global unicast address MAY be
   configured on each MANET node.

   The link state advertisements (LSAs) associated with a MANET
   interface SHOULD have the DC-bit set in the OSPFv3 Options Field and
   the DoNotAge bit set in the LS Age field as described in [OSPFv3].
   Demand Circuits are an optional feature; hence, the DC-bit setting
   recommendation level is SHOULD.

3.1.2.  LSA Formats and Examples

   LSA formats are specified in [OSPFv3].

   In order to display example LSAs, a network map is included below.
   Router names are prefixed with the letters RT, network names with the
   letter N, and router interface names with the letter I.

   o  Four MANET nodes, RT1, RT2, RT3, and RT4, reside in area 2.

   o  RT1 has one MANET interface, I11.  Through the interface, RT1 is
      full-adjacent to RT2, RT3, and RT4.

   o  RT2 has two MANET interfaces, I21 and I22, and one Ethernet
      interface, I23.  RT2 is full-adjacent to RT1 and RT4 through the
      interface I21, and full-adjacent to RT4 through the interface I22.
      Stub network N1 is attached with RT2 through the interface I23.

   o  RT3 has one MANET interface, I31, and is full-adjacent to RT1
      through the interface.

   o  RT4 has two MANET interfaces, I41 and I42.  It is full-adjacent to
      RT2 through the interface I41, and full-adjacent to RT1 and RT2
      through the interface I42.

   o  Moreover, each MANET node is configured with a management loopback

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

      +---+I11        I21+---+I23   |
      +---+ |          | +---+      |
      |                |   VI22
      |                |   +
      |                |   |
      |                |   |
      |                |   |
      |                |   |
      |                |   +
      |                |   ^I41
      +---+ |          +---+
      |RT3|-+        +-|RT4|
      +---+I31      I42+---+

   The assignment of IPv6 global unicast prefixes to network links is
   shown below.  (Note: No IPv6 global unicast addresses are configured
   on the MANET interfaces).

      RT1      LOOPBACK      2001:DB8:0001::/64
               I11           n/a
      RT2      LOOPBACK      2001:DB8:0002::/64
               I21           n/a
               I22           n/a
               I23           2001:DB8:0012::/60
      RT3      LOOPBACK      2001:DB8:0003::/64
               I31           n/a
      RT4      LOOPBACK      2001:DB8:0004::/64
               I41           n/a
               I42           n/a

   The OSPF interface IDs and the link-local addresses for the router
   interfaces in the network are shown below.  EUIxy represents the
   64-bit interface identifier of the interface Ixy, in Modified EUI-64
   format [IPV6ADD].

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      Node    Interface    Interface ID    Link-Local address
      RT1     LOOPBACK     1               n/a
              I11          2               fe80:0002::EUI11
      RT2     LOOPBACK     1               n/a
              I21          2               fe80:0002::EUI21
              I22          3               fe80:0003::EUI22
              I23          4               fe80:0004::EUI23
      RT3     LOOPBACK     1               n/a
              I31          2               fe80:0002::EUI31
      RT4     LOOPBACK     1               n/a
              I41          2               fe80:0002::EUI41
              I42          3               fe80:0003::EUI42  Router-LSAs

   As an example, consider the router-LSAs that node RT2 would
   originate.  Two MANET interfaces, consisting of 3 point-to-point
   links, are presented.

      RT2's router-LSA

      LS age = DoNotAge+0              ;newly originated
      LS type = 0x2001                 ;router-LSA
      Link State ID = 0                ;first fragment
      Advertising Router =   ;RT2's Router ID
      bit E = 0                        ;not an AS boundary router
      bit B = 0                        ;not an area border router
      Options = (V6-bit|E-bit|R-bit)
       Type = 1                        ;p2p link to RT1 over I21
       Metric = 10                     ;cost to RT1
       Interface ID = 2                ;Interface ID of I21
       Neighbor Interface ID = 2       ;Interface ID of I11
       Neighbor Router ID =  ;RT1's Router ID
       Type = 1                        ;p2p link to RT4 over I21
       Metric = 25                     ;cost to RT4
       Interface ID = 2                ;Interface ID of I21
       Neighbor Interface ID = 3       ;Interface ID of I42
       Neighbor Router ID =  ;RT4's Router ID
       Type = 1                        ;p2p link to RT4 over I22
       Metric = 15                     ;cost to RT4
       Interface ID = 3                ;Interface ID of I22
       Neighbor Interface ID = 2       ;Interface ID of I41
       Neighbor Router ID =  ;RT4's Router ID

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010  Link-LSAs

   A MANET node originates a separate link-LSA for each attached
   interface.  As an example, consider the link-LSA that RT3 will build
   for its MANET interface I31.

      RT3's link-LSA for MANET interface I31

      LS age = DoNotAge+0              ;newly originated
      LS type = 0x0008                 ;link-LSA
      Link State ID = 2                ;Interface ID of I31
      Advertising Router =   ;RT3's Router ID
      Rtr Pri = 1                      ;default priority
      Options = (V6-bit|E-bit|R-bit)
      Link-local Interface Address = fe80:0002::EUI31
      # prefixes = 0                   ;no global unicast address  Intra-Area-Prefix-LSAs

   A MANET node originates an intra-area-prefix-LSA to advertise its own
   prefixes and those of its attached stub links.  As an example,
   consider the intra-area-prefix-LSA that RT2 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 else
      Advertising Router =   ;RT2's Router ID
      # prefixes = 2
      Referenced LS type = 0x2001      ;router-LSA reference
      Referenced Link State ID = 0     ;always 0 for router-LSA
      Referenced Advertising Router =
                                       ;RT2's Router ID
       PrefixLength = 64               ;prefix on RT2's LOOPBACK
       PrefixOptions = 0
       Metric = 0                      ;cost of RT2's LOOPBACK
       Address Prefix = 2001:DB8:0002::
       PrefixLength = 60               ;prefix on I23
       PrefixOptions = 0
       Metric = 10                     ;cost of I23
       Address Prefix = 2001:DB8:0012::

   Note: MANET nodes may originate intra-area-prefix-LSAs for attached
   transit (broadcast/NBMA) networks.  This is normal behavior (defined
   in [OSPFv3]), which is irrelevant to MANET interfaces.  Please
   consult [OSPFv3] for details.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

3.2.  Incremental OSPF-MANET Hellos

   In MANETs, reducing the size of periodically transmitted packets can
   be very important in decreasing the total amount of overhead
   associated with routing.  Towards this end, removing the list of
   neighbors from Hello packets, unless that information changes, can
   reduce routing protocol overhead.  While the reduction for each Hello
   packet is small, over time it will be significant.

   A new option bit is defined in this document to facilitate the
   operation of incremental Hello packets.  A new State Check Sequence
   TLV (SCS TLV) and Neighbor Drop TLV are also defined, transmitted
   using LLS [LLS].

3.2.1.  The I Option Bit

   A new I-bit is defined in the LLS Type 1 Extended Options and Flags
   field.  The bit is defined for Hello packets and indicates that only
   incremental information is present.  See Section 5 for placement of
   the I-bit.

3.2.2.  State Check Sequence TLV (SCS TLV)

   A new TLV is defined that indicates the current state, which is
   represented by a State Check Sequence (SCS) number of the
   transmitting router.

    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                |
   |         SCS Number            |R|FS|N |   Reserved              |

   o  Type: 6

   o  Length: Set to 4.

   o  SCS Number: A circular two-octet unsigned integer indicating the
      current state of the transmitting device.  Note that when the
      incremental Hello mechanism is invoked (or re-started), an initial
      SCS value of '1' SHOULD be used for the first incremental Hello
      packet.  This sequence number is referred to as InitialSCS.  Note
      that InitialSCS also implies a full state.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

   o  R: Request bit.  If set, this is a request for current state.  The
      list of routers that should respond to this request is indicated
      in the Request From TLV (RF TLV) (defined below).  If the RF TLV
      is not present, it is assumed that the request is meant for all

   o  FS: Full State bit. If set, the Hello packet contains full state
      as far as the neighbor(s) in the Full State For TLV (FSF TLV)
      (defined below) are concerned.  If the FSF TLV is not present, the
      Hello packet contains full state for all neighbors.

   o  N: Incomplete bit.  If NOT set, the complete state associated with
      the SCS number is included in the Hello packet.  If set, this
      indicates that the appended TLVs are being sent 'persistently',
      and that there is more state associated with the SCS number that
      was sent originally, but is not included in this Hello packet.
      This bit allows any desired TLVs to be sent 'persistently' for a
      number of Hellos with the same SCS number without requiring all of
      the TLVs associated with that SCS number to be transmitted.  The
      first time an SCS number is sent, the entire state associated with
      that SCS number is transmitted, and the N-bit MUST NOT be set.

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

   A Hello with the SCS TLV appended and with the R-bit set will be
   referred to as a Hello request.

3.2.3.  Neighbor Drop TLV

   A new TLV is defined in this document that indicates neighbor(s) that
   have been removed from the list of known neighbors.

    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              |
   | Dropped Neighbor(s)                                           |
   | ....

   o  Type: 7
   o  Length: Set to the number of dropped neighbors included in the TLV
      multiplied by 4.

   o  Dropped Neighbor(s) - Router ID of the neighbor being dropped.

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3.2.4.  Request From TLV (RF TLV)

   A new TLV is defined in this document that indicates neighbor(s) from
   which the latest Hello state is being requested.

    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              |
   |                   Request From Neighbor(s)                    |
   | ....

   o  Type: 8

   o  Length: Set to the number of neighbors included in the TLV
      multiplied by 4.

   o  Request From Neighbor(s) - Router ID of the neighbor(s) from which
      Hello state is being requested.

3.2.5.  Full State For TLV (FSF TLV)

   A new TLV is defined in this document that indicates neighbor(s) to
   which the transmitting node is responding with full state.

    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              |
   |                   Full State For Neighbor(s)                  |
   | ....

   o  Type: 9

   o  Length: Set to the number of neighbors included in the TLV
      multiplied by 4.

   o  Full State For Neighbor(s) - Router ID of the neighbor(s) should
      process this packet.

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3.2.6.  Neighbor Adjacencies

   This section describes building neighbor adjacencies and the failure
   of such adjacencies using the incremental Hello signaling.  Building Neighbor Adjacencies

   Hello packets are sent periodically in accordance with [OSPF] and
   [OSPFv3].  An OSPF implementation that supports sending only partial
   neighbor information in Hello packets SHOULD always set the I-bit in
   its transmitted Hello packets, except as described elsewhere in this
   document.  Hello packets MAY be suppressed from being transmitted
   every HelloInterval if other packet transmissions are sent by the
   router during that time.

   On receiving a Hello packet from a new neighbor (in this context, a
   new neighbor is a neighbor in less than Init state as defined in
   Section 10.1 [OSPF]), if the Hello has the I-bit set, a router will:

   o  Place the new neighbor in the neighbor list described in [OSPFv3],
      Appendix A.3.2.

   o  Increment the router's SCS number that it will use in its next
      Hello (indicated in the SCS TLV).

   o  Remove the neighbor from the neighbor list described in [OSPFv3],
      Appendix A.3.2, when the neighbor has reached the Exchange state
      (as described in [OSPF], Section 10.1).

   o  Remove the neighbor from the neighbor list described in [OSPFv3],
      Appendix A.3.2, if the neighbor is not a DR or backup designated
      router (BDR) on an OSPF broadcast link, and if the neighbor is
      advertised as connected in the network-LSA advertised by the DR.  Adjacency Failure

   On discovering an adjacency failure (going to state less than
   Exchange), a router using I-bit signaling SHOULD:

   o  Remove the adjacent router from local tables, and take the
      appropriate actions for a failed adjacency described in [OSPF] and

   o  Add the formerly adjacent router to a Neighbor Drop TLV.

   o  Increment the router's SCS number that it will transmit in its
      next Hello.

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   o  Transmit Hellos with this Neighbor Drop TLV.  It may be desirable
      to send the Neighbor Drop TLV in three consecutive Hellos to
      increase the probability of reception.  In this case, 'persistent'
      Hello packets would be sent with the same SCS number, the Neighbor
      Drop TLV, and the N-bit set.  Thus, the receiver knows that the
      Neighbor Drop TLV is being sent persistently, and there is more
      state associated with the SCS in case it must request missing
      state presumably transmitted in a previous Hello.

3.2.7.  Sending Hellos

   When a device is first attached to a network (whether by being
   brought within range of another device, powering the device on,
   enabling the device's radio interface, etc.), it will need to obtain
   complete neighbor state from each of its neighbors before it can
   utilize the incremental Hello mechanism.  Thus, upon initialization,
   a device MAY send a multicast Hello request (and omit the Request
   From TLV).  Neighbors will receive the request and respond with a
   Hello with their complete neighbor state.

   If a device is in INIT state with a neighbor and receives a Hello
   from the neighbor without its router ID listed in the neighbor list,
   the device SHOULD request the current state from the neighbor.  Note
   that this is to avoid a "race" condition, since the received Hello
   can either mean that the device is NOT SEEN by the neighbor, or that
   the device is adjacent and not listed in the incremental list.  Thus,
   by receiving a Hello request, the neighbor will respond with its
   neighbor state for the neighbor.

   The first Hello packet with a particular SCS number MUST contain the
   full state associated with that SCS number, i.e., all state changes
   since the last SCS number.  The N-bit MUST NOT be set in the State
   Check Sequence TLV.

   Incremental Hello packets can be sent persistently (sent in k
   successive Hello packets), with flexibility in the actual amount of
   information being sent.  The three options include:

   o  The entire incremental Hello packet is sent persistently.  This is
      accomplished by simply sending the entire state associated with a
      SCS number for k successive Hellos.  Since the SCS number remains
      the same, the N-bit is not set in these incremental Hello packets.

   o  Partial information for a particular SCS number is sent
      persistently.  After the first Hello packet with a particular SCS
      number is sent, only the TLVs that are desired to be sent

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      persistently are sent in subsequent Hellos with the same SCS
      number and the N-bit set.

   o  No information is sent persistently.  This is simply the default
      behavior where an incremental Hello packet with a particular SCS
      number is only sent once.

3.2.8.  Receiving Hellos

   Each OSPF device supporting incremental Hello signaling, as described
   in this document, MUST keep the last known SCS number from each
   neighbor it has received Hellos from as long as the neighbor
   adjacency structure is maintained.

   If a device receives a Hello from an adjacent neighbor with an SCS
   number less than the last known SCS number from that neighbor, it
   MUST first check if the SCS number is a wrap around.  "Wrap around"
   is a condition when the last known SCS number is MAX_SCS (65535) and
   the new SCS number is 1.  If it is not a wrap around, then the device
   MUST send a Hello request to the neighbor.

   If it is a wrap around, or if a device receives a Hello from an
   adjacent neighbor with an SCS number one greater than the last known
   SCS number from that neighbor, it MUST:

   o  Examine the neighbor list described in [OSPFv3], Appendix A.3.2.
      If any neighbors are contained in this list, increment the SCS
      number contained in the adjacent neighbor's data structure.

   o  Examine the Neighbor Drop TLV as described in Section  If
      this list contains a neighbor other than the local router,
      increment the SCS number contained in the adjacent neighbor's data

   o  Examine the Neighbor Drop TLV as described in Section  If
      the local router identifier is contained in this list, destroy the
      transmitting adjacent neighbor's data structures.

   o  Examine any other TLVs incrementally signaled, as described in
      documents referring to this RFC.  If there are other state changes
      indicated, increment the SCS number contained in the adjacent
      neighbor's data structure.

   o  If no state change information is contained in the received Hello,
      send a request for current state (by setting the 'R'-bit) in the
      next Hello.

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   If a device receives a Hello from an adjacent neighbor with an SCS
   number greater than the last known SCS number + 1 from that neighbor,
   it MUST send a Hello request to the neighbor, since it may be missing
   some neighbor state.  Receiving Hellos with the N-bit Set

   If a device receives a Hello with the SCS TLV included and the N-bit
   set in this TLV, it MUST verify that it has already received the SCS
   number with the N-bit NOT set from the neighbor.  If the device
   determines that this is the first receipt of the SCS number from this
   neighbor, then it MUST send a Hello request to the neighbor, since it
   missed the initial Hello packet with the SCS number and thus is
   missing state.  Receiving Hellos with the R-bit Set

   If a device receives a Hello with the SCS TLV included and the R-bit
   set, it looks for the RF TLV.  If its router ID is listed in the RF
   TLV or the TLV is not found, it includes its full state in the next
   Hello.  This MUST include:

   o  The neighbor ID of the requesting neighbor(s) in the list of
      neighbors described in [OSPFv3], Appendix A.3.2.

   o  An SCS TLV with the transmitter's current SCS number and the
      FS-bit set.  Note that the transmitter's SCS number is NOT

   o  Any other TLVs, defined in other documents referencing this RFC,
      indicating the current state of the local system.

   o  The neighbor ID of all the neighbors who have requested current
      state, in the FSF TLV.

   If the full state is being sent to a large number of existing
   neighbors, an implementation could choose to instead generate a full
   state for all neighbors and omit the FSF TLV.  Receiving Hellos with the FS-bit Set

   When a device receives a Hello with the SCS TLV included and the
   FS-bit set, the Hello packet contains the neighbor's full state for
   the device.  The packet SHOULD be processed as follows:

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   o  If the received SCS number is equal to the last known SCS number,
      the packet SHOULD be ignored, since the device already has the
      latest state information.

   o  If the received SCS number is different than the last known SCS
      number, this Hello has new information and MUST be parsed.

   o  If it is listed in the FSF TLV, or if the FSF TLV is not present,
      the device MUST save the SCS number, process the Hello as
      described in Section 3.2.8, and process any other appended TLVs.

3.2.9.  Interoperability

   On receiving a Hello packet from a new neighbor without the I-bit
   set, the local router will continue to place that router's identifier
   in transmitted Hellos on this link as described in [OSPFv3],
   Appendix A.3.2.

3.2.10.  Support for OSPF Graceful Restart

   OSPF graceful restart, as described in [OSPFREST] and [OSPFGR],
   relies on the lack of neighbors in the list of neighbors described in
   [OSPFv3], Appendix A.3.2, to determine that an adjacent router has
   restarted, and other signaling to determine that the adjacency should
   not be torn down.  If all Hello packets transmitted by a given router
   have an empty Hello list, reliance on an empty Hello packet to signal
   a restart (or to reliably tear down an OSPF adjacency) is no longer
   possible.  Hence, this signaling must be slightly altered.  When a
   router would like to tear down all adjacencies, or signal that it has

   o  On initially restarting, during the first RouterDeadInterval after
      restart, the router will transmit Hello packets with an empty
      neighbor list and the I-bit cleared.  Any normal restart or other
      signaling may be included in these initial Hello packets.

   o  As adjacencies are learned, these newly learned adjacent routers
      are included in the multicast Hellos transmitted on the link.

   o  After one RouterDeadInterval has passed, the incremental Hello
      mechanism is invoked.  An incremental Hello packet with full state
      is sent with the I-bit set, the SCS TLV included with the FS-bit
      set, and the InitialSCS value (e.g., SCS of '1').  Subsequent
      Hello packets will include only incremental state.

   Routers that are neighboring with a restarting router MUST continue
   sending their Hello packets with the I-bit set.

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3.3.  Optimized Flooding (Overlapping Relays)

   A component that may influence the scalability and convergence
   characteristics of OSPF ([OSPF], [OSPFv3]) in a MANET environment is
   how much information needs to be flooded.  The ideal solution is that
   a router will receive a particular routing update only once.
   However, there must be a trade-off between protocol complexity and
   ensuring that every speaker in the network receives all of the
   information.  Note that a speaker refers to any node in the network
   that is running the routing protocol and transmitting routing updates
   and Hello messages.

   Controlling the amount of information on the link has increased
   importance in a MANET environment due to the potential transmission
   costs and resource availability in general.

   In some environments, a group of speakers that share the same logical
   segment may not be directly visible to each other; some of the
   possible causes are the following: low signal strength, long distance
   separation, environmental disruptions, partial VC (virtual circuit)
   meshing, etc.  In these networks, a logical segment refers to the
   local flooding domain dynamically determined by transmission radius.
   In these situations, some speakers (the ones not able to directly
   reach the sender) may never be able to synchronize their databases.
   To solve the synchronization issues encountered in these
   environments, a mechanism is needed through which all the nodes on
   the same logical segment can receive the routing information,
   regardless of the state of their adjacency to the source.

3.3.1.  Operation Overview

   The optimized flooding operation relies on the ability of a speaker
   to advertise all of its locally connected neighbors.  In OSPF, this
   ability is realized through the use of link state advertisements
   (LSA)s ([OSPF], [OSPFv3]).

   A speaker receives router-LSAs from its adjacent neighbors.  A
   speaker's router-LSA conveys the list of the adjacent speakers of the
   originator ("neighbor list").  The local speaker can compare the
   neighbor list reported by each speaker to its own neighbor list.  If
   the local neighbor list contains adjacent speakers that the
   originator cannot reach directly (i.e., those speakers that are not
   in the originator's neighbor list), then these speakers are locally
   known as non-overlapping neighbors for the originator.

   The local speaker should relay any routing information to non-
   overlapping neighbors of the sender based on the algorithm outlined
   in Section 3.3.8.  Because more than one such speaker may exist, the

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   mechanism is called "overlapping relays".  The algorithm, however,
   does select the set of overlapping relays that should transmit first.
   This set is known as the active set of overlapping relays for a

3.3.2.  Determination of Overlapping Relays

   The first step in the process is for each speaker to build and
   propagate their neighbor lists in router-LSA packets.  Every speaker
   is then in a position to determine their 2-hop neighborhood, i.e.,
   those nodes that are neighbors of the speaker's 1-hop neighbors.

   A bidirectional neighbor is considered an overlapping relay for a
   speaker if it can reach a node in the 2-hop neighborhood of the
   speaker, i.e., if it has 1-hop neighbors (excluding the speaker

   The set of Active Overlapping Relays for a speaker is the minimum set
   of direct neighbors such that every node in the 2-hop neighborhood of
   the speaker is a neighbor of at least one overlapping relay in the
   active set.

   Each speaker SHOULD select a set of Active Overlapping Relays based
   on a selection algorithm (one such algorithm is suggested in
   Section 3.3.4 and is based on the multipoint relay (MPR) selection
   algorithm described in [OLSR]).  The behavior of the overlapping
   relays MUST follow that specified in Section 3.3.8.

   Note that a speaker MUST NOT choose a neighbor to serve as an Active
   Overlapping Relay if that neighbor set the N-bit in its Active
   Overlapping Relay TLV as defined in Section 3.3.6, unless the
   neighbor is the only neighbor to reach a 2-hop neighbor.

   Election of Active Overlapping Relays is done across interfaces, and
   thus, it is node-based and not link-based.

3.3.3.  Terminology

   The following heuristic and terminology for Active Overlapping Relay
   selection is largely taken from [OLSR]:

   o  FULL: Neighbor state FULL as defined in [OSPF] and [OSPFv3].  Note
      that all neighbor references in this document are assumed to be
      FULL neighbors.

   o  N: N is the set of FULL neighbors of the node.

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   o  2-hop FULL neighbors (N2): The list of 2-hop neighbors of the node
      that are FULL and that can be reached from direct neighbors,
      excluding any directly connected neighbors.

   o  Active Set: A (sub)set of the neighbors selected, such that
      through these selected nodes, all 2-hop FULL neighbors are

   o  D(y): The degree of a 1-hop neighbor node y (where y is a member
      of N) is defined as the number of FULL neighbors of node y,
      EXCLUDING all the members of N and EXCLUDING the node performing
      the computation.

3.3.4.  Overlapping Relay Discovery Process

   A possible algorithm for discovering overlapping relays is the

   1. Start with an active set made of all members of N that have set
      the A-bit in their Active Overlapping Relay TLV (AOR TLV) as
      defined in Section 3.3.6.

   2. Calculate D(y), where y is a member of N, for all nodes in N.

   3. Add to the active set those nodes in N, which are the *only* nodes
      to provide reachability to a node in N2, i.e., if node b in N2 can
      be reached only through a symmetric link to node a in N, then add
      node a to the active set.  Remove the nodes from N2 that are now
      covered by a node in the active set.

   4. While there exist nodes in N2 that are not covered by at least one
      node in the active set:

      A. For each node in N, calculate the reachability, i.e., the
         number of nodes in N2 that are not yet covered by at least one
         node in the active set and that are reachable through this
         1-hop neighbor.

      B. Select as an Active Overlapping Relay the node with the highest
         Willingness value (Section 3.3.7) among the nodes in N with
         non-zero reachability.  In the case of multiple choices, select
         the node that provides reachability to the maximum number of
         nodes in N2.  In the case of multiple nodes providing the same
         amount of reachability, select as active the node whose D(y) is
         greater.  As a final tie breaker, the node with the highest
         router ID should be chosen.  Remove the nodes from N2 that are
         now covered by a node in the active set.

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   5. As an optimization, process each node, y, in the active set in
      increasing order of Willingness value.  If all nodes in N2 are
      still covered by at least one node in the active set, excluding
      node y, and if Willingness of node y is smaller than
      MAX_WILLINGNESS, then node y should be removed from the active

3.3.5.  The F Option Bit

   A single new option bit, the F-bit, is defined in the LLS Type 1
   Extended Options and Flags field.  The F-bit indicates that the node
   supports the optimized flooding mechanism as specified in this
   document.  See Section 5 for placement of the F-bit.

3.3.6.  Active Overlapping Relay TLV (AOR TLV)

   A new TLV is defined so that each speaker can convey its set of
   Active Overlapping Relays in the Hello messages.  The TLV is
   transmitted using LLS [LLS].

       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                 |
      |  Relays Added |A|N|           Reserved                        |
      |    Router ID(s) of Active Overlapping Relay(s)                |
      |                              ...                              |

   o  Type: 10

   o  Length - variable.  Length of TLV in bytes, NOT including Type and

   o  Relays Added - variable.  Number of Active Overlapping Relays that
      are being added.  Note that the number of Active Overlapping
      Relays that are being dropped is then given by
      [(Length - 4)/4 - Relays Added].

   o  A-bit - If this bit is set, the node is specifying that it will
      always flood routing updates that it receives, regardless of
      whether it is selected as an Active Overlapping Relay.

   o  N-bit - If this bit is set, the node is specifying that it most
      likely will not flood routing updates.  The node SHOULD NOT be

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      chosen to be an Active Overlapping Relay unless it is the *only*
      neighbor that can reach 2-hop neighbor(s).  Note that if the node
      is selected as an Active Overlapping Relay and the node cannot
      perform the required duties, network behavior is not compromised,
      since it results in the same behavior as if the node was not
      chosen as an Active Overlapping Relay.

   o  Reserved - Reserved for future use. MUST be set to zero by the
      sender, and MUST be ignored upon receipt.

   o  Router ID(s) of Active Overlapping Relay(s) - The router ID(s) of
      neighbor(s) that are either chosen to serve as an Active
      Overlapping Relay or removed from serving as an Active Overlapping
      Relay.  The Active Overlapping Relays that are being added MUST be
      listed first, and the number of such relays MUST equal Add Length.
      The remaining listed relays are being dropped as Active
      Overlapping Relays, and the number of such relays MUST equal
      [(Length - 4)/4 - Relays Added].

   Note that the A-bit and N-bit are independent of any particular
   selection algorithm to determine the set of Active Overlapping
   Relays.  However, the bits SHOULD be considered as input into the
   selection algorithm.

   If a node is selected as an Active Overlapping Relay and it does not
   support the Incremental Hello mechanism defined in Section 3.2, then
   it SHOULD always be included as an Active Overlapping Relay in the
   TLV.  Note that while a node needs to know whether it is an Active
   Overlapping Relay, it does not necessarily have to know the
   identities of the other Active Overlapping Relays.

3.3.7.  Willingness TLV

   A new TLV is defined so that each speaker can convey its willingness
   to serve as an Active Overlapping Relay in the Hello message.  The
   TLV is transmitted using the LLS [LLS].

       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                 |
      |   Willingness |                       Reserved                |

   o  Type: 11

   o  Length - 4 bytes.  It does not include the Type and Length fields.

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   o  Willingness - 1 byte to indicate the willingness of the node to
      serve as an Active Overlapping Relay for its neighbors.
         *  0: MIN_WILLINGNESS
         *  128: DEFAULT_WILLINGNESS
         *  255: MAX_WILLINGNESS

   The TLV is optional and MUST be silently ignored if not understood.
   If the Willingness TLV is not included in the Hello packet, the
   Willingness value SHOULD be taken as DEFAULT_WILLINGNESS.

3.3.8.  Flooding and Relay Decisions

   The decision whether to relay any received LSAs, and when to relay
   such information, is made depending on the topology and whether the
   node is part of the set of Active Overlapping Relays.

   Upon receiving an LSA from a bi-directional neighbor, a node makes
   flooding decisions based on the following algorithm:

   1. If the node is an Active Overlapping Relay for the adjacent
      speaker, then the router SHOULD immediately relay any information
      received from the adjacent speaker.

   2. If the node is a non-Active Overlapping Relay for the adjacent
      speaker, then the router SHOULD wait a specified amount of time
      (PushbackInterval plus jitter (see Section 3.3.10)) to decide
      whether to transmit.  [Jitter is used to try to avoid several non-
      Active Overlapping Relays from propagating redundant information.]
      Note that a node with the N-bit set in the 'Active Overlapping
      Relays' extension will not be chosen as an Active Overlapping
      Relay unless it is the only node to provide reachability to a
      2-hop neighbor.  However, it MUST perform the duties of a non-
      Active Overlapping Relay as required.  Non-Active Overlapping
      Relays MUST follow the acknowledgment mechanism outlined in
      Section 3.3.9.

      A. During this time, if the node determines that flooding the LSA
         will only result in a redundant transmission, the node MUST
         suppress its transmission.  Otherwise, it MUST transmit upon
         expiration of PushbackInterval plus jitter.

      B. If a non-Active Overlapping Relay hears a re-flood from another
         node that covers its non-overlapping neighbors before its timer
         to transmit expires, it SHOULD reset its PushbackInterval plus
         jitter timer.  (Note that the implementation should take care
         to avoid resetting the PushbackInterval timer based on
         transmissions from Active Overlapping Relays.)  During this
         time, if the node determines that flooding the update will only

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         result in a redundant transmission, the node MUST suppress its
         transmission.  Otherwise, it MUST transmit upon expiration of
         PushbackInterval plus jitter.

      C. If a non-Active Overlapping Relay hears an old instance of the
         LSA during this time, it SHOULD ignore the LSA, and it SHOULD
send a unicast packet to the neighbor with the most recent
         LSA as specified in [OSPFv3].

   3. For LSAs that are received unicast because of retransmission by
      the originator, the node MUST determine whether it has already
      received the LSA from another speaker.  If it already has the
      current instance of the LSA in its database, it MUST do nothing
      further in terms of flooding the LSA (since it would have taken
      appropriate action when it initially received the LSA).  However,
      if it does not have the current instance of the LSA in its
      database, it MUST take action according to the rules above, just
      as if it received the multicast LSA.  The acknowledgment mechanism
      outlined in Section 3.3.9 MUST be followed, and any timeout
      mechanism for unicast LSAs MAY be followed.

   Note that a node can determine whether further flooding an LSA will
   only result in a redundant transmission by already having heard link
   state acknowledgments (ACKs) or floods for the LSA from all of its

   Due to the dynamic nature of a network, the set of Active Overlapping
   Relays may not be up to date at the time the relay decision is made
   or may not be able to perform the flooding duties, e.g., due to poor
   link quality.  The non-Active Overlapping Relays prevent this
   situation from causing database synchronization issues and, thus,
   packet loss.

   Since the originator of the information, the relay, and the receiver
   are all in the same dynamically determined local flooding domain, the
   relay MUST NOT change the routing update information.  In general,
   LSAs SHOULD be sent to a well-known multicast address.  In some
   cases, routing updates MAY be sent using unicast packets.

3.3.9.  Intelligent Transmission of Link State Acknowledgments

   In order to optimize the bandwidth utilization on the link, a speaker
   MUST follow these recommendations related to ACK transmissions:

   1. All ACKs MUST be sent via multicast.

   2. Typically, LSAs are acknowledged by all of the adjacent speakers.
      In the case of relayed information, the relay MUST only expect

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

      either explicit or implicit acknowledgments from neighbors that
      have not previously acknowledged this LSA.

   3. Because routing updates are sent via multicast, the set of
      overlapping speakers will usually receive the same update more
      than once.  A speaker SHOULD only acknowledge the first update
      received on the link.

   4. An Active Overlapping Relay SHOULD NOT explicitly acknowledge
      information that it is relaying.  The relayed information will
      serve as an acknowledgment to the sender.  If no information is
      being relayed, then an explicit ACK MUST be sent.

   5. Several ACKs MAY be bundled into a single packet.  The wait
      (AckInterval) before sending one such packet reduces the number of
      packet transmissions required in acknowledging multiple LSAs.

   6. All ACK packets SHOULD reset the RouterDeadInterval at the
      receiver.  If there is no state waiting to be transmitted in a
      Hello packet at the sender, then the HelloInterval at the sender
      SHOULD be reset.  Note that an ACK serves as a Hello packet with
      no state change.

   7. Any LSA received via unicast MUST be acknowledged.  (Note that
      acknowledgment is via multicast as specified in rule (1) above.)

   An ACK received from a non-overlapping neighbor should prevent
   redundant transmission of the information to it by another
   overlapping relay.

3.3.10.  Important Timers

   This section details the timers that were introduced in Sections
   3.3.8 and 3.3.9.

   o  PushbackInterval: The length of time in seconds that a non-Active
      Overlapping Relay SHOULD wait before further flooding an LSA if
      needed.  This timer MUST be less than 1/2 of the RxmtInterval
      ([OSPF], [OSPFv3]) minus propagation delays, i.e.,
      (PushbackInterval + propagation delay) < RxmtInterval/2.  The
      PushbackInterval is set by a non-Active Overlapping Relay upon
      receipt of an LSA.

   o  AckInterval: After a node determines that it must transmit an ACK
      and the AckInterval timer is not already set, the node SHOULD set
      the AckInterval timer.  The AckInterval is the length of time in
      seconds that a node should wait in order to transmit many ACKs in
      the acknowledgment packet.  This wait reduces the number of packet

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

      transmissions required in acknowledging multiple LSAs.  The
      AckInterval MUST be less than the PushbackInterval minus
      propagation delays, i.e.,
      (AckInterval + propagation delay) < PushbackInterval.

3.3.11.  Miscellaneous Protocol Considerations

   The mechanism described refers to the operation of relays on a common
   media segment.  In other words, an LSA is only relayed out the same
   interface through which it was received.  However, the concept of
   information relay may be extended to the flooding of all link state
   advertisements received on any interface (and forwarded on any other
   interface).  OSPF works on the premise that all of the nodes in a
   flooding domain will receive all of the routing information.  Note
   that one of the important properties is that the routing information
   is not altered when relayed.

   If each speaker advertised all of its adjacent neighbors on all
   interfaces, then the overlap check would result in the determination
   of which speakers are adjacent to both speakers.  As a result, link
   state information should only be flooded to non-overlapping neighbors
   (taking all of the interfaces into account).

   The flooding mechanism in OSPF relies on a designated router to
   guarantee that any new LSA received by one router attached to the
   broadcast network will be re-flooded properly to all the other
   routers attached to the broadcast network.  Such designated routers
   must be able to reach all of the other speakers on the same subnet.
   A designated router SHOULD NOT be elected if overlapping relays are

   If such designated routers already exist, then the relays MUST be
   capable of differentiating them and then making the relaying
   decisions based on the OSPF's normal operation.  As a result, there
   may be groups of neighbors to which some information should not be
   relayed.  This mode of operation is NOT RECOMMENDED, as it adds to
   the complexity of the system.

   The intent of the overlapping relay mechanism is to optimize flooding
   of routing control information.  However, other information (such as
   data) may also be relayed in some networks using the same mechanism.

3.3.12.  Interoperability

   On receiving a Hello packet from a new neighbor without the F-bit
   set, the local router will assume that the new neighbor will flood
   normally as described in [OSPFv3].  Thus, the local router SHOULD
   include the neighbor in its overlapping relay set since the neighbor

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   will flood by default.  This will allow the local router to more
   optimally select its entire overlapping relay set.

   If the F-bit is set and the I-bit as defined in Section 3.2 is not
   set in the neighbor's Hello, and the neighbor is selected as an
   overlapping relay by the local router, the local router will continue
   to include the neighbor's identifier in its active relay set.

3.4.  New Bits in LLS Type 1 Extended Options and Flags

   Two new option bits are defined in the "LLS Type 1 Extended Options
   and Flags" Field [LLS] as follows:

   o  I-bit - defined in Section 3.2.1: The I-bit is only defined for
      Hello packets and indicates that only incremental information is

   o  F-bit - defined in Section 3.3.5: The F-bit indicates that the
      node supports the optimized flooding mechanism as specified in
      this document.

3.5.  Smart Peering

   There is significant overhead in OSPF when a router has to establish
   adjacencies with every peer with whom it can verify 2-way
   connectivity.  OSPF supports the broadcast network type for these
   scenarios, where you only have to peer with the designated router
   (DR).  However, a full mesh of connectivity is required for proper
   operation, and this doesn't help in networks with overlapping partial
   meshes of connectivity.  This document proposes a technique to reduce
   the number of adjacencies based on shortest path tree (SPT)
   reachability information.

3.5.1.  Rationale for Smart Peering

   In OSPF ([OSPF], [OSPFv3]), nodes establish an adjacency by first
   verifying 2-way connectivity between them and then synchronizing
   their link state databases.  Once the peering relationship is
   complete and the adjacency is established, the nodes will continue to
   advertise each other in their LSAs.  As a result, the peers are
   maintained in the link state database and are included in all SPF
   (Shortest Path First) calculations.  During the reliable flooding
   process, a node must ensure that each peer has indeed received the
   flooded routing update via an acknowledgment and retransmission

   Consequently, maintaining an adjacency for a particular peer is a
   trade-off between the added redundancy in routing paths and network

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   reachability versus the associated overhead (memory consumption, SPF
   computations, routing overhead, and network convergence).

   Consider the possibility of reducing the number of adjacencies that a
   node maintains without compromising reachability and redundancy.
   This will have direct implications on network scalability and is
   especially attractive in environments where the network topology is
   dynamic.  For example, in a mobile ad hoc network (MANET), where
   nodes are mobile and the topology is constantly changing, it seems
   highly desirable to 'intelligently' become adjacent with only
   selected peers and not establish a peering session with every node
   that comes within transmission range.  Selective peering can be
   particularly useful in avoiding the peering process for unstable
   nodes, i.e., nodes that come in and out of transmission range.

3.5.2.  Previous Related Work

   The formation of a FULL adjacency requires discovery (2-way
   relationship) and database synchronization.  To prevent achieving the
   FULL state, others have taken the approach of modifying link state
   protocols to use periodic advertisements (instead of a database
   exchange).  The result is that neighbor discovery is still required,
   but routing information is learned over time.  An example of this
   approach is:

   o  OSPFv2 Wireless Interface Type [WINTF]

      *  where the use of periodic advertisements "eliminates the
         formation of full adjacencies on wireless interfaces; all
         neighbor states beyond 2-Way are not reached,and no database
         synchronization is performed".

   What we propose in this specification goes a step further by not
   requiring the formation and maintenance of neighbor state (2-way, or
   other) *and* without changing the route distribution mechanisms in
   the link state protocols.  In other words, the mechanism described is
   completely backward compatible.

3.5.3.  Smart Peering Solution

   Two routers are defined as synchronized when they have identical link
   state databases.  To limit the number of neighbors that are formed,
   an algorithm is needed to select which neighbors with whom to peer.

   The algorithm MUST provide reachability to every possible destination
   in the network, just as when normal adjacency formation processes are
   used.  We should always peer with a neighbor if it provides our only
   path to currently unreachable destinations.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010  SPT Reachability Heuristics

   The peering decision is really a local matter to a router.  If a
   router can ensure that reachability to other nodes is available
   without bringing up a new adjacency, it can choose not to bring up
   the new adjacency.

   We propose an algorithm that uses the existing information about a
   new neighbor's reachability in the SPT.  If the two routers can
   already reach each other in the SPT, it is not necessary to form an
   adjacency between them.

   The decision to peer or not is made when a Hello is received.  When a
   Hello is received from a new neighbor or a neighbor in a state lower
   than Exchange:

   o  A check is made in the link state database to see if the peer is
      already reachable in the SPT.

      *  If the peer is either not known in the SPT or is not reachable,
         we start the Exchange process.

      *  If the peer is reachable, then bringing up adjacency with this
         neighbor does not provide reachability to any new destinations.

   Let's take an example of a single OSPF area.  This check would look
   for the neighbor's router-LSA.  If the LSA is present in the database
   and is reachable in the SPT, we have a chance to suppress adjacency

   It's worth noting that as the number of links and redundancy in the
   network is reduced, the likelihood of suboptimal routing increases.  State Machine

   The state machine of a basic implementation of this algorithm is
   provided below.  An implementation MAY use some heuristics (Step (3)
   below), beyond the SPT reachability, to decide whether or not it
   considers a new adjacency to be of value.

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                        |Receive a Hello     |
                    (1) |from a new potential|
                        |neighbor            |
                        |Check to see if there |
                    (2) |is a router-LSA from  |----no--(4)form a
                        |the new potential     |          new
                        |neighbor in the link  |          neighbor
                        |state database, which |
                        |is reachable in SPT   |
         (3)                      |
      |                            (3b)........................ |
      |(3a),______________________     |Determine if the      | |
      |    |Determine if the new |     |number of redundant   | |
      |    |link cost is better  |     |paths to the potential| |
      |    |than the current path|     |neighbor is < the     | |
      |    |cost by a configured |     |maximum configured    | |
      |    |amount               |     |value                 | |
      |    '`'''''''''''''''''''''     '`'''''''''''''''''''''' |
      |                       \             /                   |
      |                   .....\.........../....                |
      |                   |User configurable   |                |
      |                   |selection algorithm |                |
      |                   '`'''/'''''''\''''''''                |
      |                       /         \                       |
                            /             \
                     requirements     requirements
                        met              not met
                        /                    \
                       /                      \
           (4) form a new neighbor      (5) do not become

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3.5.4.  Advertising 2-Way Links in Router-LSAs

   The technique described in Section 3.5.3 minimizes the number of
   adjacencies in highly meshed environments.  This is especially useful
   when the network is in motion and the average adjacency lifetime is

   However, it suffers from an undesirable side effect of limiting the
   number of transit links available to forward traffic.

   An implementation may choose to allow some (or even all) of these
   2-way state neighbors to be announced in the router-LSA.  Since the
   state remains 2-way, we don't incur control plane (database sync and
   flooding) overhead.  However, advertising the link in the router-LSA
   makes the link available to the data plane.

   This can be safely done if the neighbor is reachable in a special SPT
   constructed by ignoring any other 2-way links in the network.  This
   optional optimization is described below.  Unsynchronized Adjacencies

   If the new neighbor is already reachable in the SPT, there is no
   urgency in doing a full database sync with it.  These are the steps
   we need to perform when a neighbor has reached 2-way state.

   Note that when we say "SPT" in this section, we mean the special SPT
   constructed based on rules in Section

   o  After a 2-WayReceived event, check if the neighbor is reachable in
      the SPT.  If yes, mark the neighbor as FULL with respect to link

   o  This means that the router-LSA or network-LSA link corresponding
      to the neighbor is advertised as if the neighbor is FULL.

   o  The adjacency information is constructed with the U-bit (see

   o  Database synchronization is postponed:

         *  By a configured amount of time -OR-

         *  Until the time it's absolutely "necessary"

   In either case, if a database sync is currently pending, it is
   started as soon as we detect that the neighbor is no longer reachable
   in the SPT.  The database sync can be done by Out-of-Band Sync [OOB],

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   which maintains the current adjacency and does the sync in the
   background.  A normal resync can alternately be done with the
   drawback of adjacency flap.

   In standard OSPF, we first bring up adjacency and then announce a
   transit link.  The approach described above allows the link to be
   used as a forwarding path very quickly and still allows the database
   to be synchronized in a timely fashion when the alternate flooding
   path has recently been broken.

   There is a circular dependency issue that also needs to be resolved.
   Once you start announcing the link, the shortest path will likely be
   via this very link.  So it's non-trivial to detect when the alternate
   dependent path is gone.  We would like to be able to detect that the
   neighbor is reachable via a path that doesn't traverse an
   unsynchronized path.

   We have generally solved this class of problems by running an SPF and
   pretending that the link in question doesn't exist.  It doesn't
   require a full SPF, but just enough to see if ANY other path is
   available to reach the neighbor.  The worst case is when the
   alternate path is really gone, which we find that out by building a
   full SPT.  This needs to be done every time the link state database
   changes, and for EACH link that has SPT dependence for its viability.
   This approach has scalability concerns and is not considered further

   We can achieve the same results with just ONE additional SPF that is
   capable of ignoring these Unsynchronized links.  The result from this
   SPT can be used to satisfy the reachability condition for ANY number
   of Unsynchronized Adjacencies.  This basically requires that we can
   actually tell the difference between a normal FULL adjacency and this
   new Unsynchronized Adjacency.  We can do this in one of two ways:

   (A) Defining LD Options and using a bit in it, as shown below:

      |     Type      |   LD Options  |          Metric               |
      |                      Interface ID                             |
      |                   Neighbor Interface ID                       |
      |                    Neighbor Router ID                         |

                     Link Description in a Router-LSA

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      LD Options
         Link Description options.  Used to specify some special
         capability or state of a link.

                               | | | | | | | |U|

                                   LD Options

         The "Unsynchronized" bit.  This is set if the adjacency is
         being announced before databases are fully synchronized.

      This approach is backward compatible, because the only routers
      looking at this bit are those that support the mechanisms
      specified in this document.

   (B) Introducing a new link type in router-LSA.

      This is a much more complex solution, with backward compatibility
      concerns, due to the fact that unknown link type handling is not
      defined in the OSPF standard [OSPF].  Hence, this solution isn't
      considered further.  Unsynchronized SPT

   Whenever link state changes happen, we need to run ONE additional SPF
   by ignoring all links with the U-bit set.  This SPT is then consulted
   to see if any of our Unsynchronized Adjacencies need to start
   database sync.  This SPT is also consulted when a new neighbor goes
   into 2-way state to decide if we should form the adjacency
   immediately or defer it for later.  Flooding Considerations

   One of the main goals in trying to delay the database synchronization
   is to be able to reduce unnecessary OSPF packets traversing these
   links.  Since the unsynchronized Adjacencies remain in 2-way state,
   OSPF updates will not be flooded over the corresponding interfaces,
   resulting in additional savings.

   An option is provided to enable or disable flooding over these
   Unsynchronized Adjacencies.  The advantage of allowing flooding is
   being able to use more links for control plane purposes.  We will
   still have the savings of not having to form the adjacency.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010  Overlapping Relay (OR) Election Impact

   The overlapping relay election algorithm uses the 2-hop neighborhood
   it gleans from our neighbor's router-LSAs.  The introduction of
   Unsynchronized Adjacencies needs to be considered in the relay
   election algorithm.

   If flooding is enabled on unsynchronized Adjacencies, no change is
   needed in the relay election algorithm.  If flooding is disabled,
   then the relay election algorithm needs to prune neighbors that are
   connected via an Unsynchronized Adjacency from our 1-hop and 2-hop
   neighbor lists.

4.  Security Considerations

   In a MANET, security is both more difficult and important, due to the
   wireless nature of the medium.  Controlling the ability of devices to
   connect to a MANET at Layer 2 will be relegated to Layer 2 security
   mechanisms, such as 802.1x, and others.  Controlling the ability of
   attached devices to transmit traffic will require some type of
   security system (outside the scope of this document) that can
   authenticate, and provide authorization for, individual members of
   the routing domain.

   Additional security considerations are similar to any MANET protocol
   extension.  The following text is from [MDR]:

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

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

   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 [THREATS] 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.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

   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 identity 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).

   [OSPFv3-SEC] 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
   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 [OSPFv3-SEC].  Future
   specifications can explore the use of automated key management
   protocols that may be suitable for MANETs.

   As discussed in [OSPFv3-SEC], 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 [AH] and [ESP] 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.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

   When an OSPF routing domain includes both MANETs 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 MANETs 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 MANETs.  Additionally, OSPF
   ranges can be configured to aggregate prefixes for the areas
   supporting MANETs.  With separate OSPF instances, more conservative
   local policies can be employed to limit the volume of updates
   emanating from the MANETs.

5.  IANA Considerations

   IANA has made the assignments as explained below using the policies
   outlined in [IANA].

   o  I-bit and F-bit from "LLS Type 1 Extended Options and Flags"
      registry as defined below:

   +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
   | * | * | * | * | * | * | * |...| * | * | * | * | F | I | RS| LR|
   +---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+

                  Bits in Extended Options and Flags TLV

   o  New TLV types from the "Link Local Signalling TLV Identifiers (LLS
      Types)" registry as defined below:

      TLV Name                      TLV Type
      --------                      --------
      State Check Sequence TLV          6
      Neighbor Drop TLV                 7
      Request From TLV                  8
      Full State For TLV                9
      Active Overlapping Relay TLV      10
      Willingness TLV                   11

   o  A new registry has been defined for LD Options as defined in
      Section  The U-bit is allocated by this document.

      All future additions to LD Options are subject to OSPF WG review
      and require IETF Review.

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

6.  Contributors

   The following persons are contributing authors to the document:

   Fred Baker                         Dave Cook
   Cisco Systems                      Cisco Systems
   1121 Via Del Rey                   7025-4 Kit Creek Road
   Santa Barbara, CA 93117            Research Triangle Park, NC 27709
   USA                                USA
   EMail: fred@cisco.com              EMail: dacook@cisco.com

   Alvaro Retana                      Yi Yang
   Cisco Systems                      Cisco Systems
   7025-4 Kit Creek Road              7025-4 Kit Creek Road
   Research Triangle Park, NC 27709   Research Triangle Park, NC 27709
   USA                                USA
   EMail: aretana@cisco.com           EMail: yiya@cisco.com

   Russ White
   Cisco Systems
   7025-4 Kit Creek Road
   Research Triangle Park, NC 27709
   EMail: riw@cisco.com

7.  Acknowledgments

   The authors and contributors would like to thank Pratap Pellakuru and
   Stan Ratliff for their feedback and implementation of the document.
   Thanks to Richard Ogier and John Avery for doing a final review.

8.  References

8.1.  Normative References

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

   [OSPFv3]       Coltun, R., Ferguson, D., Moy, J., and A. Lindem,
                  "OSPF for IPv6", RFC 5340, July 2008.

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

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

   [IANA]         Narten, T. and H. Alvestrand, "Guidelines for Writing
                  an IANA Considerations Section in RFCs", BCP 26,
                  RFC 5226, May 2008.

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

8.2.  Informative References

   [IPV6ADD]      Hinden, R. and S. Deering, "IP Version 6 Addressing
                  Architecture", RFC 4291, February 2006.

   [OSPFGR]       Moy, J., Pillay-Esnault, P., and A. Lindem, "Graceful
                  OSPF Restart", RFC 3623, November 2003.

   [OSPFREST]     Nguyen, L., Roy, A., and A. Zinin, "OSPF Restart
                  Signaling", RFC 4812, March 2007.

   [OOB]          Nguyen, L., Roy, A., and A. Zinin, "OSPF Out-of-Band
                  Link State Database (LSDB) Resynchronization",
                  RFC 4811, March 2007.

   [OLSR]         Clausen, T., Ed., and P. Jacquet, Ed., "Optimized Link
                  State Routing Protocol (OLSR)", RFC 3626,
                  October 2003.

   [WINTF]        Ahrenholz J., et al., "OSPFv2 Wireless Interface
                  Type", Work in Progress, May 2004.

   [MDR]          Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network
                  (MANET) Extension of OSPF Using Connected Dominating
                  Set (CDS) Flooding", RFC 5614, August 2009.

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

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

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

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

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RFC 5820          Extensions to OSPF to Support MANETs        March 2010

Authors' Addresses

   Abhay Roy (Editor)
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA 95134
   EMail: akr@cisco.com

   Madhavi W. Chandra (Editor)
   113 Holmhurst Court
   Cary, NC 27519

   EMail: mw.chandra@gmail.com

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