Mobile Ad hoc Networking (MANET)                              U. Herberg
Internet-Draft                           Fujitsu Laboratories of America
Intended status: Informational                                     J. Yi
Expires: October 11, 2012                                     T. Clausen
                                                LIX, Ecole Polytechnique
                                                           April 9, 2012

                       Security Threats for NHDP


   This document analyses common security threats of the Neighborhood
   Discovery Protocol (NHDP), and describes their potential impacts on
   MANET routing protocols using NHDP.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 11, 2012.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  NHDP Threat Overview . . . . . . . . . . . . . . . . . . . . .  4
   4.  Detailed Threat Description  . . . . . . . . . . . . . . . . .  4
     4.1.  Jamming  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Eavesdropping  . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  Incorrect HELLO Message Generation . . . . . . . . . . . .  5
       4.3.1.  Identity Spoofing  . . . . . . . . . . . . . . . . . .  6
       4.3.2.  Link Spoofing  . . . . . . . . . . . . . . . . . . . .  6
     4.4.  Replay Attack  . . . . . . . . . . . . . . . . . . . . . .  7
     4.5.  Sequence Number Attack . . . . . . . . . . . . . . . . . .  8
     4.6.  Message Timing Attacks . . . . . . . . . . . . . . . . . .  8
       4.6.1.  Interval Time Attack . . . . . . . . . . . . . . . . .  8
       4.6.2.  Validity Time Attack . . . . . . . . . . . . . . . . .  8
     4.7.  Indirect Jamming . . . . . . . . . . . . . . . . . . . . .  9
   5.  Impact of inconsistent Information Bases on Protocols
       using NHDP . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  MPR Calculation  . . . . . . . . . . . . . . . . . . . . . 10
       5.1.1.  Flooding Disruption due to Identity Spoofing . . . . . 10
       5.1.2.  Flooding Disruption due to Link Spoofing . . . . . . . 11
       5.1.3.  Broadcast Storm  . . . . . . . . . . . . . . . . . . . 12
     5.2.  Routing Loops  . . . . . . . . . . . . . . . . . . . . . . 13
     5.3.  Invalid or Non-Existing Paths to Destinations  . . . . . . 13
     5.4.  Data Sinkhole  . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   The Neighborhood Discovery Protocol (NHDP) [RFC6130] allows routers
   to acquire topological information up to two hops away from
   themselves, by way of periodic HELLO message exchanges.  The
   information acquired by NHDP is used by other protocols, such as
   OLSRv2 [OLSRv2] and SMF [SMF].  The topology information, acquired by
   way of NHDP, serves these routing protocols for calculating paths to
   all destinations in the MANET, for relay set selection for network-
   wide transmissions, etc.

   As NHDP is typically used in wireless environments, it is potentially
   exposed to different kinds of security threats, some of which are of
   particular significance as compared to wired networks.  As wireless
   radio waves can be captured as well as transmitted by any wireless
   device within radio range, there is commonly no physical protection
   as otherwise known for wired networks.  [RFC6130] does not define any
   explicit security measures for protecting the integrity of the
   information it acquires, however suggests that this be addressed in a
   fashion appropriate to the deployment of the network.

   This document analyses possible attacks on NHDP and outlines the
   consequences of such attacks to the state maintained by NHDP in each
   router (and, thus, made available to protocols using this state).

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   This document uses the terminology and notation defined in [RFC5444]
   and [RFC6130].

   Additionally, this document introduces the following terminology:

   NHDP Router:  A MANET router, running NHDP as specified in [RFC6130].

   Attacker:  A device, present in the network and which intentionally
      seeks to compromise the information bases in NHDP routers.

   Compromised NHDP Router:  An attacker, present in the network and
      which generates syntactically correct NHDP control messages.
      Control messages emitted by a Compromised NHDP router may contain
      additional information, or omit information, as compared to a
      control message generated by a non-compromized NHDP router located

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      in the same topological position in the network.

   Legitimate NHDP Router:  An NHDP router, which is not a Compromised
      NHDP Router.

3.  NHDP Threat Overview

   [RFC6130] defines a HELLO messages exchange, enabling each NHDP
   router to acquire topological information describing its 1-hop and
   2-hop neighbors, and specifies information bases for recording this

   An NHDP router running [RFC6130] periodically transmits HELLO
   messages using a link-local multicast on each of its interfaces with
   a hop-limit of 1 (i.e., HELLOs are never forwarded).  In these HELLO
   messages, an NHDP router announces the IP addresses as heard,
   symmetric or lost neighbor interface addresses.

   An adversary has several ways of harming this neighbor discovery
   process: It can announce "wrong" information about its identity,
   postulate non-existent links, and replay HELLO messages.  These
   attacks are presented in detail in Section 4.

   The different ways of attacking an NHDP deployment may eventually
   lead to inconsistent information bases, not accurately reflecting the
   correct topology of the MANET.  The consequence hereof is that
   protocols using NHDP will base their operation on incorrect
   information, causing routing protocols to not be able to calculate
   correct (or any) paths, degrade the performance of flooding
   operations based on reduced relay sets, etc.  These consequences to
   protocols using NHDP are described in detail in Section 5.

4.  Detailed Threat Description

   For each threat, described in the below, a description of the
   mechanism of the corresponding attack is given, followed by a
   description of how the attack affects NHDP.  The impacts from each
   attack on protocols using NHDP are given in Section 5.

   For simplicity in the description, examples given assume that NHDP
   routers have a single interface with a single IP address configured.
   All the attacks apply, however, for NHDP routers with multiple
   interfaces and multiple addresses as well.

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4.1.  Jamming

   One vulnerability, common for all protocols operating a wireless ad
   hoc network, is that of "jamming", i.e., that a device generates
   massive amounts of interfering radio transmissions, which will
   prevent legitimate traffic (e.g.,control traffic as well as data
   traffic) on part of a network.

   Depending on lower layers, this may not affect transmissions: HELLO
   messages from an NHDP router with "jammed" interfaces may be received
   by other NHDP routers.  As [RFC6130] identifies and uses only bi-
   directional links, a link from a jammed NHDP router to a non-jammed
   NHDP router would not be considered, and the jammed NHDP router
   appear simply as "disconnected" for the un-jammed part of the network
   - which is able to maintain accurate topology maps.

   If, due to a jamming attack, a considerable amount of HELLO messages
   are lost or corrupted due to collisions, neighbor NHDP routers are
   not able to establish links between them any more.  Thus, NHDP will
   present empty information bases to the protocols using it.

4.2.  Eavesdropping

   Eavesdropping is a common and easy passive attack in a wireless
   environment.  Once a packet is transmitted, any adjacent NHDP router
   can potentially obtain a copy, for immediate or later processing.
   Neither the source nor the intended destination can detect this.  A
   malicious NHDP router can eavesdrop on the NHDP message exchange and
   thus learn the local topology.  It may also eavesdrop on data traffic
   to learn source and destination addresses of data packets, or other
   header information, as well as the packet payload.

   Eavesdropping does not pose a direct threat to the network nor to
   NHDP, in as much as that it does not alter the information recorded
   by NHDP in its information bases and presented to other protocols
   using it, but it can provide network information required for
   enabling other attacks, such as the identity of communicating NHDP
   routers, link characteristic, NHDP router configuration, etc.

4.3.  Incorrect HELLO Message Generation

   An NHDP router running [RFC6130] performs two distinct tasks: it
   periodically generates HELLO messages, and it processes incoming
   HELLO messages from neighbor NHDP routers.  This section describes
   security attacks involving the HELLO generation.

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4.3.1.  Identity Spoofing

   Identity spoofing implies that a Compromised NHDP router sends HELLO
   messages, pretending to have the identity of another NHDP router.  A
   Compromised NHDP router can accomplish this by using another NHDP
   router's IP address in an address block of a HELLO message, and
   associating this address with a LOCAL_IF Address Block TLV.

   An NHDP router receiving the HELLO message from a neighbor, will
   assume that it originated from the NHDP router with the spoofed
   interface address.  As a consequence, it will add a Link Tuple to
   that neighbor with the spoofed address, and include it in its next
   HELLO messages as a heard neighbor (and possibly as symmetric
   neighbor after another HELLO exchange).

   Identity spoofing is particular harmful if a Compromised NHDP router
   spoofs the identity of another NHDP router that exists in the same
   routing domain.  With respect to NHDP, such a duplicated, spoofed
   address can lead to an inconsistent state up to two hops from an NHDP
   router.  Figure 1 depicts a simple example.  In that example, NHDP
   router A is in radio range of C, but not of the Compromised NHDP
   router X. If X spoofs the address of A, that can lead to conflicts
   for upper-layer routing protocols, and therefore for wrong path
   calculations as well as incorrect data traffic forwarding.

                          .---.    .---.    .---.
                          | A |----| C |----| X |
                          '---'    '---'    '---'

                                 Figure 1

   Figure 2 depicts another example.  In this example, A is two hops
   away from NHDP router C, reachable through NHDP router B. If the
   Compromised NHDP router X spoofs the address of A, C may think that A
   is indeed reachable through NHDP router D.

                 .---.    .---.    .---.    .---.    .---.
                 | A |----| B |----| C |----| D |----| X |
                 '---'    '---'    '---'    '---'    '---'

                                 Figure 2

4.3.2.  Link Spoofing

   Similar to identity spoofing, link spoofing implies that a
   Compromised NHDP router sends HELLO messages, signaling an incorrect
   set of neighbors.  This may take either of two forms:

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   o  A Compromised NHDP Router can postulate addresses of non-present
      neighbor NHDP routers in an address block of a HELLO, associated
      with LINK_STATUS TLVs.

   o  A Compromised NHDP router can "ignore" otherwise existing
      neighbors by not advertising them in its HELLO messages.

   The effect of link spoofing with respect to NHDP are twofold,
   depending on the two cases mentioned above: If the Compromised NHDP
   router ignores existing neighbors in its advertisements, links will
   be missing in the information bases maintained by other routers, and
   there may not be any connectivity to or from these NHDP routers to
   others NHDP routers in the MANET.  If, on the other hand, the
   Compromised NHDP router advertises non-existing links, this will lead
   to inclusion of topological information in the information base,
   describing non-existing links in the network (which, then, may be
   used by other protocols using NHDP in place of other, existing,

4.4.  Replay Attack

   A replay attack implies that control traffic from one region of the
   network is recorded and replayed in a different region (this type of
   attack is also known as the Wormhole attack).  This may, for example,
   happen when two Compromised NHDP routers collaborate on an attack,
   one recording traffic in its proximity and tunneling it to the other
   Compromised NHDP router, which replays the traffic.  In a protocol
   where links are discovered by testing reception, this will result in
   extraneous link creation (basically, a "virtual" link between the two
   Compromised NHDP routers will appear in the information bases of
   neighboring NHDP routers).

   While this situation may result from an attack, it may also be
   intentional: if data-traffic also is relayed over the "virtual" link,
   the link being detected is indeed valid for use.  This is, for
   instance, used in wireless repeaters.  If data traffic is not carried
   over the virtual link, an imaginary, useless, link between the two
   Compromised NHDP routers, has been advertised, and is being recorded
   in the information bases of their neighboring NHDP routers.

   Replay attacks can be especially damaging if coupled with spoofing
   and tampering with sequence numbers in the replayed messages,
   potentially destroying some important topology information in NHDP
   routers all over the network, as described in Section 4.5.

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4.5.  Sequence Number Attack

   [RFC6130] uses message sequence numbers, to avoid processing and
   forwarding the same message more than once.  An attack may consist of
   a Compromised NHDP router, spoofing the identity of another
   Legitimate NHDP router in the network and transmitting a large number
   of HELLO messages, each with different message sequence numbers.
   Subsequent HELLOs with the same sequence numbers, originating from
   theLegitimate NHDP router whose identity was spoofed, would hence be
   ignored, until eventually information concerning these "spoofed"
   HELLO messages expires.

   As illustrated in Figure 1, if the Compromised NHDP router X spoofs
   the identify of NHDP router A, and broadcasts several HELLO messages,
   all the valid HELLO messages sent by A with the same sequence numbers
   will be discarded by C, until the information concerning these HELLOs

4.6.  Message Timing Attacks

   In [RFC6130], each HELLO message contains a "validity time" and may
   contain an "interval time" field, identifying the time for which
   information in that control message should be considered valid until
   discarded, and the time until the next control message of the same
   type should be expected [RFC5497].

4.6.1.  Interval Time Attack

   A use of the expected interval between two successive HELLO messages
   is for determining the link quality in [RFC6130]: if messages are not
   received within the expected intervals (e.g., a certain fraction of
   messages are missing), then this may be used to exclude a link from
   being considered as useful, even if (some) bi-directional
   communication has been verified.  If a Compromised NHDP router X
   spoofs the identity of an existing NHDP router A, and sends HELLOs
   indicating a low interval time, an NHDP router B receiving this HELLO
   will expect the following HELLO to arrive within the interval time
   indicated - or otherwise, decrease the link quality for the link A-B.
   Thus, X may cause NHDP router B's estimate of the link quality for
   the link A-B to fall below the limit, where it is no longer
   considered as useful and, thus, not used.

4.6.2.  Validity Time Attack

   A Compromised NHDP router X can spoof the identity of an NHDP router
   A and send a HELLO using a low validity time (e.g.,1 ms).  A
   receiving NHDP router B will discard the information upon expiration
   of that interval, i.e., a link between NHDP router A and B will be

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   "torn down" by X.

4.7.  Indirect Jamming

   Indirect jamming is when a Compromised NHDP router X by its actions
   causes other legitimate NHDP routers to generate inordinate amounts
   of control traffic.  This increases channel occupation, and the
   overhead in each receiving NHDP router processing this control
   traffic.  With this traffic originating from Legitimate NHDP routers,
   the malicious device may remain undetected to the wider network.

   Figure 3 illustrates indirect jamming of [RFC6130].  A Compromised
   NHDP router X advertises a symmetric spoofed link to the non-existing
   NHDP router B (at time t0).  Router A selects X as MPR upon reception
   of the HELLO, and will trigger a HELLO at t1.  Overhearing this
   triggered HELLO, the attacker sends another HELLO at t2, advertising
   the link to B as lost, which leads to NHDP router A deselecting the
   attacker as MPR, and another triggered message at t3.  The cycle may
   be repeated, alternating advertising the link X-B as LOST and SYM.

                             MPRs(X)                   MPRs()
                .---.        .---.        .---.        .---.
                | A |        | A |        | A |        | A |
                '---'        '---'        '---'        '---'
                  |            |            |            |
                  | SYM(B)     |            | LOST(B)    |
                  |            |            |            |
                .---.        .---.        .---.        .---.
                | X |        | X |        | X |        | X |
                '---'        '---'        '---'        '---'
                  .            .
                  .            .
                  .            .
                .....        .....
                . B .        . B .
                .....        .....

                 t0           t1           t2           t3

                                 Figure 3

5.  Impact of inconsistent Information Bases on Protocols using NHDP

   This section describes the impact on protocols, using NHDP, of NHDP
   failing to obtain and represent accurate information, possibly as a
   consequence of the attacks described in Section 4.  This description
   emphasizes the impacts on the MANET protocols OLSRv2 [OLSRv2], and

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   SMF [SMF].

5.1.  MPR Calculation

   MPR selection (as used in e.g., [OLSRv2] and [SMF]) uses information
   about a router's 1-hop and 2-hop neighborhood, assuming that (i) this
   information is accurate, and (ii) all 1-hop neighbors are apt to act
   as as MPR, depending on the willingness they report.  Thus, a
   Compromised NHDP router will seek to manipulate the 1-hop and 2-hop
   neighborhood information in a router such as to cause the MPR
   selection to fail, leading to a flooding disruption of TC messages.

5.1.1.  Flooding Disruption due to Identity Spoofing

   A Compromised NHDP router can spoof the identify of other routers, to
   disrupt the MPR selection, so as to cache certain parts of the
   network from the flooding traffic.

   In Figure 4, a Compromised NHDP router X spoofs the identity of B.
   The link between X and C is correctly detected and listed in X's
   HELLOs.  Router A will receive HELLOs indicating links from,
   respectively B:{B-E}, X:{X-C, X-E}, and D:{D-E, D-C}.  For router A,
   X and D are equal candidates for MPR selection.  To make sure the X
   can be selected as MPR for router A, X can set its willingness to the
   maximum value.

                         .---.    .---.    .---.
                         | E |----| D |----| C |
                         '---'    '---'    '---'
                           |        |        .
                           |        |        .
                         .---.    .---.    .---.
                         | B |----| A |----| X |
                         '---'    '---'    '---'
                                           spoofs B

                                 Figure 4

   If B and X (i) accept MPR selection and (ii) forward flooded traffic
   as-if they were both B, identity spoofing by X is harmless.  However,
   if X does not forward flooded traffic (i.e., does not accept MPR
   selection), its presence entails flooding disruption: selecting B
   over D renders C unreachable by flooded traffic.

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                                  | D |
                .---.    .---.    .---.    .---.    .---.
                | X |----| A |----| B |----| C |----| E |...
                '---'    '---'    '---'    '---'    '---'
               spoofs E

                                 Figure 5

   In Figure 5, the Compromised NHDP router X spoofs the identity of E,
   i.e.,router A and C both receive HELLOs from a router identifying as
   E. For router B, A and C present the same neighbor sets, and are
   equal candidates for MPR selection.  If router B selects only router
   A as MPR, C will not relay flooded traffic from or transiting via B,
   and router X (and routers to the "right" of it) will not receive
   flooded traffic.

5.1.2.  Flooding Disruption due to Link Spoofing

   A Compromised NHDP router can also spoof links to other NHDP routers,
   and thereby makes itself appear as the most appealing candidate of
   MPR for its neighbors, possibly to the exclusion of other NHDP
   routers in the neighborhood (this, in particular, if the Compromised
   NHDP router spoof links to all other NHDP routers in the
   neighborhood, plus to one other NHDP router).  By thus excluding
   other legitimate NHDP routers from being selected as MPR, the
   Compromised NHDP router will receive and be expected to relay all
   flooded traffic (e.g., TC messages in OLSRv2 or data traffic in SMF)
   - which it can then drop or otherwise manipulate.

   In the network in Figure 6, the Compromised NHDP router X spoofs
   links to the existing router C, as well as to a fictitious W. Router
   A receives HELLOs from X and B, reporting X: {X-C, X-W}, b: {B-C}.
   All else being equal, X appears a better choice as MPR than B, as X
   appears to cover all neighbors of B, plus W.

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                                             ,---.    .....
                                             | S |    . C .
                                             '---'    .....
                                               |        .
                                               |        .
                  .---.    .---.    .---.    .---.    .---.
                  | D |----| C |----| B |----| A |----| X |
                  '---'    '---'    '---'    '---'    '---'
                                                      . w .

                                 Figure 6

   As router A will not select B as MPR, B will not relay flooded
   messages received from router A. The NHDP routers on the left of B
   (starting with C) will, thus, not receive any flooded messages from
   or transiting NHDP router A (e.g., a message originating from S).

5.1.3.  Broadcast Storm

   Compromised NHDP router may attack the network by attempting to
   degrade the performance of optimized flooding algorithms so as to be
   equivalent to classic flooding.  This can be achieved by forcing an
   NHDP router into choosing all its 1-hop neighbors as MPRs.  In
   MANETs, a broadcast storm caused by classic flooding is a serious
   problem which can result in redundancy, contention and collisions

   As shown in Figure 7, the Compromised NHDP router X spoofs the
   identity of NHDP router B and, spoofs a link to router Y {B-Y} (Y
   does not have to be exist).  By doing so, the legitimate NHDP router
   A has to select the legitimate NHDP router B as its MPR, in order for
   it to reach all its 2-hop neighbors.  The Compromised NHDP router Y
   can perform this identity+link spoofing for all of NHDP router A's
   1-hop neighbors, thereby forcing NHDP router A to select all its
   neighbors as MPR - disabling the optimization sought by the MPR

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                         | B |
                         .---.    .---.      .....
                         | A |----| X | .  . . Y .
                         '---'    '---'      .....
                                  spoofs B

                                 Figure 7

5.2.  Routing Loops

   Inconsistent information bases, provided by NHDP to other protocols,
   can also cause routing loops.  In Figure 8, the Compromised NHDP
   router X spoofs the identity of NHDP router E. NHDP router D has data
   traffic to send to NHDP router A. The topology recorded in the
   information base of router D indicates that the shortest path to
   router A is {D->E->A}, because of the link {A-E} reported by X.
   Therefore, the data traffic will be routed to the NHDP router E. As
   the link {A-E} does not exist in NHDP router E's information bases,
   it will identify the next hop for data traffic to NHDP router A as
   being NHDP router D. A loop between the NHDP routers D and E is thus

                    .---.    .---.    .---.    .---.    .---.
                    | A |----| B |----| C |----| D |----| E |
                    '---'    '---'    '---'    '---'    '---'
                    | X |
                    spoofs E

                                 Figure 8

5.3.  Invalid or Non-Existing Paths to Destinations

   By reporting inconsistent topology information in NHDP, the invalid
   links/routers can be propagated as link state information with TC
   messages and results in route failure.  As illustrated in Figure 8,
   if NHDP router B tries to send data packets to NHDP router E, it will
   choose router A as its next hop, based on the information of non-
   existing link {A-E} reported by the Compromised NHDP router X.

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5.4.  Data Sinkhole

   With the ability to spoof multiple identities of legitimate NHDP
   routers (by eavesdropping, for example), the Compromised NHDP router
   can represent a "data sinkhole" for its 1-hop and 2-hop neighbors.
   Data packets that come across its neighbors may be forwarded to the
   Compromised NHDP router instead of to the real destination.  The
   packet can then be dropped, manipulated, duplicated, etc., by the
   Compromised NHDP router.  As shown in Figure 8, if the Compromised
   NHDP router X spoofs the identity of NHDP router E, all the data
   packets to E that cross NHDP routers A and B will be sent to NHDP
   router X, instead of to E.

6.  Security Considerations

   This document does not specify a protocol or a procedure.  The
   document, however, reflects on security considerations for NHDP and
   MANET routing protocols using NHDP for neighborhood discovery.

7.  IANA Considerations

   This document contains no actions for IANA.

8.  References

8.1.  Normative References

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

   [RFC5444]  Clausen, T., Dearlove, C., Dean, J., and C. Adjih,
              "Generalized MANET Packet/Message Format", RFC 5444,
              February 2009.

   [RFC5497]  Clausen, T. and C. Dearlove, "Representing Multi-Value
              Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497,
              March 2009.

   [RFC6130]  Clausen, T., Dean, J., and C. Dearlove, "MANET
              Neighborhood Discovery Protocol (NHDP)", RFC 6130,
              April 2011.

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8.2.  Informative References

   [OLSRv2]   Clausen, T., Dearlove, C., Philippe, P., and U. Ulrich,
              "The Optimized Link State Routing Protocol version 2",
              work in progress draft-ietf-manet-olsrv2-14.txt,
              March 2012.

   [SMF]      Macker, J., "Simplified Multicast Forwarding", work in
              progress draft-ietf-manet-smf-14.txt, March 2012.

              Ni, S., Tseng, Y., Chen, Y., and J. Sheu, "The Broadcast
              Storm Problem in a Mobile Ad Hoc Network", Proceedings of
              the 5th annual ACM/IEEE international conference on Mobile
              computing and networking, 1999.

Authors' Addresses

   Ulrich Herberg
   Fujitsu Laboratories of America
   1240 E Arques Ave
   Sunnyvale, CA 94085


   Jiazi Yi
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 1 69 33 40 31
   Email: jiazi@jiaziyi@com

   Thomas Heide Clausen
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 6 6058 9349

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