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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 3561

Mobile Ad Hoc Networking Working Group                    Charles E. Perkins
INTERNET DRAFT                                         Nokia Research Center
24 November 2000                                          Elizabeth M. Royer
                                     University of California, Santa Barbara
                                                                Samir R. Das
                                                    University of Cincinnati


              Ad hoc On-Demand Distance Vector (AODV) Routing
                        draft-ietf-manet-aodv-07.txt
Status of This Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at
   any time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at:
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at:
        http://www.ietf.org/shadow.html.

   This document is a submission by the Mobile Ad Hoc Networking Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the manet@itd.nrl.navy.mil mailing list.

   Distribution of this memo is unlimited.

Abstract

    The Ad Hoc On-Demand Distance Vector (AODV) routing protocol is
    intended for use by mobile nodes in an ad hoc network.  It offers
    quick adaptation to dynamic link conditions, low processing and
    memory overhead, low network utilization, and determines unicast

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    between sources and destinations.  It uses destination sequence
    numbers to ensure loop freedom at all times (even in the face of
    anomalous delivery of routing control messages), solving problems
    (such as ``counting to infinity'') associated with classical distance
    vector protocols.

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                                   Contents
Status of This Memo                                                         i


Abstract                                                                    i


 1.  Introduction                                                           1


 2.  Overview                                                               2


 3.  AODV Terminology                                                       3


 4.  Route Request (RREQ) Message Format                                    4


 5.  Route Reply (RREP) Message Format                                      5


 6.  Route Error (RERR) Message Format                                      7


 7.  Route Reply Acknowledgment (RREP-ACK) Message Format                   8


 8.  AODV Operation                                                         8
      8.1. Maintaining Route Utilization Records . . . . . . . . . .        8
      8.2. Generating Route Requests . . . . . . . . . . . . . . . .     9
            8.2.1.  Controlling Route Request broadcasts  . . . . . .    10
      8.3. Forwarding Route Requests . . . . . . . . . . . . . . . .     11
            8.3.1.  Processing Route Requests . . . . . . . . . . . .    11
      8.4. Generating Route Replies  . . . . . . . . . . . . . . . .     13
            8.4.1.  Route Reply Generation by the Destination    . . .   13
            8.4.2.  Route Reply Generation by an Intermediate Node  .    14
      8.5. Generating Gratuitous RREPs . . . . . . . . . . . . . . .    14
      8.6. Forwarding Route Replies  . . . . . . . . . . . . . . . .    15
      8.7. Hello Messages  . . . . . . . . . . . . . . . . . . . . .    16
      8.8. Maintaining Local Connectivity  . . . . . . . . . . . . .    17
      8.9. Route Error Messages  . . . . . . . . . . . . . . . . . .    18
            8.9.1.  Local Repair  . . . . . . . . . . . . . . . . . .   19
     8.10. Route Expiry and Deletion . . . . . . . . . . . . . . . .    21
     8.11. Actions After Reboot  . . . . . . . . . . . . . . . . . .    21
     8.12. Interfaces  . . . . . . . . . . . . . . . . . . . . . . .    22


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 9.  AODV and Aggregated Networks                                       22


10.  Using AODV with Other Networks                                     23


11.  Extensions                                                         24
     11.1. Hello Interval Extension Format . . . . . . . . . . . . .    24


12.  Configuration Parameters                                           25


13.  Security Considerations                                            26


14.  Acknowledgments                                                    27

1.  Introduction

    The Ad Hoc On-Demand Distance Vector (AODV) algorithm enables
    dynamic, self-starting, multihop routing between participating mobile
    nodes wishing to establish and maintain an ad hoc network.  AODV
    allows mobile nodes to obtain routes quickly for new destinations,
    and does not require nodes to maintain routes to destinations that
    are not in active communication.  AODV allows mobile nodes to respond
    quickly to link breakages and changes in network topology.  The
    operation of AODV is loop-free, and by avoiding the Bellman-Ford
    ``counting to infinity'' problem offers quick convergence when the
    ad hoc network topology changes (typically, when a node moves in the
    network).  When links break, AODV causes the affected set of nodes to
    be notified so that they are able to invalidate the routes using the
    broken link.


    One distinguishing feature of AODV is its use of a destination
    sequence number for each route entry.  The destination sequence
    number is created by the destination for any route information it
    sends to requesting nodes.  Using destination sequence numbers
    ensures loop freedom and is simple to program.  Given the choice
    between two routes to a destination, a requesting node always selects
    the one with the greatest sequence number.

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2.  Overview


    Route Requests (RREQs), Route Replies (RREPs), and Route Errors
    (RERRs) are the message types defined by AODV. These message types
    are handled by UDP, and normal IP header processing applies.  So, for
    instance, the requesting node is expected to use its IP address as
    the source IP address for the messages.  The range of dissemination
    of broadcast RREQs can be indicated by the TTL in the IP header.
    Fragmentation is typically not required.


    As long as the endpoints of a communication connection have valid
    routes to each other, AODV does not play any role.  When a route to
    a new destination is needed, the node uses a broadcast RREQ to find
    a route to the destination.  A route can be determined when the RREQ
    reaches either the destination itself, or an intermediate node with
    a 'fresh enough' route to the destination.  A 'fresh enough' route
    is an unexpired route entry for the destination whose associated
    sequence number is at least as great as that contained in the RREQ.
    The route is made available by unicasting a RREP back to the source
    of the RREQ. Since each node receiving the request caches a route
    back to the source of the request, the RREP can be unicast back from
    the destination to the source, or from any intermediate node that
    is able to satisfy the request back to the source.  A RREQ can be
    conditioned by requirements on the path to the destination, namely
    bandwidth or delay bounds.


    Nodes monitor the link status of next hops in active routes.  When a
    link break in an active route is detected, a RERR message is used to
    notify other nodes that the loss of that link has occurred.  The RERR
    message indicates which destinations are now unreachable due to the
    loss of the link.


    AODV is a routing protocol, and it deals with route table management.
    Route table information must be kept even for ephemeral routes, such
    as are created to temporarily store reverse paths towards nodes
    originating RREQs.  AODV uses the following fields with each route
    table entry:


     -  Destination IP Address

     -  Destination Sequence Number

     -  Interface
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     -  Hop Count (number of hops needed to reach destination)

     -  Last Hop Count (described in subsection 8.2.1)

     -  Next Hop

     -  List of Precursors (described in Section 8.1)

     -  Lifetime (expiration or deletion time of the route)

     -  Routing Flags

3.  AODV Terminology

    This protocol specification uses conventional meanings [1] for
    capitalized words such as MUST, SHOULD, etc., to indicate requirement
    levels for various protocol features.  This section defines other
    terminology used with AODV that is not already defined in [3].


       active route


          A routing table entry with a finite metric in the Hop Count
          field.  A routing table may contain entries that are not active
          (invalid routes or entries).  They have an infinite metric
          in the Hop Count field.  Only active entries can be used to
          forward data packets.  Invalid entries are eventually deleted.


       forwarding node


          A node which agrees to forward packets destined for another
          destination node, by retransmitting them to a next hop which is
          closer to the unicast destination along a path which has been
          set up using routing control messages.


       forward route


          A route set up to send data packets from a source to a
          destination.


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       reverse route


          A route set up to forward a reply (RREP) packet back to the
          source from the destination or from an intermediate node having
          a route to the destination.

4.  Route Request (RREQ) Message Format

     0                     1                     2                     3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type       |J|R|G|        Reserved           |  Hop Count|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Broadcast ID                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Destination IP Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Destination Sequence Number                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Source IP Address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Source Sequence Number                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    The format of the Route Request message is illustrated above, and
    contains the following fields:


       Type            1


       J               Join flag; reserved for multicast.


       R               Repair flag; reserved for multicast.


       G               Gratuitous RREP flag; indicates whether a
                       gratuitous RREP should be unicast to the node
                       specified in the Destination IP Address field (see
                       sections 8.2, 8.5)


       Reserved        Sent as 0; ignored on reception.


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       Hop Count       The number of hops from the Source IP Address to
                       the node handling the request.


       Broadcast ID    A sequence number uniquely identifying the
                       particular RREQ when taken in conjunction with the
                       source node's IP address.


       Destination IP Address
                       The IP address of destination for which a route is
                       desired.


       Destination Sequence Number
                       The last sequence number received in the past by
                       the source for any route towards the destination.


       Source IP Address
                       The IP address of the node which originated the
                       Route Request.


       Source Sequence Number
                       The current sequence number to be used for route
                       entries pointing to (and generated by) the source
                       of the route request.

5.  Route Reply (RREP) Message Format

     0                     1                     2                     3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type       |R|A|     Reserved      |Prefix Sz|  Hop Count|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Destination IP address                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Destination Sequence Number                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Source IP address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              Lifetime                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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    The format of the Route Reply message is illustrated above, and
    contains the following fields:


       Type           2


       R              Repair flag; used for multicast.


       A              Acknowledgment required; see sections 7 and 8.6.


       Reserved       Sent as 0; ignored on reception.


       Prefix Size    If nonzero, the 5-bit Prefix Size specifies that the
                      indicated next hop may be used for any nodes with
                      the same routing prefix (as defined by the Prefix
                      Size) as the requested destination.


       Hop Count      The number of hops from the Source IP Address to
                      the Destination IP Address.  For multicast route
                      requests this indicates the number of hops to the
                      multicast tree member sending the RREP.


       Destination IP Address
                      The IP address of the destination for which a route
                      is supplied.


       Destination Sequence Number
                      The destination sequence number associated to the
                      route.


       Source IP Address
                      The IP address of the source node which issued the
                      RREQ for which the route is supplied.


       Lifetime       The time for which nodes receiving the RREP consider
                      the route to be valid.


    Note that the Prefix Size allows a Subnet Leader to supply a route
    for every host in the subnet defined by the routing prefix, which
    is determined by the IP address of the Subnet Leader and the Prefix
    Size.  In order to make use of this feature, the Subnet Leader has to
    guarantee reachability to all the hosts sharing the indicated subnet


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    prefix.  The Subnet Leader is also responsible for maintaining the
    Destination Sequence Number for the whole subnet.

6.  Route Error (RERR) Message Format

     0                     1                     2                     3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type       |N|           Reserved            | DestCount |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Unreachable Destination IP Address (1)            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Unreachable Destination Sequence Number (1)          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
    |  Additional Unreachable Destination IP Addresses (if needed)  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Additional Unreachable Destination Sequence Numbers (if needed)|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    The format of the Route Error message is illustrated above, and
    contains the following fields:


       Type         3


       N            No delete flag; set when a node has performed a local
                    repair of a link, and upstream nodes should not delete
                    the route.


       Reserved     Sent as 0; ignored on reception.


       DestCount    The number of unreachable destinations included in the
                    message; MUST be at least 1.


       Unreachable Destination IP Address
                    The IP address of the destination which has become
                    unreachable due to a link break.


       Unreachable Destination Sequence Number
                    The last known sequence number, incremented by one,
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                    of the destination listed in the previous Unreachable
                    Destination IP Address field.


    The RERR message is sent whenever a link break causes one or more
    destinations to become unreachable.  The unreachable destination
    addresses included are those of all lost destinations which are now
    unreachable due to the loss of that link.

7.  Route Reply Acknowledgment (RREP-ACK) Message Format

     0                     1
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type       |  Reserved   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Type         4


       Reserved     Sent as 0; ignored on reception.


    The RREP-ACK message is used to acknowledge receipt of a RREP
    message.  It is used in cases where the link over which the RREP
    message is sent may be unreliable.

8.  AODV Operation

    This section describes the scenarios under which nodes generate
    RREQs, RREPs and RERRs for unicast communication, and how the message
    data are handled.

8.1.  Maintaining Route Utilization Records

    For each valid route maintained by a node (containing a finite Hop
    Count metric) as a routing table entry, the node also maintains a
    list of precursors that may be forwarding packets on this route.
    These precursors will receive notifications from the node in the
    event of detection of the loss of the next hop link.  The list of
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    precursors in a routing table entry contains those neighboring nodes
    to which a route reply was generated or forwarded.


    Each time a route is used to forward a data packet, its Lifetime
    field is updated to be current time plus ACTIVE_ROUTE_TIMEOUT.

8.2.  Generating Route Requests

    A node broadcasts a RREQ when it determines that it needs a route
    to a destination and does not have one available.  This can happen
    if the destination is previously unknown to the node, or if a
    previously valid route to the destination expires or is broken
    (i.e., an infinite metric is associated with the route).  The
    Destination Sequence Number field in the RREQ message is the last
    known destination sequence number for this destination and is copied
    from the Destination Sequence Number field in the routing table.  If
    no sequence number is known, a sequence number of zero is used.  The
    Source Sequence Number in the RREQ message is the node's own sequence
    number.  The Broadcast ID field is incremented by one from the last
    broadcast ID used by the current node.  Each node maintains only one
    broadcast ID. The Hop Count field is set to zero.


    A source node often expects to have bidirectional communications with
    a destination node.  In such cases, it is not sufficient for the
    source node to have a route to the destination node; the destination
    must also have a route back to the source node.  In order to cause
    this to happen as efficiently as possible, any generation of an RREP
    by an intermediate node (as in section 8.4) for delivery to the
    source node, should be accompanied by some action which notifies the
    destination about a route back to the source node.  The source node
    selects this mode of operation in the intermediate nodes by setting
    the `G' flag.  See section 8.5 for details about actions taken by the
    intermediate node in response to a RREQ with the `G' flag set.


    After broadcasting a RREQ, a node waits for a RREP. If the RREP is
    not received within NET_TRAVERSAL_TIME milliseconds, the node MAY
    rebroadcast the RREQ, up to a maximum of RREQ_RETRIES times.  Each
    rebroadcast MUST increment the Broadcast ID field.


    Data packets waiting for a route (i.e., waiting for a RREP after RREQ
    has been sent) SHOULD be buffered.  The buffering SHOULD be FIFO. If
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    a RREQ has been rebroadcast RREQ_RETRIES times without receiving any
    RREP, all data packets destined for the corresponding destination
    SHOULD be dropped from the buffer and a Destination Unreachable
    message delivered to the application.

8.2.1.  Controlling Route Request broadcasts

    To prevent unnecessary network-wide broadcasts of RREQs, the
    source node SHOULD use an expanding ring search technique as an
    optimization.  In an expanding ring search, the source node initially
    uses a TTL = TTL_START in the RREQ packet IP header and sets the
    timeout for receiving a RREP to 2 * TTL * NODE_TRAVERSAL_TIME
    milliseconds.  Upon timeout, the source rebroadcasts the RREQ with
    the TTL incremented by TTL_INCREMENT. This continues until the
    TTL set in the RREQ reaches TTL_THRESHOLD, beyond which a TTL =
    NET_DIAMETER is used for each rebroadcast.  Each time, the timeout
    for receiving a RREP is calculated as before.  Each rebroadcast
    increments the Broadcast ID field in the RREQ packet.  The RREQ
    can be rebroadcast with TTL = NET_DIAMETER up to a maximum of
    RREQ_RETRIES times.


    When a RREP is received, the Hop Count used in the RREP packet is
    remembered as Last Hop Count in the routing table.  When a new route
    to the same destination is required at a later time (e.g., upon route
    loss), the TTL in the RREQ IP header is initially set to this Last
    Hop Count plus TTL_INCREMENT. Thereafter, following each timeout the
    TTL is incremented by TTL_INCREMENT until TTL = TTL_THRESHOLD is
    reached.  Beyond this TTL = NET_DIAMETER is used as before.


    As a further optimization, timeouts MAY be determined dynamically via
    measurements, instead of using a statically configured value related
    to NODE_TRAVERSAL_TIME. To accomplish this, the RREQ may carry the
    timestamp via an extension field as defined in Section 11 to be
    carried back by the RREP packet (again via an extension field).  The
    difference between the current time and this timestamp will determine
    the route discovery latency.  The timeout may be set to be a small
    factor times the average of the last few route discovery latencies
    for the concerned destination.  These latencies may be recorded as
    additional fields in the routing table.
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    If the optimizations described in this section are used, an expired
    routing table entry SHOULD NOT be expunged before DELETE_PERIOD.
    Otherwise, the soft state corresponding to the route (e.g., Last Hop
    Count) will be lost.  In such cases, a longer routing table entry
    expunge time may be specified.  Any routing table entry waiting for a
    RREP should not be expunged before RREP_WAIT_TIME.

8.3.  Forwarding Route Requests

    When a node receives a broadcast RREQ, it first checks to determine
    whether it has received a RREQ with the same Source IP Address
    and Broadcast ID within at least the last BROADCAST_RECORD_TIME
    milliseconds.  If such a RREQ has been received, the node silently
    discards the newly received RREQ. The rest of this subsection
    describes actions taken for RREQs that are not discarded.

8.3.1.  Processing Route Requests

    When a node receives a RREQ, the node checks to determine whether it
    has an active route to the destination.  If the node does not have
    an active route, it rebroadcasts the RREQ from its interface(s) but
    using its own IP address in the IP header of the outgoing RREQ. The
    Destination Sequence Number in the RREQ is updated to the maximum
    of the existing Destination Sequence Number in the RREQ and the
    destination sequence number in the routing table (if an entry exists)
    of the current node.  The TTL or hop limit field in the outgoing IP
    header is decreased by one.  The Hop Count field in the broadcast
    RREQ message is incremented by one, to account for the new hop
    through the intermediate node.


    If the node, on the other hand, does has an active route for the
    destination, it compares the destination sequence number for that
    route with the Destination Sequence Number field of the incoming
    RREQ. If the existing destination sequence number is smaller than
    the Destination Sequence Number field of the RREQ, the node again
    rebroadcasts the RREQ just as if it did not have an active route to
    the destination.


    The node generates a RREP (as discussed further in section 8.4) if
    either:
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       (i)        it has an active route to the destination, and the
                  node's existing destination sequence number is greater
                  than or equal to the Destination Sequence Number of the
                  RREQ, or


       (ii)       it is itself the destination.


    The node always creates or updates a reverse route to the Source IP
    Address in its routing table.  If a route to the Source IP Address
    already exists, it is updated only if either


       (i)        the Source Sequence Number in the RREQ is higher than
                  the destination sequence number of the Source IP Address
                  in the route table, or


       (ii)       the sequence numbers are equal, but the hop count as
                  specified by the RREQ is now smaller than the existing
                  hop count in the routing table.


    When a reverse route is created or updated, the following actions are
    carried out:


     1. the Source Sequence Number from the RREQ is copied to the
        corresponding destination sequence number;


     2. the next hop in the routing table becomes the node broadcasting
        the RREQ (it is obtained from the source IP address in the IP
        header and is often not equal to the Source IP Address field in
        the RREQ message);


     3. the hop count is copied from the Hop Count in the RREQ message;


     4. the lifetime of the route is the higher of its current lifetime
        (for an active route) and current time plus REV_ROUTE_LIFE.


    Even if the route is not updated because the existing route has a
    higher destination sequence number, but if it is scheduled to expire
    before REV_ROUTE_LIFE, its lifetime is still updated to be current
    time plus REV_ROUTE_LIFE.

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    This reverse route would be needed in case the node receives an
    eventual RREP back to the node which originated the RREQ (identified
    by the Source IP Address).

8.4.  Generating Route Replies

    If a node receives a route request for a destination, and either
    has a fresh enough route to satisfy the request or is itself the
    destination, the node generates a RREP message and unicasts it back
    to the node indicated by the Source IP Address field of the received
    RREQ. The node generating the RREP message copies the Source and
    Destination IP Addresses in RREQ message into the corresponding
    fields in the RREP message which is to be sent back toward the
    source of the RREQ. Additional operations are slightly different,
    depending on whether the node is itself the requested destination, or
    instead if it is an intermediate node with an admissible route to the
    destination.


    As the RREP is forwarded to the source, the Hop Count field is
    incremented by one at each hop.  Thus, when the RREP reaches the
    source, the Hop Count represents the distance, in hops, of the
    destination from the source.

8.4.1.  Route Reply Generation by the Destination

    If the generating node is the destination itself, it uses a
    destination sequence number at least equal to a sequence number
    generated after the last detected change in its neighbor set and at
    least equal to the destination sequence number in the RREQ. If the
    destination node has not detected any change in its set of neighbors
    since it last incremented its destination sequence number, it MAY use
    the same destination sequence number.  The destination node places
    the value zero in the Hop Count field of the RREP.


    The destination node copies the value MY_ROUTE_TIMEOUT into
    the Lifetime field of the RREP. Each node MAY make a separate
    determination about its value MY_ROUTE_TIMEOUT.

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8.4.2.  Route Reply Generation by an Intermediate Node


    If node generating the RREP is not the destination node, but
    instead is an intermediate hop along the path from the source to the
    destination, it copies the last known destination sequence number in
    the Destination Sequence Number field in the RREP message.


    The intermediate node places its distance in hops from the
    destination (indicated by the hop count in the routing table) in the
    Hop Count field in the RREP.


    When the intermediate node updates its route table for the source
    of the RREQ, it puts the last hop node (from which it received the
    RREQ, as indicated by the source IP address field in the IP header)
    into the precursor list for the forward path route entry -- i.e., the
    entry for the Destination IP Address.  Furthermore, the intermediate
    node puts the next hop towards the destination in the precursor list
    for the reverse route entry -- i.e., the entry for the Source IP
    Address field of the RREQ message data.


    The intermediate node calculates the Lifetime field of the RREP by
    subtracting the current time from the expiration time in its route
    table entry.

8.5.  Generating Gratuitous RREPs

    When a node receives a RREQ and responds with a RREP, it does not
    forward the RREQ any further.  If all incarnations of a single
    RREQ are replied to by intermediate nodes, the destination does
    not receive any copies of the RREQ. Hence, it does not learn of a
    route to the source node.  This can be problematic if the source is
    attempting to establish a TCP session.  In order that the destination
    learn of routes to the source node, the source node SHOULD set the
    gratuitous RREP ('G') flag in the RREQ if the session is going to be
    run over TCP, or if the destination should receive the gratuitous
    RREP for any other reason.  Intermediate nodes receiving a RREQ
    with the 'G' flag set and responding with a RREP SHOULD unicast a
    gratuitous RREP to the destination node.

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    The RREP that is sent to the source of the RREQ is the same as
    before.  The gratuitous RREP that is to be sent to the desired
    destination contains the following values in the RREP message fields:


       Hop Count   The Hop Count as received in the RREQ


       Destination IP Address
                   The IP address of the node that generated the RREQ


       Destination Sequence Number
                   The Source Sequence Number from the RREQ


       Source IP Address
                   The IP address of the destination node


       Lifetime    The remaining lifetime of the route towards the
                   destination node, as known by the intermediate node.


    The gratuitous RREP is then sent to the next hop along the path to
    the destination node.

8.6.  Forwarding Route Replies

    When a node receives a RREP message, it first compares the
    Destination Sequence Number in the message with its own copy of
    destination sequence number for the Destination IP Address.  The
    forward route for this destination is created or updated only if
    (i) the Destination Sequence Number in the RREP is greater than the
    node's copy of the destination sequence number, or (ii) the sequence
    numbers are the same, but the route is no longer active or the Hop
    Count in RREP is smaller than the hop count in route table entry.  If
    a new route is created or the old route is updated, the next hop is
    the node from which the RREP is received, which is indicated by the
    source IP address field in the IP header; the hop count is the Hop
    Count in the RREP message plus one; the expiry time is the current
    time plus the Lifetime in the RREP message; the destination sequence
    number is the Destination Sequence Number in the RREP message.


    The current node can now begin using this route to send data packets
    to the destination.


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    If the current node is not the source node as indicated by the Source
    IP Address in the RREP message AND a forward route has been created
    or updated as described before, the node consults its route table
    entry for the source node to determine the next hop for the RREP
    packet, and then forwards the RREP towards the source with its Hop
    Count incremented by one.


    When any node generates or forwards a RREP, the precursor list for
    the corresponding destination node is updated by adding to it the
    next hop node to which the RREP is forwarded.  Also, at each node the
    (reverse) route used to forward a RREP has its lifetime changed to
    current time plus ACTIVE_ROUTE_TIMEOUT.


    If a node forwards a RREP over a link that is likely to have errors,
    the node MAY set the `A' flag to require that the recipient of the
    RREP acknowledge receipt of the RREP by sending a RREP-ACK message
    back.

8.7.  Hello Messages

    A node MAY offer connectivity information by broadcasting local
    Hello messages as follows.  Every HELLO_INTERVAL milliseconds, the
    node checks whether it has sent a broadcast (e.g., a RREQ or an
    appropriate layer 2 message) within the last HELLO_INTERVAL. If it
    has not, it MAY generate a broadcast RREP with TTL = 1, called a
    Hello message, with the message fields set as follows:


       Destination IP Address
                    The node's IP address.


       Destination Sequence Number
                    The node's latest sequence number.


       Hop Count    0


       Lifetime     ALLOWED_HELLO_LOSS * HELLO_INTERVAL


    A node MAY determine connectivity by listening for packets from
    its set of neighbors.  If it receives no packets for more than
    ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD


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    assume that the link to this neighbor is currently broken.  When this
    happens, the node SHOULD proceed as in Section 8.9.

8.8.  Maintaining Local Connectivity

    Each forwarding node SHOULD keep track of its active next hops (i.e.,
    which next hops have been used to forward packets towards some
    destination within the last ACTIVE_ROUTE_TIMEOUT milliseconds).  This
    is done by updating the Lifetime field of a routing table entry used
    to forward data packets to current time plus ACTIVE_ROUTE_TIMEOUT
    milliseconds.  For purposes of efficiency, each node may try to learn
    which of these active next hops are really in the neighborhood at the
    current time using one or more of the available link or network layer
    mechanisms, as described below.


     -  Any suitable link layer notification, such as those provided by
        IEEE 802.11, can be used to determine connectivity, each time
        a packet is transmitted to an active next hop.  For example,
        absence of a link layer ACK or failure to get a CTS after sending
        RTS, even after the maximum number of retransmission attempts,
        will indicate loss of the link to this active next hop.


     -  Passive acknowledgment can be used when the next hop is expected
        to forward the packet, by listening to the channel for a
        transmission attempt made by the next hop.  If transmission is
        not detected within NEXT_HOP_WAIT milliseconds or the next hop is
        not a forwarding node (and thus is never supposed to transmit the
        packet) one of the following methods should be used to determine
        connectivity.


         *  Receiving an ICMP ACK message from the next hop.  The ICMP
            ACK message SHOULD be sent to a forwarding node by a next hop
            which is also the destination as in the in the IP header of
            the packet.  This should be done only when this destination
            has not sent any packets to the concerned forwarding node
            within the last HELLO_INTERVAL milliseconds.


         *  A RREQ unicast to the next hop, asking for a route to the
            next hop.


         *  An ICMP Echo Request message unicast to the next hop.
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    If a link to the next hop cannot be detected by any of these methods,
    the forwarding node SHOULD assume that the link is broken, and take
    corrective action by following the methods specified in Section 8.9.

8.9.  Route Error Messages

    A node initiates a RERR message in three situations:


       (i)        if it detects a link break for the next hop of an active
                  route in its routing table, or


       (ii)       if it gets a data packet destined to a node for which it
                  does not have an active route, or


       (iii)      if it receives a RERR from a neighbor for one or more
                  active routes.


    For cases (i) and (ii), the destination sequence numbers in the
    routing table for the unreachable destination(s) are incremented by
    one.  Then RERR is broadcast with the unreachable destination(s) and
    their incremented destination sequence number(s) included in the
    packet.  For case (i), the unreachable destinations are the broken
    next hop, and any additional destinations which are now unreachable
    due to the loss of this next hop link.  For case (ii), there is only
    one unreachable destination, which is the destination of the data
    packet that cannot be delivered.  The DestCount field of the RERR
    packet indicates the number of unreachable destinations included in
    the packet.


    For cases (i) and (ii), for each unreachable destination the node
    copies the value in the Hop Count route table field into the Last
    Hop Count field, and marks the Hop Count for this destination as
    infinity, and thus invalidates the route.


    For case (iii) when a node receives a RERR message, for each
    unreachable destination included in the packet, the node determines
    whether the source node (as indicated by the source IP address in the
    IP header) forwarding the RERR packet is its own next hop used to
    reach this destination.  If so, the node takes the following actions:
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       (a)        updates the corresponding destination sequence number
                  with the Destination Sequence Number in the packet, and


       (b)        marks the Hop Count for this destination as infinity,
                  and thus invalidates the route.


       (c)        checks the precursor list for this destination.  If one
                  or more of these precursor lists are non-empty, the node
                  creates a RERR message, including as unreachable each
                  destination with a non-empty precursor list.  It also
                  includes their destination sequence numbers, and then
                  broadcasts this RERR message.


    When a node receives a RERR message, it always updates its
    destination sequence number(s) for the unreachable destination(s)
    included in the packet using the corresponding sequence numbers
    included in the message.  When a node broadcasts a RERR message, it
    always deletes the precursor list of each unreachable destination
    included in the message.


    When a node invalidates a route to a neighboring node, it must also
    delete that neighbor from any precursor lists for routes to other
    nodes.  This prevents precursor lists from containing stale entries
    of neighbors with which the node is no longer able to communicate.
    The node should inspect the precursor list of each destination entry
    in its routing table, and delete the lost neighbor from any list in
    which it appears.

8.9.1.  Local Repair

    When a link break in an active route occurs, the node upstream of
    that break MAY choose to repair the link locally if the destination
    is no farther than MAX_REPAIR_TTL hops away.  To repair the link
    break itself, it increments the sequence number for the destination
    and then broadcasts a RREQ for that destination.  The TTL of the RREQ
    should initially be set to the following value:
              max(MIN_REPAIR_TTL, 0.5 TTL to source) + LOCAL_ADD_TTL

    Thus, local repair attempts should never be visible to the source
    node, and will always have minimum TTL equal to MIN_REPAIR_TTL
    + LOCAL_ADD_TTL. The node initiating the repair then waits the
    discovery period to receive RREPs in response to the RREQ. If, at
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    the end of the discovery period, it has not received a RREP for that
    destination, it proceeds as described in Section 8.9 by creating a
    RERR message for that destination.


    On the other hand, if the nodes does receive one or more RREPs during
    the discovery period, the node proceeds as described in Section 8.6,
    creating a route table entry for that destination.  It then compares
    the hop count of the new route with the value in the last hop count
    route table entry for that destination.  If the hop count of the
    newly determined route to the destination is greater than the hop
    count of the previously known route, as recorded in the last hop
    count field, the node MAY create a RERR message for the destination
    and send this message to the source node.  The node sets the 'N' flag
    of the RERR, and then broadcasts this message if it has one or more
    precursor nodes for this route table entry.


    A node which receives a RERR message with the 'N' flag set MUST
    NOT delete the route to that destination.  The only action taken
    should be the retransmission of the message, if the RERR arrived
    from the next hop along that route, and if there are one or more
    precursor nodes for that route to the destination.  When the source
    node receives a RERR message with the 'N' flag set, if this message
    came from its next hop along its route to the destination then the
    source node MAY choose to reinitiate route discovery, as described in
    Section 8.2.


    Local repair of link breaks in active routes sometimes results in
    increased path lengths to those destinations.  Repairing the link
    locally is likely to increase the number of data packets which are
    able to be delivered to the destinations, since data packets will not
    be dropped as the RERR travels to the source node.  Sending a RERR
    to the source node after locally repairing the link break allows the
    source to find a fresh route to the destination which is more optimal
    based on current node positions.  However, it does not require the
    source node to rebuild the route, as the source may be done, or
    nearly done, with the data session.


    When a link breaks along an active route, there are often multiple
    destinations which become unreachable.  The node which is upstream
    of the broken link tries an immediate local repair for only the one
    destination towards which the packet was traveling.  Other routes
    using the same link MUST be marked as broken, but the node handling
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    the local repair MAY flag each such newly broken route as locally
    repairable; this local repair flag in the route table MUST be reset
    when the route times out (i.e., after the route has been not been
    active for ACTIVE_ROUTE_TIMEOUT). Before the timeout occurs, these
    other routes will be repaired as needed when packets arrive for the
    other destinations.  Alternatively, depending upon local congestion,
    the node MAY begin the process of establishing local repairs for the
    other routes, without waiting for new packets to arrive.

8.10.  Route Expiry and Deletion

    If the Lifetime of an active routing entry expires, the following
    actions are taken.


     1. The entry is invalidated by copying the Hop Count to the Last Hop
        Count field and then making the Hop Count infinity.


     2. The destination sequence number of this routing entry is
        incremented by one.


     3. The Lifetime field is updated to current time plus DELETE_PERIOD.
        Before this time, the entry MUST NOT be deleted.


    Note that the Lifetime field plays dual role -- for an active route
    it is the expiry time, and for an invalid route it is the deletion
    time.


    These actions are also taken whenever a route entry is invalidated
    for any reason, for example, for link breakage or receiving a RERR.


    If a data packet is received for an invalid route, the Lifetime
    field is always updated to current time plus DELETE_PERIOD. The
    determination of DELETE_PERIOD is discussed in Section 12

8.11.  Actions After Reboot

    A node participating in the ad hoc network must take certain
    actions after reboot as it will have lost its prior sequence
    number and as well as its last known sequence numbers for various
    other destinations.  However, there may be neighboring nodes which
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    are using this node as an active next hop.  This can potentially
    create routing loops.  To prevent this possibility, each node on
    reboot waits for DELETE_PERIOD. In this time, it does not respond
    to any routing packets.  However, if it receives a data packet,
    it broadcasts a RERR as described in subsection 8.9 and resets
    the waiting timer (Lifetime) to expire after current time plus
    DELETE_PERIOD.


    It can be shown that by the time the rebooted node comes out of
    the waiting phase and becomes an active router again, none of its
    neighbors will be using it as an active next hop any more.  Its own
    sequence number gets updated once it receives a RREQ from any other
    node, as the RREQ always carries the maximum destination sequence
    number seen en route.

8.12.  Interfaces

    Because AODV should operate smoothly over wired, as well as wireless,
    networks, and because it is likely that AODV will also be used with
    multi-homed radios, the interface over which packets arrive must
    be known to AODV whenever a packet is received.  This includes the
    reception of RREQ, RREP, and RERR messages.  Whenever a packet is
    received from a new neighbor, the interface on which that packet was
    received is recorded into the route table entry for that neighbor,
    along with all the other appropriate routing information.  Similarly,
    whenever a route to a new destination is learned, the interface
    through which the destination can be reached is also recorded into
    the destination's route table entry.


    When multiple interfaces are available, a node receiving and
    rebroadcasting a RREQ message rebroadcasts that message on all
    interfaces.  Similarly, when a node needs to transmit a RERR, it
    should only broadcast it on those interfaces which have precursor
    nodes for that route.

9.  AODV and Aggregated Networks

    AODV has been designed for use by mobile nodes with IP addresses
    that are not necessarily related to each other, to create an ad hoc
    network.  However, in some cases a collection of mobile nodes MAY
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    operate in a fixed relationship to each other and share a common
    subnet prefix, moving together within an area where an ad hoc network
    has formed.  Call such a collection of nodes a ``subnet''.  In this
    case, it is possible for a single node within the subnet to advertise
    reachability for all other nodes on the subnet, by responding with
    a RREP message to any RREQ message requesting a route to any node
    with the subnet routing prefix.  Call the single node the ``subnet
    router''.  In order for a subnet router to operate the AODV protocol
    for the whole subnet, it has to maintain a destination sequence
    number for the entire subnet.  In any such RREP message sent by the
    subnet router, the Prefix Size field of the RREP message MUST be
    set to the length of the subnet prefix.  Other nodes sharing the
    subnet prefix SHOULD NOT issue RREP messages, and SHOULD forward RREQ
    messages to the subnet leader.

10.  Using AODV with Other Networks

    In some configurations, an ad hoc network may be able to provide
    connectivity between external routing domains that do not use AODV.
    If the points of contact to the other networks can act as subnet
    routers (see Section 9) for any relevant networks within the external
    routing domains, then the ad hoc network can maintain connectivity to
    the external routing domains.  Indeed, the external routing networks
    can use the ad hoc network defined by AODV as a transit network.


    In order to provide this feature, a point of contact to an external
    network (call it an Infrastructure Router) has to act as the subnet
    router for every subnet of interest within the external network for
    which the Infrastructure Router can provide reachability.  This
    includes the need for maintaining a destination sequence number for
    that external subnet.


    If multiple Infrastructure Routers offer reachability to the same
    external subnet, those Infrastructure Routers have to cooperate (by
    means outside the scope of this specification) to provide consistent
    AODV semantics for ad hoc access to those subnets.

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11.  Extensions


    RREQ and RREP messages have extensions defined in the following
    format:

     0                     1                     2                     3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type       |     Length      |      type-specific data ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    where:


       Type      1


       Length    The length of the type-specific data, not including the
                 Type and Length fields of the extension.


    Extensions with types between 128 and 255 may NOT be skipped.  The
    rules for extensions will be spelled out more fully, and conform with
    the rules for handling IPv6 options.

11.1.  Hello Interval Extension Format

     0                     1                     2                     3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type       |     Length      |   Hello Interval ...      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | ... Hello Interval, continued |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Type      2


       Length    4


       Hello Interval
                 The number of milliseconds between successive
                 transmissions of a Hello message.

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    The Hello Interval extension MAY be appended to a RREP message with
    TTL == 1, to be used by a neighboring receiver in determine how long
    to wait for subsequent such RREP messages (i.e., Hello messages; see
    section 8.7).

12.  Configuration Parameters

    This section gives default values for some important values
    associated with AODV protocol operations.  A particular mobile
    node may wish to change certain of the parameters, in particular
    the NET_DIAMETER, NODE_TRAVERSAL_TIME, MY_ROUTE_TIMEOUT,
    ALLOWED_HELLO_LOSS, RREQ_RETRIES, and possibly the HELLO_INTERVAL. In
    the latter case, the node should advertise the HELLO_INTERVAL in its
    Hello messages, by appending a Hello Interval Extension to the RREP
    message.  Choice of these parameters may affect the performance of
    the protocol.


       Parameter Name            Value
       ----------------------    -----
       ACTIVE_ROUTE_TIMEOUT      3,000 Milliseconds
       ALLOWED_HELLO_LOSS        2
       BROADCAST_RECORD_TIME     2 * NET_TRAVERSAL_TIME
       DELETE_PERIOD             see note below
       HELLO_INTERVAL            1,000 Milliseconds
       LOCAL_ADD_TTL             2
       MAX_REPAIR_TTL            0.3 * NET_DIAMETER
       MY_ROUTE_TIMEOUT          2 * ACTIVE_ROUTE_TIMEOUT
       NET_DIAMETER              35
       NEXT_HOP_WAIT             NODE_TRAVERSAL_TIME + 10
       NODE_TRAVERSAL_TIME       40
       REV_ROUTE_LIFE            NET_TRAVERSAL_TIME
       NET_TRAVERSAL_TIME        3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2
       RREQ_RETRIES              2
       TTL_START                 1
       TTL_INCREMENT             2
       TTL_THRESHOLD             7
    DELETE_PERIOD should be an upper bound on the time for which
    an upstream node A can have a neighbor B to be an active next

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    hop for destination D, while B has invalidated the route to D.
    Beyond this time B can delete the route to D. The determination
    of the upper bound somewhat depends on the characteristics of
    the underlying link layer.  For example, if the link layer
    feedback is used to detect loss of link DELETE_PERIOD must be
    at least ACTIVE_ROUTE_TIMEOUT. If there is no feedback and hello
    messages must be used, DELETE_PERIOD must be at least maximum of
    ACTIVE_ROUTE_TIMEOUT and ALLOWED_HELLO_LOSS * HELLO_INTERVAL. If
    hello messages are received from a neighbor but data packets to that
    neighbor are lost, (due to temporary link asymmetry, e.g.)  we have
    to make more concrete assumptions about the underlying link layer.
    We assume that such asymmetry cannot persist beyond a certain certain
    time, say, a multiple K of ALLOWED_HELLO_LOSS * HELLO_INTERVAL.
    In other words, it cannot not be the case that a node receives K
    subsequent hello messages from a neighbor, while that same neighbor
    fails to receive any data packet from the node in this period.  This
    is a reasonable assumption as this AODV specification works only with
    symmetric links.  Covering all possibilities,


              DELETE_PERIOD = K * max (ACTIVE_ROUTE_TIMEOUT,
       ALLOWED_HELLO_LOSS * HELLO_INTERVAL) (K = 5 is recommended).


    NET_DIAMETER measures the maximum possible number of hops between
    two nodes in the network.  NODE_TRAVERSAL_TIME is a conservative
    estimate of the average one hop traversal time for packets and should
    include queueing delays, interrupt processing times and transfer
    times.  ACTIVE_ROUTE_TIMEOUT SHOULD be set to a longer value (at
    least 10,000 milliseconds) if link-layer indications are used to
    detect link breakages such as in IEEE 802.11 [2] standard.  TTL_START
    should be set to at least 2 if Hello messages are used for local
    connectivity information.  Performance of the AODV protocol is
    sensitive to the chosen values of these constants, which often depend
    on the characteristics of the underlying link layer protocol, radio
    technologies etc.

13.  Security Considerations

    Currently, AODV does not specify any special security measures.
    Route protocols, however, are prime targets for impersonation
    attacks, and must be protected by use of authentication techniques
    involving generation of unforgeable and cryptographically strong
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    message digests or digital signatures.  It is expected that, in
    environments where security is an issue, that IPSec authentication
    headers will be deployed along with the necessary key management to
    distribute keys to the members of the ad hoc network using AODV.

14.  Acknowledgments

    We acknowledge with gratitude the work done at University of
    Pennsylvania within Carl Gunter's group, as well as at Stanford and
    CMU, to determine some conditions (especially involving reboots and
    lost RERRs) under which previous versions of AODV could suffer from
    routing loops.  Contributors to those efforts include Karthikeyan
    Bhargavan, Joshua Broch, Dave Maltz, Madanlal Musuvathi, and
    Davor Obradovic.  The idea of a DELETE_PERIOD, for which expired
    routes (and, in particular, the sequence numbers) to a particular
    destination must be maintained, was also suggested by them.


    We also acknowledge the comments and improvements suggested by SJ Lee
    (especially regarding local repair) and Mahesh Marina.
References


    [1] S. Bradner.  Key words to use in RFCs to indicate requirement
        levels.  RFC 2119, March 1997.


    [2] IEEE Standards Department.  Wireless LAN medium access control
        (MAC) and physical layer (PHY) specifications, IEEE standard
        802.11--1997, 1997.


    [3] C. E. Perkins.  Mobile ad hoc networking terminology.  IETF
        Internet Draft, draft-ietf-manet-term-00.txt (Work in Progress),
        October 1997.







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Author's Addresses

    Questions about this memo can be directed to:

       Charles E. Perkins
       Communications Systems Laboratory
       Nokia Research Center
       313 Fairchild Drive
       Mountain View, CA 94303
       USA
       +1 650 625 2986
       +1 650 691 2170 (fax)
       charliep@iprg.nokia.com

       Elizabeth M. Royer
       Dept. of Electrical and Computer Engineering
       University of California, Santa Barbara
       Santa Barbara, CA 93106
       +1 805 893 7788
       +1 805 893 3262 (fax)
       eroyer@alpha.ece.ucsb.edu

       Samir R. Das
       Department of Electrical and Computer Engineering
       & Computer Science
       University of Cincinnati
       Cincinnati, OH 45221-0030
       +1 513 556 2594
       +1 513 556 7326 (fax)
       sdas@ececs.uc.edu

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