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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
19 January 2002                               Elizabeth M. Belding-Royer
                                 University of California, Santa Barbara
                                                            Samir R. Das
                                                University of Cincinnati

            Ad hoc On-Demand Distance Vector (AODV) Routing
                      draft-ietf-manet-aodv-10.txt


Status of This Memo

   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.

   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.


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 routes to destinations within the ad hoc network.  It uses
   destination sequence numbers to ensure loop freedom at all times
   (even in the face of anomalous delivery of routing control messages),
   avoiding 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                                                           1

 3. AODV Terminology                                                   3

 4. Message Formats                                                    4
     4.1. Route Request (RREQ) Message Format . . . . . . . . . . .    4
     4.2. Route Reply (RREP) Message Format . . . . . . . . . . . .    5
     4.3. Route Error (RERR) Message Format . . . . . . . . . . . .    7

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

 6. AODV Operation                                                     8
     6.1. Maintaining Sequence Numbers  . . . . . . . . . . . . . .    8
     6.2. Maintaining Route Table Entries and Precursor Lists . . .   10
     6.3. Generating Route Requests . . . . . . . . . . . . . . . .   10
     6.4. Controlling Dissemination of Route Request Messages . . .   11
     6.5. Processing and Forwarding Route Requests  . . . . . . . .   12
     6.6. Generating Route Replies  . . . . . . . . . . . . . . . .   14
           6.6.1. Route Reply Generation by the Destination . . . .   14
           6.6.2. Route Reply Generation by an Intermediate Node  .   14
           6.6.3. Generating Gratuitous RREPs . . . . . . . . . . .   15
     6.7. Receiving and Forwarding Route Replies  . . . . . . . . .   16
     6.8. Operation over Unidirectional Links . . . . . . . . . . .   17
     6.9. Hello Messages  . . . . . . . . . . . . . . . . . . . . .   17
    6.10. Maintaining Local Connectivity  . . . . . . . . . . . . .   18
    6.11.  Route Error Messages, Route Expiry and Route Deletion  .   19
    6.12. Local Repair  . . . . . . . . . . . . . . . . . . . . . .   20
    6.13. Actions After Reboot  . . . . . . . . . . . . . . . . . .   22
    6.14. Interfaces  . . . . . . . . . . . . . . . . . . . . . . .   22

 7. AODV and Aggregated Networks                                      23

 8. Using AODV with Other Networks                                    23

 9. Extensions                                                        24
     9.1. Hello Interval Extension Format . . . . . . . . . . . . .   24
     9.2. Timestamp Extension Format  . . . . . . . . . . . . . . .   25




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10. Configuration Parameters                                          25

11. Security Considerations                                           27

12. Acknowledgments                                                   28

 A. Draft Modifications                                               29


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
   to link breakages and changes in network topology in a timely manner.
   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.


2. Overview

   Route Requests (RREQs), Route Replies (RREPs), and Route Errors
   (RERRs) are the message types defined by AODV. These message
   types are received at port 654, over UDP, and normal IP header
   processing applies.  So, for instance, the requesting node is
   expected to use its IP address as the Originator IP address for the
   messages.  For broadcast messages, the IP limited broadcast address
   (255.255.255.255) is used.  This means that such messages are not
   blindly forwarded.  However, AODV operation does require certain
   messages (e.g., RREQ) to be disseminated widely, perhaps throughout
   the ad hoc network.  The range of dissemination of such RREQs is
   indicated by the TTL in the IP header.  Fragmentation is typically
   not required.




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   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 broadcasts a 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 origination of
   the RREQ. Each node receiving the request caches a route back to the
   originator of the request, so that the RREP can be unicast from the
   destination along a path to that originator, or likewise from any
   intermediate node that is able to satisfy the request.

   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 those destinations which are now unreachable due to
   the loss of the link.  In order to enable this reporting mechanism,
   each node keeps a ``precursor list'', containing the IP address for
   each its neighbors that are likely to use it as a next hop towards
   the destination that is now unreachable.  The information in the
   precursor lists is most easily acquired during the processing for
   generation of a RREP message, which by definition has to be sent to a
   node in a precursor list (see section 6.6).

   A RREQ may also be received for a multicast IP address.  In this
   document, full processing for such messages is not specified.  For
   example, the originator of such a RREQ for a multicast IP address
   may have to follow special rules.  However, it is important to
   enable correct multicast operation by intermediate nodes that are
   not enabled as originating or destination nodes for IP multicast
   addresses, and likewise are not equipped for any special multicast
   protocol processing.  For such multicast-unaware nodes, processing
   for a multicast IP address as a destination IP address MUST be
   carried out in the same way as for any other destination IP address.

   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











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   uses the following fields with each route table entry:

       -  Destination IP Address
       -  Destination Sequence Number
       -  Interface
       -  Hop Count (number of hops needed to reach destination)
       -  Last Hop Count (described in subsections 6.4 and 6.11)
       -  Next Hop
       -  List of Precursors (described in Section 6.2)
       -  Lifetime (expiration or deletion time of the route)
       -  Routing Flags

   Managing the sequence number is crucial to avoiding routing loops,
   even when links break and a node is no longer reachable to supply
   its own information about its sequence number.  A destination
   becomes unreachable when a link breaks or is deactivated.  When these
   conditions occur, the route is invalidated by operations involving
   the sequence number and metric (hop count).  See section 6.1 for
   details.


3. AODV Terminology

   This protocol specification uses conventional meanings [2] 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.

      broadcast

         Broadcasting means transmitting to the IP Limited Broadcast
         address, 255.255.255.255.  A broadcast packet may not be
         blindly forwarded, but broadcasting is useful to enable
         dissemination of AODV messages throughout the ad hoc network.

      forwarding node

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



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      forward route

         A route set up to send data packets from a node originating a
         Route Discovery operation towards its desired destination.

      originating node

         A node that initiates an AODV message to be processed and
         possibly retransmitted by other nodes in the ad hoc network.
         For instance, the node initiating a Route Discovery process and
         broadcasting the RREQ message is called the originating node of
         the RREQ message.

      reverse route

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


4. Message Formats

4.1. 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   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            RREQ ID                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination IP Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Originator IP Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Originator 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.




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      G              Gratuitous RREP flag; indicates whether a
                     gratuitous RREP should be unicast to the node
                     specified in the Destination IP Address field (see
                     sections 6.3, 6.6.3)

      Reserved       Sent as 0; ignored on reception.

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

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

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

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

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

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


4.2. 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                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Originator 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 5 and 6.7.

      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 Originator 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.

      Originator IP Address
                    The IP address of the node which originated 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
   prefix.  The Subnet Leader is also responsible for maintaining the
   Destination Sequence Number for the whole subnet.  See section 7 for
   details.








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4.3. 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 that has become
                  unreachable due to a link break.

      Unreachable Destination Sequence Number
                  The sequence number in the route table entry for
                  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 from some of the node's neighbors.
   See section 6.2 for information about how to maintain the appropriate
   records for this determination, and section 6.11 for specification
   about how to create the list of destinations.







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5. 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 may be 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 or unidirectional.


6. AODV Operation

   This section describes the scenarios under which nodes generate Route
   Request (RREQ), Route Reply (RREP) and Route Error (RERR) messages
   for unicast communication towards a destination, and how the message
   data are handled.  In order to process the messages correctly,
   certain state information has to be maintained in the route table
   entries for the destinations of interest.

   All AODV messages are sent to port 654 using UDP.


6.1. Maintaining Sequence Numbers

   AODV depends on each node in the network to own and maintain a
   sequence number to guarantee the loop-freedom of all routes towards
   that node.  A node increments its own sequence number in two
   circumstances:

    -  Immediately before a node originates a route discovery, it MUST
       increment its own sequence number.  This prevents problems with
       deleted reverse routes to the originator of a RREQ.
    -  Immediately before a destination node originates a RREP in
       response to a RREQ, it MUST update its own sequence number to
       the maximum of its current sequence number and the destination
       sequence number in the RREQ packet.

   When the destination increments its sequence number, it MUST do so by
   treating the sequence number value as if it were an unsigned number.
   Thus, if the sequence number has already been assigned to be the
   largest possible number representable as a 32-bit unsigned integer



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   (i.e., 4294967295), then when it is incremented it will then have a
   value of zero (0).  Similarly, if the sequence number currently has
   the value 2147483647, which is the largest possible positive integer
   when if 2's complement arithmetic is in use, the next value will be
   2147483648, which is the most negative possible integer in the same
   numbering system.  The representation of negative numbers is not
   relevant to the incrementation of AODV sequence numbers.  This is
   in contrast to the manner in which the result of comparing two AODV
   sequence numbers is to be treated (see below).

   Every route table entry at every node MUST include the latest
   information available about the sequence number for the IP address of
   the destination node for which the route table entry is maintained.
   This sequence number is called the "destination sequence number".  It
   is updated whenever a node receives new (i.e., not stale) information
   about the sequence number from RREQ, RREP, or RERR messages that may
   be received related to that destination.  In order to ascertain that
   information about a destination is not stale, the node compares its
   current numerical value for the sequence number with that obtained
   from the incoming AODV message.  This comparison MUST be done using
   signed 32-bit arithmetic.  If the result of subtracting the currently
   stored sequence number from the value of the incoming sequence number
   is less than zero, then the information related to that destination
   in the AODV message MUST be discarded, since that information is
   stale compared to the node's currently stored information.

   The only other circumstance in which a node may change the
   destination sequence number in one of its route table entries is in
   response to a broken or expired link to the next hop towards that
   destination.  The node determines which destinations use a broken
   next hop by consulting its routing table.  In this case, for each
   destination that uses the next hop, the node increments the sequence
   number and puts the Hop Count to be "infinity" (for the case of
   broken links, see also see sections 6.11, 6.12).

   A node may change the sequence number in the routing table entry of a
   destination only if:

    -  it is itself the destination node, and offers a new route to
       itself, or

    -  it receives an AODV message with new information about the
       sequence number for a destination node, or

    -  the path towards the destination node expires or breaks.







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6.2. Maintaining Route Table Entries and Precursor Lists

   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
   precursors in a routing table entry contains those neighboring nodes
   to which a route reply was generated or forwarded.

   When a node receives an AODV control packet from a neighbor, it
   checks its route table for an entry for that neighbor.  In the event
   that there is no corresponding entry for that neighbor, an entry
   is created.  The sequence number is either determined from the
   information contained in the control packet (i.e., the neighbor is
   the originator of a RREQ), or else it is initialized to zero if the
   sequence number for that node can not be determined.  The Lifetime
   field of the routing table entry is either determined from the
   control packet (i.e., the neighbor is the originator of a RREP for
   itself), or it is initialized to ALLOWED_HELLO_LOSS * HELLO_INTERVAL.
   In other words, the reception of a control packet has the same
   meaning as the reception of an explicit Hello message, in that it
   signifies an active connection to that neighbor.  The hop count to
   the neighbor is set to one.

   Each time a route is used to forward a data packet, its Active Route
   Lifetime field of both the destination and the next hop on the path
   to the destination is updated to be no less than the current time
   plus ACTIVE_ROUTE_TIMEOUT. Since the route between each originator
   and destination pair are expected to be symmetric, the Active Route
   Lifetime for the previous hop, along the reverse path back to the
   IP source, is also updated to be no less than the current time plus
   ACTIVE_ROUTE_TIMEOUT.


6.3. 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
   Originator Sequence Number in the RREQ message is the node's own
   sequence number.  The RREQ ID field is incremented by one from the




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   last RREQ ID used by the current node.  Each node maintains only one
   RREQ ID. The Hop Count field is set to zero.

   Before broadcasting the RREQ, the originating node buffers the RREQ
   ID and the Originator IP address (its own address) of the RREQ
   for PATH_TRAVERSAL_TIME milliseconds.  In this way, when the node
   receives the packet again from its neighbors, it will not reprocess
   and re-forward the packet.

   An originating node often expects to have bidirectional
   communications with a destination node.  In such cases, it is
   not sufficient for the originating node to have a route to the
   destination node; the destination must also have a route back to
   the originating node.  In order for this to happen as efficiently
   as possible, any generation of a RREP by an intermediate node (as
   in section 6.6) for delivery to the originating node SHOULD be
   accompanied by some action that notifies the destination about a
   route back to the originating node.  The originating node selects
   this mode of operation in the intermediate nodes by setting the `G'
   flag.  See section 6.6.3 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 try
   again to discover a route by broadcasting a RREQ, up to a maximum
   of RREQ_RETRIES times.  Each new attempt MUST increment the RREQ ID
   field.

   Data packets waiting for a route (i.e., waiting for a RREP after a
   RREQ has been sent) SHOULD be buffered.  The buffering SHOULD be
   "first-in, first-out" (FIFO). If a route discovery has been attempted
   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.


6.4. Controlling Dissemination of Route Request Messages

   To prevent unnecessary network-wide dissemination of RREQs, the
   originating node SHOULD use an expanding ring search technique as
   an optimization.  In an expanding ring search, the originating
   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.  If the RREQ times out without a
   corresponding RREP, the originator broadcasts the RREQ again 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 attempt.  Each time, the timeout for receiving a



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   RREP is calculated as before.  Each attempt increments the RREQ ID
   field in the RREQ packet.  The RREQ can be broadcast with TTL =
   NET_DIAMETER up to a maximum of RREQ_RETRIES times.

   When a RREP is received, the Hop Count indicated in the RREP packet
   is stored as the 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.

   Timeouts MAY be more accurately determined dynamically via
   measurement, 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 9.2 to be
   carried back by the RREP packet (again via an extension field).  The
   difference between the current time and this timestamp determines 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.

   An expired routing table entry SHOULD NOT be expunged before
   (current_time + DELETE_PERIOD) (see section 6.11).  Otherwise, the
   soft state corresponding to the route (e.g., Last Hop Count) will be
   lost.  Furthermore, a longer routing table entry expunge time MAY be
   configured.  Any routing table entry waiting for a RREP SHOULD NOT be
   expunged before (current_time + PATH_TRAVERSAL_TIME).


6.5. Processing and Forwarding Route Requests

   When a node receives a RREQ, it first checks to determine whether it
   has received a RREQ with the same Originator IP Address and RREQ ID
   within at least the last PATH_TRAVERSAL_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.

   The node always creates a reverse route to the Originator IP Address
   in its routing table if one does not already exist.  If a route to
   the Originator IP Address already exists, it is updated only if
   either

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




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

   This reverse route will be needed if the node receives a RREP back
   to the node that originated the RREQ (identified by the Originator
   IP Address).  When the reverse route is created or updated, the
   following actions are carried out:

    1. the Originator Sequence Number from the RREQ is copied to the
       corresponding destination sequence number in the route table
       entry;

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

    3. the hop count is copied from the Hop Count in the RREQ message
       and incremented by one;

   Whenever a RREQ message is received, the Lifetime of the reverse
   route entry for the Originator IP address is set to be the maximum of
   (ExistingLifetime, MinimalLifetime), where

      MinimalLifetime =    (current time + PATH_TRAVERSAL_TIME -
                           2*HopCount*NODE_TRAVERSAL_TIME).

   The node generates a RREP (as discussed further in section 6.6) if
   either:

      (i)       it is itself the destination (see section 6.6.1), or

      (ii)      it has an active route to the destination, and the
                destination sequence number in the node's existing
                route table entry for the destination is greater than
                or equal to the Destination Sequence Number of the
                RREQ (comparison using signed 32-bit arithmetic).  See
                section 6.6.2 for further information about generating
                the RREP in this case.

   When either of these conditions is satisfied, the node does not
   rebroadcast the RREQ.

   Otherwise, if the incoming IP header has TTL larger than 1, the node
   updates and broadcasts the RREQ to address 255.255.255.255 on all of
   its configured interface(s) (see section 6.14).  To update the RREQ,
   the TTL or hop limit field in the outgoing IP header is decreased by




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   one, and the Hop Count field in the RREQ message is incremented by
   one, to account for the new hop through the intermediate node.


6.6. 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.  This node copies
   the Destination IP Address and the Originator Sequence Number in
   RREQ message into the corresponding fields in the RREP message.
   Processing is 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.  These scenarios
   are described in the sections below.

   Once created, the RREP is unicast to the next hop toward the
   originator of the RREQ, as indicated by the route table entry for
   that originator.  As the RREP is forwarded back towards the node
   which originated the RREQ message, the Hop Count field is incremented
   by one at each hop.  Thus, when the RREP reaches the originator, the
   Hop Count represents the distance, in hops, of the destination from
   the originator.


6.6.1. Route Reply Generation by the Destination

   If the generating node is the destination itself, it MUST update its
   own sequence number to the maximum of its current sequence number and
   the destination sequence number in the RREQ packet.  The destination
   node places its sequence number into the Destination Sequence Number
   field of the RREP, and enters the value zero in the Hop Count field
   of the RREP.

   The destination node copies the value MY_ROUTE_TIMEOUT (see
   section 10) into the Lifetime field of the RREP. Each node MAY
   reconfigure its value for MY_ROUTE_TIMEOUT, within mild constraints
   (see section 10).


6.6.2. Route Reply Generation by an Intermediate Node

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





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   The intermediate node updates the forward path route entry by placing
   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.  The intermediate node also updates its route
   table entry for the node originating the RREQ by placing the next hop
   towards the destination in the precursor list for the reverse route
   entry -- i.e., the entry for the Originator IP Address field of the
   RREQ message data.

   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. The Lifetime field of the RREP is
   calculated by subtracting the current time from the expiration time
   in its route table entry.


6.6.3. Generating Gratuitous RREPs

   After a node receives a RREQ and responds with a RREP, it discards
   the RREQ. If intermediate nodes reply to every transmission of a
   given RREQ, the destination does not receive any copies of it.  In
   this situation, it does not learn of a route to the originating node.
   This could cause the destination to initiate a network-wide route
   discovery (for example, if the originator is attempting to establish
   a TCP session).  In order that the destination learn of routes to the
   originating node, the originating node SHOULD set the ``gratuitous
   RREP'' ('G') flag in the RREQ if for any reason the destination is
   likely to need a route to the originating node.  If, in response to a
   RREQ with the 'G' flag set, an intermediate node returns a RREP, it
   MUST also unicast a gratuitous RREP to the destination node.

   The RREP that is sent to the originator 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 indicated in the node's route table
                 entry for the originator

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

      Destination Sequence Number
                 The Originator Sequence Number from the RREQ

      Originator IP Address
                 The IP address of the Destination node in the RREQ





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

   The gratuitous RREP is then sent to the next hop along the path to
   the destination node, just as if the destination node had already
   issued a RREQ for the originating node and this RREP was produced in
   response to that (fictitious) RREQ.


6.7. Receiving and Forwarding Route Replies

   When a node receives a RREP message, it compares the Destination
   Sequence Number in the message with its own copy of destination
   sequence number for the Destination IP Address in the RREP message.
   The forward route for this destination is created if it does not
   already exist, or it is 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 (iii) the sequence numbers are the
   same, and the Hop Count in the RREP is smaller than the hop count
   in route table entry.  In either of these cases, the next hop in
   the route entry is assigned to be 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; and the destination sequence number is the Destination
   Sequence Number in the RREP message.  The current node can now begin
   using this route to forward data packets to the destination.

   If the current node is not the node indicated by the Originator IP
   Address in the RREP message AND a forward route has been created or
   updated as described above, the node consults its route table entry
   for the originating node to determine the next hop for the RREP
   packet, and then forwards the RREP towards the originator using the
   information in the route table entry.

   When any node transmits 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 be the maximum of (existing-lifetime, (current time +
   ACTIVE_ROUTE_TIMEOUT)).

   If a node forwards a RREP over a link that is likely to have errors
   or be unidirectional, the node SHOULD set the `A' flag to require
   that the recipient of the RREP acknowledge receipt of the RREP by
   sending a RREP-ACK message back (see section 6.8).




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6.8. Operation over Unidirectional Links

   It is possible that a RREP transmission may fail, especially if the
   RREQ transmission triggering the RREP occurs over a unidirectional
   link.  If no other RREP generated from the same route discovery
   attempt reaches the node which originated the RREQ message, the
   originator will reattempt network-wide route discovery after a
   timeout (see section 6.3).  However, the same scenario might well
   be repeated, and no route would be discovered even after repeated
   retries.  Unless corrective action is taken, this can happen even
   when bidirectional routes between originator and destination do
   exist.  Link layers using broadcast transmissions for the RREQ will
   not be able to detect the presence of such unidirectional links.  In
   AODV, any node acts on only the first RREQ with the same RREQ ID
   and ignores any subsequent RREQs.  Suppose, for example, that the
   first RREQ arrives along a path that has one or more unidirectional
   link(s).  A subsequent RREQ may arrive via a bidirectional path
   (assuming such paths exist), but it will be ignored.

   To prevent this problem, when a node detects that its transmission of
   a RREP message has failed, it remembers the next-hop of the failed
   RREP in a ``blacklist'' set.  Such failures can be detected via
   the absence of a link-layer or network-layer acknowledgment (e.g.,
   RREP-ACK). A node ignores all RREQs received from any node in its
   blacklist set.  Nodes are removed from the blacklist set after a
   BLACKLIST_TIMEOUT period (see section 10).  This period should be set
   to the upper bound of the time it takes to perform the allowed number
   of route request retry attempts as described in section 6.3.


6.9. 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 broadcast a RREP with TTL = 1, called a Hello
   message, with the RREP 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




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   A node MAY determine connectivity by listening for packets from its
   set of neighbors.  If, within the past DELETE_PERIOD, it has received
   a Hello message from a neighbor, and then for that neighbor does
   not receive any packets (Hello messages or otherwise) for more than
   ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD
   assume that the link to this neighbor is currently broken.  When this
   happens, the node SHOULD proceed as in Section 6.11.

   Whenever a node receives a Hello message from a neighbor, the
   node SHOULD make sure that it has an active route to the neighbor,
   and create one if necessary.  If a route already exists, then the
   Lifetime for the route should be increased, if necessary, to be at
   least ALLOWED_HELLO_LOSS * HELLO_INTERVAL. The route to the neighbor,
   if it exists, MUST subsequently contain the latest Destination
   Sequence Number from the Hello message.  Routes that are newly
   created from the reception of Hello messages might have empty
   precursor lists, and in that case would not trigger RERR messages
   when the neighbor moves away and the neighbor route expires.


6.10. Maintaining Local Connectivity

   Each forwarding node SHOULD keep track of its continued connectivity
   to its active next hops (i.e., which next hops or precursors have
   forwarded packets to or from the forwarding node during the last
   ACTIVE_ROUTE_TIMEOUT), as well as neighbors that have transmitted
   Hello messages during the last (ALLOWED_HELLO_LOSS * HELLO_INTERVAL).
   A node can maintain accurate information about its continued
   connectivity to these active next hops, 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,
       indicates loss of the link to this active next hop.

    -  If possible, passive acknowledgment SHOULD 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 the destination (and thus is never supposed to
       transmit the packet) one of the following methods should be used
       to determine connectivity.

        *  Receiving any packet (including a Hello message) from the
           next hop.




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        *  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.

   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 6.11.


6.11.  Route Error Messages, Route Expiry and Route Deletion

   A Route Error (RERR) message MAY be either broadcast (if there
   are many precursors), unicast (if there is only 1 precursor),
   or iteratively unicast to all precursors (if broadcast is
   inappropriate).  Even when the RERR message is iteratively unicast to
   several precursors, it is considered to be a single control message
   for the purposes of the description in the text that follows.

   A node initiates processing for 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 if the routing table
                entry for the next hop expires (also see section 6.1),
                or

      (ii)      if it gets a data packet destined to a node for which it
                does not have an active route, and has already made an
                attempt at local repair (if local repair is being used),
                or

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

   For case (i), the node first makes a list of unreachable destinations
   consisting of the unreachable neighbor and any additional
   destinations in the local routing table that use the unreachable
   neighbor as the next hop.  For case (ii), there is only one
   unreachable destination, which is the destination of the data packet
   that cannot be delivered.  For case (iii), the list should consist of
   those destinations in the RERR for which there exists a corresponding
   entry in the local routing table that has the transmitter of the
   received RERR as the next hop.

   Some of the unreachable destinations in the list could be used by
   neighboring nodes, and it may therefore be necessary to send a (new)
   RERR. The RERR should contain those destinations that are part of
   the created list of unreachable destinations and have a non-empty
   precursor list.



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   The neighboring node(s) that should receive the RERR are all those
   that belong to a precursor list of at least one of the unreachable
   destination(s) in the newly created RERR. In case there is only one
   unique neighbor that needs to receive the RERR, the RERR SHOULD be
   unicast to that destination.  Otherwise the RERR is typically sent
   to the local broadcast address (Destination IP == 255.255.255.255,
   TTL == 1) with the unreachable destinations, and their corresponding
   destination sequence numbers, included in the packet.  The DestCount
   field of the RERR packet indicates the number of unreachable
   destinations included in the packet.

   Just before transmitting the RERR, certain updates are made on the
   routing table that may affect the destination sequence numbers for
   the unreachable destinations.  For each one of these destinations,
   the corresponding routing table entry is updated as follows:

    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, if it
       exists, is incremented by one for cases (i) and (ii) above, and
       copied from the incoming RERR in case (iii) above.

    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 in the routing table plays dual role
   -- for an active route it is the expiry time, and for an invalid
   route it is the deletion time.  If a data packet is received for an
   invalid route, the Lifetime field is updated to current time plus
   DELETE_PERIOD. The determination of DELETE_PERIOD is discussed in
   Section 10.


6.12. 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
   was no farther than MAX_REPAIR_TTL hops away.  To repair the link
   break, the node 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 * #hops to originator) +
      LOCAL_ADD_TTL.

   Thus, local repair attempts should never be visible to the
   originating node, and will always have TTL >= MIN_REPAIR_TTL
   + LOCAL_ADD_TTL. The node initiating the repair then waits the



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   discovery period to receive RREPs in response to the RREQ. If, at
   the end of the discovery period, it has not received a RREP for that
   destination, it proceeds as described in Section 6.11 by transmitting
   a RERR message for that destination.

   On the other hand, if the node receives one or more RREPs during the
   discovery period, it proceeds as described in Section 6.7, updating
   its 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 SHOULD create a RERR message for the destination, with the
   'N' bit set.

   A node that 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 originating 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 originating node MAY choose to reinitiate route discovery, as
   described in Section 6.3.

   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 that are
   able to be delivered to the destinations, since data packets will not
   be dropped as the RERR travels to the originating node.  Sending a
   RERR to the originating node after locally repairing the link break
   may allow the originator to find a fresh route to the destination
   that is better, based on current node positions.  However, it
   does not require the originating node to rebuild the route, as the
   originator may be done, or nearly done, with the data session.

   When a link breaks along an active route, there are often multiple
   destinations that become unreachable.  The node that is upstream of
   the broken link tries an immediate local repair for only the one
   destination towards which the data packet was traveling.  Other
   routes using the same link MUST be marked as broken, but the node
   handling 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 (e.g., 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




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   repairs for the other routes, without waiting for new packets to
   arrive.


6.13. Actions After Reboot

   A node participating in the ad hoc network must take certain actions
   after reboot as it might lose all sequence number records for all
   destinations, including its own sequence number.  However, there
   may be neighboring nodes that 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. During
   this time, the node does not transmit any RREP messages.  If the
   node receives a RREQ, RREP, or RERR control packet, it SHOULD create
   route entries as appropriate given the sequence number information
   in the control packets.  If the node receives a data packet for
   some other destination, it MUST broadcast a RERR as described in
   subsection 6.11 and reset the waiting timer to expire after current
   time plus DELETE_PERIOD.

   It can be shown [1] 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.


6.14. 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 retransmitting a RREQ
   message rebroadcasts that message on all interfaces that have been
   configured for operation in the ad-hoc network, except those on which
   it is known that all of the nodes neighbors have already received
   the RREQ For instance, for some broadcast media (e.g., Ethernet) it
   may be presumed that all nodes on the same link receive a brodacast
   message at the same time.  When a node needs to transmit a RERR, it



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   should only transmit it on those interfaces that have precursor nodes
   for that route.


7. 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
   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.


8. 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 7) 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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9. 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 to
   the rules for handling IPv6 options.


9.1. 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.

   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 6.9).







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9.2. Timestamp 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     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Timestamp in NTP Format                    +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type     3

      Length   8

      Timestamp
               The number of seconds and fractional seconds since the
               Timestamp Extension was added to the control message as
               transmitted by the originator (e.g., of a RREQ message).

   The Timestamp value is structured according to the format for NTP
   timestamps specified in RFC 2030 [5].  For convenience, the following
   text is taken from that document, but should not be used as a
   substitute for consulting RFC 2030 for details.

   NTP timestamps are represented as a 64-bit unsigned fixed-point
   number, in seconds relative to 0h on 1 January 1900.  The integer
   part is in the first 32 bits and the fraction part in the last 32
   bits.  In the fraction part, the non-significant low order can be
   set to 0.  It is advisable to fill the non-significant low order
   bits of the timestamp with a random, unbiased bitstring, both to
   avoid systematic roundoff errors and as a means of loop detection and
   replay detection (see below).  One way of doing this is to generate a
   random bitstring in a 64-bit word, then perform an arithmetic right
   shift a number of bits equal to the number of significant bits of the
   timestamp, then add the result to the original timestamp.


10. Configuration Parameters

   This section gives default values for some important parameters
   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



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   the protocol.  The configured value for MY_ROUTE_TIMEOUT MUST be at
   least 2 * REV_ROUTE_LIFE.

      Parameter Name           Value
      ----------------------   -----
      ACTIVE_ROUTE_TIMEOUT     3,000 Milliseconds
      ALLOWED_HELLO_LOSS       2
      BLACKLIST_TIMEOUT        RREQ_RETRIES * NET_TRAVERSAL_TIME
      DELETE_PERIOD            see note below
      HELLO_INTERVAL           1,000 Milliseconds
      LOCAL_ADD_TTL            2
      MAX_REPAIR_TTL           0.3 * NET_DIAMETER
      MIN_REPAIR_TTL           see note below
      MY_ROUTE_TIMEOUT         2 * ACTIVE_ROUTE_TIMEOUT
      NET_DIAMETER             35
      NEXT_HOP_WAIT            NODE_TRAVERSAL_TIME + 10
      NODE_TRAVERSAL_TIME      40
      NET_TRAVERSAL_TIME       3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2
      PATH_DISCOVERY_TIME 2 * NET_TRAVERSAL_TIME2RREQ_RETRIES
      TTL_START                1
      TTL_INCREMENT            2
      TTL_THRESHOLD            7


   The MIN_REPAIR_TTL should be the last known hop count to
   the destination.  If Hello messages are used, then the
   ACTIVE_ROUTE_TIMEOUT parameter value MUST be more than the
   value (ALLOWED_HELLO_LOSS * HELLO_INTERVAL).

   DELETE_PERIOD should be an upper bound on the time for which an
   upstream node A can have a neighbor B as an active next 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.  If Hello messages are used to determine the continued
   availability of links to next hop nodes, DELETE_PERIOD must be at
   least ALLOWED_HELLO_LOSS * HELLO_INTERVAL. If the link layer feedback
   is used to detect loss of link, DELETE_PERIOD must be at least
   ACTIVE_ROUTE_TIMEOUT. 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 time, say, a multiple K of HELLO_INTERVAL.
   In other words, a node will invariably receive at least one out
   of K subsequent Hello messages from a neighbor if the link is
   working and the neighbor is sending no other traffic.  Covering all
   possibilities,





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DELETE_PERIOD = K * max (ACTIVE_ROUTE_TIMEOUT, 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 [4] 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.  BLACKLIST_TIMEOUT should be suitably increased
   if an expanding ring search is used.  In such cases, it should be
   [(TTL_THRESHOLD - TTL_START)/TTL_INCREMENT] + 1 + RREQ_RETRIES. This
   is to account for possible additional route discovery attempts.


11. Security Considerations

   Currently, AODV does not specify any special security measures.
   Route protocols, however, are prime targets for impersonation
   attacks.  If there is danger of such attacks, AODV control messages
   must be protected by use of authentication techniques, such as those
   involving generation of unforgeable and cryptographically strong
   message digests or digital signatures.  In particular, RREP messages
   SHOULD be authenticated to avoid creation of spurious routes to a
   desired destination.  Otherwise, an attacker could masquerade as the
   desired destination, and maliciously deny service to the destination
   and/or maliciously inspect and consume traffic intended for delivery
   to the destination.  RERR messages, while less dangerous, SHOULD be
   authenticated in order to prevent malicious nodes from disrupting
   valid routes between nodes that are communication partners.

   Since AODV does not make any assumption about the nature of the
   address assignment to the mobile nodes except that they are presumed
   to have unique IP addresses, no definite statements can be made about
   the applicability of IPsec authentication headers or key exchange
   mechanisms.  However, if the mobile nodes in the ad hoc network have
   pre-established security associations, they should be able to use the
   same authentication mechanisms based on their IP addresses as they
   would have used otherwise.








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12. 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
   Sung-Ju Lee (especially regarding local repair), Mahesh Marina, Erik
   Nordstrom (who provided text for section 6.11), Yves Prelot, Manel
   Guerrero Zapata, Philippe Jacquet, Ian Chakeres, and Fred Baker.


References

   [1] Karthikeyan Bhargavan, Carl A. Gunter, and Davor Obradovic.
       Fault Origin Adjudication.  In Proceedings of the Workshop on
       Formal Methods in Software Practice, Portland, OR, August 2000.

   [2] S. Bradner.  Key words for use in RFCs to Indicate Requirement
       Levels.  Request for Comments (Best Current Practice) 2119,
       Internet Engineering Task Force, March 1997.

   [3] J. Manner et al.  Mobility Related Terminology (work in
       progress).  draft-manner-seamoby-terms-02.txt, July 2001.

   [4] IEEE 802.11 Committee, AlphaGraphics #35, 10201 N.35th Avenue,
       Phoenix AZ 85051.  Wireless LAN Medium Access Control MAC and
       Physical Layer PHY Specifications, June 1997.  IEEE Standard
       802.11-97.

   [5] D. Mills.  Simple Network Time Protocol (SNTP) Version 4 for
       IPv4, IPv6 and OSI.  Request for Comments (Informational) 2030,
       Internet Engineering Task Force, October 1996.













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A. Draft Modifications

   The following are major changes between this version (10) of the AODV
   draft and the previous version (09):

    -  Specified that the next hop towards the originator of a RREQ
       must be added to the precursor list for the destination, when an
       intermediate node sends a Gratuitous RREP to the next hop towards
       that destination (see section 6.6.3).

    -  Specified that sequence numbers are to be compared as signed
       integers.

    -  Clarified that "broadcast" means transmission to 255.255.255.255,
       and replaced terminology about "flooding" by "network-wide route
       discovery", since that is what AODV does.

    -  In line with last point, replaced "Flooding ID" by "RREQ ID", and
       FLOOD_RECORD_TIME by RREQ_RECORD_TIME.

    -  Changed name of "Source IP Address" field to be "Originator
       IP Address" in RREQ message format, and changed the ``Source
       Sequence Number'' field to be the ``Originator Sequence Number''
       field in the RREQ and RREP message formats.

    -  Clarified that RREQ messages do not have to be rebroadcast over
       some types of network interfaces, when it may be presumed that
       all nodes reachable from the network interface have already
       received the same incoming RREQ message as the node processing
       the RREQ (see section 6.14).

    -  Made section 4-7 in version 09 subsections of one section in
       version 10.

    -  Changed the Lifetime field in section 6.2 to be set to
       HELLO_INTERVAL * ALLOWED_HELLO_LOSS on reception of a control
       packet.

    -  Added that the lifetime for the route to the next hop towards a
       destination should be updated when a data packet is forwarded to
       that node.

    -  Updated the calculation of MinimalLifetime in section 6.5.

    -  Clarified section 6.11.

    -  Added a definition for the timestamp extension field.





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    -  Introduced a new parameter, PATH_DISCOVERY_TIME, to replace the
       former RREQ_RECORD_TIME, REV_ROUTE_LIFE, and RREP_WAIT_TIME
       parameters.


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. Belding-Royer
      Dept. of Computer Science
      University of California, Santa Barbara
      Santa Barbara, CA 93106
      +1 805 893 3411
      +1 805 893 8553 (fax)
      ebelding@cs.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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