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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 June 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-11.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@ietf.org 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                                                    5
     4.1. Route Request (RREQ) Message Format . . . . . . . . . . .    5
     4.2. Route Reply (RREP) Message Format . . . . . . . . . . . .    6
     4.3. Route Error (RERR) Message Format . . . . . . . . . . . .    8
     4.4. Route Reply Acknowledgment (RREP-ACK) Message Format  . .    9

 5. AODV Operation                                                     9
     5.1. Maintaining Sequence Numbers  . . . . . . . . . . . . . .    9
     5.2. Route Table Entries and Precursor Lists . . . . . . . . .   11
     5.3. Generating Route Requests . . . . . . . . . . . . . . . .   12
     5.4. Controlling Dissemination of Route Request Messages . . .   13
     5.5. Processing and Forwarding Route Requests  . . . . . . . .   13
     5.6. Generating Route Replies  . . . . . . . . . . . . . . . .   15
           5.6.1. Route Reply Generation by the Destination . . . .   15
           5.6.2. Route Reply Generation by an Intermediate Node  .   16
           5.6.3. Generating Gratuitous RREPs . . . . . . . . . . .   16
     5.7. Receiving and Forwarding Route Replies  . . . . . . . . .   17
     5.8. Operation over Unidirectional Links . . . . . . . . . . .   18
     5.9. Hello Messages  . . . . . . . . . . . . . . . . . . . . .   19
    5.10. Maintaining Local Connectivity  . . . . . . . . . . . . .   20
    5.11. Route Error Messages, Route Expiry and Route Deletion . .   21
    5.12. Local Repair  . . . . . . . . . . . . . . . . . . . . . .   22
    5.13. Actions After Reboot  . . . . . . . . . . . . . . . . . .   24
    5.14. Interfaces  . . . . . . . . . . . . . . . . . . . . . . .   24

 6. AODV and Aggregated Networks                                      25

 7. Using AODV with Other Networks                                    25

 8. Extensions                                                        26
     8.1. Hello Interval Extension Format . . . . . . . . . . . . .   26

 9. Configuration Parameters                                          27




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10. Security Considerations                                           28

11. IPv6 Considerations                                               29

12. Acknowledgments                                                   29

 A. Draft Modifications                                               31


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
   lost 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 via 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 a valid
   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 5.6).

   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 short-lived 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
       -  Vaild Destination Sequence Number
       -  Interface
       -  Hop Count (number of hops needed to reach destination)
       -  Next Hop
       -  List of Precursors (described in Section 5.2)
       -  Lifetime (expiration or deletion time of the route)
       -  Routing Flags
       -  State

   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 marking the route table entry state as
   invalid.  See section 5.1 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 route towards a destination that has a routing table entry
         that is marked as valid.  Only active routes can be used to
         forward data packets.

      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.

      destination

         An IP address to which data packets are to be transmitted.
         Same as "destination node".  A node knows it is the destination
         node for a data packet when its address appears in the
         appropriate field of the IP header.  Routes for destination
         nodes are supplied by action of the AODV protocol, which



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         carries the IP address of the destination node in route
         discovery messages.

      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.

      forward route

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

      invalid route

         A route that has expired, denoted by a state of invalid in
         the routing table.  An invalid route is used to store the
         previously valid route information for an extended period
         of time.  An invalid route may not be used to forward data
         packets.

      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.

      sequence number

         An increasing number maintained by each originating node.  When
         used in control messages it is used by other nodes to determine
         the freshness of the information contained from the originating
         node.

      valid route

         See active route.





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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|D|U|   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.

      G              Gratuitous RREP flag; indicates whether a
                     gratuitous RREP should be unicast to the node
                     specified in the Destination IP Address field (see
                     sections 5.3, 5.6.3)

      D              Destination only flag; indicates only the
                     destination may respond to this RREQ (see
                     section 5.5).

      U              Unknown sequence number; indicates the destination
                     sequence number is unknown(see section 5.3).

      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.



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      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                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 4.4 and 5.7.

      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




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                    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 in milliseconds 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 6 for
   details.

   The 'A' bit is used in cases where the link over which the RREP
   message is sent may be unreliable or unidirectional.  When the
   RREP message contains the 'A' bit set, the receiver of the RREP is
   expected to return a RREP-ACK message.  See section 5.8.
















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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 5.2 for information about how to maintain the appropriate
   records for this determination, and section 5.11 for specification
   about how to create the list of destinations.







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

   The Route Reply Acknowledgment (RREP-ACK) message MUST be sent in
   response to a RREP message with the 'A' bit set (see section 4.2.
   This is typically done when there is danger of unidirectional
   links preventing the completion of a Route Discovery cycle (see
   section 5.8).

    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.


5. 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.


5.1. Maintaining Sequence Numbers

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



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    -  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.
   To accomplish sequence number rollover, if the sequence number has
   already been assigned to be the largest possible number representable
   as a 32-bit unsigned integer (i.e., 4294967295), then when it is
   incremented it will then have a value of zero (0).  On the other
   hand, if the sequence number currently has the value 2147483647,
   which is the largest possible positive integer if 2's complement
   arithmetic is in use with 32-bit integers, 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).

   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, this is
   necessary to accomplish sequence number rolloever.  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 lost or expired link to the next hop towards that
   destination.  The node determines which destinations use a particular
   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 marks the route as invalid (see also sections 5.11, 5.12).
   Whenever any fresh enough (i.e., containing a sequence number at
   least equal to the recorded sequence number) routing information for
   an affected destination is received by a node that has marked that
   route table entry as invalid, the node SHOULD update its route table
   information according to the information contained in the update.

   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



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    -  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.


5.2. Route Table Entries and Precursor Lists

   When a node receives an AODV control packet from a neighbor, or
   creates or updates a route for a particular destination, it checks
   its route table for an entry for the destination.  In the event
   that there is no corresponding entry for that destination, an entry
   is created.  The sequence number is either determined from the
   information contained in the control packet, or else the valid
   sequence number field is set to false.  The route is only updated if
   the new sequence number is either

      (i)       higher than the destination sequence number in the route
                table, or

      (ii)      the sequence numbers are equal, but the hop count (of
                the new information) plus one, is smaller than the
                existing hop count in the routing table, or

      (iiI)     the sequence number is unknown.

   The Lifetime field of the routing table entry is either
   determined from the control packet, or it is initialized to
   ACTIVE_ROUTE_TIMEOUT. This route may now be used to send any queued
   data packets and fufills any outstanding route requests.

   Each time a route is used to forward a data packet, its Active
   Route Lifetime field of the source, 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.

   For each valid route maintained by a node 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.






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5.3. Generating Route Requests

   A node disseminates 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 marked as invalid.  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, the unknown sequence number flag MUST
   be set.  The Originator Sequence Number in the RREQ message is the
   node's own sequence number, which is incremented prior to insertion
   in a RREQ. The RREQ ID field is incremented by one from the 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_DISCOVERY_TIME. 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 5.6) 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 5.6.3 for details about actions taken by the
   intermediate node in response to a RREQ with the `G' flag set.

   A node SHOULD NOT generate more than RREQ_RATELIMIT RREQ messages
   per second.  After broadcasting a RREQ, a node waits for a RREP (or
   other control message with current information regarding a route to
   the appropriate destination).  If a route is not received within
   NET_TRAVERSAL_TIME milliseconds, the node MAY try again to discover a
   route by broadcasting another RREQ, up to a maximum of RREQ_RETRIES
   times at the maximum TTL value.  Each new attempt MUST increment and
   update the RREQ ID. For each attempt, the TTL field of the IP header
   is set according to the mechanism specified in section 5.4, in order
   to enable control over how far the RREQ is disseminated for the each
   retry.

   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



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   "first-in, first-out" (FIFO). If a route discovery has been attempted
   RREQ_RETRIES times at the maximum TTL without receiving any RREP, all
   data packets destined for the corresponding destination SHOULD be
   dropped from the buffer and a Destination Unreachable message SHOULD
   be delivered to the application.


5.4. Controlling Dissemination of Route Request Messages

   To prevent unnecessary network-wide dissemination of RREQs, the
   originating node SHOULD use an expanding ring search technique.  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 NET_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 RREP is calculated as described in Section 5.4.  When
   it is desired to have all retries traverse the entire ad hoc network,
   this can be achieved by configuring TTL_START and TTL_INCREMENT both
   to be the same value as NET_DIAMETER.

   The Hop Count stored in an invalid routing table entry indicates
   the last known hop count to that destination 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 the 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.

   Timeouts MAY be more accurately determined dynamically via
   measurement, instead of using a statically configured value related
   to NODE_TRAVERSAL_TIME.

   An expired routing table entry SHOULD NOT be expunged before
   (current_time + DELETE_PERIOD) (see section 5.11).  Otherwise, the
   soft state corresponding to the route (e.g., last known 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 + 2 * NET_TRAVERSAL_TIME).


5.5. Processing and Forwarding Route Requests

   When a node receives a RREQ, it first creates or updates a route to
   the previous hop without a valid sequence number (see section 5.2)
   then checks to determine whether it has received a RREQ with
   the same Originator IP Address and RREQ ID within at least the



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   last PATH_DISCOVERY_TIME. 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.

   First, it first increments the hop count value in the RREQ by one,
   to account for the new hop through the intermediate node.  Then the
   node creates or updates a reverse route to the Originator IP Address
   (see section 5.2) using the Originator Sequence Number from the RREQ
   in its 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 on the route are also carried out:

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

    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;

   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 + 2*NET_TRAVERSAL_TIME -
                           2*HopCount*NODE_TRAVERSAL_TIME).

   The current node can now begin using the reverse route to forward
   data packets.

   The node generates a RREP (as discussed further in section 5.6) if
   either:

      (i)       it is itself the destination (see section 5.6.1), or

      (ii)      it has an active route to the destination, the
                destination sequence number in the node's existing route
                table entry for the destination is valid and greater
                than or equal to the Destination Sequence Number of the
                RREQ (comparison using signed 32-bit arithmetic), and
                the ``destination only'' ('D') flag is NOT set.  See
                section 5.6.2 for further information about generating
                the RREP in this case.





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   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 5.14).  To update the RREQ,
   the TTL or hop limit field in the outgoing IP header is decreased
   by one, and the Hop Count field in the RREQ message is incremented
   by one, to account for the new hop through the intermediate node.
   Lastly, the Destination Sequence number for the requested destination
   is set to the maximum of the corresponding value received in the RREQ
   message, and the destination sequence value currently maintained by
   the node for the requested destination.  However, the forwarding node
   MUST NOT modify its maintained value for the destination sequence
   number, even if the value received in the incoming RREQ is larger
   than the value currently maintained by the forwarding node.


5.6. 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 fresh enough 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.


5.6.1. Route Reply Generation by the Destination

   If the generating node is the destination itself, it MUST increment
   its own sequence number by one if the sequence number in the
   RREQ packet is equal to that incremented value.  Otherwise, the
   destination does not change its sequence number before generating
   the RREP message.  The destination node places its (perhaps newly
   incremented) sequence number into the Destination Sequence Number
   field of the RREP, and enters the value zero in the Hop Count field
   of the RREP.



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   The destination node copies the value MY_ROUTE_TIMEOUT (see
   section 9) into the Lifetime field of the RREP. Each node MAY
   reconfigure its value for MY_ROUTE_TIMEOUT, within mild constraints
   (see section 9).


5.6.2. 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 known sequence number for the
   destination into the Destination Sequence Number field in the RREP
   message.

   The intermediate node updates the forward 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 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) 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.


5.6.3. 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, the destination does not learn of a route to the
   originating node.  This could cause the destination to initiate a
   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.






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   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

      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.


5.7. Receiving and Forwarding Route Replies

   When a node receives a RREP message, it first creates or updates
   a route to the previous hop without a valid sequence number (see
   section 5.2) then increments the hop count value in the RREP by one,
   to account for the new hop through the intermediate node.  Call this
   incremented value the "New Hop Count".  Then the forward route for
   this destination is created if it does not already exist.  Otherwise,
   the node compares the Destination Sequence Number in the message with
   its own stored destination sequence number for the Destination IP
   Address in the RREP message.  Upon comparison, the existing entry is
   updated only if either

      (i)       the sequence number in the routing table is invalid in
                route table entry.

      (ii)      the Destination Sequence Number in the RREP is greater
                than the node's copy of the destination sequence number
                and the known value is valid, or

      (iii)     the sequence numbers are the same, but the route is no
                longer active, or




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      (iiii)    the sequence numbers are the same, and the New Hop Count
                is smaller than the hop count in route table entry.

   If either the route table entry to the destination is created or
   updated, 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 New Hop Count;
   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 that route table entry.  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 5.8).

   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)). Finally, the precursor list for the next hop
   towards the destination is updated to contain the next hop towards
   the source.


5.8. 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 route discovery after a timeout (see
   section 5.3).  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



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   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 9).  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 5.3.

   Note that the RREP-ACK packet does not contain any information about
   which RREP it is acknowledging.  The time at which the RREP-ACK is
   received will likely come just after the time when the RREP was sent
   with the 'A' bit.  This information is expected to be sufficient
   to provide assurance to the sender of the RREP that the link is
   currently bidirectional.  However, that assurance cannot be always
   expected to remain permanently.


5.9. Hello Messages

   A node MAY offer connectivity information by broadcasting local Hello
   messages.  A node SHOULD only use hello messages if it is part of an
   active route.  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

   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 lost.  When this
   happens, the node SHOULD proceed as in Section 5.11.



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   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.  The current node can now
   begin using this route to forward data packets.  Routes that are
   created by hello messages and not used by any other active routes
   will have empty precursor lists and would not trigger a RERR message
   when the neighbor moves away and a neighbor timeout occurs.

   Also, whenever a node receives any control packet it has the same
   meaning as the reception of an explicit Hello message, in that it
   signifies an active connection to the node indicated by the Source IP
   Address of the IP header of the control message packet.


5.10. 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.

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



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        *  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 lost, and take
   corrective action by following the methods specified in Section 5.11.


5.11. 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.
   With that understanding, a node SHOULD NOT generate more than
   RERR_RATELIMIT RERR messages per second.

   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 while transmitting data, or

      (ii)      if it gets a data packet destined to a node for which it
                does not have an active route and is not repairing (if
                using local repair), 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.

   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



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   one unique neighbor that needs to receive the RERR, the RERR
   SHOULD be unicast toward 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 destination sequence number of this routing entry, if it
       exists and is valid, is incremented for cases (i) and (ii) above,
       and copied from the incoming RERR in case (iii) above.

    2. The entry is invalidated by marking the route entry as invalid

    3. The Lifetime field is updated to current time plus DELETE_PERIOD.
       Before this time, the entry SHOULD 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 9.


5.12. 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) + LOCAL_ADD_TTL,

   where #hops is the number of hops to the sender (originator) of the
   currently undeliverable packet.  Thus, local repair attempts will
   often be invisible to the originating node, and will always have TTL
   >= MIN_REPAIR_TTL + LOCAL_ADD_TTL. The node initiating the repair
   then waits the discovery period to receive RREPs in response to the
   RREQ. During local repair data packets SHOULD be buffered.  If, at
   the end of the discovery period, it has not received a RREP (or other
   control message creating or updating the route) for that destination,



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   it proceeds as described in Section 5.11 by transmitting a RERR
   message for that destination.

   On the other hand, if the node receives one or more RREPs (or
   other control message creating or updating the route to the desired
   destination) during the discovery period, it first compares the hop
   count of the new route with the value in the hop count field of the
   invalid 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 the node SHOULD issue a RERR
   message for the destination, with the 'N' bit set.  Then it proceeds
   as described in Section 5.7, updating its route table entry for that
   destination.

   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 5.3.

   Local repair of link breaks in 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 lost 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 invalid, but the node
   handling the local repair MAY flag each such newly lost 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
   repairs for the other routes, without waiting for new packets to
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5.13. 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, but MUST not forward any control packets.  If the
   node receives a data packet for some other destination, it SHOULD
   broadcast a RERR as described in subsection 5.11 and MUST 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.


5.14. 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 broadcast
   message at the same time.  When a node needs to transmit a RERR, it
   SHOULD only transmit it on those interfaces that have precursor nodes
   for that route.





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6. 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.

   If several nodes in the subnet advertise reachability to the subnet
   defined by the subnet prefix, the node with the lowest IP address
   is elected to be the subnet leader, and all other nodes MUST stop
   advertising reachability.

   The behavior of default routes (i.e., routes with routing prefix
   length 0) is not defined in this specification.  Selection of routes
   sharing prefix bits should be according to longest match first.


7. 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 6) 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



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   means outside the scope of this specification) to provide consistent
   AODV semantics for ad hoc access to those subnets.


8. Extensions

   In this section, the format of extensions to the RREQ and RREP
   messages is specified.  All such extensions appear after the message
   data, and have 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     1-255

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

   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.


8.1. 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




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   to wait for subsequent such RREP messages (i.e., Hello messages; see
   section 5.9).


9. 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, 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.
   Changing NODE_TRAVERSAL_TIME also changes the node's estimate
   of the NET_TRAVERSAL_TIME, and so can only be done with suitable
   knowledge about the behavior of other nodes in the ad hoc network.
   The configured value for MY_ROUTE_TIMEOUT MUST be at least 2 *
   PATH_DISCOVERY_TIME.

      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       2 * NODE_TRAVERSAL_TIME * NET_DIAMETER
      PATH_DISCOVERY_TIME      2 * NET_TRAVERSAL_TIME
      RERR_RATELIMIT           10
      RREQ_RETRIES             2
      RREQ_RATELIMIT           10
      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).




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   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,

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 queuing 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 *
   NET_TRAVERSAL_TIME. This is to account for possible additional route
   discovery attempts.


10. 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



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   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.


11. IPv6 Considerations

   See [6] for detailed operation for IPv6.  The only changes to the
   protocol are that the address fields are enlarged.


12. Acknowledgments

   Special thanks to Ian Chakeres, UCSB, for his extensive suggestions
   and contributions to this revision.

   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 5.11), Yves Prelot, Marc
   Mosko, Manel Guerrero Zapata, Philippe Jacquet, 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.



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   [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.

   [6] C. Perkins, E. Royer, and S. Das.  Ad Hoc On Demand Distance
       Vector (AODV) Routing for IP version 6 (work in progress).
       Internet Draft, Internet Engineering Task Force.
       draft-perkins-manet-aodv6-01.txt, November 2001.
































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A. Draft Modifications

   The following are major changes between this version (11) of the AODV
   draft and the previous version:

    -  Added definitions for valid route, invalid route and sequence
       number.

    -  Re-added discussion in processing and forwarding RREQ that it is
       necessary to update the destination sequence number in a RREQ
       being forwarded to the maximum known sequence number.

    -  Added ``destination only'' ('D') flag.

    -  Specify hello messages should only be used by nodes on active
       routes.

    -  Clarify RERR messages are generated only in response failing to
       send data.

    -  Removed term ``broken''.

    -  Routes should be created or updated to the previous hop when a
       control message is received.

    -  Added comments near route creation instructions that routes may
       be used once created and should cancel further route discovery or
       local repair for those destinations.

    -  Removed NTP timestamp extension

    -  Added ``route state'' field to routing table.

    -  Removed ``last hop count'' field from routing table.

    -  Removed ``infinite hop count'' from draft.

    -  Added ``unknown sequence number'' flag to RREQ.

    -  Restructured Route Table Entries and Precursor Lists (section 5.2
       to inlcude how to create and update routes.

    -  Route Lifetime to source must be updated when forwarding data
       packets.

    -  Specify that during reboot no control messages should be
       forwarded.





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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. Belding-Royer
      Department 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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