[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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
9 November 2001                               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-09.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
   between sources and destinations.  It uses destination sequence
   numbers to ensure loop freedom at all times (even in the face of
   anomalous delivery of routing control messages), solving problems
   (such as ``counting to infinity'') associated with classical distance
   vector protocols.








Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page i]


Internet Draft                   AODV                    9 November 2001




                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Overview                                                           1

 3. AODV Terminology                                                   3

 4. Route Request (RREQ) Message Format                                4

 5. Route Reply (RREP) Message Format                                  6

 6. Route Error (RERR) Message Format                                  7

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

 8. AODV Operation                                                     8
     8.1. Maintaining Sequence Numbers  . . . . . . . . . . . . . .    8
     8.2. Maintaining Route Table Entries and Route Utilization
             Records  . . . . . . . . . . . . . . . . . . . . . . .    9
     8.3. Generating Route Requests . . . . . . . . . . . . . . . .   10
     8.4. Controlling Dissemination of Route Request Messages . . .   11
     8.5. Processing and Forwarding Route Requests  . . . . . . . .   12
     8.6. Generating Route Replies  . . . . . . . . . . . . . . . .   13
           8.6.1. Route Reply Generation by the Destination . . . .   14
           8.6.2. Route Reply Generation by an Intermediate Node  .   14
           8.6.3. Generating Gratuitous RREPs . . . . . . . . . . .   15
     8.7. Forwarding Route Replies  . . . . . . . . . . . . . . . .   15
     8.8. Operation over Unidirectional Links . . . . . . . . . . .   16
     8.9. Hello Messages  . . . . . . . . . . . . . . . . . . . . .   17
    8.10. Maintaining Local Connectivity  . . . . . . . . . . . . .   18
    8.11. Route Error Messages  . . . . . . . . . . . . . . . . . .   18
    8.12. Local Repair  . . . . . . . . . . . . . . . . . . . . . .   20
    8.13. Route Expiry and Deletion . . . . . . . . . . . . . . . .   21
    8.14. Actions After Reboot  . . . . . . . . . . . . . . . . . .   22
    8.15. Interfaces  . . . . . . . . . . . . . . . . . . . . . . .   22

 9. AODV and Aggregated Networks                                      23

10. Using AODV with Other Networks                                    23

11. Extensions                                                        24



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page ii]


Internet Draft                   AODV                    9 November 2001


    11.1. Hello Interval Extension Format . . . . . . . . . . . . .   24

12. Configuration Parameters                                          25

13. Security Considerations                                           26

14. Acknowledgments                                                   27

 A. Draft Modifications                                               28


1. Introduction

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

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


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 source 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 that
   certain messages (e.g., RREQ) have to be disseminated widely,
   perhaps throughout the ad hoc network.  The range of dissemination




Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 1]


Internet Draft                   AODV                    9 November 2001


   of such flooded RREQs is indicated by the TTL in the IP header.
   Fragmentation is typically not required.

   As long as the endpoints of a communication connection have valid
   routes to each other, AODV does not play any role.  When a route to
   a new destination is needed, the node uses a broadcast RREQ to find
   a route to the destination.  A route can be determined when the RREQ
   reaches either the destination itself, or an intermediate node with
   a 'fresh enough' route to the destination.  A 'fresh enough' route
   is an unexpired route entry for the destination whose associated
   sequence number is at least as great as that contained in the RREQ.
   The route is made available by unicasting a RREP back to the source
   of the RREQ. Each node receiving the request caches a route back to
   the originator of the request, so that the 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 which destinations 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 which 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 8.6).

   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 source of such an 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 source 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 uses the following fields with each route
   table entry:

    -  Destination IP Address
    -  Destination Sequence Number
    -  Interface



Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 2]


Internet Draft                   AODV                    9 November 2001


    -  Hop Count (number of hops needed to reach destination)
    -  Last Hop Count (described in subsections 8.4 and 8.11)
    -  Next Hop
    -  List of Precursors (described in Section 8.2)
    -  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 node detecting the condition increments the
   destination's sequence number and the metric in the route table entry
   is assigned to be infinite.  See section 8.1 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
         flooding.

      flood

         Flooding means to send a message to every node of the ad hoc
         network, or to every node in an region of the ad hoc network.
         In AODV, a message is flooded by iterated use of broadcast, for
         which receivers must also rebroadcast after their processing
         steps have been completed for that message.







Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 3]


Internet Draft                   AODV                    9 November 2001


      forwarding node

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

      forward route

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

      originating node

         A node which initiates an AODV message which is the processed
         and possibly retransmitted by other nodes in the ad hoc
         network.  For instance, the node initiating a Route Discovery
         process and flooding 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
         source from the destination or from an intermediate node having
         a route to the destination.


4. Route Request (RREQ) Message Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |J|R|G|       Reserved          |   Hop Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Flooding ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination IP Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source IP Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Source Sequence Number                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Type           1



Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 4]


Internet Draft                   AODV                    9 November 2001


      J              Join flag; reserved for multicast.

      R              Repair flag; reserved for multicast.

      G              Gratuitous RREP flag; indicates whether a
                     gratuitous RREP should be unicast to the node
                     specified in the Destination IP Address field (see
                     sections 8.3, 8.6.3)

      Reserved       Sent as 0; ignored on reception.

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

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

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

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

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

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


















Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 5]


Internet Draft                   AODV                    9 November 2001


5. Route Reply (RREP) Message Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |R|A|    Reserved     |Prefix Sz|   Hop Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination IP address                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source IP address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Lifetime                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Type          2

      R             Repair flag; used for multicast.

      A             Acknowledgment required; see sections 7 and 8.7.

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

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

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

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




Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 6]


Internet Draft                   AODV                    9 November 2001


      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.


6. Route Error (RERR) Message Format

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


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

      Type        3

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

      Reserved    Sent as 0; ignored on reception.

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

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

      Unreachable Destination Sequence Number
                  The last known sequence number, incremented by one,



Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 7]


Internet Draft                   AODV                    9 November 2001


                  of the destination listed in the previous Unreachable
                  Destination IP Address field.

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


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

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


      Type        4

      Reserved    Sent as 0; ignored on reception.

   The RREP-ACK message 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.


8. AODV Operation

   This section describes the scenarios under which nodes generate Route
   Request (RREQ), Route Replie (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 for the route table
   entries for the destinations of interest.

   All AODV messages are sent to port 654 using UDP.


8.1. 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 RREQ flood, it MUST
       increment its own sequence number.  This prevents problems with
       deleted reverse routes to the originator of a RREQ.



Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 8]


Internet Draft                   AODV                    9 November 2001


    -  Immediately before a destination node orginates 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.

   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 information about the
   sequence number from RREQ, RREP, or RERR messages that may be
   received related to that destination.

   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 can easily determine which destinations
   use a broken next hop by consulting its precursor lists for the
   next hop.  In this case, for each destination which 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 8.11, 8.12).

   In summary, a node may change the sequence number for a particular
   destination only if:

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

    -  it receives an AODV message with new information about the
       sequence number for some other destination node

    -  the path towards the destination node expires or breaks.


8.2. Maintaining Route Table Entries and Route Utilization Records

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



Perkins, Belding-Royer, Das         Expires 9 May 2002          [Page 9]


Internet Draft                   AODV                    9 November 2001


   the information contained in the control packet (i.e., the neighbor
   is the source of a RREQ), or else it is initialized to zero if the
   sequence number for that node can not be determined.  The lifetime
   for 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 MY_ROUTE_TIMEOUT. The hopcount to the
   neighbor is set to one.

   Each time a route is used to forward a data packet, its Lifetime
   field is updated to be no less than the current time plus
   ACTIVE_ROUTE_TIMEOUT. Since the route between each source and
   destination pair are expected to be symmetric, the 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.


8.3. Generating Route Requests

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

   Before flooding the RREQ, the source node buffers the Flooding
   ID and the Source IP address (its own address) of the RREQ for
   FLOOD_RECORD_TIME milliseconds.  In this way, when the node receives
   the packet again as it is flooded by its neighbors, it will not
   reprocess and re-forward the packet.

   A source node often expects to have bidirectional communications with
   a destination node.  In such cases, it is not sufficient for the
   source node to have a route to the destination node; the destination
   must also have a route back to the source node.  In order for this
   to happen as efficiently as possible, any generation of an RREP
   by an intermediate node (as in section 8.6) for delivery to the
   source node, should be accompanied by some action which notifies the
   destination about a route back to the source node.  The source node
   selects this mode of operation in the intermediate nodes by setting




Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 10]


Internet Draft                   AODV                    9 November 2001


   the `G' flag.  See section 8.6.3 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 flood the RREQ, up to a maximum of RREQ_RETRIES times.  Each
   new attempt MUST increment the Flooding ID field.

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


8.4. Controlling Dissemination of Route Request Messages

   To prevent unnecessary network-wide floods of RREQs, the source node
   SHOULD use an expanding ring search technique as an optimization.
   In an expanding ring search, the source node initially uses a TTL
   = TTL_START in the RREQ packet IP header and sets the timeout for
   receiving a RREP to 2 * TTL * NODE_TRAVERSAL_TIME milliseconds.  If
   the RREQ times out without a corresponding RREP, the source floods
   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 flood.  Each time, the
   timeout for receiving a RREP is calculated as before.  Each attempt
   increments the Flooding ID field in the RREQ packet.  The RREQ can
   be flooded with TTL = NET_DIAMETER up to a maximum of RREQ_RETRIES
   times.

   When a RREP is received, the Hop Count used in the RREP packet is
   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
   measurements, instead of using a statically configured value related
   to NODE_TRAVERSAL_TIME. To accomplish this, the RREQ may carry the
   timestamp via an extension field as defined in Section 11 to be
   carried back by the RREP packet (again via an extension field).  The
   difference between the current time and this timestamp will determine
   the route discovery latency.  The timeout may be set to be a small
   factor times the average of the last few route discovery latencies




Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 11]


Internet Draft                   AODV                    9 November 2001


   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 8.13).  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 + RREP_WAIT_TIME).


8.5. Processing and Forwarding Route Requests

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

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

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

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

   This reverse route would be needed in case the node receives an
   eventual RREP back to the node which originated the RREQ (identified
   by the Source IP Address).  When the reverse route is created or
   updated, the following actions are carried out:

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

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

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





Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 12]


Internet Draft                   AODV                    9 November 2001


   Under all circumstances whenever a RREQ message is received, the
   Lifetime of the reverse route entry for the source IP address is set
   to be the maximum of (ExistingLifetime, MinimalLifetime), where

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

   After updating the reverse route, the node checks to determine
   whether it has an active route to the destination.  If the node
   does not have an active route, and the incoming IP header has TTL
   larger than 1, it broadcasts the RREQ from all of its configured
   interface(s) (see section 8.15).  The Destination Sequence Number
   in the RREQ is updated to the maximum of the existing Destination
   Sequence Number in the RREQ and the destination sequence number in
   the routing table (if an entry exists) of the current node.  The TTL
   or hop limit field in the outgoing IP header is decreased by one.
   The Hop Count field in the broadcast RREQ message is incremented by
   one, to account for the new hop through the intermediate node.

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

   The node generates a RREP (as discussed further in section 8.6) if
   either:

      (i)       it has an active route to the destination, and the
                node's existing destination sequence number is greater
                than or equal to the Destination Sequence Number of the
                RREQ, or

      (ii)      it is itself the destination.

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


8.6. 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 Source and Destination IP Addresses in RREQ message into the
   corresponding fields in the RREP message which is to be sent back
   toward the source of the RREQ. Additional operations are slightly



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 13]


Internet Draft                   AODV                    9 November 2001


   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
   below.  In either case, the RREP is unicast to the node's next hop en
   route to the originating node.

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


8.6.1. 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 the value zero in the Hop Count field of the RREP.

   The destination node copies the value MY_ROUTE_TIMEOUT (see
   section 12) into the Lifetime field of the RREP. Each node MAY
   reconfigure its value for MY_ROUTE_TIMEOUT, within mild constraints
   (see section 12).


8.6.2. Route Reply Generation by an Intermediate Node

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

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

   The intermediate node 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.






Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 14]


Internet Draft                   AODV                    9 November 2001


8.6.3. Generating Gratuitous RREPs

   After a node receives a RREQ and responds with a RREP, it discards
   the RREQ. If all incarnations of a single RREQ are replied to by
   intermediate nodes, the destination does not receive any copies of
   the RREQ. Hence, it does not learn of a route to the source node.
   This would cause the destination to initiate a route discovery flood,
   if for example the source 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 the session is going to be run over TCP, or if
   the destination should receive the gratuitous RREP for any other
   reason.  If an intermediate node returns a RREP in response to a RREQ
   with the 'G' flag set, it MUST also unicast a gratuitous RREP to the
   destination node.

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

      Hop Count  The Hop Count as received in the RREQ

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

      Destination Sequence Number
                 The Source Sequence Number from the RREQ

      Source IP Address
                 The IP address of the destination node in the RREQ

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

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


8.7. Forwarding Route Replies

   When a node receives a RREP message, it first increments the hop
   count value in the RREP by one, to account for the new hop through
   the intermediate node.  It then 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 or updated only if
   (i) the Destination Sequence Number in the RREP is greater than the
   node's copy of the destination sequence number, or (ii) the sequence
   numbers are the same, but the route is no longer active or the



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 15]


Internet Draft                   AODV                    9 November 2001


   incremented Hop Count in RREP is smaller than the hop count in route
   table entry.  If a new route is created or the old route is updated,
   the next hop is the node from which the RREP is received, which is
   indicated by the source IP address field in the IP header; the hop
   count is the Hop Count in the RREP message plus one; the expiry
   time is the current time plus the Lifetime in the RREP message; the
   destination sequence number is the Destination Sequence Number in the
   RREP message.

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

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

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

   If a node forwards a RREP over a link that is likely to have errors
   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 8.8).


8.8. Operation over Unidirectional Links

   It is possible that a RREP transmission may fail if a RREQ
   transmission may occur over a unidirectional link.  If no other RREP
   generated from the same RREQ flood reaches the source, the source
   will attempt to flood the RREQ after a timeout (see section 8.3).
   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
   source and destination do exist.  In AODV, any node acts on only
   the first RREQ with the same Flooding 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
   an RREP message has failed, it remembers the next-hop of the failed



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 16]


Internet Draft                   AODV                    9 November 2001


   RREP in a ``blacklist'' set.  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.  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 8.3.

   Link layers using broadcast transmissions for RREQ will not be able
   to detect the presence of such unidirectional links.  Such failure
   can be detected via the absence of a link-layer or network-layer
   acknowledgment (e.g., RREP-ACK).


8.9. 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

   A node MAY determine connectivity by listening for packets from
   its set of neighbors.  If it receives no packets for more than
   ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD
   assume that the link to this neighbor is currently broken.  When this
   happens, the node SHOULD proceed as in Section 8.11.

   Whenever a node receives a HELLO packet 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 ACTIVE_ROUTE_TIMEOUT. In any case, the route to the neighbor
   should be updated to contain the latest Destination Sequence Number
   from the HELLO message.  Routes which are newly created from the
   reception of HELLO messages have empty precursor lists, and so
   typically do not trigger RERR messages when the neighbor moves away
   and the neighbor route expires.





Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 17]


Internet Draft                   AODV                    9 November 2001


8.10. Maintaining Local Connectivity

   Each forwarding node SHOULD keep track of its continued connectivity
   to its active next hops (i.e.  which next hops have forwarded,
   or used to forward packets, within the last ACTIVE_ROUTE_TIMEOUT
   milliseconds, as well as neighbors that have transmitted HELLO
   messages within 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.

        *  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 8.11.


8.11. Route Error Messages

   A node initiates a RERR message in three situations:

      (i)       if it detects a link break for the next hop of an active
                route in its routing table (also see section 8.1), 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, or



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 18]


Internet Draft                   AODV                    9 November 2001


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

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

   For case (i), the node first makes a list of destinations which use
   the next hop which has been detected to be broken.  For case (iii),
   the node instead makes the list of affected destinations which use
   the transmitter of the received RERR as the next hop, from among
   those destinations listed in the received RERR message.  Then, in
   either case (i) or (iii), the the node uses the constructed list of
   affected destinations to disseminate information about the broken
   route to the appropriate other nodes; if there are no affected
   destinations, the node does not disseminate the RERR message

   For each one of the affected destinations, the node takes the
   following actions:

      (a)       updates the corresponding destination sequence number(s)
                with the Destination Sequence Number(s) in the packet

      (b)       copies the old value of Hop Count into the Last Hop
                Count field.

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

      (d)       checks the precursor list for each destination for
                emptiness.  If the list is empty, don't follow steps (e)
                -- (g)

      (e)       Otherwise, the node creates or updates the data in a
                RERR message to be transmitted.  Each destination with
                a non-empty precursor list is included as unreachable
                along with its destination sequence numbers

      (f)       transmit the RERR message.  If there is only one
                previous hop that needs to receive the RERR, the node
                SHOULD unicast the RERR to the previous hop.  Otherwise,
                the node SHOULD transmit the RERR message to the IP
                broadcast address.

      (g)       delete the precursor list of each unreachable
                destination





Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 19]


Internet Draft                   AODV                    9 November 2001


   The RERR is locally broadcast (Destination IP == 255.255.255.255,
   TTL == 1) with the unreachable destination(s) and the destination
   sequence number for each one included in the packet.  For case
   (i), the unreachable destinations are the broken next hop, and any
   additional destinations which are now unreachable due to the loss of
   this next hop link.  For case (ii), there is only one unreachable
   destination, which is the destination of the data packet that cannot
   be delivered.  The DestCount field of the RERR packet indicates the
   number of unreachable destinations included in the packet.

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


8.12. 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
   is no farther than MAX_REPAIR_TTL hops away.  To repair the link
   break itself, it increments the sequence number for the destination
   and then floods a RREQ for that destination.  The TTL of the
   broadcast RREQ should initially be set to the following value:
         max(MIN_REPAIR_TTL, 0.5 distance to source) + LOCAL_ADD_TTL.
   Thus, local repair attempts should never be visible to the source
   node, and will always have minimum TTL equal to MIN_REPAIR_TTL
   + LOCAL_ADD_TTL. The node initiating the repair then waits the
   discovery period to receive RREPs in response to the RREQ. If, at
   the end of the discovery period, it has not received a RREP for that
   destination, it proceeds as described in Section 8.11 by transmitting
   a RERR message for that destination.

   On the other hand, if the nodes does receive one or more RREPs
   during the discovery period, the node proceeds as described in
   Section 8.7, 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 which receives a RERR message with the 'N' flag set MUST
   NOT delete the route to that destination.  The only action taken
   should be the retransmission of the message, if the RERR arrived



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 20]


Internet Draft                   AODV                    9 November 2001


   from the next hop along that route, and if there are one or more
   precursor nodes for that route to the destination.  When the source
   node receives a RERR message with the 'N' flag set, if this message
   came from its next hop along its route to the destination then the
   source node MAY choose to reinitiate route discovery, as described in
   Section 8.3.

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

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


8.13. Route Expiry and Deletion

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

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

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

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






Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 21]


Internet Draft                   AODV                    9 November 2001


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

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

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


8.14. 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 which 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 packets, 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 8.11 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.


8.15. 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.



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 22]


Internet Draft                   AODV                    9 November 2001


   When multiple interfaces are available, a node retransmitting a RREQ
   message rebroadcasts that message on all interfaces which have been
   configured for operation in the ad-hoc network.  When a node needs to
   transmit a RERR, it should only transmit it on those interfaces which
   have precursor nodes for that route.


9. AODV and Aggregated Networks

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


10. Using AODV with Other Networks

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

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

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



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 23]


Internet Draft                   AODV                    9 November 2001


11. Extensions

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

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

   where:

      Type     1

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

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


11.1. Hello Interval Extension Format

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

      Type     2

      Length   4

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

   The Hello Interval extension MAY be appended to a RREP message with
   TTL == 1, to be used by a neighboring receiver in determine how long
   to wait for subsequent such RREP messages (i.e., Hello messages; see
   section 8.9).







Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 24]


Internet Draft                   AODV                    9 November 2001


12. Configuration Parameters

   This section gives default values for some important values
   associated with AODV protocol operations.  A particular mobile
   node may wish to change certain of the parameters, in particular
   the NET_DIAMETER, NODE_TRAVERSAL_TIME, MY_ROUTE_TIMEOUT,
   ALLOWED_HELLO_LOSS, RREQ_RETRIES, and possibly the HELLO_INTERVAL. In
   the latter case, the node should advertise the HELLO_INTERVAL in its
   Hello messages, by appending a Hello Interval Extension to the RREP
   message.  Choice of these parameters may affect the performance of
   the protocol.  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
      FLOOD_RECORD_TIME        2 * NET_TRAVERSAL_TIME
      DELETE_PERIOD            see note below
      HELLO_INTERVAL           1,000 Milliseconds
      LOCAL_ADD_TTL            2
      MAX_REPAIR_TTL           0.3 * NET_DIAMETER
      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
      REV_ROUTE_LIFE           NET_TRAVERSAL_TIME
      NET_TRAVERSAL_TIME       3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2
      RREQ_RETRIES             2
      TTL_START                1
      TTL_INCREMENT            2
      TTL_THRESHOLD            7


   The MIN_REPAIR_TTL should be the last known hop count to the
   destination.

   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.  For example, if the link layer feedback is used to detect
   loss of link DELETE_PERIOD must be at least ACTIVE_ROUTE_TIMEOUT.
   If there is no feedback and hello messages must be used,
   DELETE_PERIOD must be at least maximum of ACTIVE_ROUTE_TIMEOUT
   and ALLOWED_HELLO_LOSS * HELLO_INTERVAL. If hello messages are



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 25]


Internet Draft                   AODV                    9 November 2001


   received from a neighbor but data packets to that neighbor are
   lost, (due to temporary link asymmetry, e.g.)  we have to make more
   concrete assumptions about the underlying link layer.  We assume
   that such asymmetry cannot persist beyond a certain certain time,
   say, a multiple K of ALLOWED_HELLO_LOSS * HELLO_INTERVAL. In other
   words, it cannot not be the case that a node receives K subsequent
   hello messages from a neighbor, while that same neighbor fails to
   receive any data packet from the node in this period.  Covering all
   possibilities,

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

   NET_DIAMETER measures the maximum possible number of hops between
   two nodes in the network.  NODE_TRAVERSAL_TIME is a conservative
   estimate of the average one hop traversal time for packets and should
   include queueing delays, interrupt processing times and transfer
   times.  ACTIVE_ROUTE_TIMEOUT SHOULD be set to a longer value (at
   least 10,000 milliseconds) if link-layer indications are used to
   detect link breakages such as in IEEE 802.11 [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 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.


13. 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 which 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



Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 26]


Internet Draft                   AODV                    9 November 2001


   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.


14. Acknowledgments

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

   We also acknowledge the comments and improvements suggested by SJ
   Lee (especially regarding local repair), Mahesh Marina, Yves Prelot,
   Manel Guerrero Zapata, and Philippe Jacquet.


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.











Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 27]


Internet Draft                   AODV                    9 November 2001


A. Draft Modifications

   The following are major changes between this version (09) of the AODV
   draft and the previous version (08):

    -  Added section specifically about sequence number management.

    -  Added the port number 654 to the specification, since it has
       already been allocated.

    -  Rewrote the Security Considerations section to include more
       details about the specific exposures relevant to AODV instead of
       only for routing protocols in general.

    -  Clarified that nodes increment the sequence number for a
       destination on the other side of a broken link at the time the
       link breaks, and not as part of any later message processing.

    -  Clarified that "broadcast" means transmission to 255.255.255.255,
       and "flooding" means iterated broadcast by each node in turn
       until every node in the network has received the message.

    -  Promoted former section 8.2.1 ("Controlling Route Request
       broadcasts") to be its own major section.

    -  Fine-tuned specification for lifetime for reverse routes.

    -  Removed references to unused, nonexistent, and unspecified ICMP
       ACK message.

    -  Added paragraph about creating/updating routes to neighbors when
       receive control packets from them (section 8.3).

    -  Added action for a source initiating a RREQ - it records the
       Flooding ID and source IP address of the RREQ so that it will
       not reprocess the packet as it receives it from its neighbors
       (section 8.5).

    -  Clarified when to increment the sequence number in a RREQ in
       section 8.6.1.

    -  Reordered the paragraphs in section 8.6.1 so that they follow
       temporal order.

    -  Clarified that RREPs are unicast to the next hop en route to the
       source, not to the actual source node, so that the intermediate
       nodes can process the RREP (section 8.7)





Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 28]


Internet Draft                   AODV                    9 November 2001


    -  Made terminology changes so that routes to neighbors advertising
       HELLO messages are considered active routes (section 8.9).


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
















Perkins, Belding-Royer, Das         Expires 9 May 2002         [Page 29]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/