Mobile Ad Hoc Networking Working Group                Charles E. Perkins
INTERNET DRAFT                                     Nokia Research Center
24 November 2000
2 March 2001                                          Elizabeth M. Royer
                                 University of California, Santa Barbara
                                                            Samir R. Das
                                                University of Cincinnati

            Ad hoc On-Demand Distance Vector (AODV) Routing
                        draft-ietf-manet-aodv-07.txt
                      draft-ietf-manet-aodv-08.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.

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

   Distribution of this memo is unlimited.

Abstract

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

                                Contents

Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Overview                                                               2                                                           1

 3. AODV Terminology                                                       3                                                   2

 4. Route Request (RREQ) Message Format                                    4                                3

 5. Route Reply (RREP) Message Format                                      5                                  4

 6. Route Error (RERR) Message Format                                      7                                  6

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

 8. AODV Operation                                                         8                                                     7
     8.1. Maintaining Route Utilization Records . . . . . . . . . .        8    7
     8.2. Generating Route Requests . . . . . . . . . . . . . . . .     9    7
           8.2.1. Controlling Route Request broadcasts  . . . . . .    10    8
     8.3. Forwarding Route Requests . . . . . . . . . . . . . . . .     11    9
           8.3.1. Processing Route Requests . . . . . . . . . . . .    11    9
     8.4. Generating Route Replies  . . . . . . . . . . . . . . . .     13   11
           8.4.1. Route Reply Generation by the Destination   . . .   13   11
           8.4.2. Route Reply Generation by an Intermediate Node  .    14
      8.5.   11
           8.4.3. Generating Gratuitous RREPs . . . . . . . . . . .   12
     8.5. Forwarding Route Replies  . . . .    14
      8.6. Forwarding Route Replies . . . . . . . . . . . .   13
     8.6. Operation over Unidirectional Links . . . . .    15 . . . . . .   14
     8.7. Hello Messages  . . . . . . . . . . . . . . . . . . . . .    16   14
     8.8. Maintaining Local Connectivity  . . . . . . . . . . . . .    17   15
     8.9. Route Error Messages  . . . . . . . . . . . . . . . . . .    18   16
           8.9.1. Local Repair  . . . . . . . . . . . . . . . . . .   19   17
    8.10. Route Expiry and Deletion . . . . . . . . . . . . . . . .    21   18
    8.11. Actions After Reboot  . . . . . . . . . . . . . . . . . .    21   19
    8.12. Interfaces  . . . . . . . . . . . . . . . . . . . . . . .    22   19

 9. AODV and Aggregated Networks                                       22                                      20

10. Using AODV with Other Networks                                     23                                    20

11. Extensions                                                         24                                                        20
    11.1. Hello Interval Extension Format . . . . . . . . . . . . .    24   21

12. Configuration Parameters                                           25                                          21

13. Security Considerations                                            26                                           23

14. Acknowledgments                                                    27                                                   23

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 handled by UDP, and normal IP header processing applies.  So, for
   instance, the requesting node is expected to use its IP address as
   the source IP address for the messages.  The range of dissemination
   of broadcast RREQs can be indicated by the TTL in the IP header.
   Fragmentation is typically not required.

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

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

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

    -  Destination IP Address

    -  Destination Sequence Number
    -  Interface

    -  Hop Count (number of hops needed to reach destination)
    -  Last Hop Count (described in subsection 8.2.1)

    -  Next Hop
    -  List of Precursors (described in Section 8.1)

    -  Lifetime (expiration or deletion time of the route)
    -  Routing Flags

3. AODV Terminology

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

      active route

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

      forwarding node

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

      forward route

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

      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| Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Broadcast ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination IP Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source IP Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Source Sequence Number                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Type           1

      J              Join flag; reserved for multicast.

      R              Repair flag; reserved for multicast.

      G              Gratuitous RREP flag; indicates whether a
                     gratuitous RREP should be unicast to the node
                     specified in the Destination IP Address field (see
                     sections 8.2, 8.5) 8.4.3)

      Reserved       Sent as 0; ignored on reception.

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

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

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

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

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

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

5. Route Reply (RREP) Message Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |R|A|    Reserved     |Prefix Sz|   Hop Count| 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 8.6. 8.5.

      Reserved      Sent as 0; ignored on reception.

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

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

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

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

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

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

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

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

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

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

      Type        4

      Reserved    Sent as 0; ignored on reception.

   The RREP-ACK message is 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
   RREQs, RREPs and RERRs for unicast communication, and how the message
   data are handled.

8.1. Maintaining Route Utilization Records

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

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

8.2. Generating Route Requests

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

   A source node often expects to have bidirectional communications with
   a destination node.  In such cases, it is not sufficient for the
   source node to have a route to the destination node; the destination
   must also have a route back to the source node.  In order to cause for this
   to happen as efficiently as possible, any generation of an RREP
   by an intermediate node (as in section 8.4) for delivery to the
   source node, should be accompanied by some action which notifies the
   destination about a route back to the source node.  The source node
   selects this mode of operation in the intermediate nodes by setting
   the `G' flag.  See section 8.5 8.4.3 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
   rebroadcast the RREQ, up to a maximum of RREQ_RETRIES times.  Each
   rebroadcast MUST increment the Broadcast ID field.

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

8.2.1. Controlling Route Request broadcasts

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

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

   As a further optimization, timeouts MAY be determined dynamically via
   measurements, instead of using a statically configured value related
   to NODE_TRAVERSAL_TIME. To accomplish this, the RREQ may carry the
   timestamp via an extension field as defined in Section 11 to be
   carried back by the RREP packet (again via an extension field).  The
   difference between the current time and this timestamp will determine
   the route discovery latency.  The timeout may be set to be a small
   factor times the average of the last few route discovery latencies
   for the concerned destination.  These latencies may be recorded as
   additional fields in the routing table.

   If the optimizations described in this section are used, an expired
   routing table entry SHOULD NOT be expunged before DELETE_PERIOD.
   Otherwise, the soft state corresponding to the route (e.g., Last Hop
   Count) will be lost.  In such cases, a longer routing table entry
   expunge time may be specified.  Any routing table entry waiting for a
   RREP should not be expunged before RREP_WAIT_TIME.

8.3. Forwarding Route Requests

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

8.3.1. Processing Route Requests

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

   If the node, on the other hand, does has 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
   rebroadcasts the RREQ just as if it did not have an active route to
   the destination.

   The node generates a RREP (as discussed further in section 8.4) if
   either:

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

      (ii)      it is itself the destination.

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

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

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

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

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

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

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

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

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

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

8.4. Generating Route Replies

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

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

8.4.1. Route Reply Generation by the Destination

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

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

8.4.2. Route Reply Generation by an Intermediate Node

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

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

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

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

8.5.

8.4.3. Generating Gratuitous RREPs

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

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

      Hop Count  The Hop Count as received in the RREQ

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

      Destination Sequence Number
                 The Source Sequence Number from the RREQ
      Source IP Address
                 The IP address of the destination node

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

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

8.6.

8.5. Forwarding Route Replies

   When a node receives a RREP message, it first compares the
   Destination Sequence Number in the message with its own copy of
   destination sequence number for the Destination IP Address. 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 Hop Count in RREP is smaller than the hop count in
   route table entry.  If a new route is created or the old route is
   updated, the next hop is the node from which the RREP is received,
   which is indicated by the source IP address field in the IP header;
   the hop count is the Hop Count in the RREP message plus one; the
   expiry time is the current time plus the Lifetime in the RREP
   message; the destination sequence number is the Destination Sequence
   Number in the RREP message.

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

   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, errors
   or be unidirectional, the node MAY set the `A' flag to require that
   the recipient of the RREP acknowledge receipt of the RREP by sending
   a RREP-ACK message back.

8.7.  Hello Messages

    A node MAY offer connectivity information by broadcasting local
    Hello messages as follows.  Every HELLO_INTERVAL milliseconds, the
    node checks whether

8.6. Operation over Unidirectional Links

   When unidirectional links are present, it has sent is possible that a broadcast (e.g., RREP
   transmission may fail.  Such failure can be detected via the absence
   of a RREQ link-layer or an
    appropriate layer 2 message) within the last HELLO_INTERVAL. network-layer acknowledgment (e.g., RREP-ACK). If it
    has not, it MAY generate a broadcast
   no other RREP with TTL = 1, called a
    Hello message, with generated from the message fields set as same route request broadcast reaches
   the source, the source will redo the broadcast after a timeout (see
   section 8.2).  However, the same scenario will repeat, and no route
   will be discovered even after repeated retries.  This is possible
   even when bidirectional routes between source and destination do
   exist.  This happens because a RREQ transmission may occur over a
   unidirectional link.  Link layers using broadcast transmissions for
   RREQ will not be able to detect the presence of such unidirectional
   links.  Also, in AODV any node acts on only the first RREQ with
   the same broadcast ID and ignores any subsequent RREQs.  It is
   possible that the first RREQ arrives along a path that has one or
   more unidirectional link(s).  However, a subsequent RREQ may arrive
   via a bidirectional path (assuming such paths exist), but it will be
   ignored.

   To prevent this problem, a node that fails to transmit a RREP
   remembers the next-hop of the failed 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 8.2.

8.7. Hello Messages

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

      Destination IP Address
                  The node's IP address.

      Destination Sequence Number
                  The node's latest sequence number.

      Hop Count   0

      Lifetime    ALLOWED_HELLO_LOSS * HELLO_INTERVAL
   A node MAY determine connectivity by listening for packets from
   its set of neighbors.  If it receives no packets for more than
   ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD
   assume that the link to this neighbor is currently broken.  When this
   happens, the node SHOULD proceed as in Section 8.9.

8.8. Maintaining Local Connectivity

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

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

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

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

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

        *  An ICMP Echo Request message unicast to the next hop.

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

8.9. Route Error Messages

   A node initiates a RERR message in three situations:

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

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

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

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

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

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

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

      (b)       marks the Hop Count for this destination as infinity,
                and thus invalidates the route.  The old value of Hop
                Count is copied into the Last Hop Count field.

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

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

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

8.9.1. Local Repair

   When a link break in an active route occurs, the node upstream of
   that break MAY choose to repair the link locally if the destination
   is no farther than MAX_REPAIR_TTL hops away.  To repair the link
   break itself, it increments the sequence number for the destination
   and then broadcasts a RREQ for that destination.  The TTL of the RREQ
   should initially be set to the following value:
          max(MIN_REPAIR_TTL, 0.5 TTL 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 8.9 by creating a
   RERR message for that destination.

   On the other hand, if the nodes does receive one or more RREPs during
   the discovery period, the node proceeds as described in Section 8.6, 8.5,
   creating a route table entry for that destination.  It then compares
   the hop count of the new route with the value in the last hop count
   route table entry for that destination.  If the hop count of the
   newly determined route to the destination is greater than the hop
   count of the previously known route, as recorded in the last hop
   count field, the node MAY create a RERR message for the destination
   and send this message to the source node.  The node sets the 'N' flag
   of the RERR, and then broadcasts this message if it has one or more
   precursor nodes for this route table entry.

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

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

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

8.10. Route Expiry and Deletion

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

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

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

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

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

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

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

8.11. Actions After Reboot

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

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

8.12. Interfaces

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

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

9. AODV and Aggregated Networks

   AODV has been designed for use by mobile nodes with IP addresses
   that are not necessarily related to each other, to create an ad hoc
   network.  However, in some cases a collection of mobile nodes MAY
   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.

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 8.7).

12. Configuration Parameters

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

      Parameter Name           Value
      ----------------------   -----
      ACTIVE_ROUTE_TIMEOUT     3,000 Milliseconds
      ALLOWED_HELLO_LOSS       2
      BLACKLIST_TIMEOUT        RREQ_RETRIES * NET_TRAVERSAL_TIME
      BROADCAST_RECORD_TIME    2 * NET_TRAVERSAL_TIME
      DELETE_PERIOD            see note below
      HELLO_INTERVAL           1,000 Milliseconds
      LOCAL_ADD_TTL            2
      MAX_REPAIR_TTL           0.3 * NET_DIAMETER
      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 to be 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
   received from a neighbor but data packets to that neighbor are
   lost, (due to temporary link asymmetry, e.g.)  we have to make more
   concrete assumptions about the underlying link layer.  We assume
   that such asymmetry cannot persist beyond a certain certain time,
   say, a multiple K of ALLOWED_HELLO_LOSS * HELLO_INTERVAL. In other
   words, it cannot not be the case that a node receives K subsequent
   hello messages from a neighbor, while that same neighbor fails to
   receive any data packet from the node in this period.  This
    is a reasonable assumption as this AODV specification works only with
    symmetric links.  Covering all
   possibilities,

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

   NET_DIAMETER measures the maximum possible number of hops between
   two nodes in the network.  NODE_TRAVERSAL_TIME is a conservative
   estimate of the average one hop traversal time for packets and should
   include queueing delays, interrupt processing times and transfer
   times.  ACTIVE_ROUTE_TIMEOUT SHOULD be set to a longer value (at
   least 10,000 milliseconds) if link-layer indications are used to
   detect link breakages such as in IEEE 802.11 [2] standard.  TTL_START
   should be set to at least 2 if Hello messages are used for local
   connectivity information.  Performance of the AODV protocol is
   sensitive to the chosen values of these constants, which often depend
   on the characteristics of the underlying link layer protocol, radio
   technologies etc.  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, and must be protected by use of authentication techniques
   involving generation of unforgeable and cryptographically strong
   message digests or digital signatures.  It is expected that, in
   environments where security is an issue, that IPSec authentication
   headers will be deployed along with the necessary key management to
   distribute keys to the members of the ad hoc network using AODV.

14. Acknowledgments

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

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

References

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

   [2] IEEE Standards Department. 802.11 Committee, AlphaGraphics #35, 10201 N.35th Avenue,
       Phoenix AZ 85051.  Wireless LAN medium access control
        (MAC) and physical layer (PHY) specifications, IEEE standard
        802.11--1997, Medium Access Control MAC and
       Physical Layer PHY Specifications, June 1997.  IEEE Standard
       802.11-97.

   [3] C. Charles E. Perkins.  Mobile ad hoc networking terminology.  IETF
        Internet Draft, draft-ietf-manet-term-00.txt (Work  Terminology for Ad-Hoc Networking (work in Progress),
        October
       progress).  draft-ietf-manet-terms-00.txt, November 1997.

Author's Addresses

   Questions about this memo can be directed to:

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

      Elizabeth M. Royer
      Dept. of Electrical and Computer Engineering Science
      University of California, Santa Barbara
      Santa Barbara, CA 93106
      +1 805 893 7788 3411
      +1 805 893 3262 8553 (fax)
       eroyer@alpha.ece.ucsb.edu
      eroyer@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