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

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

            Ad Hoc On-Demand Distance Vector (AODV) Routing
                      draft-ietf-manet-aodv-05.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 both unicast
   and multicast routes between sources and destinations.  It uses
   destination sequence numbers to ensure loop freedom at all times
   (even in the face of anomalous delivery of routing control messages),
   solving problems (such as ``counting to infinity'') associated with
   classical distance vector protocols.








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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Overview                                                           2

 3. AODV Terminology                                                   4

 4. Route Request (RREQ) Message Format                                5

 5. Route Reply (RREP) Message Format                                  7

 6. Route Error (RERR) Message Format                                  8

 7. Multicast Activation (MACT) Message Format                         9

 8. Group Hello (GRPH) Message Format                                 10

 9. Node Operation - Unicast                                          11
     9.1. Maintaining Route Utilization Records . . . . . . . . . .   11
     9.2. Generating Route Requests . . . . . . . . . . . . . . . .   11
           9.2.1. Controlling Route Request broadcasts  . . . . . .   12
     9.3. Forwarding Route Requests . . . . . . . . . . . . . . . .   13
           9.3.1. Processing Route Requests . . . . . . . . . . . .   13
     9.4. Generating Route Replies  . . . . . . . . . . . . . . . .   14
     9.5. Forwarding Route Replies  . . . . . . . . . . . . . . . .   15
     9.6. Hello Messages  . . . . . . . . . . . . . . . . . . . . .   16
     9.7. Maintaining Local Connectivity  . . . . . . . . . . . . .   17
     9.8. Route Error Messages  . . . . . . . . . . . . . . . . . .   17
     9.9. Route Expiry and Deletion . . . . . . . . . . . . . . . .   19
    9.10. Actions After Reboot  . . . . . . . . . . . . . . . . . .   19

10. Node Operation - Multicast                                        20
    10.1. Maintaining Multicast Tree Utilization Records  . . . . .   20
    10.2. Generating Route Requests . . . . . . . . . . . . . . . .   20
    10.3. Forwarding Route Requests . . . . . . . . . . . . . . . .   21
    10.4. Generating Route Replies  . . . . . . . . . . . . . . . .   21
    10.5. Forwarding Route Replies  . . . . . . . . . . . . . . . .   22
    10.6. Route Activation  . . . . . . . . . . . . . . . . . . . .   23
    10.7. Multicast Tree Pruning  . . . . . . . . . . . . . . . . .   24
    10.8. Repairing Link Breakages  . . . . . . . . . . . . . . . .   24
    10.9. Tree Partitions . . . . . . . . . . . . . . . . . . . . .   25



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   10.10. Reconnecting Two Trees  . . . . . . . . . . . . . . . . .   26
   10.11. Group Hello Messages  . . . . . . . . . . . . . . . . . .   27
   10.12. Actions After Reboot  . . . . . . . . . . . . . . . . . .   28

11. Broadcast                                                         28

12. Quality of Service                                                29

13. AODV and Aggregated Networks                                      29

14. Using AODV with Other Networks                                    30

15. Address Autoconfiguration                                         30

16. Extensions                                                        31
    16.1. Hello Interval Extension Format . . . . . . . . . . . . .   31
    16.2. Multicast Group Leader Extension Format . . . . . . . . .   32
    16.3. Multicast Group Rebuild Extension Format  . . . . . . . .   33
    16.4. Multicast Group Information Extension Format  . . . . . .   33
    16.5. Maximum Delay Extension Format  . . . . . . . . . . . . .   34
    16.6. Minimum Bandwidth Extension Format  . . . . . . . . . . .   34

17. Configuration Parameters                                          35

18. Security Considerations                                           37

19. Acknowledgements                                                  37

 A. Draft Modifications                                               39


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 for the formation
   of multicast groups whose membership is free to change during the
   lifetime of the network.  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.





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   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 or the multicast group leader
   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), Route Errors (RERRs),
   Multicast Activations (MACTs), and Group Hellos (GRPHs) 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 (either a single node or a multicast group)
   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.

   RREQs are also used when a node wishes to join a multicast group.
   A join flag in the RREQ informs nodes that when receiving the
   RREP, they are not just setting route pointers but are also setting
   multicast route pointers, which will be used if the route is selected




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   to be added onto the tree.  If the route is chosen for addition to
   the multicast tree, it will be activated by a MACT message.

   For multicast groups, a Group Hello message is periodically broadcast
   across the network by the multicast group leader.  The message
   carries multicast group and corresponding group leader IP addresses.
   This information is used for repairing multicast trees after a
   previously disconnected portion of the network containing part of the
   multicast tree becomes reachable once again.

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

    -  Last Hop Count (described in subsection 9.2.1)
    -  Next Hop

    -  List of Precursors (described in Section 9.1)
    -  Lifetime (expiration or deletion time of the route)

    -  Routing Flags

   The following information is stored in each entry of the multicast
   route table for multicast tree routes:

    -  Multicast Group IP Address
    -  Multicast Group Leader IP Address

    -  Multicast Group Sequence Number
    -  Next Hops

    -  Hop Count to next Multicast Group Member

    -  Hop Count to Multicast Group Leader

   The Next Hops field is a linked list of structures, each of which
   contains the following fields:

    -  Next Hop IP Address
    -  Link Direction

    -  Activated Flag



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   The direction of the link is relative to the location of the group
   leader, i.e.  UPSTREAM is a next hop towards the group leader, and
   DOWNSTREAM is a next hop away from the group leader.  A node on
   the multicast tree must necessarily have only one UPSTREAM link.
   The IP Address of a Next Hop MUST NOT be used to forward multicast
   messages until after a MACT message has activated the route (see
   Section 10.6).


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 [4].

      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 inifnite 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.

      group leader

         A node which is a member of the given multicast group
         and which is typically the first such group member in the
         connected portion of the network.  This node is responsible for
         initializing and maintaining the multicast group destination
         sequence number.

      group leader table

         The table where ad hoc nodes keep information concerning each
         multicast group and its corresponding group leader.  There is




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         one entry in the table for each multicast group for which the
         node has received a Group Hello (see Section 10.2).

      multicast tree

         The tree containing all nodes which are members of the
         multicast group and all nodes which are needed to connect the
         multicast group members.

      multicast route table

         The table where ad hoc nodes keep routing (including next hops)
         information for various multicast groups.

      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.

      subnet leader

         A node which is a member of the subnet defined by a specific
         routing prefix, and which offers reachability to every other
         node with the same routing prefix.  The subnet leader is
         responsible for initializing and maintaining the destination
         sequence number for every node on the subnet.


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

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



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      Type           1

      J              Join flag; set when source node wants to join a
                     multicast group.

      R              Repair flag; set when a node wants to initiate
                     a repair to connect two previously disconnected
                     portions of the multicast tree.

      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.

   When a node wishes to repair a multicast tree, it appends the
   Multicast Group Rebuild extension (see Section 16.3).  When a node
   wishes to unicast the RREQ for a multicast group to the group leader,
   it includes the Multicast Group Leader extension (see Section 16.2).













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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|    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; set when a node is responding
                    to a repair request to connect two previously
                    disconnected portions of the multicast tree.

      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.




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

   When the RREP is sent for a multicast destination, the Multicast
   Group Information extension is appended (see Section 16.4).

   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      |           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

      Reserved    Sent as 0; ignored on reception.

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

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

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




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

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


7. Multicast Activation (MACT) 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      |P|G|U|R|    Reserved           |   Hop Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Multicast Group IP address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source IP address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Source Sequence Number                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The format of the Multicast Activation message is illustrated above,
   and contains the following fields:

      Type        4

      P           Prune flag; set when a node wishes to prune itself
                  from the tree, unset when the node is activating a
                  tree link.

      G           Group Leader flag; set by a multicast tree member that
                  fails to repair a multicast tree link breakage, and
                  indicates to the group member receiving the message
                  that it should become the new multicast group leader.

      U           Update flag; set when a multicast tree member has
                  repaired a broken tree link and is now a new distance
                  from the group leader.

      R           Reboot flag; set when a node has just rebooted (see
                  Section 10.12).

      Reserved    Sent as 0; ignored on reception.






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      Hop Count   The distance of the sending node from the multicast
                  group leader.  Used only when the 'U' flag is set;
                  otherwise sent as 0.

      Multicast Group IP Address
                  The IP address of the Multicast Group for which a
                  route is supplied.

      Source IP Address
                  The IP address of the sending node.

      Source Sequence Number
                  The current sequence number for route information
                  generated by the source of the route request.

   To prune itself from the tree (i.e., inactivate its last link to the
   multicast tree), a multicast tree member sends a MACT with the 'P'
   flag = 1 to its next hop on the multicast tree.  A multicast tree
   member that has more than one next hop to the multicast tree SHOULD
   NOT prune itself from the multicast tree.


8. Group Hello (GRPH) 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      |U|M|       Reserved            |   Hop Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Group Leader IP address                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Multicast Group IP address                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Multicast Group Sequence Number                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The format of the Group Hello message is illustrated above, and
   contains the following fields:

      Type        5

      U           Update flag; set when there has been a change in group
                  leader information.

      M           Off_Mtree flag; set by a node receiving the group
                  hello that is not on the multicast tree.

      Reserved    Sent as 0; ignored on reception.



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      Hop Count   The number of hops the packet has traveled.  Used by
                  multicast tree nodes to update their distance from the
                  group leader when the M flag is not set.

      Group Leader IP Address
                  The IP address of the group leader.

      Multicast Group IP Address
                  The IP address of the Multicast Group for which the
                  sequence number is supplied.

      Multicast Group Sequence Number
                  The current sequence number of the multicast group.


9. Node Operation - Unicast

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


9.1. 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.


9.2. 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



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   broadcast ID used by the current node for the same destination.  The
   Hop Count field is set to zero.

   After broadcasting a RREQ, a node waits for a RREP. If the RREP
   is not received within RREP_WAIT_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.


9.2.1. 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 16 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 of the average of the last few route discovery latencies




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   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 too early.  Otherwise, the
   soft states 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.  In general, any routing table entry waiting for a RREP
   should not be expunged before the timeout for receiving RREP.


9.3. 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 the last BCAST_ID_SAVE 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.


9.3.1. Processing Route Requests

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

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

   The node generates a RREP (as discussed further in section 9.4) if
   either:

      (i)       it has an active route to the destination, and the
                node's existing destination sequence number is greater




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                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 destination sequence number 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 will be used by an eventual RREP back to the node
   which originated the RREQ (identified by the Source IP Address).


9.4. 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. If the node is not the destination node, it copies the last



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   known destination sequence number in the Destination Sequence Number
   field in the RREP message.  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 Source and Destination IP Addresses in RREQ message are copied to
   corresponding fields in the RREP message.

   If the generating node is not the destination node, then the
   generating 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. If the generating node is the destination node, it
   places the value zero in the Hop Count field.  The Hop Count field
   is incremented by one at each hop as the RREP is forwarded to the
   source.  When the RREP reaches the source, the Hop Count represents
   the distance, in hops, of the destination from the source.

   If the node is not the destination node, it calculates the Lifetime
   field of the RREP by subtracting the current time from the expiration
   time in its route table entry.  Otherwise, if the generating node
   is also the destination node, it 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.

   If the generating node is not the node indicated by the Destination
   IP Address, then it puts the next hop towards the destination in the
   precursor list for the reverse route entry.  (This is the entry for
   Source IP Address.)  In addition, the generating node 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.  (This is the entry for the Destination IP
   Address).


9.5. 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.  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



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   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.


9.6. 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 9.8.





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9.7. 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 9.8.


9.8. Route Error Messages

   A node initiates a RERR message in three situations:





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

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

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

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

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

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

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

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

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

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



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   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.


9.9. 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 17


9.10. 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 9.8 and resets



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   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.


10. Node Operation - Multicast

   This section describes the scenarios under which nodes generate
   control messages for multicast communication, and how the fields in
   the messages are handled.


10.1. Maintaining Multicast Tree Utilization Records

   For each multicast tree to which a node belongs, either because it
   is a member of the group or because it is a router for the multicast
   tree, the node maintains a list of next hops -- i.e., those 1-hop
   neighbors that are likewise a part of the multicast tree.  This
   list of next hops is used for forwarding messages received for
   the multicast group.  A node will forward a multicast message to
   every such next hop, except that neighbor from which the message
   arrived.  If there are multiple next hops, the forwarding operation
   MAY be performed by broadcasting the multicast packet to the node's
   neighbors; only the neighbors that belong to the multicast tree and
   have the sending node as a next hop continue to forward the multicast
   packet.


10.2. Generating Route Requests

   A node sends a RREQ either when it determines that it should be a
   part of a multicast group, and it is not already a member of that
   group, or when it has a message to send to the multicast group but
   does not have a route to that group.  If the node wishes to join the
   multicast group, it sets the `J' flag in the RREQ; otherwise, it
   leaves the flag unset.  The destination address of the RREQ is always
   set to the multicast group address.  If the node knows the group
   leader and has a route to it, the node places the group leader's
   address in the Multicast Group Leader extension (Section 16.2), and
   unicasts the RREQ to the corresponding next hop for that destination.
   Otherwise, if the node does not have a route to the group leader, or
   if it does not know who the multicast group leader is, it broadcasts
   the RREQ and does not include the extension field.



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   The process of waiting for a RREP to a RREQ with a multicast
   destination address is the same as that described in Section 9.2.
   The node may resend the RREQ up to RREQ_RETRIES additional times if
   a RREP is not received.  If a RREQ was unicast to a group leader
   and a RREP is not received within RREP_WAIT_TIME milliseconds, the
   node broadcasts subsequent RREQs for that multicast group across the
   network.  If a RREP is not received after RREQ_RETRIES additional
   requests, the node may assume that there are no other members of that
   particular group within the connected portion of the network.  If it
   wanted to join the multicast group, it then becomes the multicast
   group leader for that multicast group and initializes the sequence
   number of the multicast group.  Otherwise, if it only wanted to send
   packets to that group without actually joining the group, it drops
   the packets it had for that group and aborts the session.

   When the node wishes to join or send a message to a multicast group,
   it first consults its Group Leader Table.  Based on the existence
   of an entry for the multicast group in this table, the node then
   formulates and sends the RREQ as described at the beginning of this
   section.


10.3. Forwarding Route Requests

   The operation of nodes forwarding RREQs for multicast is similar
   to that for the reception and forwarding of RREQs as described in
   Section 9.3, with one exception.  If the RREQ is a join request, it
   creates a multicast group next hop entry for the node from which it
   received the RREQ. The generation of the route reply (RREP) message
   is discussed in the following section.


10.4. Generating Route Replies

   If a node receives a join RREQ for a multicast group, and it is
   already a member of the multicast tree for that group, the node
   updates its Multicast Route Table and then generates a RREP message.
   It unicasts the RREP back to the node indicated by the Source IP
   Address field of the received RREQ. The RREP contains the current
   sequence number for the multicast group and the IP address of the
   group leader.  Furthermore, it initializes the Hop Count field of
   the RREP to zero.  Additional information about the multicast group
   leader is entered into the Multicast Group Information extension (see
   Section 16.4).

   A node can only respond to a join RREQ if it is a member of the
   multicast tree.  If a node receives a multicast route request that
   is not a join message, it can reply if it has a current route to the
   multicast tree.  Otherwise it continues forwarding the request.  If a



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   node receives a join RREQ for a multicast group and it is not already
   a member of the multicast tree for that group, it rebroadcasts the
   RREQ to its neighbors.

   In the event that a node receives a unicasted multicast route request
   that specifies its own IP address as the destination address (i.e.,
   the source node believes this destination node to be the multicast
   group leader), but the node is in fact not the group leader, it
   can simply ignore the RREQ. The source node will time out after
   RREP_WAIT_TIME milliseconds and will broadcast a new RREQ without the
   group leader address specified.

   Regardless of whether the multicast group leader or a multicast tree
   member generates the RREP, the RREP fields are set as follows:

      Hop Count    0

      Destination IP Address
                   The IP address of the multicast group.

      Destination Sequence Number
                   The current multicast group sequence number.

      Lifetime     The time for which nodes receiving the RREP consider
                   the route to be valid (only used it the RREQ is not a
                   join request).

   The Multicast Group Information extension described in Section 16.4
   is also included for join requests.  If the node generating the RREP
   is not on the multicast tree (because the RREQ was not a join RREQ),
   it places its distance from the multicast tree in the Hop Count
   field, instead of 0.


10.5. Forwarding Route Replies

   If an intermediate node receives a RREP in response to a RREQ that
   it has transmitted (or retransmitted on behalf of some other node),
   it increments the Hop Count and Multicast Group Hop Count fields and
   forwards the RREP along the path to the source of the RREQ.

   When the node receives more than one RREP for the same RREQ, it saves
   the route information with the greatest sequence number, and beyond
   that the lowest hop count; it discards all other RREPs.  This node
   forwards the first RREP towards the source of the RREQ, and then
   forwards later RREPs only if they have a greater sequence number or
   smaller metric.





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10.6. Route Activation

   When a node broadcasts a RREQ message, it is likely to receive more
   than one reply since any node in the multicast tree can respond.  If
   the RREQ was not a join request, then once the source node receives
   the first RREP, it may begin using this route to forward data
   packets.  On the other hand, if the RREQ was a join request, the RREP
   message sets up route pointers as it travels back to the source node.
   These route pointers may eventually graft a branch onto the multicast
   tree.  If multiple branches to the same destination are created in
   such a manner, a loop will be formed.  Hence, in order to prevent
   the formation of any such loops, it is necessary to activate only
   one of the routes created by the RREP messages.  The RREP containing
   the largest destination sequence number is chosen to be the added
   branch to the multicast tree.  In the event that a node receives more
   than one RREP with the same (largest) sequence number, it selects the
   first one with the smallest hop count, i.e., the shortest distance to
   a member of the multicast tree.

   After waiting RREP_WAIT_TIME milliseconds, the node must select the
   route it wishes to use as its link to the multicast tree.  This is
   accomplished by sending a Multicast Activation (MACT) message.  The
   Destination IP Address field of the MACT packet is set to the IP
   address of the multicast group.  The node unicasts this message to
   the selected next hop, effectively activating the route.  It then
   sets the Activated flag in the next hop Multicast Route Table entry
   associated with that node.  After receiving this message, the node
   to which the MACT was sent activates the route entry for the link in
   its multicast route table, thereby finalizing the creation of the
   tree branch.  All neighbors not receiving this message time out and
   delete that node as a next hop for the multicast group in their route
   tables, having never activated the route entry for that next hop.

   Two scenarios exist for a neighboring node receiving the MACT
   message.  If this node was previously a member of the multicast
   tree, it does not propagate the MACT message any further.  However,
   if the next hop selected by the source node's MACT message was not
   previously a multicast tree member, it will have propagated the
   original RREQ further up the network in search of nodes which are
   tree members.  Thus it is possible that this node also received more
   than one RREP, as noted in section 10.5.

   When the node receives a MACT selecting it as the next hop, it
   unicasts its own MACT to the node it has chosen as its next hop,
   and so on up the tree, until a node which was already a part of the
   multicast tree is reached.






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10.7. Multicast Tree Pruning

   A multicast group member can revoke its member status at any time.
   However, it can only actually leave the multicast tree if it is not a
   tree router for any other nodes in the multicast group (i.e., if it
   is a leaf node).  If a node wishing to leave the multicast group is
   a leaf node, it unicasts to its next hop on the tree a MACT message
   with the 'P' flag set and with the Destination IP Address set to the
   IP address of the multicast group.  It then deletes the multicast
   group information for that group from its Multicast Route Table.
   When its next hop receives this message, it deletes the sending
   node's information from its list of next hops for the multicast tree.
   If the removal of the sending node causes this node to become a leaf
   node, and if this node is also not a member of the multicast group,
   it may in turn prune itself by sending its own MACT message up the
   tree.

   When the multicast group leader wishes to leave the multicast group,
   it proceeds in a manner similar to the one just described.  If it
   is a leaf node, it may leave the group and unicast a prune message
   to its next hop.  The next hop acts in the manner described in
   Section 10.10, since the prune message is coming from its upstream
   neighbor.  Otherwise, if the group leader is not a leaf node, it may
   not prune itself from the tree.  It takes the actions described in
   Section 10.9, where it selects one of its next hops and unicasts to
   it the MACT with set `G' flag.


10.8. Repairing Link Breakages

   Branches of the multicast tree become invalid if a broken link
   results in an infinite metric being associated with the route.  When
   a broken link is detected between two nodes on the multicast tree,
   the two nodes should delete the link from their list of next hops for
   the multicast group.  The node downstream of the break (i.e., the
   node which is further from the multicast group leader) is responsible
   for initiating the repair of the broken link.  In order to repair
   the tree, the downstream node broadcasts a RREQ with destination IP
   address set to the IP address of the multicast group and with the `J'
   flag set.  The destination sequence number of the RREQ is the last
   known sequence number of the multicast group.  The node also includes
   the Multicast Group Leader Extension.  The Multicast Group Hop Count
   field of this extension is set to the distance of the source node
   from the multicast group leader.  A node MUST have a hop count to
   the multicast group leader less than or equal to the indicated value
   in order to respond.  This hop count requirement prevents nodes on
   the same side of the break as the node initiating the repair from
   replying to the RREQ.




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   The RREQ is broadcast using an expanding rings search.  Because of
   the high probability that other nearby nodes can be used to rebuild
   the route, the original RREQ is broadcast with a TTL (time to live)
   field value equal to two more than the Multicast Group Hop Count.  In
   this way, the effects of the link breakage may be localized.  If no
   reply is received within RREP_WAIT_TIME milliseconds, all subsequent
   RREQs (up to RREQ_RETRIES additional attempts) are broadcast across
   the entire network.  Any node that is a part of the multicast tree
   and that has a hop count to the multicast group leader smaller than
   that contained in the RREQ can return a RREP. If there is more than
   one RREP received at the originating node, route deletions occur as
   described in the previous section.

   At the end of the discovery period, the node selects its next hop
   and unicasts a MACT message to that node to activate the link, as
   described in Section 10.7.  Since the node was repairing a tree
   break, it is likely that it is now a different distance from the
   group leader than it was before the break.  If this is the case, it
   must inform its DOWNSTREAM next hops of their new distance from the
   group leader.  It does this by broadcasting a MACT message with the
   'U' flag set, and the Hop Count field set to the node's new distance
   from the group leader.  This 'U' flag indicates that multicast tree
   nodes should update their distance from the group leader.  If these
   nodes have downstream next hops, they in turn must send a MACT
   message with a set 'U' flag to their next hops, and so on.  The Hop
   Count field is incremented by one each time the packet is received.
   When a node on the multicast tree receives the MACT message with the
   'U' flag set, in determines whether this packet arrived from its
   UPSTREAM neighbor.  If it did not, the node discards the packet.

   When a link break occurs, it is possible that the tree will be
   repaired through different intermediate nodes.  Hence, if the node
   UPSTREAM of the break is not a group member, and if the loss of that
   link causes it to become a leaf node, it sets a prune timer to wait
   for the link to be repaired.  This PRUNE_TIMEOUT should be larger
   than RREP_WAIT_TIMEOUT to give the link time to be repaired.  If,
   when this timer expires, the node has not received a MACT message
   selecting it to be a part of the repaired tree branch, it prunes
   itself from the tree by sending a MACT with set 'P' flag to its next
   hop, as previously described.


10.9. Tree Partitions

   It is possible that after a link breaks, the tree cannot be repaired
   due to a network partition.  If the node attempting to repair a
   tree link breakage does not receive a response after RREQ_RETRIES
   attempts, it can be assumed that the network has become partitioned
   and the multicast tree cannot be repaired at this time.  In this



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   situation, if the node which initiated the route rebuilding is a
   multicast group member, it becomes the new multicast group leader
   for its part of the multicast tree partition.  It broadcasts a Group
   Hello for this multicast group.  The `U' flag in the Group Hello is
   set, indicating that there has been a change in the group leader
   information.  All nodes receiving this message update their Group
   Leader Table to indicate the new group leader information.  Nodes
   which are a part of the multicast tree also update the group leader
   information for that group in their Multicast Route Table to indicate
   the new group leader.

   On the other hand, if the node which had initiated the repair is not
   a multicast group member, there are two possibilities.  If it only
   has one next hop for the multicast tree, it prunes itself from the
   tree by unicasting a MACT message, with the 'P' flag set, to its next
   hop.  The node receiving this message notes that the message came
   from its upstream link, i.e., from a node that is closer to the group
   leader than it is.  If the node receiving this message is a multicast
   group member, it becomes the new group leader and broadcasts a
   Group Hello message as indicated above.  Otherwise, if it is not a
   multicast group member and it only has one other next hop link, it
   similarly prunes itself from the tree.  This process continues until
   a multicast group member is reached.

   The other possibility is that the node which initiated the rebuilding
   is not a group member and has more than one next hop for the tree.
   In this case, it cannot prune itself, since doing so would partition
   the tree.  It instead chooses one of its next hops and unicasts a
   MACT with the 'G' flag set to that node.  This flag indicates that
   the next group member to receive this message should become the
   new group leader.  It then changes the direction of that link to
   be UPSTREAM. If the node's next hop is a group member, this node
   becomes the group leader.  Otherwise, the node unicasts its own MACT
   message with the 'G' flag set to one of its next hops, and changes
   the direction of that link.  Once a group member is reached, the new
   group leader is determined.


10.10. Reconnecting Two Trees

   In the event that a link break can not be repaired, the multicast
   tree remains partitioned until the two parts of the network become
   connected once again.  A node from one partition of the network knows
   that it has come into contact with a node from the other partition of
   the network by noting the difference in the GRPH message multicast
   group leader information.  The multicast group leader with the lower
   IP address initiates the tree repair.  For the purposes of this
   explanation, call this node GL1.  GL1 unicasts a RREQ with both
   the 'J' and 'R' flags set to the group leader of the other network



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   partition (GL2), using the node from which it received the GRPH
   as the next hop.  This RREQ contains the current value of GL1's
   multicast group sequence number.  If any node that receives the RREQ
   is a member of GL2's multicast tree, it MUST forward the RREQ along
   its upstream link, i.e.  towards GL2.  This prevents any loops from
   being formed after the repair.  Upon receiving the RREQ, GL2 takes
   the larger of its and the received multicast group sequence number,
   increments this value by one, and responds with a RREP. This is the
   group leader which becomes the leader of the reconnected multicast
   tree.  The 'R' flag of the RREP is set, indicating that this RREP is
   in response to a repair request.

   As the RREP is propagated back to GL1, nodes add the incoming and
   outgoing links to the Multicast Route Table next hop entries if
   these entries do not already exist.  The nodes also activate these
   entries, thereby adding the branch on to the multicast tree.  If a
   node that was previously a member of GL1's tree receives the RREP, it
   MUST forward the packet along its link to its previous group leader
   (GL1).  It then updates its group leader information to reflect GL2
   as the new group leader, changes the direction of the next hop link
   associated with GL1 to DOWNSTREAM, and sets the direction of the
   link on which it received the RREP to UPSTREAM. When GL1 receives
   the RREP, it updates its group leader information and sets the
   link from which it received the RREP as its upstream link.  The
   tree is now reconnected.  The next time GL2 broadcasts a GRPH, it
   sets the `U' flag to indicate that there is a change in the group
   leader information and group members should update the corresponding
   information.  All network nodes update their Group Leader Table to
   reflect the new group leader information.


10.11. Group Hello Messages

   If a node sends a RREQ to join a multicast group (`J' flag set)
   and after RREQ_RETRIES attempts does not receives a response, it
   then becomes the multicast group leader.  The node initializes the
   multicast group sequence number and then broadcasts a Group Hello
   message to inform network nodes that it is now the group leader
   for the multicast group.  To ensure nodes maintain consistent and
   up-to-date information about who the multicast group leaders are,
   any node which is a group leader for a multicast group broadcasts
   such a Group Hello across the network every GROUP_HELLO_INTERVAL
   milliseconds.  The contents of the GRPH fields are set as follows:

      U Flag      0

      M Flag      0

      Hop Count   0



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      Group Leader IP Address
                  The IP Address of the group leader.

      Multicast Group IP Address
                  The IP Address of the Multicast Group for which the
                  node is the group leader.

      Multicast Group Sequence Number
                  One plus the last known sequence number of the
                  multicast group.

   Nodes receiving the Group Hello increment the Hop Count field by one
   before forwarding the message.  When a node not on the multicast
   tree receives the GRPH message, it sets the M flag.  This indicates
   that this incarnation of the message has traveled off the multicast
   tree, and hence cannot be used by group members to verify their
   distance from the group leader.  The U flag is set by the group
   leader whenever there has been a change in group leader information.
   It informs nodes that they should update the group leader information
   associated with the indicated multicast group.


10.12. Actions After Reboot

   A node participating in the multicast tree that reboots (or restarts
   the routing daemon) loses all of its multicast tree information.
   Upon reboot, a node should broadcast a MACT message with set Reboot
   ('R') flag to inform neighboring nodes that it has lost its multicast
   group information.  Since the rebooted node does not know whether it
   was previously a member of the multicast tree, it should broadcast
   this packet unconditionally upon starting the daemon.  When a node
   on the multicast tree receives the reboot MACT message, it checks
   whether this message came from one of its next hops on the multicast
   tree.  If so, one of two situations exists.

   If the reboot MACT came from a downstream link, the node deletes that
   link from its list of next hops and sets a prune timer according to
   the guidelines in Section 10.8.  Otherwise, if the reboot MACT came
   from a node's upstream link, it must rebuild the tree branch as is
   also indicated in Section 10.8.


11. Broadcast

   When a node wishes to generate a broadcast, it sends the broadcast
   packet to address 255.255.255.255.  AODV does not specify
   transmissions to any directed broadcast address.





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   Every node maintains a list to keep track of which broadcast packets
   have already been received and retransmitted.  The list contains, for
   each distinct broadcast packet received, the source IP address and
   the IP ident value from the IP header of the broadcast packet.

   When a node receives a packet broadcast to address 255.255.255.255,
   it checks the source IP address and the IP ident value of the
   broadcast packet's IP header.  The node then checks to see whether
   the broadcast packet has already been received in the past, and thus
   whether it has already retransmitted the broadcast packet.  If there
   is no existing list entry containing the same IP source address and
   IP ident value, the node retransmits the broadcast packet.  If there
   is such a list entry with matching source IP address and IP ident
   field, the node silently discards the broadcast packet.

   List entries SHOULD be kept for at least BROADCAST_RECORD_TIME
   before the node expunges the record.  BROADCAST_RECORD_TIME
   is a configurable parameter, but it MUST be at least equal to
   RREP_WAIT_TIME.


12. Quality of Service

   AODV currently provides some minimal controls to enable mobile nodes
   in an ad hoc network to specify, as part of a RREQ, certain Quality
   of Service parameters that a route to a destination must satisfy.
   In particular, a RREQ MAY include a Maximum Delay extension (see
   Section 16.5) or a Minimum Bandwidth extension (see Section 16.6).

   If, after establishment of such a route, any node along the path
   detects that the requested Quality of Service parameters can no
   longer be maintained, that node MUST originate a ICMP QOS_LOST
   message back to the node which had originally requested the now
   unavailable parameters.


13. 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



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   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.


14. 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 13) 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.


15. Address Autoconfiguration

   When a node in an ad hoc network wishes to obtain an IP address, it
   may be difficult or impossible to contact any address allocation
   agency in the network.  In such cases, the node should attempt to
   select a random address on network 169.253/16, analogous to the way
   that Autonet allocations are done and as is proposed in the zeroconf
   working group [2].

   Following the suggestions for Duplicate Address Detection (DAD) as
   with IPv6 Stateless Address Autoconfiguration [5] and zeroconf, the
   node first picks a random IP address in the range 2048-65534 from
   169.253/16.  Then, the node issues a RREQ for that randomly selected
   address.  If no RREP is returned for the selected address, the node
   retries the RREQ up to RREQ_RETRIES times.  If, after all retries,
   no RREP is still received, the node assumes that the address is not
   already in use, and assumes that the address can safely be taken for



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   its own.  Otherwise, the node randomly picks another address from the
   same range and begins the ad hoc DAD procedure again.

   In order for a return route to be built for a possible RREP, the node
   performing DAD has to have use of some temporary IP address.  This
   temporary IP address is to be selected from the range 1-2047 of the
   class B network 169.253/16.  No address in that range should ever be
   selected for permanent assignment by any node in the ad hoc network;
   all such addresses are only to be used for the purpose of targeting
   possible RREP messages produced during DAD. It is expected that this
   will provide enough addresses for the purpose, since each address
   would never be used for more than a few seconds or a few hundreds of
   milliseconds.  The timeout parameters for the RREQ messages issued
   during DAD are the same as the usual timeout parameters for RREQ
   messages.


16. 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.


16.1. 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 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      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 9.6).


16.2. Multicast Group Leader Extension Format

   This extension is appended to a RREQ by a node wishing to repair a
   multicast tree.

      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     | Multicast Group Leader IP ... |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   ... Address (continued)     | Previous Hop IP Address ...   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   ... (continued)             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type     3

      Length   8

      Multicast Group Leader IP Address
               The IP Address of the Multicast Group Leader.

      Previous Hop IP Address
               The IP Address of the node which previously received the
               RREQ. This field is used when the RREQ is unicast to
               the group leader when a node wishes to join a multicast
               group.

   This extension is used when unicasting the RREQ to the group leader.
   Each node receiving the RREQ updates the Previous Hop IP Address
   field to reflect its address.







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16.3. Multicast Group Rebuild Extension Format

   This extension is appended to a RREQ by a node wishing to repair a
   multicast tree.

      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     |   Multicast Group Hop Count   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type     4

      Length   2

      Multicast Group Hop Count
               The distance in hops of the node sending the RREQ from
               the Multicast Group Leader.

   This extension is used for rebuilding a multicast tree branch.  It is
   used to ensure that only nodes as least as close to the group leader
   as indicated by the Multicast Group Hop Count field respond to the
   request.


16.4. Multicast Group Information Extension Format

   The following extension is used to carry additional information for
   the RREP message (see Section 5) when sent to establish a route to a
   multicast destination.

    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     |   Multicast Group Hop Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Multicast Group Leader IP Address               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Type     5

      Length   6

      Multicast Group Hop Count
               The distance of the node from the Multicast Group Leader.

      Multicast Group Leader IP Address
               The IP Address of the current Multicast Group Leader.



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   This extension is included when responding to a RREQ to join a
   multicast group.  The node responding to the RREQ places its distance
   from the group leader in the Multicast Group Hop Count field.


16.5. Maximum Delay 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     |          Max Delay            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type         6

      Length       2

      Max Delay    The number of seconds allowed for a transmission from
                   the source to the destination.

   The Maximum Delay Extension can be appended to a RREQ by a requesting
   node in order to place a maximum bound on the acceptable time
   delay experienced on any acceptable path from the source to the
   destination.

   Before forwarding the RREQ, an intermediate node MUST compare its
   NODE_TRAVERSAL_TIME to the (remaining) Max Delay indicated in the
   Maximum Delay Extension.  If the Max Delay is less, the node MUST
   discard the RREQ and not process it any further.  Otherwise, the
   node subtracts NODE_TRAVERSAL_TIME from the Max Delay value in
   the extension and continues processing the RREQ as specified in
   Section 9.3.


16.6. Minimum Bandwidth 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     |    Minimum Bandwidth ...      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | ...  Minimum Bandwidth        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type         7

      Length       4





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      Minimum Bandwidth
                   The amount of bandwidth (in kilobits/sec) needed
                   for acceptable transmission from the source to the
                   destination.

   The Minimum Bandwidth Extension can be appended to a RREQ by a
   requesting node in order to specify the minimal amount of bandwidth
   that must be made available along acceptable path from the source to
   the destination.

   Before forwarding the RREQ, an intermediate node MUST compare its
   available link capacity to the Minimum Bandwidth indicated in the
   extension.  If the requested amount of bandwidth is not available,
   the node MUST discard the RREQ and not process it any further.
   Otherwise, the node continues processing the RREQ as specified in
   Section 9.3.


17. 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, 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.
























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      Parameter Name           Value
      ----------------------   -----
      ACTIVE_ROUTE_TIMEOUT     3,000 Milliseconds
      ALLOWED_HELLO_LOSS       2
      BAD_LINK_LIFETIME        2 * RREP_WAIT_TIME
      BCAST_ID_SAVE            30,000 Milliseconds
      BROADCAST_RECORD_TIME    RREP_WAIT_TIME
      DELETE_PERIOD            see note below
      GROUP_HELLO_INTERVAL     5,000 Milliseconds
      HELLO_INTERVAL           1,000 Milliseconds
      MTREE_BUILD              2 * REV_ROUTE_LIFE
      MY_ROUTE_TIMEOUT         2 * ACTIVE_ROUTE_TIMEOUT
      NET_DIAMETER             35
      NEXT_HOP_WAIT            NODE_TRAVERSAL_TIME + 10
      NODE_TRAVERSAL_TIME      40
      PRUNE_TIMEOUT            ACTIVE_ROUTE_TIMEOUT
      REV_ROUTE_LIFE           RREP_WAIT_TIME
      RREP_WAIT_TIME           3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2
      RREQ_RETRIES             2
      TTL_START                1
      TTL_INCREMENT            2
      TTL_THRESHOLD            7



   DELETE_PERIOD should be an upper bound on the time for which
   an upstream node A can have a neighbor B to be an active next
   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).



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   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 [3] 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.


18. 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.


19. Acknowledgements

   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
   and Mahesh Marina.












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References

   [1] 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.

   [2] E. Guttman and S. Cheshire (chairs).  Zero Configuration
       Networking (zeroconf), June 1999.
       http://www.ietf.org/html.charters/zeroconf-charter.html.

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

   [4] Charles E. Perkins.  Terminology for Ad-Hoc Networking.
       draft-ietf-manet-terms-00.txt, November 1997.  (work in
       progress).

   [5] S. Thomson and T. Narten.  IPv6 Stateless Address
       Autoconfiguration.  Request for Comments (Draft Standard)
       2462, Internet Engineering Task Force, December 1998.































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A. Draft Modifications

   The following are major changes between this version (05) of the AODV
   draft and the previous version (04):

    -  Processing Route Requests section.  This section has been
       modified so that the Destination Sequence Number field of the
       RREQ always contains the greatest sequence number seen along the
       route.

    -  Forwarding Route Replies section.  This section has been modified
       so that only RREPs with a greater sequence number than what was
       previously known are forwarded.  RREPs with smaller sequence
       number are suppressed.

    -  RERR section modifications.  This section has been altered to
       more clearly indicate when a RERR is sent, and the actions to
       be taken on reception of a RERR. Additionally, the RERR message
       has been modified so that the sequence number of each listed
       destination, incremented by one, is included.  This section also
       now includes the process of deleting neighbors from precursor
       lists.

    -  Addition of Route Expiry and Deletion section.  This section
       describes the purpose of the DELETE_PERIOD, where a node must
       keep a record of an expired route for at least DELETE_PERIOD
       before it may delete the route entirely.

    -  Addition of Actions After Reboot section.  This section describes
       the actions to be taken after a node reboots.  Specifically,
       because a rebooted node will have lost all its routes, it must
       wait DELETE_PERIOD before responding to any routing packets.
       Additionally, it must broadcast a RERR packet for any data
       packets that are sent to it within this time and then reset its
       DELETE_PERIOD timer.

    -  Addition of Actions After Reboot section for multicast.  A Reboot
       flag has been added to the MACT message.  Since a rebooted node
       has lost all of its multicast tree information and does not know
       whether it was participating in multicast before it was rebooted,
       it must broadcast a reboot MACT message upon boot to inform its
       neighbors it has lost all multicast routing information.

    -  Addition of Address Autoconfiguration section.  This section
       describes the procedure for an AODV node to obtain an IP address.
       This method is intended to be compliant with that proposed by the
       zeroconf working group [2].

    -  Type numbers have been assigned to the extensions.



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Author's Addresses

   Questions about this memo can be directed to:

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


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


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




















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