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Mobile Ad Hoc Networking Working Group                   Charles Perkins
INTERNET DRAFT                                          Sun Microsystems
20 November 1997

            Ad Hoc On Demand Distance Vector (AODV) Routing
                      draft-ietf-manet-aodv-00.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.  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.''

   To learn the current status of any Internet-Draft, please check
   the ``1id-abstracts.txt'' listing contained in the Internet-Drafts
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   Europe), ftp.nis.garr.it (South Europe), munnari.oz.au (Pacific Rim),
   ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).


Abstract

   The Ad Hoc On-Demand Distance Vector (AODV) routing protocol is
   intended for use by mobile nodes in an ad hoc network characterized
   by frequent changes in link connectivity to each other caused
   by relative movement.  It offers quick adaptation to dynamic
   link conditions, low processing and memory overhead, low network
   utilization, and establishment of routes between sources and
   destination which are loop free at all times.  It makes use of
   destination sequence numbers, which are a novel means of ensuring
   loop freedom even in the face of anomalous delivery of routing
   control messages, and which solve classical problems associated with
   distance vector protocols, including the problem of ''counting to
   infinity''.








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                                Contents


Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1

 2. Overview                                                           2

 3. AODV Terminology                                                   3

 4. Route Request Message Format                                       4

 5. Route Reply Message Format                                         5

 6. Node Operation                                                     5
     6.1. Maintaining Route Utilization Records . . . . . . . . . .    5
     6.2. Generating Route Requests . . . . . . . . . . . . . . . .    6
     6.3. Forwarding Route Requests . . . . . . . . . . . . . . . .    6
     6.4. Generating Route Replies  . . . . . . . . . . . . . . . .    7
     6.5. Generating Hello Messages . . . . . . . . . . . . . . . .    8
     6.6. Initiating Triggered Route Replies  . . . . . . . . . . .    8
     6.7. Detecting Link Breakage . . . . . . . . . . . . . . . . .    9

 7. Configuration Parameters                                           9

 8. Extensions                                                        10

 9. Security Considerations                                           10


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 also defines timely responses
   to link breakages.  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).





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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 itself for any usable 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 one with the greatest sequence number.

   Another feature of AODV is that link breakages cause immediate
   notifications to be sent to the affected set of nodes, but only that
   set.


2. Overview

   Route Requests (RREQs) and Route Replies (RREPs) are the two message
   types defined by AODV. These message types are handled by UDP,
   and normal IP header processing applies.  So, for instance, the
   requesting node is expected to use its IP address as the source IP
   address for the messages.  The range of dissemination of broadcast
   RREQs can be indicated by the TTL in the IP header.  Fragmentation is
   typically not required.

   As long as the endpoints of a communication connection have valid
   routes to each other, AODV does not play any role.  When a route to a
   new destination is needed, the node uses a broadcast RREQ to find a
   route to the destination.  A route can be determined when the request
   reaches either the destination itself, or an intermediate node with a
   fresh enough route to the destination.  The route is made available
   by unicasting a RREP back to the source of the RREQ. Since each node
   receiving the request keeps track of a route back to the source of
   the request, the RREP Reply 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.

   If a RREP is broadcast to the limited broadcast address
   (255.255.255.255), and has a TTL of one, and a destination address of
   the node itself with metric 0, then it is received by all the node's
   neighbors, and treated by them as a "hello" message.  This hello
   message is a local advertisement for the continued presence of the
   node.  Neighbors that are using routes through the broadcasting node
   will continue to mark the routes as valid.  If hello messages from a
   particular node stop coming, the neighbor can assume that the node
   has moved away.  When that happens, the neighbor will mark the link
   to the node as broken, and may trigger a notification to some of its
   other neighbors that the link has broken.






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   Since AODV is a routing protocol, it deals with route table
   management.  AODV assumes the following fields exist in each route
   table entry:

      - Destination IP Address
      - Destination Sequence Number
      - Hop Count
      - Next Hop
      - Lifetime


   This information must be kept even for ephemeral routes, such as are
   created to temporarily keep track of reverse paths towards nodes
   originating RREQs.


3. AODV Terminology

   This section defines terminology used with AODV that is not already
   defined in [2].

      route table

         The table where ad-hoc nodes keep routing (including next hop)
         information for various destinations.  For IPv6, this can be
         associated with the Destination Cache.

      triggered update

         An unsolicited route update transmitted by an intermediate node
         along the path to the destination.

   This protocol specification uses conventional meanings [1] for
   capitalized words such as MUST, SHOULD, etc., to indicate requirement
   levels for various protocol features.

















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4. Route Request 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           |   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:

      Type     xx

      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 identifying the particular RREQ
               uniquely when taken in conjunction with the source node's
               IP address.

      Destination IP Address
               The IP address of the 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 for route information
               generated by the source of the route request.




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5. Route Reply 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      |L|          Reserved           |   Hop Count   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Destination IP address                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Destination Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Lifetime                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Type     xx

      Reserved Sent as 0; ignored on reception.

      Hop Count The number of hops from the Source IP Address to the
               Destination IP Address.

      L        If the 'L' bit is set, the message is a "hello" message
               and contains a list of the node's neighbors.

      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

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


6. Node Operation

   This section describes the scenarios under which nodes generate RREQs
   and RREPs, and how the fields in the message are handled.


6.1. Maintaining Route Utilization Records

   For each valid route maintained by a node (containing a finite
   metric), the node also maintains a list of those neighbors that are



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   actively using the route.  This active-list of neighbors will receive
   notifications from the node in the event of detection of a link
   breakage.


6.2. 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.  Routes can become invalid
   if they time out (the Lifetime associated with the route expires),
   or else if a link breakage results in an infinite metric being
   associated with the route.  When a route table entry is marked
   with an infinite metric, its expiration time is also updated to be
   the current time plus BAD_LINK_LIFETIME milliseconds.  After the
   expiration time, the route MAY be expunged from the node's route
   table.

   After broadcasting a RREQ a node waits for a RREP, and if the
   reply is not received within RREP_WAIT_TIME seconds, the node may
   rebroadcast the RREQ. The RREQ may be rebroadcast up to a maximum of
   RREQ_RETRIES times.  Each rebroadcast has to increment the Broadcast
   ID field.  The node MAY choose to use larger TTL values in the IP
   header field, or wait for longer times for the RREP to arrive.


6.3. Forwarding Route Requests

   When a node receives a broadcast RREQ, it first checks to see
   whether it has received a RREQ with the same Source IP Address field
   within the last BCAST_ID_SAVE milliseconds.  If such a RREQ has
   been received, the node silently discards the newly received RREQ.
   Otherwise, the node checks to see whether it has a route to the
   destination.  If the node does not have a route, it rebroadcasts the
   RREQ from its interface(s) with the same field values, but using its
   own IP address in the IP header of the outgoing RREQ. 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 to the new hop through the intermediate node.  In this
   case, the node also creates a reverse route to the Source IP Address
   in its routing table with next hop equal to the IP address of the
   neighboring node that sent the broadcast RREQ (often not equal to the
   Source IP Address field in the RREQ message).  This reverse route
   might be used for an eventual RREP back to the original node making
   the RREQ (identified by the Source IP Address).  The reverse route is
   put into the route table with lifetime REV_ROUTE_LIFE milliseconds.





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   If, on the other hand, the node does have a route for the
   destination, it compares the destination sequence number (dest-seqno)
   for that route with the Destination Sequence Number field of the
   incoming RREQ. If the node's existing dest-seqno is smaller than
   the Destination Sequence Number field of the RREQ, the node again
   rebroadcasts the RREQ just as if it did not have a route to the
   destination at all.

      In this case, the intermediate node MAY also transmit a RREQ
      to the active-list associated with the stale route to that
      destination?

   If the node has a route to the destination, and the node's existing
   dest-seqno is greater than or equal to the Destination Sequence
   Number of the RREQ, then the node generates a RREP as discussed
   further in section 6.4.


6.4. Generating Route Replies

   If a node receives a route request for a destination, and has a
   fresh enough route to satisfy the request, the node generates a RREP
   message and unicasts it back to the node indicated by the Source
   IP Address field of the received RREQ. First, the node copies over
   its destination sequence number from the entry in its route table,
   or if the generating node is the node itself, it uses a destination
   sequence number at least equal to a sequence number generated after
   the last detected change in its neighbor set.  If the node has not
   detected any change in its set of neighbors since it last incremented
   it destination sequence number, it may use the same destination
   sequence number.

   As part of the process of generating the RREP, the generating node
   creates or updates an entry in its routing table for the Source
   IP Address, if necessary as described in section 6.3.  The Source
   Sequence Number is put into the route entry, along with the Hop Count
   from the RREQ. The expiration time for the route table entry is set
   to the current time plus ACTIVE_ROUTE_TIMEOUT seconds.

   If the generating node is not the destination node, then the
   generating node calculates the Hop Count between the Source IP
   Address and the Destination IP Address by adding together the Hop
   Count from the RREQ and the hop count stored in the route table entry
   for the destination node.  If, on the other hand, the generating node
   is the destination node itself, the Hop Count field in the RREP is
   simply equal to the Hop Count received in the RREQ.

   If the node is not the destination node, it calculates the Lifetime
   field of the RREQ by subtracting the current time from the expiration



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   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.

   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
   active-list for the reverse path route entry.


6.5. Generating Hello Messages

   Every node generates a "hello" message once every HELLO_INTERVAL
   milliseconds.  This hello message is a broadcast IP RREP with TTL =
   1, and the message fields set as follows:

      Destination IP Address
                  the node's IP address,

      Destination Sequence Number
                  the latest sequence number

      Hop Count   0

      Lifetime    (1 + ALLOWED_HELLO_LOSS) * HELLO_INTERVAL


6.6. Initiating Triggered Route Replies

   A node can trigger an unsolicited RREP if either it detects a link
   breakage for a next hop along an active route in its route table, or
   if it receives a RREP from a neighbor with an infinite metric for an
   active route (i.e., containing a Destination IP Address for which
   there is a route table entry with a nonempty active-list)

   The unsolicited RREP is unicast to each neighbor in the nonempty
   active-list for the route to that destination.  The contents of the
   RREP fields are set as follows:

      L           0

      Hop Count   65,535

      Destination IP Address
                  The destination in the broken route

      Destination Sequence Number
                  One plus the destination sequence number recorded in
                  the route.




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6.7. Detecting Link Breakage

   A node can detect a link breakage by listening for "hello" messages
   from its set of neighbors.  If it has received hello messages from
   a particular neighbor, but misses more than ALLOWED_HELLO_LOSS
   consecutive hello messages from that neighbor, the node can presume
   that the particular neighbor is no longer able to maintain a direct
   link with the mobile node.  When this happens, the node should assume
   that its link with the former neighbor has been broken, and proceed
   as in Section 6.6.  A node should assume that a hello message has
   been missed if it is not received within 1.5 times the duration of
   the HELLO_INTERVAL.

   Alternatively, the node can use any physical-layer or link-layer
   methods to detect link breakages with nodes it has considered as
   neighbors.


7. Configuration Parameters

   This section gives default values for some important values
   associated with AODV protocol operations.

      ACTIVE_ROUTE_TIMEOUT   300

      ALLOWED_HELLO_LOSS     2

      BAD_LINK_LIFETIME      3000

      BCAST_ID_SAVE          3000

      HELLO_INTERVAL         1000

      NETWORK_DIAMETER       100

      NODE_TRAVERSAL_TIME    400

      MY_ROUTE_TIMEOUT       600

      REV_ROUTE_LIFE         3000

      RREP_WAIT_TIME         3 * NODE_TRAVERSAL_TIME * NETWORK_DIAMETER

      RREQ_RETRIES           3








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8. Extensions

   RREQ and RREP messages may have extensions defined in future versions
   of the protocol.  These extensions will have the following format:

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

   where:

      Type     xx

      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.


9. 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.



















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References

   [1] S. Bradner.  Key words for use in RFCs to Indicate Requirement
       Levels.  RFC 2119, March 1997.

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


Author's Address

   Questions about this memo can be directed to:

      Charles E. Perkins
      Sun Microsystems
      901 San Antonio Rd.
      Palo Alto, CA, 94303
      USA
      1 650 786 6464
      1 650 786 6445 (fax)
      cperkins@eng.sun.com






























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