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rfc:rfc3561

Network Working Group C. Perkins Request for Comments: 3561 Nokia Research Center Category: Experimental E. Belding-Royer

                               University of California, Santa Barbara
                                                                S. Das
                                              University of Cincinnati
                                                             July 2003
          Ad hoc On-Demand Distance Vector (AODV) Routing

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 The Ad hoc On-Demand Distance Vector (AODV) routing protocol is
 intended for use by mobile nodes in an ad hoc network.  It offers
 quick adaptation to dynamic link conditions, low processing and
 memory overhead, low network utilization, and determines unicast
 routes to destinations within the ad hoc network.  It uses
 destination sequence numbers to ensure loop freedom at all times
 (even in the face of anomalous delivery of routing control messages),
 avoiding problems (such as "counting to infinity") associated with
 classical distance vector protocols.

Table of Contents

 1.  Introduction ...............................................  2
 2.  Overview  ..................................................  3
 3.  AODV Terminology ...........................................  4
 4.  Applicability Statement ....................................  6
 5.  Message Formats ............................................  7
     5.1. Route Request (RREQ) Message Format ...................  7
     5.2. Route Reply (RREP) Message Format .....................  8
     5.3. Route Error (RERR) Message Format ..................... 10
     5.4. Route Reply Acknowledgment (RREP-ACK) Message Format .. 11
 6.  AODV Operation ............................................. 11
     6.1. Maintaining Sequence Numbers .......................... 11
     6.2. Route Table Entries and Precursor Lists ............... 13

Perkins, et. al. Experimental [Page 1] RFC 3561 AODV Routing July 2003

     6.3. Generating Route Requests ............................. 14
     6.4. Controlling Dissemination of Route Request Messages ... 15
     6.5. Processing and Forwarding Route Requests .............. 16
     6.6. Generating Route Replies .............................. 18
          6.6.1. Route Reply Generation by the Destination ...... 18
          6.6.2. Route Reply Generation by an Intermediate
                 Node ........................................... 19
          6.6.3. Generating Gratuitous RREPs .................... 19
     6.7. Receiving and Forwarding Route Replies ................ 20
     6.8. Operation over Unidirectional Links ................... 21
     6.9. Hello Messages ........................................ 22
     6.10 Maintaining Local Connectivity ........................ 23
     6.11 Route Error (RERR) Messages, Route Expiry and Route
          Deletion .............................................. 24
     6.12 Local Repair .......................................... 26
     6.13 Actions After Reboot  ................................. 27
     6.14 Interfaces ............................................ 28
 7.  AODV and Aggregated Networks ............................... 28
 8.  Using AODV with Other Networks ............................. 29
 9.  Extensions ................................................. 30
     9.1. Hello Interval Extension Format ....................... 30
 10. Configuration Parameters ................................... 31
 11. Security Considerations .................................... 33
 12. IANA Considerations ........................................ 34
 13. IPv6 Considerations ........................................ 34
 14. Acknowledgments ............................................ 34
 15. Normative References ....................................... 35
 16. Informative References ..................................... 35
 17. Authors' Addresses ......................................... 36
 18. Full Copyright Statement ................................... 37

1. Introduction

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

Perkins, et. al. Experimental [Page 2] RFC 3561 AODV Routing July 2003

 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 to be included along with 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 is required to select the one with the greatest sequence number.

2. Overview

 Route Requests (RREQs), Route Replies (RREPs), and Route Errors
 (RERRs) are the message types defined by AODV.  These message types
 are received via UDP, and normal IP header processing applies. So,
 for instance, the requesting node is expected to use its IP address
 as the Originator IP address for the messages.  For broadcast
 messages, the IP limited broadcast address (255.255.255.255) is used.
 This means that such messages are not blindly forwarded.  However,
 AODV operation does require certain messages (e.g., RREQ) to be
 disseminated widely, perhaps throughout the ad hoc network.  The
 range of dissemination of such RREQs is indicated by the TTL in the
 IP header.  Fragmentation is typically not required.
 As long as the endpoints of a communication connection have valid
 routes to each other, AODV does not play any role.  When a route to a
 new destination is needed, the node broadcasts a RREQ to find a route
 to the destination.  A route can be determined when the RREQ reaches
 either the destination itself, or an intermediate node with a 'fresh
 enough' route to the destination.  A 'fresh enough' route is a valid
 route entry for the destination whose associated sequence number is
 at least as great as that contained in the RREQ.  The route is made
 available by unicasting a RREP back to the origination of the RREQ.
 Each node receiving the request caches a route back to the originator
 of the request, so that the RREP can be unicast from the destination
 along a path to that originator, or likewise from any intermediate
 node that is able to satisfy the request.
 Nodes monitor the link status of next hops in active routes.  When a
 link break in an active route is detected, a RERR message is used to
 notify other nodes that the loss of that link has occurred.  The RERR
 message indicates those destinations (possibly subnets) which are no
 longer reachable by way of the broken link.  In order to enable this
 reporting mechanism, each node keeps a "precursor list", containing
 the IP address for each its neighbors that are likely to use it as a
 next hop towards each destination.  The information in the precursor
 lists is most easily acquired during the processing for generation of
 a RREP message, which by definition has to be sent to a node in a
 precursor list (see section 6.6).  If the RREP has a nonzero prefix

Perkins, et. al. Experimental [Page 3] RFC 3561 AODV Routing July 2003

 length, then the originator of the RREQ which solicited the RREP
 information is included among the precursors for the subnet route
 (not specifically for the particular destination).
 A RREQ may also be received for a multicast IP address.  In this
 document, full processing for such messages is not specified.  For
 example, the originator of such a RREQ for a multicast IP address may
 have to follow special rules.  However, it is important to enable
 correct multicast operation by intermediate nodes that are not
 enabled as originating or destination nodes for IP multicast
 addresses, and likewise are not equipped for any special multicast
 protocol processing.  For such multicast-unaware nodes, processing
 for a multicast IP address as a destination IP address MUST be
 carried out in the same way as for any other destination IP address.
 AODV is a routing protocol, and it deals with route table management.
 Route table information must be kept even for short-lived routes,
 such as are created to temporarily store reverse paths towards nodes
 originating RREQs.  AODV uses the following fields with each route
 table entry:
  1. Destination IP Address
  2. Destination Sequence Number
  3. Valid Destination Sequence Number flag
  4. Other state and routing flags (e.g., valid, invalid, repairable,

being repaired)

  1. Network Interface
  2. Hop Count (number of hops needed to reach destination)
  3. Next Hop
  4. List of Precursors (described in Section 6.2)
  5. Lifetime (expiration or deletion time of the route)
 Managing the sequence number is crucial to avoiding routing loops,
 even when links break and a node is no longer reachable to supply its
 own information about its sequence number.  A destination becomes
 unreachable when a link breaks or is deactivated.  When these
 conditions occur, the route is invalidated by operations involving
 the sequence number and marking the route table entry state as
 invalid.  See section 6.1 for details.

3. AODV Terminology

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

Perkins, et. al. Experimental [Page 4] RFC 3561 AODV Routing July 2003

    active route
       A route towards a destination that has a routing table entry
       that is marked as valid.  Only active routes can be used to
       forward data packets.
    broadcast
       Broadcasting means transmitting to the IP Limited Broadcast
       address, 255.255.255.255.  A broadcast packet may not be
       blindly forwarded, but broadcasting is useful to enable
       dissemination of AODV messages throughout the ad hoc network.
    destination
       An IP address to which data packets are to be transmitted.
       Same as "destination node".  A node knows it is the destination
       node for a typical data packet when its address appears in the
       appropriate field of the IP header.  Routes for destination
       nodes are supplied by action of the AODV protocol, which
       carries the IP address of the desired destination node in route
       discovery messages.
    forwarding node
       A node that agrees to forward packets destined for another
       node, by retransmitting them to a next hop that is closer to
       the unicast destination along a path that has been set up using
       routing control messages.
    forward route
       A route set up to send data packets from a node originating a
       Route Discovery operation towards its desired destination.
    invalid route
       A route that has expired, denoted by a state of invalid in the
       routing table entry.  An invalid route is used to store
       previously valid route information for an extended period of
       time.  An invalid route cannot be used to forward data packets,
       but it can provide information useful for route repairs, and
       also for future RREQ messages.

Perkins, et. al. Experimental [Page 5] RFC 3561 AODV Routing July 2003

    originating node
       A node that initiates an AODV route discovery message to be
       processed and possibly retransmitted by other nodes in the ad
       hoc network.  For instance, the node initiating a Route
       Discovery process and broadcasting the RREQ message is called
       the originating node of the RREQ message.
    reverse route
       A route set up to forward a reply (RREP) packet back to the
       originator from the destination or from an intermediate node
       having a route to the destination.
    sequence number
       A monotonically increasing number maintained by each
       originating node.  In AODV routing protocol messages, it is
       used by other nodes to determine the freshness of the
       information contained from the originating node.
    valid route
       See active route.

4. Applicability Statement

 The AODV routing protocol is designed for mobile ad hoc networks with
 populations of tens to thousands of mobile nodes.  AODV can handle
 low, moderate, and relatively high mobility rates, as well as a
 variety of data traffic levels.  AODV is designed for use in networks
 where the nodes can all trust each other, either by use of
 preconfigured keys, or because it is known that there are no
 malicious intruder nodes.  AODV has been designed to reduce the
 dissemination of control traffic and eliminate overhead on data
 traffic, in order to improve scalability and performance.

Perkins, et. al. Experimental [Page 6] RFC 3561 AODV Routing July 2003

5. Message Formats

5.1. Route Request (RREQ) Message Format

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |J|R|G|D|U|   Reserved          |   Hop Count   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            RREQ ID                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Destination IP Address                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Destination Sequence Number                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Originator IP Address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Originator Sequence Number                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format of the Route Request message is illustrated above, and
 contains the following fields:
    Type           1
    J              Join flag; reserved for multicast.
    R              Repair flag; reserved for multicast.
    G              Gratuitous RREP flag; indicates whether a
                   gratuitous RREP should be unicast to the node
                   specified in the Destination IP Address field (see
                   sections 6.3, 6.6.3).
    D              Destination only flag; indicates only the
                   destination may respond to this RREQ (see
                   section 6.5).
    U              Unknown sequence number; indicates the destination
                   sequence number is unknown (see section 6.3).
    Reserved       Sent as 0; ignored on reception.
    Hop Count      The number of hops from the Originator IP Address
                   to the node handling the request.

Perkins, et. al. Experimental [Page 7] RFC 3561 AODV Routing July 2003

    RREQ ID        A sequence number uniquely identifying the
                   particular RREQ when taken in conjunction with the
                   originating node's IP address.
    Destination IP Address
                   The IP address of the destination for which a route
                   is desired.
    Destination Sequence Number
                   The latest sequence number received in the past
                   by the originator for any route towards the
                   destination.
    Originator IP Address
                   The IP address of the node which originated the
                   Route Request.
    Originator Sequence Number
                   The current sequence number to be used in the route
                   entry pointing towards the originator of the route
                   request.

5.2. Route Reply (RREP) Message Format

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |R|A|    Reserved     |Prefix Sz|   Hop Count   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Destination IP address                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Destination Sequence Number                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Originator IP address                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           Lifetime                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format of the Route Reply message is illustrated above, and
 contains the following fields:
    Type          2
    R             Repair flag; used for multicast.
    A             Acknowledgment required; see sections 5.4 and 6.7.
    Reserved      Sent as 0; ignored on reception.

Perkins, et. al. Experimental [Page 8] RFC 3561 AODV Routing July 2003

    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 Originator IP Address
                  to the Destination IP Address.  For multicast route
                  requests this indicates the number of hops to the
                  multicast tree member sending the RREP.
    Destination IP Address
                  The IP address of the destination for which a route
                  is supplied.
    Destination Sequence Number
                  The destination sequence number associated to the
                  route.
    Originator IP Address
                  The IP address of the node which originated the RREQ
                  for which the route is supplied.
    Lifetime      The time in milliseconds for which nodes receiving
                  the RREP consider the route to be valid.
 Note that the Prefix Size allows a subnet router 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 router and the Prefix
 Size.  In order to make use of this feature, the subnet router has to
 guarantee reachability to all the hosts sharing the indicated subnet
 prefix.  See section 7 for details.  When the prefix size is nonzero,
 any routing information (and precursor data) MUST be kept with
 respect to the subnet route, not the individual destination IP
 address on that subnet.
 The 'A' bit is used when the link over which the RREP message is sent
 may be unreliable or unidirectional.  When the RREP message contains
 the 'A' bit set, the receiver of the RREP is expected to return a
 RREP-ACK message.  See section 6.8.

Perkins, et. al. Experimental [Page 9] RFC 3561 AODV Routing July 2003

5.3. Route Error (RERR) Message Format

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |N|          Reserved           |   DestCount   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Unreachable Destination IP Address (1)             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Unreachable Destination Sequence Number (1)           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |  Additional Unreachable Destination IP Addresses (if needed)  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Additional Unreachable Destination Sequence Numbers (if needed)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format of the Route Error message is illustrated above, and
 contains the following fields:
    Type        3
    N           No delete flag; set when a node has performed a local
                repair of a link, and upstream nodes should not delete
                the route.
    Reserved    Sent as 0; ignored on reception.
    DestCount   The number of unreachable destinations included in the
                message; MUST be at least 1.
    Unreachable Destination IP Address
                The IP address of the destination that has become
                unreachable due to a link break.
    Unreachable Destination Sequence Number
                The sequence number in the route table entry for
                the destination listed in the previous Unreachable
                Destination IP Address field.
 The RERR message is sent whenever a link break causes one or more
 destinations to become unreachable from some of the node's neighbors.
 See section 6.2 for information about how to maintain the appropriate
 records for this determination, and section 6.11 for specification
 about how to create the list of destinations.

Perkins, et. al. Experimental [Page 10] RFC 3561 AODV Routing July 2003

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

 The Route Reply Acknowledgment (RREP-ACK) message MUST be sent in
 response to a RREP message with the 'A' bit set (see section 5.2).
 This is typically done when there is danger of unidirectional links
 preventing the completion of a Route Discovery cycle (see section
 6.8).
  0                   1
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |   Reserved    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type        4
    Reserved    Sent as 0; ignored on reception.

6. AODV Operation

 This section describes the scenarios under which nodes generate Route
 Request (RREQ), Route Reply (RREP) and Route Error (RERR) messages
 for unicast communication towards a destination, and how the message
 data are handled.  In order to process the messages correctly,
 certain state information has to be maintained in the route table
 entries for the destinations of interest.
 All AODV messages are sent to port 654 using UDP.

6.1. Maintaining Sequence Numbers

 Every route table entry at every node MUST include the latest
 information available about the sequence number for the IP address of
 the destination node for which the route table entry is maintained.
 This sequence number is called the "destination sequence number".  It
 is updated whenever a node receives new (i.e., not stale) information
 about the sequence number from RREQ, RREP, or RERR messages that may
 be received related to that destination.  AODV depends on each node
 in the network to own and maintain its destination sequence number to
 guarantee the loop-freedom of all routes towards that node.  A
 destination node increments its own sequence number in two
 circumstances:
  1. Immediately before a node originates a route discovery, it MUST

increment its own sequence number. This prevents conflicts with

    previously established reverse routes towards the originator of a
    RREQ.

Perkins, et. al. Experimental [Page 11] RFC 3561 AODV Routing July 2003

  1. Immediately before a destination node originates a RREP in

response to a RREQ, it MUST update its own sequence number to the

    maximum of its current sequence number and the destination
    sequence number in the RREQ packet.
 When the destination increments its sequence number, it MUST do so by
 treating the sequence number value as if it were an unsigned number.
 To accomplish sequence number rollover, if the sequence number has
 already been assigned to be the largest possible number representable
 as a 32-bit unsigned integer (i.e., 4294967295), then when it is
 incremented it will then have a value of zero (0).  On the other
 hand, if the sequence number currently has the value 2147483647,
 which is the largest possible positive integer if 2's complement
 arithmetic is in use with 32-bit integers, the next value will be
 2147483648, which is the most negative possible integer in the same
 numbering system.  The representation of negative numbers is not
 relevant to the increment of AODV sequence numbers.  This is in
 contrast to the manner in which the result of comparing two AODV
 sequence numbers is to be treated (see below).
 In order to ascertain that information about a destination is not
 stale, the node compares its current numerical value for the sequence
 number with that obtained from the incoming AODV message.  This
 comparison MUST be done using signed 32-bit arithmetic, this is
 necessary to accomplish sequence number rollover.  If the result of
 subtracting the currently stored sequence number from the value of
 the incoming sequence number is less than zero, then the information
 related to that destination in the AODV message MUST be discarded,
 since that information is stale compared to the node's currently
 stored information.
 The only other circumstance in which a node may change the
 destination sequence number in one of its route table entries is in
 response to a lost or expired link to the next hop towards that
 destination.  The node determines which destinations use a particular
 next hop by consulting its routing table.  In this case, for each
 destination that uses the next hop, the node increments the sequence
 number and marks the route as invalid (see also sections 6.11, 6.12).
 Whenever any fresh enough (i.e., containing a sequence number at
 least equal to the recorded sequence number) routing information for
 an affected destination is received by a node that has marked that
 route table entry as invalid, the node SHOULD update its route table
 information according to the information contained in the update.

Perkins, et. al. Experimental [Page 12] RFC 3561 AODV Routing July 2003

 A node may change the sequence number in the routing table entry of a
 destination only if:
  1. it is itself the destination node, and offers a new route to

itself, or

  1. it receives an AODV message with new information about the

sequence number for a destination node, or

  1. the path towards the destination node expires or breaks.

6.2. Route Table Entries and Precursor Lists

 When a node receives an AODV control packet from a neighbor, or
 creates or updates a route for a particular destination or subnet, it
 checks its route table for an entry for the destination.  In the
 event that there is no corresponding entry for that destination, an
 entry is created.  The sequence number is either determined from the
 information contained in the control packet, or else the valid
 sequence number field is set to false.  The route is only updated if
 the new sequence number is either
 (i)       higher than the destination sequence number in the route
           table, or
 (ii)      the sequence numbers are equal, but the hop count (of the
           new information) plus one, is smaller than the existing hop
           count in the routing table, or
 (iii)     the sequence number is unknown.
 The Lifetime field of the routing table entry is either determined
 from the control packet, or it is initialized to
 ACTIVE_ROUTE_TIMEOUT.  This route may now be used to send any queued
 data packets and fulfills any outstanding route requests.
 Each time a route is used to forward a data packet, its Active Route
 Lifetime field of the source, destination and the next hop on the
 path to the destination is updated to be no less than the current
 time plus ACTIVE_ROUTE_TIMEOUT.  Since the route between each
 originator and destination pair is expected to be symmetric, the
 Active Route Lifetime for the previous hop, along the reverse path
 back to the IP source, is also updated to be no less than the current
 time plus ACTIVE_ROUTE_TIMEOUT.  The lifetime for an Active Route is
 updated each time the route is used regardless of whether the
 destination is a single node or a subnet.

Perkins, et. al. Experimental [Page 13] RFC 3561 AODV Routing July 2003

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

6.3. Generating Route Requests

 A node disseminates a RREQ when it determines that it needs a route
 to a destination and does not have one available.  This can happen if
 the destination is previously unknown to the node, or if a previously
 valid route to the destination expires or is marked as invalid.  The
 Destination Sequence Number field in the RREQ message is the last
 known destination sequence number for this destination and is copied
 from the Destination Sequence Number field in the routing table.  If
 no sequence number is known, the unknown sequence number flag MUST be
 set.  The Originator Sequence Number in the RREQ message is the
 node's own sequence number, which is incremented prior to insertion
 in a RREQ.  The RREQ ID field is incremented by one from the last
 RREQ ID used by the current node.  Each node maintains only one RREQ
 ID.  The Hop Count field is set to zero.
 Before broadcasting the RREQ, the originating node buffers the RREQ
 ID and the Originator IP address (its own address) of the RREQ for
 PATH_DISCOVERY_TIME.  In this way, when the node receives the packet
 again from its neighbors, it will not reprocess and re-forward the
 packet.
 An originating node often expects to have bidirectional
 communications with a destination node.  In such cases, it is not
 sufficient for the originating node to have a route to the
 destination node; the destination must also have a route back to the
 originating node.  In order for this to happen as efficiently as
 possible, any generation of a RREP by an intermediate node (as in
 section 6.6) for delivery to the originating node SHOULD be
 accompanied by some action that notifies the destination about a
 route back to the originating node.  The originating node selects
 this mode of operation in the intermediate nodes by setting the 'G'
 flag.  See section 6.6.3 for details about actions taken by the
 intermediate node in response to a RREQ with the 'G' flag set.
 A node SHOULD NOT originate more than RREQ_RATELIMIT RREQ messages
 per second.  After broadcasting a RREQ, a node waits for a RREP (or
 other control message with current information regarding a route to
 the appropriate destination).  If a route is not received within
 NET_TRAVERSAL_TIME milliseconds, the node MAY try again to discover a
 route by broadcasting another RREQ, up to a maximum of RREQ_RETRIES

Perkins, et. al. Experimental [Page 14] RFC 3561 AODV Routing July 2003

 times at the maximum TTL value.  Each new attempt MUST increment and
 update the RREQ ID.  For each attempt, the TTL field of the IP header
 is set according to the mechanism specified in section 6.4, in order
 to enable control over how far the RREQ is disseminated for the each
 retry.
 Data packets waiting for a route (i.e., waiting for a RREP after a
 RREQ has been sent) SHOULD be buffered.  The buffering SHOULD be
 "first-in, first-out" (FIFO).  If a route discovery has been
 attempted RREQ_RETRIES times at the maximum TTL without receiving any
 RREP, all data packets destined for the corresponding destination
 SHOULD be dropped from the buffer and a Destination Unreachable
 message SHOULD be delivered to the application.
 To reduce congestion in a network, repeated attempts by a source node
 at route discovery for a single destination MUST utilize a binary
 exponential backoff.  The first time a source node broadcasts a RREQ,
 it waits NET_TRAVERSAL_TIME milliseconds for the reception of a RREP.
 If a RREP is not received within that time, the source node sends a
 new RREQ.  When calculating the time to wait for the RREP after
 sending the second RREQ, the source node MUST use a binary
 exponential backoff.  Hence, the waiting time for the RREP
 corresponding to the second RREQ is 2 * NET_TRAVERSAL_TIME
 milliseconds.  If a RREP is not received within this time period,
 another RREQ may be sent, up to RREQ_RETRIES additional attempts
 after the first RREQ.  For each additional attempt, the waiting time
 for the RREP is multiplied by 2, so that the time conforms to a
 binary exponential backoff.

6.4. Controlling Dissemination of Route Request Messages

 To prevent unnecessary network-wide dissemination of RREQs, the
 originating node SHOULD use an expanding ring search technique.  In
 an expanding ring search, the originating node initially uses a TTL =
 TTL_START in the RREQ packet IP header and sets the timeout for
 receiving a RREP to RING_TRAVERSAL_TIME milliseconds.
 RING_TRAVERSAL_TIME is calculated as described in section 10.  The
 TTL_VALUE used in calculating RING_TRAVERSAL_TIME is set equal to the
 value of the TTL field in the IP header.  If the RREQ times out
 without a corresponding RREP, the originator broadcasts the RREQ
 again with the TTL incremented by TTL_INCREMENT.  This continues
 until the TTL set in the RREQ reaches TTL_THRESHOLD, beyond which a
 TTL = NET_DIAMETER is used for each attempt.  Each time, the timeout
 for receiving a RREP is RING_TRAVERSAL_TIME.  When it is desired to
 have all retries traverse the entire ad hoc network, this can be
 achieved by configuring TTL_START and TTL_INCREMENT both to be the
 same value as NET_DIAMETER.

Perkins, et. al. Experimental [Page 15] RFC 3561 AODV Routing July 2003

 The Hop Count stored in an invalid routing table entry indicates the
 last known hop count to that destination in the routing table.  When
 a new route to the same destination is required at a later time
 (e.g., upon route loss), the TTL in the RREQ IP header is initially
 set to the Hop Count plus TTL_INCREMENT.  Thereafter, following each
 timeout the TTL is incremented by TTL_INCREMENT until TTL =
 TTL_THRESHOLD is reached.  Beyond this TTL = NET_DIAMETER is used.
 Once TTL = NET_DIAMETER, the timeout for waiting for the RREP is set
 to NET_TRAVERSAL_TIME, as specified in section 6.3.
 An expired routing table entry SHOULD NOT be expunged before
 (current_time + DELETE_PERIOD) (see section 6.11).  Otherwise, the
 soft state corresponding to the route (e.g., last known hop count)
 will be lost.  Furthermore, a longer routing table entry expunge time
 MAY be configured.  Any routing table entry waiting for a RREP SHOULD
 NOT be expunged before (current_time + 2 * NET_TRAVERSAL_TIME).

6.5. Processing and Forwarding Route Requests

 When a node receives a RREQ, it first creates or updates a route to
 the previous hop without a valid sequence number (see section 6.2)
 then checks to determine whether it has received a RREQ with the same
 Originator IP Address and RREQ ID within at least the last
 PATH_DISCOVERY_TIME.  If such a RREQ has been received, the node
 silently discards the newly received RREQ.  The rest of this
 subsection describes actions taken for RREQs that are not discarded.
 First, it first increments the hop count value in the RREQ by one, to
 account for the new hop through the intermediate node.  Then the node
 searches for a reverse route to the Originator IP Address (see
 section 6.2), using longest-prefix matching.  If need be, the route
 is created, or updated using the Originator Sequence Number from the
 RREQ in its routing table.  This reverse route will be needed if the
 node receives a RREP back to the node that originated the RREQ
 (identified by the Originator IP Address).  When the reverse route is
 created or updated, the following actions on the route are also
 carried out:
 1. the Originator Sequence Number from the RREQ is compared to the
    corresponding destination sequence number in the route table entry
    and copied if greater than the existing value there
 2. the valid sequence number field is set to true;
 3. the next hop in the routing table becomes the node from which the
    RREQ was received (it is obtained from the source IP address in
    the IP header and is often not equal to the Originator IP Address
    field in the RREQ message);

Perkins, et. al. Experimental [Page 16] RFC 3561 AODV Routing July 2003

 4. the hop count is copied from the Hop Count in the RREQ message;
 Whenever a RREQ message is received, the Lifetime of the reverse
 route entry for the Originator IP address is set to be the maximum of
 (ExistingLifetime, MinimalLifetime), where
    MinimalLifetime =    (current time + 2*NET_TRAVERSAL_TIME -
                         2*HopCount*NODE_TRAVERSAL_TIME).
 The current node can use the reverse route to forward data packets in
 the same way as for any other route in the routing table.
 If a node does not generate a RREP (following the processing rules in
 section 6.6), and if the incoming IP header has TTL larger than 1,
 the node updates and broadcasts the RREQ to address 255.255.255.255
 on each of its configured interfaces (see section 6.14).  To update
 the RREQ, the TTL or hop limit field in the outgoing IP header is
 decreased by one, and the Hop Count field in the RREQ message is
 incremented by one, to account for the new hop through the
 intermediate node.  Lastly, the Destination Sequence number for the
 requested destination is set to the maximum of the corresponding
 value received in the RREQ message, and the destination sequence
 value currently maintained by the node for the requested destination.
 However, the forwarding node MUST NOT modify its maintained value for
 the destination sequence number, even if the value received in the
 incoming RREQ is larger than the value currently maintained by the
 forwarding node.
 Otherwise, if a node does generate a RREP, then the node discards the
 RREQ.  Notice that, if intermediate nodes reply to every transmission
 of RREQs for a particular destination, it might turn out that the
 destination does not receive any of the discovery messages.  In this
 situation, the destination does not learn of a route to the
 originating node from the RREQ messages.  This could cause the
 destination to initiate a route discovery (for example, if the
 originator is attempting to establish a TCP session).  In order that
 the destination learn of routes to the originating node, the
 originating node SHOULD set the "gratuitous RREP" ('G') flag in the
 RREQ if for any reason the destination is likely to need a route to
 the originating node.  If, in response to a RREQ with the 'G' flag
 set, an intermediate node returns a RREP, it MUST also unicast a
 gratuitous RREP to the destination node (see section 6.6.3).

Perkins, et. al. Experimental [Page 17] RFC 3561 AODV Routing July 2003

6.6. Generating Route Replies

 A node generates a RREP if either:
 (i)       it is itself the destination, or
 (ii)      it has an active route to the destination, the destination
           sequence number in the node's existing route table entry
           for the destination is valid and greater than or equal to
           the Destination Sequence Number of the RREQ (comparison
           using signed 32-bit arithmetic), and the "destination only"
           ('D') flag is NOT set.
 When generating a RREP message, a node copies the Destination IP
 Address and the Originator Sequence Number from the RREQ message into
 the corresponding fields in the RREP message.  Processing is slightly
 different, depending on whether the node is itself the requested
 destination (see section 6.6.1), or instead if it is an intermediate
 node with an fresh enough route to the destination (see section
 6.6.2).
 Once created, the RREP is unicast to the next hop toward the
 originator of the RREQ, as indicated by the route table entry for
 that originator.  As the RREP is forwarded back towards the node
 which originated the RREQ message, the Hop Count field is incremented
 by one at each hop.  Thus, when the RREP reaches the originator, the
 Hop Count represents the distance, in hops, of the destination from
 the originator.

6.6.1. Route Reply Generation by the Destination

 If the generating node is the destination itself, it MUST increment
 its own sequence number by one if the sequence number in the RREQ
 packet is equal to that incremented value.  Otherwise, the
 destination does not change its sequence number before generating the
 RREP message.  The destination node places its (perhaps newly
 incremented) sequence number into the Destination Sequence Number
 field of the RREP, and enters the value zero in the Hop Count field
 of the RREP.
 The destination node copies the value MY_ROUTE_TIMEOUT (see section
 10) into the Lifetime field of the RREP.  Each node MAY reconfigure
 its value for MY_ROUTE_TIMEOUT, within mild constraints (see section
 10).

Perkins, et. al. Experimental [Page 18] RFC 3561 AODV Routing July 2003

6.6.2. Route Reply Generation by an Intermediate Node

 If the node generating the RREP is not the destination node, but
 instead is an intermediate hop along the path from the originator to
 the destination, it copies its known sequence number for the
 destination into the Destination Sequence Number field in the RREP
 message.
 The intermediate node updates the forward route entry by placing the
 last hop node (from which it received the RREQ, as indicated by the
 source IP address field in the IP header) into the precursor list for
 the forward route entry -- i.e., the entry for the Destination IP
 Address.  The intermediate node also updates its route table entry
 for the node originating the RREQ by placing the next hop towards the
 destination in the precursor list for the reverse route entry --
 i.e., the entry for the Originator IP Address field of the RREQ
 message data.
 The intermediate node places its distance in hops from the
 destination (indicated by the hop count in the routing table) Count
 field in the RREP.  The Lifetime field of the RREP is calculated by
 subtracting the current time from the expiration time in its route
 table entry.

6.6.3. Generating Gratuitous RREPs

 After a node receives a RREQ and responds with a RREP, it discards
 the RREQ.  If the RREQ has the 'G' flag set, and the intermediate
 node returns a RREP to the originating node, it MUST also unicast a
 gratuitous RREP to the destination node.  The gratuitous RREP that is
 to be sent to the desired destination contains the following values
 in the RREP message fields:
 Hop Count                        The Hop Count as indicated in the
                                  node's route table entry for the
                                  originator
 Destination IP Address           The IP address of the node that
                                  originated the RREQ
 Destination Sequence Number      The Originator Sequence Number from
                                  the RREQ
 Originator IP Address            The IP address of the Destination
                                  node in the RREQ

Perkins, et. al. Experimental [Page 19] RFC 3561 AODV Routing July 2003

 Lifetime                         The remaining lifetime of the route
                                  towards the originator of the RREQ,
                                  as known by the intermediate node.
 The gratuitous RREP is then sent to the next hop along the path to
 the destination node, just as if the destination node had already
 issued a RREQ for the originating node and this RREP was produced in
 response to that (fictitious) RREQ.  The RREP that is sent to the
 originator of the RREQ is the same whether or not the 'G' bit is set.

6.7. Receiving and Forwarding Route Replies

 When a node receives a RREP message, it searches (using longest-
 prefix matching) for a route to the previous hop.  If needed, a route
 is created for the previous hop, but without a valid sequence number
 (see section 6.2).  Next, the node then increments the hop count
 value in the RREP by one, to account for the new hop through the
 intermediate node.  Call this incremented value the "New Hop Count".
 Then the forward route for this destination is created if it does not
 already exist.  Otherwise, the node compares the Destination Sequence
 Number in the message with its own stored destination sequence number
 for the Destination IP Address in the RREP message.  Upon comparison,
 the existing entry is updated only in the following circumstances:
 (i)       the sequence number in the routing table is marked as
           invalid in route table entry.
 (ii)      the Destination Sequence Number in the RREP is greater than
           the node's copy of the destination sequence number and the
           known value is valid, or
 (iii)     the sequence numbers are the same, but the route is is
           marked as inactive, or
 (iv)      the sequence numbers are the same, and the New Hop Count is
           smaller than the hop count in route table entry.
 If the route table entry to the destination is created or updated,
 then the following actions occur:
  1. the route is marked as active,
  1. the destination sequence number is marked as valid,
  1. the next hop in the route entry is assigned to be the node from

which the RREP is received, which is indicated by the source IP

    address field in the IP header,

Perkins, et. al. Experimental [Page 20] RFC 3561 AODV Routing July 2003

  1. the hop count is set to the value of the New Hop Count,
  1. the expiry time is set to the current time plus the value of the

Lifetime in the RREP message,

  1. and the destination sequence number is the Destination Sequence

Number in the RREP message.

 The current node can subsequently use this route to forward data
 packets to the destination.
 If the current node is not the node indicated by the Originator IP
 Address in the RREP message AND a forward route has been created or
 updated as described above, the node consults its route table entry
 for the originating node to determine the next hop for the RREP
 packet, and then forwards the RREP towards the originator using the
 information in that route table entry.  If a node forwards a RREP
 over a link that is likely to have errors or be unidirectional, the
 node SHOULD set the 'A' flag to require that the recipient of the
 RREP acknowledge receipt of the RREP by sending a RREP-ACK message
 back (see section 6.8).
 When any node transmits a RREP, the precursor list for the
 corresponding destination node is updated by adding to it the next
 hop node to which the RREP is forwarded.  Also, at each node the
 (reverse) route used to forward a RREP has its lifetime changed to be
 the maximum of (existing-lifetime, (current time +
 ACTIVE_ROUTE_TIMEOUT).  Finally, the precursor list for the next hop
 towards the destination is updated to contain the next hop towards
 the source.

6.8. Operation over Unidirectional Links

 It is possible that a RREP transmission may fail, especially if the
 RREQ transmission triggering the RREP occurs over a unidirectional
 link.  If no other RREP generated from the same route discovery
 attempt reaches the node which originated the RREQ message, the
 originator will reattempt route discovery after a timeout (see
 section 6.3).  However, the same scenario might well be repeated
 without any improvement, and no route would be discovered even after
 repeated retries.  Unless corrective action is taken, this can happen
 even when bidirectional routes between originator and destination do
 exist.  Link layers using broadcast transmissions for the RREQ will
 not be able to detect the presence of such unidirectional links.  In
 AODV, any node acts on only the first RREQ with the same RREQ ID and
 ignores any subsequent RREQs.  Suppose, for example, that the first

Perkins, et. al. Experimental [Page 21] RFC 3561 AODV Routing July 2003

 RREQ arrives along a path that has one or more unidirectional
 link(s).  A subsequent RREQ may arrive via a bidirectional path
 (assuming such paths exist), but it will be ignored.
 To prevent this problem, when a node detects that its transmission of
 a RREP message has failed, it remembers the next-hop of the failed
 RREP in a "blacklist" set.  Such failures can be detected via the
 absence of a link-layer or network-layer acknowledgment (e.g., RREP-
 ACK).  A node ignores all RREQs received from any node in its
 blacklist set.  Nodes are removed from the blacklist set after a
 BLACKLIST_TIMEOUT period (see section 10).  This period should be set
 to the upper bound of the time it takes to perform the allowed number
 of route request retry attempts as described in section 6.3.
 Note that the RREP-ACK packet does not contain any information about
 which RREP it is acknowledging.  The time at which the RREP-ACK is
 received will likely come just after the time when the RREP was sent
 with the 'A' bit.  This information is expected to be sufficient to
 provide assurance to the sender of the RREP that the link is
 currently bidirectional, without any real dependence on the
 particular RREP message being acknowledged.  However, that assurance
 typically cannot be expected to remain in force permanently.

6.9. Hello Messages

 A node MAY offer connectivity information by broadcasting local Hello
 messages.  A node SHOULD only use hello messages if it is part of an
 active route.  Every HELLO_INTERVAL milliseconds, the node checks
 whether it has sent a broadcast (e.g., a RREQ or an appropriate layer
 2 message) within the last HELLO_INTERVAL.  If it has not, it MAY
 broadcast a RREP with TTL = 1, called a Hello message, with the RREP
 message fields set as follows:
    Destination IP Address         The node's IP address.
    Destination Sequence Number    The node's latest sequence number.
    Hop Count                      0
    Lifetime                       ALLOWED_HELLO_LOSS * HELLO_INTERVAL
 A node MAY determine connectivity by listening for packets from its
 set of neighbors.  If, within the past DELETE_PERIOD, it has received
 a Hello message from a neighbor, and then for that neighbor does not
 receive any packets (Hello messages or otherwise) for more than

Perkins, et. al. Experimental [Page 22] RFC 3561 AODV Routing July 2003

 ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD
 assume that the link to this neighbor is currently lost.  When this
 happens, the node SHOULD proceed as in Section 6.11.
 Whenever a node receives a Hello message from a neighbor, the node
 SHOULD make sure that it has an active route to the neighbor, and
 create one if necessary.  If a route already exists, then the
 Lifetime for the route should be increased, if necessary, to be at
 least ALLOWED_HELLO_LOSS * HELLO_INTERVAL.  The route to the
 neighbor, if it exists, MUST subsequently contain the latest
 Destination Sequence Number from the Hello message.  The current node
 can now begin using this route to forward data packets.  Routes that
 are created by hello messages and not used by any other active routes
 will have empty precursor lists and would not trigger a RERR message
 if the neighbor moves away and a neighbor timeout occurs.

6.10. Maintaining Local Connectivity

 Each forwarding node SHOULD keep track of its continued connectivity
 to its active next hops (i.e., which next hops or precursors have
 forwarded packets to or from the forwarding node during the last
 ACTIVE_ROUTE_TIMEOUT), as well as neighbors that have transmitted
 Hello messages during the last (ALLOWED_HELLO_LOSS * HELLO_INTERVAL).
 A node can maintain accurate information about its continued
 connectivity to these active next hops, using one or more of the
 available link or network layer mechanisms, as described below.
  1. Any suitable link layer notification, such as those provided by

IEEE 802.11, can be used to determine connectivity, each time a

    packet is transmitted to an active next hop.  For example, absence
    of a link layer ACK or failure to get a CTS after sending RTS,
    even after the maximum number of retransmission attempts,
    indicates loss of the link to this active next hop.
  1. If layer-2 notification is not available, passive acknowledgment

SHOULD be used when the next hop is expected to forward the

    packet, by listening to the channel for a transmission attempt
    made by the next hop.  If transmission is not detected within
    NEXT_HOP_WAIT milliseconds or the next hop is the destination (and
    thus is not supposed to forward the packet) one of the following
    methods SHOULD be used to determine connectivity:
  • Receiving any packet (including a Hello message) from the next

hop.

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

hop.

Perkins, et. al. Experimental [Page 23] RFC 3561 AODV Routing July 2003

  • An ICMP Echo Request message unicast to the next hop.
 If a link to the next hop cannot be detected by any of these methods,
 the forwarding node SHOULD assume that the link is lost, and take
 corrective action by following the methods specified in Section 6.11.

6.11. Route Error (RERR) Messages, Route Expiry and Route Deletion

 Generally, route error and link breakage processing requires the
 following steps:
  1. Invalidating existing routes
  1. Listing affected destinations
  1. Determining which, if any, neighbors may be affected
  1. Delivering an appropriate RERR to such neighbors
 A Route Error (RERR) message MAY be either broadcast (if there are
 many precursors), unicast (if there is only 1 precursor), or
 iteratively unicast to all precursors (if broadcast is
 inappropriate).  Even when the RERR message is iteratively unicast to
 several precursors, it is considered to be a single control message
 for the purposes of the description in the text that follows.  With
 that understanding, a node SHOULD NOT generate more than
 RERR_RATELIMIT RERR messages per second.
 A node initiates processing for a RERR message in three situations:
 (i)       if it detects a link break for the next hop of an active
           route in its routing table while transmitting data (and
           route repair, if attempted, was unsuccessful), or
 (ii)      if it gets a data packet destined to a node for which it
           does not have an active route and is not repairing (if
           using local repair), or
 (iii)     if it receives a RERR from a neighbor for one or more
           active routes.
 For case (i), the node first makes a list of unreachable destinations
 consisting of the unreachable neighbor and any additional
 destinations (or subnets, see section 7) in the local routing table
 that use the unreachable neighbor as the next hop.  In this case, if
 a subnet route is found to be newly unreachable, an IP destination
 address for the subnet is constructed by appending zeroes to the

Perkins, et. al. Experimental [Page 24] RFC 3561 AODV Routing July 2003

 subnet prefix as shown in the route table entry.  This is
 unambiguous, since the precursor is known to have route table
 information with a compatible prefix length for that subnet.
 For case (ii), there is only one unreachable destination, which is
 the destination of the data packet that cannot be delivered.  For
 case (iii), the list should consist of those destinations in the RERR
 for which there exists a corresponding entry in the local routing
 table that has the transmitter of the received RERR as the next hop.
 Some of the unreachable destinations in the list could be used by
 neighboring nodes, and it may therefore be necessary to send a (new)
 RERR.  The RERR should contain those destinations that are part of
 the created list of unreachable destinations and have a non-empty
 precursor list.
 The neighboring node(s) that should receive the RERR are all those
 that belong to a precursor list of at least one of the unreachable
 destination(s) in the newly created RERR.  In case there is only one
 unique neighbor that needs to receive the RERR, the RERR SHOULD be
 unicast toward that neighbor.  Otherwise the RERR is typically sent
 to the local broadcast address (Destination IP == 255.255.255.255,
 TTL == 1) with the unreachable destinations, and their corresponding
 destination sequence numbers, included in the packet.  The DestCount
 field of the RERR packet indicates the number of unreachable
 destinations included in the packet.
 Just before transmitting the RERR, certain updates are made on the
 routing table that may affect the destination sequence numbers for
 the unreachable destinations.  For each one of these destinations,
 the corresponding routing table entry is updated as follows:
 1. The destination sequence number of this routing entry, if it
    exists and is valid, is incremented for cases (i) and (ii) above,
    and copied from the incoming RERR in case (iii) above.
 2. The entry is invalidated by marking the route entry as invalid
 3. The Lifetime field is updated to current time plus DELETE_PERIOD.
    Before this time, the entry SHOULD NOT be deleted.
 Note that the Lifetime field in the routing table plays dual role --
 for an active route it is the expiry time, and for an invalid route
 it is the deletion time.  If a data packet is received for an invalid
 route, the Lifetime field is updated to current time plus
 DELETE_PERIOD.  The determination of DELETE_PERIOD is discussed in
 Section 10.

Perkins, et. al. Experimental [Page 25] RFC 3561 AODV Routing July 2003

6.12. Local Repair

 When a link break in an active route occurs, the node upstream of
 that break MAY choose to repair the link locally if the destination
 was no farther than MAX_REPAIR_TTL hops away.  To repair the link
 break, the node increments the sequence number for the destination
 and then broadcasts a RREQ for that destination.  The TTL of the RREQ
 should initially be set to the following value:
    max(MIN_REPAIR_TTL, 0.5 * #hops) + LOCAL_ADD_TTL,
 where #hops is the number of hops to the sender (originator) of the
 currently undeliverable packet.  Thus, local repair attempts will
 often be invisible to the originating node, and will always have TTL
 >= MIN_REPAIR_TTL + LOCAL_ADD_TTL.  The node initiating the repair
 then waits the discovery period to receive RREPs in response to the
 RREQ.  During local repair data packets SHOULD be buffered.  If, at
 the end of the discovery period, the repairing node has not received
 a RREP (or other control message creating or updating the route) for
 that destination, it proceeds as described in Section 6.11 by
 transmitting a RERR message for that destination.
 On the other hand, if the node receives one or more RREPs (or other
 control message creating or updating the route to the desired
 destination) during the discovery period, it first compares the hop
 count of the new route with the value in the hop count field of the
 invalid route table entry for that destination.  If the hop count of
 the newly determined route to the destination is greater than the hop
 count of the previously known route the node SHOULD issue a RERR
 message for the destination, with the 'N' bit set.  Then it proceeds
 as described in Section 6.7, updating its route table entry for that
 destination.
 A node that receives a RERR message with the 'N' flag set MUST NOT
 delete the route to that destination.  The only action taken should
 be the retransmission of the message, if the RERR arrived from the
 next hop along that route, and if there are one or more precursor
 nodes for that route to the destination.  When the originating node
 receives a RERR message with the 'N' flag set, if this message came
 from its next hop along its route to the destination then the
 originating node MAY choose to reinitiate route discovery, as
 described in Section 6.3.
 Local repair of link breaks in routes sometimes results in increased
 path lengths to those destinations.  Repairing the link locally is
 likely to increase the number of data packets that are able to be
 delivered to the destinations, since data packets will not be dropped
 as the RERR travels to the originating node.  Sending a RERR to the

Perkins, et. al. Experimental [Page 26] RFC 3561 AODV Routing July 2003

 originating node after locally repairing the link break may allow the
 originator to find a fresh route to the destination that is better,
 based on current node positions.  However, it does not require the
 originating node to rebuild the route, as the originator may be done,
 or nearly done, with the data session.
 When a link breaks along an active route, there are often multiple
 destinations that become unreachable.  The node that is upstream of
 the lost link tries an immediate local repair for only the one
 destination towards which the data packet was traveling.  Other
 routes using the same link MUST be marked as invalid, but the node
 handling the local repair MAY flag each such newly lost route as
 locally repairable; this local repair flag in the route table MUST be
 reset when the route times out (e.g., after the route has been not
 been active for ACTIVE_ROUTE_TIMEOUT).  Before the timeout occurs,
 these other routes will be repaired as needed when packets arrive for
 the other destinations.  Hence, these routes are repaired as needed;
 if a data packet does not arrive for the route, then that route will
 not be repaired.  Alternatively, depending upon local congestion, the
 node MAY begin the process of establishing local repairs for the
 other routes, without waiting for new packets to arrive.  By
 proactively repairing the routes that have broken due to the loss of
 the link, incoming data packets for those routes will not be subject
 to the delay of repairing the route and can be immediately forwarded.
 However, repairing the route before a data packet is received for it
 runs the risk of repairing routes that are no longer in use.
 Therefore, depending upon the local traffic in the network and
 whether congestion is being experienced, the node MAY elect to
 proactively repair the routes before a data packet is received;
 otherwise, it can wait until a data is received, and then commence
 the repair of the route.

6.13. Actions After Reboot

 A node participating in the ad hoc network must take certain actions
 after reboot as it might lose all sequence number records for all
 destinations, including its own sequence number.  However, there may
 be neighboring nodes that are using this node as an active next hop.
 This can potentially create routing loops.  To prevent this
 possibility, each node on reboot waits for DELETE_PERIOD before
 transmitting any route discovery messages.  If the node receives a
 RREQ, RREP, or RERR control packet, it SHOULD create route entries as
 appropriate given the sequence number information in the control
 packets, but MUST not forward any control packets.  If the node
 receives a data packet for some other destination, it SHOULD
 broadcast a RERR as described in subsection 6.11 and MUST reset the
 waiting timer to expire after current time plus DELETE_PERIOD.

Perkins, et. al. Experimental [Page 27] RFC 3561 AODV Routing July 2003

 It can be shown [4] 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.  If no such RREQ arrives, the node MUST
 initialize its own sequence number to zero.

6.14. Interfaces

 Because AODV should operate smoothly over wired, as well as wireless,
 networks, and because it is likely that AODV will also be used with
 multiple wireless devices, the particular interface over which
 packets arrive must be known to AODV whenever a packet is received.
 This includes the reception of RREQ, RREP, and RERR messages.
 Whenever a packet is received from a new neighbor, the interface on
 which that packet was received is recorded into the route table entry
 for that neighbor, along with all the other appropriate routing
 information.  Similarly, whenever a route to a new destination is
 learned, the interface through which the destination can be reached
 is also recorded into the destination's route table entry.
 When multiple interfaces are available, a node retransmitting a RREQ
 message rebroadcasts that message on all interfaces that have been
 configured for operation in the ad-hoc network, except those on which
 it is known that all of the nodes neighbors have already received the
 RREQ For instance, for some broadcast media (e.g., Ethernet) it may
 be presumed that all nodes on the same link receive a broadcast
 message at the same time.  When a node needs to transmit a RERR, it
 SHOULD only transmit it on those interfaces that have neighboring
 precursor nodes for that route.

7. AODV and Aggregated Networks

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

Perkins, et. al. Experimental [Page 28] RFC 3561 AODV Routing July 2003

 length of the subnet prefix.  Other nodes sharing the subnet prefix
 SHOULD NOT issue RREP messages, and SHOULD forward RREQ messages to
 the subnet router.
 The processing for RREPs that give routes to subnets (i.e., have
 nonzero prefix length) is the same as processing for host-specific
 RREP messages.  Every node that receives the RREP with prefix size
 information SHOULD create or update the route table entry for the
 subnet, including the sequence number supplied by the subnet router,
 and including the appropriate precursor information.  Then, in the
 future the node can use the information to avoid sending future RREQs
 for other nodes on the same subnet.
 When a node uses a subnet route it may be that a packet is routed to
 an IP address on the subnet that is not assigned to any existing node
 in the ad hoc network.  When that happens, the subnet router MUST
 return ICMP Host Unreachable message to the sending node.  Upstream
 nodes receiving such an ICMP message SHOULD record the information
 that the particular IP address is unreachable, but MUST NOT
 invalidate the route entry for any matching subnet prefix.
 If several nodes in the subnet advertise reachability to the subnet
 defined by the subnet prefix, the node with the lowest IP address is
 elected to be the subnet router, and all other nodes MUST stop
 advertising reachability.
 The behavior of default routes (i.e., routes with routing prefix
 length 0) is not defined in this specification.  Selection of routes
 sharing prefix bits should be according to longest match first.

8. 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 7) 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.

Perkins, et. al. Experimental [Page 29] RFC 3561 AODV Routing July 2003

 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.

9. Extensions

 In this section, the format of extensions to the RREQ and RREP
 messages is specified.  All such extensions appear after the message
 data, and have the following format:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |     type-specific data ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 where:
 Type     1-255
 Length   The length of the type-specific data, not including the Type
          and Length fields of the extension in bytes.
 Extensions with types between 128 and 255 may NOT be skipped.  The
 rules for extensions will be spelled out more fully, and conform to
 the rules for handling IPv6 options.

9.1. Hello Interval Extension Format

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |       Hello Interval ...      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ... Hello Interval, continued |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type     1
 Length   4
 Hello Interval
          The number of milliseconds between successive transmissions
          of a Hello message.

Perkins, et. al. Experimental [Page 30] RFC 3561 AODV Routing July 2003

 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 6.9).

10. Configuration Parameters

 This section gives default values for some important parameters
 associated with AODV protocol operations.  A particular mobile node
 may wish to change certain of the parameters, in particular the
 NET_DIAMETER, MY_ROUTE_TIMEOUT, ALLOWED_HELLO_LOSS, RREQ_RETRIES, and
 possibly the HELLO_INTERVAL.  In the latter case, the node should
 advertise the HELLO_INTERVAL in its Hello messages, by appending a
 Hello Interval Extension to the RREP message.  Choice of these
 parameters may affect the performance of the protocol.  Changing
 NODE_TRAVERSAL_TIME also changes the node's estimate of the
 NET_TRAVERSAL_TIME, and so can only be done with suitable knowledge
 about the behavior of other nodes in the ad hoc network.  The
 configured value for MY_ROUTE_TIMEOUT MUST be at least 2 *
 PATH_DISCOVERY_TIME.
 Parameter Name           Value
 ----------------------   -----
 ACTIVE_ROUTE_TIMEOUT     3,000 Milliseconds
 ALLOWED_HELLO_LOSS       2
 BLACKLIST_TIMEOUT        RREQ_RETRIES * NET_TRAVERSAL_TIME
 DELETE_PERIOD            see note below
 HELLO_INTERVAL           1,000 Milliseconds
 LOCAL_ADD_TTL            2
 MAX_REPAIR_TTL           0.3 * NET_DIAMETER
 MIN_REPAIR_TTL           see note below
 MY_ROUTE_TIMEOUT         2 * ACTIVE_ROUTE_TIMEOUT
 NET_DIAMETER             35
 NET_TRAVERSAL_TIME       2 * NODE_TRAVERSAL_TIME * NET_DIAMETER
 NEXT_HOP_WAIT            NODE_TRAVERSAL_TIME + 10
 NODE_TRAVERSAL_TIME      40 milliseconds
 PATH_DISCOVERY_TIME      2 * NET_TRAVERSAL_TIME
 RERR_RATELIMIT           10
 RING_TRAVERSAL_TIME      2 * NODE_TRAVERSAL_TIME *
                          (TTL_VALUE + TIMEOUT_BUFFER)
 RREQ_RETRIES             2
 RREQ_RATELIMIT           10
 TIMEOUT_BUFFER           2
 TTL_START                1
 TTL_INCREMENT            2
 TTL_THRESHOLD            7
 TTL_VALUE                see note below

Perkins, et. al. Experimental [Page 31] RFC 3561 AODV Routing July 2003

 The MIN_REPAIR_TTL should be the last known hop count to the
 destination.  If Hello messages are used, then the
 ACTIVE_ROUTE_TIMEOUT parameter value MUST be more than the value
 (ALLOWED_HELLO_LOSS * HELLO_INTERVAL).  For a given
 ACTIVE_ROUTE_TIMEOUT value, this may require some adjustment to the
 value of the HELLO_INTERVAL, and consequently use of the Hello
 Interval Extension in the Hello messages.
 TTL_VALUE is the value of the TTL field in the IP header while the
 expanding ring search is being performed.  This is described further
 in section 6.4.  The TIMEOUT_BUFFER is configurable.  Its purpose is
 to provide a buffer for the timeout so that if the RREP is delayed
 due to congestion, a timeout is less likely to occur while the RREP
 is still en route back to the source.  To omit this buffer, set
 TIMEOUT_BUFFER = 0.
 DELETE_PERIOD is intended to provide an upper bound on the time for
 which an upstream node A can have a neighbor B as an active next hop
 for destination D, while B has invalidated the route to D.  Beyond
 this time B can delete the (already invalidated) route to D.  The
 determination of the upper bound depends somewhat on the
 characteristics of the underlying link layer.  If Hello messages are
 used to determine the continued availability of links to next hop
 nodes, DELETE_PERIOD must be at least ALLOWED_HELLO_LOSS *
 HELLO_INTERVAL.  If the link layer feedback is used to detect loss of
 link, DELETE_PERIOD must be at least ACTIVE_ROUTE_TIMEOUT.  If hello
 messages are received from a neighbor but data packets to that
 neighbor are lost (e.g., due to temporary link asymmetry), we have to
 make more concrete assumptions about the underlying link layer. We
 assume that such asymmetry cannot persist beyond a certain time, say,
 a multiple K of HELLO_INTERVAL.  In other words, a node will
 invariably receive at least one out of K subsequent Hello messages
 from a neighbor if the link is working and the neighbor is sending no
 other traffic.  Covering all possibilities,
    DELETE_PERIOD = K * max (ACTIVE_ROUTE_TIMEOUT, HELLO_INTERVAL)
                       (K = 5 is recommended).
 NET_DIAMETER measures the maximum possible number of hops between two
 nodes in the network.  NODE_TRAVERSAL_TIME is a conservative estimate
 of the average one hop traversal time for packets and should include
 queuing delays, interrupt processing times and transfer times.
 ACTIVE_ROUTE_TIMEOUT SHOULD be set to a longer value (at least 10,000
 milliseconds) if link-layer indications are used to detect link
 breakages such as in IEEE 802.11 [5] 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

Perkins, et. al. Experimental [Page 32] RFC 3561 AODV Routing July 2003

 characteristics of the underlying link layer protocol, radio
 technologies etc.  BLACKLIST_TIMEOUT should be suitably increased if
 an expanding ring search is used.  In such cases, it should be
 {[(TTL_THRESHOLD - TTL_START)/TTL_INCREMENT] + 1 + RREQ_RETRIES} *
 NET_TRAVERSAL_TIME.  This is to account for possible additional route
 discovery attempts.

11. Security Considerations

 Currently, AODV does not specify any special security measures. Route
 protocols, however, are prime targets for impersonation attacks.  In
 networks where the node membership is not known, it is difficult to
 determine the occurrence of impersonation attacks, and security
 prevention techniques are difficult at best.  However, when the
 network membership is known and there is a danger of such attacks,
 AODV control messages must be protected by use of authentication
 techniques, such as those involving generation of unforgeable and
 cryptographically strong message digests or digital signatures.
 While AODV does not place restrictions on the authentication
 mechanism used for this purpose, IPsec AH is an appropriate choice
 for cases where the nodes share an appropriate security association
 that enables the use of AH.
 In particular, RREP messages SHOULD be authenticated to avoid
 creation of spurious routes to a desired destination.  Otherwise, an
 attacker could masquerade as the desired destination, and maliciously
 deny service to the destination and/or maliciously inspect and
 consume traffic intended for delivery to the destination.  RERR
 messages, while less dangerous, SHOULD be authenticated in order to
 prevent malicious nodes from disrupting valid routes between nodes
 that are communication partners.
 AODV does not make any assumption about the method by which addresses
 are assigned to the mobile nodes, except that they are presumed to
 have unique IP addresses.  Therefore, no special consideration, other
 than what is natural because of the general protocol specifications,
 can be made about the applicability of IPsec authentication headers
 or key exchange mechanisms.  However, if the mobile nodes in the ad
 hoc network have pre-established security associations, it is
 presumed that the purposes for which the security associations are
 created include that of authorizing the processing of AODV control
 messages.  Given this understanding, the mobile nodes should be able
 to use the same authentication mechanisms based on their IP addresses
 as they would have used otherwise.

Perkins, et. al. Experimental [Page 33] RFC 3561 AODV Routing July 2003

12. IANA Considerations

 AODV defines a "Type" field for messages sent to port 654.  A new
 registry has been created for the values for this Type field, and the
 following values have been assigned:
    Message Type                    Value
    ---------------------------     -----
    Route Request (RREQ)            1
    Route Reply (RREP)              2
    Route Error (RERR)              3
    Route-Reply Ack (RREP-ACK)      4
 AODV control messages can have extensions.  Currently, only one
 extension is defined.  A new registry has been created for the Type
 field of the extensions:
    Extension Type                  Value
    ---------------------------     -----
    Hello Interval                  1
 Future values of the Message Type or Extension Type can be allocated
 using standards action [2].

13. IPv6 Considerations

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

14. Acknowledgments

 Special thanks to Ian Chakeres, UCSB, for his extensive suggestions
 and contributions to recent revisions.
 We acknowledge with gratitude the work done at University of
 Pennsylvania within Carl Gunter's group, as well as at Stanford and
 CMU, to determine some conditions (especially involving reboots and
 lost RERRs) under which previous versions of AODV could suffer from
 routing loops.  Contributors to those efforts include Karthikeyan
 Bhargavan, Joshua Broch, Dave Maltz, Madanlal Musuvathi, and Davor
 Obradovic.  The idea of a DELETE_PERIOD, for which expired routes
 (and, in particular, the sequence numbers) to a particular
 destination must be maintained, was also suggested by them.
 We also acknowledge the comments and improvements suggested by Sung-
 Ju Lee (especially regarding local repair), Mahesh Marina, Erik
 Nordstrom (who provided text for section 6.11), Yves Prelot, Marc
 Mosko, Manel Guerrero Zapata, Philippe Jacquet, and Fred Baker.

Perkins, et. al. Experimental [Page 34] RFC 3561 AODV Routing July 2003

15. Normative References

 [1]  Bradner, S. "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [2]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

16. Informative References

 [3]  Manner, J., et al., "Mobility Related Terminology", Work in
      Progress, July 2001.
 [4]  Karthikeyan Bhargavan, Carl A. Gunter, and Davor Obradovic.
      Fault Origin Adjudication.  In Proceedings of the Workshop on
      Formal Methods in Software Practice, Portland, OR, August 2000.
 [5]  IEEE 802.11 Committee, AlphaGraphics #35, 10201 N.35th Avenue,
      Phoenix AZ 85051.  Wireless LAN Medium Access Control MAC and
      Physical Layer PHY Specifications, June 1997.  IEEE Standard
      802.11-97.
 [6]  Perkins, C., Royer, E. and S. Das, "Ad hoc on demand distance
      vector (AODV) routing for ip version 6", Work in Progress.

Perkins, et. al. Experimental [Page 35] RFC 3561 AODV Routing July 2003

17. Authors' Addresses

 Charles E. Perkins
 Communications Systems Laboratory
 Nokia Research Center
 313 Fairchild Drive
 Mountain View, CA 94303
 USA
 Phone: +1 650 625 2986
 Fax: +1 650 691 2170 (fax)
 EMail: Charles.Perkins@nokia.com
 Elizabeth M. Belding-Royer
 Department of Computer Science
 University of California, Santa Barbara
 Santa Barbara, CA 93106
 Phone: +1 805 893 3411
 Fax: +1 805 893 8553
 EMail: ebelding@cs.ucsb.edu
 Samir R. Das
 Department of Electrical and Computer Engineering
 & Computer Science
 University of Cincinnati
 Cincinnati, OH 45221-0030
 Phone: +1 513 556 2594
 Fax: +1 513 556 7326
 EMail: sdas@ececs.uc.edu

Perkins, et. al. Experimental [Page 36] RFC 3561 AODV Routing July 2003

18. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Perkins, et. al. Experimental [Page 37]

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