Mobile Ad Hoc Networking Working Group Charles E. Perkins
INTERNET DRAFT Nokia Research Center
17 February 2003 Elizabeth M. Belding-Royer
University of California, Santa Barbara
Samir R. Das
University of Cincinnati
Ad hoc On-Demand Distance Vector (AODV) Routing
draft-ietf-manet-aodv-13.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@ietf.org mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
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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.
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Contents
Status of This Memo i
Abstract i
1. Introduction 1
2. Overview 1
3. AODV Terminology 3
4. Applicability Statement 5
5. Message Formats 5
5.1. Route Request (RREQ) Message Format . . . . . . . . . . . 5
5.2. Route Reply (RREP) Message Format . . . . . . . . . . . . 7
5.3. Route Error (RERR) Message Format . . . . . . . . . . . . 8
5.4. Route Reply Acknowledgment (RREP-ACK) Message Format . . 9
6. AODV Operation 9
6.1. Maintaining Sequence Numbers . . . . . . . . . . . . . . 10
6.2. Route Table Entries and Precursor Lists . . . . . . . . . 11
6.3. Generating Route Requests . . . . . . . . . . . . . . . . 12
6.4. Controlling Dissemination of Route Request Messages . . . 13
6.5. Processing and Forwarding Route Requests . . . . . . . . 14
6.6. Generating Route Replies . . . . . . . . . . . . . . . . 16
6.6.1. Route Reply Generation by the Destination . . . . 16
6.6.2. Route Reply Generation by an Intermediate Node . 17
6.6.3. Generating Gratuitous RREPs . . . . . . . . . . . 17
6.7. Receiving and Forwarding Route Replies . . . . . . . . . 18
6.8. Operation over Unidirectional Links . . . . . . . . . . . 19
6.9. Hello Messages . . . . . . . . . . . . . . . . . . . . . 20
6.10. Maintaining Local Connectivity . . . . . . . . . . . . . 21
6.11. Route Error (RERR) Messages, Route Expiry and Route
Deletion . . . . . . . . . . . . . . . . . . . . . . . 22
6.12. Local Repair . . . . . . . . . . . . . . . . . . . . . . 23
6.13. Actions After Reboot . . . . . . . . . . . . . . . . . . 25
6.14. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 26
7. AODV and Aggregated Networks 26
8. Using AODV with Other Networks 27
9. Extensions 28
9.1. Hello Interval Extension Format . . . . . . . . . . . . . 28
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10. Configuration Parameters 29
11. Security Considerations 31
12. IANA Considerations 32
13. IPv6 Considerations 32
14. Acknowledgments 32
A. Draft Modifications 34
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.
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
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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
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
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uses the following fields with each route table entry:
- Destination IP Address
- Destination Sequence Number
- Valid Destination Sequence Number flag
-
- Other state and routing flags (e.g., valid, invalid, repairable,
being repaired)
- Network Interface
- Hop Count (number of hops needed to reach destination)
- Next Hop
- List of Precursors (described in Section 6.2)
- 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].
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
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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.
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.
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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.
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)
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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.
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.
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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.
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.
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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.
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.
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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.
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.
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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:
- 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.
- 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.
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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.
A node may change the sequence number in the routing table entry of a
destination only if:
- it is itself the destination node, and offers a new route to
itself, or
- it receives an AODV message with new information about the
sequence number for a destination node, or
- 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.
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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.
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
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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
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 receivied 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 calculcated as described in section 10. The
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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.
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:
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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);
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
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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).
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
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reconfigure its value for MY_ROUTE_TIMEOUT, within mild constraints
(see section 10).
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
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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:
- the route is marked as active,
- the destination sequence number is marked as valid,
- 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,
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- the hop count is set to the value of the New Hop Count,
- the expiry time is set to the current time plus the value of
the Lifetime in the RREP message,
- 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 RREQ arrives along a path that has one or more unidirectional
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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
ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD
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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.
- Any suitable link layer notification, such as those provided by
IEEE 802.11, can be used to determine connectivity, each time
a packet is transmitted to an active next hop. For example,
absence of a link layer ACK or failure to get a CTS after sending
RTS, even after the maximum number of retransmission attempts,
indicates loss of the link to this active next hop.
- If 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.
* An ICMP Echo Request message unicast to the next hop.
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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:
- Invalidating existing routes
- Listing affected destinations
- Determining which, if any, neighbors may be affected
- 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 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.
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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.
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
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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 reparing 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
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.
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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.
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
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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 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.
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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 partcular 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.
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.
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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.
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).
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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
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
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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, (due to temporary link asymmetry, e.g.) we have
to make more concrete assumptions about the underlying link layer.
We assume that such asymmetry cannot persist beyond a certain 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 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.
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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.
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12. IANA Considerations
AODV defines a "Type" field for messages sent to port 654. A new
registry is to be created for the values for this Type field, and the
following values 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 is to be 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.
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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.
References
[1] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
[2] T. Narten and H. Alvestrand. Guidelines for Writing an IANA
Considerations Section in RFCs. Request for Comments (Best
Current Practice) 2434, Internet Engineering Task Force, October
1998.
[3] J. Manner et al. Mobility Related Terminology (work in
progress). draft-manner-seamoby-terms-02.txt, 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] C. Perkins, E. Royer, and S. Das. Ad hoc on demand distance
vector (AODV) routing for ip version 6 (work in progress).
Internet Draft, Internet Engineering Task Force, November 2001.
References [1] and [2] are normative; all others are informative.
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A. Draft Modifications
The following are major changes between this version (13) of the AODV
draft and previous versions:
- RERR clarifications for handling subnet routes. A processing
step was added to eliminate host routes that are redundant with
subnet routes.
- Added explicit specification to clarify that subnet routes are
handled the same way as host routes for managing timeouts and
route table entries.
- Applicability Statement and IANA Considerations sections added.
- Normative References placed at beginning of bibliography.
- Updated Security Considerations section. AH is suggested but not
mandated as a good choice for authenticating control messages.
- Updated the AODV and Aggregated Networks section to include the
transmission of an ICMP Host Unreachable message for data packets
sent to non-existent destinations.
Author's Addresses
Questions about this memo can be directed to:
Charles E. Perkins
Communications Systems Laboratory
Nokia Research Center
313 Fairchild Drive
Mountain View, CA 94303
USA
+1 650 625 2986
+1 650 691 2170 (fax)
charliep@iprg.nokia.com
Elizabeth M. Belding-Royer
Department of Computer Science
University of California, Santa Barbara
Santa Barbara, CA 93106
+1 805 893 3411
+1 805 893 8553 (fax)
ebelding@cs.ucsb.edu
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Samir R. Das
Department of Electrical and Computer Engineering
& Computer Science
University of Cincinnati
Cincinnati, OH 45221-0030
+1 513 556 2594
+1 513 556 7326 (fax)
sdas@ececs.uc.edu
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