Mobile Ad Hoc Networking Working Group Charles E. Perkins
INTERNET DRAFT Nokia Research Center
19 January 2002 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-10.txt
Status of This Memo
This document is a submission by the Mobile Ad Hoc Networking Working
Group of the Internet Engineering Task Force (IETF). Comments should
be submitted to the manet@itd.nrl.navy.mil mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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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. Message Formats 4
4.1. Route Request (RREQ) Message Format . . . . . . . . . . . 4
4.2. Route Reply (RREP) Message Format . . . . . . . . . . . . 5
4.3. Route Error (RERR) Message Format . . . . . . . . . . . . 7
5. Route Reply Acknowledgment (RREP-ACK) Message Format 8
6. AODV Operation 8
6.1. Maintaining Sequence Numbers . . . . . . . . . . . . . . 8
6.2. Maintaining Route Table Entries and Precursor Lists . . . 10
6.3. Generating Route Requests . . . . . . . . . . . . . . . . 10
6.4. Controlling Dissemination of Route Request Messages . . . 11
6.5. Processing and Forwarding Route Requests . . . . . . . . 12
6.6. Generating Route Replies . . . . . . . . . . . . . . . . 14
6.6.1. Route Reply Generation by the Destination . . . . 14
6.6.2. Route Reply Generation by an Intermediate Node . 14
6.6.3. Generating Gratuitous RREPs . . . . . . . . . . . 15
6.7. Receiving and Forwarding Route Replies . . . . . . . . . 16
6.8. Operation over Unidirectional Links . . . . . . . . . . . 17
6.9. Hello Messages . . . . . . . . . . . . . . . . . . . . . 17
6.10. Maintaining Local Connectivity . . . . . . . . . . . . . 18
6.11. Route Error Messages, Route Expiry and Route Deletion . 19
6.12. Local Repair . . . . . . . . . . . . . . . . . . . . . . 20
6.13. Actions After Reboot . . . . . . . . . . . . . . . . . . 22
6.14. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 22
7. AODV and Aggregated Networks 23
8. Using AODV with Other Networks 23
9. Extensions 24
9.1. Hello Interval Extension Format . . . . . . . . . . . . . 24
9.2. Timestamp Extension Format . . . . . . . . . . . . . . . 25
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10. Configuration Parameters 25
11. Security Considerations 27
12. Acknowledgments 28
A. Draft Modifications 29
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
broken link.
One distinguishing feature of AODV is its use of a destination
sequence number for each route entry. The destination sequence
number is created by the destination for any route information it
sends to requesting nodes. Using destination sequence numbers
ensures loop freedom and is simple to program. Given the choice
between two routes to a destination, a requesting node always selects
the one with the greatest sequence number.
2. Overview
Route Requests (RREQs), Route Replies (RREPs), and Route Errors
(RERRs) are the message types defined by AODV. These message
types are received at port 654, over 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.
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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 an
unexpired route entry for the destination whose associated sequence
number is at least as great as that contained in the RREQ. The route
is made available by unicasting a RREP back to the 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 which are now unreachable due to
the loss of the 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
the destination that is now unreachable. 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).
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 ephemeral 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
- Interface
- Hop Count (number of hops needed to reach destination)
- Last Hop Count (described in subsections 6.4 and 6.11)
- Next Hop
- List of Precursors (described in Section 6.2)
- Lifetime (expiration or deletion time of the route)
- Routing Flags
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 metric (hop count). See section 6.1 for
details.
3. AODV Terminology
This protocol specification uses conventional meanings [2] for
capitalized words such as MUST, SHOULD, etc., to indicate requirement
levels for various protocol features. This section defines other
terminology used with AODV that is not already defined in [3].
active route
A routing table entry with a finite metric in the Hop Count
field. A routing table may contain entries that are not active
(invalid routes or entries). They have an infinite metric
in the Hop Count field. Only active entries can be used to
forward data packets. Invalid entries are eventually deleted.
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.
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.
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forward route
A route set up to send data packets from a node originating a
Route Discovery operation towards its desired destination.
originating node
A node that initiates an AODV 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.
4. Message Formats
4.1. 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| 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.
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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)
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 greatest 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 for
route entries pointing to (and generated by) the
originator of the route request.
4.2. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 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.
Lifetime The time for which nodes receiving the RREP consider
the route to be valid.
Note that the Prefix Size allows a Subnet Leader to supply a route
for every host in the subnet defined by the routing prefix, which
is determined by the IP address of the Subnet Leader and the Prefix
Size. In order to make use of this feature, the Subnet Leader has to
guarantee reachability to all the hosts sharing the indicated subnet
prefix. The Subnet Leader is also responsible for maintaining the
Destination Sequence Number for the whole subnet. See section 7 for
details.
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4.3. 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.
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5. Route Reply Acknowledgment (RREP-ACK) Message Format
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 4
Reserved Sent as 0; ignored on reception.
The RREP-ACK message may be used to acknowledge receipt of a RREP
message. It is used in cases where the link over which the RREP
message is sent may be unreliable or unidirectional.
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
AODV depends on each node in the network to own and maintain a
sequence number to guarantee the loop-freedom of all routes towards
that node. A 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 problems with
deleted reverse routes to 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.
Thus, if the sequence number has already been assigned to be the
largest possible number representable as a 32-bit unsigned integer
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(i.e., 4294967295), then when it is incremented it will then have a
value of zero (0). Similarly, if the sequence number currently has
the value 2147483647, which is the largest possible positive integer
when if 2's complement arithmetic is in use, 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 incrementation 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).
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. 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. 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 broken or expired link to the next hop towards that
destination. The node determines which destinations use a broken
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 puts the Hop Count to be "infinity" (for the case of
broken links, see also see sections 6.11, 6.12).
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.
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6.2. Maintaining Route Table Entries and Precursor Lists
For each valid route maintained by a node (containing a finite Hop
Count metric) as a routing table entry, the node also maintains a
list of precursors that may be forwarding packets on this route.
These precursors will receive notifications from the node in the
event of detection of the loss of the next hop link. The list of
precursors in a routing table entry contains those neighboring nodes
to which a route reply was generated or forwarded.
When a node receives an AODV control packet from a neighbor, it
checks its route table for an entry for that neighbor. In the event
that there is no corresponding entry for that neighbor, an entry
is created. The sequence number is either determined from the
information contained in the control packet (i.e., the neighbor is
the originator of a RREQ), or else it is initialized to zero if the
sequence number for that node can not be determined. The Lifetime
field of the routing table entry is either determined from the
control packet (i.e., the neighbor is the originator of a RREP for
itself), or it is initialized to ALLOWED_HELLO_LOSS * HELLO_INTERVAL.
In other words, the reception of a control packet has the same
meaning as the reception of an explicit Hello message, in that it
signifies an active connection to that neighbor. The hop count to
the neighbor is set to one.
Each time a route is used to forward a data packet, its Active Route
Lifetime field of both the 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 are 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.
6.3. Generating Route Requests
A node broadcasts a RREQ when it determines that it needs a route
to a destination and does not have one available. This can happen
if the destination is previously unknown to the node, or if a
previously valid route to the destination expires or is broken
(i.e., an infinite metric is associated with the route). The
Destination Sequence Number field in the RREQ message is the last
known destination sequence number for this destination and is copied
from the Destination Sequence Number field in the routing table. If
no sequence number is known, a sequence number of zero is used. The
Originator Sequence Number in the RREQ message is the node's own
sequence number. The RREQ ID field is incremented by one from the
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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_TRAVERSAL_TIME milliseconds. 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.
After broadcasting a RREQ, a node waits for a RREP. If the RREP is
not received within NET_TRAVERSAL_TIME milliseconds, the node MAY try
again to discover a route by broadcasting a RREQ, up to a maximum
of RREQ_RETRIES times. Each new attempt MUST increment the RREQ ID
field.
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 without receiving any RREP, all data packets
destined for the corresponding destination SHOULD be dropped from
the buffer and a Destination Unreachable message delivered to the
application.
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 as
an optimization. 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 2 * TTL *
NODE_TRAVERSAL_TIME milliseconds. 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
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RREP is calculated as before. Each attempt increments the RREQ ID
field in the RREQ packet. The RREQ can be broadcast with TTL =
NET_DIAMETER up to a maximum of RREQ_RETRIES times.
When a RREP is received, the Hop Count indicated in the RREP packet
is stored as the Last Hop Count in the routing table. When a new
route to the same destination is required at a later time (e.g., upon
route loss), the TTL in the RREQ IP header is initially set to this
Last Hop Count plus TTL_INCREMENT. Thereafter, following each timeout
the TTL is incremented by TTL_INCREMENT until TTL = TTL_THRESHOLD is
reached. Beyond this TTL = NET_DIAMETER is used as before.
Timeouts MAY be more accurately determined dynamically via
measurement, instead of using a statically configured value related
to NODE_TRAVERSAL_TIME. To accomplish this, the RREQ may carry the
timestamp via an extension field as defined in Section 9.2 to be
carried back by the RREP packet (again via an extension field). The
difference between the current time and this timestamp determines the
route discovery latency. The timeout may be set to be a small factor
times the average of the last few route discovery latencies for the
concerned destination. These latencies may be recorded as additional
fields in the routing table.
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 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 + PATH_TRAVERSAL_TIME).
6.5. Processing and Forwarding Route Requests
When a node receives a RREQ, it first checks to determine whether it
has received a RREQ with the same Originator IP Address and RREQ ID
within at least the last PATH_TRAVERSAL_TIME milliseconds. If such a
RREQ has been received, the node silently discards the newly received
RREQ. The rest of this subsection describes actions taken for RREQs
that are not discarded.
The node always creates a reverse route to the Originator IP Address
in its routing table if one does not already exist. If a route to
the Originator IP Address already exists, it is updated only if
either
(i) the Originator Sequence Number in the RREQ is higher
than the destination sequence number of the Originator
IP Address in the route table, or
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(ii) the sequence numbers are equal, but the hop count as
specified by the RREQ, plus one, is now smaller than the
existing hop count in the 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 are carried out:
1. the Originator Sequence Number from the RREQ is copied to the
corresponding destination sequence number in the route table
entry;
2. 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);
3. the hop count is copied from the Hop Count in the RREQ message
and incremented by one;
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 + PATH_TRAVERSAL_TIME -
2*HopCount*NODE_TRAVERSAL_TIME).
The node generates a RREP (as discussed further in section 6.6) if
either:
(i) it is itself the destination (see section 6.6.1), or
(ii) it has an active route to the destination, and the
destination sequence number in the node's existing
route table entry for the destination is greater than
or equal to the Destination Sequence Number of the
RREQ (comparison using signed 32-bit arithmetic). See
section 6.6.2 for further information about generating
the RREP in this case.
When either of these conditions is satisfied, the node does not
rebroadcast the RREQ.
Otherwise, if the incoming IP header has TTL larger than 1, the node
updates and broadcasts the RREQ to address 255.255.255.255 on all of
its configured interface(s) (see section 6.14). To update the RREQ,
the TTL or hop limit field in the outgoing IP header is decreased by
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one, and the Hop Count field in the RREQ message is incremented by
one, to account for the new hop through the intermediate node.
6.6. Generating Route Replies
If a node receives a route request for a destination, and either
has a fresh enough route to satisfy the request or is itself the
destination, the node generates a RREP message. This node copies
the Destination IP Address and the Originator Sequence Number in
RREQ message into the corresponding fields in the RREP message.
Processing is slightly different, depending on whether the node is
itself the requested destination, or instead if it is an intermediate
node with an admissible route to the destination. These scenarios
are described in the sections below.
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 update its
own sequence number to the maximum of its current sequence number and
the destination sequence number in the RREQ packet. The destination
node places its 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).
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 last known sequence number for the
destination into the Destination Sequence Number field in the RREP
message.
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The intermediate node updates the forward path 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 path 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) in
the Hop 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 intermediate nodes reply to every transmission of a
given RREQ, the destination does not receive any copies of it. In
this situation, it does not learn of a route to the originating node.
This could cause the destination to initiate a network-wide 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.
The RREP that is sent to the originator of the RREQ is the same
as before. The gratuitous RREP that is to be sent to the desired
destination contains the following values in the RREP message fields:
Hop Count The Hop Count as 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.
6.7. Receiving and Forwarding Route Replies
When a node receives a RREP message, it compares the Destination
Sequence Number in the message with its own copy of destination
sequence number for the Destination IP Address in the RREP message.
The forward route for this destination is created if it does not
already exist, or it is updated only if (i) the Destination Sequence
Number in the RREP is greater than the node's copy of the destination
sequence number, or (ii) the sequence numbers are the same, but the
route is no longer active, or (iii) the sequence numbers are the
same, and the Hop Count in the RREP is smaller than the hop count
in route table entry. In either of these cases, 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; the hop count is the Hop Count in the RREP message plus
one; the expiry time is the current time plus the Lifetime in the
RREP message; and the destination sequence number is the Destination
Sequence Number in the RREP message. The current node can now begin
using 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 the route table entry.
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)).
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).
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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 network-wide route discovery after a
timeout (see section 6.3). However, the same scenario might well
be repeated, 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
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.
6.9. Hello Messages
A node MAY offer connectivity information by broadcasting local
Hello messages as follows. Every HELLO_INTERVAL milliseconds, the
node checks whether it has sent a broadcast (e.g., a RREQ or an
appropriate layer 2 message) within the last HELLO_INTERVAL. If
it has not, it MAY 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
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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
assume that the link to this neighbor is currently broken. 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. Routes that are newly
created from the reception of Hello messages might have empty
precursor lists, and in that case would not trigger RERR messages
when the neighbor moves away and the neighbor route expires.
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 possible, 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 never supposed to
transmit the packet) one of the following methods should be used
to determine connectivity.
* Receiving any packet (including a Hello message) from the
next hop.
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* A RREQ unicast to the next hop, asking for a route to the
next hop.
* An ICMP Echo Request message unicast to the next hop.
If a link to the next hop cannot be detected by any of these methods,
the forwarding node SHOULD assume that the link is broken, and take
corrective action by following the methods specified in Section 6.11.
6.11. Route Error Messages, Route Expiry and Route Deletion
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.
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, or if the routing table
entry for the next hop expires (also see section 6.1),
or
(ii) if it gets a data packet destined to a node for which it
does not have an active route, and has already made an
attempt at local repair (if local repair is being used),
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 in the local routing table that use the unreachable
neighbor as the next hop. 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.
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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 to that destination. 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 entry is invalidated by copying the Hop Count to the Last Hop
Count field and then making the Hop Count infinity.
2. The destination sequence number of this routing entry, if it
exists, is incremented by one for cases (i) and (ii) above, and
copied from the incoming RERR in case (iii) above.
3. The Lifetime field is updated to current time plus DELETE_PERIOD.
Before this time, the entry MUST NOT be deleted.
Note that the Lifetime field 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
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 to originator) +
LOCAL_ADD_TTL.
Thus, local repair attempts should never be visible to the
originating node, and will always have TTL >= MIN_REPAIR_TTL
+ LOCAL_ADD_TTL. The node initiating the repair then waits the
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discovery period to receive RREPs in response to the RREQ. If, at
the end of the discovery period, it has not received a RREP for that
destination, it proceeds as described in Section 6.11 by transmitting
a RERR message for that destination.
On the other hand, if the node receives one or more RREPs during the
discovery period, it proceeds as described in Section 6.7, updating
its route table entry for that destination. It then compares the hop
count of the new route with the value in the last hop count route
table entry for that destination. If the hop count of the newly
determined route to the destination is greater than the hop count of
the previously known route, as recorded in the last hop count field,
the node SHOULD create a RERR message for the destination, with the
'N' bit set.
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 active 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.
When a link breaks along an active route, there are often multiple
destinations that become unreachable. The node that is upstream of
the broken 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 broken, but the node
handling the local repair MAY flag each such newly broken route as
locally repairable; this local repair flag in the route table MUST be
reset when the route times out (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. Alternatively, depending upon local
congestion, the node MAY begin the process of establishing local
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repairs for the other routes, without waiting for new packets to
arrive.
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. During
this time, the node does not transmit any RREP 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. If the node receives a data packet for
some other destination, it MUST broadcast a RERR as described in
subsection 6.11 and reset the waiting timer to expire after current
time plus DELETE_PERIOD.
It can be shown [1] 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.
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
multi-homed radios, the interface over which packets arrive must
be known to AODV whenever a packet is received. This includes the
reception of RREQ, RREP, and RERR messages. Whenever a packet is
received from a new neighbor, the interface on which that packet was
received is recorded into the route table entry for that neighbor,
along with all the other appropriate routing information. Similarly,
whenever a route to a new destination is learned, the interface
through which the destination can be reached is also recorded into
the destination's route table entry.
When multiple interfaces are available, a node 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 brodacast
message at the same time. When a node needs to transmit a RERR, it
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should only transmit it on those interfaces that have 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 leader.
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
RREQ and RREP messages have extensions defined in the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | type-specific data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type 1
Length The length of the type-specific data, not including the
Type and Length fields of the extension.
Extensions with types between 128 and 255 may NOT be skipped. The
rules for extensions will be spelled out more fully, and conform 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 2
Length 4
Hello Interval
The number of milliseconds between successive
transmissions of a Hello message.
The Hello Interval extension MAY be appended to a RREP message with
TTL == 1, to be used by a neighboring receiver in determine how long
to wait for subsequent such RREP messages (i.e., Hello messages; see
section 6.9).
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9.2. Timestamp 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp in NTP Format +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 3
Length 8
Timestamp
The number of seconds and fractional seconds since the
Timestamp Extension was added to the control message as
transmitted by the originator (e.g., of a RREQ message).
The Timestamp value is structured according to the format for NTP
timestamps specified in RFC 2030 [5]. For convenience, the following
text is taken from that document, but should not be used as a
substitute for consulting RFC 2030 for details.
NTP timestamps are represented as a 64-bit unsigned fixed-point
number, in seconds relative to 0h on 1 January 1900. The integer
part is in the first 32 bits and the fraction part in the last 32
bits. In the fraction part, the non-significant low order can be
set to 0. It is advisable to fill the non-significant low order
bits of the timestamp with a random, unbiased bitstring, both to
avoid systematic roundoff errors and as a means of loop detection and
replay detection (see below). One way of doing this is to generate a
random bitstring in a 64-bit word, then perform an arithmetic right
shift a number of bits equal to the number of significant bits of the
timestamp, then add the result to the original timestamp.
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, NODE_TRAVERSAL_TIME, MY_ROUTE_TIMEOUT,
ALLOWED_HELLO_LOSS, RREQ_RETRIES, and possibly the HELLO_INTERVAL. In
the latter case, the node should advertise the HELLO_INTERVAL in its
Hello messages, by appending a Hello Interval Extension to the RREP
message. Choice of these parameters may affect the performance of
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the protocol. The configured value for MY_ROUTE_TIMEOUT MUST be at
least 2 * REV_ROUTE_LIFE.
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
NEXT_HOP_WAIT NODE_TRAVERSAL_TIME + 10
NODE_TRAVERSAL_TIME 40
NET_TRAVERSAL_TIME 3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2
PATH_DISCOVERY_TIME 2 * NET_TRAVERSAL_TIME2RREQ_RETRIES
TTL_START 1
TTL_INCREMENT 2
TTL_THRESHOLD 7
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).
DELETE_PERIOD should be 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 route to D. The determination of the upper
bound somewhat depends 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,
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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 queueing delays, interrupt processing times and transfer
times. ACTIVE_ROUTE_TIMEOUT SHOULD be set to a longer value (at
least 10,000 milliseconds) if link-layer indications are used to
detect link breakages such as in IEEE 802.11 [4] 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. 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. If there is 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. 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.
Since AODV does not make any assumption about the nature of the
address assignment to the mobile nodes except that they are presumed
to have unique IP addresses, no definite statements 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, they 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. Acknowledgments
We acknowledge with gratitude the work done at University of
Pennsylvania within Carl Gunter's group, as well as at Stanford and
CMU, to determine some conditions (especially involving reboots and
lost RERRs) under which previous versions of AODV could suffer from
routing loops. Contributors to those efforts include Karthikeyan
Bhargavan, Joshua Broch, Dave Maltz, Madanlal Musuvathi, and
Davor Obradovic. The idea of a DELETE_PERIOD, for which expired
routes (and, in particular, the sequence numbers) to a particular
destination must be maintained, was also suggested by them.
We also acknowledge the comments and improvements suggested by
Sung-Ju Lee (especially regarding local repair), Mahesh Marina, Erik
Nordstrom (who provided text for section 6.11), Yves Prelot, Manel
Guerrero Zapata, Philippe Jacquet, Ian Chakeres, and Fred Baker.
References
[1] 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.
[2] 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.
[3] J. Manner et al. Mobility Related Terminology (work in
progress). draft-manner-seamoby-terms-02.txt, July 2001.
[4] 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.
[5] D. Mills. Simple Network Time Protocol (SNTP) Version 4 for
IPv4, IPv6 and OSI. Request for Comments (Informational) 2030,
Internet Engineering Task Force, October 1996.
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A. Draft Modifications
The following are major changes between this version (10) of the AODV
draft and the previous version (09):
- Specified that the next hop towards the originator of a RREQ
must be added to the precursor list for the destination, when an
intermediate node sends a Gratuitous RREP to the next hop towards
that destination (see section 6.6.3).
- Specified that sequence numbers are to be compared as signed
integers.
- Clarified that "broadcast" means transmission to 255.255.255.255,
and replaced terminology about "flooding" by "network-wide route
discovery", since that is what AODV does.
- In line with last point, replaced "Flooding ID" by "RREQ ID", and
FLOOD_RECORD_TIME by RREQ_RECORD_TIME.
- Changed name of "Source IP Address" field to be "Originator
IP Address" in RREQ message format, and changed the ``Source
Sequence Number'' field to be the ``Originator Sequence Number''
field in the RREQ and RREP message formats.
- Clarified that RREQ messages do not have to be rebroadcast over
some types of network interfaces, when it may be presumed that
all nodes reachable from the network interface have already
received the same incoming RREQ message as the node processing
the RREQ (see section 6.14).
- Made section 4-7 in version 09 subsections of one section in
version 10.
- Changed the Lifetime field in section 6.2 to be set to
HELLO_INTERVAL * ALLOWED_HELLO_LOSS on reception of a control
packet.
- Added that the lifetime for the route to the next hop towards a
destination should be updated when a data packet is forwarded to
that node.
- Updated the calculation of MinimalLifetime in section 6.5.
- Clarified section 6.11.
- Added a definition for the timestamp extension field.
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- Introduced a new parameter, PATH_DISCOVERY_TIME, to replace the
former RREQ_RECORD_TIME, REV_ROUTE_LIFE, and RREP_WAIT_TIME
parameters.
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
Dept. 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
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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