Request for Comments: 3561 Nokia Research Center
Category: Experimental E. Belding-Royer
University of California, Santa Barbara
S. Das
University of Cincinnati
July 2003
Ad hoc On-Demand Distance Vector (AODV) Routing
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Ad hoc On-Demand Distance Vector (AODV) routing protocol is
intended for use by mobile nodes in an ad hoc network. It offers
quick adaptation to dynamic link conditions, low processing and
memory overhead, low network utilization, and determines unicast
routes to destinations within the ad hoc network. It uses
destination sequence numbers to ensure loop freedom at all times
(even in the face of anomalous delivery of routing control messages),
avoiding problems (such as "counting to infinity") associated with
classical distance vector protocols.
Table of Contents
1. Introduction ............................................... 2
2. Overview .................................................. 3
3. AODV Terminology ........................................... 4
4. Applicability Statement .................................... 6
5. Message Formats ............................................ 7
5.1. Route Request (RREQ) Message Format ................... 7
5.2. Route Reply (RREP) Message Format ..................... 8
5.3. Route Error (RERR) Message Format ..................... 10
5.4. Route Reply Acknowledgment (RREP-ACK) Message Format .. 11
6. AODV Operation ............................................. 11
6.1. Maintaining Sequence Numbers .......................... 11
6.2. Route Table Entries and Precursor Lists ............... 13
6.3. Generating Route Requests ............................. 14
6.4. Controlling Dissemination of Route Request Messages ... 15
6.5. Processing and Forwarding Route Requests .............. 16
6.6. Generating Route Replies .............................. 18
6.6.1. Route Reply Generation by the Destination ...... 18
6.6.2. Route Reply Generation by an Intermediate
Node ........................................... 19
6.6.3. Generating Gratuitous RREPs .................... 19
6.7. Receiving and Forwarding Route Replies ................ 20
6.8. Operation over Unidirectional Links ................... 21
6.9. Hello Messages ........................................ 22
6.10 Maintaining Local Connectivity ........................ 23
6.11 Route Error (RERR) Messages, Route Expiry and Route
Deletion .............................................. 24
6.12 Local Repair .......................................... 26
6.13 Actions After Reboot ................................. 27
6.14 Interfaces ............................................ 28
7. AODV and Aggregated Networks ............................... 28
8. Using AODV with Other Networks ............................. 29
9. Extensions ................................................. 30
9.1. Hello Interval Extension Format ....................... 30
10. Configuration Parameters ................................... 31
11. Security Considerations .................................... 33
12. IANA Considerations ........................................ 34
13. IPv6 Considerations ........................................ 34
14. Acknowledgments ............................................ 34
15. Normative References ....................................... 35
16. Informative References ..................................... 35
17. Authors' Addresses ......................................... 36
18. Full Copyright Statement ................................... 37
1. Introduction
The Ad hoc On-Demand Distance Vector (AODV) algorithm enables
dynamic, self-starting, multihop routing between participating mobile
nodes wishing to establish and maintain an ad hoc network. AODV
allows mobile nodes to obtain routes quickly for new destinations,
and does not require nodes to maintain routes to destinations that
are not in active communication. AODV allows mobile nodes to respond
to link breakages and changes in network topology in a timely manner.
The operation of AODV is loop-free, and by avoiding the Bellman-Ford
"counting to infinity" problem offers quick convergence when the ad
hoc network topology changes (typically, when a node moves in the
network). When links break, AODV causes the affected set of nodes to
be notified so that they are able to invalidate the routes using the
lost link.
One distinguishing feature of AODV is its use of a destination
sequence number for each route entry. The destination sequence
number is created by the destination to be included along with any
route information it sends to requesting nodes. Using destination
sequence numbers ensures loop freedom and is simple to program.
Given the choice between two routes to a destination, a requesting
node is required to select the one with the greatest sequence number.
2. Overview
Route Requests (RREQs), Route Replies (RREPs), and Route Errors
(RERRs) are the message types defined by AODV. These message types
are received via UDP, and normal IP header processing applies. So,
for instance, the requesting node is expected to use its IP address
as the Originator IP address for the messages. For broadcast
messages, the IP limited broadcast address (255.255.255.255) is used.
This means that such messages are not blindly forwarded. However,
AODV operation does require certain messages (e.g., RREQ) to be
disseminated widely, perhaps throughout the ad hoc network. The
range of dissemination of such RREQs is indicated by the TTL in the
IP header. Fragmentation is typically not required.
As long as the endpoints of a communication connection have valid
routes to each other, AODV does not play any role. When a route to a
new destination is needed, the node broadcasts a RREQ to find a route
to the destination. A route can be determined when the RREQ reaches
either the destination itself, or an intermediate node with a 'fresh
enough' route to the destination. A 'fresh enough' route is a valid
route entry for the destination whose associated sequence number is
at least as great as that contained in the RREQ. The route is made
available by unicasting a RREP back to the origination of the RREQ.
Each node receiving the request caches a route back to the originator
of the request, so that the RREP can be unicast from the destination
along a path to that originator, or likewise from any intermediate
node that is able to satisfy the request.
Nodes monitor the link status of next hops in active routes. When a
link break in an active route is detected, a RERR message is used to
notify other nodes that the loss of that link has occurred. The RERR
message indicates those destinations (possibly subnets) which are no
longer reachable by way of the broken link. In order to enable this
reporting mechanism, each node keeps a "precursor list", containing
the IP address for each its neighbors that are likely to use it as a
next hop towards each destination. The information in the precursor
lists is most easily acquired during the processing for generation of
a RREP message, which by definition has to be sent to a node in a
precursor list (see section 6.6). If the RREP has a nonzero prefix
length, then the originator of the RREQ which solicited the RREP
information is included among the precursors for the subnet route
(not specifically for the particular destination).
A RREQ may also be received for a multicast IP address. In this
document, full processing for such messages is not specified. For
example, the originator of such a RREQ for a multicast IP address may
have to follow special rules. However, it is important to enable
correct multicast operation by intermediate nodes that are not
enabled as originating or destination nodes for IP multicast
addresses, and likewise are not equipped for any special multicast
protocol processing. For such multicast-unaware nodes, processing
for a multicast IP address as a destination IP address MUST be
carried out in the same way as for any other destination IP address.
AODV is a routing protocol, and it deals with route table management.
Route table information must be kept even for short-lived routes,
such as are created to temporarily store reverse paths towards nodes
originating RREQs. AODV uses the following fields with each route
table entry:
- 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
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.
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).
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.
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.
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.
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.
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.
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.
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
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 received within this time period,
another RREQ may be sent, up to RREQ_RETRIES additional attempts
after the first RREQ. For each additional attempt, the waiting time
for the RREP is multiplied by 2, so that the time conforms to a
binary exponential backoff.
6.4. Controlling Dissemination of Route Request Messages
To prevent unnecessary network-wide dissemination of RREQs, the
originating node SHOULD use an expanding ring search technique. In
an expanding ring search, the originating node initially uses a TTL =
TTL_START in the RREQ packet IP header and sets the timeout for
receiving a RREP to RING_TRAVERSAL_TIME milliseconds.
RING_TRAVERSAL_TIME is calculated as described in section 10. The
TTL_VALUE used in calculating RING_TRAVERSAL_TIME is set equal to the
value of the TTL field in the IP header. If the RREQ times out
without a corresponding RREP, the originator broadcasts the RREQ
again with the TTL incremented by TTL_INCREMENT. This continues
until the TTL set in the RREQ reaches TTL_THRESHOLD, beyond which a
TTL = NET_DIAMETER is used for each attempt. Each time, the timeout
for receiving a RREP is RING_TRAVERSAL_TIME. When it is desired to
have all retries traverse the entire ad hoc network, this can be
achieved by configuring TTL_START and TTL_INCREMENT both to be the
same value as NET_DIAMETER.
The Hop Count stored in an invalid routing table entry indicates the
last known hop count to that destination in the routing table. When
a new route to the same destination is required at a later time
(e.g., upon route loss), the TTL in the RREQ IP header is initially
set to the Hop Count plus TTL_INCREMENT. Thereafter, following each
timeout the TTL is incremented by TTL_INCREMENT until TTL =
TTL_THRESHOLD is reached. Beyond this TTL = NET_DIAMETER is used.
Once TTL = NET_DIAMETER, the timeout for waiting for the RREP is set
to NET_TRAVERSAL_TIME, as specified in section 6.3.
An expired routing table entry SHOULD NOT be expunged before
(current_time + DELETE_PERIOD) (see section 6.11). Otherwise, the
soft state corresponding to the route (e.g., last known hop count)
will be lost. Furthermore, a longer routing table entry expunge time
MAY be configured. Any routing table entry waiting for a RREP SHOULD
NOT be expunged before (current_time + 2 * NET_TRAVERSAL_TIME).
6.5. Processing and Forwarding Route Requests
When a node receives a RREQ, it first creates or updates a route to
the previous hop without a valid sequence number (see section 6.2)
then checks to determine whether it has received a RREQ with the same
Originator IP Address and RREQ ID within at least the last
PATH_DISCOVERY_TIME. If such a RREQ has been received, the node
silently discards the newly received RREQ. The rest of this
subsection describes actions taken for RREQs that are not discarded.
First, it first increments the hop count value in the RREQ by one, to
account for the new hop through the intermediate node. Then the node
searches for a reverse route to the Originator IP Address (see
section 6.2), using longest-prefix matching. If need be, the route
is created, or updated using the Originator Sequence Number from the
RREQ in its routing table. This reverse route will be needed if the
node receives a RREP back to the node that originated the RREQ
(identified by the Originator IP Address). When the reverse route is
created or updated, the following actions on the route are also
carried out:
1. the Originator Sequence Number from the RREQ is compared to the
corresponding destination sequence number in the route table entry
and copied if greater than the existing value there
2. the valid sequence number field is set to true;
3. the next hop in the routing table becomes the node from which the
RREQ was received (it is obtained from the source IP address in
the IP header and is often not equal to the Originator IP Address
field in the RREQ message);
4. the hop count is copied from the Hop Count in the RREQ message;
Whenever a RREQ message is received, the Lifetime of the reverse
route entry for the Originator IP address is set to be the maximum of
(ExistingLifetime, MinimalLifetime), where
MinimalLifetime = (current time + 2*NET_TRAVERSAL_TIME -
2*HopCount*NODE_TRAVERSAL_TIME).
The current node can use the reverse route to forward data packets in
the same way as for any other route in the routing table.
If a node does not generate a RREP (following the processing rules in
section 6.6), and if the incoming IP header has TTL larger than 1,
the node updates and broadcasts the RREQ to address 255.255.255.255
on each of its configured interfaces (see section 6.14). To update
the RREQ, the TTL or hop limit field in the outgoing IP header is
decreased by one, and the Hop Count field in the RREQ message is
incremented by one, to account for the new hop through the
intermediate node. Lastly, the Destination Sequence number for the
requested destination is set to the maximum of the corresponding
value received in the RREQ message, and the destination sequence
value currently maintained by the node for the requested destination.
However, the forwarding node MUST NOT modify its maintained value for
the destination sequence number, even if the value received in the
incoming RREQ is larger than the value currently maintained by the
forwarding node.
Otherwise, if a node does generate a RREP, then the node discards the
RREQ. Notice that, if intermediate nodes reply to every transmission
of RREQs for a particular destination, it might turn out that the
destination does not receive any of the discovery messages. In this
situation, the destination does not learn of a route to the
originating node from the RREQ messages. This could cause the
destination to initiate a route discovery (for example, if the
originator is attempting to establish a TCP session). In order that
the destination learn of routes to the originating node, the
originating node SHOULD set the "gratuitous RREP" ('G') flag in the
RREQ if for any reason the destination is likely to need a route to
the originating node. If, in response to a RREQ with the 'G' flag
set, an intermediate node returns a RREP, it MUST also unicast a
gratuitous RREP to the destination node (see section 6.6.3).
6.6. Generating Route Replies
A node generates a RREP if either:
(i) it is itself the destination, or
(ii) it has an active route to the destination, the destination
sequence number in the node's existing route table entry
for the destination is valid and greater than or equal to
the Destination Sequence Number of the RREQ (comparison
using signed 32-bit arithmetic), and the "destination only"
('D') flag is NOT set.
When generating a RREP message, a node copies the Destination IP
Address and the Originator Sequence Number from the RREQ message into
the corresponding fields in the RREP message. Processing is slightly
different, depending on whether the node is itself the requested
destination (see section 6.6.1), or instead if it is an intermediate
node with an fresh enough route to the destination (see section
6.6.2).
Once created, the RREP is unicast to the next hop toward the
originator of the RREQ, as indicated by the route table entry for
that originator. As the RREP is forwarded back towards the node
which originated the RREQ message, the Hop Count field is incremented
by one at each hop. Thus, when the RREP reaches the originator, the
Hop Count represents the distance, in hops, of the destination from
the originator.
6.6.1. Route Reply Generation by the Destination
If the generating node is the destination itself, it MUST increment
its own sequence number by one if the sequence number in the RREQ
packet is equal to that incremented value. Otherwise, the
destination does not change its sequence number before generating the
RREP message. The destination node places its (perhaps newly
incremented) sequence number into the Destination Sequence Number
field of the RREP, and enters the value zero in the Hop Count field
of the RREP.
The destination node copies the value MY_ROUTE_TIMEOUT (see section
10) into the Lifetime field of the RREP. Each node MAY reconfigure
its value for MY_ROUTE_TIMEOUT, within mild constraints (see section
10).
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
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,
- 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
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
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