Mobile Ad Hoc Networking Working Group Charles E. Perkins
INTERNET DRAFT Nokia Research Center
10 March 2000 Elizabeth M. Royer
University of California, Santa Barbara
Samir R. Das
University of Cincinnati
Ad Hoc On-Demand Distance Vector (AODV) Routing
draft-ietf-manet-aodv-05.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 both unicast
and multicast routes between sources and destinations. It uses
destination sequence numbers to ensure loop freedom at all times
(even in the face of anomalous delivery of routing control messages),
solving 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 2
3. AODV Terminology 4
4. Route Request (RREQ) Message Format 5
5. Route Reply (RREP) Message Format 7
6. Route Error (RERR) Message Format 8
7. Multicast Activation (MACT) Message Format 9
8. Group Hello (GRPH) Message Format 10
9. Node Operation - Unicast 11
9.1. Maintaining Route Utilization Records . . . . . . . . . . 11
9.2. Generating Route Requests . . . . . . . . . . . . . . . . 11
9.2.1. Controlling Route Request broadcasts . . . . . . 12
9.3. Forwarding Route Requests . . . . . . . . . . . . . . . . 13
9.3.1. Processing Route Requests . . . . . . . . . . . . 13
9.4. Generating Route Replies . . . . . . . . . . . . . . . . 14
9.5. Forwarding Route Replies . . . . . . . . . . . . . . . . 15
9.6. Hello Messages . . . . . . . . . . . . . . . . . . . . . 16
9.7. Maintaining Local Connectivity . . . . . . . . . . . . . 17
9.8. Route Error Messages . . . . . . . . . . . . . . . . . . 17
9.9. Route Expiry and Deletion . . . . . . . . . . . . . . . . 19
9.10. Actions After Reboot . . . . . . . . . . . . . . . . . . 19
10. Node Operation - Multicast 20
10.1. Maintaining Multicast Tree Utilization Records . . . . . 20
10.2. Generating Route Requests . . . . . . . . . . . . . . . . 20
10.3. Forwarding Route Requests . . . . . . . . . . . . . . . . 21
10.4. Generating Route Replies . . . . . . . . . . . . . . . . 21
10.5. Forwarding Route Replies . . . . . . . . . . . . . . . . 22
10.6. Route Activation . . . . . . . . . . . . . . . . . . . . 23
10.7. Multicast Tree Pruning . . . . . . . . . . . . . . . . . 24
10.8. Repairing Link Breakages . . . . . . . . . . . . . . . . 24
10.9. Tree Partitions . . . . . . . . . . . . . . . . . . . . . 25
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10.10. Reconnecting Two Trees . . . . . . . . . . . . . . . . . 26
10.11. Group Hello Messages . . . . . . . . . . . . . . . . . . 27
10.12. Actions After Reboot . . . . . . . . . . . . . . . . . . 28
11. Broadcast 28
12. Quality of Service 29
13. AODV and Aggregated Networks 29
14. Using AODV with Other Networks 30
15. Address Autoconfiguration 30
16. Extensions 31
16.1. Hello Interval Extension Format . . . . . . . . . . . . . 31
16.2. Multicast Group Leader Extension Format . . . . . . . . . 32
16.3. Multicast Group Rebuild Extension Format . . . . . . . . 33
16.4. Multicast Group Information Extension Format . . . . . . 33
16.5. Maximum Delay Extension Format . . . . . . . . . . . . . 34
16.6. Minimum Bandwidth Extension Format . . . . . . . . . . . 34
17. Configuration Parameters 35
18. Security Considerations 37
19. Acknowledgements 37
A. Draft Modifications 39
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 for the formation
of multicast groups whose membership is free to change during the
lifetime of the network. AODV allows mobile nodes to respond quickly
to link breakages and changes in network topology. 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.
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One distinguishing feature of AODV is its use of a destination
sequence number for each route entry. The destination sequence
number is created by the destination or the multicast group leader
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), Route Errors (RERRs),
Multicast Activations (MACTs), and Group Hellos (GRPHs) are the
message types defined by AODV. These message types are handled by
UDP, and normal IP header processing applies. So, for instance, the
requesting node is expected to use its IP address as the source IP
address for the messages. The range of dissemination of broadcast
RREQs can be indicated by the TTL in the IP header. Fragmentation is
typically not required.
As long as the endpoints of a communication connection have valid
routes to each other, AODV does not play any role. When a route
to a new destination (either a single node or a multicast group)
is needed, the node uses a broadcast 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 source of the RREQ. Since
each node receiving the request caches a route back to the source of
the request, the RREP can be unicast back from the destination to
the source, or from any intermediate node that is able to satisfy
the request back to the source. A RREQ can be conditioned by
requirements on the path to the destination, namely bandwidth or
delay bounds.
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 which destinations are now unreachable due to the
loss of the link.
RREQs are also used when a node wishes to join a multicast group.
A join flag in the RREQ informs nodes that when receiving the
RREP, they are not just setting route pointers but are also setting
multicast route pointers, which will be used if the route is selected
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to be added onto the tree. If the route is chosen for addition to
the multicast tree, it will be activated by a MACT message.
For multicast groups, a Group Hello message is periodically broadcast
across the network by the multicast group leader. The message
carries multicast group and corresponding group leader IP addresses.
This information is used for repairing multicast trees after a
previously disconnected portion of the network containing part of the
multicast tree becomes reachable once again.
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 uses the following fields with each route
table entry:
- Destination IP Address
- Destination Sequence Number
- Hop Count (number of hops needed to reach destination)
- Last Hop Count (described in subsection 9.2.1)
- Next Hop
- List of Precursors (described in Section 9.1)
- Lifetime (expiration or deletion time of the route)
- Routing Flags
The following information is stored in each entry of the multicast
route table for multicast tree routes:
- Multicast Group IP Address
- Multicast Group Leader IP Address
- Multicast Group Sequence Number
- Next Hops
- Hop Count to next Multicast Group Member
- Hop Count to Multicast Group Leader
The Next Hops field is a linked list of structures, each of which
contains the following fields:
- Next Hop IP Address
- Link Direction
- Activated Flag
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The direction of the link is relative to the location of the group
leader, i.e. UPSTREAM is a next hop towards the group leader, and
DOWNSTREAM is a next hop away from the group leader. A node on
the multicast tree must necessarily have only one UPSTREAM link.
The IP Address of a Next Hop MUST NOT be used to forward multicast
messages until after a MACT message has activated the route (see
Section 10.6).
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 [4].
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 inifnite metric
in the Hop Count field. Only active entries can be used to
forward data packets. Invalid entries are eventually deleted.
forwarding node
A node which agrees to forward packets destined for another
destination node, by retransmitting them to a next hop which is
closer to the unicast destination along a path which has been
set up using routing control messages.
forward route
A route set up to send data packets from a source to a
destination.
group leader
A node which is a member of the given multicast group
and which is typically the first such group member in the
connected portion of the network. This node is responsible for
initializing and maintaining the multicast group destination
sequence number.
group leader table
The table where ad hoc nodes keep information concerning each
multicast group and its corresponding group leader. There is
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one entry in the table for each multicast group for which the
node has received a Group Hello (see Section 10.2).
multicast tree
The tree containing all nodes which are members of the
multicast group and all nodes which are needed to connect the
multicast group members.
multicast route table
The table where ad hoc nodes keep routing (including next hops)
information for various multicast groups.
reverse route
A route set up to forward a reply (RREP) packet back to the
source from the destination or from an intermediate node having
a route to the destination.
subnet leader
A node which is a member of the subnet defined by a specific
routing prefix, and which offers reachability to every other
node with the same routing prefix. The subnet leader is
responsible for initializing and maintaining the destination
sequence number for every node on the subnet.
4. 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| Reserved | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Broadcast ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the Route Request message is illustrated above, and
contains the following fields:
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Type 1
J Join flag; set when source node wants to join a
multicast group.
R Repair flag; set when a node wants to initiate
a repair to connect two previously disconnected
portions of the multicast tree.
Reserved Sent as 0; ignored on reception.
Hop Count The number of hops from the Source IP Address to
the node handling the request.
Broadcast ID A sequence number uniquely identifying the
particular RREQ when taken in conjunction with the
source node's IP address.
Destination IP Address
The IP address of destination for which a route is
desired.
Destination Sequence Number
The last sequence number received in the past by
the source for any route towards the destination.
Source IP Address
The IP address of the node which originated the
Route Request.
Source Sequence Number
The current sequence number to be used for route
entries pointing to (and generated by) the source
of the route request.
When a node wishes to repair a multicast tree, it appends the
Multicast Group Rebuild extension (see Section 16.3). When a node
wishes to unicast the RREQ for a multicast group to the group leader,
it includes the Multicast Group Leader extension (see Section 16.2).
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5. 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| Reserved |Prefix Sz| Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the Route Reply message is illustrated above, and
contains the following fields:
Type 2
R Repair flag; set when a node is responding
to a repair request to connect two previously
disconnected portions of the multicast tree.
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 Source 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.
Source IP Address
The IP address of the source node which issued the
RREQ for which the route is supplied.
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Lifetime The time for which nodes receiving the RREP consider
the route to be valid.
When the RREP is sent for a multicast destination, the Multicast
Group Information extension is appended (see Section 16.4).
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.
6. 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 | 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
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 which has become
unreachable due to a link break.
Unreachable Destination Sequence Number
The last known sequence number, incremented by one,
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of 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. The unreachable destination
addresses included are those of all lost destinations which are now
unreachable due to the loss of that link.
7. Multicast Activation (MACT) 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 |P|G|U|R| Reserved | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the Multicast Activation message is illustrated above,
and contains the following fields:
Type 4
P Prune flag; set when a node wishes to prune itself
from the tree, unset when the node is activating a
tree link.
G Group Leader flag; set by a multicast tree member that
fails to repair a multicast tree link breakage, and
indicates to the group member receiving the message
that it should become the new multicast group leader.
U Update flag; set when a multicast tree member has
repaired a broken tree link and is now a new distance
from the group leader.
R Reboot flag; set when a node has just rebooted (see
Section 10.12).
Reserved Sent as 0; ignored on reception.
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Hop Count The distance of the sending node from the multicast
group leader. Used only when the 'U' flag is set;
otherwise sent as 0.
Multicast Group IP Address
The IP address of the Multicast Group for which a
route is supplied.
Source IP Address
The IP address of the sending node.
Source Sequence Number
The current sequence number for route information
generated by the source of the route request.
To prune itself from the tree (i.e., inactivate its last link to the
multicast tree), a multicast tree member sends a MACT with the 'P'
flag = 1 to its next hop on the multicast tree. A multicast tree
member that has more than one next hop to the multicast tree SHOULD
NOT prune itself from the multicast tree.
8. Group Hello (GRPH) 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 |U|M| Reserved | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Leader IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the Group Hello message is illustrated above, and
contains the following fields:
Type 5
U Update flag; set when there has been a change in group
leader information.
M Off_Mtree flag; set by a node receiving the group
hello that is not on the multicast tree.
Reserved Sent as 0; ignored on reception.
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Hop Count The number of hops the packet has traveled. Used by
multicast tree nodes to update their distance from the
group leader when the M flag is not set.
Group Leader IP Address
The IP address of the group leader.
Multicast Group IP Address
The IP address of the Multicast Group for which the
sequence number is supplied.
Multicast Group Sequence Number
The current sequence number of the multicast group.
9. Node Operation - Unicast
This section describes the scenarios under which nodes generate
RREQs, RREPs and RERRs for unicast communication, and how the message
data are handled.
9.1. Maintaining Route Utilization Records
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.
Each time a route is used to forward a data packet, its Lifetime
field is updated to be current time plus ACTIVE_ROUTE_TIMEOUT.
9.2. Generating Route Requests
A node broadcasts a RREQ when it determines that it needs a route
to a destination and does not have one available. This can happen
if the destination is previously unknown to the node, or if a
previously valid route to the destination expires 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
Source Sequence Number in the RREQ message is the node's own sequence
number. The Broadcast ID field is incremented by one from the last
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broadcast ID used by the current node for the same destination. The
Hop Count field is set to zero.
After broadcasting a RREQ, a node waits for a RREP. If the RREP
is not received within RREP_WAIT_TIME milliseconds, the node MAY
rebroadcast the RREQ, up to a maximum of RREQ_RETRIES times. Each
rebroadcast MUST increment the Broadcast ID field.
Data packets waiting for a route (i.e., waiting for a RREP after RREQ
has been sent) SHOULD be buffered. The buffering SHOULD be FIFO. If
a RREQ has been rebroadcast 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.
9.2.1. Controlling Route Request broadcasts
To prevent unnecessary network-wide broadcasts of RREQs, the
source node SHOULD use an expanding ring search technique as an
optimization. In an expanding ring search, the source 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. Upon timeout, the source rebroadcasts the RREQ 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 rebroadcast. Each time, the timeout
for receiving a RREP is calculated as before. Each rebroadcast
increments the Broadcast ID field in the RREQ packet. The RREQ
can be rebroadcast with TTL = NET_DIAMETER up to a maximum of
RREQ_RETRIES times.
When a RREP is received, the Hop Count used in the RREP packet is
remembered as 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.
As a further optimization, timeouts MAY be determined dynamically via
measurements, 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 16 to be
carried back by the RREP packet (again via an extension field). The
difference between the current time and this timestamp will determine
the route discovery latency. The timeout may be set to be a small
factor of the average of the last few route discovery latencies
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for the concerned destination. These latencies may be recorded as
additional fields in the routing table.
If the optimizations described in this section are used, an expired
routing table entry should not be expunged too early. Otherwise, the
soft states corresponding to the route (e.g., Last Hop Count) will be
lost. In such cases, a longer routing table entry expunge time may
be specified. In general, any routing table entry waiting for a RREP
should not be expunged before the timeout for receiving RREP.
9.3. Forwarding Route Requests
When a node receives a broadcast RREQ, it first checks to determine
whether it has received a RREQ with the same Source IP Address and
Broadcast ID within the last BCAST_ID_SAVE 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.
9.3.1. Processing Route Requests
When a node receives a RREQ, the node checks to determine whether it
has an active route to the destination. If the node does not have
an active route, it rebroadcasts the RREQ from its interface(s) but
using its own IP address in the IP header of the outgoing RREQ. The
Destination Sequence Number in the RREQ is updated to the maximum
of the existing Destination Sequence Number in the RREQ and the
destination sequence number in the routing table (if an entry exists)
of the current node. The TTL or hop limit field in the outgoing IP
header is decreased by one. The Hop Count field in the broadcast
RREQ message is incremented by one, to account for the new hop
through the intermediate node.
If the node, on the other hand, does has an active route for the
destination, it compares the destination sequence number for that
route with the Destination Sequence Number field of the incoming
RREQ. If the existing destination sequence number is smaller than
the Destination Sequence Number field of the RREQ, the node again
rebroadcasts the RREQ just as if it did not have an active route to
the destination.
The node generates a RREP (as discussed further in section 9.4) if
either:
(i) it has an active route to the destination, and the
node's existing destination sequence number is greater
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than or equal to the Destination Sequence Number of the
RREQ, or
(ii) it is itself the destination.
The node always creates or updates a reverse route to the Source IP
Address in its routing table. If a route to the Source IP Address
already exists, it is updated only if either
(i) the Source Sequence Number in the RREQ is higher than
the destination sequence number of the Source IP Address
in the destination sequence number table, or
(ii) the sequence numbers are equal, but the hop count as
specified by the RREQ is now smaller than the existing
hop count in the routing table.
When a reverse route is created or updated, the following actions are
carried out:
1. the Source Sequence Number from the RREQ is copied to the
corresponding destination sequence number;
2. the next hop in the routing table becomes the node broadcasting
the RREQ (it is obtained from the source IP address in the IP
header and is often not equal to the Source IP Address field in
the RREQ message);
3. the hop count is copied from the Hop Count in the RREQ message;
4. the lifetime of the route is the higher of its current lifetime
(for an active route) and current time plus REV_ROUTE_LIFE.
Even if the route is not updated because the existing route has a
higher destination sequence number, but if it is scheduled to expire
before REV_ROUTE_LIFE, its lifetime is still updated to be current
time plus REV_ROUTE_LIFE.
This reverse route will be used by an eventual RREP back to the node
which originated the RREQ (identified by the Source IP Address).
9.4. 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 and unicasts it back
to the node indicated by the Source IP Address field of the received
RREQ. If the node is not the destination node, it copies the last
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known destination sequence number in the Destination Sequence Number
field in the RREP message. If the generating node is the destination
itself, it uses a destination sequence number at least equal to a
sequence number generated after the last detected change in its
neighbor set and at least equal to the destination sequence number in
the RREQ. If the destination node has not detected any change in its
set of neighbors since it last incremented its destination sequence
number, it MAY use the same destination sequence number.
The Source and Destination IP Addresses in RREQ message are copied to
corresponding fields in the RREP message.
If the generating node is not the destination node, then the
generating 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. If the generating node is the destination node, it
places the value zero in the Hop Count field. The Hop Count field
is incremented by one at each hop as the RREP is forwarded to the
source. When the RREP reaches the source, the Hop Count represents
the distance, in hops, of the destination from the source.
If the node is not the destination node, it calculates the Lifetime
field of the RREP by subtracting the current time from the expiration
time in its route table entry. Otherwise, if the generating node
is also the destination node, it copies the value MY_ROUTE_TIMEOUT
into the Lifetime field of the RREP. Each node MAY make a separate
determination about its value MY_ROUTE_TIMEOUT.
If the generating node is not the node indicated by the Destination
IP Address, then it puts the next hop towards the destination in the
precursor list for the reverse route entry. (This is the entry for
Source IP Address.) In addition, the generating node puts 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. (This is the entry for the Destination IP
Address).
9.5. Forwarding Route Replies
When a node receives a RREP message, it first compares the
Destination Sequence Number in the message with its own copy of
destination sequence number for the Destination IP Address. The
forward route for this destination is created or 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 the Hop
Count in RREP is smaller than the hop count in route table entry. If
a new route is created or the old route is updated, the next hop is
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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; the destination sequence
number is the Destination Sequence Number in the RREP message.
The current node can now begin using this route to send data packets
to the destination.
If the current node is not the source node as indicated by the Source
IP Address in the RREP message AND a forward route has been created
or updated as described before, the node consults its route table
entry for the source node to determine the next hop for the RREP
packet, and then forwards the RREP towards the source with its Hop
Count incremented by one.
When any node generates or forwards 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
current time plus ACTIVE_ROUTE_TIMEOUT.
9.6. 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 generate a broadcast RREP with TTL = 1, called a
Hello message, with the 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 it receives no packets 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 9.8.
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9.7. Maintaining Local Connectivity
Each forwarding node SHOULD keep track of its active next hops (i.e.,
which next hops have been used to forward packets towards some
destination within the last ACTIVE_ROUTE_TIMEOUT milliseconds). This
is done by updating the Lifetime field of a routing table entry used
to forward data packets to current time plus ACTIVE_ROUTE_TIMEOUT
milliseconds. For purposes of efficiency, each node may try to learn
which of these active next hops are really in the neighborhood at the
current time 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,
will indicate loss of the link to this active next hop.
- Passive acknowledgment can 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
not a forwarding node (and thus is never supposed to transmit the
packet) one of the following methods should be used to determine
connectivity.
* Receiving an ICMP ACK message from the next hop. The ICMP
ACK message SHOULD be sent to a forwarding node by a next hop
which is also the destination as in the in the IP header of
the packet. This should be done only when this destination
has not sent any packets to the concerned forwarding node
within the last HELLO_INTERVAL milliseconds.
* 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 9.8.
9.8. Route Error Messages
A node initiates a RERR message in three situations:
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(i) if it detects a link break for the next hop of an active
route in its routing table, or
(ii) if it gets a data packet destined to a node for which it
does not have an active route, or
(iii) if it receives a RERR from a neighbor for one or more
active routes.
For cases (i) and (ii), the destination sequence numbers in the
routing table for the unreachable destination(s) are incremented by
one. Then RERR is broadcast with the unreachable destination(s) and
their incremented destination sequence number(s) included in the
packet. For case (i), the unreachable destinations are the broken
next hop, and any additional destinations which are now unreachable
due to the loss of this next hop link. For case (ii), there is only
one unreachable destination, which is the destination of the data
packet that cannot be delivered. The DestCount field of the RERR
packet indicates the number of unreachable destinations included in
the packet.
For cases (i) and (ii), for each unreachable destination the node
copies the value in the Hop Count route table field into the Last
Hop Count field, and marks the Hop Count for this destination as
infinity, and thus invalidates the route.
For case (iii) when a node receives a RERR message, for each
unreachable destination included in the packet, the node determines
whether the source node (as indicated by the source IP address in the
IP header) forwarding the RERR packet is its own next hop used to
reach this destination. If so, the node takes the following actions:
(a) updates the corresponding destination sequence number
with the Destination Sequence Number in the packet, and
(b) marks the Hop Count for this destination as infinity,
and thus invalidates the route.
(c) checks the precursor list for this destination. If one
or more of these precursor lists are non-empty, the node
creates a RERR message, including as unreachable each
destination with a non-empty precursor list. It also
includes their destination sequence numbers, and then
broadcasts this RERR message.
When a node receives a RERR message, it always updates its
destination sequence number(s) for the unreachable destination(s)
included in the packet using the corresponding sequence numbers
included in the message. When a node broadcasts a RERR message, it
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always deletes the precursor list of each unreachable destination
included in the message.
When a node invalidates a route to a neighboring node, it must also
delete that neighbor from any precursor lists for routes to other
nodes. This prevents precursor lists from containing stale entries
of neighbors with which the node is no longer able to communicate.
The node should inspect the precursor list of each destination entry
in its routing table, and delete the lost neighbor from any list in
which it appears.
9.9. Route Expiry and Deletion
If the Lifetime of an active routing entry expires, the following
actions are taken.
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 is
incremented by one.
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 plays dual role -- for an active route
it is the expiry time, and for an invalid route it is the deletion
time.
These actions are also taken whenever a route entry is invalidated
for any reason, for example, for link breakage or receiving a RERR.
If a data packet is received for an invalid route, the Lifetime
field is always updated to current time plus DELETE_PERIOD. The
determination of DELETE_PERIOD is discussed in Section 17
9.10. Actions After Reboot
A node participating in the ad hoc network must take certain
actions after reboot as it will have lost its prior sequence
number and as well as its last known sequence numbers for various
other destinations. However, there may be neighboring nodes which
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. In this time, it does not respond
to any routing packets. However, if it receives a data packet,
it broadcasts a RERR as described in subsection 9.8 and resets
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the waiting timer (Lifetime) to expire after current time plus
DELETE_PERIOD.
It can be shown 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.
10. Node Operation - Multicast
This section describes the scenarios under which nodes generate
control messages for multicast communication, and how the fields in
the messages are handled.
10.1. Maintaining Multicast Tree Utilization Records
For each multicast tree to which a node belongs, either because it
is a member of the group or because it is a router for the multicast
tree, the node maintains a list of next hops -- i.e., those 1-hop
neighbors that are likewise a part of the multicast tree. This
list of next hops is used for forwarding messages received for
the multicast group. A node will forward a multicast message to
every such next hop, except that neighbor from which the message
arrived. If there are multiple next hops, the forwarding operation
MAY be performed by broadcasting the multicast packet to the node's
neighbors; only the neighbors that belong to the multicast tree and
have the sending node as a next hop continue to forward the multicast
packet.
10.2. Generating Route Requests
A node sends a RREQ either when it determines that it should be a
part of a multicast group, and it is not already a member of that
group, or when it has a message to send to the multicast group but
does not have a route to that group. If the node wishes to join the
multicast group, it sets the `J' flag in the RREQ; otherwise, it
leaves the flag unset. The destination address of the RREQ is always
set to the multicast group address. If the node knows the group
leader and has a route to it, the node places the group leader's
address in the Multicast Group Leader extension (Section 16.2), and
unicasts the RREQ to the corresponding next hop for that destination.
Otherwise, if the node does not have a route to the group leader, or
if it does not know who the multicast group leader is, it broadcasts
the RREQ and does not include the extension field.
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The process of waiting for a RREP to a RREQ with a multicast
destination address is the same as that described in Section 9.2.
The node may resend the RREQ up to RREQ_RETRIES additional times if
a RREP is not received. If a RREQ was unicast to a group leader
and a RREP is not received within RREP_WAIT_TIME milliseconds, the
node broadcasts subsequent RREQs for that multicast group across the
network. If a RREP is not received after RREQ_RETRIES additional
requests, the node may assume that there are no other members of that
particular group within the connected portion of the network. If it
wanted to join the multicast group, it then becomes the multicast
group leader for that multicast group and initializes the sequence
number of the multicast group. Otherwise, if it only wanted to send
packets to that group without actually joining the group, it drops
the packets it had for that group and aborts the session.
When the node wishes to join or send a message to a multicast group,
it first consults its Group Leader Table. Based on the existence
of an entry for the multicast group in this table, the node then
formulates and sends the RREQ as described at the beginning of this
section.
10.3. Forwarding Route Requests
The operation of nodes forwarding RREQs for multicast is similar
to that for the reception and forwarding of RREQs as described in
Section 9.3, with one exception. If the RREQ is a join request, it
creates a multicast group next hop entry for the node from which it
received the RREQ. The generation of the route reply (RREP) message
is discussed in the following section.
10.4. Generating Route Replies
If a node receives a join RREQ for a multicast group, and it is
already a member of the multicast tree for that group, the node
updates its Multicast Route Table and then generates a RREP message.
It unicasts the RREP back to the node indicated by the Source IP
Address field of the received RREQ. The RREP contains the current
sequence number for the multicast group and the IP address of the
group leader. Furthermore, it initializes the Hop Count field of
the RREP to zero. Additional information about the multicast group
leader is entered into the Multicast Group Information extension (see
Section 16.4).
A node can only respond to a join RREQ if it is a member of the
multicast tree. If a node receives a multicast route request that
is not a join message, it can reply if it has a current route to the
multicast tree. Otherwise it continues forwarding the request. If a
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node receives a join RREQ for a multicast group and it is not already
a member of the multicast tree for that group, it rebroadcasts the
RREQ to its neighbors.
In the event that a node receives a unicasted multicast route request
that specifies its own IP address as the destination address (i.e.,
the source node believes this destination node to be the multicast
group leader), but the node is in fact not the group leader, it
can simply ignore the RREQ. The source node will time out after
RREP_WAIT_TIME milliseconds and will broadcast a new RREQ without the
group leader address specified.
Regardless of whether the multicast group leader or a multicast tree
member generates the RREP, the RREP fields are set as follows:
Hop Count 0
Destination IP Address
The IP address of the multicast group.
Destination Sequence Number
The current multicast group sequence number.
Lifetime The time for which nodes receiving the RREP consider
the route to be valid (only used it the RREQ is not a
join request).
The Multicast Group Information extension described in Section 16.4
is also included for join requests. If the node generating the RREP
is not on the multicast tree (because the RREQ was not a join RREQ),
it places its distance from the multicast tree in the Hop Count
field, instead of 0.
10.5. Forwarding Route Replies
If an intermediate node receives a RREP in response to a RREQ that
it has transmitted (or retransmitted on behalf of some other node),
it increments the Hop Count and Multicast Group Hop Count fields and
forwards the RREP along the path to the source of the RREQ.
When the node receives more than one RREP for the same RREQ, it saves
the route information with the greatest sequence number, and beyond
that the lowest hop count; it discards all other RREPs. This node
forwards the first RREP towards the source of the RREQ, and then
forwards later RREPs only if they have a greater sequence number or
smaller metric.
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10.6. Route Activation
When a node broadcasts a RREQ message, it is likely to receive more
than one reply since any node in the multicast tree can respond. If
the RREQ was not a join request, then once the source node receives
the first RREP, it may begin using this route to forward data
packets. On the other hand, if the RREQ was a join request, the RREP
message sets up route pointers as it travels back to the source node.
These route pointers may eventually graft a branch onto the multicast
tree. If multiple branches to the same destination are created in
such a manner, a loop will be formed. Hence, in order to prevent
the formation of any such loops, it is necessary to activate only
one of the routes created by the RREP messages. The RREP containing
the largest destination sequence number is chosen to be the added
branch to the multicast tree. In the event that a node receives more
than one RREP with the same (largest) sequence number, it selects the
first one with the smallest hop count, i.e., the shortest distance to
a member of the multicast tree.
After waiting RREP_WAIT_TIME milliseconds, the node must select the
route it wishes to use as its link to the multicast tree. This is
accomplished by sending a Multicast Activation (MACT) message. The
Destination IP Address field of the MACT packet is set to the IP
address of the multicast group. The node unicasts this message to
the selected next hop, effectively activating the route. It then
sets the Activated flag in the next hop Multicast Route Table entry
associated with that node. After receiving this message, the node
to which the MACT was sent activates the route entry for the link in
its multicast route table, thereby finalizing the creation of the
tree branch. All neighbors not receiving this message time out and
delete that node as a next hop for the multicast group in their route
tables, having never activated the route entry for that next hop.
Two scenarios exist for a neighboring node receiving the MACT
message. If this node was previously a member of the multicast
tree, it does not propagate the MACT message any further. However,
if the next hop selected by the source node's MACT message was not
previously a multicast tree member, it will have propagated the
original RREQ further up the network in search of nodes which are
tree members. Thus it is possible that this node also received more
than one RREP, as noted in section 10.5.
When the node receives a MACT selecting it as the next hop, it
unicasts its own MACT to the node it has chosen as its next hop,
and so on up the tree, until a node which was already a part of the
multicast tree is reached.
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10.7. Multicast Tree Pruning
A multicast group member can revoke its member status at any time.
However, it can only actually leave the multicast tree if it is not a
tree router for any other nodes in the multicast group (i.e., if it
is a leaf node). If a node wishing to leave the multicast group is
a leaf node, it unicasts to its next hop on the tree a MACT message
with the 'P' flag set and with the Destination IP Address set to the
IP address of the multicast group. It then deletes the multicast
group information for that group from its Multicast Route Table.
When its next hop receives this message, it deletes the sending
node's information from its list of next hops for the multicast tree.
If the removal of the sending node causes this node to become a leaf
node, and if this node is also not a member of the multicast group,
it may in turn prune itself by sending its own MACT message up the
tree.
When the multicast group leader wishes to leave the multicast group,
it proceeds in a manner similar to the one just described. If it
is a leaf node, it may leave the group and unicast a prune message
to its next hop. The next hop acts in the manner described in
Section 10.10, since the prune message is coming from its upstream
neighbor. Otherwise, if the group leader is not a leaf node, it may
not prune itself from the tree. It takes the actions described in
Section 10.9, where it selects one of its next hops and unicasts to
it the MACT with set `G' flag.
10.8. Repairing Link Breakages
Branches of the multicast tree become invalid if a broken link
results in an infinite metric being associated with the route. When
a broken link is detected between two nodes on the multicast tree,
the two nodes should delete the link from their list of next hops for
the multicast group. The node downstream of the break (i.e., the
node which is further from the multicast group leader) is responsible
for initiating the repair of the broken link. In order to repair
the tree, the downstream node broadcasts a RREQ with destination IP
address set to the IP address of the multicast group and with the `J'
flag set. The destination sequence number of the RREQ is the last
known sequence number of the multicast group. The node also includes
the Multicast Group Leader Extension. The Multicast Group Hop Count
field of this extension is set to the distance of the source node
from the multicast group leader. A node MUST have a hop count to
the multicast group leader less than or equal to the indicated value
in order to respond. This hop count requirement prevents nodes on
the same side of the break as the node initiating the repair from
replying to the RREQ.
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The RREQ is broadcast using an expanding rings search. Because of
the high probability that other nearby nodes can be used to rebuild
the route, the original RREQ is broadcast with a TTL (time to live)
field value equal to two more than the Multicast Group Hop Count. In
this way, the effects of the link breakage may be localized. If no
reply is received within RREP_WAIT_TIME milliseconds, all subsequent
RREQs (up to RREQ_RETRIES additional attempts) are broadcast across
the entire network. Any node that is a part of the multicast tree
and that has a hop count to the multicast group leader smaller than
that contained in the RREQ can return a RREP. If there is more than
one RREP received at the originating node, route deletions occur as
described in the previous section.
At the end of the discovery period, the node selects its next hop
and unicasts a MACT message to that node to activate the link, as
described in Section 10.7. Since the node was repairing a tree
break, it is likely that it is now a different distance from the
group leader than it was before the break. If this is the case, it
must inform its DOWNSTREAM next hops of their new distance from the
group leader. It does this by broadcasting a MACT message with the
'U' flag set, and the Hop Count field set to the node's new distance
from the group leader. This 'U' flag indicates that multicast tree
nodes should update their distance from the group leader. If these
nodes have downstream next hops, they in turn must send a MACT
message with a set 'U' flag to their next hops, and so on. The Hop
Count field is incremented by one each time the packet is received.
When a node on the multicast tree receives the MACT message with the
'U' flag set, in determines whether this packet arrived from its
UPSTREAM neighbor. If it did not, the node discards the packet.
When a link break occurs, it is possible that the tree will be
repaired through different intermediate nodes. Hence, if the node
UPSTREAM of the break is not a group member, and if the loss of that
link causes it to become a leaf node, it sets a prune timer to wait
for the link to be repaired. This PRUNE_TIMEOUT should be larger
than RREP_WAIT_TIMEOUT to give the link time to be repaired. If,
when this timer expires, the node has not received a MACT message
selecting it to be a part of the repaired tree branch, it prunes
itself from the tree by sending a MACT with set 'P' flag to its next
hop, as previously described.
10.9. Tree Partitions
It is possible that after a link breaks, the tree cannot be repaired
due to a network partition. If the node attempting to repair a
tree link breakage does not receive a response after RREQ_RETRIES
attempts, it can be assumed that the network has become partitioned
and the multicast tree cannot be repaired at this time. In this
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situation, if the node which initiated the route rebuilding is a
multicast group member, it becomes the new multicast group leader
for its part of the multicast tree partition. It broadcasts a Group
Hello for this multicast group. The `U' flag in the Group Hello is
set, indicating that there has been a change in the group leader
information. All nodes receiving this message update their Group
Leader Table to indicate the new group leader information. Nodes
which are a part of the multicast tree also update the group leader
information for that group in their Multicast Route Table to indicate
the new group leader.
On the other hand, if the node which had initiated the repair is not
a multicast group member, there are two possibilities. If it only
has one next hop for the multicast tree, it prunes itself from the
tree by unicasting a MACT message, with the 'P' flag set, to its next
hop. The node receiving this message notes that the message came
from its upstream link, i.e., from a node that is closer to the group
leader than it is. If the node receiving this message is a multicast
group member, it becomes the new group leader and broadcasts a
Group Hello message as indicated above. Otherwise, if it is not a
multicast group member and it only has one other next hop link, it
similarly prunes itself from the tree. This process continues until
a multicast group member is reached.
The other possibility is that the node which initiated the rebuilding
is not a group member and has more than one next hop for the tree.
In this case, it cannot prune itself, since doing so would partition
the tree. It instead chooses one of its next hops and unicasts a
MACT with the 'G' flag set to that node. This flag indicates that
the next group member to receive this message should become the
new group leader. It then changes the direction of that link to
be UPSTREAM. If the node's next hop is a group member, this node
becomes the group leader. Otherwise, the node unicasts its own MACT
message with the 'G' flag set to one of its next hops, and changes
the direction of that link. Once a group member is reached, the new
group leader is determined.
10.10. Reconnecting Two Trees
In the event that a link break can not be repaired, the multicast
tree remains partitioned until the two parts of the network become
connected once again. A node from one partition of the network knows
that it has come into contact with a node from the other partition of
the network by noting the difference in the GRPH message multicast
group leader information. The multicast group leader with the lower
IP address initiates the tree repair. For the purposes of this
explanation, call this node GL1. GL1 unicasts a RREQ with both
the 'J' and 'R' flags set to the group leader of the other network
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partition (GL2), using the node from which it received the GRPH
as the next hop. This RREQ contains the current value of GL1's
multicast group sequence number. If any node that receives the RREQ
is a member of GL2's multicast tree, it MUST forward the RREQ along
its upstream link, i.e. towards GL2. This prevents any loops from
being formed after the repair. Upon receiving the RREQ, GL2 takes
the larger of its and the received multicast group sequence number,
increments this value by one, and responds with a RREP. This is the
group leader which becomes the leader of the reconnected multicast
tree. The 'R' flag of the RREP is set, indicating that this RREP is
in response to a repair request.
As the RREP is propagated back to GL1, nodes add the incoming and
outgoing links to the Multicast Route Table next hop entries if
these entries do not already exist. The nodes also activate these
entries, thereby adding the branch on to the multicast tree. If a
node that was previously a member of GL1's tree receives the RREP, it
MUST forward the packet along its link to its previous group leader
(GL1). It then updates its group leader information to reflect GL2
as the new group leader, changes the direction of the next hop link
associated with GL1 to DOWNSTREAM, and sets the direction of the
link on which it received the RREP to UPSTREAM. When GL1 receives
the RREP, it updates its group leader information and sets the
link from which it received the RREP as its upstream link. The
tree is now reconnected. The next time GL2 broadcasts a GRPH, it
sets the `U' flag to indicate that there is a change in the group
leader information and group members should update the corresponding
information. All network nodes update their Group Leader Table to
reflect the new group leader information.
10.11. Group Hello Messages
If a node sends a RREQ to join a multicast group (`J' flag set)
and after RREQ_RETRIES attempts does not receives a response, it
then becomes the multicast group leader. The node initializes the
multicast group sequence number and then broadcasts a Group Hello
message to inform network nodes that it is now the group leader
for the multicast group. To ensure nodes maintain consistent and
up-to-date information about who the multicast group leaders are,
any node which is a group leader for a multicast group broadcasts
such a Group Hello across the network every GROUP_HELLO_INTERVAL
milliseconds. The contents of the GRPH fields are set as follows:
U Flag 0
M Flag 0
Hop Count 0
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Group Leader IP Address
The IP Address of the group leader.
Multicast Group IP Address
The IP Address of the Multicast Group for which the
node is the group leader.
Multicast Group Sequence Number
One plus the last known sequence number of the
multicast group.
Nodes receiving the Group Hello increment the Hop Count field by one
before forwarding the message. When a node not on the multicast
tree receives the GRPH message, it sets the M flag. This indicates
that this incarnation of the message has traveled off the multicast
tree, and hence cannot be used by group members to verify their
distance from the group leader. The U flag is set by the group
leader whenever there has been a change in group leader information.
It informs nodes that they should update the group leader information
associated with the indicated multicast group.
10.12. Actions After Reboot
A node participating in the multicast tree that reboots (or restarts
the routing daemon) loses all of its multicast tree information.
Upon reboot, a node should broadcast a MACT message with set Reboot
('R') flag to inform neighboring nodes that it has lost its multicast
group information. Since the rebooted node does not know whether it
was previously a member of the multicast tree, it should broadcast
this packet unconditionally upon starting the daemon. When a node
on the multicast tree receives the reboot MACT message, it checks
whether this message came from one of its next hops on the multicast
tree. If so, one of two situations exists.
If the reboot MACT came from a downstream link, the node deletes that
link from its list of next hops and sets a prune timer according to
the guidelines in Section 10.8. Otherwise, if the reboot MACT came
from a node's upstream link, it must rebuild the tree branch as is
also indicated in Section 10.8.
11. Broadcast
When a node wishes to generate a broadcast, it sends the broadcast
packet to address 255.255.255.255. AODV does not specify
transmissions to any directed broadcast address.
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Every node maintains a list to keep track of which broadcast packets
have already been received and retransmitted. The list contains, for
each distinct broadcast packet received, the source IP address and
the IP ident value from the IP header of the broadcast packet.
When a node receives a packet broadcast to address 255.255.255.255,
it checks the source IP address and the IP ident value of the
broadcast packet's IP header. The node then checks to see whether
the broadcast packet has already been received in the past, and thus
whether it has already retransmitted the broadcast packet. If there
is no existing list entry containing the same IP source address and
IP ident value, the node retransmits the broadcast packet. If there
is such a list entry with matching source IP address and IP ident
field, the node silently discards the broadcast packet.
List entries SHOULD be kept for at least BROADCAST_RECORD_TIME
before the node expunges the record. BROADCAST_RECORD_TIME
is a configurable parameter, but it MUST be at least equal to
RREP_WAIT_TIME.
12. Quality of Service
AODV currently provides some minimal controls to enable mobile nodes
in an ad hoc network to specify, as part of a RREQ, certain Quality
of Service parameters that a route to a destination must satisfy.
In particular, a RREQ MAY include a Maximum Delay extension (see
Section 16.5) or a Minimum Bandwidth extension (see Section 16.6).
If, after establishment of such a route, any node along the path
detects that the requested Quality of Service parameters can no
longer be maintained, that node MUST originate a ICMP QOS_LOST
message back to the node which had originally requested the now
unavailable parameters.
13. 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
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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.
14. 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 13) 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.
15. Address Autoconfiguration
When a node in an ad hoc network wishes to obtain an IP address, it
may be difficult or impossible to contact any address allocation
agency in the network. In such cases, the node should attempt to
select a random address on network 169.253/16, analogous to the way
that Autonet allocations are done and as is proposed in the zeroconf
working group [2].
Following the suggestions for Duplicate Address Detection (DAD) as
with IPv6 Stateless Address Autoconfiguration [5] and zeroconf, the
node first picks a random IP address in the range 2048-65534 from
169.253/16. Then, the node issues a RREQ for that randomly selected
address. If no RREP is returned for the selected address, the node
retries the RREQ up to RREQ_RETRIES times. If, after all retries,
no RREP is still received, the node assumes that the address is not
already in use, and assumes that the address can safely be taken for
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its own. Otherwise, the node randomly picks another address from the
same range and begins the ad hoc DAD procedure again.
In order for a return route to be built for a possible RREP, the node
performing DAD has to have use of some temporary IP address. This
temporary IP address is to be selected from the range 1-2047 of the
class B network 169.253/16. No address in that range should ever be
selected for permanent assignment by any node in the ad hoc network;
all such addresses are only to be used for the purpose of targeting
possible RREP messages produced during DAD. It is expected that this
will provide enough addresses for the purpose, since each address
would never be used for more than a few seconds or a few hundreds of
milliseconds. The timeout parameters for the RREQ messages issued
during DAD are the same as the usual timeout parameters for RREQ
messages.
16. 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 with
the rules for handling IPv6 options.
16.1. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 9.6).
16.2. Multicast Group Leader Extension Format
This extension is appended to a RREQ by a node wishing to repair a
multicast tree.
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 | Multicast Group Leader IP ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Address (continued) | Previous Hop IP Address ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 3
Length 8
Multicast Group Leader IP Address
The IP Address of the Multicast Group Leader.
Previous Hop IP Address
The IP Address of the node which previously received the
RREQ. This field is used when the RREQ is unicast to
the group leader when a node wishes to join a multicast
group.
This extension is used when unicasting the RREQ to the group leader.
Each node receiving the RREQ updates the Previous Hop IP Address
field to reflect its address.
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16.3. Multicast Group Rebuild Extension Format
This extension is appended to a RREQ by a node wishing to repair a
multicast tree.
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 | Multicast Group Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 4
Length 2
Multicast Group Hop Count
The distance in hops of the node sending the RREQ from
the Multicast Group Leader.
This extension is used for rebuilding a multicast tree branch. It is
used to ensure that only nodes as least as close to the group leader
as indicated by the Multicast Group Hop Count field respond to the
request.
16.4. Multicast Group Information Extension Format
The following extension is used to carry additional information for
the RREP message (see Section 5) when sent to establish a route to a
multicast destination.
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 | Multicast Group Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Leader IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 5
Length 6
Multicast Group Hop Count
The distance of the node from the Multicast Group Leader.
Multicast Group Leader IP Address
The IP Address of the current Multicast Group Leader.
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This extension is included when responding to a RREQ to join a
multicast group. The node responding to the RREQ places its distance
from the group leader in the Multicast Group Hop Count field.
16.5. Maximum Delay 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 | Max Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 6
Length 2
Max Delay The number of seconds allowed for a transmission from
the source to the destination.
The Maximum Delay Extension can be appended to a RREQ by a requesting
node in order to place a maximum bound on the acceptable time
delay experienced on any acceptable path from the source to the
destination.
Before forwarding the RREQ, an intermediate node MUST compare its
NODE_TRAVERSAL_TIME to the (remaining) Max Delay indicated in the
Maximum Delay Extension. If the Max Delay is less, the node MUST
discard the RREQ and not process it any further. Otherwise, the
node subtracts NODE_TRAVERSAL_TIME from the Max Delay value in
the extension and continues processing the RREQ as specified in
Section 9.3.
16.6. Minimum Bandwidth 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 | Minimum Bandwidth ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Minimum Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 7
Length 4
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Minimum Bandwidth
The amount of bandwidth (in kilobits/sec) needed
for acceptable transmission from the source to the
destination.
The Minimum Bandwidth Extension can be appended to a RREQ by a
requesting node in order to specify the minimal amount of bandwidth
that must be made available along acceptable path from the source to
the destination.
Before forwarding the RREQ, an intermediate node MUST compare its
available link capacity to the Minimum Bandwidth indicated in the
extension. If the requested amount of bandwidth is not available,
the node MUST discard the RREQ and not process it any further.
Otherwise, the node continues processing the RREQ as specified in
Section 9.3.
17. Configuration Parameters
This section gives default values for some important values
associated with AODV protocol operations. A particular mobile node
may wish to change certain of the parameters, in particular the
NET_DIAMETER, MY_ROUTE_TIMEOUT, ALLOWED_HELLO_LOSS, RREQ_RETRIES,
and possibly the HELLO_INTERVAL. In the latter case, the node should
advertise the HELLO_INTERVAL in its Hello messages, by appending
a Hello Interval Extension to the RREP message. Choice of these
parameters may affect the performance of the protocol.
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Parameter Name Value
---------------------- -----
ACTIVE_ROUTE_TIMEOUT 3,000 Milliseconds
ALLOWED_HELLO_LOSS 2
BAD_LINK_LIFETIME 2 * RREP_WAIT_TIME
BCAST_ID_SAVE 30,000 Milliseconds
BROADCAST_RECORD_TIME RREP_WAIT_TIME
DELETE_PERIOD see note below
GROUP_HELLO_INTERVAL 5,000 Milliseconds
HELLO_INTERVAL 1,000 Milliseconds
MTREE_BUILD 2 * REV_ROUTE_LIFE
MY_ROUTE_TIMEOUT 2 * ACTIVE_ROUTE_TIMEOUT
NET_DIAMETER 35
NEXT_HOP_WAIT NODE_TRAVERSAL_TIME + 10
NODE_TRAVERSAL_TIME 40
PRUNE_TIMEOUT ACTIVE_ROUTE_TIMEOUT
REV_ROUTE_LIFE RREP_WAIT_TIME
RREP_WAIT_TIME 3 * NODE_TRAVERSAL_TIME * NET_DIAMETER / 2
RREQ_RETRIES 2
TTL_START 1
TTL_INCREMENT 2
TTL_THRESHOLD 7
DELETE_PERIOD should be an upper bound on the time for which
an upstream node A can have a neighbor B to be 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. For example, if the link layer
feedback is used to detect loss of link DELETE_PERIOD must be
at least ACTIVE_ROUTE_TIMEOUT. If there is no feedback and hello
messages must be used, DELETE_PERIOD must be at least maximum of
ACTIVE_ROUTE_TIMEOUT and ALLOWED_HELLO_LOSS * HELLO_INTERVAL. 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 certain
time, say, a multiple K of ALLOWED_HELLO_LOSS * HELLO_INTERVAL.
In other words, it cannot not be the case that a node receives K
subsequent hello messages from a neighbor, while that same neighbor
fails to receive any data packet from the node in this period. This
is a reasonable assumption as this AODV specification works only with
symmetric links. Covering all possibilities,
DELETE_PERIOD = K * max (ACTIVE_ROUTE_TIMEOUT,
ALLOWED_HELLO_LOSS * HELLO_INTERVAL) (K = 5 is recommended).
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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 [3] 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.
18. Security Considerations
Currently, AODV does not specify any special security measures.
Route protocols, however, are prime targets for impersonation
attacks, and must be protected by use of authentication techniques
involving generation of unforgeable and cryptographically strong
message digests or digital signatures. It is expected that, in
environments where security is an issue, that IPSec authentication
headers will be deployed along with the necessary key management to
distribute keys to the members of the ad hoc network using AODV.
19. Acknowledgements
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 SJ Lee
and Mahesh Marina.
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References
[1] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
[2] E. Guttman and S. Cheshire (chairs). Zero Configuration
Networking (zeroconf), June 1999.
http://www.ietf.org/html.charters/zeroconf-charter.html.
[3] Wireless LAN Medium Access Control MAC and Physical Layer
PHY Specifications. IEEE Standard 802.11-97, Jun 1997.
AlphaGraphics #35, 10201 N.35th Avenue, Phoenix AZ 85051.
[4] Charles E. Perkins. Terminology for Ad-Hoc Networking.
draft-ietf-manet-terms-00.txt, November 1997. (work in
progress).
[5] S. Thomson and T. Narten. IPv6 Stateless Address
Autoconfiguration. Request for Comments (Draft Standard)
2462, Internet Engineering Task Force, December 1998.
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A. Draft Modifications
The following are major changes between this version (05) of the AODV
draft and the previous version (04):
- Processing Route Requests section. This section has been
modified so that the Destination Sequence Number field of the
RREQ always contains the greatest sequence number seen along the
route.
- Forwarding Route Replies section. This section has been modified
so that only RREPs with a greater sequence number than what was
previously known are forwarded. RREPs with smaller sequence
number are suppressed.
- RERR section modifications. This section has been altered to
more clearly indicate when a RERR is sent, and the actions to
be taken on reception of a RERR. Additionally, the RERR message
has been modified so that the sequence number of each listed
destination, incremented by one, is included. This section also
now includes the process of deleting neighbors from precursor
lists.
- Addition of Route Expiry and Deletion section. This section
describes the purpose of the DELETE_PERIOD, where a node must
keep a record of an expired route for at least DELETE_PERIOD
before it may delete the route entirely.
- Addition of Actions After Reboot section. This section describes
the actions to be taken after a node reboots. Specifically,
because a rebooted node will have lost all its routes, it must
wait DELETE_PERIOD before responding to any routing packets.
Additionally, it must broadcast a RERR packet for any data
packets that are sent to it within this time and then reset its
DELETE_PERIOD timer.
- Addition of Actions After Reboot section for multicast. A Reboot
flag has been added to the MACT message. Since a rebooted node
has lost all of its multicast tree information and does not know
whether it was participating in multicast before it was rebooted,
it must broadcast a reboot MACT message upon boot to inform its
neighbors it has lost all multicast routing information.
- Addition of Address Autoconfiguration section. This section
describes the procedure for an AODV node to obtain an IP address.
This method is intended to be compliant with that proposed by the
zeroconf working group [2].
- Type numbers have been assigned to the extensions.
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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. Royer
Dept. of Electrical and Computer Engineering
University of California, Santa Barbara
Santa Barbara, CA 93106
+1 805 893 7788
+1 805 893 3262 (fax)
eroyer@alpha.ece.ucsb.edu
Samir R. Das
Department of Electrical and Computer Enginnering
& 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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