ad_hoc.txt

283个中文RFC文档

RFC3561 - Ad hoc On-Demand Distance Vector (AODV) Routing
Network Working Group C. Perkins
Request for Comments: 3561 Nokia Research Center
Category: Experimental E. Belding-Royer
University of California, Santa Barbara
S. Das
University of Cincinnati
July 2003

Ad hoc On-Demand Distance Vector (AODV) Routing

Status of this Memo

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

Copyright Notice

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

Abstract

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

Table of Contents

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

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