Mobile Ad-hoc Networks (MANET) E. Baccelli
Internet-Draft INRIA
Intended status: Informational C. Perkins
Expires: January 12, 2014 Futurewei
July 11, 2013
Multi-hop Ad Hoc Wireless Communication
draft-baccelli-manet-multihop-communication-02
Abstract
This document describes characteristics of communication between
nodes in a multi-hop ad hoc wireless network, that protocol engineers
and system analysts should be aware of when designing solutions for
ad hoc networks at the IP layer.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Multi-hop Ad Hoc Wireless Networks . . . . . . . . . . . . . 2
3. Common Packet Transmission Characteristics in Multi-hop Ad
Hoc Wireless Networks . . . . . . . . . . . . . . . . . . . . 3
3.1. Asymmetry, Time-Variation, and Non-Transitivity . . . . . 3
3.2. Radio Range and Wireless Irregularities . . . . . . . . . 4
4. Alternative Terminology . . . . . . . . . . . . . . . . . . . 7
5. IP over Multi-hop Ad Hoc Wireless . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10
1. Introduction
Experience gathered with ad hoc routing protocol development,
deployment and operation, shows that wireless communication presents
specific challenges [RFC2501] [DoD01], which Internet protocol
designers should be aware of, when designing solutions for ad hoc
networks at the IP layer. This document briefly describes these
challenges.
2. Multi-hop Ad Hoc Wireless Networks
For the purposes of this document, a multi-hop ad hoc wireless
network will be considered to be a collection of devices that each
have a radio transceiver, that are using the same physical and medium
access protocols, that are moreover configured to self-organize and
provide store-and-forward functionality on top of these protocols as
needed to enable communications. The devices providing network
connectivity are considered to be routers. Other non-routing
wireless devices, if present in the ad hoc network, are considered to
be "end-hosts". The considerations in this document apply equally to
routers or end-hosts; we use the term "node" to refer to any such
network device in the ad hoc network.
Examples of multi-hop ad hoc wireless network deployment and
operation include wireless community networks such as
Funkfeuer[FUNKFEUER] and Freifunk[FREIFUNK]; these use routers
running OLSR (Optimized Link State Routing [RFC3626]) on IEEE 802.11
in ad hoc mode with the same ESSID (Extended Service Set
Identification) at the link layer. Multi-hop ad hoc wireless
networks may also run on link layers other than 802.11, and may use
routing protocols other than OLSR (for instance, AODV[RFC3561],
TBRPF[RFC3684], DSR[RFC4728], or OSPF-MPR[RFC5449]).
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In contrast, simple hosts communicating through an 802.11 access
point in infrastructure mode do not form a multi-hop ad hoc wireless
network, since the central role of the access point is predetermined,
and since nodes other than the access point do not generally provide
store-and-forward functionality.
3. Common Packet Transmission Characteristics in Multi-hop Ad Hoc
Wireless Networks
Let A and B be two nodes in a multi-hop ad hoc wireless network N.
Suppose that, when node A transmits a packet through its interface on
network N, that packet is correctly received by node B without
requiring storage and/or forwarding by any other device. We will
then say that B can "detect" packets transmitted by A, or more simply
that B detects A. Note that therefore, when B detects an IP packet
transmitted by A, the TTL of the IP packet detected by B will be
precisely the same as it was when A transmitted that packet.
Let S be the set of nodes that can detect packets transmitted by node
A through its interface on network N. The following section gathers
common characteristics concerning packet transmission over such
networks, which were observed through experience with MANET routing
protocol development (OLSR[RFC3626], AODV[RFC3561], TBRPF[RFC3684],
DSR[RFC4728], or OSPF-MPR[RFC5449]), as well as deployment and
operation (Freifunk[FREIFUNK], Funkfeuer[FUNKFEUER]).
3.1. Asymmetry, Time-Variation, and Non-Transitivity
First, even though a node C in set S can (by definition) detect
packets transmitted by node A, there is no guarantee that node C can,
conversely, send IP packets directly to node A. In other words, even
though C can detect packets transmitted by A (since it is a member of
set S), there is no guarantee that A can detect packets transmitted
by C. Thus, multi-hop ad hoc wireless communications may be
"asymmetric". Such cases are common.
Second, there is no guarantee that, as a set, S is at all stable,
i.e. the membership of set S may in fact change at any rate, at any
time. Thus, multi-hop ad hoc wireless communications may be "time-
variant". Time variation is often observed in multi-hop ad hoc
wireless networks due to variability of the wireless medium, and to
node mobility.
Now, conversely, let V be the set of nodes which A detects -- in
other words, IP packets transmitted by any node in set V are received
directly by A, without TTL decrement. Suppose that node A is
communicating at time t0 through its interface on network N. As a
consequence of time variation and asymmetry, we observe that A:
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1. cannot assume that S = V,
2. cannot assume that S and/or V are unchanged at time t1 later than
t0.
Furthermore, transitivity is not guaranteed over multi-hop ad hoc
wireless networks. Indeed, let's assume that, through their
respective interfaces within network N:
1. node B and node A can detect one another (i.e. node B is a member
of sets S and V), and,
2. node A and node C can also detect one another (i.e. node C is a
also a member of sets S and V).
These assumptions do not imply that node B can detect node C, nor
that node C can detect node B (through their interface on network N).
Such "non-transitivity" is common on multi-hop ad hoc wireless
networks.
In a nutshell: multi-hop ad hoc wireless communications can be
asymmetric, non-transitive, and time-varying.
3.2. Radio Range and Wireless Irregularities
Section 3.1 presents an abstract description of some common
characteristics concerning packet transmission over multi-hop ad hoc
wireless networks. This section describes practical examples, which
illustrate the characteristics listed in Section 3.1 as well as other
common effects.
Wireless communication links are subject to limitations to the
distance across which they may be established. The range-limitation
factor creates specific problems on multi-hop ad hoc wireless
networks. In this context, the radio ranges of several nodes often
partially overlap. Such partial overlap causes communication to be
non-transitive and/or asymmetric, as described in Section 3.1.
Moreover, the range varies from one node to another, depending on
location and environmental factors. This is in addition to the time
variation of range and signal strength caused by variability in the
local environment.
For example, as depicted in Figure 1, it may happen that a node B
detects a node A which transmits at high power, whereas B transmits
at lower power. In such cases, B detects A, but A cannot detect B.
This examplifies the asymmetry in multi-hop ad hoc wireless
communications as defined in Section 3.1.
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Radio Ranges for Nodes A and B
<~~~~~~~~~~~~~+~~~~~~~~~~~~~>
| <~~~~~~+~~~~~~>
+--|--+ +--|--+
| A |======>| B |
+-----+ +-----+
Figure 1: Asymmetric Link example. Node A can communicate with
node B, but B cannot communicate with A.
Another example, depicted in Figure 2, is known as the "hidden node"
problem. Even though the nodes all have equal power for their radio
transmissions, they cannot all detect one another. In the figure,
nodes A and B can detect one another, and A and C can also detect one
another. On the other hand, nodes B and C cannot detect one another.
When nodes B and C try to communicate with node A simultaneously,
their radio signals collide. Node A will only be able to detect
noise, and may even be unable to determine the source of the noise.
The hidden terminal problem illustrates the property of non-
transitivity in multi-hop ad hoc wireless communications as described
in Section 3.1.
Radio Ranges for Nodes A, B, C
<~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~~>
|<~~~~~~~~~~~~~+~~~~~~~~~~~~~>|
+--|--+ +--|--+ +--|--+
| B |=======>| A |<=======| C |
+-----+ +-----+ +-----+
Figure 2: The hidden node problem. Nodes C and B
try to communicate with node A at the same time,
and their radio signals collide.
Another situation, shown in Figure 3, is known as the "exposed node"
problem. In the figure, node A is transmitting (to node B). As
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shown, node C cannot reliably communicate with node D, because of the
on-going transmission of node A, presenting interference within C's
radio-range. Node C cannot detect D, but node D can detect C because
D is outside A's radio range. Node C is then called an "exposed
node", because it is exposed to co-channel interference from node A
and thereby prevented from exchanging protocol messages to enable
data transmission to node D -- even though the transmission would be
successful and would not interfere with the reception of data sent
from node A to node B.
Radio Ranges for Nodes A, B, C, D
<~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~~~+~~~~~~~~~~~>
|<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~>
+--|--+ +--|--+ +--|--+ +--|--+
| B |<======| A | | C |======>| D |
+-----+ +-----+ +-----+ +-----+
Figure 3: The exposed node problem. When node A is communicating
with node B, node C is an "exposed node".
Hidden and exposed node situations are often observed in multi-hop ad
hoc wireless networks. Problems with asymmetric links may also arise
for reasons other than power inequality (e.g., multipath
interference). Such problems are often resolved by specific
mechanisms below the IP layer. However, depending on the link layer
technology in use and the position of the nodes, such problems due to
range-limitation and partial overlap may affect the IP layer.
Besides radio range limitations, wireless communications are affected
by irregularities in the shape of the geographical area over which
nodes may effectively communicate (see for instance [MC03], [MI03]).
For example, even omnidirectional wireless transmission is typically
non-isotropic (i.e. non-circular). Signal strength often suffers
frequent and significant variations, which are not a simple function
of distance. Instead, it is a complex function of the environment
including obstacles, weather conditions, interference, and other
factors that change over time. Because each individual link has to
encounter different terrain, path, obstructions, atmospheric
conditions and other phenomena, analytical formulation of signal
strength is considered intractable [VTC99], and the radio engineering
community has thus developed numerous radio propagation models,
relying on median values observed in specific environments [SAR03].
The above irregularities also cause communications on multi-hop ad
hoc wireless networks to be non-transitive, asymmetric, or time-
varying, as described in Section 3.1, and may impact protocols at the
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IP layer and above. There may be no indication to IP when a
previously established communication channel becomes unusable; "link
down" triggers are generally absent in multi-hop ad hoc wireless
networks, since the absence of detectable radio energy (e.g., in
carrier waves) may simply indicate that neighboring nodes are not
currently transmitting. Such an absence of detectable radio energy
does not therefore indicate whether or not transmissions have failed
to reach the intended destination.
4. Alternative Terminology
Many terms have been used in the past to describe the relationship of
nodes in a multi-hop ad hoc wireless network based on their ability
to send or receive packets to/from each other. The terms used in
this document have been selected because the authors believe they are
unambiguous, with respect to the goal of this document (see
Section 1).
Nevertheless, here are a few other terms that describe the same
relationship between nodes in multi-hop ad hoc wireless networks. In
the following, let network N be, again, a multi-hop ad hoc wireless
network. Let the set S be, as before, the set of nodes that can
directly receive packets transmitted by node A through its interface
on network N. In other words, any node B belonging to S can detect
packets transmitted by A. Then, due to the asymmetry characteristic
of wireless links:
- We may say that node B hears node A. In this terminology, there
is no guarantee that node A is hears node B, even if node B hears
node A.
- We may say that node B is reachable from node A. In this
terminology, there is no guarantee that node A is reachable from
node B, even if node B is reachable from node A.
- We may say that node A has a link to node B. In this
terminology, there is no guarantee that node B has a link to node
A, even if node A has a link to node B.
- We may say that node B is adjacent to node A. In this
terminology, there is no guarantee that node A is adjacent to node
B, even if node B is adjacent to node A.
- We may say that node B is downstream from node A. In this
terminology, there is no guarantee that node A is downstream from
node B, even if node B is downstream from node A.
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- We may say that node B is a neighbor of node A. In this
terminology, there is no guarantee that node A is a neighbor of
node B, even if node B a neighbor of node A. As it happens, the
terminology for "neighborhood" is quite confusing for asymmetric
links. When B can detect signals from A, but A cannot detect B,
it is not clear whether B should be considered a neighbor of A at
all, since A would not necessarily be aware that B was a neighbor.
Perhaps it is best to avoid the "neighbor" terminology except for
symmetric links.
This list of alternative terminologies is given here for illustrative
purposes only, and is not suggested to be complete or even
representative of the breadth of terminologies that have been used in
various ways to explain the properties mentioned in Section 3.
5. IP over Multi-hop Ad Hoc Wireless
The characteristics of packet transmission over multi-hop ad hoc
wireless networks, described in previous sections, are not the
typical characteristics expected by IP [RFC6250]. Nevertheless, it
is possible and desirable to run IP over such networks, through the
use of:
IP interface configuration, such as described in RFC 5889
[RFC5889], or
routing protocols designed for operation over wireless interfaces,
for example OLSR[RFC3626], AODV[RFC3561], or OSPF-MPR[RFC5449].
Thus, even though the physical effects described in this document
require robust protocol designs for routing and topology management,
the experience in the projects described in the cited references
shows that useful networks can be designed and operated using well-
understood techniques. Protocols running above the IP layer can be
shielded somewhat from the unusual characteristics experienced over
multi-hop ad hoc wireless networks. Note however that some protocols
are nevertheless more appropriate than others when interfaces to
multi-hop ad hoc wireless networks are involved in the communication.
For instance, for applications written to run over both UDP and TCP,
the latter choice may be preferred in situations with relatively high
packet loss rates. But such choices must be based on application
requirements.
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6. Security Considerations
This document does not have any security considerations.
7. IANA Considerations
This document does not have any IANA actions.
8. Informative References
[RFC2501] Corson, M. and J. Macker, "Mobile Ad hoc Networking
(MANET): Routing Protocol Performance Issues and
Evaluation Considerations", RFC 2501, January 1999.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561, July
2003.
[RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol (OLSR)", RFC 3626, October 2003.
[RFC3684] Ogier, R., Templin, F., and M. Lewis, "Topology
Dissemination Based on Reverse-Path Forwarding (TBRPF)",
RFC 3684, February 2004.
[RFC4728] Johnson, D., Hu, Y., and D. Maltz, "The Dynamic Source
Routing Protocol (DSR) for Mobile Ad Hoc Networks for
IPv4", RFC 4728, February 2007.
[RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903, June
2007.
[RFC5449] Baccelli, E., Jacquet, P., Nguyen, D., and T. Clausen,
"OSPF Multipoint Relay (MPR) Extension for Ad Hoc
Networks", RFC 5449, February 2009.
[RFC5889] Baccelli, E. and M. Townsley, "IP Addressing Model in Ad
Hoc Networks", RFC 5889, September 2010.
[RFC6250] Thaler, D., "Evolution of the IP Model", RFC 6250, May
2011.
[DoD01] Freebersyser, J. and B. Leiner, "A DoD perspective on
mobile ad hoc networks", Addison Wesley C. E. Perkins,
Ed., 2001, pp. 29--51, 2001.
[FUNKFEUER]
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, "Austria Wireless Community Network,
http://www.funkfeuer.at", 2013.
[MC03] Corson, S. and J. Macker, "Mobile Ad hoc Networking:
Routing Technology for Dynamic, Wireless Networks", IEEE
Press Mobile Ad hoc Networking, Chapter 9, 2003.
[SAR03] Sarkar, T., Ji, Z., Kim, K., Medour, A., and M. Salazar-
Palma, "A Survey of Various Propagation Models for Mobile
Communication", IEEE Press Antennas and Propagation
Magazine, Vol. 45, No. 3, 2003.
[VTC99] Kim, D., Chang, Y., and J. Lee, "Pilot power control and
service coverage support in CDMA mobile systems", IEEE
Press Proceedings of the IEEE Vehicular Technology
Conference (VTC), pp.1464-1468, 1999.
[MI03] Kotz, D., Newport, C., and C. Elliott, "The Mistaken
Axioms of Wireless-Network Research", Dartmouth College
Computer Science Technical Report TR2003-467, 2003.
[FREIFUNK]
, "Freifunk Wireless Community Networks,
http://www.freifunk.net", 2013.
Appendix A. Acknowledgements
This document stems from discussions with the following people, in
alphabetical order: Jari Arkko, Teco Boot, Carlos Jesus Bernardos
Cano, Ian Chakeres, Thomas Clausen, Christopher Dearlove, Ralph
Droms, Brian Haberman, Ulrich Herberg, Paul Lambert, Kenichi Mase,
Thomas Narten, Erik Nordmark, Alexandru Petrescu, Stan Ratliff, Zach
Shelby, Shubhranshu Singh, Fred Templin, Dave Thaler, Mark Townsley,
Ronald Velt in't, and Seung Yi.
Authors' Addresses
Emmanuel Baccelli
INRIA
Phone: +33-169-335-511
EMail: Emmanuel.Baccelli@inria.fr
URI: http://www.emmanuelbaccelli.org/
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Charles E. Perkins
Futurewei
Phone: +1-408-330-5305
EMail: charlie.perkins@huawei.com
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