Multi-hop Ad Hoc Wireless CommunicationINRIAEmmanuel.Baccelli@inria.frhttp://www.emmanuelbaccelli.org/Futurewei+1-408-330-4586charlie.perkins@huawei.com
Internet
Internet AreaI-DInternet Draft
This document describes characteristics of communication
between interfaces 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.
Experience gathered with ad hoc routing protocol development,
deployment and operation, shows that wireless communication presents
specific challenges ,
which Internet protocol designers should be aware of, when designing
solutions for ad hoc networks at the IP layer.
This document does not prescribe solutions, but instead briefly
describes these challenges in hopes of increasing that awareness.
As background, RFC 3819 provides an excellent
reference for higher-level considerations when designing protocols for
shared media. From MTU to subnet design, from security to considerations
about retransmissions, RFC 3819 provides guidance and design rationale
to help with many aspects of higher-level protocol design.
The present document focuses more specifically on challenges in
multi-hop ad hoc wireless networking. For example, in that context,
even though a wireless link may experience high variability as a
communications channel, such variation does not mean that the link
is "broken". Many layer-2 technologies serve to reduce error
rates by various means. Nevertheless, such errors as noted in this
document may still become visible above layer-2 and so become relevant
to the operation of higher layer protocols.
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 at least one radio transceiver (i.e., wireless
network interface), and that are moreover configured
to self-organize and provide store-and-forward functionality
as needed to enable communications.
This document focuses on the characteristics
of communications through such a network interface.
Although the characteristics of packet transmission over multi-hop
ad hoc wireless networks, described below, are not the typical
characteristics expected by IP , it is
desirable and possible to run IP over such
networks, as demonstrated in certain deployments currently in
operation, such as Freifunk , and
Funkfeuer . These deployments use
routers running IP protocols e.g., OLSR (Optimized Link State Routing
) on top of
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 IEEE 802.11, and may
use routing protocols other than OLSR. The following documents provide
a number of examples:
AODV , OLSRv2 ,
TBRPF ,
OSPF (,
and ), or DSR .
Note that in contrast, devices communicating via an IEEE 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 devices other than the access point
do not generally provide store-and-forward functionality.
In the following, we will consider several devices in a multi-hop
ad hoc wireless network N. Each device will be considered only
through its own wireless interface to network N. For conciseness
and readability, this document uses the expressions "device A"
(or simply "A") as a
synonym for "the wireless interface of device A to network N".
Let A and B be two devices in network N.
Suppose that, when device A transmits an IP packet through its
interface on network N, that packet is correctly and directly received
by device B without requiring storage and/or forwarding by any other
device. We will then say that B can "detect" A. Note that therefore,
when B detects A, an IP packet transmitted by A will be rigorously
identical to the corresponding IP packet received by B.
Let S be the set of devices that detect device A through its wireless
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 (for instance,
OLSR, AODV,
TBRPF, DSR,
and OSPF-MPR), as well as deployment and
operation (e.g., Freifunk,
Funkfeuer).
First, even though a device C in set S can (by definition) detect
device A, there is no guarantee that C can,
conversely, send IP packets directly to A. In other words, even
though C can detect A (since it is a member of set S), there is no
guarantee that A can detect 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 device mobility.
Now, conversely, let V be the set of devices which A detects.
Suppose that A is
communicating at time t0 through its interface on network N.
As a consequence of time variation and asymmetry,
we observe that A:
cannot assume that S = V, and
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. Suppose that, through their
respective interfaces within network N:
device B and device A can detect one another (i.e. B is a member of
sets S and V), and,
device A and device C can also detect one another (i.e. C is a
also a member of sets S and V).
These assumptions do not imply that B can detect C, nor that
C can detect B (through their interface on network N).
Such "non-transitivity" is common on multi-hop ad hoc
wireless networks.
In summary: multi-hop ad hoc wireless communications can be
asymmetric, non-transitive, and time-varying.
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
as well as other common effects.
Wireless communications are particularly subject to limitations on the
distance across which they may be established. The range-limitation
factor creates specific problems on multi-hop ad hoc wireless
networks. Due to the lack of isolation between the transmitters,
the radio ranges of several devices
often partially overlap, causing
communication to be non-transitive and/or asymmetric as described
in . Moreover, the range of each device may
depend on location and environmental factors.
This is in addition to possible time variations of range and signal
strength.
For example it may happen that a device B detects a device A which
transmits at high power, whereas B transmits at lower power. In such
cases, as depicted in , B can detect A,
but A cannot detect B. This exemplifies asymmetry in wireless
communications as defined in .
Another example, depicted in , is known as
the "Hidden Terminal"
problem. Even though the devices all have equal power for their radio
transmissions, they cannot all detect one another. In the figure,
devices A and B can detect one another, and devices A and C can also
detect one another. Nevertheless, B and C cannot detect one
another. When B and C simultaneously try to communicate with A, their
radio signals collide. Device A may then receive incoherent noise,
and may even be unable to determine the source of the noise. The hidden
terminal problem is a consequence of the property of non-transitivity
in multi-hop ad hoc wireless communications as described
in .
Another situation, shown in , is known as
the "Exposed Terminal"
problem. In the figure, device A and device B can detect each other,
and A is transmitting packets to B, thus A cannot detect device C --
but C can detect A. As shown in Figure 3, during the on-going
transmission of A, device C cannot reliably communicate with
device D because of interference within C's radio range due to A's
transmissions. Device C is then said to be "exposed", because it is
exposed to co-channel interference from A and is thereby prevented
from reliably exchanging protocol messages with D -- even though these
transmissions would not interfere with the reception of data sent from
A destined to B.
Hidden and exposed terminal situations are often observed in multi-hop
ad hoc wireless networks. Asymmetry issues with wireless communication
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; CSMA/CA, for example, requires that the physical medium be
unoccupied from the point of view of both devices before starting transmission.
Nevertheless, depending on the link layer technology in use and the
position of the devices, such problems may affect the IP layer due to
range limitation and partial overlap.
Besides radio range limitations, wireless communications are
affected by irregularities in the shape of the geographical area
over which devices may effectively communicate (see for instance
, ).
For example, even omnidirectional wireless transmission is
typically non-isotropic (i.e. non-circular).
Signal strength often suffers frequent and significant variations,
which do not have a simple dependence on distance. Instead, the
dependence is a
complex function of the environment including obstacles, weather
conditions, interference, and other factors that change over time.
Because wireless communications often encounter different terrain,
path, obstructions, atmospheric conditions and other phenomena,
analytical formulation of signal strength is considered intractable
. The radio engineering community has
developed numerous radio propagation approximations, relying on median
values observed in specific environments .
These irregularities cause communications on multi-hop ad hoc
wireless networks to be non-transitive, asymmetric, or time-varying,
as described in , and may impact protocols at
the IP layer and above. There may be no indication to the IP layer
when a previously established communication channel becomes unusable;
"link down" triggers are often 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 devices are not
currently transmitting.
Many terms have been used in the past to describe the relationship of
devices 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
previous sections of this document have been selected because the
authors believe they are unambiguous, with respect to the
goal of this document as formulated in .
In this section, we exhibit some other terms that describe the same
relationship between devices 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
devices that can directly receive packets transmitted by device A
through its interface on network N. In other words, any device B
belonging to S can detect packets transmitted by A. Then,
due to the asymmetric nature of wireless communications:
- We may say that device A "reaches" device B. In this
terminology, there is no guarantee that B reaches
A, even if A reaches B.
- We may say that device B "hears" device A. In this
terminology, there is no guarantee that A hears
B, even if B hears A.
- We may say that device A "has a link" to device B. In this
terminology, there is no guarantee that B has a link to A, even if
A has a link to B.
- We may say that device B "is adjacent to" device A. In this
terminology, there is no guarantee that A is adjacent to B, even
if B is adjacent to A.
- We may say that device B "is downstream from" device A. In this
terminology, there is no guarantee that A is downstream from
B, even if B is downstream from A.
- We may say that device B "is a neighbor of" device A. In this
terminology, there is no guarantee that A is a neighbor of B, even if
B a neighbor of A. Terminology based on "neighborhood"
is quite confusing for multi-hop wireless communications.
For example, when B can detect A, but A cannot detect B, it is not
clear whether or not B should be considered a neighbor of A;
A would not necessarily be aware that B was a neighbor, as it cannot
detect B. It is thus best to avoid the "neighbor" terminology, except
when bidirectionality has been established.
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
. Note that bidirectionality
is not synonymous with symmetry. For example, the error statistics
in either direction are often different for a link that is otherwise
considered bidirectional.
Section 18 of RFC 3819 provides an excellent
overview of security considerations at the subnetwork layer. Beyond
the material there, multi-hop ad hoc wireless networking (i) is not
limited to subnetwork layer operation, and (ii) makes use of wireless
communications.
On one hand, a detailed description of security implications of
wireless communications in general is outside of the scope of this
document. It is true that eavesdropping on a wireless link is much
easier than for wired media (although significant progress has
been made in the field of wireless monitoring of wired transmissions).
As a result, traffic analysis attacks can be even more subtle and
difficult to defeat in this context. Furthermore, such communications
over a shared media are particularly prone
to theft of service and denial of service (DoS) attacks.
On the other hand, the potential multi-hop aspect of the networks we
consider in this document goes beyond traditional scope of subnetwork
design. In practice, unplanned relaying of network traffic (both user
traffic and control traffic) happens routinely. Due to the physical
nature of wireless media, Man in the Middle
(MITM) attacks are facilitated, which may significantly alter
network performance. This highlights the importance of the
"end-to-end principle": L3 security, end-to-end, becomes a primary goal,
independently of securing layer-2 and layer-1
protocols (though L2 and L1 security often help to reach this
goal).
This document does not have any IANA actions.
A DoD perspective on mobile ad hoc networkshttp://www.funkfeuer.atAustria Wireless Community Network,
http://www.funkfeuer.athttps://map.funkfeuer.at/wien/Mobile Ad hoc Networking: Routing Technology
for Dynamic, Wireless NetworksA Survey of Various Propagation Models for Mobile CommunicationPilot power control and service coverage support in CDMA mobile systemsThe Mistaken Axioms of Wireless-Network ResearchFreifunk Wireless Community Networks, http://www.freifunk.nethttp://www.freifunk.net
This document stems from discussions with the following people,
in alphabetical order:
Jari Arkko,
Teco Boot,
Brian Carpenter,
Carlos Jesus Bernardos Cano,
Zhen Cao,
Ian Chakeres,
Thomas Clausen,
Robert Cragie,
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.