| Internet Area | E. Baccelli |
| Internet-Draft | INRIA |
| Intended status: Informational | C. Perkins |
| Expires: May 4, 2016 | Futurewei |
| November 1, 2015 |
Multi-hop Ad Hoc Wireless Communication
draft-ietf-intarea-adhoc-wireless-com-00
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.
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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 does not prescribe solutions, but instead briefly describes these challenges in hopes of increasing that awareness. For example, even though a wireless link may experience high variability as a communications channel, such variation does not mean that the link is "broken"; indeed 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 a 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 [RFC6250], it is desirable and possible to run IP over such networks, as demonstrated in certain deployments currently in operation, such as Freifunk [FREIFUNK], and Funkfeuer [FUNKFEUER]. These deployments use routers running IP protocols e.g., OLSR (Optimized Link State Routing [RFC3626]) 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 (for instance, AODV [RFC3561], TBRPF [RFC3684], DSR [RFC4728], or OSPF-MPR [RFC5449]).
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[RFC3626], AODV[RFC3561], TBRPF[RFC3684], DSR[RFC4728], and OSPF-MPR[RFC5449]), as well as deployment and operation (Freifunk[FREIFUNK], Funkfeuer[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:
Furthermore, transitivity is not guaranteed over multi-hop ad hoc wireless networks. Indeed, let's assume that, through their respective interfaces within network N:
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 a nutshell: multi-hop ad hoc wireless communications can be asymmetric, non-transitive, and time-varying.
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 communications 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 devices often partially overlap. Such partial overlap causes communication to be non-transitive and/or asymmetric, as described in Section 3.1. Moreover, the range may vary from one device 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 device B detects a device 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.
Radio Ranges for Devices A and B
<~~~~~~~~~~~~~+~~~~~~~~~~~~~>
| <~~~~~~+~~~~~~>
+--|--+ +--|--+
| A |======>| B |
+-----+ +-----+
Figure 1: Asymmetric wireless communication example. Device A can communicate with device B, but B cannot communicate with A.
Another example, depicted in Figure 2, 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. On the other hand, B and C cannot detect one another. When B and C simultaneously try to communicate with A, their radio signals may collide. Device A may receive incoherent 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 Devices A, B, C
<~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~~>
|<~~~~~~~~~~~~~+~~~~~~~~~~~~~>|
+--|--+ +--|--+ +--|--+
| B |=======>| A |<=======| C |
+-----+ +-----+ +-----+
Figure 2: The hidden terminal problem. Devices C and B
try to communicate with device A at the same time,
and their radio signals collide.
Another situation, shown in Figure 3, 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.
Radio Ranges for Devices A, B, C, D
<~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~+~~~~~~~~~~~>
|<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~>
+--|--+ +--|--+ +--|--+ +--|--+
| B |<======| A | | C |======>| D |
+-----+ +-----+ +-----+ +-----+
Figure 3: The exposed terminal problem. When device A is communicating
with device B, and device C is "exposed".
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, for example, CSMA/CA, which ensures transmission in periods perceived to be unoccupied by other transmissions. However, 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 [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 wireless communications have 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 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 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 devices 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.
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 (see Section 1).
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:
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. We do not discuss bidirectionality, but as a final observation it is worthwhile to 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.
This document does not make any detailed description about the security implications of wireless communications. Notably, 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). Nevertheless, the need for securing high-level (layer-3 and above) protocols for wireless media is a priori independent from the need to secure the layer-2 and layer-1 protocols for such media.
This document does not have any IANA actions.
This document stems from discussions with the following people, in alphabetical order: Jari Arkko, Teco Boot, Carlos Jesus Bernardos Cano, 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.