Multi-hop Ad Hoc Wireless Communication
draft-baccelli-manet-multihop-communication-00

Mobile Ad-hoc Networks (MANET)                               E. Baccelli
Internet-Draft                                                     INRIA
Intended status: Informational                                C. Perkins
Expires: May 26, 2012                                            Tellabs
                                                            Nov 23, 2011

                Multi-hop Ad Hoc Wireless Communication
             draft-baccelli-manet-multihop-communication-00

Abstract

   This document describes some characteristics of communication between
   nodes in a multi-hop ad hoc wireless network.  These are not
   requirements in the sense usually understood as applying to
   formulation of a requirements document.  Nevertheless, protocol
   engineers and system analysts involved with designing solutions for
   ad hoc networks must maintain awareness of these characteristics.

Status of This Memo

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   This Internet-Draft will expire on May 26, 2012.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Multi-hop Ad Hoc Wireless Networks  . . . . . . . . . . . . . . 3
   3.  Common Packet Transmission Characteristics in Multi-hop Ad
       Hoc Wireless Networks . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Asymmetry, Time-Variation, and Non-Transitivity . . . . . . 4
     3.2.  Radio Range and Wireless Irregularities . . . . . . . . . . 5
   4.  Alternative Terminology . . . . . . . . . . . . . . . . . . . . 7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   7.  Informative References  . . . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   The goal of this document is to describe some aspects of multi-hop ad
   hoc wireless communication.  Experience gathered with [RFC3626]
   [RFC3561] [RFC3684] [RFC4728] [RFC5449] [RFC2501] [DoD01] shows that
   this type of communication presents specific challenges.  This
   document briefly describes these challenges, which one should
   maintain awareness of, when designing Internet protocols for ad hoc
   networks.

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.

   An example of multi-hop ad hoc wireless network is a wireless
   community network such as Funkfeuer [FUNKFEUER] or Freifunk
   [FREIFUNK], that consists in routers running OLSR [RFC3626] on 802.11
   in ad hoc mode with the same ESSID at link layer.  Multi-hop ad hoc
   wireless networks may also run on link layers other than 802.11.

   Note however that simple hosts communicating through an access point
   with 802.11 in infrastructure mode do not form a multi-hop ad hoc
   wireless network, since the central role of the access point is
   determined a priori, 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 "hears" packets from A. Note that therefore, when B
   can hear IP packets from A, the TTL of the IP packet heard by B will
   be precisely the same as it was when A transmitted that packet.

   Let S be the set of nodes that can hear packets transmitted by node A

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   through its interface on network N. The following section gathers
   common characteristics concerning packet transmission over such
   networks, which were observed through experience with [RFC3626]
   [RFC3561] [RFC3684] [RFC4728] [RFC5449].

3.1.  Asymmetry, Time-Variation, and Non-Transitivity

   First, there is no guarantee that a node C within S can,
   symmetrically, send IP packets directly to node A. In other words,
   even though C can "hear" packets from A (since it is a member of set
   S), there is no guarantee that A can "hear" packets from C. Thus,
   multi-hop ad hoc wireless communications may be "asymmetric".  Such
   cases are not uncommon.

   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".  Such variations are not unusual 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 from which A can directly
   receive packets -- in other words, A can "hear" packets from any node
   in set V. 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:

   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 hear each other (i.e. node B is a member of
       sets S and V), and,

   2.  node A and node C can also hear each other (i.e. node C is a also
       a member of sets S and V).

   These assumptions do not imply that node B can hear node C, nor that
   node C can hear node B (through their interface on network N).  Such
   "non-transitivity" is not uncommon on multi-hop ad hoc wireless
   networks.

   In a nutshell: multi-hop ad hoc wireless communications can be

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   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, it is not uncommon that the radio ranges
   of several nodes partially overlap.  Such partial overlap causes
   communication to be non-transitive and/or asymmetric, as described in
   Section 3.1.

   For example, as depicted in Figure 1, it may happen that a node B
   hears a node A which transmits at high power, whereas B transmits at
   lower power.  In such cases, B can hear A, but A cannot hear B. This
   examplifies the asymmetry in multi-hop ad hoc wireless communications
   as defined in Section 3.1.

                 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 reach one another.  In the figure,
   nodes A and B can hear each other, and A and C can also hear each
   other.  On the other hand, nodes B and C cannot hear each other.
   When nodes B and C try to communicate with node A at the same time,
   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.

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                    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
   shown, node C cannot communicate properly with node D, because of the
   on-going transmission of node A, polluting C's radio-range.  Node C
   cannot hear D, but node D can hear 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 transmitting data 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 not uncommon 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 the link layer

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   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.  The analytical formulation of such
   variation is often considered intractable.

   These 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 the IP layer.  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.

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 (or at
   least hope) 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 "hear"
   packets transmitted by A. Then, due to the asymmetry characteristic
   of wireless links:

      - 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.

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      - 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.

      - 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 hear signals from A, but A cannot hear 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.  Security Considerations

   This document does not have any security considerations.

6.  IANA Considerations

   This document does not have any IANA actions.

7.  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.

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   [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.

   [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]  "Austria Wireless Community Network,
                http://www.funkfeuer.at", 2009.

   [IPev]       Thaler, D., "Evolution of the IP Model",
                draft-thaler-ip-model-evolution-01.txt (work in
                progress), 2008.

   [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.

   [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", 2009.

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, 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.

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Authors' Addresses

   Emmanuel Baccelli
   INRIA

   Phone: +33-169-335-511
   EMail: Emmanuel.Baccelli@inria.fr
   URI:   http://www.emmanuelbaccelli.org/

   Charles E. Perkins
   Tellabs

   Phone: +1-408-970-6560
   EMail: charliep@tellabs.com

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