Internet Area                                                E. Baccelli
Internet-Draft                                                     INRIA
Intended status: Informational                                C. Perkins
Expires: May 4, July 15, 2016                                         Futurewei
                                                        November 1, 2015
                                                        January 12, 2016

                Multi-hop Ad Hoc Wireless Communication


   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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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   4
     3.2.  Radio Range and Wireless Irregularities . . . . . . . . .   4
   4.  Alternative Terminology . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8   9
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   8   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  12

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 does not prescribe
   solutions, but instead briefly describes these challenges in hopes of
   increasing that awareness.

   As background, RFC 3819 [RFC3819] 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"; 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.

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 (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.

3.  Common Packet Transmission Characteristics in Multi-hop Ad Hoc
    Wireless Networks

   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

   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]).

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

   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:

   1.  cannot assume that S = V,

   2.  cannot assume that S and/or V are unchanged at time t1 later than

   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.  device B and device A can detect one another (i.e.  B is a member
       of sets S and V), and,

   2.  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 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 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

   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. communication: 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 problem: when device A is communicating
                  communicates 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.

4.  Alternative Terminology

   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:

      - 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.  As it happens, 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 B should be considered a
      neighbor of A at all, since 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 for when some level of symmetry
      has been verified.

   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

5.  Security Considerations

   This document does

   Section 18 of RFC 3819 [RFC3819] provides an excellent overview of
   security considerations at the subnetwork layer.  Beyond the material
   there, multi-hop ad hoc wireless networking (i) is not make any limited to
   subnetwork layer operation, and (ii) makes use of wireless

   On one hand, a detailed description about the of security implications of
   wireless communications. communications in general is outside of the scope of this
   document.  Notably, however, 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  As
   a result, traffic analysis attacks can be even more subtle and above) protocols for wireless media
   difficult to defeat in this context.  Furthermore, such
   communications over a priori independent from 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 need to secure stick to the "end-to-end principle": L3
   security, end-to-end, becomes a primary goal, independently of
   securing layer-2 and layer-1 protocols for such media. (though L2 and L1 security can
   indeed help to reach this goal).

6.  IANA Considerations

   This document does not have any IANA actions.

7.  Informative References

   [RFC2501]  Corson, S. and J. Macker, "Mobile Ad hoc Networking
              (MANET): Routing Protocol Performance Issues and
              Evaluation Considerations", RFC 2501,
              DOI 10.17487/RFC2501, January 1999,

   [RFC3561]  Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
              Demand Distance Vector (AODV) Routing", RFC 3561,
              DOI 10.17487/RFC3561, July 2003,

   [RFC3626]  Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
              State Routing Protocol (OLSR)", RFC 3626,
              DOI 10.17487/RFC3626, October 2003,

   [RFC3684]  Ogier, R., Templin, F., and M. Lewis, "Topology
              Dissemination Based on Reverse-Path Forwarding (TBRPF)",
              RFC 3684, DOI 10.17487/RFC3684, February 2004,

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,

   [RFC4728]  Johnson, D., Hu, Y., and D. Maltz, "The Dynamic Source
              Routing Protocol (DSR) for Mobile Ad Hoc Networks for
              IPv4", RFC 4728, DOI 10.17487/RFC4728, February 2007,

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              DOI 10.17487/RFC4903, June 2007,

   [RFC5449]  Baccelli, E., Jacquet, P., Nguyen, D., and T. Clausen,
              "OSPF Multipoint Relay (MPR) Extension for Ad Hoc
              Networks", RFC 5449, DOI 10.17487/RFC5449, February 2009,

   [RFC5889]  Baccelli, E., Ed. and M. Townsley, Ed., "IP Addressing
              Model in Ad Hoc Networks", RFC 5889, DOI 10.17487/RFC5889,
              September 2010, <>.

   [RFC6250]  Thaler, D., "Evolution of the IP Model", RFC 6250,
              DOI 10.17487/RFC6250, 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.

              "Austria Wireless Community Network,
    ", 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 Wireless Community Networks,
    ", 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, 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.

Authors' Addresses

   Emmanuel Baccelli


   Charles E. Perkins

   Phone: +1-408-330-4586