MANET Autoconfiguration (Autoconf) E. Baccelli Internet-Draft INRIA Intended status: Informational C. Perkins Expires: November 25, 2010 Tellabs May 24, 2010 Multi-hop Ad Hoc Wireless Communication draft-baccelli-multi-hop-wireless-communication-04 Abstract This document describes some important 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 This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 25, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. Baccelli & Perkins Expires November 25, 2010 [Page 1] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Communication in Multi-hop Ad Hoc Wireless Networks . . . . . . 3 2.1. Asymmetry, Time-Variation, and Non-Transitivity . . . . . . 3 2.2. Radio Range and Wireless Irregularities . . . . . . . . . . 4 3. Alternative Terminology . . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 6. Informative References . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 9 Appendix B. Changes from previous version . . . . . . . . . . . . 9 Baccelli & Perkins Expires November 25, 2010 [Page 2] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 1. Introduction The goal of this document is to describe some important aspects of multi-hop ad hoc wireless communication. These characteristics have been very well researched, and numerous results have been published during many years of experience dealing with wireless communications. Experience gathered with [RFC2501] [RFC3626] [RFC3561] [RFC3684] [RFC4728] [RFC5449] [DoD01] shows that this type of communication presents specific challenges that are relevant to the design of Internet protocols that are intended for establishing connectivity in multi-hop ad hoc wireless networks. This document briefly describes some of these challenges. 2. Communication in Multi-hop Ad Hoc Wireless Networks In this document, we consider a multi-hop ad hoc wireless network to be a collection of devices that all have radio transceivers using the same physical and medium access protocols. They may be configured to provide store-and-forward functionality on top of these protocols, as needed to enable communications; consequently, such nodes would usually be classified as routers in the resulting wireless network. In the following, we will just refer to these devices as nodes. 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 detectable by node B without requiring storage and/or forwarding by any other node. In this circumstance, we will say that B can receive packets directly from A. Alternatively, we may also 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 through its interface on network N. We will now describe some fundamental characteristics of multi-hop ad hoc wireless communication. Because of these characteristics, some assumptions about packet transmission that are typically made in wired networks, are often untrue in multi-hop ad hoc wireless networks. 2.1. 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 node A (since it is a member of set S), there is no guarantee that A can "hear" packets from node C. Thus, multi-hop ad hoc wireless communications may be "asymmetric". Such asymmetry is often experienced on multi-hop ad hoc wireless Baccelli & Perkins Expires November 25, 2010 [Page 3] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 networks, due to well-known properties of wireless communication. Second, there is no guarantee that, as a set, S is at all stable. The membership of set S may in fact change at any rate, any time. Thus, multi-hop ad hoc wireless communications may be "time-variant". Such variations are often experienced 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). This neither implies 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 often observed on multi-hop ad hoc wireless networks. In a nutshell: multi-hop ad hoc wireless communications often prove to be asymmetric, non-transitive, and time-varying in character. 2.2. Radio Range and Wireless Irregularities In Section 2.1 we presented an abstract description of some multi-hop ad hoc wireless communication characteristics. This section points out a practical reality, at the root of these characteristics. Wireless communication links are often subject to significant limitations to the distance across which they may be established. In the extreme cases, some radio links are measured in centimeters, not meters, although such short-range radio links are not typically considered to support multi-hop ad hoc networks. More often, radio Baccelli & Perkins Expires November 25, 2010 [Page 4] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 links are encountered with range limited to several tens or hundreds of meters. The range-limitation factor creates specific problems, observed in multi-hop ad hoc wireless networks. In this context, it is indeed not rare that the radio ranges of several nodes partially overlap. This partial overlap often causes communication on multi-hop ad hoc wireless networks to be non-transitive and/or asymmetric, as described in Section 2.1. For example, it often happens that a sending node transmits a packet at higher power than is used by the node receiving the packet. Radio Ranges for Nodes A and B <~~~~~~~~~~~~~+~~~~~~~~~~~~~> | <~~~~~~+~~~~~~> +--|--+ +--|--+ |NodeA|======>|NodeB| +-----+ +-----+ Figure 1: Asymmetric Link example. Node A can communicate with node B, but node B cannot communicate with node A. Another example, depicted in Figure 2, is known as the "hidden node" problem. Even though the nodes are shown as all having equal power for their radio transmissions, they are not at all equally accessible to each other. In the figure, nodes B and C cannot hear each other. On the other hand, nodes A and B can hear each other while A and C can also 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 noisy interference, and may even be unable to determine the source of the issue. The hidden node problem is a practical example of the non-transitivity of multi-hop ad hoc wireless communications mentioned in Section 2.1. Radio Ranges for Nodes A, B, C <~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~~> |<~~~~~~~~~~~~~+~~~~~~~~~~~~~>| +--|--+ +--|--+ +--|--+ |NodeB|=======>|NodeA|<=======|NodeC| +-----+ +-----+ +-----+ Figure 2: The hidden node problem. When nodes C and B try to communicate with node A at the same time, their radio signals collide. Baccelli & Perkins Expires November 25, 2010 [Page 5] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 A related situation is shown in Figure 3, illustrating 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, in node C's radio-range. Node C cannot hear D, but node D can hear C because node D is outside node 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 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 <~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~~~+~~~~~~~~~~~> |<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~~~~> +--|--+ +--|--+ +--|--+ +--|--+ |NodeB|<======|NodeA| |NodeC|======>|NodeD| +-----+ +-----+ +-----+ +-----+ 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 common in multi-hop ad hoc wireless networks. Problems with asymmetric links 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 situation and the link layer technology in use, such problems, and others caused by 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 [MI03]). For example, even within radio range, omnidirectional wireless transmission area is generally far from isotropic (circular). Nodes seldom hear each other perfectly, and signal strength often varies significantly. The variation is not a simple function of distance, but rather a complex function of the environment including obstacles, weather conditions, interferences as well as other factors that change over time. The analytical formulation of the functional 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 2.1. Just as with the problems introduced by limited wireless range, the irregularities of wireless communications often require resolution below the network (IP) layer, and yet nevertheless may impact the network layer. For example, there may be Baccelli & Perkins Expires November 25, 2010 [Page 6] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 no indication to IP when a previously established communication channel becomes unusable. In other words, "link down" triggers are often absent in wireless systems. This lack can potentially break various assumptions that have been used for features designed for wired communication links, and cause failures or poor performance. 3. 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 relatively unambiguous, with respect to the goal of this document (see Section 1). Nevertheless, here are a few other phrasings, describing the same relationship between wireless nodes. 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 node 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. - 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. Baccelli & Perkins Expires November 25, 2010 [Page 7] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 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 2. 4. Security Considerations This document does not have any security considerations. 5. IANA Considerations This document does not have any IANA actions. 6. Informative References [RFC2501] Corson, S. and J. Macker, "Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations", RFC 2501, 1999. [RFC3626] Clausen, T. and P. Jacquet, "The Optimized Link State Routing Protocol", RFC 3626, October 2003. [RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On- Demand Distance Vector (AODV) Routing", RFC 3561, July 2003. [RFC3684] Ogier, R., Templin, f., and M. Lewis, "Topology Dissemination Based on Reverse-Path Forwarding", 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, 2007. [RFC5449] Baccelli, E., Clausen, T., Jacquet, P., and D. Ngyuen, "OSPF Multipoint Relay (MPR) Extension for Ad Hoc Networks", RFC 5449, February 2009. [IPev] Thaler, D., "Evolution of the IP Model", draft-thaler-ip-model-evolution-01.txt (work in progress), 2008. [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. Baccelli & Perkins Expires November 25, 2010 [Page 8] Internet-Draft Multi-hop Ad Hoc Wireless Communication May 2010 [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. Appendix A. Acknowledgements This document stems from discussions with the following people, in no particular order: Thomas Clausen, Erik Nordmark, Teco Boot, Seung Yi, Stan Ratliff, Fred Templin, Thomas Narten, Ronald Velt in't, Christopher Dearlove, Shubhranshu Singh, Carlos Jesus Bernardos Cano, Kenichi Mase, Paul Lambert, Ralph Droms, Ulrich Herberg, Zach Shelby, Alexandru Petrescu, Ian Chakeres, Dave Thaler, Jari Arkko, and Mark Townsley. Appendix B. Changes from previous version o Corrected the description of exposed nodes in Section 2.2 o Added more text to describe the problem that wireless media often lack a "link-down" trigger. 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: +1-408-435-0777 x337 EMail: charliep@wichorus.com Baccelli & Perkins Expires November 25, 2010 [Page 9]