MANET Autoconfiguration (Autoconf) E. Baccelli Internet-Draft INRIA Intended status: Informational C. Perkins Expires: August 27, 2009 WiChorus February 23, 2009 Multi-hop Ad Hoc Wireless Communication draft-baccelli-multi-hop-wireless-communication-01 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), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 27, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. 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. Abstract This document describes some important aspects, experienced over the Baccelli & Perkins Expires August 27, 2009 [Page 1] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 past decade, of multi-hop ad hoc wireless communication between routers. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Communication on Multi-hop Ad Hoc Wireless Networks . . . . . . 3 3. Asymmetry, Time-Variation, and Non-Transitivity . . . . . . . . 3 4. Radio Range, Exposed Nodes and Hidden Terminals . . . . . . . . 4 5. Alternative Terminology . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 8. Informative References . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8 Baccelli & Perkins Expires August 27, 2009 [Page 2] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 1. Introduction The goal of this document is to describe some important aspects of multi-hop ad hoc wireless communication between routers, observed over the years. Experience gathered with multi-hop ad hoc wireless communication [RFC2501] [RFC3626] [RFC3561] [RFC3684] [RFC4728] [DoD01] shows that this type of communication presents specific challenges. This document briefly describes some of these challenges. 2. Communication on 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. All are configured to provide store-and-forward functionality on top of these protocols, as needed to enable communications; consequently, they can be classified as routers in the resulting wireless network. In the following, we will refer to these devices equivalently as nodes, or routers. 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 router. 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. 3. Asymmetry, Time-Variation, and Non-Transitivity First, there is no guarantee that a router C within S can, symmetrically, send IP packets directly to router 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 networks, due to well-known properties of wireless communication. Second, there is no guarantee that, as a set, S is at all stable. Baccelli & Perkins Expires August 27, 2009 [Page 3] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 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 on multi-hop ad hoc wireless networks due to variability of the wireless medium, and to router mobility. Now, conversely, let V be the set of routers from which node A can directly receive packets -- in other words, A can "hear" packets from any node in set V. Suppose that router A is communicating at time t0 through its interface on network N. As a consequence of time variation and assymetry, 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. 4. Radio Range, Exposed Nodes and Hidden Terminals 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 links are encountered with range limited to several tens or hundreds of meters. The range-limited characteristic of wireless communications creates new problems that are often observed in multi-hop ad hoc wireless networks. One such problem is shown in Figure 1. Observe that, even though the nodes are shown as all having equal communication ranges, Baccelli & Perkins Expires August 27, 2009 [Page 4] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 they are not at all equally accessible to each other. In the figure, two wireless communications are shown to be in progress; one from node D to node C, and the other one from node A to node B. As shown, this figure illustrates that while router D can hear router C without interference, router C is prevented from hearing router D because router A (in C's radio range), is already communicating with another node. This case is known as the "exposed node" problem, and is often observed on multi-hop ad hoc wireless networks. Radio Ranges for Routers A, B, C, D <~~~~~~~~~~~~+~~~~~~~~~~~~> <~~~~~~~~~~~~~~+~~~~~~~~~> |<~~~~~~~~~~~~+~~~~~~~~~~~~>|<~~~~~~~~~~~~+~~~~~~~~~~~~> +--|-+ +--|-+ +--|-+ +--|-+ |RtrD|=======>|RtrC| |RtrA|------->|RtrB| +----+ +----+ +----+ +----+ Router C becomes an Exposed Node Figure 1: The exposed node problem. Router C is prevented from hearing router D while router A is communicating with router B. Another case which is caused by the range-limited characteristic of wireless communications and is often observed in multi-hop ad hoc wireless networks, is shown in Figure 2. In this example routers B and C cannot hear each other. On the other hand, routers A and B can hear each other and furthermore A and C can also hear each other. When routers B and C try to communicate with router A at the same time, their radio signals collide. Router A will only be able to detect noisy interference, and may even be unable to determine the source of the issue. This case is known as the "hidden terminal" problem, and is often observed on multi-hop ad hoc wireless networks. Baccelli & Perkins Expires August 27, 2009 [Page 5] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 Radio Ranges for Routers A, B, C <~~~~~~~~~~~~~+~~~~~~~~~~~~~> <~~~~~~~~~~~~~+~~~~~~~~~~~~> |<~~~~~~~~~~~~~+~~~~~~~~~~~~~>| +--|-+ +--|-+ +--|-+ |RtrB|========>|RtrA|<========|RtrC| +----+ +----+ +----+ Hidden Terminals at Rtr A Figure 2: The hidden terminal problem. Router C and Router B try to communicate with router A at the same time, and their radio signals collide. 5. 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 routers that can directly receive packets transmitted by router A through its interface on network N. In other words, any router B belonging to S can "hear" packets transmitted by router A. Then, due to the asymmetry characteristic of wireless links: - We may say that router B is reachable from router A. In this terminology, there is no guarantee that router A is reachable from node B, even if router B is reachable from router A. - We may say that router A has a link to router B. In this terminology, there is no guarantee that router B has a link to router A, even if router A has a link to router B. - We may say that router B is adjacent to router A. In this terminology, there is no guarantee that router A is adjacent to router B, even if router B is adjacent to router A. Baccelli & Perkins Expires August 27, 2009 [Page 6] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 - We may say that router B is a neighbor of router A. In this terminology, there is no guarantee that router A is a neighbor of router B, even if router B a neighbor of router A. - We may say that router B is downstream from router A. In this terminology, there is no guarantee that router A is downstream from router B, even if router B is downstream from router A. 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. 6. Security Considerations This document does not have any security considerations. 7. IANA Considerations This document does not have any IANA actions. 8. 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. [IPev] Thaler, D., "Evolution of the IP Model", draft-thaler-ip-model-evolution-01.txt (work in progress), 2008. Baccelli & Perkins Expires August 27, 2009 [Page 7] Internet-Draft Multi-hop Ad Hoc Wireless Communication February 2009 [DoD01] Freebersyser, J. and B. Leiner, "A DoD perspective on mobile ad hoc networks", Addison Wesley C. E. Perkin, Ed., 2001, pp. 29--51, 2001. [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. 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. Authors' Addresses Emmanuel Baccelli INRIA Phone: +33-169-335-511 EMail: Emmanuel.Baccelli@inria.fr URI: http://www.emmanuelbaccelli.org/ Charles E. Perkins WiChorus Phone: +1-408-435-0777 x337 EMail: charliep@wichorus.com Baccelli & Perkins Expires August 27, 2009 [Page 8]