


Autoconf                                                E. Baccelli, Ed.
Internet-Draft                                                     INRIA
Intended status: Informational                          M. Townsley, Ed.
Expires: September 23, 2010                                Cisco Systems
                                                          March 22, 2010


                 IP Addressing Model in Ad Hoc Networks
                draft-ietf-autoconf-adhoc-addr-model-03

Abstract

   This document describes a model for configuring IP addresses and
   subnet prefixes on the interfaces of routers which connect to links
   with undetermined connectivity properties.

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   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on September 23, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents



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   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 BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Applicability Statement . . . . . . . . . . . . . . . . . . . . 4
   4.  IP Subnet Prefix Configuration  . . . . . . . . . . . . . . . . 4
   5.  IP Address Configuration  . . . . . . . . . . . . . . . . . . . 4
   6.  Addressing Model  . . . . . . . . . . . . . . . . . . . . . . . 5
     6.1.  IPv6 Model  . . . . . . . . . . . . . . . . . . . . . . . . 5
     6.2.  IPv4 Model  . . . . . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8



























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1.  Introduction

   The appropriate configuration of IP addresses and subnet masks for
   router network interfaces is generally a prerequisite to the correct
   functioning of routing protocols.  Consideration of various items,
   including underlying link capabilities and connectivity, geographical
   topology, available address blocks, assumed traffic patterns
   etc., are used when determining the appropriate network topology and
   the associated IP interface configuration.

   When the capabilities and connectivity of the links that connect
   routers are well-known and stable, logical network topology design
   and corresponding IP interface configuration are straightforward.
   Absent any assumption about link-level connectivity, however, there
   is no canonical method for determining a given IP interface
   configuration.

   Link-level connectivity is generally qualified as undetermined when
   it is unplanned and/or time-varying in character.  Ad hoc networks
   are typical examples of networks with undetermined link-level
   connectivity.  Routing protocols for ad hoc networks have as purpose
   to detect and maintain paths across the network, even when faced with
   links with undetermined connectivity, assuming that routers'
   interfaces are configured with IP addresses.  This document thus
   proposes a model for configuration of IP addresses and subnet
   prefixes on router interfaces to links with undetermined connectivity
   properties, to allow routing protocols and data packet forwarding to
   function.

   Note that routers may ultimately need additional IP prefixes for the
   diverse applications that could run directly on the routers
   themselves, or for assignment to attached hosts or networks.  For
   IPv6, these addresses may be global [RFC3587], Unique-Local [RFC4193]
   or Link-Local [RFC4291].  For IPv4, the addresses may be global
   (i.e., public) or private [RFC1918].  In general, global scope is
   desired over local scope, though it is understood that this may not
   always be achievable via automatic configuration mechanisms.  In this
   document however, automatic configuration of the prefixes used for
   general applications is considered as a problem that is separable
   from that of automatic configuration of addresses and prefixes
   necessary for routing protocols to function.  This document thus
   focuses on the latter: the type of IP address and subnet mask
   configuration necessary for routing protocols and data packet
   forwarding to function.







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2.  Terminology

   This document uses the vocabulary and the concepts defined in
   [RFC1918] and [RFC4632] for IPv4, as well as [RFC4291] for IPv6.


3.  Applicability Statement

   This model gives guidance about the configuration of IP addresses and
   the IP subnet prefixes on router's IP interfaces which connect to
   links with undetermined connectivity properties.

   When more specific assumptions can be made regarding the connectivity
   between interfaces, or the (persistent) reachability of some
   addresses, these should be considered when configuring subnet
   prefixes.


4.  IP Subnet Prefix Configuration

   If the link to which an interface connects enables no assumptions of
   connectivity to other interfaces, the only addresses which can be
   assumed "on link", are the address(es) of that interface itself.
   Note that while link-local addresses are assumed to be "on link", the
   utility of link-local addresses is limited as described in Section 6.

   Subnet prefix configuration on such interfaces must thus not make any
   promises in terms of direct (one hop) IP connectivity to IP addresses
   other than that of the interface itself.  This suggests the following
   principle:

   o  no on-link subnet prefix should be configured on such an
      interface.

   Note that if layer 2 communication is enabled between a pair of
   interfaces, IP packet exchange is also enabled, even if IP subnet
   configuration is absent or different on each of these interfaces.

   Also note that if, on the contrary, assumptions can be made regarding
   the connectivity between interfaces, or regarding the persistent
   reachability of some addresses, these should be considered when
   configuring IP subnet prefixes, and the corresponding interface(s)
   may in such case be configured with an on-link subnet prefix.


5.  IP Address Configuration

   Routing protocols running on a router may exhibit different



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   requirements for uniqueness of interface addresses; some have no such
   requirements, others have requirements ranging from local uniqueness
   only, to uniqueness within, at least, the routing domain (as defined
   in [RFC1136]).

   Configuring an IP address that is unique within the routing domain
   satisfies the less stringent uniqueness requirements of local
   uniqueness, while also enabling protocols which have the most
   stringent requirements of uniqueness within the routing domain.  This
   suggests the following principle:

   o  an IP address assigned to an interface that connects to a link
      with undetermined connectivity properties should be unique, at
      least within the routing domain.


6.  Addressing Model

   Section 4 and Section 5 describe principles for IP address and subnet
   prefix configuration on an interface of a router, when that interface
   connects to a link with undetermined connectivity properties.  The
   following describes guidelines that follow from these principles,
   respectively for IPv6 and IPv4.

   Note that the guidelines provided in this document slightly differ
   for IPv6 and IPv4, as IPv6 offers possibilities that IPv4 does not
   (i.e., the possibility to simply not configure any on-link subnet
   prefix on an IPv6 interface), which provide a "cleaner" model.

6.1.  IPv6 Model

   For IPv6, the principles described in Section 4 and Section 5 suggest
   the following rules:

   o  An IP address configured on this interface should be unique, at
      least within the routing domain, and

   o  No on-link subnet prefix is configured on this interface.

   Note that while an IPv6 link-local address is assigned to each
   interface as per [RFC4291], in general link-local addresses are of
   limited utility on links with undetermined connectivity, as
   connectivity to neighbors may be constantly changing.  The known
   limitations are:

   o  There is no mechanism to ensure that IPv6 link-local addresses are
      unique across multiple links, hence they cannot be used to
      reliably identify routers (it is often desirable to identify a



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      router with an IP address).

   o  Routers cannot forward any packets with link-local source or
      destination addresses to other links (as per [RFC4291]) while most
      of the time, routers need to be able to forward packets to/from
      different links.

   Therefore, autoconfiguration solutions should be encouraged to
   primarily focus on configuring IP addresses that are not IPv6 link-
   local.

6.2.  IPv4 Model

   For IPv4, the principles described in Section 4 and Section 5 suggest
   rules similar to those mentioned for IPv6 in Section 6.1, that are:

   o  An IP address configured on this interface should be unique, at
      least within the routing domain, and

   o  Any subnet prefix configured on this interface should be 32 bits
      long.

   Note that the use of IPv4 link-local addresses [RFC3927] in this
   context should be discouraged for most applications, as the
   limitations outlined in Section 6.1 for IPv6 link-local addresses
   also concern IPv4 link-local addresses.  These limitations are
   further exacerbated by the smaller pool of IPv4 link-local addresses
   to choose from and thus increased reliance on Duplicate Address
   Detection (DAD).


7.  IANA Considerations

   This document has no actions for IANA.


8.  Security Considerations

   This document thus focuses on the IP address and subnet mask
   configuration necessary for routing protocols and data packet
   forwarding to function.  [RFC4593] describes generic threats to
   routing protocols, whose applicability is not altered by the presence
   of interfaces with undetermined connectivity properties.  As such,
   the addressing model described in this document does not introduce
   new security threats.

   However, the possible lack of pre-established infrastructure or
   authority, as enabled by the use of interfaces with undetermined



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   connectivity properties, may render some of the attacks described in
   [RFC4593] easier to undertake.  In particular, detection of
   malevolent misconfiguration may be more difficult to detect and to
   locate.


9.  References

9.1.  Normative References

   [RFC1136]  Hares, S. and D. Katz, "Administrative Domains and Routing
              Domains: A Model for Routing in the Internet", RFC 1136,
              December 1989.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, 2006.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              2005.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              RFC 1918, 1996.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, 2005.

   [RFC3587]  Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
              Unicast Address Format", RFC 3587, 2003.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", RFC 4632, 2006.

9.2.  Informative References

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, 2006.


Appendix A.  Contributors

   This document reflects discussions and contributions from several
   individuals including (in alphabetical order): Teco Boot, Ulrich
   Herberg, Thomas Narten, Erik Nordmark, Charles Perkins, Zach Shelby
   and Dave Thaler.




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Authors' Addresses

   Emmanuel Baccelli
   INRIA

   Email: Emmanuel.Baccelli@inria.fr
   URI:   http://www.emmanuelbaccelli.org/


   Mark Townsley
   Cisco Systems

   Email: townsley@cisco.com






































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