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Versions: 00                                                            
Internet Draft                                                 D. Thaler
February 25, 2006                                              Microsoft
Expires August 2006

                          Multi-Subnet MANETs
           <draft-thaler-autoconf-multisubnet-manets-00.txt>

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Copyright Notice

   Copyright (C) The Internet Society (2006).  All Rights Reserved.

Abstract

   This document describes an approach to addressing nodes in Mobile
   Ad-hoc Networks (MANETs) which involves assigning a separate subnet
   to each MANET router.  This approach avoids many of the problems
   that arise in other approaches, and is intended to allow existing
   protocols and applications to work unmodified.

1. Introduction

   A Mobile Ad-hoc Network (MANET) is one in which the topology may be
   very dynamic, such as highly mobile nodes communicating over a
   wireless medium.  In this document we assume that there are
   forwarding nodes which participate in a MANET routing protocol.
   Furthermore, we assume that there may also be non-MANET nodes
   attached behind MANET forwarding nodes (e.g., via wired Ethernet).
   We will use the term "MANET router" to refer to the first type of
   nodes, and "client" to refer to the latter type of nodes.


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   There are three general approaches to architecting the data
   (forwarding) plane:

   (A)  Treat the entire MANET as a single link, and do all forwarding
        at the datalink-layer, exposing (say) normal Ethernet to IP.
        The control plane thus consists of a layer-2 routing protocol.
        In this approach, existing protocols and applications work
        normally.

   (B)  Treat the MANET as a single subnet composed of multiple links,
        such that MANET routers must do layer-3 forwarding (decrement
        TTL, etc.) between nodes in the same subnet.  The control plane
        consists of a layer-3 MANET routing protocol.  This approach
        has a number of problems as discussed in [MLSI], and may break
        existing protocols and applications.  Clients are either not
        supported at all, or some additional proxying method is
        required.

   (C)  Treat the MANET as a site containing many subnets, such that
        MANET routers act as normal routers when forwarding data
        between subnets.  The control plane consists of a layer-3 MANET
        routing protocol, as with approach (B).  This approach avoids
        most of the problems of (B), easily supports clients, and is
        what the remainder of this document outlines.

2. Overview

   The MANET is treated as a site composed of many subnets.  Each MANET
   router is assigned its own subnet prefix(es).  Each client is
   assigned its own IP address(es) on a subnet assigned to a MANET
   router to which it is attached.

   The MANET site may or may not have external connectivity.  If there
   is, then any site gateways are configured with the site prefix(es)
   and advertise them into the external routing system, as normal site
   gateways do.  If there is no external connectivity, then one or more
   MANET routers are configured with the site prefix(es), e.g., a
   Unique Local IPv6 Prefix [RFC4193], and 10/8 for IPv4 [RFC1918].  It
   may be possible to automatically generate a site prefix for the site
   in a zero-configuration MANET (e.g., by electing one node to do so),
   but such a mechanism is outside the scope of this document. Note
   that a /48 IPv6 site prefix results in the ability to support up to
   2^16 MANET routers, plus any clients behind them.

   Each router then uses a subnet allocation protocol to acquire one or
   more subnet prefixes within the site prefix.  It then uses this
   subnet prefix to number itself and any clients.

   As a result, this approach needs only two MANET-specific protocols:

        1) A routing protocol capable of exchanging subnet routes, and

        2) A subnet allocation protocol capable of allocating prefixes.

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   All other protocols and applications operate normally.

3. Client Behavior

   Clients operate with no changes, using normal IPv4 and IPv6
   mechanisms.

4. MANET Router Behavior

4.1. Obtaining a subnet prefix

   The requirements for a subnet allocation protocol are:

   1) Must be capable of allocating a subnet prefix (not just a single
      address).

   2) Must prevent duplicate subnet prefix assignments.

   3) Must handle network partitions and heals, due to the highly
      dynamic nature of a MANET.

   4) Must complete rapidly, with low overhead.

   5) Must support a bootstrap scenario where all MANET routers without
      a manually configured prefix attempt to allocate a prefix at the
      same time.

   6) Should support the ability to detect duplicate layer-2 addresses.

   7) Should support the ability to detect a duplicate IPv6 link-local
      address within the MANET site (since the dynamic nature means
      that any other router may eventually be on the same link, this
      helps avoid future conflicts).

   Details of the actual subnet allocation protocol are outside the
   scope of this draft.  At a high level, an address allocation
   protocol proposed for a multilink-subnet approach (approach B) can
   be modified to be suitable, by inclusion of a prefix length.

4.2. Using a subnet prefix

   One common practice in BGP is to assign a routable address to the
   router's loopback interface, and use that for communication which
   may be routed over different physical interfaces over time (e.g., to
   survive failures).

   In the same vein, a MANET router takes its allocated prefix, assigns
   it to its loopback interface, and advertises it in the MANET routing
   protocol.  If the MANET router also has a client interface (e.g., an
   Ethernet link with wired clients attached), then the implementation
   can either allocate a second subnet prefix for that link, or may
   simply assign its sole subnet prefix to a client interface rather
   than its loopback interface.  The MANET router then assigns its own

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   IPv4 and/or IPv6 addresses on its loopback interface by using a
   fixed address suffix (e.g., .1 in IPv4 and ::1 in IPv6).

   The MANET router's other interfaces (e.g., a wireless interface used
   to communicate with other MANET routers) is not given an IPv4
   address (i.e., left unnumbered).  An IPv6 link-local address is
   assigned per normal IPv6 stateless address autoconfiguration
   [RFC2462].  Note that normal IPv6 Duplicate Address Detection may
   occur as usual, but the MANET routers seen on that link will change
   over time.  DAD need not be redone as long as the subnet allocation
   protocol is responsible for detecting duplicate IPv6 link-local
   addresses.

   Applications running on the MANET router will use the loopback
   interface, since that is the one with the global address on it.  All
   other MANET nodes will appear to the application as being off-
   subnet.

5. Security Considerations

   Often the hardest attacks to defend against are on-link attacks
   (e.g., ARP attacks in IPv4).  Unlike the other two approaches, this
   approach narrows the set of nodes which are on-link to only those
   nodes which are nearby, as opposed to being anywhere in the MANET.
   In addition to mitigations in various protocols, this may make
   social mitigations easier as well.

   In addition, since this approach does not change the standard
   address allocation and forwarding mechanisms in IPv4 and IPv6, it
   avoids the security issues mentioned in [MLSI], resulting in a
   potentially more secure environment than with a multilink subnet
   approach.

   The routing protocol in use must be secured to avoid attackers
   misrouting packets.  The issues here are discussed in [RFC2501].

   The subnet allocation protocol in use must also be secured to avoid
   attackers consuming all available subnet space, or preventing
   legitimate MANET routers from obtaining a subnet prefix.

6. IANA Considerations

   This document has no actions for IANA.

7. Acknowledgements

   Suresh Krishnan first suggested assigning a subnet prefix to each
   MANET node.






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8. Normative References

   [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
             and E. Lear, "Address Allocation for Private Internets",
             BCP 5, RFC 1918, February 1996.

   [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.

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

9. Informative References

   [MLSI]    Thaler, D., "Issues With Protocols Proposing Multilink
             Subnets", draft-thaler-intarea-multilink-subnet-issues-
             00.txt, February 2006.

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

Authors' Addresses

   Dave Thaler
   Microsoft
   One Microsoft Way
   Redmond, WA 98052
   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com
























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