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Here is the motivation for our gateway discovery/negotiation
protocols.  The protocol is designed to treat present problems on
the C-U Community Wireless Network, to prevent future problems that
we anticipate, and to help us fulfill our future plans.  Here are
present problems, future problems and plans.

  1 It is inevitable that CWNs are going to be multihomed, whether
    it be for greater bandwidth, for shorter paths to the Internet,
    for higher reliability, or else by accident.  Multi-homing will
    cause problems for us if we do not deal with it explicitly (see
    below).  We have to design our network to cope with multi-homing.

  2 A multi-homed wireless network should be "default-free."
    Advertising a default network (0/0) on the wireless net will
    lead to routers flip-flopping between equal-cost (or near
    equal-cost) default routes.  Since we do not expect to run an
    EGP for a long while, but we do expect that our Internet gateways
    will NAT the wireless net, we anticipate that gateway flip-flops
    will sever TCP sessions.

  3 We want to be able to provide every subscriber with at least
    one routable IP number, since that is what they will need to use
    the full spectrum of Internet apps, and also to run a server.

  4 It would be foolish to design without IPv6 in mind.
    Routable IPv4 numbers are in short supply. It is not clear
    whether CWNs can get hold of them, and it appears that they
    will cost us. IPv6 numbers, on the the other hand, are plentiful
    and essentially free.  IPv6 is unquestionably the future of
    the Internet.  People are already using v6 a little bit, both
    Mac OS X and Windows XP support it, and wherever IPv4 numbers
    are "running out"---in Asia, for example---there are already
    significant economic and government incentives to adopt v6.
    We should provide for IPv6 gateways in our designs.

    IPv6 numbers can already be had very easily: if you have a
    single IPv4 number, you can use "6to4" to get a /48 in IPv6
    space. That is 2**80 IPv6 addresses, which is enough to assign
    a number to each of your home appliances.  (Yes, it is *that*
    *many*.) For more info, see this tutorial,
    <http://www.onlamp.com/pub/a/onlamp/2001/06/01/ipv6_tutorial.html>.

    I want for our nodes to be reached easily from the Internet,
    since that will help far more people to participate in network
    development, maintenance, and troubleshooting.  When our networks
    are NAT'd onto the Internet, it is awfully hard to provide a
    connection into each node from the Internet. 6to4 connectivity
    is an enormous help there.  Granted, developers will need an
    IPv6-capable computer, but you're a geek, right? (The C-U
    Wireless development server is IPv6 capable, BTW.)

References

  [1] IETF MANET discussion on gateway discovery
  [2] Personal communication w/ Perry Metzger re: 6to4

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