netdev - Re: Weak host model vs .interface down

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Date:	Sat, 12 Jun 2010 11:34:29 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Mark Smith <lk-netdev@...netdev.nosense.org>
Cc:	netdev@...r.kernel.org, Rick Jones <rick.jones2@...com>
Subject: Re: Weak host model vs .interface down

Mark Smith <lk-netdev@...netdev.nosense.org> wrote on 2010/06/12 01:57:48:
>
> On Fri, 11 Jun 2010 21:41:45 +0200
> Joakim Tjernlund <joakim.tjernlund@...nsmode.se> wrote:
>
> > Rick Jones <rick.jones2@...com> wrote on 2010/06/11 19:13:42:
> > >
> > > > The weak model doesn't go into such detail, it is assumption/impl. detail
> > > > to assume that the ip address still is part of the system even when the interface
> > > > is down. One could just as well define interface down as temporarly removing
> > > > the IP address from the system too. This makes make much more sense to me and
> > > > if you always want the system to answer on a IP adress you make it an IP alias.
> > > >
> > > > Since the current behaviour is a problem to me and routers in general, can
> > > > we change this? What is the current usage model which needs it to stay as is?
> > >
> > > Router != end-system  so I wouldn't think the weak or strong end-system model
> > > would apply to a router.  I think Stephen already posted a patch to allow that
> > > for when one's box was a router rather than an end-system.
> >
> > Not really an anwser to what I was asking but I choose to read that as
> > you agree with me. The rest is an impl. detail. :)
> > Stephen's patch is good but I would not mind making I/F down removing the
> > IP address from the system unconditionally.
> >
>
> I've asked the same question a few years back and got the same answer.
> I accept the strong host / weak host argument, however I've also
> thought about the problem a bit more, and why people get confused about
> it.
>
> The problem is the mental model. Assigning an IP address to an
> interface implies that the IP address as attached and associated with
> the interface and therefore the state of the interface. That is
> certainly the case for people like me who work with networking
> equipment, typically routers, which follow the strong host model. It is
> very convenient to know that by shutting down an interface the
> associated IP address stops working too. Other measures, such as
> ACLing, or writing down and deleting and then having put it back, are
> relatively much more effort and error prone.

Very well put!

>
> While I'm sure past operational history is likely to make this
> impractical, it would be far more intuitive for weak host model IP
> address assignments to be made to a single, forced always up virtual
> interface on the host, and strong host IP address assignments made to
> any other "non-weak host" interfaces.
>
> It'd be an interesting experiment to see if loopback could be used as a
> "host interface" in the weak host model.

Or you can use the dummy I/F too. I have used lo/dummy to assign a host/system
address and it works fine. I am not aware of any limitations but if there are
I am sure someone will point them out :)

 Jocke

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