Subject: Re: [nemo] Router lifetime in RAs from mobile routers

From: Alexandru Petrescu <[email-masked]>
Date: Thu, 01 Apr 2004 11:34:57 +0200

Hi Ted, thanks for the comment; let me add further support to Vijay's
explanation on motivating the MR's sending RA's with lifetime 0 (on the
egress interface on the home link).

Vijay Devarapalli wrote:
> Ted:
>> I'm a bit confused by this text in section 5.6:
>> 
>>> When the Mobile Router is at home, it MAY be configured to send 
>>> Router Advertisements and reply to Router Solicitations on the 
>>> interface attached to the home link.  The value of the Router 
>>> Lifetime field MUST be set to zero to prevent other nodes from 
>>> configuring the Mobile Router as the default router.
>> 
>> I don't see cases where the mobile router would have a different 
>> upstream link in its home link than a non-mobile router on this 
>> link.

Right, neither do I; we haven't considered such cases.

We didn't want the MR on the home link to be default router for any host
on the home link, so RFC2461 says we can achieve this by having the MR
to put lifetime 0 in the RA sent by MR.
RFC 2461:
> A router might want to send Router Advertisements without advertising
>  itself as a default router.  For instance, a router might advertise
>  prefixes for address autoconfiguration while not wishing to forward
>  packets.  Such a router sets the Router Lifetime field in outgoing 
> advertisements to zero.

All routers on the home link (mobile or fixed) configure their default
routes manually (or IGP), but not from received RA's; so the danger of
MR becoming default upstream router is valid only at the hosts on the
home link, not for the routers there.

> Ted:
>> Is there a use case where it is expected that all the routers on 
>> the home link might be mobile routers (so only mobile routers are 
>> replying to RSs?)

We haven't considered such use cases.

I think that you seem to agree with us about forbidding the MR to become
an upstream default router for hosts on the home link - we don't want
that either. Requiring MR to put lifetime 0 was _not_ motivated by use
cases where all the routers on the home link be mobile routers.

Alex

> actually this restriction is there to protect IPv6 hosts on the home 
> link from configuring a mobile router as the default router instead 
> of a non-mobile router.
> 
> you could have mobile nodes and mobile routers connecting to the same
>  home link when they are at home. when the mobile routers return 
> home, they can start acting like regular routers on the egress 
> interface (the interface with which they attach to the home link). 
> they can start sending router advertisements. but we dont want mobile
>  nodes (or any IPv6 host) on the home link to pick the mobile router
>  as the default router. so we recommed setting the lifetime in the 
> router advertisements to zero, so that no host on the home link picks
>  a mobile router as the default router.
> 
> do you see a need for more text to explain this?
> 
> Vijay

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From: Alexandru Petrescu <[email-masked]>
Date: Thu, 01 Apr 2004 11:34:57 +0200

Alex