Re: Multi-link subnets and MIPv6 (was Re: [Mip6] Re: WG Review: Recharter of Mobility for IPv6 (mip6))

[Julien Laganier <[e-mail]>]

Hi Vijay,

Some thoughts below:

On Friday 29 September 2006 23:54, Vijay Devarapalli wrote:
> Hi Jari and Dave,
>
> Jari Arkko wrote:
> >
> >> can you send us more information on this? specifically
> >> on what we are supposed to look for. document problems
> >> that we might encounter when MIPv6 is run over multilink
> >> subnets?
> >
> > The basic reference is the new IAB draft,
> > draft-iab-multilink-subnet-issues-00.txt.
>
> I went through this document.
>
> first I was surprised when I saw the following
> text
>
> >    However, other WGs continued to allow for this concept even
> > though it had been rejected in the IPv6 WG.  Mobile IPv6
> > [RFC3775] allows tunnels to mobile nodes to use the same subnet
> > as a home link, with the Home Agent doing layer 3 forwarding
> > between them.
>
> I was surprised to see the MIPv6 home link and the
> tunnels to each mobile node being compared to a
> multi-link subnet. its not a multi-link subnet as
> described in draft-ietf-ipv6-multilink-subnets-00.txt.
> the home agent does not bridge and forward link-scope
> traffic onto the tunnels.

My (perhaps flawed) understanding is that a multilink subnet occurs 
when a subnet prefix is used to number interfaces on different links, 
and an entity forward traffic between those two links. It is not only 
about forwarding link-scope traffic.

If you look at MIPv6 as described per section 8.4. of RFC 3775 this 
AFAICS exactly what happens: The MN numbers its tunnel interface with 
an address in the same subnet prefix that the one used on the home 
link (which is different from the tunnel interface.), and the HA 
forwards traffic:

> 8.4.  IPv6 Home Agents
> 
> [...] 
>
>   o  Every home agent MUST be able to intercept packets (using proxy
>      Neighbor Discovery [12]) addressed to a mobile node for which
>      it is currently serving as the home agent, on that mobile
>      node's home link, while the mobile node is away from home
>      (Section 10.4.1). 
>
>   o  Every home agent MUST be able to encapsulate [15] such
>      intercepted packets in order to tunnel them to the primary
>      care-of address for the mobile node indicated in its binding in
>      the home agent's Binding Cache (Section 10.4.2).

Regarding your observation that link-scope traffic is not forwarded in 
MIPv6, I think that not forwarding link-scope traffic is exactly the 
problem with multilink subnets: lot of applications assume that if 
two nodes share a same subnet prefix, then it means they share a 
link, and they can therefore communicate with link-scope traffic, and 
TTL won't get decremented. This assumption is broken when you have a 
multilink subnet but you do not forward link-scoped traffic.

> the draft later on says
>
> >    A multilink subnet model SHOULD NOT be used.  IETF WG's using,
> > or considering using, multilink subnets today should investigate
> > moving to one of the other models.  For example, Mobile IPv6
> > should investigate having the Home Agent not decrement the Hop
> > Limit, and forward multicast traffic.
>
> currently no link local traffic sent on the home
> link is forwarded over the Mobile IP tunnel to the
> mobile node.

I think that is exactly one of the issue of multilink subnets as used 
in MIPv6 (see above.)

> [...]
>
> the current use of Mobile IP tunnels as an
> extension of the home links does not introduce the
> issues described with multi-link subnets in
> draft-iab-multilink-subnet-issues. or I might be
> missing something big.

IMHO MIPv6 introduce the issues described in the multilink subnet 
draft. 

--julien


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