[e2e] modularity of address lookup and management

Joe Touch touch at ISI.EDU
Mon Oct 23 11:54:58 PDT 2006

Craig Partridge wrote:
> Hi folks:
> I'm currently working on a challenge related to how one modularizes
> problems of looking up addresses, assigning (or learning of) addresses on
> interfaces, at the like.
> To make this problem concrete, consider the requirement that a multihomed
> host accept any datagram with any one of the host's IP addresses as the
> destination. 

That presumes the weak host model; there are cases where that's not what
you want (e.g., to group sets of tunnels to associate then with
different virtual hosts/routers inside a real host/router).

> If address control is done in the MAC/device driver layer,
> then a datagram could trigger one lookup per interface.  If address control
> is done by the IP layer, then there are challenges of mapping IP addresses
> to MAC layers/interfaces (required, as I recall, on outbound where the
> source IP address usually must but that of the outbound interface).

When is this 'address control' happening? There's already one lookup per
packet on arrival to see if it matches one of the interfaces. On
departure, that typically happens only for packets whose source
addresses are unassigned.

I thought RFC1122 talked about this in detail.

> Somehow, in the back of my head, there's the idea that someone looked at
> this problem in the early 1990s.  There's text in the Router Requirements
> RFC and Host Requirements sketching out the problems, but not espousing a
> particular software modularity.  Did anyone write something on this problem?

I thought it implied that most of this happened at the network layer,
not the link layer.


> Thanks!
> Craig
> E-mail: craig at aland.bbn.com or craig at bbn.com

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