Re: Some Comments on ID/Loc Separation Proposals

[Juan Rodriguez Hervella <jrh@it.uc3m.es>]

On Thursday 13 November 2003 17:12, Erik Nordmark wrote:
> > An IP address can be used, for example, to allow ULPs to indicate
> > which physical interface to use for outbound packets by explictly
> > choosing a source address.  This can be useful for multicast traffic
> > and specialized ULPs such as routing protocols.
>
> I heard this asserted elsewhere yersterday but I haven't seen any IPv*
> stacks that do such a thing.
> The outbound interface is selected based on the destination IP address
> independently of the source address.
> Thus if the application specifies the source address for if2 and sends
> a packet to some destination where the routing table for the destination
> says to use if1, then the packet will go out if1 with the source of if2.

Hello,

Quoted from RFC 1122 ("Strong End System" model):

---beginning of quotation:

           There are two key requirement issues related to multihoming:

            (A)  A host MAY silently discard an incoming datagram whose
                 destination address does not correspond to the physical
                 interface through which it is received.

            (B)  A host MAY restrict itself to sending (non-source-
                 routed) IP datagrams only through the physical
                 interface that corresponds to the IP source address of
                 the datagrams.

                o    Strong ES Model

                      The Strong ES (End System, i.e., host) model
                      emphasizes the host/gateway (ES/IS) distinction,
                      and would therefore substitute MUST for MAY in
                      issues (A) and (B) above.  It tends to model a
                      multihomed host as a set of logical hosts within
                      the same physical host.

                      With respect to (A), proponents of the Strong ES
                      model note that automatic Internet routing
                      mechanisms could not route a datagram to a
                      physical interface that did not correspond to the
                      destination address.

                      Under the Strong ES model, the route computation
                      for an outgoing datagram is the mapping:

                         route(src IP addr, dest IP addr, TOS)
                                                        -> gateway

----end of quotation

In short, Erik, under the "strong ES" model assuption, 
your example  would _not_work.

Ciao !

-- 
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