Subject: Source address selection

From: Mohan Parthasarathy <[email-masked]>
Date: Mon, 23 Aug 1999 09:24:39 -0700 (PDT)

In section 4.2 of draft-draves-ipngwg-simple-srcaddr-01.txt. it
states that

>It is RECOMMENDED that the candidate source addresses be the set of
>unicast addresses assigned to the interface that will be used to
>send to the destination. (The "outgoing" interface.)

For a given destination, there could be multiple routes
e.g multiple default routes, by which the destination can
be reached. Each route may be reachable through a different
interface. If we choose the route and hence the interface
without looking at the scope of the destination, we may send
out a packet to global scope with the link local source
address i.e the interface chosen may not have global scoped
addreeses.

So, when there are multiple routes to the same destination,
should the route selection look at the scope of the destination
so that a proper *source address* would be selected ?

-mohan

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From: Richard Draves <[email-masked]>
Date: Tue, 24 Aug 1999 09:33:07 -0700

> so that a proper *source address* would be selected ?

If I understand correctly, you are asking if source address selection
concerns should influence the routing decision. For example, if there are
two default routes, should one default route be preferred over another
because it will lead to a better choice of source address.

I think such behavior should be allowed but not required.

Would it be helpful to have some discussion of this point in the draft?

Thanks,
Rich

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From: Mohan Parthasarathy <[email-masked]>
Date: Tue, 24 Aug 1999 15:07:27 -0700 (PDT)

> because it will lead to a better choice of source address.

Correct. 

> Would it be helpful to have some discussion of this point in the draft?
 
As this draft specifically talks about source address selection,
the above could be one of the scenarios where one needs to
select the route properly so that a proper source address is
chosen. Is there a practical case where this could happen i.e
having 2 default routes but the right scoped addresses are
not available on both the interfaces ?

-mohan

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From: Richard Draves <[email-masked]>
Date: Thu, 26 Aug 1999 11:34:39 -0700

> not available on both the interfaces ?

I think it's an unlikely scenario, almost certainly indicating
misconfiguration.

I would rather not require (or even recommend) that source address selection
considerations influence the routing decision. I like to keep the two
modules separate in my implementation and I imagine that most implementors
would feel similarly. But for those that want to do it, it should be
allowed.

Rich