Subject: RE: (ION) widest acceptable prefix

From: Nick Weeds <[email-masked]>
Date: Wed, 21 Oct 1998 11:05:21 +0100

Joel,

Thanks for this explanation.  It makes the intention much clearer.

If I understand you correctly, it is important for NHRP routers which
forward NHRP Resolution Requests to update the prefix length.

Suppose a transit NHRP router gets a request for 1.2.3.4/8, looks up
1.2.3.4 in its routing table and finds an entry for 1.2.0.0/16.
Presumably in this case the NHRP router MUST update the prefix length in
the request to be at least 16.  Is this correct ?

What if the NHRP router has routing table entries for 1.2.0.0/16 and for
1.2.4.0/24 ?
The router might update the request to be any of the following:

(1) 1.2.3.4/16
This uses a simple routing table lookup of 1.2.3.4 but gives a
sub-optimal route for 1.2.4.*.

(2) 1.2.3.4/24
This requires a more complicated routing table lookup to spot the
1.2.4.0/24 route.

(3) 1.2.3.4/0xFF
This avoids sub-optimal routing and complicated routing table lookup by
setting prefix length = 0xFF in all cases.

Which of these 3 actions is acceptable ?

Thanks,

	Nick.

>-----Original Message-----
>From:	Joel Halpern [SMTP:[email-masked]]
>Sent:	Monday, October 19, 1998 9:04 AM
>To:	Fox, Barbara
>Cc:	'Albert Manfredi'; [email-masked]'
>Subject:	RE: (ION) widest acceptable prefix
>
>Sorry, I got lost in the (1)s and (2)s.  The intend was:
>
>If you request an answer of length x, you will never get an answer shorter
>than length x.
>
>This is not so much for the originator  (he is probably only willing to
>accept /32, or willing to accept anything), but for the progression.  It
>is important that the match only get lengthened as it goes from router to
>router.  If the mask got shortened, it could produce incorrect overlapping
>resolutions.
>
>Yours,
>Joel M. Halpern                         [email-masked]
>Newbridge Networks Inc.
>
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---

From: Srinivasan Balaji <[email-masked]>
Date: Wed, 21 Oct 1998 09:49:46 -0400

If I have understood your question correctly,
  1.2.4.x/24 is anyway incorrect as we want to reach 1.2.3.x/24 and 1.2.4.x/24
doesn't take us there. Only (1) would be the right one[ultimately the egress
NHRP router would set the final value greater or equal to 16], as far as (3) is
concerned, there is nothing like prefix length of 0xff, the largest is 32 (host
route) and since this router doesn't have a host specific route, 1.2.3.4/32
would also be incorrect.

> the request to be at least 16.  Is this correct ?

Yes.

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From: Joel Halpern <[email-masked]>
Date: Tue, 27 Oct 1998 00:34:16 -0500 (EST)

Providing the correct answer to these questions is what the two confusing
addressing constraints in the r2r document are trying to do.  I would
still appreciate assistance from any of the original authors of that text
in order to make it clearer correctly.

To answer the specific question:

> the request to be at least 16.  Is this correct ?
Yep, the easy case.

> What if the NHRP router has routing table entries for 1.2.0.0/16 and for
The obvious overlap case.  Any answer as long or longer than what I state
below is going to provide acceptable forwarding.  (So, yes /32 or /FF are
ok, but lose possible information.)

> The router might update the request to be any of the following: > 
> (1) 1.2.3.4/16
Assuming that 1.2.4.0/24 is actually pointing to a different interfacethan
1.2.0.0/16, then
Answering 1.2.3.4/16 is NOT permitted.  It would give incorrect
information relative to 1.2.4.0/24.

Yes, this requires more complicated logic.  It is also the correct answer.
The router must look for such overlaps, and lengthen the prefix
accordingly so as to avoid giving incorrect answers.  Whether this is
actually easy or hard depends upon how one has build ones RIB/FIB.

Yours,