Subject: (ION) widest acceptable prefix

From: Nick Weeds <[email-masked]>
Date: Thu, 15 Oct 1998 12:49:43 +0100

RFC 2332 section 5.2.1 says that the prefix length in an NHRP Resolution
Request "may be used to qualify the widest acceptable prefix length
which may be used to satisfy the NHRP Resolution Request".

What is meant by "widest" here ?
Is it
(1) the longest (widest) prefix length in bits, matching the smallest
range of IP addresses
(2) the shortest prefix length in bits, matching the largest (widest)
range of IP addresses ?

If (1) then the prefix length in an NHRP Resolution Request can be used
to request the next hop to a subnet rather than a host.

If (2) then the prefix length in an NHRP Resolution Request can be used
to request the next hop to a host rather than a subnet.

Which interpretation is correct ?

Thanks,

	Nick.

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From: "Albert Manfredi" <[email-masked]>
Date: Thu, 15 Oct 1998 09:53:21 -0400

-----Original Message-----
From: Nick Weeds <[email-masked]>
To: [email-masked]' <[email-masked]>
Date: Thursday, October 15, 1998 8:02 AM
Subject: (ION) widest acceptable prefix

>Which interpretation is correct ?

I think #1, if I understand it correctly. You're looking for the
shortcut that gets you as close to the IP host as possible. So I
suppose you call that "the smallest range of IP addresses?"

The goal here is to try to prevent loops, so clearly that means
bypassing as many IP routers as possible. Although proof that this
avoids loop is explicitly not provided in the RFC.

Bert
[email-masked]

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From: Nick Weeds <[email-masked]>
Date: Thu, 15 Oct 1998 15:28:33 +0100

I think that with interpretation (1) a request with prefix length 24
means that the longest (widest) acceptable prefix length in the reply is
24, so the reply prefix length must be <= 24).
That stops the shortcut from getting too close to the destination host.
After all, the destination host has reply prefix length = 32 which is
inconsistent with the request for a reply prefix length <= 24.
But that doesn't seem to fit the objective of getting as close to the IP
host as possible.

On the other hand, with interpretation (2) a request with prefix length
24 means that the reply prefix length must be >= 24, so the resulting
shortcut will be guaranteed to get pretty close to the destination host
-probably at least to a machine on the right subnet.

So do we actually want interpretation (2) ?

Thanks,

	Nick.

>From:	Albert Manfredi [SMTP:[email-masked]]
>Sent:	Thursday, October 15, 1998 2:53 PM
>To:	Nick Weeds; [email-masked]'
>Subject:	Re: (ION) widest acceptable prefix
>
>

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From: "Albert Manfredi" <[email-masked]>
Date: Thu, 15 Oct 1998 11:53:43 -0400

However you word it, the end result you want is that the shortcut be
based on the longest possible prefix, as I read it. And it makes
sense. The longest prefix ensures that the routing options after the
shortcut is established are as few as possible.

Example: What ATM edge router should I use to get to IP address
x.y.z.w?

Option 1: ATM address j is associated with IP address x.y.n.m

Option 2: ATM address k is associated with IP address x.y.z.p

Use Option 2.

Bert

>>>If (1) then the prefix length in an NHRP Resolution Request can
be
>>used
>>>If (2) then the prefix length in an NHRP Resolution Request can
be
>>used