Re: [dhcwg] Gratuitous ARP in DHCP vs. IPv4 ACD Draft

Ralph Droms <rdroms@cisco.com> Mon, 02 August 2004 22:55 UTC

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Date: Mon, 02 Aug 2004 16:39:04 -0400
To: Stuart Cheshire <cheshire@apple.com>
From: Ralph Droms <rdroms@cisco.com>
Subject: Re: [dhcwg] Gratuitous ARP in DHCP vs. IPv4 ACD Draft
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Cc: DHCP discussion list <dhcwg@ietf.org>, babatke@ra.rockwell.com, Ted Lemon <mellon@nominum.com>
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At 12:49 PM 8/2/2004 -0700, Stuart Cheshire wrote:

>The question of whether to announce via ARP request or response is
>something that really doesn't matter, and is therefore the thing
>guaranteed to generate the greatest amount of debate. As John Schnizlein
>correctly pointed out, "RFC 826 explicitly does not discriminate between
>request or reply messages before updating its table".

If it really doesn't matter, why are we having this discussion?  Are you
proposing we change DHCP to use an ARP request for announcements?

>Here's what I plan to put in the next draft:
>
>Why are ARP Announcements performed using ARP Request packets and not
>ARP Reply packets?
>
>There are two reasons, one is historical precedent, and the other is
>practicality.
>
>The historical precedent is that Gratuitous ARP is described in Stevens
>Networking [Ste94] as using ARP Request packets.

It would be better to cite an historical precedent from an RFC.  I seem
to remember (I looked it up once before but am in the airport without
access to reference materials) that Comer describes gratuitous ARP
as using an ARP Reply message.

>The practical reason is that ARP Request packets are more likely to work
>correctly with more existing ARP implementations, some of which may not
>implement RFC 826 correctly.

Do you have specific examples of stacks that work when an ARP Request
is received but not when an ARP Reply is received?

- Ralph


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