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Re: DAD question

"Duncan, Richard (Jeremy)" <jeremy.duncan@salientfed.com> Fri, 10 August 2012 22:32 UTC

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From: "Duncan, Richard (Jeremy)" <jeremy.duncan@salientfed.com>
To: Jared Mauch <jared@puck.nether.net>, "ipv6@ietf.org" <ipv6@ietf.org>
Subject: Re: DAD question
Thread-Topic: DAD question
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Date: Fri, 10 Aug 2012 22:32:40 +0000
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References: <36AA0AF8-95FD-4751-AE2E-A7A3D07038EB@cisco.com>, <C34ADAF7-8125-4176-AC52-21BD5BCD07A2@puck.nether.net>
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Cc: "Fred Baker (fred)" <fred@cisco.com>
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Fred-

That's not the case at all.  In testing I have done on Unix, Linux and Windows systems they all do (1).  There are a few variations with BSD, but for the most part they just stop trying.  In fact, RFC 4862 allows for that behavior and actually encourages it:

5.4.5. When Duplicate Address Detection Fails

A tentative address that is determined to be a duplicate as described above MUST NOT be assigned to an interface, and the node SHOULD log a system management error.

If the address is a link-local address formed from an interface identifier based on the hardware address, which is supposed to be uniquely assigned (e.g., EUI-64 for an Ethernet interface), IP operation on the interface SHOULD be disabled. By disabling IP operation, the node will then:

- not send any IP packets from the interface,

- silently drop any IP packets received on the interface, and

- not forward any IP packets to the interface (when acting as a router or processing a packet with a Routing header).

In this case, the IP address duplication probably means duplicate hardware addresses are in use, and trying to recover from it by configuring another IP address will not result in a usable network. In fact, it probably makes things worse by creating problems that are harder to diagnose than just disabling network operation on the interface; the user will see a partially working network where some things work, and other things do not.

On the other hand, if the duplicate link-local address is not formed from an interface identifier based on the hardware address, which is supposed to be uniquely assigned, IP operation on the interface MAY be continued.


010100110110010101101101011100000110010101110010001000000100011001101001

Jeremy Duncan
Senior Director, IPv6 Network Architect
Salient Federal Solutions, Inc. (Now including SGIS & Command Information Inc.)
4000 Legato Road, Suite 600
Fairfax, VA 22033
Google Voice: 540.440.1193
jeremy.duncan@salientfed.com

Jared Mauch <jared@puck.nether.net> wrote:


On Aug 10, 2012, at 6:17 PM, Fred Baker (fred) wrote:

> Is it fair to assume that implementations do DAD and follow (2)?

This is the logical thing that I personally would do..

- Jared
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