[dhcwg] IPv4 Address Conflict Detection
Stuart Cheshire <cheshire@apple.com> Mon, 04 February 2002 20:26 UTC
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From: Stuart Cheshire <cheshire@apple.com>
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Subject: [dhcwg] IPv4 Address Conflict Detection
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At the DHC WG meeting in Salt Lake City we briefly talked about address conflict detection. The feedback I got was that while it is definitely useful to nail down a clear specification of how do do address conflict detection properly, it doesn't have an obvious natural "home" in any current IETF WG, so I should just solicit feedback and then send it in as an individual submission. The two working groups we felt had expertise in this area are DHC and MOBILEIP, hence this email: ---------------- From time to time people ask how Mac OS, Windows, and other OSs do that thing they do where they give an error message if two hosts are accidentally configured with the same IP address. Detecting address conflicts is not difficult, but to date there has been no IETF Standard specifying how to do it. The only RFC I could find that even mentions IPv4 address conflict detection is RFC 2131, where it says things like: If the client detects that the address is already in use (e.g., through the use of ARP), the client MUST send a DHCPDECLINE message to the server Unfortunately, RFC 2131 doesn't go into much detail about trivia like how many ARP packets to send, how long to wait, etc. This is not a criticism of RFC 2131, because defining IPv4 address conflict detection is rightly outside the scope of RFC 2131. Ideally, there should have been an existing specification for RFC 2131 to reference, like this: If the client detects that the address is already in use [RFC xxxx], the client MUST send a DHCPDECLINE message to the server Sadly, that specification did not exist when RFC 2131 was written. To remedy this, I have written a short draft specifying how to detect address conflicts. <http://www.ietf.org/internet-drafts/draft-cheshire-ipv4-acd-00.txt> I'm sending this email not because I think that DHC or MOBILEIP ought to take on this work, but because I think that if it eventually becomes an RFC then DHC and MOBILEIP may want to reference it in their own standards, so I want to give everyone a chance to take a look and see if they like what it says. DHCP depends on a conflict detection mechanism in order to trigger a DHCP DECLINE packet. My hope is that this draft is a clear specification of how to perform that conflict detection, so that if/when RFC 2131 is updated, it can reference this specification instead of again saying, "e.g., through the use of ARP." I think it makes sense to publish IPv4 Address Conflict Detection as a separate standard, because while address conflict detection is important for a DHCP client, it is useful no matter how a host is configured. If a host is configured manually, then address conflict detection allows the host to display an error message if two hosts are accidentally given the same address. If a host is using a Zeroconf self-assigned link-local address, then address conflict detection is the mechanism that tells the host it needs to select a different address. Right now, as written, draft-00 specifies that a host probe the network for 8-10 seconds before beginning to use an IP address. For a desktop machine using DHCP, this is probably fine. For a small mobile device, it may not be fine. A small mobile device may want to be allowed to access the network much quicker than that. For this reason, feedback from MOBILEIP would be good. One of the problems on today's networks is that Ethernet switches that implement spanning tree often silently discard all packets for many seconds, which makes it hard to say how long a host should probe before using an address. One possibility is that we could revise the draft to say that on networks where successful connectivity can be determined by the hardware with some acceptable degree of certainty, all the timeouts can be ten times shorter than currently specified: i.e. 0-200ms initial delay, four packets 200ms apart, for a total probing time of 800-1000ms. Thoughts? Stuart Cheshire <cheshire@apple.com> * Wizard Without Portfolio, Apple Computer * Chairman, IETF ZEROCONF * www.stuartcheshire.org _______________________________________________ dhcwg mailing list dhcwg@ietf.org https://www1.ietf.org/mailman/listinfo/dhcwg
- [dhcwg] IPv4 Address Conflict Detection Stuart Cheshire
- [dhcwg] Re: [mobile-ip] IPv4 Address Conflict Det… Gerald Maguire
- [dhcwg] Re: [mobile-ip] IPv4 Address Conflict Det… s. goswami
- Re: [dhcwg] Re: [mobile-ip] IPv4 Address Conflict… dworley