[dhcwg] Proposed resolution to DNA issue 11: Reachability test
Bernard Aboba <aboba@internaut.com> Wed, 28 January 2004 11:26 UTC
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Date: Mon, 26 Jan 2004 12:48:31 -0800
From: Bernard Aboba <aboba@internaut.com>
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Subject: [dhcwg] Proposed resolution to DNA issue 11: Reachability test
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The DNA issues list is available at: http://www.drizzle.com/~aboba/DNA/dnaissues.html The text of DNA issue 11 is enclosed below. The proposed resolution is as follows: Add the following definition to the terminology section (1.2): "Valid address In this specification, the term "valid address" refers to either a staticly configured IPv4 address, or an address assigned via DHCPv4 which has not been relinquished, and whose lease has not expired." Change Section 2.1 to the following: "2.1. Reachability Test The purpose of the reachability test is to determine whether the host is connected to a network on which it has a valid routable IPv4 address. The host skips the reachability test in the following circumstances: [a] If the host does not have good reason to believe that is connected to a network on which it has a valid routable IPv4 address. Since confirming failure of the reachability test requires considerable latency, mistakes are costly. In the absence of other evidence, a host SHOULD instead send a DHCPREQUEST from the INIT-REBOOT state, as described in [RFC2131], Section 3.2 and 4.3.2. [b] If the host believes that it is connected to a network on which it does not have a valid routable IPv4 address. A Link-Local IPv4 address does not count as a valid routable IPv4 address, nor does an IPv4 address assigned via DHCPv4, but whose lease has expired. In this case, the host SHOULD send a DHCPDISCOVER from the INIT state, as described in [RFC2131] Section 4.4. [c] If the host does not have information on default gateway(s) on the network to which it believes it is connected. The reachability test is performed by attempting to verify reachability of default gateway(s) on a former point of attachment. The host may probe only the primary default gateway, or it may probe primary and secondary default gateways, in series or in parallel. However, the host MUST only configure default gateway(s) which pass the reachability test. If the test is successful, the host may continue to use a valid routable IPv4 address without having to re-acquire it. This reduces roaming latency by allowing the host to bypass DHCP as well as subsequent Duplicate Address Detection (DAD). In contrast to a DHCP exchange, which may be between a DHCP client and an offlink DHCP server, the reachability test occurs between a host and its next hop router." --------------------------------------------------------------------------- Issue 11: Reachability test Submitter: Mark Stapp Submitter email address: mjs@cisco.com Date first submitted: November 11, 2003 Reference: http://www1.ietf.org/mail-archive/working-groups/dhcwg/current/msg02702.html Document: DNAv4-04 Comment type: T Priority: S Section: 2.1 Rationale/Explanation of issue: I have some comments on draft-ietf-dhc-dna-ipv4-04.txt - on section 2.1 in particular. [2.1] Reachability Test Doesn't this sentence in 2.1 sort of turn DHCP on its head? "The purpose of the reachability test is to determine whether the host is connected to a network on which it had previously obtained a still valid routable IPv4 address." As I understand it, it's only the DHCP server who knows authoritatively whether the address binding that the client remembers is still valid. Working around that authority in circumstances where the server has failed, if the client could actually determine that, would be one thing. But working around that without the client's even attempting to confirm its binding with the server is problematic. This section states that a host may skip confirmation of its DHCP address when it reconnects under some circumstances. The DHCP INIT-REBOOT state is actually pretty important, and skipping it could have some undesireable consequences. As 2.1 is worded now, a host could have a week-old memory of an address it got on network A, and could re-use it upon reconnecting to network A without attempting INIT-REBOOT. That's a pretty dramatic change, if I've read the text accurately, and I'm not comfortable with it. Is the goal of 2.1 to assist clients who have marginal connections - wireless clients whose associations are flapping, for example? Maybe a time-based, stateful heuristic would be appropriate here. For example, if the host believes that it has reconnected to network A, and that it last communicated with the DHCP server on network A within the last - one minute? five minutes? - then it could proceed without INIT-REBOOT. If the host had a) been off of network A for more than five minutes, or b) been attached to some other network since it last attached to network A then it would go through INIT-REBOOT normally. Or are there folks who think that the client should always do INIT-REBOOT if it can? In that case, if there's a latency issue for some types of client or some types of link, maybe we should try to make a low-latency answer available from the server without changing the client behavior at all. [Ted Lemon] Some people have made a lot of what I would consider a misreading of RFC2131 - that INIT-REBOOT is optional. I believe it's optional because some hosts may not have stable store, not because hosts that *do* have stable store should ever skip INIT-REBOOT. I think that in order to get good behavior in the case where we have flapping going on on an 802.11 link, it's going to require a compromise on one of three directions. The choices are (remember, this is just for 802.11): - Try to do it right. This means that when we see a change, we look for a new configuration but don't immediately throw out the old, in the sense that we keep the old address configured alongside the new, and only kill it off after a timeout has passed. I don't think that anybody is going to implement this, and I'm not seriously proposing it - just wanted to mention it. - Compromise in the direction of switching quickly. This means that when we detect a possible change in attachment, we assume that the old connection is gone and never coming back. So we throw it away immediately and switch. MacOS X does this now. My wife Andrea was unable to get her email for quite a while this afternoon as a result, because she has a Titanium, which has, shall we say, antenna issues. She kept losing her access point in the middle of downloading her email, so she got the same ten messages quite a few times before she gave up. - Compromise in the direction of switching slowly. AFAIK nobody does this now. What this means is that when we notice a network transition, we keep it in mind, but don't try to reconfigure immediately - we wait 90 seconds, long enough for any active TCP sessions to time out. If, during that timeout period, the old network comes back, we continue using it. After this period has passed, we give up and move to the new network. This is a point solution for 802.11. I think that aggressive switching probably works fine for ethernet and similar media, because a change in media is usually a physical process. The solutions proposed in DNAv4 having to do with not trying to reconfigure until we actually detect a new link, as opposed to the loss of the old link, make a lot of sense - this means that if the switch is power cycled, we don't lose our address, but if we're plugged into a different switch, we get a new address quickly. [Mark Stapp] Couldn't quite tell from this whether you agreed with me that the text in the draft was ... too general. Like your wife, I've occasionally had 802.11 troubles, and I'd be interested in providing some guidelines that would make it easier for an 802.11 client to deal with those troubles better. I don't read INIT-REBOOT to mean 'throw out your configuration.' I've read it as 'try to confirm that your configuration is still valid'; there are lots of ways of optimizing the confirmation process. The dna draft proposes optimizing it by avoiding it - skipping the confirmation message based on some heuristic. The heuristic that's in the text isn't restricted to 802.11 clients, and isn't time-based or damped. The draft also doesn't discuss interactions between L2 (which notices that the link has gone down and been reconnected), the IP stack, and the DHCP client, and I agree with you that users are most likely to notice a transient link problem if it takes down their TCP connections. I'd like to distinguish 'reconfiguration' from 'confirmation'. I agree that helping 802.11 clients avoid unnecessary reconfiguration is desirable, and I'd support text that made specific recommendations for those clients. But the -04 draft loses the benefits of confirmation in too many situations. _______________________________________________ dhcwg mailing list dhcwg@ietf.org https://www1.ietf.org/mailman/listinfo/dhcwg
- [dhcwg] Proposed resolution to DNA issue 11: Reac… Bernard Aboba