<p dir="ltr">If we are going to ignore the IP address changing use-case, can we just make the default infinity? Then nobody ever has to worry about control plane outages for existing client. 24 hours is way too long to be useful anyway. </p>
<div class="gmail_quote">On Jan 28, 2015 12:44 PM, "Salvatore Orlando" <<a href="mailto:sorlando@nicira.com">sorlando@nicira.com</a>> wrote:<br type="attribution"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On 28 January 2015 at 20:19, Brian Haley <span dir="ltr"><<a href="mailto:brian.haley@hp.com" target="_blank">brian.haley@hp.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Hi Kevin,<br>
<span><br>
On 01/28/2015 03:50 AM, Kevin Benton wrote:<br>
> Hi,<br>
><br>
> Approximately a year and a half ago, the default DHCP lease time in Neutron was<br>
> increased from 120 seconds to 86400 seconds.[1] This was done with the goal of<br>
> reducing DHCP traffic with very little discussion (based on what I can see in<br>
> the review and bug report). While it it does indeed reduce DHCP traffic, I don't<br>
> think any bug reports were filed showing that a 120 second lease time resulted<br>
> in too much traffic or that a jump all of the way to 86400 seconds was required<br>
> instead of a value in the same order of magnitude.<br>
><br>
> Why does this matter?<br>
><br>
> Neutron ports can be updated with a new IP address from the same subnet or<br>
> another subnet on the same network. The port update will result in anti-spoofing<br>
> iptables rule changes that immediately stop the old IP address from working on<br>
> the host. This means the host is unreachable for 0-12 hours based on the current<br>
> default lease time without manual intervention[2] (assuming half-lease length<br>
> DHCP renewal attempts).<br>
<br>
</span>So I'll first comment on the problem. You're essentially "pulling the rug" out<br>
from under these VMs by changing their IP (and that of their router and DHCP/DNS<br>
server), but you expect they should fail quickly and come right back online. In<br>
a non-Neutron environment wouldn't the IT person that did this need some pretty<br>
good heat-resistant pants for all the flames from pissed-off users? Sure, the<br>
guy on his laptop will just bounce the connection, but servers (aka VMs) should<br>
stay pretty static. VMs are servers (and cows according to some).<br></blockquote><div><br></div><div>I actually expect this kind operation to not be one Neutron users will do very often, mostly because regardless of whether you're in the cloud or not, you'd still need to wear those heat resistant pants.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<br>
The correct solution is to be able to renumber the network so there is no issue<br>
with the anti-spoofing rules dropping packets, or the VMs having an unreachable<br>
IP address, but that's a much bigger nut to crack.<br></blockquote><div><br></div><div>Indeed. In my opinion the "update IP" operation sets false expectations in users. I have considered disallowing PUT on fixed_ips in the past but that did not go ahead because there were users leveraging it.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<span><br>
> Why is this on the mailing list?<br>
><br>
> In an attempt to make the VMs usable in a much shorter timeframe following a<br>
> Neutron port address change, I submitted a patch to reduce the default DHCP<br>
> lease time to 8 minutes.[3] However, this was upsetting to several people,[4] so<br>
> it was suggested I bring this discussion to the mailing list. The following are<br>
> the high-level concerns followed by my responses:<br>
><br>
</span>> * 8 minutes is arbitrary<br>
> o Yes, but it's no more arbitrary than 1440 minutes. I picked it as an<br>
<span>> interval because it is still 4 times larger than the last short value,<br>
> but it still allows VMs to regain connectivity in <5 minutes in the<br>
> event their IP is changed. If someone has a good suggestion for another<br>
> interval based on known dnsmasq QPS limits or some other quantitative<br>
> reason, please chime in here.<br>
<br>
</span>We run 48 hours as the default in our public cloud, and I did some digging to<br>
remind myself of the multiple reasons:<br>
<br>
1. Too much DHCP traffic. Sure, only that initial request is broadcast, but<br>
dnsmasq is very verbose and loves writing to syslog for everything it does -<br>
less is more. Do a scale test with 10K VMs and you'll quickly find out a large<br>
portion of traffic is DHCP RENEWs, and syslog is huge.<br></blockquote><div><br></div><div>This is correct, and something I overlooked in my previous post. Nevertheless I still think that it is really impossible to find an optimal default which is regarded as such by every user. The current default has been chosen mostly for the reason you explain below, and I don't see a strong reason for changing it.</div><div> <br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<br>
2. During a control-plane upgrade or outage, having a short DHCP lease time will<br>
take all your VMs offline. The old value of 2 minutes is not a realistic value<br>
for an upgrade, and I don't think 8 minutes is much better. Yes, when DHCP is<br>
down you can't boot a new VM, but as long as customers can get to their existing<br>
VMs they're pretty happy and won't scream bloody murder.<br></blockquote><div><br></div><div>In our cloud we were continuously hit bit this. We could not take our dhcp agents out, otherwise all VMs would lose their leases, unless the downtime of the agent was very brief. </div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<br>
There's probably more, but those were the top two, with #2 being most important.<br></blockquote><div><br></div><div>Summarizing, I think that Kevin is exposing a real, albeit well-know problem (sorry about my dhcp release faux pas - I can use jet lag as a justification!), and he's proposing a mitigation to it. On the other hand, this mitigation, as Brian explains, is going to cause real operational issues. Still, we're arguing on the a default value for a configuration parameter. I therefore think the best thing that we can do is explicitly stating what happens when setting long or short lease times.</div><div>I expected this to be documented in [1], but it's not. I think that place and neutron.conf might contain this kind of documentation, such as:</div><div><br></div><div><div><font face="monospace, monospace"># DHCP Lease duration (in seconds). </font></div><div><font face="monospace, monospace"># Use<span style="white-space:pre-wrap"> -1 to</span> tell dnsmasq to use infinite lease times. <span style="white-space:pre-wrap"> </span></font></div><div><font face="monospace, monospace"># dhcp_lease_duration = 86400</font></div></div><div><font face="monospace, monospace"># Note that long DHCP leases will result in delays</font></div><div><font face="monospace, monospace"># in instances acquiring updated IP addresses. This</font></div><div><font face="monospace, monospace"># may result in downtime for those instance as anti</font></div><div><font face="monospace, monospace"># spoof policy will then block all traffic in and out of</font></div><div><font face="monospace, monospace"># them. In order to minimise this downtime window</font></div><div><font face="monospace, monospace"># the lease time should be shorter, for example</font></div><div><font face="monospace, monospace"># dhcp_lease_duration = 480</font><br></div><div><br></div><div>However, I would not change the current system default, as this might affect operational systems.</div><div><br></div><div>Apologies again for my stupid dhcp-release note,</div><div>Salvatore</div><div><br></div><div>[1] <a href="http://developer.openstack.org/api-ref-networking-v2.html" target="_blank">http://developer.openstack.org/api-ref-networking-v2.html</a></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
<br>
> * other datacenters use long lease times<br>
> o This is true, but it's not really a valid comparison. In most regular<br>
<span>> datacenters, updating a static DHCP lease has no effect on the data<br>
> plane so it doesn't matter that the client doesn't react for hours/days<br>
> (even with DHCP snooping enabled). However, in Neutron's case, the<br>
> security groups are immediately updated so all traffic using the old<br>
> address is blocked.<br>
<br>
</span>Yes, and choosing the lease time is a deployment decision that needs to take a<br>
lot of things into account. Like I said, we don't even use the default. The<br>
default should just be a good guess for a standard deployment, not a value that<br>
caters towards the edge cases, especially when the value is tunable in neutron.conf.<br>
<br>
> * dhcp traffic is scary because it's broadcast<br>
> o ARP traffic is also broadcast and many clients will expire entries every<br>
<span>> 5-10 minutes and re-ARP. L2population may be used to prevent ARP<br>
> propagation, so the comparison between DHCP and ARP isn't always<br>
> relevant here.<br>
<br>
</span>I don't recall anyone being scared of broadcast, and can't find any comments<br>
regarding it in <a href="https://review.openstack.org/#/c/150595/" target="_blank">https://review.openstack.org/#/c/150595/</a><br>
<span><br>
> Please reply back with your opinions/anecdotes/data related to short DHCP lease<br>
> times.<br>
<br>
</span>I can only speculate on why 24 hours was chosen as the default back in 2013,<br>
possibly because a lot of wireless router firmware defaults are set as such?<br>
<span><br>
> 1. <a href="https://github.com/openstack/neutron/commit/d9832282cf656b162c51afdefb830dacab72defe" target="_blank">https://github.com/openstack/neutron/commit/d9832282cf656b162c51afdefb830dacab72defe</a><br>
> 2. Manual intervention could be an instance reboot, a dhcp client invocation via<br>
> the console, or a delayed invocation right before the update. (all significantly<br>
> more difficult to script than a simple update of a port's IP via the API).<br>
> 3. <a href="https://review.openstack.org/#/c/150595/" target="_blank">https://review.openstack.org/#/c/150595/</a><br>
> 4. <a href="http://i.imgur.com/xtvatkP.jpg" target="_blank">http://i.imgur.com/xtvatkP.jpg</a><br>
<br>
</span>I was a much bigger baby than that :)<br>
<span><font color="#888888"><br>
-Brian<br>
</font></span><div><div><br>
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