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>But they will if we document it well, which is what Salvatore suggested.

I don't think this is a good approach, and it's a big part of why I started
this thread. Most of the deployers/operators I have worked with only read
the bare minimum documentation to get a Neutron deployment working and they
only adjust the settings necessary for basic functionality.

We have an overwhelming amount of configuration options and adding a note
specifying that a particular setting for DHCP leases has been optimized to
reduce logging at the cost of long downtimes during port IP address updates
is a waste of time and effort on our part.

>I think the current default value is also more indicative of something
you'd find in your house, or at work - i.e. stable networks.

Tenants don't care what the DHCP lease time is or that it matches what they
would see from a home router. They only care about connectivity.

>One solution is to disallow this operation.

I want this feature to be useful in deployments by default, not strip it
away. You can probably do this with /etc/neutron/policy.json without a code
change if you wanted to block it in a deployment like yours where you have
such a high lease time.

>Perhaps letting the user set it, but allow the admin to set the valid
range for min/max?  And if they don't specify they get the default?

Tenants wouldn't have any reason to adjust this default. They would be even
less likely than the operator to know about this weird relationship between
a DHCP setting and the amount of time they lose connectivity after updating
their ports' IPs.

>It impacts anyone that hasn't changed from the default since July 2013 and
later
(Havana), since if they don't notice, they might get bitten by it.

Keep in mind that what I am suggesting with the lease-renewal-time would be
separate from the lease expiration time. The only difference that an
operator would see on upgrade (if using the defaults) is increased DHCP
traffic and more logs to syslog from dnsmasq. The lease time would still be
the same so the downtime windows for DHCP agents would be maintained. That
is much less of an impact than many of the non-config changes we make
between cycles.

To clarify, even with an option for dhcp-renewal-time I am proposing, you
are still opposed to setting it to anything low because of logging and the
~24 bps background DHCP traffic per VM?

On Thu, Jan 29, 2015 at 7:11 PM, Brian Haley <brian.haley at hp.com> wrote:

> On 01/29/2015 05:28 PM, Kevin Benton wrote:
> >>How is Neutron breaking this?  If I move a port on my physical switch to
> a
> > different subnet, can you still communicate with the host sitting on it?
> > Probably not since it has a view of the world (next-hop router) that no
> longer
> > exists, and the network won't route packets for it's old IP address to
> the new
> > location.  It has to wait for it's current DHCP lease to tick down to
> the point
> > where it will use broadcast to get a new one, after which point it will
> work.
> >
> > That's not just moving to a different subnet. That's moving to a
> different
> > broadcast domain. Neutron supports multiple subnets per network
> (broadcast
> > domain). An address on either subnet will work. The router has two
> interfaces
> > into the network, one on each subnet.[2]
> >
> >
> >>Does it work on Windows VMs too?  People run those in clouds too.  The
> point is
> > that if we don't know if all the DHCP clients will support it then it's a
> > non-starter since there's no way to tell from the server side.
> >
> > It appears they do.[1] Even for clients that don't, the worst case
> scenario is
> > just that they are stuck where we are now.
> >
> >>"... then the deployer can adjust the value upwards...", hmm, can they
> adjust it
> > downwards as well?  :)
> >
> > Yes, but most people doing initial openstack deployments don't and
> wouldn't
> > think to without understanding the intricacies of the security groups
> filtering
> > in Neutron.
>
> But they will if we document it well, which is what Salvatore suggested.
>
> >>I'm glad you're willing to "boil the ocean" to try and get the default
> changed,
> > but is all this really worth it when all you have to do is edit the
> config file
> > in your deployment?  That's why the value is there in the first place.
> >
> > The default value is basically incompatible with port IP changes. We
> shouldn't
> > be shipping defaults that lead to half-broken functionality. What I'm
> > understanding is that the current default value is to workaround
> shortcomings in
> > dnsmasq. This is an example of implementation details leaking out and
> leading to
> > bad UX.
>
> I think the current default value is also more indicative of something
> you'd
> find in your house, or at work - i.e. stable networks.
>
> I had another thought on this Kevin, hoping that we could come to some
> resolution, because sure, shipping broken functionality isn't great.  But
> here's
> the rub - how do we make a change in a fixed IP work in *all* deployments?
> Since the end-user can't set this value, they'll run into this problem in
> my
> deployment, or any other that has some not-very-short lease time.  One
> solution
> is to disallow this operation.  The other is to fix neutron to make this
> work
> better (I don't know what that involves, but there's bound to be a way).
> Perhaps letting the user set it, but allow the admin to set the valid
> range for
> min/max?  And if they don't specify they get the default?
>
> > If we had an option to configure how often iptables rules were refreshed
> to
> > match their security group, there is no way we would have a default of
> 12 hours.
> > This is essentially the same level of connectivity interruption, it just
> happens
> > to be a narrow use case so it hasn't been getting any attention.
> >
> > To flip your question around, why do you care if the default is lower?
> You
> > already adjust it beyond the 1 day default in your deployment, so how
> would a
> > different default impact you?
>
> It impacts anyone that hasn't changed from the default since July 2013 and
> later
> (Havana), since if they don't notice, they might get bitten by it.
>
> -Brian
>
>
> >
> > 1. http://support.microsoft.com/kb/121005
> > 2. Similar to using the "secondary" keyword on Cisco devices. Or just
> the "ip
> > addr add" command on linux.
> >
> > On Thu, Jan 29, 2015 at 1:34 PM, Brian Haley <brian.haley at hp.com
> > <mailto:brian.haley at hp.com>> wrote:
> >
> >     On 01/29/2015 03:55 AM, Kevin Benton wrote:
> >     >>Why would users want to change an active port's IP address anyway?
> >     >
> >     > Re-addressing. It's not common, but the entire reason I brought
> this up is
> >     > because a user was moving an instance to another subnet on the
> same network and
> >     > stranded one of their VMs.
> >     >
> >     >> I worry about setting a default config value to handle a very
> unusual use case.
> >     >
> >     > Changing a static lease is something that works on normal networks
> so I don't
> >     > think we should break it in Neutron without a really good reason.
> >
> >     How is Neutron breaking this?  If I move a port on my physical
> switch to a
> >     different subnet, can you still communicate with the host sitting on
> it?
> >     Probably not since it has a view of the world (next-hop router) that
> no longer
> >     exists, and the network won't route packets for it's old IP address
> to the new
> >     location.  It has to wait for it's current DHCP lease to tick down
> to the point
> >     where it will use broadcast to get a new one, after which point it
> will work.
> >
> >     > Right now, the big reason to keep a high lease time that I agree
> with is that it
> >     > buys operators lots of dnsmasq downtime without affecting running
> clients. To
> >     > get the best of both worlds we can set DHCP option 58 (a.k.a
> dhcp-renewal-time
> >     > or T1) to 240 seconds. Then the lease time can be left to be
> something large
> >     > like 10 days to allow for tons of DHCP server downtime without
> affecting running
> >     > clients.
> >     >
> >     > There are two issues with this approach. First, some simple dhcp
> clients don't
> >     > honor that dhcp option (e.g. the one with Cirros), but it works
> with dhclient so
> >     > it should work on CentOS, Fedora, etc (I verified it works on
> Ubuntu). This
> >     > isn't a big deal because the worst case is what we have already
> (half of the
> >     > lease time). The second issue is that dnsmasq hardcodes that
> option, so a patch
> >     > would be required to allow it to be specified in the options file.
> I am happy to
> >     > submit the patch required there so that isn't a big deal either.
> >
> >     Does it work on Windows VMs too?  People run those in clouds too.
> The point is
> >     that if we don't know if all the DHCP clients will support it then
> it's a
> >     non-starter since there's no way to tell from the server side.
> >
> >     > If we implement that fix, the remaining issue is Brian's other
> comment about too
> >     > much DHCP traffic. I've been doing some packet captures and the
> standard
> >     > request/reply for a renewal is 2 unicast packets totaling about
> 725 bytes.
> >     > Assuming 10,000 VMs renewing every 240 seconds, there will be an
> average of 242
> >     > kbps background traffic across the entire network. Even at a
> density of 50 VMs,
> >     > that's only 1.2 kbps per compute node. If that's still too much,
> then the
> >     > deployer can adjust the value upwards, but that's hardly a reason
> to have a high
> >     > default.
> >
> >     "... then the deployer can adjust the value upwards...", hmm, can
> they adjust it
> >     downwards as well?  :)
> >
> >     > That just leaves the logging problem. Since we require a change to
> dnsmasq
> >     > anyway, perhaps we could also request an option to suppress logs
> from renewals?
> >     > If that's not adequate, I think 2 log entries per vm every 240
> seconds is really
> >     > only a concern for operators with large clouds and they should
> have the
> >     > knowledge required to change a config file anyway. ;-)
> >
> >     I'm glad you're willing to "boil the ocean" to try and get the
> default changed,
> >     but is all this really worth it when all you have to do is edit the
> config file
> >     in your deployment?  That's why the value is there in the first
> place.
> >
> >     Sorry, I'm still unconvinced we need to do anything more than
> document this.
> >
> >     -Brian
> >
> >
> >
> >
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> >
> >
> > --
> > Kevin Benton
> >
> >
> >
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-- 
Kevin Benton
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