[openstack-dev] [Quantum] continuing todays discussion about the l3 agents

Vinay Bannai vbannai at gmail.com
Sun Dec 2 18:29:13 UTC 2012

My understanding of the "scheduler" approach based on what I read on the
ML's is to have a mechanism where the DHCP agents can land on different
nodes. For example, just like we have compute hosts in nova, we have a
bunch of DHCP capable hosts (and L3 capable hosts etc) that can be selected
to host the network service for a tenant when the network/subnet is
created. The process of selecting the host to run the service is based on a
"scheduler". This allows a graceful horizontal scaling. This approach is
similar to what nova does. You have a bunch of hosts capable of providing a
network service and the "scheduler" picks them based on filters and other
tunable knobs. I think you already know this:-). I  was spelling it out so
that you can see where I am coming from.

Either way we look at it, I think it will be helpful if we decoupled the
horizontal (scaling to multiple nodes) and vertical scaling (redundancy and
failover). One should not imply the other. In your last paragraph, you
mention "orchestration tool" and dhcp agents configured to handle specific
networks. I have not been able to wrap my head around this completely but
it appears to b ea different variant of the "scheduler" approach where it
is configured manually. Is my understanding correct? Or if you don't mind,
can you elaborate further on that idea.


On Sun, Dec 2, 2012 at 7:16 AM, Gary Kotton <gkotton at redhat.com> wrote:

>  On 12/01/2012 03:31 AM, gong yong sheng wrote:
> On 12/01/2012 07:49 AM, Vinay Bannai wrote:
> Gary and Mark,
>  You brought up the issue of scaling horizontally and vertically in your
> earlier email. In the case of horizontal scaling, I would agree that it
> would have to be based on the "scheduler" approach proposed by Gong and
> Nachi.
> I am not sure that I understand the need for a scheduler when it comes to
> the DHCP agent.  In my opinion this is unnecessary overhead and it is not
> necessarily required.
> Last week Mark addressed the problem with all of the DHCP agents all
> listening on the same message queue. In theory we are able to run more than
> one DHCP agents in parallel. This offers HA at the expense of an IP per
> DHCP agent per subnet.
> I think that for the DHCP agents we need to look into enabling the DHCP
> agents to treat specific networks. This can be done in a very rudimentary
> way - have a configuration variable for the DHCP agent indicating a list of
> networks to be treated by the agent. A orchestration tool can just
> configure the network ID's and launch the service - then we will have
> scalable and highly available DHCP service. I would prefer not to have to
> add this into the Quantum API as it just complicates things.
>  On the issue of vertical scaling (I am using the DHCP redundancy as an
> example), I think it would be good to base our discussions on the various
> methods that have been discussed and do pro/con analysis in terms of scale,
> performance and other such metrics.
>  - Split scope DHCP (two or more servers split the IP address and there
> is no overlap)
>   pros: simple
>   cons: wastes IP addresses,
>  - Active/Standby model (might have run VRRP or hearbeats to dictate who
> is active)
>   pros: load evenly shared
>   cons: needs shared knowledge of address assignments,
>             need hearbeats or VRRP to keep track of failovers
> another one is the IP address waste. we need one VIP, and 2+ more address
> for VRRP servers. ( we can use dhcp server's ip if we don't want to do load
> balancing behind the VRRP servers)
> another one is it will make system complicated.
>  - LB method (use load balancer to fan out to multiple dhcp servers)
>   pros: scales very well
>   cons: the lb becomes the single point of failure,
>            the lease assignments needs to be shared between the dhcp
> servers
>  LB method will also wast ip address. First we at lease need a VIP
> address. then we will need more dhcp servers running for one network.
> If we need to VRRP the VIP, we will need 2+ more addresses.
> another one is it will make system complicated.
> I see that the DHCP agent and the quantum server communicate using RPC. Is
> the plan to leave it alone or migrate it towards something like AMQP based
> server in the future when the "scheduler" stuff is implemented.
> I am not very clear your point. But current RPC is on AMQP.
>  Vinay
>  On Wed, Nov 28, 2012 at 8:03 AM, Mark McClain <mark.mcclain at dreamhost.com
> > wrote:
>> On Nov 28, 2012, at 8:03 AM, gong yong sheng <gongysh at linux.vnet.ibm.com>
>> wrote:
>> > On 11/28/2012 08:11 AM, Mark McClain wrote:
>> >> On Nov 27, 2012, at 6:33 PM, gong yong sheng <
>> gongysh at linux.vnet.ibm.com> wrote:
>> >>
>> >> Just wanted to clarify two items:
>> >>
>> >>>> At the moment all of the dhcp agents receive all of the updates. I
>> do not see why we need the quantum service to indicate which agent runs
>> where. This will change the manner in which the dhcp agents work.
>> >>> No. currently, we can run only one dhcp agent  since we are using a
>> topic queue for notification.
>> >> You are correct.  There is a bug in the underlying Oslo RPC
>> implementation that sets the topic and queue names to be same value.  I
>> didn't get a clear explanation of this problem until today and will have to
>> figure out a fix to oslo.
>> >>
>> >>> And one problem with multiple agents serving the same ip is:
>> >>> we will have more than one agents want to update the ip's leasetime
>> now and than.
>> >> This is not a problem.  The DHCP protocol was designed for multiple
>> servers on a network.  When a client accepts a lease, the server that
>> offered the accepted lease will be the only process attempting to update
>> the lease for that port.  The other DHCP instances will not do anything, so
>> there won't be any chance for a conflict.  Also, when a client renews it
>> sends a unicast message to that previous DHCP server and so there will only
>> be one writer in this scenario too.  Additionally, we don't have to worry
>> about conflicting assignments because the dhcp agents use the same static
>> allocations from the Quantum database.
>> > I mean dhcp agent is trying to update leasetime to quantum server. If
>> we have more than one dhcp agents, this will cause confusion.
>> >    def update_lease(self, network_id, ip_address, time_remaining):
>> >        try:
>> >            self.plugin_rpc.update_lease_expiration(network_id,
>> ip_address,
>> >                                                    time_remaining)
>> >        except:
>> >            self.needs_resync = True
>> >            LOG.exception(_('Unable to update lease'))
>> > I think it is our dhcp agent's defect. Why does our dhcp agent need the
>> lease time? all the IPs are managed in our quantum server, there is not
>> need for dynamic ip management in dhcp server managed by dhcp agent.
>>  There cannot be confusion.  The dhcp client selects only one server to
>> accept a lease, so only one agent will update this field at a time. (See
>> RFC2131 section 4.3.2 for protocol specifics).  The dnsmasq allocation
>> database is static in Quantum's setup, so the lease renewal needs to
>> propagate to the Quantum Server.  The Quantum server then uses the lease
>> time to avoid allocating IP addresses before the lease has expired.  In
>> Quantum, we add an additional restriction that expired allocations are not
>> reclaimed until the associated port has been deleted as well.
>> mark
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>  --
> Vinay Bannai
> Email: vbannai at gmail.com
> Google Voice: 415 938 7576
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Vinay Bannai
Email: vbannai at gmail.com
Google Voice: 415 938 7576
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