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On 11/27/2012 07:50 PM, Gary Kotton wrote:
> On 11/27/2012 06:06 AM, Mark McClain wrote:
>> All-
>>
>> I wanted to continue the discussion from the today's meeting about the L3 agents.  The two proposed solutions take different approaches, so I think we first should agree on what we're trying to solve: scaling or availability or both.
>>
>> Nachi and Yong call their proposal "scheduler", but really it is really a horizontal scale out model.  Scheduling is the means they've chosen to distribute the load.  While their solution scales out horizontally, it does not address fully availability.  Gary's proposal fronts the l3 services with a load balancing like service.  It addresses availability by using an active/standby setup, but does not cover what happens when vertical scaling maxes out due to too many tenant networks and/or routers to fit on a physical node.
>>
>> I think the answer is to do both by incorporating a combination of the two proposals.  The L3 and DHCP agents are different enough that we may not be able to find a universal solution and that's is ok.
>>
>> Lastly, deployers have different SLAs and may even have different SLAs for different tenants, so we need to make sure we have a foundation for vendors and deployers to meet their varying SLAs.
>>
>> Thoughts?
>
> It would be great if we could have a universal solution. I feel that 
> due to the different roles of the services this will be very 
> challenging to achieve. I'll try and explain in more detail below.
>
> *DHCP agents*:
> At the moment each DHCP agent is able to allocate the IP address for a 
> specific mac address. Each agent has this information as it is 
> received from the notifications from the Quantum service. The problems 
> with the DHCP agent are as follows (please feel free to add or remove):
> i. For each networking providing DHCP services (currently only 
> implemented by dnsmasq in Quantum) a dnsmasq process is created by the 
> agent. This is problematic when the number of networks is large.
> ii. When interacting with Nova firewall rules are created to enable 
> the traffic to arrive from the DHCP server to the VM. This is 
> problematic if the DHCP agent terminates and the VM wishes to renew a IP.
>
> Originally I suggested that we use a load balancer to distribute the 
> traffic amongst the DHCP agents. Sadly this is not relevant for two 
> reasons:
> i. HA proxy does not have UDP support. This would have enabled a 
> virtual IP address for the DHCP server => no changes to the nova 
> rules. The load balancer would have detected if agents were down and 
> redirect traffic to agents that are up.
> ii. It does not address point #ii above. I suggested to have a flag or 
> configuration variable for each agent that indicate a list of networks 
> that the agent can service.  This will enable the agent to limit the 
> resources that can be consumed on a specific host. Naturally the devil 
> is in the details on how one can go about this if it is relevant.
>
> I think that if we had the "supported list of networks" configurable 
> for the DHCP agents then the vendor can deploy as many DHCP agents as 
> she/he wishes. I would prefer that this information is not on the 
> Quantum service but locally on the agents. This will offer a solution 
> for scale and high availability of DHCP resources.
Why do we need supported list of networks on dhcp agents? Do u want some 
agents have the same network data, right? If this is the case, we need 
to modify our dhcp agents. we will create as many as dhcp server ports 
on one network. (This is not too bad)
My design will use scheduler to distribute networks among dhcp agents.  
So if agents have configured "supported list of networks", we can 
schedule these networks to these agents. But now, it is a list of agents 
which can host one network at the same time. In addition to the 
'supported list of networks', we can support 'supported list of tenants' 
scheduling hint configured on agents. With tenants hint, quantum 
scheduler will schedule the networks of a given list tenants to this agent.

>
> Only problem is the ensuring that the DHCP traffic gets to the VM :). 
> I do not think that it is feasible to update the hosts each time with 
> a rule for a new DHCP agent that is added. One option to consider to 
> to rewrite the source IP of the traffic sent from the DHCP agent. This 
> is essentially what is done by a load balancer.
For this problem, Dan said we will replace the dhcp anti-spoofing with 
our own quantum implementation. I think this is a chance.
So if user wants to use the HA configuration of dhcp server, quantum 
version must be used, otherwise, he cannot configure HA (i.e. more than 
one l3 agent serves one network), he can live with simple server side 
scheduling (scheduling network to only one dhcp agent)
>
> *L3 agents*:
> Problems here are:
> i. HA - what if a L3 agent goes down.
> ii. Scale - how can we deploy a number of l3 agents
> iii. Amount of firewall rules
>
> In the first case if the L3 agent goes down then someone accessing a 
> floating IP will be unable to access that IP. This is something that 
> is critical for anyone running a cloud.
>
> I have thought about a number of options but each has its shortcomings:
> i. L3 agents to run VRRP. This will enable l3 agents to work in an 
> active backup pair. This requires a number of changes to the agent. 
> Each agent will have the same configuration enabling them to treat 
> inbound and outbound traffic.
I like this way, But I don't how to implement. If it works, we can have 
same scheduling ways:
on l3 agents, we configure it to support a list of routers or routers of 
a list of tenants.
we will use same scheduling algorithm  as we use for dhcp agents
> ii. Transparent load balancing - HA proxy does not support this
> iii. Having the agents rewrite the destination MAC address of the 
> default GW (the l3 agent). This solves outbound traffic but inbound is 
> problematic
> iv. Running l3 agents on each host could ensure that the traffic 
> generated on those hosts has floating IP's. This would require us to 
> change the implementation of the l3 agents to only build firewall 
> rules for devices on the HOST.
I don't like multi-host way.
>
> None of the above deal with the firewall rules. This is something that 
> can be addressed in a similar way to the DHCP agent with the L3 agents 
> specifically indicating which routers it will support (this is already 
> implemented when namespaces are not supported)
>
> Thanks
> Gary
>
Thanks
Yong Sheng Gong
>> mark
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