Subject: RE: [Forces-protocol] topic 7: HA

From: "Khosravi, Hormuzd M" <[email-masked]>
Date: Tue, 6 Apr 2004 13:51:29 -0700

Hi Jamal,

-----Original Message-----
From: Jamal Hadi Salim [mailto:[email-masked]] 

> 2) PL Level:
> 
> Depending on the scheme implemented by the TML level, the PL
> level may have no role to play i.e HA being transparent to it.
> [HK] There might be a role for synchronization

Yes, this is true for some of the schemes but not all. More below.

> The TML layer
> -------------
> To describe the detail,  a good example would be  to show multiple
> CEs trying to control an FE.
> 
> [HK] The schemes below seem to be different ways of synchronizing
state
> between CEs and FEs, correct me if I am wrong ? Some of this
> intelligence should be part of the PL rather than everything being
part
> of TML. Do you agree ?

Theres two things: 1) The management of the links/locgical
links/channels etc. This clearly belongs in the TML.
2) If you assume #1 above then next thing is to ask if you can take
advantage of that detail in synchronization - this is what the text
below attempts to do. For example, the FE PL may be totaly unaware of
the fact that there is another FE involved as its backup or that there
is a HA a pair of CEs in HA mode controliing it (as in case 3 which i
labeled soprano).

[HK] I agree with #1, here what you mean is things like keep-alive need
to be taken care of by the TML. I also agree with the example you have
given for #2 that the FE PL doesn't need to be aware of the fact that
there are 2 CEs in HA mode controlling it. However, don't you think the
CE, CE PL should have some of this intelligence and instruct the CE TML
how it should behave ? How would the TML figure this out ? 

Let me know what you think.
May be if we work through a complete use-case, this will be more clear.

Regards
Hormuzd

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---

From: Jamal Hadi Salim <[email-masked]>
Date: 07 Apr 2004 07:41:45 -0400

Hormuzd,

On Tue, 2004-04-06 at 16:51, Khosravi, Hormuzd M wrote:
> to be taken care of by the TML. 

Also the failover of those links. Of course keepalive is one scheme for
detecting need for failover, but there could be notifications either
from the PL (note the "move" example i gave) which could force such a
connection failover.

> I also agree with the example you have

Not sure i see the CE-PL involved (but not ruling it out); 
the TML, yes - perhaps via an admin interface like CEM.

I think that CEs working together in HA mode to control an FE may need
to have a CE-CE communication to decide which is the master for that FE.
This is what i described in scheme #3 - i refered to it as an election
process. There are several schemes that could use the result of the
election at the CE:

1) The TML is NOT aware of the election results. An example of such a
method is what VRRP uses and is described in scheme #3. In such a
scenario, an IP or MAC address is taken over by a master CE. The master
FE is unaware since at the TML layer it continues to receive from the
same address.
2) The #2  scheme.  
After the election, all the participating CEs TMLs are told about the
result via some "move" protocol instruction.
I think from the CE side, this is probably the better approach.
We should probably strive to make the FE side very simple and move any
required complexity to the CE; so from that perspective, something like
a move instruction at the CE which takes over all the logical and
physical addressing may be a better approach.
On the FE side, i would even go as far as saying to keep it simple,
an FE being backed up should not be aware of its backup.

> May be if we work through a complete use-case, this will be more clear.

Note, i presented the three schemes in the x-CEs to single FE in the
first email. Is this sufficient for that case?
What do you have in mind?

cheers,
jamal