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

From: "Khosravi, Hormuzd M" <[email-masked]>
Date: Tue, 6 Apr 2004 00:01:57 -0700

Hi Jamal,

Sorry for the delay, see my comments below...

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

This text does not talk about state synchronization or restarts
or rollovers. Those details are considered solvable if the
discussion below is resolvable.

Two levels of HA  responsibilities that i see:

1) TML level:

a) The TML controls logical connection availability and failover. 
[HK] Agreed

b) The TML also controls peer HA managements.
[HK] Not sure what you mean here but any state synchronization needed
should be part of the PL.

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

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 ?

For all three schemes, a heartbeat that terminates at the TML layer 
is needed for meeting requirement 1). The are three schemes i can
think of which i will list here:

Scheme 1) Le brute force:
The FE TML is aware of the multiple CEs i.e knows their logical
connections. 
It may receive commands from all the CEs but will process the one with 
highest priority only in given time instances.

Protocol requirement: To have priorities in the headers. We already do.

Advantages: KISS. 
Disadvatanges: too much traffic

Scheme 2) the Tenor:
The FE TML is aware of all CEs controlling it however responds to only
one at any one point. 
Something akin to HA as achieved by SCTP multihoming or some
mobility protocols. i.e at some point the current known master is
obsoleted for example because of some missing heartbeats or physical
disconnect or by explicit instructions to migrate. 

Protocol requirement: 
A "move" command to instruct to a migration to a new CE being issued 
to the FE. 

Advantages: less traffic
Disadvantages: slower in recovery compared to 1).

Scheme 3) the soprano:
The FE is NOT aware of the CEs unlike 1 and 2 above.
The election of which CE is controlling an FE is done elsewhere
at the CE level.
At any point the controlling CE will use a logical address.
This makes it transparent to the FE how many CEs are behind the control
point. Very easily doable with a broadcast or multicast protocol.
Example protocol which does this is VRRP.

Protocol requirement: None, the TML is burdened with providing either
multicast or broadcast.

In terms of compute requirements on the FE a) > b) > c)
In terms of bandwidth requirements on the wire a > b > c

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---

From: Jamal Hadi Salim <[email-masked]>
Date: 06 Apr 2004 14:06:02 -0400

Hi Hormuzd,

> should be part of the PL.

In some of the schemes i mentioned this is true.

> [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

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).

I have listed the advantages and disadvantages below which i will leave
in for clarity.
But also refer to #1 and #2 above since they form the basis.

> In terms of bandwidth requirements on the wire a > b > c

cheers,
jamal