Subject: [Forces-protocol] topic 7: HA

From: Jamal Hadi Salim <[email-masked]>
Date: 24 Mar 2004 09:15:10 -0500

Heres something to reduce the noisy silence ;->

cheers,
jamal

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. 
b) The TML also controls peer HA managements.

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.

The TML layer
-------------
To describe the detail,  a good example would be  to show multiple
CEs trying to control an FE.

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

The PL Layer
------------
The PL layer provides HA management of the LFBs and their availability.

Scheme 1)
The PL heartbeats terminate at the TML layer.
The TML layer aggregates info on its PLs into its own heartbeats
to its peers.
The TML peer uses the knowledge to make calls on failover. 

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