Subject: reqs - failover support

From: Christopher Ross <[email-masked]>
Date: Fri, 26 Jan 2001 12:49:26 +0100

Dear all,

I've been thinking about how a failover would work when first detected
by the ENRP server.

I'm considering an example where we have two PEs, PE1 and PE2, which
somehow share state, and a PU requesting a service, starting up with
using PE1. The application I have in mind is handling of (voice)
calls, to keep things simple. So the PU uses PE1 to set up a call, and
call state is held at PE1, and by some means exchanged with PE2.
Assuming that the call reaches the state "talk" and soon after PE1
fails, what happens next? If we have no communication between the
server pool and the PU during the call in state "talk", we really need
the ENRP server to tell the PU to use PE2 to when PU wants to tear
down his call, rather than having the PU find out the hard way.

As far as I can make out, in the example 2.5.1 (see stage 12) it's the
PU who discovers the failure of PE(1,A), and who then switches over to
PE(1,B). This makes sense for the FTP example, but not in the case of
a failure during a stable call.

Has the communication from the ENRP server to the PU been catered for?

Thanks

Chris

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From: Qiaobing Xie <[email-masked]>
Date: Fri, 26 Jan 2001 15:21:45 -0600

> down his call, rather than having the PU find out the hard way.

The key is that the PU (to be exact, the A-SAP layer in the PU) knows
both PE1 and PE2 from its first ENRP query. When the PU tries to tear
down the call and finds PE1 is unreachable, it already has PE2 in its
local cache and only needs to simply redirect the "tear down" message to
PE2. And since PE2 is sharing state and hence knows the call, it will
perform the tear down correctly.

-Qiaobing