Subject: m2ua loss of association

From: [email-masked]
Date: Tue, 20 Nov 2001 17:03:35 -0500

All,

General question about the handling of the loss of association between the 
SG and the ASP. 

Scenario is for example:

STP connected to SG with SG M2UA connected to the AS, everything is active 
and handling traffic. The SCTP association fails while the MTP-2 link is 
in service. Several possible handlings on the SG may be made, the MTP-2 
link is taken out of service on the SG or the MTP-2 goes into a local 
processor outage mode. On the AS, several handlings are also possible, the 
MTP-3 gets an indication of RPO or gets an OOS indication.

It seems for purposes of interoperability that the handling on both the AS 
and the SG should be consistent (e.g., if one side handled as an OOS and 
the other as a PO, the link would never come back into service in certain 
scenarios).

I would propose the use of the OS on the SG (ie, taking the links down 
when the association fails) because the PO mechanisms are more complex 
(short term and long term handling) and this is expected to be an 
extremely rare situation not justifiying the additional logic. This would 
result in an immediate COO from the STP with the resulting retrieval after 
exchange of mesages, noting that because the association is down there is 
no retrieval of messages which are possibly in the TB on the SG (i.e, the 
MTP3 would not be able to get the BSNT and would force a hard changeover). 
Similarly on the AS, if the MTP-3 received an OOS for the link when the 
association failed it would initiate a changeover (can not do retrieval - 
so some loss of messages). This would then be followed by restoration in 
which the MTP-3 would continue to align the link through start messages 
which would result in the continuous attempts to bring the association 
back up.

Comments?? Should this be captured in the spec?

Regards,

Seamus

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 21 Nov 2001 01:24:02 -0600

Seamus,

[email-masked] wrote:              (Tue, 20 Nov 2001 17:03:35)
> MTP-3 gets an indication of RPO or gets an OOS indication.

This is completely consistent with SS7.  Once MTP L3 gets a PO for an
link it cannot know its status until it recovers.  The link may actually
be OOS.

> the other as a PO, the link would never come back into service in certain 

I couldn't disagree more.  We had several months of discussion on this
and came to the conclusion that how this is to be treated is
implementation dependent.  M2UA certainly does not need to (and must
not) describe what the peers are to do when there is no longer
communication between them.

> extremely rare situation not justifiying the additional logic.

That depends on the implementation, as well as performance and
reliability considerations.

> This would result in an immediate COO from the STP with the resulting
> retrieval after exchange of mesages, noting that because the association is
> down there is no retrieval of messages which are possibly in the TB on the
> SG (i.e, the MTP3 would not be able to get the BSNT and would force a hard
> changeover). 

Another implementation assumption.  M2UA permits implementations where
BSNT and retrieval are possible even though the association is gone
(buffered on the AS side).  Immediate changeovers might not be desirable
in implementations that are subject to brief association loss or
switchover.  One of the possible implementation methods for handling
SCTP congestion is also processor outage.

> so some loss of messages).

Again, M2UA permits implementations that can do both retrieval and FSNC
even though the association is lost.  And again, if switchover or
restoration of the assocation is possible in a short period, performance
and reliability considerations will lead some implementations to use the
LPO facility as provided in MTP rather than failing the link and
increasing the occurence of changeover.

> This would then be followed by restoration in which the MTP-3 would continue
> to align the link through start messages which would result in the
> continuous attempts to bring the association back up.

If the implementation has a hope (configuration dependent) of bringing
the association up within T1, there is no reason to perform a
changeover.

> Comments?? Should this be captured in the spec?

After long discussions in the WG, implementation dependent
considerations such as these were removed from the spec by concensus.
There is no general rule that fits all implementations, configurations,
operational considerations, reliability and performance concerns.  Even
examples are hard to provide without condideration for the implemenation
architecture.

The MTP protocol is fully capable of handling all combinations: OS/OS
OS/PO PO/PO PO/OS, as all combinations are possible in the SS7 protocol
today.  There are no interoperability concerns, just performance
considerations.  Even the SS7 specifications leave the selection of
procedure to the implementation based on local conditions with
consideration for the performance objectives that the implementation is
to meet.

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

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From: [email-masked]
Date: Wed, 28 Nov 2001 16:11:49 -0500

Brian,

I disagree with some of your comments on interoperability - in the past, 
SS7 implementations at the MTP2/MTP3 level have come from a single vendor 
and the handling between the MTP3 and the MTP2 even in a distributed 
environment has always been consistent though implementation specific. At 
this point gateway vendors look at producing products that may be coupled 
with MTP3 implementations from a number of vendors and the beauty of 
having a standard is that this is now a possibility and hence, the 
standard should define mechanisms which produce interoperability (this is 
totally independent of the ability of the MTP3 to handle OS/OS, OS/PO, 
etc..).

As previously mentionned it is possible to create scenarios within the 
MTP3/ M2UA/MTP2 that do not work and cannot be recovered from  - Hence, I 
think they should be specified to support the interoperability of vendors 
that produce M2UA gateways and those building STPs for example.

Seamus

"Brian F. G. Bidulock" <[email-masked]>
11/21/2001 02:24 AM
Please respond to sigtran

        To:     "sigtran" <[email-masked]>
        cc:     [email-masked]
        Subject:        Re: m2ua loss of association

Seamus,

[email-masked] wrote:              (Tue, 20 Nov 2001 
17:03:35)

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 28 Nov 2001 15:24:03 -0600

Seamus,

[email-masked] wrote:              (Wed, 28 Nov 2001 15:28:31)
> totally independent of the ability of the MTP3 to handle OS/OS, OS/PO, 

Can you propose text?

> that produce M2UA gateways and those building STPs for example.

Could you cite a scenario which "cannot be recovered from"?

--brian

> Please respond to bidulock
> 
>  
>         To:     [email-masked]
>         cc:     sigtran <[email-masked]>

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

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