Subject: [Sigtran] Another Congestion/SCON Question

From: [email-masked]
Date: Wed, 4 Dec 2002 09:06:12 -0500

All:

I've come across an issue I'd like some clarification and/or further
opinions.

If the latest version of the implementor's guide, para. 3.9.2 states that
where the SGP maintains the congestion status of a destination, the SGP
MUST additionally respond with an SCON before the DAVA/DRST message.  It
also states that where the SGP does not maintain the congestion status of a
destination, the response to a DAUD should always be only a DAVA/DRS/DUNA
(no SCON).

An agreement reached at the plugtest was when congestion for a concerned
destination has abated, the SGP must send an SCON with congestion status of
0 (whether or not the SGP maintains status) in response to an audit.

This agreement seems to run counter to what the guide states.  Is the SGP
supposed to always maintain a destination's congestion status?

Opinions, comments or further clarifications?

Regards,
Mark Erickson

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 4 Dec 2002 12:23:26 -0700

mark.erickson,

[email-masked] wrote:                                             (Wed, 04 Dec 2002 09:06:12)
> 0 (whether or not the SGP maintains status) in response to an audit.

There are several problems with that:

1) MTP is not always capable of determining abatement, particularly when
   congestion has resulted from attempting to send a message on local link.
   In this case, the SG does not maintain state (as the response is
   statistical in nature) and it cannot signal abatement.

2) There is no primitive to signal abatement between MTP and it users.
   MTP-STAUS is only used to signal congestion and never abatement.
   Therefore, the NIF could likely have no mechanism to signal abatement to
   M3UA.

3) For local link congestion, MTP does not remember to which users it has sent
   congestion, and because it does not send abatement, does no know to which
   users it would send abatement if if one wanted to try to make it do so.

Because of the above, "the SGP is only required to send SCON in response to
DAUD where it maintains congestion state" was meaning that where MTP maintains
congestion state to its users, SCON is required in response to DAUD.  MTP does
not maintain congestion state for local link congestion and therefore is can
never really be required to send SCON in response to DAUD.  But when SCON is
sent it MUST be sent before DAVA/DRST to avoid the period where the AS might
think that a destination was uncongested briefly otherwise.

So, when the SGP does not maintain state (and most MTP's do not maintain
congestion state to their users) it is not required to respond with SCON
in response to DAUD, and cannot be expected to signal abatement with
SCON 0 either.

It is certainly obvious that an SG MAY send SCON 0 to signal abatement;
however, it cannot be required of an SG, just as the SG MUST send SCON
in response to DAUD (but only if it maintains state to its users, which
most MTPs do not).

The inconsistency is in sending SCON 0 to signal abatement: it cannot be
required of an SG if it does not maintain congestion state to its users.

--brian

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

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From: Tolga Asveren <[email-masked]>
Date: Wed, 4 Dec 2002 11:31:19 -0800 (PST)

Hi Mark,

The scenario you are asking is not clear to me, if SGP
does not keep congestion status(in fact it always
keeps, just for ITU it updates congestion status only
based on local link congestion status but not with
received TFC), then how can it know at a given time
(when DAUD is received from ASP) congestion status for
a destination? Or are you referring to the case, where
SGP genereates RCT on behalf of ASP, and want
clarification, what should be done, when SGP decides
that congestion abates based on congestion test
procedure? In such a case, SGP should send SCON(0) to
the relevant ASP. 

      Tolga

> Opinions, comments or further clarifications?

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