Subject: RE: [Sigtran] ASP load sharing..

From: "Mohan Parthasarathy" <[email-masked]>
Date: Thu, 12 Dec 2002 10:04:20 -0800

Brian,

Thanks for your answers.

> 
> > 
> > 3) If an ASP fails where an ASP itself could not indicate 
> to the SG but
> >     e.g. SCTP heartbeat indicates that an ASP failed, will 
> the messages be
> >     sent to the other ASPs because there was no ACK for the 
> messages ?
> 
> Sending unacknowledged messages can lead to duplication.  See 
> draft-bidulock-sigtran-corid-00.txt for an analysis of the 
> situation an a method using the Correlation Id parameter 
> which can avoid both duplication and message loss in this scenario.
> 
thanks for the pointer. Is there any mechanism for the responses to
go back to the same ASP ? On a distrbuted system, a request might
originate from one ASP and one would expect the response to go back
to the same ASP. Is there anything at the SUA level to turn off
load sharing so that it can be directed back to the same SCTP
association on which the request arrived ? 

> > 
> > 4) When an ASP fails, most likely it will not be able to 
> indicate to the
> >    SG that it failed. SCTP heartbeat may be slow to 
> discover that ASP failed.
> >    Is there a mechanism that can be used to indicate to the 
> SG that an ASP
> >    failed ?
> 
> No.
> 
Isn't this a limitation ? Don't you want to detect the ASP failures
much more faster than this so that the bad ASP can be reomved from
the active list soon ? 

-thanks
mohan

> --brian
> -- 
> Brian F. G. Bidulock
> [email-masked]
> http://www.openss7.org/
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From: Tolga Asveren <[email-masked]>
Date: Thu, 12 Dec 2002 11:00:05 -0800 (PST)

Mohan,

> much more faster than this so that the bad ASP can
[TOLGA] How can you do this fast enough? I think the
answer is you have a good IP network, and so adjust
your SCTP timeout values low. I don't think this is a
limitation of the protocol, afterall everyhting is
limited by the physicall infrastructure you have. 

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Thu, 12 Dec 2002 13:27:43 -0700

Mohan,

Mohan Parthasarathy wrote:                  (Thu, 12 Dec 2002 10:04:20)
> Brian,
> 
> Thanks for your answers.
> 
> > 
> > > 
> > > 3) If an ASP fails where an ASP itself could not indicate to the SG but
> > >     e.g. SCTP heartbeat indicates that an ASP failed, will the messages be
> > >     sent to the other ASPs because there was no ACK for the messages ?
> > 

For TCAP in SUA there is the concept of the TID Label which is
placed in the ASPAC and tells the SG which TID ranges to direct to
which ASPs in Loadshare AS.  (See 4.7.3.1)  However, this mechanism
was a last minute addition and did not receive sufficient discussion.
As such is somewhat broken for loadshare: it doesn't handle failure
cases very well at all and there is little to no description in the
RFC as to what should be done in ASP failure situations.  And of
course this does not work with non-TCAP Class 0/1 SCCP-Users or mixed
CONS/CLNS SCCP-Users such as BSSAP.  For BSSAP one would prefer to
loadshare on components of the Complete Level 3 Information.

A more general mechanism which permits the ASP to direct the SG as
to which loadsharing traffic flows it wishes to accept are described
in draft-bidulock-sigtran-loadsel-00.txt.  SUA examples for a
loadshare AS with load selection based on Global Title are described
in example in Section 5.3.  This is a generic mechanism which also
includes adding the Load Selection parameter into the NTFY message
and procedures so that the remaining ASPs in the pool can assume the
load of a failed ASP under ASP control (rather than the SG throwing
the traffic at whatever ASP it wants).

Even finer control can be achieved with load grouping on top of
load selection as defined in draft-bidulock-sigtran-loadgrp-00.txt.
> 
> > > 
> > > 4) When an ASP fails, most likely it will not be able to indicate to the
> > >    SG that it failed. SCTP heartbeat may be slow to discover that ASP failed.
> > >    Is there a mechanism that can be used to indicate to the SG that an ASP
> > >    failed ?
> > 
> > No.
> > 

It is the normal situation that the SG must detect the failure of
ASPs and notify the ASPs of the failure and not visa versa.  This is
perhaps the case because the SG, being an SS7 component is expected
to meet 5 or 6 9's while the ASP may be less reliable.

--brian