Subject: [Sigtran] message distribution at ASP

From: Dario <[email-masked]>
Date: Wed, 30 May 2007 09:56:36 -0700 (PDT)

Hi,

I have a simple question regarding message distribution at ASP.

The RFC 4666 says about message distribution the following:

- at SGP (section 1.4.2.4.): "....by comparing elements of the incoming SS7
message to currently defined Routing Keys in the SGP"

- at ASP (section 1.4.2.5.): "by observing the Destination Point Code (and
possibly other elements of outgoing message, such as the SLS value). The ASP
must also take into account whether the related Routing Context is active or
not"

Do these descriptions converge to the same thing?

Here is one example where I am unsure:
An ASP processes traffic for two ASs:

AS1 ~ RK1 = (DPC X, OPC Y, SI=3)
AS2 ~ RK2= (DPC X, OPC Y, SI=5)

ASP is connected to two SGs, namely SG1 and SG2.
Both SGs have available connection to SS7 destination Y.

ASP is registered and active for AS1 and AS2 at SG1.
ASP is registered and active for AS1 at SG2.

My question is the following:
how message with routing label DPC=Y, OPC=X, SI=5 can be routed from ASP:

a) if we observe Destination PC only, it can be send to either SG1 or SG2
b) if we observe Routing Label and match it to AS, it can be sent over SG1
only 

what is the correct distribution?

thanks,
Dario
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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 30 May 2007 15:26:20 -0600

Dario,

Dario wrote:                                                                         (Wed, 30 May 2007 09:56:36)
> what is the correct distribution?

Section 1.4.2.4 is for distributing messages from the SGP to an ASP.  Section
1.4.2.5 is for distributing messages from the ASP to the SGP, which is your
case.  Also note that in Section 1.4.2.5 it says, as you quote, the
"Destination Point Code (and possibly other elements of the outgoing message,
such as the SLS value)."   Note that SI is an element of the outgoing message.

In SS7 the local network, PC and SI value are associated with an MTP-SAP and thus
with a specific MTP-User (ISUP in your case).  All the messages you describe
are associated with AS2.  Therefore, the ASP should send the message to an SGP
for which it is fully active for AS2 (otherwise, it will risk message loss).
When active for multiple SGP within the same SG, it is typically to loadshare
over the SGP by SLS value, or to mirror the traffic mode of the AS, such as
sending the message to an SGP for which the ASP is active in Active/Standby
mode).  When there are multiple SG via which the message can be routed, as in
your AS1 above, the selection between SG for which the ASP is active should
normally be based on SLS value (to avoid requiring multiple SG acting as STPs
to pass message over C-Links under non-failure conditions).  For multiple SG,
the availability of the intended destination via the SG should also be
considered: see the implementation note under Section 1.4.2.5.

--brian

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Brian F. G. Bidulock
[email-masked]
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From: Dario <[email-masked]>
Date: Wed, 30 May 2007 15:05:59 -0700 (PDT)

Hi Brian,
thanks a lot. what I had in mind is that for distribution, routing label of
outgoing message should match the Routing Key,and then the message should ne
sent to the remote M3UA peer where related AS is active
(if LSH applies, implementation dependent function, usually based on SLS
will distribute the message among active peers).
hope I got it right.
best regards, Dario

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 30 May 2007 16:24:27 -0600

Dario,

Dario wrote:                                    (Wed, 30 May 2007 15:05:59)
> will distribute the message among active peers).

Yes, matching against the SGP Routing Key with PC fields reversed is one
way to go about routing messages from the ASP.  For some
implementations, however, messages generated by the process associated
with the ASP on a handle (e.g. file descriptor) associated with the AS
inherently implies (by MTP-SAP) a routing key match of this form.  For
example, if at the ASP your AS2 is associated with a file descriptor
associated with trunk groups between X and Y, and the AS application
(ISUP) only sends messages associated with circuits in that trunks group
on that file descriptor, then there is no need to introduce an
extraneous and additional match against the RK.  We took some care in
the preparation of the M3UA approach to equally allow both of these
valid implementation approaches: one requiring a handle for each AS, the
other needing only one handle for all AS, and then matching reverse-RKs
within the M3UA layer at the ASP.  In the first case, consider it as
though the RC is bound to the handle.  In the second case, it is not.
Of course, both approaches have their advantages and disadvantages in
real implementations.  (Q.701 too discusses both approaces to the MTP
MTP/User interface.)

In allowing the designer this freedom, in some places the language
might appear vauge, or at least to skirt the matter of RK matching or
RC binding at the ASP.  It helps to bear in mind that the is a 1:1:1
replationship between AS:RC:RK.  That is, given the RC, the RK is known;
given the RK, the RC is known.

--brian

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Brian F. G. Bidulock
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