Subject: RE: [Sigtran] DUNA applies to SG as a whole

From: "Daniel Cohn" <[email-masked]>
Date: Fri, 6 Jan 2006 11:35:34 -0600

Does this apply to M3UA as well?

The RFC (3332bis) seems to contain contradictory statements in this
regard.  Consider the following excerpt from 1.3.2.5:

   As shown in Figure 1 an ASP may be connected to multiple SGPs. In
   such a case a particular SS7 destination may be reachable via more
   than one SGP and/or SG, i.e., via more than one route. As MTP3 users
   only maintain status on a destination and not on a route basis, the
   M3UA layer must maintain the status (availability, restriction,
   and/or congestion of route to destination) of the individual routes,
   derive the overall availability or congestion status of the
   destination from the status of the individual routes, and inform the
   MTP3 users of this derived status whenever it changes.

and this excerpt from A.2.2:

   From the perspective of the M3UA layer at an ASP, a particular SG is
   capable of transferring traffic to a provisioned SS7 destination X if
   an SCTP association with at least one SGP of the SG is established,
   the SGP has returned an acknowledgement to the ASP to indicate that
   the ASP is actively handling traffic for that destination X, the SGP
   has not indicated that the destination X is inaccessible and the SGP
   has not indicated MTP Restart.  When an ASP is configured to use
   multiple SGPs for transferring traffic to the SS7 network, the ASP
   must maintain knowledge of the current capability of the SGPs to
   handle traffic to destinations of interest.

Also, the statement in 1.4.1 regarding coordination of states across
SGPs is worded as "SHOULD", which implies that other implementations are
possible.

To me this suggests that the ASP is responsible for maintaining the
status of each destination on a per-SGP basis.  Is it not possible for
an SGP to be partially isolated from the SS7 network, such that some SS7
destinations are reachable and others are not?  Since the SGP generally
operates in server mode, it cannot simply terminate its SCTP association
with the ASPs.  The ASPs will try to re-establish.  So, the only way it
can inform the ASPs that a particular destination is unreachable is via
the DUNA message.

The example redundancy model contained in A.1 suggests that SGPs are
processes within the same host.  However, this is a restrictive
definition, and I do not believe it is intended to be taken as normative
text.  In fact, SGPs are individually addressable, which suggests to me
that they can indeed exist on separate physical hosts.  In this case,
the isolation of one SGP from the SS7 network should not be taken to
mean that the whole SG is isolated.  Since there is no "SGP Inactive"
message, I see no other way to communicate this type of situation to the
ASP.

Comments?

Thanks,
Dan

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On
Behalf Of Brian F. G. Bidulock
Sent: Friday, January 06, 2006 10:37 AM
To: Ilie Glib
Cc: SIGTRAN
Subject: Re: [Sigtran] DUNA applies to SG as a whole

Ilie,

Ilie Glib wrote:
(Fri, 06 Jan 2006 10:39:00)
> Hello Folks
> 
> Let's consider an SG with multiple SGPs.
> 
> RFC 3868 says (1.2.2.  Terminology)
> 
>    Where an SG contains more than one SGP, the
>    SG is a logical entity and the contained SGPs are assumed to be
>    coordinated into a single management view to the SS7 network and to
>    the supported Application Servers.
> 
> I interpret this statement so that SGPs of the same SG have identical
> connectivity to SS7 NW, in particular DUNA, DAVA messages sent from
> one SGP apply to all SGPs of this SG. Thus if an SGP sends a DUNA to
> an ASP, the ASP assumes that Affected SPC is unavailable through the
> SG, and this applies to all SGPs in the SG.
> 
> section 3.4.1.  of SUA RFC says
> 
> 3.4.1.  Destination Unavailable (DUNA)
> 
>    The DUNA message
>    is sent from the SG or relay node to all concerned ASPs (servicing
>    SCCP-users considered local to the SG or relay node, see chapter
>    1.3.1.1), when a destination or SCCP-user has become unreachable.
The
>    SUA-User at the ASP is expected to stop traffic to the affected
>    destination or SCCP-user through the SG or relay node initiating
the
>    DUNA.
> 
> Here SG is mentioned. Thus in case of multiple SGPs DUNA applies to SG
> as a whole. The same is valid for DUPU, and DAVA.
> 
> Is this reading of the RFC statements correct?

Yes.

--brian

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---

From: Ilie Glib <[email-masked]>
Date: Fri, 6 Jan 2006 19:08:59 +0100

Dan,

On 1/6/06, Daniel Cohn <[email-masked]> wrote:
> with the ASPs.

[Ilie ] the SGP can send ASP-Inactive-Ack to go inactive, and answer
incoming ASPAC with an error "management blocking" to stay Inactive.
But this does not solve your concerns.

> The ASPs will try to re-establish.  So, the only way it

[Ilie ] I agree.

Regards

--
Ilie

---

From: "Tolga Asveren" <[email-masked]>
Date: Fri, 6 Jan 2006 13:07:59 -0500

Daniel,

> From: [email-masked] [mailto:[email-masked]]On
> Behalf Of Daniel Cohn
> Sent: Friday, January 06, 2006 12:36 PM
> To: [email-masked]
> Subject: RE: [Sigtran] DUNA applies to SG as a whole
>
>
[TOLGA]Yes.
> can inform the ASPs that a particular destination is unreachable is via
[TOLGA]SG is a single entity form MTP3 point of view. Actually this is
pretty much the practical meaning of SG, a group of SGPs with a unique MTP3
view.

> message, I see no other way to communicate this type of situation to the
[TOLGA]I agree that it makes more sense to have SGPs in different hosts to
provide host redundancy -BTW, I believe this is the case in Figure A-1 as
well, please note that SGP1.1. and SGP1.2 are part of SG1, and they reside
on different hosts-.

What I don't understand is why this causes a problem from current M3UA
procedures point of view. One has a distributed MTP3 implementation spanning
multiple hosts, which acts as a single entity. Each of those hosts are
different SGPs part of the same SG.

For the problem of "SGP being isolated from SS7 network":
a)A multi-SGP SG probably will have an internal messaging mechanism to
provide a unique MTP3-view. So, messages can be forwarded to another SGP of
the same SG, which have active links to SS7 network.
b)Like Ilie has suggested, ASPIA-ACK could be used.

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 6 Jan 2006 11:57:00 -0700

Daniel,

Daniel Cohn wrote:                           (Fri, 06 Jan 2006 11:35:34)
> can inform the ASPs that a particular destination is unreachable is via

The proper procedure for partial isolation is unsolicited ASP Inactive Ack
to affected AS.  Yes, the ASP will periodically attempt to reactivate the
AS, as it should.

Sending DUNA for partial isolation is incorrect: it tells all ASPs in the
AS that no SGP in the SG can reach the destination.

> message, I see no other way to communicate this type of situation to the

Unsolicited ASP Inactive Ack is effectively an "SGP Inactive" message.

--brian

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

---

From: Ilie Glib <[email-masked]>
Date: Fri, 6 Jan 2006 19:58:55 +0100

Dan,

Draft draft-ietf-sigtran-rfc3332bis-05.txt has the following statement

3.4.1 Destination Unavailable (DUNA)

   The DUNA message is sent from an SGP in an SG to all concerned ASPs
   to indicate that the SG has determined that one or more SS7
   destinations are unreachable.  It is also sent by an SGP in response
   to a message from the ASP to an unreachable SS7 destination. As an
   implementation option the SG may suppress the sending of subsequent
   "response" DUNA messages regarding a certain unreachable SS7
   destination for a certain period to give the remote side time to
   react. If there is no alternate route via another SG, the MTP3-User
   at the ASP is expected to stop traffic to the affected destination
   via the SG as per the defined MTP3-User procedures.

Thus I believe that in M3UA every SGP upon receiving a message towards
an unreachable destination sooner or later will answer with a DUNA to
the ASP. But of couse some traffic can be lost, which can be saved if
there is another SG with connectivity to the affected SPC.

Apropo, SUA RFC does not have a statement like "DUNA is also sent by
an SGP in response to a message from the ASP to an unreachable SS7
destination"
So I could imagine an implementation that sends DUNAs from one SGP only.

Regards

Ilie

--
Ilie

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 6 Jan 2006 12:25:13 -0700

Ilie,

Ilie Glib wrote:                                                      (Fri, 06 Jan 2006 19:58:55)
> So I could imagine an implementation that sends DUNAs from one SGP only.

What is sent to the MTP-User and what the MTP-User expects of the
MTP layer is specified in SS7 standards, not in M3UA.  The bis document
makes statements that are out of scope here and should have moved the
statement to the non-normative appendix.

An SG would be best generate DUNA to each ASP in each affected AS
under the following conditions:

a) In the case where the AS point code is hosted by the SG, DUNA is
   generated under the same conditions that MTP-PAUSE would be generated
   to local MTP-Users are the SG.

b) In the case of the multiple SG as STP configuration, DUNA is
   generated under the same conditions that TFP would be generated and
   sent to the AS point code, were the AS point code a normal SS7 SEP.

Understand?

--brian

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

---

From: Ilie Glib <[email-masked]>
Date: Mon, 9 Jan 2006 18:39:06 +0100

Brian

thank you very much indeed for your kind help and patience

Ilie