RE: DUNA applies to SG as a whole

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 protected] [mailto:[email protected]] 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