H.225- CRV

[Paul E. Jones]

Chris,

Considering that V1 only allowed a single call over a single TCP connection,
what would have been the point of a CRV in the Setup message?  If they could
all be the same, it makes them worthless.  (Of course, I've found CRVs in
packet-switched systems mostly worthless, but not entirely.)

>From H.323:
``One CRV is used to associate the call signalling messages. This CRV shall
be used in all call signalling messages between two entities (endpoint to
Gatekeeper, endpoint-to-endpoint, etc.) related to the same call.''

Note the text that says "between two entities".  If the intent was that
these values were only unique "per connection", it would say so.  The CRVs
are unique between two entities.

>From H.225.0:
``Note that the CRV is only unique on a particular part of a call, e.g.
between two terminals, or between a terminal and a gatekeeper. If a given
terminal has two calls in the same conference, each shall have the same
conference ID but different CRVs.''

The latter sentence is more important: when communicating between two
entities, it states clearly that there must be different CRVs.  Again, with
V1 those two calls would have been using different TCP connections, yet the
text clearly says the CRVs must be different.

Now, I may be wrong-- this is just my interpretation of the text.  I'd like
to hear other comments.

Paul

----- Original Message -----
From: "Chris Wayman Purvis" <cwp at isdn-comms.co.uk>
To: "Paul E. Jones" <paulej at packetizer.com>
Cc: <ITU-SG16 at mailbag.cps.intel.com>
Sent: Tuesday, January 16, 2001 5:16 AM
Subject: Re: H.225- CRV


> Paul,
>
> I disagree.  CRV is surely unique per connection.
> One may be well advised to keep it more unique than that, because in
> gatekeepered environments you're liable to end up with problems if you're
not
> careful because of your note 1 - but if I receive Q.931 messages on
separate
> TCP connections giving the same CRV I shouldn't expect them to refer to
the
> same call.  I argue that this is rooted in the principal of
TransportAddress
> throughout the H.323 standards including the SAPI (IP port number) as well
as
> actual address.
>
> Regards,
> Chris
>
> "Paul E. Jones" wrote:
> >
> > Archana,
> >
> > (a) is closer to the way things actually work.
> >
> > Essentially, you can use whatever values you wish so long as you never
place
> > two calls to the same endpoint using the same CRV value.  So, whether
two
> > calls to an endpoint use the same TCP connection or different
connections,
> > the CRV must be unique.
> >
> > You should also consider the possibility that an endpoint may have two
call
> > signaling addresses.  How would you know if this were the case?  The
safest
> > thing to do is never have two calls with the same CRV at the same time.
> > However, if you know that the destination endpoints are unique, you
could
> > re-use the CRV value.. at least when a Gatekeeper is not being used.
> >
> > There are also so other properties of CRVs:
> >   1) The CRV sent in an ARQ to place a call shall be the same as the
> >      CRV in the resulting Setup message
> >   2) The CRV sent in an ARQ to accept a call should *not* be taken
> >      from the incoming Setup message-- it should be locally generated so
> >      that it is a unique between the endpoint and the GK
> >
> > Paul
> >
> > ----- Original Message -----
> > From: "Nehru, Archana" <archie at trillium.com>
> > To: "'Paul E. Jones'" <paulej at PACKETIZER.COM>
> > Cc: <ITU-SG16 at mailbag.cps.intel.com>
> > Sent: Monday, January 15, 2001 10:28 PM
> > Subject: H.225- CRV
> >
> > > hi all,
> > >
> > > I have a question about H.323 -CRV values. Q.931 specs says that a CRV
> > value
> > > should be unique per D-channel layer 2 logical link.
> > >
> > > In H.225 context, does the above mean that:
> > >
> > > a) CRV values should be unique across all outgoing H.225 calls that
have
> > the
> > > same "destination" IP address ?
> > >
> > > b) CRV value only needs to be unique across all H.225 connections
using
> > the
> > > same TCP connection? In other words, if an endpoint A makes two
> > simultaneous
> > > calls to endpoint B, then the same CRV can be used on both the calls,
as
> > > long as they use different TCP connections?
> > >
> > > I would appreciate any help on this.
> > >
> > > Regards
> > > Archana
> > >
> > >
> > >
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > For help on this mail list, send "HELP ITU-SG16" in a message to
> > listserv at mailbag.intel.com
>
> --
> Dr Chris Purvis -- Development Manager
> ISDN Communications Ltd, The Stable Block, Ronans, Chavey Down Road
> Winkfield Row, Berkshire.  RG42 6LY  ENGLAND
> Phone: +44 1344 899 007
> Fax:   +44 1344 899 001
>

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