Paul,
Considering that V1 only allowed a single call over a single TCP connection,
REALLY? First I've heard of it! Clarifications were added later as to the mechanics, but I don't believe multiple calls over single call signalling connection was ever forbidden!
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.)
It was part of a job lot stolen from Q.931, remember! Besides, I'm surprised at you suggesting that nothing in H.323v1 was worthless...
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.
Yup. All messages between two given entities associated with a given call have the same CRV. That does not imply that all messages between two given entities with the same CRV are associated with the same call (i.e. => is not the same as <=).
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.
This is much more pertinent! It's also impossible to be certain about. What constitutes a different "terminal" in this context? For example, if a single piece of equipment has two IP addresses, is it one terminal or two?
Now, I may be wrong-- this is just my interpretation of the text. I'd like to hear other comments.
It's opened a can of worms, for sure. Maybe CRV is only unique per combination of IP addresses, ignoring port numbers. Seems strange to me, but possible. I really don't recall anything ever having said that only a single call was allowed per TCP call signalling connection, however.
Philosophical question (not seriously intended for detailed discussion): If the call signalling connection were dropped (still, I believe, permitted, although not recommended), and a fresh call signalling connection made between the same address/port pairs, is this the same connection? Are these the same endpoints as those involved in the initial call? Consider a signalling proxy (no media, doesn't handle H.245 directly within the same box on the same IP address), which crashes, and has its IP address taken over by a similar signalling proxy, all while a "call" is going on...
Regards, Chris
----- Original Message ----- From: "Chris Wayman Purvis" cwp@isdn-comms.co.uk To: "Paul E. Jones" paulej@packetizer.com Cc: ITU-SG16@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:
- The CRV sent in an ARQ to place a call shall be the same as the CRV in the resulting Setup message
- 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@trillium.com To: "'Paul E. Jones'" paulej@PACKETIZER.COM Cc: ITU-SG16@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@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