Subject: [Megaco] supplied address and ServiceChangeAddress

From: "Jan Lindquist \(AL/EAB\)" <[email-masked]>
Date: Thu, 28 Apr 2005 16:35:17 +0200

Hello,
 
I would like assistance in the interpretation in the text in H.248
specifically in section 7.2.8 relating to ServiceChangeAddress.  Here is
a copy of the text in question:
 
"The MG may specify the transport ServiceChangeAddress to be used by the
MGC for sending messages in the ServiceChangeAddress parameter in the
input ServiceChangeDescriptor. The MG may specify an address in the
ServiceChangeAddress parameter of the ServiceChange request, and the MGC
may also do so in the ServiceChange reply. In either case, the recipient
must use the supplied address as the destination for all subsequent
transaction requests within the association."
 
The H.248 transport to be used is UDP/IP.
 
The question relates to the last sentence in the above paragraph.  A
common technique with UDP transport of H.248 is to copy the source IP
address in the IP header that is in the first ServiceChange from the MG.
So all subsequent message to the MG will use that MG IP address.  There
is no need for a ServiceChangeAddress.
 
The question is in the reverse direction.  If I want to change the IP
address that the MG should use for any subsequent messages to the MGC.
The MGC sends a ServiceChange reply with a new IP address to itself.
Shouldn't the MG use that IP address for subsequent messages?
 
There is a subtle implication in the text that this is acceptable but
what does "supplied address" mean.  For ServiceChange coming from the MG
the IP address in the IP header is the supplied address.  If this
accepted technique for indicating supplied address it is possible to
interprete the ServiceChange reply could indicate the supplied address
for the MGC.
 
Is this interpretation correct?
 
Regards,
JanL
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---

From: "Jan Lindquist \(AL/EAB\)" <[email-masked]>
Date: Fri, 29 Apr 2005 13:07:03 +0200

Hi Albrecht,

Very good question.  I am trying to get this clarified in the megaco
mailing list in order to determine if something needs to be documented
and how.  I agree that H.248 does not mention these assumptions and I
need to get this straightened in order to improve interoperability and
performance of profiles being created in ETSI.

If H.248 is not clear on these assumptions we need to add it to the ETSI
H.248 profiles in general.  I do not think that H.248 specification can
explain these assumptions because it cannot make any statement against
the transport layer.  The profile on the other hand does refer to the
transport layer and therefor can explain interaction between the H.248
layer and transport layer.

I would like to propose that two assumptions are documented in the
TISPAN AGW profile (only applies to UDP transport):

1. The MGC will store the source IP address retrieveed from the first
ServiceChange.  All subsequent messages delivered to the MG will use
this IP address.

2. The MG will store the source IP address retrieved from the reply to
the ServiceChange(1).  All 
subsequent messages delivered to the MGC will use this IP address.

Note (1) - The MG will use the DNS lookup in order to retrieve the
initial IP address to send the first ServiceChange.  This IP address may
be different than the IP address returned in the reply to ServiceChange.

Regards,
JanL

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] 
Sent: den 29 april 2005 12:02
To: Jan Lindquist (AL/EAB)
Cc: [email-masked]; [email-masked]
Subject: RE: [Megaco] supplied address and ServiceChangeAddress

Hi Jan,

just an additional comment to the sentence:
"A common
technique with UDP transport of H.248 is to copy the source IP address
in the IP header that is in the first ServiceChange from the MG."

I don't know how to handle such a argument. This seems to be an
assumption, or a specific interpretation, or a "best current practice"
because it is not mentioned in any H.248 Rec. Right?

My question is: do you want to make this implicit assumption as an
explicit proposal, e.g., for ETSI H.248 Profiles? ["because the
subsequent argumentation bases on this hypothesis"]

Best rgds/Albrecht

                      "Kevin Boyle"

                      <[email-masked]         To:      "Jan Lindquist
(AL/EAB)" <[email-masked]>, [email-masked]      
                      om>                      cc:

                      Sent by:                 Subject: RE: [Megaco]
supplied address and ServiceChangeAddress                      
                      megaco-bounces@i

                      etf.org

                      28.04.2005 17:00

There is additional text in that section: "Note that the use of
ServiceChangeAddress is not encouraged."  This is advice that ought to
be followed.

That aside, to answer your question, the paragraph is discussing the
SCAddress parameter alone.  It only indicates that if the SCAddress
parameter is present, then the recipient of the SC command must switch
to that new address.

Note that if the MGC wishes to change its address, it should use Handoff
and the SCMGCID to announce that change.

Kevin

  _____

From: [email-masked] [mailto:[email-masked]] On Behalf
Of Jan Lindquist (AL/EAB)
Sent: Thursday, April 28, 2005 10:35 AM
To: [email-masked]
Subject: [Megaco] supplied address and ServiceChangeAddress

Hello,

Regards,
JanL