[itu-sg16] AVD-3813 Handling Of Error Conditions in H.323

[Sasha Ruditsky]

Hi Paul,

 

Yes, I am looking for a Broader statement. 

Specifically, my problem is the fact that ETSI TS 101 804 - 2 defines
procedures referring to Q.931 sections.

 

Let consider one example from ETSI TS 101 804 - 2:

BCC_TE_S_U00_07 clause 5.8.6.1 [4]

Ensure that the IUT in the Null call state U0, on receipt of a SETUP
message with a mandatory information element

missing,

sends a RELEASE COMPLETE message containing a Cause information element
indicating the cause value 96

"mandatory information element missing" and remains in the Null call
state U0.

 

Which suggest to the reader that Q.931 section 5.8.6.1 (and some other
explicitly referenced sections) 

is the way H.323 should be implemented.

 

What I want to achieve is an explicit statement in H.323 or H.225.0,
which would say that this is not the case,

that these sections do not apply, instead... and here we do need to
provide something which would 

close the gap created by this unspecified Cause code value.

 

The problem to select such cause is stemmed from the fact that syntax of
Q.931 messages and syntax of 

H.225.0 messages are quite different.

Q.931 status codes cover syntax errors in Q.931 part of the message.
However, very significant part of the H.225.0 

is PER encoded, so "is the perfectly encoded Q.931 message with ASN.1
part encoded incorrectly encoded correctly?" 

and should the response be STATUS and if STATUS then with which cause
code?

 

I believe it does not make sense to respond to syntactical errors in
Q.931 part and ignore such errors in ASN.1. 

On the other hand we did not have and do not have any fine grained
definition of what to do with different ASN.1 error cases 

and this most probably created the situation where different
implementations behave differently. 

 

BTW: In our implementation for example we use cause value 95: "Invalid
message, unspecified" for any syntactical mistake 

in any H.225.0 message. If somebody has a different approach, please
share with us!

 

So my ultimate goal is to make clear that H.323 is not working according
to ETSI TS 101 804 - 2 and at the same to create minimal 

possible impact on existing implementations. 

 

Regards,

Sasha

 

 

 

 

From: Paul E. Jones [mailto:paulej at packetizer.com] 
Sent: Friday, October 02, 2009 2:29 PM
To: Sasha Ruditsky; itu-sg16 at lists.packetizer.com
Subject: RE: [itu-sg16] AVD-3813 Handling Of Error Conditions in H.323

 

Sasha,

 

The fact that H.225.0 says a device shall send a Status message for an
unknown message, yet leaves the Cause code unspecified, is certainly an
issue we should close on.  My suggestion would be to use 97: "Message
type non-existent or not

Implemented."

 

I think an "unknown message" would be one that is not currently defined
today in H.225.0.  If the message is syntactically invalid, then I
believe that is a protocol error.  In that case, either 100 or 111 would
be good choices depending on whether it is just an invalid IE or
something that is impossible to decode.

 

Those are fairly minor changes, though the impact might be significant.
However, your problem description suggests you are looking for a broader
statement.  Do you have a specific proposal in mind, either a new
paragraph or section on error handling or a reference to Q.931?

 

Paul

 

From: itu-sg16-bounces at lists.packetizer.com
[mailto:itu-sg16-bounces at lists.packetizer.com] On Behalf Of Sasha
Ruditsky
Sent: Wednesday, September 30, 2009 6:01 PM
To: itu-sg16 at lists.packetizer.com
Subject: [itu-sg16] AVD-3813 Handling Of Error Conditions in H.323

 

Q2 Experts,

 

The conclusion for the discussion of the AVD-3813 during the last SG16
meeting's was to ask the experts opinion through the mailing list.

My apology for the short notice. I hope that we still have time before
the next meeting for people to understand the problem and express their
opinions. 

 

The problem discussed in AVD 3813 is more or less as follows:

 

"

H.225.0 gives very little attention to the specification of processing
of the H.225.0 Call Signaling messages errors. The only place dedicated
to this subject is Clause 7.1 of H.225.0.  

 

On the other hand, ITU-T Recommendation Q.931 on which H.225.0 messages
are based provides quite detailed information on the same subject. While
H.225.0 states that Implementations shall follow ITU-T Rec. Q.931 as
specified in H.225.0, there is a lot of confusion surrounding the cases
which H.225.0 does not cover and Q.931 does.

 

In addition, ETSI TS 101 804 - 2 defines Conformance Test Specification
for ITU-T H.225.0. This test specification apparently based on Q.931
procedures, not on the corresponding H.225.0 ones. More than this, in
many cases ETSI TS 101 804 - 2 requests behavior which claims to be
based on Q.931, however is not defined neither in Q.931, nor H.225.0. 

"

 

Apparently "H.323 conformant" not always means "ETSI TS 101 804 - 2
conformant" and I believe we need to find some solution to at least make
this particular point clear.

 

I'm going to resubmit AVD-3813. 

It would be great to be able to get some ideas from the group into the
resubmitted document.

 

Thank you,

Sasha 

 

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