PDU & ASN parser

Pete Cordell pete at TECH-KNOW-WARE.COM
Thu Jul 1 05:32:59 EDT 1999

Dear All,

I support the principle that only one H.225 call signalling message should
be sent per TPKT.  Firstly I believe it was agreed between early
implementers that this is the way they would work.  Secondly, even though it
is possible to send multiple PER messages per TPKT there are other reasons
why this is not totally desirable.  In our case of working on a high volume
GK, we only need to process a fraction of H.245 messages.  If there is only
one message per TPKT we can easily work out what the message is by looking
at the first few bytes.  However, if there are multiple messages per packet
we have no choice but to decode each and every message.  In most cases this
represents a totally unnecessary processing overhead and is therefore best


Pete Cordell
pete at tech-know-ware.com

-----Original Message-----
From: Paul Long <Plong at SMITHMICRO.COM>
To: 'h323implementors at imtc.org' <h323implementors at imtc.org>
Cc: ITU-SG16 at mailbag.cps.intel.com <ITU-SG16 at mailbag.cps.intel.com>
Date: 30 June 1999 17:53
Subject: RE: PDU & ASN parser

>I raised this issue over a year ago. In addition, H.225.0 does not address
>this issue regarding RAS (it does regarding call signaling), so I suppose
>could pack multiple RAS messages per unreliable-transport PDU. As you say,
>doesn't make sense to pack messages, and I don't think anyone does it. We
>should probably add something like the following to section 6.1 of the next
>revision of H.225.0.
>"When messages are sent on the H.225.0 RAS channel, only one whole message
>shall be sent within the boundaries defined by the transport; there shall
>no fragmentation of H.225.0 RAS messages across transport PDUs."
>We should also insert the indicated text, below, into existing text in the
>same section.
>"When messages are sent on the reliable H.245 control channel, >>>exactly
>message should be sent but<<< more than one message may be sent within the
>boundaries defined by the reliable transport PDU as long as whole messages
>Paul Long
>Smith Micro Software, Inc.
> -----Original Message-----
> From: Guntram Diehl [SMTP:gdiehl at innovaphone.com]
> Sent: Wednesday, June 30, 1999 4:14 AM
> To: 'Bancroft Scott'; 'Srikanth Nayani'
> Cc: 'h323implementors at imtc.org'
> Subject: RE: PDU & ASN parser
> Yes this can be done, but does any implementation actually do it? Why
> would anyone want to do this, it just saves 4 bytes tpkt header per
> additional H.245 message and it does not change the fragmentation of
> whole thing by IP.
> Or is there another reason why this was specified in the standard this
> way?
> Guntram
> -----Original Message-----
> From: Bancroft Scott [mailto:baos at OSS.COM]
> Sent: Wednesday, June 30, 1999 3:54 AM
> To: Srikanth Nayani
> Cc: h323implementors at imtc.org
> Subject: Re: PDU & ASN parser
> On Tue, 29 Jun 1999, Srikanth Nayani wrote:
> >   In the H.225 document  chapter 6 mentions that multiple messages
>can be
> > sent over the reliable H.245 control channel within the boundaries
>of the
> > reliable transport channel . Does that mean that two H.245 PDUs
> > using PER and whose total encoded length is within the boundary of
> > H.245 TCP channel can be merely concatenated and sent as  a single
> > of the H.245 TCP channel.
> > If so how does a ASN.1 decoder know that there are two
>messages in the
> > payload and the offset at which the second message starts.
> An PER decoder does not know that there are two messages in the
> It does know where the first message ends, so the location of the next
> message can be derived from this knowledge.
> --
> Bancroft Scott                                Toll Free
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