PDU & ASN parser

[Paul Long]

Guntram,

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 one
could pack multiple RAS messages per unreliable-transport PDU. As you say, it
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 be
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 one
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 are
sent;"

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
the
        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
encoded
        > using PER and whose total encoded length is within the boundary of
the
        > H.245 TCP channel can be merely concatenated and sent as  a single
payload
        > 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
payload.
        It does know where the first message ends, so the location of the next
        message can be derived from this knowledge.


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