The meaning of the reception of a "new" capability set

Paul Long plong at SMITHMICRO.COM
Mon Nov 1 11:20:08 EST 1999


No, no, no!!! :-) Each TCS message is a _delta_ to be applied to the remote
EP's current capability set stored in the EP. This issue was discussed a few
months ago on the reflector. My reply follows my signature.

BTW, the pause feature added to H.323v2 redefines the meaning of an empty
TCS. In H.323v1, an empty TCS causes no change to the current cap set--a
NO-OP, so to speak. As of v2, however, an empty TCS means close all outgoing
channels, assume that the remote EP has no receive or transmit caps (flush
the current capapbility set for the remote EP), and upon receipt of the next
presumably non-empty TCS, re-open outgoing channels based on the possibly
new set of receive caps. From what I've heard, though, many if not all v2
EPs still exhibit v1 behavior in this regard. I doubt whether "pause" will
ever be a dependable feature. IMO, overloading a message like this is just
asking for trouble. The same thing could have been better accomplished with

Paul Long
Smith Micro Software, Inc.

>>>>>>>>> My reply from July 1, 1999
Ramana, Chris, et al.:

An incoming terminalCapabilitySet message _modifies_ the current capability
set in the receiving EP. The entire set is not necessarily transmitted
within every message. This has been true since H.245v1. In fact, H.245 says
that one should not transmit unchanged capabilities. One starts out with an
empty set of descriptors. Incoming descriptors replace any extant ones with
the same descriptor number. If one does not already exist with the same
number, this adds a new descriptor; if one does exist, the new one replaces
the old one; if the new descriptor does not contain
simultaneousCapabilities, this removes any existing descriptor for the given
number. (This is the same scheme used to modify the capability table.) Here
is an illustrative chronological scenario:

        Current capability set:

Incoming termCapSet 1
0: {{H.261qcif},{G.711}}
1: {{H.261cif},{G.711}}

        Current capability set:
        0: {{H.261qcif},{G.711}}
        1: {{H.261cif},{G.711}}

Incoming termCapSet 2
1: {}

        Current capability set:
        0: {{H.261qcif},{G.711}}

Incoming termCapSet 3
1: {{H.261cif},{G.711}}
2: {{H.263sqcif&qcif&cif},{G.723,G.711}}

        Current capability set:
        0: {{H.261qcif},{G.711}}
        1: {{H.261cif},{G.711}}
        2: {{H.263sqcif&qcif&cif},{G.723,G.711}}

Incoming termCapSet 4
1: {}
0: {{H.261qcif&cif},{G.723.1,G.711)}

        Current capability set:
        0: {{H.261qcif&cif},{G.723.1,G.711)}
        2: {{H.263sqcif&qcif&cif},{G.723,G.711}}

Paul Long
Smith Micro Software, Inc.

-----Original Message-----
From: Ilya Freytsis [mailto:IFREYTSIS at VIDEOSERVER.COM]
Sent: Monday, November 01, 1999 8:57 AM
Subject: Re: The meaning of the reception of a "new" capability set

I believe every new capability set should be treated as a complete
replacement for the previously received one.  Otherwise conferencing
(MCS) and Gateways will not be able exercise enough control over the
modes of the multipoint conferences and gateway sessions.

Ilya Freytsis

                -----Original Message-----
                From:   frank.derks at PHILIPS.COM
[mailto:frank.derks at PHILIPS.COM]
                Sent:   Monday, November 01, 1999 8:31 AM
                To:     ITU-SG16 at
                Subject:        The meaning of the reception of a "new"
capability set

                Neither H.323 nor H.245 are too clear about what it
when an endpoint receives another set of capabilities after having
received one. In previous E-mail messages on this list, I have read that
capability sets are cumulative, i.e. anything
                "new" received in a TCS message adds to the capabilities
transmitted in a previous TCS message. I have, however, found nothing in
recommendations that backs this up.

                So what is the intended procedure? Does a newly received
of capabilities replace the previously received set? Do capabilities
received in a TCS message add to the already received capabilities? How
capabilities removed? What constitutes an "empty
                capability set" etc.

                I would appreciate it if somebody could point me in the
right direction.



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