H.450.x related

Sunil Chandra schandra at HSS.HNS.COM
Fri Oct 22 13:57:37 EDT 1999


First, I apologize to the reflector if this question has been answered
multiple times before. Is there a FAQ for this mailing list?
IMO, option 1 is absolutely valid. Some apps may not honor the odd/even
rule, and if I recall correctly this requirement has been relaxed a bit in
H.323 (correct me if I am wrong). It has been suggested in a few occasions
that the odd/even rule is an RTP rule and not an H.225.0 despite the fact
that it is mentioned in the spec.
Also it is not only "+1", rather the following odd number since the "+1"
port may be busy.
You can specify an RTCP odd address in OLC for RTCP receive reports and once
you get the OLC from the remote endpoint, use the same odd RTCP for Tx
reports, and use the lower even number for RTP port and that way you will
still be fully compliant.

Here is an example

START_PDU: ============================= START PDU
============================
LAYER:     H245
DIRECTION: Sent
PDU_TYPE: OpenLogicalChannel
 Forward Logical Channel Number: 257
   Unicast Address: IP, 209.218.18.91
        TSAPIdentifier: 1161
 END_PDU: ================================ END PDU
=============================

START_PDU: ============================= START PDU
============================
LAYER:     H245
DIRECTION: Sent
PDU_TYPE: OpenLogicalChannelAck
 Forward Logical Channel Number: 257
 frwrdMltplxAckPrmtrs:
  H2250LCAPs_mdChnnl:
   Unicast Address: IP, 209.218.18.91
    TSAPIdentifier: 1160
  H2250LCAPs_mdCntrlChnnl:
   Unicast Address: IP, 209.218.18.91
    TSAPIdentifier: 1161
END_PDU: ================================ END PDU
=============================

START_PDU: ============================= START PDU
============================
LAYER:     H245
DIRECTION: Received
PDU_TYPE: OpenLogicalChannelAck
 Forward Logical Channel Number: 257
 frwrdMltplxAckPrmtrs:
  H2250LCAPs_mdChnnl:
   Unicast Address: IP, 209.218.18.82
    TSAPIdentifier: 1030
  H2250LCAPs_mdCntrlChnnl:
   Unicast Address: IP, 209.218.18.82
    TSAPIdentifier: 1031
END_PDU: ================================ END PDU
=============================

START_PDU: ============================= START PDU
============================
LAYER:     H245
DIRECTION: Received
PDU_TYPE: OpenLogicalChannel
 Forward Logical Channel Number: 257
  H2250LCPs_mdCntrlChnnl:
   Unicast Address: IP, 209.218.18.82
    TSAPIdentifier: 1031
END_PDU: ================================ END PDU
=============================

regards,

Hani

 -----Original Message-----
From: Mohamed Mustafa [mailto:M.Mustafa at SDXPLC.COM]
Sent: Friday, October 22, 1999 6:15 AM
To: ITU-SG16 at MAILBAG.INTEL.COM <mailto:ITU-SG16 at MAILBAG.INTEL.COM>
Subject: Re: RTCP port number



Hani,

Thanks for your reply. There are still a few points I'm not clear on.

Your first four points are consistent with my current understanding of the
OLC/OLCA messages. I'll elaborate on these points just to try and avoid the
ambiguities of my previous email:

        There are two optional TransportAddress fields (mediaChannel and
mediaControlChannel) within the forward logical channel parameters section
of the OLC request message. H.245 specifies that the mediaChannel field
should not be included in the OLC request when the transport is unicast. The
mediaControlChannel field, if present in the OLC request, specifies the
address on which the sender of the OLC will be listening for media control
messages. If the mediaControlChannel field is not present in the OLC
request, this indicates that a control session is not required.

        The mediaChannel and mediaControlChannel fields are also present in
the OLCA response message. The mediaChannel field must be present in a OLCA
response when the transport is unicast, and it specifies the address to
which the sender of the corresponding OLC must transmit the media. The
mediaControlChannel field in the OLCA response specifies the address on
which the sender of the OLCA will be listening for media control messages
for this session. The mediaControlChannel should only be included in the
OLCA response if a media control channel is required (i.e., only if the
mediaControlChannel field was present in the corresponding OLC request).

Therefore, assuming a control channel is required, each endpoint must
transmit and receive three transport address in order to establish a
bi-directional media stream; 1) RTCP address in OLC (a1), 2) RTP address in
OLCA (a2), and 3) RTCP address in OLCA (a3). To use a single bi-directional
RTCP channel, each endpoint must specify the same address in a1 and a3. The
+1 rule requires the port numbers in a1 and a3 to be odd, and that a2 and a3
are identical except for the port number where it is one less in a2 compared
to a3.

My initial question was regarding the case where one of the endpoints
supplies a set of addresses where a1 and a2 do not conform to the +1 rule.
There are two possibilities in this case:

1) a1 and a3 are the identical - i.e., single bi-directional RTCP channel,
but a2 and a3 do not conform to the +1 rule.
2) a2 and a3 conform to the +1 rule - i.e., 2 unidirectional RTCP channels.

My original question can be more precisely worded as: Is option 1 valid?


Thanks again,

Mohamed



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