14 SDP offer/ answer procedures
24.5813GPPMission Critical Video (MCVideo) media plane controlProtocol specificationRelease 18TS
14.1 General
The capabilities described in clause 4.3 are negotiated in the signalling and application plane using SDP offer / answer in SIP requests or SIP responses as specified in 3GPP TS 24.281 [2].
This clause describes the details of the SDP offer / answer procedures with regards to the MCVideo fmtp attribute in the "m=application" m-line.
14.2 Generating an SDP offer
14.2.1 General
When the offerer generates an SDP offer, in order to negotiate the establishment of a media plane control channel, the offerer shall include a media description ("m=" line) associated with the media plane control channel. In addition, the offerer may associate an SDP fmtp attribute with the media description.
NOTE: "Initial offer" refers to the offer when the media plane control channel is initially negotiated. It might, or might not, be the initial offer within the session.
14.2.2 "mc_queueing" fmtp attribute
The MCVideo client shall include the "mc_queueing" fmtp attribute in SDP offers when queueing of Transmission request is supported.
The controlling MCVideo function shall include the "mc_queueing" fmtp attribute in SDP offers when queueing of Transmission request is supported.
The non-controlling MCVideo function shall include the "mc_queueing" fmtp attribute in SDP offers if the controlling MCVideo included the "mc_queueing" fmtp attribute in the SDP offer.
14.2.3 "mc_priority" fmtp attribute
The MCVideo client shall include the "mc_priority" fmtp attribute when a transmission priority different than the default priority is required. The MCVideo client should base transmission priority on the configured value in 3GPP TS 24.484 [13].
When inviting an MCVideo client or an MCVideo group to a pre-arranged group call, the controlling MCVideo function and the non-controlling MCVideo function shall include in the "mc_priority" fmtp attribute with the value of the <user-priority> element in the <entry> element specified in 3GPP TS 24.481 [12].
14.2.4 "mc_granted" fmtp attribute
The MCVideo client shall include the "mc_granted" fmtp attribute in the SDP offer of an initial SIP INVITE request when it is acceptable for the MCVideo client to receive a granted indication in the SIP 200 (OK) response to an initial INVITE request.
14.2.5 "mc_implicit_request" fmtp attribute
The MCVideo client shall include the "mc_implicit_request" fmtp attribute when a SIP request shall be interpreted as an implicit Transmission request. If not explicitly stated in procedures in the present document or in procedures in 3GPP TS 24.281 [2] that the "mc_implicit_request" fmtp attribute shall be included, the decision to include the "mc_implicit_request" fmtp attribute or not, is an implementation option.
14.2.6 "mc_reception_priority" fmtp attribute
The MCVideo client shall include the "mc_reception_priority" fmtp attribute when a reception priority different than the default reception priority is required. The MCVideo client should base reception priority on the configured value in 3GPP TS 24.484 [13].
14.3 Generating the SDP answer
14.3.1 General
When the answerer receives an SDP offer, which contains a media description for a media plane control channel, the answerer shall include a media description associated with the media plane control channel in the answer. In addition, the answerer may associate an SDP fmtp attribute with the media description. The SDP fmtp attribute shall not include parameters that were not present in the associated SDP offer.
14.3.2 "mc_queueing" fmtp attribute
The MCVideo client shall include the "mc_queueing" fmtp attribute in SDP answers when queueing of Transmission request is supported.
The controlling MCVideo function shall include the "mc_queueing" fmtp attribute in SDP answers when queueing of Transmission request is supported and the "mc_queueing" fmtp attribute was included in the SDP offer.
The non-controlling MCVideo function shall include the "mc_queueing" fmtp attribute in SDP answers if the controlling MCVideo function included the "mc_queueing" fmtp attribute in the SDP offer.
NOTE: For MCVideo clients connected to the non-controlling MCVideo function, the determination of if queueing is supported or not is determine case by case using the Track Info field for each individual transmission control participant.
14.3.3 "mc_priority" fmtp attribute
If the "mc_priority" fmtp attribute is included in an SDP offer, the controlling MCVideo function:
1. if the <on-network-recvonly> element is present in the <entry> element as specified in 3GPP TS 24.481 [12] for the MCVideo user identified by the <entry> element, shall not include a "mc_priority" fmtp attribute in the SDP answer;
2. if the <on-network-recvonly> element is not present in the <entry> element as specified in 3GPP TS 24.481 [12] for the MCVideo user identified by the <entry> element in the MCVideo group document:
a. shall determine the priority value to include in "mc_priority" fmtp attribute of the SDP answer, by choosing the lowest value from the following inputs:
i. the value of transmission priority in the "mc_priority" fmtp attribute included in the SDP offer;
ii. the value of the <user-priority> element in the MCVideo group document as specified in 3GPP TS 24.481 [12]; and
iii the value of the <num-levels-priority-hierarchy> element in the MCVideo service configuration document as specified in 3GPP TS 24.484 [13]; and
b. shall include the priority value determined above in step 2a, in the "mc_priority" fmtp attribute of the SDP answer.
If the "mc_priority" fmtp attribute is included in an SDP offer, the MCVideo client and the non-controlling MCVideo function shall return the value included in the SDP offer in the SDP answer.
14.3.4 "mc_granted" fmtp attribute
If the mc_granted" fmtp attribute is included in an SDP offer, the controlling MCVideo function:
1. if the MCVideo call is not a temporary group session, may use the SIP 200 (OK) response to indicate that the implicit Transmission request is granted; and
2. if the MCVideo call is a temporary group session, shall not indicate that the implicit Transmission request is granted.
NOTE: A MCVideo group call is a temporary group session when the <on-network-temporary> element is present in the <list-service> element as specified in 3GPP TS 24.481 [12].
If the controlling MCVideo function grants the implicit request and decide to use the SIPP 200 (OK) response to grant the implicit request, the controlling MCVideo function shall include the "mc_granted" fmtp attribute in the SDP answer.
If the controlling MCVideo function decides not to grant the implicit request, the controlling MCVideo function shall not include the "mc_granted" fmtp attribute in the SDP answer.
14.3.5 "mc_implicit_request" fmtp attribute
If the "mc_implicit_request" fmtp attribute is included in an SDP offer, the MCVideo server shall accept the initial INVITE request to be an implicit request for media transmission unless the MCVideo client is joining a chat group call or an ongoing pre-arranged call and include the "mc_implicit_request" fmtp attribute in responses to the SIP request.
14.3.6 "mc_reception_priority" fmtp attribute
If the "mc_reception_priority" fmtp attribute is included in an SDP offer, the controlling MCVideo function:
1. shall determine the reception priority value to include in "mc_reception_priority" fmtp attribute of the SDP answer, by choosing the lowest value from the following inputs:
a. the value of reception priority in the "mc_reception_priority" fmtp attribute included in the SDP offer; and
b. the value of the <user-reception-priority> element in the MCVideo group document as specified in 3GPP TS 24.481 [12].
14.4 Offerer processing of the SDP answer
When the offerer receives an SDP answer, if an SDP fmtp attribute is associated with the media description associated with the media plane control channel, and if the attribute contains attribute parameters that were not present in the associated offer, the offerer shall discard those attribute parameters.
14.5 Modifying the media plane control channel
When an offerer generates a subsequent SDP offer, the offerer follows the rules for generating an initial offer, as described in clause 14.2, with the exception that the offerer shall not include the ‘mc_granted’ SDP fmtp attribute parameter in a subsequent offer. No semantics has been defined for the ‘mc_granted’ attribute parameter in a subsequent offer.
For the ‘mc_implicit_request’ semantic is only defined for a subsequent SDP offer when upgrading from normal to an emergency call using an SIP re-INVITE request as specified in 3GPP TS 24.281 [2].
14.6 The use of SDP offer / answer procedures in off-network mode
14.6.1 General
The MCVideo client can negotiate the capabilities used in a private call using PRIVATE CALL SETUP REQUEST message and PRIVATE CALL ACCEPT message as specified in 3GPP TS 24.281 [2]. The PRIVATE CALL SETUP REQUEST message contains the SDP offer and the PRIVATE CALL ACCEPT message contains the SDP answer.
When initiating a group call or a broadcast group call using the GROUP CALL ANNOUNCEMENT message or GROUP CALL BROADCAST message as specified in 3GPP TS 24.281 [2] the capabilities to be used during the call cannot be negotiated.
14.6.2 fmtp attribute "mc_queueing"
When initiating a private call and if the MCVideo client supports queueing of Transmission requests, the MCVideo client shall include the fmtp attribute "mc_queueing" in the SDP offer.
When an MCVideo client accepting the invitation to the private call receives an SDP offer containing the fmtp attribute "mc_queueing" and if the MCVideo client supports queueing of Transmission requests, the MCVideo client shall include the fmtp attribute "mc_queueing" in the SDP answer.
When initiating a group call and if queueing of Transmission requests is supported, the MCVideo client shall include the fmtp attribute "mc_queueing" in the SDP offer.
NOTE: Participants in a group call where queueing is used acknowledge the support of queueing in the Transmission control Indicator field in the Transmission request message.
Annex A (informative):
Change history
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Change history |
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Date |
Meeting |
TDoc |
CR |
Rev |
Cat |
Subject/Comment |
New version |
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2017-01 |
Initial version. |
0.0.0 |
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2017-01 |
Implementing the following agreed P-CRs after CT1#101bis: C1-170281. |
0.1.0 |
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2017-04 |
Implementing the following agreed P-CRs after CT1#103: C1-171504. |
0.2.0 |
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2017-05 |
Implementing the following agreed P-CRs after CT1#104: C1-172173, C1-172174, C1-172369, C1-172521, C1-172524, C1-172527, C1-172529. |
0.3.0 |
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2017-06 |
CT-76 |
CP-171109 |
Version 1.0.0 created for presentation for information at CT76 |
1.0.0 |
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2017-06 |
CT-76 |
Version 14.0.0 created after approval at CT76 |
14.0.0 |
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2017-09 |
CT-77 |
CP-172104 |
0001 |
1 |
F |
Proposal for clause 2 on References |
14.1.0 |
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2017-09 |
CT-77 |
CP-172104 |
0002 |
1 |
F |
Proposal for clause 4 on General overview |
14.1.0 |
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2017-09 |
CT-77 |
CP-172104 |
0003 |
1 |
F |
Proposal for clause 5 on Entities |
14.1.0 |
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2017-09 |
CT-77 |
CP-172104 |
0004 |
1 |
F |
Proposal for clause 12 on Extensions |
14.1.0 |
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2017-09 |
CT-77 |
CP-172104 |
0005 |
1 |
F |
Proposal for clause 13 on media plane security |
14.1.0 |
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2017-09 |
CT-77 |
CP-172104 |
0006 |
1 |
F |
Proposal for clause 14 on SDP procedures |
14.1.0 |
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2017-09 |
CT-77 |
CP-172104 |
0007 |
1 |
F |
Off-network call release |
14.1.0 |
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2017-12 |
CT-78 |
CP-173066 |
0008 |
4 |
F |
Proposal for on-network timers and counters used in transmission control |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0009 |
F |
Corrections to transmission control messages |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0010 |
3 |
F |
Adding missing transmission control messages |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0011 |
F |
Addition of MCVideo transmission control Definitions and Abbreviations |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0012 |
1 |
F |
Corrections to instances of floor control usage in MCVideo |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0013 |
F |
Corrections to transmission control participant state machines |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0014 |
1 |
F |
Corrections to transmission control server state machines |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0015 |
3 |
F |
Effective priority |
14.2.0 |
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2017-12 |
CT-78 |
CP-173066 |
0016 |
F |
Reject causes |
14.2.0 |
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2018-03 |
CT-79 |
CP-180074 |
0017 |
F |
Correction to MCVideo transmission control message coding |
14.3.0 |
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2018-06 |
CT-80 |
CP-181055 |
0018 |
1 |
F |
MCVideo message and field fixes |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0019 |
1 |
F |
Corrections on Transmission Idle and Transmission control Ack |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0020 |
1 |
F |
Corrections on Message Type and Subtype |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0021 |
2 |
F |
Corrections on Override Indicator |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0023 |
1 |
F |
Corrections on Reception Indicator |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0024 |
1 |
F |
Corrections on Reception Priority |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0027 |
2 |
F |
Corrections on Overriding ID and Overridden ID |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0029 |
1 |
F |
Corrections on Transmission Arbitration |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0030 |
1 |
F |
Corrections on Transmission Granted |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0031 |
F |
Corrections on Transmission revoked cause codes |
14.4.0 |
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2018-06 |
CT-80 |
CP-181055 |
0032 |
1 |
F |
Corrections on Transmission Release |
14.4.0 |
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2018-06 |
CT-80 |
CP-181065 |
0025 |
4 |
B |
Corrections on Media Reception |
15.0.0 |
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2018-06 |
CT-80 |
CP-181065 |
0028 |
3 |
B |
Corrections on Receive Media Reception Notification |
15.0.0 |
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2018-06 |
CT-80 |
CP-181065 |
0033 |
2 |
B |
Usage of MBMS for MCVideo – media plane |
15.0.0 |
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2018-09 |
CT-81 |
CP-182148 |
0036 |
1 |
B |
Application Group Paging procedure |
15.1.0 |
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2018-09 |
CT-81 |
CP-182153 |
0037 |
2 |
B |
MBMS bearer announcement over an MBMS bearer |
15.1.0 |
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2018-12 |
CT-82 |
CP-183057 |
0039 |
1 |
B |
MBMS procedures for group dynamic data |
15.2.0 |
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2018-12 |
CT-82 |
CP-183046 |
0041 |
F |
Correction on MCVideo Group Identity and SSRC field |
15.2.0 |
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2018-12 |
CT-82 |
CP-183062 |
0038 |
1 |
F |
Incorrect reference to table for MBMS Subchannel Control |
16.0.0 |
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2019-03 |
CT-83 |
CP-190080 |
0044 |
1 |
A |
MCVideo Reception control |
16.1.0 |
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2019-06 |
CT-84 |
CP-191127 |
0046 |
A |
Adding SSRC of Transmitter in reception control messages |
16.2.0 |
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2019-06 |
CT-84 |
CP-191119 |
0049 |
A |
Corrections in description for Offnetwork Transmission participant state diagram – basic operation |
16.2.0 |
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2019-06 |
CT-84 |
CP-191119 |
0052 |
1 |
A |
Removal of creating another instance of ‘Transmission participant state transition diagram for basic transmission control operation’ |
16.2.0 |
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2019-06 |
CT-84 |
CP-191119 |
0055 |
2 |
A |
Correcting improper handling of Transmission cancel request |
16.2.0 |
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2019-06 |
CT-84 |
CP-191119 |
0058 |
1 |
A |
Handle Transmission End request while waiting for response for Tranmission request message |
16.2.0 |
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2019-06 |
CT-84 |
CP-191142 |
0062 |
1 |
F |
Removing Transmission cancel request and Transmission cancel response |
16.2.0 |
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2019-06 |
CT-84 |
CP-191119 |
0065 |
A |
Corrections in generating Transmission control Ack message. |
16.2.0 |
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2019-09 |
CT-85 |
CP-192065 |
0066 |
1 |
F |
Minor corrections in transmission control state machine |
16.3.0 |
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2019-09 |
CT-85 |
CP-192065 |
0067 |
F |
Remove Media ID |
16.3.0 |
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2020-03 |
CT-87e |
CP-200121 |
0068 |
1 |
C |
FEC encoding by the BM-SC |
16.4.0 |
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2020-06 |
CT-88e |
CP-201121 |
0069 |
1 |
F |
Corrections in 6.3.5.2.2 and 6.3.5.3.3 |
16.5.0 |
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2020-06 |
CT-88e |
CP-201121 |
0070 |
F |
Corrections in 6.3.5.4.2 |
16.5.0 |
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2020-06 |
CT-88e |
CP-201121 |
0071 |
F |
Corrections in 6.3.6.3.6 |
16.5.0 |
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2020-06 |
CT-88e |
CP-201121 |
0072 |
F |
Incorrect counter Cx upper limit check |
16.5.0 |
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2020-06 |
CT-88e |
CP-201121 |
0073 |
F |
New instance creation and release for basic / general reception control state m/c. |
16.5.0 |
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2020-06 |
CT-88e |
CP-201091 |
0074 |
1 |
A |
Simultaneous reception of media at transmission control server (reception control) |
16.5.0 |
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2020-09 |
CT-89e |
CP-202143 |
0077 |
2 |
A |
Method to handle no active receiver in MCVideo System |
16.6.0 |
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2020-09 |
CT-89e |
CP-202176 |
0079 |
1 |
B |
usage in Transmission Control |
17.0.0 |
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2021-12 |
CT-84e |
CP-213023 |
0086 |
1 |
A |
Non-controlling MCVideo function |
17.1.0 |
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2022-03 |
CT-85e |
CP-220221 |
0089 |
1 |
A |
Auto-Receive Reception Mode |
17.2.0 |
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2022-09 |
CT-87e |
CP-222173 |
0090 |
1 |
F |
Corrections to acknowledgment required bit of transmission control ack message |
18.0.0 |
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2022-09 |
CT-87e |
CP-222173 |
0091 |
1 |
F |
Corrections to acknowledgment required bit of transmission idle message |
18.0.0 |
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2022-09 |
CT-87e |
CP-222173 |
0092 |
3 |
F |
Clarification of the SSRC to be used in video, audio and transmission control (TC) streams in MCVideo |
18.0.0 |