RTP-mixer formatting of multi-party Real-time text

Document Type Replaced Internet-Draft (individual)
Author Gunnar Hellstrom 
Last updated 2020-04-09
Replaced by RFC 9071
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IESG IESG state Replaced by draft-ietf-avtcore-multi-party-rtt-mix
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


Real-time text mixers for multi-party sessions need to identify the source of each transmitted group of text so that the text can be presented by endpoints in suitable grouping with other text from the same source. Regional regulatory requirements specify provision of real-time text in multi-party calls. RFC 4103 mixer implementations can use traditional RTP functions for source identification, but the mixer source switching performance is limited when using the default transmission with redundancy. An enhancement for RFC 4103 real-time text mixing is provided in the present specification, suitable for a centralized conference model that enables source identification and efficient source switching. The intended use is for real-time text mixers and multi-party-aware participant endpoints. The mechanism builds on use of the CSRC list in the RTP packet. A capability exchange is specified so that it can be verified that a participant can handle the multi-party coded real-time text stream. The capability is indicated by an sdp media attribute "rtt-mix". A brief description about how a mixer can format text for the case when the endpoint is not multi-party aware is also provided.


Gunnar Hellstrom (gunnar.hellstrom@ghaccess.se)

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)