Network Working Group A. Roach Internet-Draft Mozilla Intended status: Standards Track S. Nandakumar Expires: August 21, 2016 Cisco Systems P. Thatcher Google February 18, 2016 RTP Stream Identifier (RID) Source Description (SDES) draft-ietf-avtext-rid-00 Abstract This document defines and registers an RTCP SDES item, RID, for identification of RTP streams associated with Encoded Streams and Dependent Streams. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 21, 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Roach, et al. Expires August 21, 2016 [Page 1]
Internet-Draft RID SDES February 2016 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Key Words for Requirements . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Usage of 'rid' in RTP and RTCP . . . . . . . . . . . . . . . 3 4.1. RTCP 'RID' SDES Extension . . . . . . . . . . . . . . . . 4 4.2. RTP 'RID' Header Extension . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5.1. New SDES item . . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 8.1. Normative References . . . . . . . . . . . . . . . . . . 5 8.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction RTP sessions frequently consist of multiple streams, each of which is identified at any given time by its SSRC; however, the SSRC associated with a stream is not guaranteed to be stable over its lifetime. Within a session, these streams can be tagged with a number of identifiers, including CNAMEs and MSIDs [I-D.ietf-mmusic-msid]. Unfortunately, none of these have the proper ordinality to refer to an individual stream; all such identifiers can appear in more than one stream at a time. While approaches that use unique Payload Types (PTs) per stream have been used in some applications, this is a semantic overloading of that field, and one for which its size is inadequate: in moderately complex systems that use PT to uniquely identify every potential combination of codec configuration and unique stream, it is possible to simply run out of values. To address this situation, we define a new RTCP SDES identifier that uniquely identifies a single stream. A key motivator for defining this identifier is the ability to differentiate among different encodings of a single Source Stream that are sent simultaneously (i.e., simulcast). This need for unique identification extends to Dependent Streams (i.e., layers used by a layered codec). At the same time, when Redundancy RTP Streams are in use, we also need an identifier that connects such streams to the RTP stream for which they are providing redundancy. To that end, when this new identifier is in use, it appears (and contains the same value) in Roach, et al. Expires August 21, 2016 [Page 2]
Internet-Draft RID SDES February 2016 both in the Redundancy RTP Stream as well as the stream it is correcting. For lack of a better term, we have elected to call this term "RID," which loosely stands for "RTP stream IDentifier." It should be noted that this isn't an overly-precise use of the term "RTP Stream," due to the lack of an existing well-defined term for the construct we are attempting to identify. See Section 3 for a formal definition of the exact scope of a RID. The use of RIDs in SDP is described in [I-D.ietf-mmusic-rid]. 2. Key Words for Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] 3. Terminology In this document, the terms "Source Stream", "Encoded Stream," "RTP Stream", "Source RTP Stream", "Dependent Stream", "Received RTP Stream", and "Redundancy RTP Stream" are used as defined in [RFC7656]. For Encoded Streams, the RID refers to the "Source RTP Stream" as defined by [RFC7656] Section 2.1.10. For Dependent Streams, it refers to the RTP Stream that, like the Source RTP Stream of an Encoded Stream, is the RTP Stream that is not a Redundancy RTP Stream. For conciseness, we define the term "RID RTP Stream" to refer to this construct. For clarity, when RID is used, Redundancy RTP Streams that can be used to repair Received RTP Streams will use the same RID value as the Received RTP Stream they are intended to be combined with. 4. Usage of 'rid' in RTP and RTCP The RTP fixed header includes the payload type number and the SSRC values of the RTP stream. RTP defines how you de-multiplex streams within an RTP session; however, in some use cases, applications need further identifiers in order to effectively map the individual RID RTP Streams to their equivalent payload configurations in the SDP. This specification defines a new RTCP SDES item [RFC3550], 'RID', which is used to carry these identifiers within RTCP SDES packets. This makes it possible for a receiver to associate received RTP Roach, et al. Expires August 21, 2016 [Page 3]
Internet-Draft RID SDES February 2016 packets (identifying the RID RTP Stream) with a media description having the format constraint specified. This specification also uses the RTP header extension for RTCP SDES items [I-D.ietf-avtext-sdes-hdr-ext] to allow carrying RID information in RTP packets. This allowes correlation at stream startup, or after stream changes where the use of RTCP may not be sufficiently responsive. 4.1. RTCP 'RID' SDES Extension 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RID=TBD | length | rid ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The rid payload is UTF-8 encoded and is not null-terminated. RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value. 4.2. RTP 'RID' Header Extension Because recipients of RTP packets will typically need to know which "a=rid" constraints they correspond to immediately upon receipt, this specification also defines a means of carrying RID identifiers in RTP extension headers, using the technique described in [I-D.ietf-avtext-sdes-hdr-ext]. As described in that document, the header extension element can be encoded using either the one-byte or two-byte header, and the identification-tag payload is UTF-8 encoded, as in SDP. As the identification-tag is included in an RTP header extension, there should be some consideration about the packet expansion caused by the identification-tag. To avoid Maximum Transmission Unit (MTU) issues for the RTP packets, the header extension's size needs to be taken into account when the encoding media. Note that set of header extensions included in the packet needs to be padded to the next 32-bit boundary [RFC5285]. It is RECOMMENDED that the identification-tag is kept short. In many cases, a one-byte tag will be sufficient; it is RECOMMENDED that implementations use the shortest identifier that fits their purposes. Roach, et al. Expires August 21, 2016 [Page 4]
Internet-Draft RID SDES February 2016 5. IANA Considerations 5.1. New SDES item RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this document. RFC EDITOR NOTE: Please replace TBD with the assigned SDES identifier value. This document adds the MID SDES item to the IANA "RTCP SDES item types" registry as follows: Value: TBD Abbrev.: RID Name: RTP Stream Identifier Reference: RFCXXXX 6. Security Considerations The actual identifiers used for RIDs are expected to be opaque. As such, they are not expected to contain information that would be sensitive, were it observed by third-parties. 7. Acknowledgements Many thanks for review and input from Cullen Jennings, Magnus Westerlund, Colin Perkins, Peter Thatcher, Jonathan Lennox, and Paul Kyzivat. 8. References 8.1. Normative References [I-D.ietf-avtext-sdes-hdr-ext] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP Header Extension for RTCP Source Description Items", draft-ietf-avtext-sdes-hdr-ext-02 (work in progress), July 2015. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. Roach, et al. Expires August 21, 2016 [Page 5]
Internet-Draft RID SDES February 2016 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003, <http://www.rfc-editor.org/info/rfc3550>. [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July 2008, <http://www.rfc-editor.org/info/rfc5285>. [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms for Real-Time Transport Protocol (RTP) Sources", RFC 7656, DOI 10.17487/RFC7656, November 2015, <http://www.rfc-editor.org/info/rfc7656>. 8.2. Informative References [I-D.ietf-mmusic-msid] Alvestrand, H., "WebRTC MediaStream Identification in the Session Description Protocol", draft-ietf-mmusic-msid-11 (work in progress), October 2015. [I-D.ietf-mmusic-rid] Thatcher, P., Zanaty, M., Nandakumar, S., Burman, B., Roach, A., and B. Campen, "RTP Payload Format Constraints", draft-ietf-mmusic-rid-04 (work in progress), February 2016. Authors' Addresses Adam Roach Mozilla Email: adam@nostrum.com Suhas Nandakumar Cisco Systems Email: snandaku@cisco.com Peter Thatcher Google Email: pthatcher@google.com Roach, et al. Expires August 21, 2016 [Page 6]