MMUSIC Working Group T. Schierl Internet Draft S. Wenger Document: draft-schierl-mmusic-layered-codec-02 Expires: June 2007 December 2006 Signaling media decoding dependency in Session Description Protocol (SDP) Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on June 28, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This memo defines semantics that allow for signaling the decoding dependency of different media descriptions with the same media type in the Session Description Protocol (SDP). This is required, for example, if media data is separated and transported in different network streams INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 as a result of the use of a layered or multiple descriptive media coding process. A new grouping type "DDP" -- decoding dependency -- is defined, to be used in conjunction with RFC 3388 entitled "Grouping of Media Lines in the Session Description Protocol". In addition, an attribute is specified describing the relationship of the media streams in a "DDP" group. Schierl Standards Track [page 2]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 Table of Content 1. Introduction.................................................4 2. Terminology..................................................4 3. Definitions..................................................4 4. Motivation, Use Cases, and Architecture......................6 4.1. Motivation.................................................6 4.2. Use cases..................................................7 5. Signaling Media Dependencies.................................8 5.1. Design Principles..........................................8 5.2. Semantics..................................................8 5.2.1. SDP grouping semantics for decoding dependency............8 5.2.2. Attribute for dependency signaling per media-stream.......8 6. Usage of new semantics in SDP................................9 6.1.1. Usage with the SDP Offer/Answer Model.....................9 6.1.2. Declarative usage.........................................9 6.2. Examples..................................................10 7. Security Considerations.....................................11 8. IANA Consideration..........................................11 9. Acknowledgements............................................11 10. References..................................................11 10.1. Normative References......................................11 10.2. Informative References....................................12 11. Author's Addresses..........................................12 12. Intellectual Property Statement.............................12 13. Disclaimer of Validity......................................13 14. Copyright Statement.........................................13 15. RFC Editor Considerations...................................13 16. Change Log:.................................................13 Schierl Standards Track [page 3]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 1. Introduction An SDP session description may contain one or more media descriptions, each identifying a single media stream. A media description is identified by one "m=" line. Today, if more than one "m=" lines exist that indicate the same media type, a receiver cannot identify a specific relationship between those media. A layered or Multiple Description Coded (MDC) Media Bitstream contains, by definition, one or more Media Partitions that are conveyed in their own media stream. The only case we are interested in is a non-degraded layered/MDC Bitstream with two or more Media Partitions. Carrying more than one Media Partition in its own session is one of the key use cases for employing layered or MDC coded media. Senders, network elements, or receivers can suppress sending/forwarding/subscribing /decoding individual Media Partitions and still preserve perhaps suboptimal, but useful media quality. One property of all Media Bitstreams relevant to this memo is that their Media Partitions have a well-defined usage relationship. For example, in layered coding, "higher" Media Partitions are useless without "lower" ones. In MDC coding, Media Partitions are complementary -- the more Media Partitions one receives, the better the reproduced quality gets. At present, SDP and its supporting infrastructure of RFCs lack the means to express such a usage relationship. Trigger for the present memo has been the standardization process of the RTP payload format for the Scalable Video Coding extension to ITU-T Rec. H.264 / MPEG-4 AVC [SVCpayld]. When drafting [SVCpayld], it was observed that the aforementioned lack in signaling support is one that's not specific to SVC, but applies to all layered or MDC codecs. Therefore, this memo presents a generic solution. The mechanisms defined herein are media transport protocol independent, i.e. applicable beyond the use of RTP [RFC3550]. 2. Terminology 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 BCP 14, RFC 2119 [RFC2119]. 3. Definitions Media stream: As per [RFC4466]. Schierl Standards Track [page 4]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 Media Bitstream: A valid, decodable stream, containing ALL media partitions generated by the encoder. An Media Bitstream normally conforms to a media coding standard. Media Partition: A subset of a Media Bitstream indented for independent transportation. An integer number of Media Partitions form a Media Bitstream. In layered coding, a Media Partition represents one or more layers, that are handled as one unit. In MDC coding, a Media Partition represents one or more descriptions that are handled as a unit. Decoding dependency: The class of relationships media partitions have to each other. At present, this memo defines two decoding dependencies: layering and multiple description. Layered/hierarchical coding dependency: Each Media Partition is only useful (i.e. can be decoded) when ALL of the Media Partitions it depends on are available. The dependencies between the Media Partitions create a directed graph. Note: normally, in layered/hierarchical coding, the more Media Partitions are employed (following the rule above), the better a reproduced quality is possible. Multi description coding (MDC) dependency: N of M Media Partitions are required to form a Media Bitstream, but there is no hierarchy between these Media Partitions. Most MDC schemes aim at an increase of reproduced media quality when more media partitions are decoded than are necessarily required to form an Operation Point. Operation point: In layered coding, a subset of a layered Media Bitstream that includes all Media Partitions required for reconstruction at a certain point of quality or error resilience, and does not include any other Media Partitions. In MDC coding, a subset of a MDC Media Bitstream that is compliant with the MDC coding standard in question. The following terms are included for clarification on RTP [RFC3550] multiplexing techniques. Further discussion can be found in section 5.2 of [RFC3550]. Session multiplexing: The Media Bitstream is distributed onto different RTP sessions, whereby each RTP session carries one RTP packet stream with one or more Media Partitions of the Media Bitstream. Each RTP session requires a separate signaling and has a Schierl Standards Track [page 5]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 separate Timestamp, Sequence Number, and SSRC space. Dependency between sessions MUST be signaled according to this memo. 4. Motivation, Use Cases, and Architecture 4.1. Motivation Two types of dependencies are discussed in the following in more detail: layered or hierarchical decoding dependency, and multi- description decoding dependency. The transport of layered/hierarchical and multi description coding share as key motivators the desire for media adaptation to network conditions, i.e. related to bandwidth, error rates, connectivity of endpoints in multicast or broadcast scenarios, and similar. o Layered/hierarchical decoding dependency: In layered coding, the partitions of a Media Bitstream are known as media layers or simply layers. One or more layers may be transported in different network streams. A classic use case is known as receiver-driven layered multicast, in which a receiver selects a combination of media streams in response to quality or bit-rate requirements. Back in the mid 1990s, the then available layered media formats and codecs envisioned primarily (or even exclusively) a one-dimensional hierarchy of layers. That is, each so-called enhancement layer referred to exactly one layer "below". The single exception has been the base layer, which is self-contained. Therefore, the identification of one enhancement layer fully specifies the operation point of a layered decoding scheme, including knowledge about all the other layers that need to be decoded. [RFC4466] contains rudimentary support for exactly this use case and media formats, in that it allows for signaling a range of transport addresses for a certain media description. By definition, a higher transport address identifies a higher layer in the one-dimensional hierarchy. A receiver needs only to decode data conveyed over this transport address and lower transport addresses to decode this Operation Point. Newer media formats depart from this simple one-dimensional hierarchy, in that highly complex (at least tree-shaped) dependency hierarchies can be implemented. Compelling use cases for these complex hierarchies have been identified by industry. Support for it is therefore desirable. However, SDP in its current form does not allow for the signaling of these complex relationships. Therefore, receivers cannot make an informed decision on which layers to subscribe (in case of layered multicast). Schierl Standards Track [page 6]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 o Multi descriptive decoding dependency: In the most basic form of multiple description coding (MDC), each partition forms an independent representation of the media. That is, decoding of any of the Media Partition yields useful reproduced media data. When more than one Media Partition is available, then a decoder can process them jointly, and the resulting media quality increases. The highest reproduced quality is available if all original Media Partitions are available for decoding. More complex forms of multiple description coding can also be envisioned, i.e. where, as a minimum, N out of M total Media Partitions need to be available to allow meaningful decoding. MDC has not yet been embraced heavily by the media standardization community, though it is subject of a lot of academic research. As an example, we refer to [MDC]. We cover MDC in this memo because we a) envision that MDC media formats will come into the picture within the lifetime of this memo, and b) the solution for its signaling is very similar to the one of layered coding. 4.2. Use cases o Receiver driven layered multicast This technology is discussed in [RFC3550] and references therein. We refrain from elaborating further; the subject is well known and understood. o Multiple end-to-end transmission with different properties Assume a unicast and point-to-point topology, wherein one endpoint sends media to another. Assume further that different forms of media transmission are available. The difference may lie in the cost of the transmission (free, charged), in the available protection (unprotected/secure), in the quality of service (guaranteed quality / best effort), or other factors. Layered and MDC coding allow to match the media characteristics to the available transmission path(s). For example, in layered coding it makes sense to convey the base layer over high QoS and/or over an encrypted transmission path. Enhancement layers, on the other hand, can be conveyed over best effort, as they are "optional" in their characteristic -- nice to have, but non-essential for media consumption. Similarly, while it is essential that the base layer is encrypted, there is (at least conceptually) a lesser need to encrypt the enhancement layer, as the enhancement layer is less important and useful without the (encrypted) base layer in terms of resulting video Schierl Standards Track [page 7]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 quality. (This, of course depends highly on the media coding technology involved -- if the enhancement layer has some value in its own right, then it perhaps needs as strong encryption as the base layer.) In a different scenario, the base layer may be offered in a non-encrypted session as a free preview. An encrypted enhancement layer references this base layer and allows optimal quality play- back; however, it is only accessible to users who have the key, which may have been distributed by a conditional access mechanism. 5. Signaling Media Dependencies 5.1. Design Principles The dependency signaling is only feasible between media descriptions described with an "m="-line and with an assigned media identification attribute ("mid"), as defined in RFC3388. 5.2. Semantics 5.2.1. SDP grouping semantics for decoding dependency This specification defines the new grouping semantics Decoding Dependency "DDP": DDP associates a media stream, identified by its mid attribute, with a DDP group. Each media stream MUST be composed of an integer number of Media Partitions. All media streams of a DDP group MUST have the same type of coding dependency (as signaled by attribute defined in 5.2.2) and MUST belong to one media bitstream. All media streams MUST contain at least one operation point. The DDP group type informs a receiver about the requirement for treating the media streams of the group according to the new media level attribute "depend", as defined in 5.2.2. 5.2.2. Attribute for dependency signaling per media-stream This memo defines a new media-level value attribute, "depend", with the following BNF [RFC2234]. The "identification-tag" (if used) is defined in [RFC3388]: depend-attribute = "a=depend:" dependency-type-tag *(space identification-tag) dependency-type-tag = dependency dependency = "lay" / "mdc" The "depend"-attribute describes the decoding dependency. The "depend"-attribute may be followed by a sequence of identification- tag(s) which identify all related media streams. The attribute MAY Schierl Standards Track [page 8]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 be used with multicast as well as with unicast transport addresses. The following types of dependencies are defined: o lay: Layered decoding dependency -- identifies the described media stream as one or more partitions of a layered media bitstream. When lay is used, all media streams MUST be identified by the following identification-tag(s) that are required for a successful use of the media stream. The identification-tag(s) MUST be present when lay is in use. Further the described media stream represents one operation point of the layered media bitstream. As a result, all other media streams belonging to the same dependency group, but not identified by an identification-tag in the media description, are not required for a successful reproduction of the operation point represented by the media stream in question. Hence, a media sender MAY omit sending them when that is advantageous from a scalability or transport viewpoint. o mdc: Multi descriptive decoding dependency -- signals that the described media stream is or one more partitions of a multi description coding (MDC) media bitstream. By definition, at least N out of M streams of the group MUST be received for allowing decoding the media, whereby N and M are media stream dependent and not signaled. Receiving more than one media stream of the group may enhance the decodable quality of the media bitstream. This type of dependency does not require the signaling of the depended media streams. 6. Usage of new semantics in SDP 6.1.1. Usage with the SDP Offer/Answer Model If an Answerer does not understand the decoding dependency signaling, it should be able to detect the 'base' media only for a layered media session or should be able to detect only one partition of MDC media session. That is, the session description SHOULD offer a backward compatible partition of the media stream with a separate media description. This media description MAY point to the same transport address as used for an extended media session description using the features defined in this memo. Following this reasoning, an Answerer can obtain minimum service quality (by decoding the base layer or a single description) even when it does not understand the mechanisms of this memo. Edt. note: need more text here. 6.1.2. Declarative usage Edt. note: To be written Schierl Standards Track [page 9]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 6.2. Examples a.) Example for signaling transport of operation points of a layered video bitstream in different network streams: v=0 o=svcsrv 289083124 289083124 IN IP4 host.example.com s=LAYERED VIDEO SIGNALING Seminar t=0 0 c=IN IP4 224.2.17.12/127 a=group:DDP 1 2 3 4 m=video 40000 RTP/AVP 94 b=AS:96 a=framerate:15 a=rtpmap:94 h264/90000 a=mid:1 m=video 40002 RTP/AVP 95 b=AS:64 a=framerate:15 a=rtpmap:95 svc1/90000 a=mid:2 a=depend:lay 1 m=video 40004 RTP/AVP 96 b=AS:128 a=framerate:30 a=rtpmap:96 svc1/90000 a=mid:3 a=depend:lay 1 m=video 40004 RTP/SAVP 100 c=IN IP4 224.2.17.13/127 b=AS:512 k=uri:conditional-access-server.example.com a=framerate:30 a=rtpmap:100 svc1/90000 a=mid:4 a=depend:lay 1 3 b.) Example for signaling transport of streams of a multi description (MDC) video bitstream in different network streams: v=0 o=mdcsrv 289083124 289083124 IN IP4 host.example.com s=MULTI DESCRIPTION VIDEO SIGNALING Seminar Schierl Standards Track [page 10]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 t=0 0 c=IN IP4 224.2.17.12/127 a=group:DDP 1 2 3 m=video 40000 RTP/AVP 94 a=mid:1 a=depend:mdc m=video 40002 RTP/AVP 95 a=mid:2 a=depend:mdc m=video 40004 RTP/AVP 96 c=IN IP4 224.2.17.13/127 a=mid:3 a=depend:mdc 7. Security Considerations 8. IANA Consideration 9. Acknowledgements Funding for the RFC Editor function is currently provided by the Internet Society. Further, the author Thomas Schierl of Fraunhofer HHI is sponsored by the European Commission under the contract number FP6-IST-0028097, project ASTRALS. 10. References 10.1. Normative References [RFC4466] Handley, M., Jacobson, V, and C. Perkins, "SDP: Session Description Protocol", IETF work in progress, July 2006. [RFC3388] Camarillo, G., Holler, J., and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997 Schierl Standards Track [page 11]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 10.2. Informative References [SVCpayld] Wenger,S., Wang, Y.-K., and T. Schierl, "RTP Payload Format for SVC Video", "draft-ietf-avt-rtp-svc-00.txt", December 2006 [MDC] Vitali, A., Borneo, A., Fumagalli, M., and R. Rinaldo, "Video over IP using Standard-Compatible Multiple Description Coding: an IETF proposal", Packet Video Workshop, April 2006, Hangzhou, China 11. Author's Addresses Thomas Schierl Phone: +49-30-31002-227 Fraunhofer HHI Email: schierl@hhi.fhg.de Einsteinufer 37 D-10587 Berlin Germany Stephan Wenger Phone: +358-50-486-0637 Nokia Research Center Email: stewe@stewe.org P.O. Box 100 FIN-33721 Tampere Finland 12. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Schierl Standards Track [page 12]
INTERNET-DRAFT draft-schierl-mmusic-layered-codec-02 December 2006 13. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 14. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 15. RFC Editor Considerations none 16. Change Log: 19Dec06 / TS: removed SSRC multiplexing and with that various information about RTP draft title correction corrected SDP reference editorial modifications throughout the document added Stephan Wenger to the list of authors removed section "network elements not supporting dependency singaling" 20-28Dec06 / TS, StW: Editorial improvements Schierl Standards Track [page 13]