Network Working Group S. Wenger Internet Draft Y.-K. Wang Document: draft-wenger-avt-rtp-svc-00.txt Expires: April 2006 October 2005 RTP Payload Format for SVC Video 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 March 29, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This memo describes an RTP Payload format for the scalable extension of the ITU-T Recommendation H.264 video codec which is the technically identical to ISO/IEC International Standard 14496-10 video codec. The RTP payload format allows for packetization of one or more Network Abstraction Layer Units (NALUs), produced by the video encoder, in each RTP payload. The payload format has wide applicability, as it supports applications from simple low bit-rate conversational usage, to Internet video streaming with interleaved transmission, to high bit-rate video-on-demand. INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 Table of Content RTP Payload Format for SVC Video...............................1 1. Introduction............................................3 1.1. SVC - the scalable enhancement of H.264/AVC...............3 2. Conventions.............................................3 3. The SVC Codec ...........................................3 3.1. Overview..............................................3 3.2. Parameter Set Concept...................................4 3.3. Network Abstraction Layer Unit Header ....................4 4. Scope...................................................6 5. Definitions and Abbreviations.............................7 5.1. Definitions............................................7 5.2. Abbreviations..........................................7 6. RTP Payload Format.......................................7 6.1. RTP Header Usage.......................................7 6.2. Common Structure of the RTP Payload Format................8 6.3. NAL Unit Header Usage...................................8 6.4. Packetization Modes.....................................8 6.5. Decoding Order Number (DON).............................8 6.6. Single NAL Unit Packet..................................8 6.7. Aggregation Packets.....................................8 6.8. Fragmentation Units (FUs)...............................9 7. Packetization Rules......................................9 8. De-Packetization Process (Informative).....................9 9. Payload Format Parameters.................................9 9.1. MIME Registration......................................9 9.1.1. Mapping of MIME Parameters to SDP......................10 9.1.2. Usage with the SDP Offer/Answer Model..................11 9.1.3. Usage in Declarative Session Descriptions..............11 10. Examples...............................................11 11. Parameter Set Considerations.............................11 12. Security Considerations .................................11 13. Congestion Control......................................11 14. IANA Consideration......................................12 15. Acknowledgements........................................12 16. References.............................................12 16.1. Normative References...................................12 16.2. Informative References.................................12 17. Author's Addresses......................................12 RFC Editor Considerations ....................................13 Open Issues.................................................13 18. Changes Log............................................13 Wenger, Wang Standards Track [page 2] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 1. Introduction 1.1. SVC - the scalable enhancement of H.264/AVC This memo specifies an RTP [RFC3550] payload format for a forthcoming new mode of the H.264/AVC video codec, known as Scalable Video Coding (SVC). Formally, SVC will take the form of an Amendment to ISO/IEC 14496 Part 10 [MPEG4-10], and likely as one or more new Annexes of ITU-T Rec. H.264 [H.264]. It is planned to keep the technical alignment between the two mentioned specifications, as well as backward compatibility with previous versions of H.264/AVC. The current working draft of SVC is available for public review [SVC]. Technical maturity will be reached perhaps around mid 2006 for which timeframe the ISO/IEC Committee Draft is expected. In this memo, SVC is used as an acronym for the mentioned scalable extensions of H.264/AVC. SVC covers all of H.264/AVC's applications, ranging from all forms of digital compressed video from, low bit-rate Internet streaming applications to HDTV broadcast and Digital Cinema applications with nearly lossless coding. This memo tries to follow a similar philosophy by keeping as close an alignment to the H.264/AVC payload RFC [RFC3984] as possible. It basically documents the enhancements relevant from an RTP transport viewpoint, defines signaling support for SVC, and deprecates the single NAL unit mode of RFC 3984. 2. Conventions 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]. This specification uses the notion of setting and clearing a bit when bit fields are handled. Setting a bit is the same as assigning that bit the value of 1 (On). Clearing a bit is the same as assigning that bit the value of 0 (Off). 3. The SVC Codec 3.1. Overview SVC provides scalable video bitstreams. A scalable video bitstream contains a non-scalable base layer and one or more enhancement layers. An enhancement layer may enhance the temporal resolution (i.e. the frame rate), the spatial resolution, or the quality of the video content represented by the lower layer or part thereof. The scalable layers can be aggregated to a single RTP stream, or transported independently. Wenger, Wang Standards Track [page 3] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 The concept of video coding layer (VCL) and network abstraction layer (NAL) is inherited from AVC. The VCL contains the signal processing functionality of the codec; mechanisms such as transform, quantization, motion-compensated prediction, loop filter, inter- layer prediction. A coded picture of a base or enhancement layer consists of one or more slices. The Network Abstraction Layer (NAL) encapsulates each slice generated by the VCL into one or more Network Abstraction Layer Units (NAL units). Please consult RFC 3984 for a more in-depth discussion of the NAL unit concept. Each SVC layer is formed by NAL units, representing the coded video bits of the layer. An RTP stream carrying only one layer would carry NAL units belonging to that layer only. An RTP stream carrying a complete scalable video bit stream would carry NAL units of a base layer and one or more enhancement layers. SVC specifies the decoding order of these NAL units. The concept of scaling the visual content quality by omitting the transport and decoding of entire enhancement layers is denoted as coarse-grained scalability (CGS). In some cases, the bit rate of a given enhancement layer can be reduced by truncating bits from individual NAL units. Truncation leads to a graceful degradation of the video quality of the reproduced enhancement layer. This concept is known as fine-grained (granularity) scalability (FGS). 3.2. Parameter Set Concept The parameter set concept is inherited from AVC. In SVC, pictures from different layers may use the same sequence or picture parameter set and may also use different sequence or picture parameter sets. If different sequence parameter sets are used, then at any time instant during the decoding process, there may be more than one active sequence picture parameter set. Any specific active sequence parameter set remains unchanged throughout a coded video sequence, and any active picture parameter set remains unchanged within a coded picture. 3.3. Network Abstraction Layer Unit Header An SVC NAL unit consists of a header of one, two or three bytes and the payload byte string. The header indicates the type of the NAL unit, the (potential) presence of bit errors or syntax violations in the NAL unit payload, information regarding the relative importance of the NAL unit for the decoding process, and (optionally, when the header is of three bytes) the scalable layer decoding dependency information. This RTP payload specification is designed to be unaware of the bit string in the NAL unit payload. The NAL unit header co-serves as the payload header of this RTP payload format. The payload of a NAL unit follows immediately. Wenger, Wang Standards Track [page 4] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 The syntax and semantics of the NAL unit header are specified in [SVC], but the essential properties of the NAL unit header are summarized below. The first byte of the NAL unit header has the following format (it's the same as in H.264/AVC and RFC 3984): +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ |F|NRI| Type | +---------------+ F: 1 bit forbidden_zero_bit. The H.264 specification declares a value of 1 as a syntax violation. NRI: 2 bits nal_ref_idc. A value of 00 indicates that the content of the NAL unit is not used to reconstruct reference pictures for inter picture prediction. Such NAL units can be discarded without risking the integrity of the reference pictures in the same layer. Values greater than 00 indicate that the decoding of the NAL unit is required to maintain the integrity of the reference pictures. For a slice or slice data partitioning NAL unit, a NRI value of 11 indicates that the NAL unit contains data of a key picture, as specified in [SVC]. Informative Note: The concept of a key picture has been introduced in SVC, and no assumption should be made that any pictures in bit streams compliant with the 2003 and 2005 versions of H.264 follow this rule. Type: 5 bits nal_unit_type. This component specifies the NAL unit payload type as defined in table 7-1 of [SVC], and later within this memo. For a reference of all currently defined NAL unit types and their semantics, please refer to section 7.4.1 in [SVC]. Previously, NAL unit types 20 and 21 (among others) have been reserved for future extensions. SVC is using these two NAL unit types. They indicate the presence of one more byte that is helpful from a transport viewpoint. +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ | PRID |D|E| +---------------+ PRID: 6 bits simple_priority_id. This component specifies a priority identifier for the NAL unit. When extension_flag is equal to 0, simple_priority_id is used for inferring the values of dependency_id, temporal_level, and quality_level. When Wenger, Wang Standards Track [page 5] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 simple_priority_id is not present, it shall be inferred to be equal to 0. D: 1 bit discardable_flag. A value of 1 indicates that the content of the NAL unit (dependency_id = currDependencyId) is not used in the decoding process of NAL units with dependency_id > currDependencyId. Such NAL units can be discarded without risking the integrity of higher scalable layers with larger values of dependency_id. discardable_flag equal to 0 indicates that the decoding of the NAL unit is required to maintain the integrity of higher scalable layers with larger values of dependency_id. E: 1 bit extension_flag. A value of 1 indicates that the third byte of the NAL unit header is present. When the E-bit of the second byte is 1, then the NAL unit header extends to a third byte: +---------------+ |0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+ | TL | DID | QL| +---------------+ TL: 3 bits temporal_level. This component is used to indicate temporal scalability or frame rate. A layer consisted of pictures of a smaller temporal_level value has a smaller frame rate. DID: 3 bits dependency_id. This component is used to indicate the inter-layer coding dependency hierarchy. At any temporal location, a picture of a smaller dependency_id value may be used for inter-layer prediction for coding of a picture with a larger dependency_id value. QL: 2 bits quality_level. This component is used to indicate FGS layer hierarchy. At any temporal location and with identical dependency_id value, an FGS picture with quality_level value equal to QL uses the FGS picture or base quality picture (the non-FGS picture when QL-1 = 0) with quality_level value equal to QL-1 for inter-layer prediction. When QL is larger than 0, the NAL unit contains an FGS slice or part thereof. This memo introduces new NAL unit types, which are presented in section 5.2. The NAL unit types defined in this memo are marked as unspecified in [SVC]. Moreover, this specification extends the semantics of F, NRI, PRID, D, TL, DID and QL as described in section 5.3. 4. Scope Wenger, Wang Standards Track [page 6] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 This payload specification can only be used to carry the "naked" SVC NAL unit stream over RTP, and not the bitstream format discussed in Annex B of SVC. Likely, the applications of this specification will be in the IP based multimedia communications fields including conversational multimedia, video telephony or video conferencing, Internet streaming and TV over IP. 5. Definitions and Abbreviations 5.1. Definitions This document uses the definitions of [SVC] and [H.264]. The following terms, defined in [SVC], are summed up for convenience: scalable bitstream: an SVC compliant bit stream containing a base layer and at least one enhancement layer access unit: A set of NAL units pertaining to a certain temporal location. An access unit includes the slice data of the pictures of all scalable layers at that temporal location and possibly other associated data e.g. SEI messages and parameter sets. coded video sequence: A sequence of access units that consists, in decoding order, of an instantaneous decoding refresh (IDR) access unit followed by zero or more non-IDR access units including all subsequent access units up to but not including any subsequent IDR access unit. IDR access unit: An access unit in which all the primary coded pictures are IDR pictures. IDR picture: A coded picture with the property that the decoding of this coded picture and all the following coded pictures in decoding order, in the same layer (i.e. with the same values of dependency_id and quality_level, respectively), can be performed without inter prediction from any picture prior to the coded picture in decoding order in the same layer. An IDR picture causes a "reset" in the decoding process of the scalable layer containing the IDR picture. 5.2. Abbreviations In addition to the abbreviations defined in [RFC3984], the following ones are defined. CGS: Coarse granularity scalability FGS: Fine granularity scalability MANE: Media-aware network element 6. RTP Payload Format 6.1. RTP Header Usage Please see section 5.1 of RFC3984 [RFC3984]. Wenger, Wang Standards Track [page 7] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 6.2. Common Structure of the RTP Payload Format Please see section 5.2 of RFC3984 [RFC3984]. 6.3. NAL Unit Header Usage The structure and semantics of the NAL unit header were introduced in section 3.3. This section specifies the semantics of F, NRI, PRID, D, TL, DID and QL according to this specification. The semantics of F specified in section 5.3 of [RFC3984] also applies herein. For NRI, for the bitstream that is compliant with AVC, the semantics specified in section 5.3 of [H.264] are applicable, otherwise only the semantics specified in SVC [SVC] is applicable. For PRID, in addition to the semantics specified in [SVC], according to this RTP payload specification, values of PRID indicate the relative transport priority, as determined by the encoder, which is typically increasing from a lower layer to a higher layer. MANEs can use this information to protect more important NAL units better than they do less important NAL units. The transport priority increases as the PRID value increases. For D, MANEs can use this information to protect NAL units with D equal to 0 better than they do NAL units with D equal to 1. For TL, DID and QL, in addition to the semantics specified in [SVC], according to this RTP payload specification, values of TL, DID or QL indicate the relative transport priority. MANEs can use this information to protect more important NAL units better than they do less important NAL units. A higher value of TL, DID or QL indicates a higher priority if the other two components are identical correspondingly. Informative note: Using of SPID, D, TL, DID and QL in combination may better indicate the relative transport priority. 6.4. Packetization Modes Please see section 5.4 of RFC3984 [RFC3984]. The single NAL unit mode SHALL NOT be used. 6.5. Decoding Order Number (DON) Please see section 5.5 of RFC3984 [RFC3984]. 6.6. Single NAL Unit Packet Please see section 5.6 of RFC3984 [RFC3984]. 6.7. Aggregation Packets Please see section 5.7 of RFC3984 [RFC3984]. Wenger, Wang Standards Track [page 8] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 6.8. Fragmentation Units (FUs) Please see section 5.8 of RFC3984 [RFC3984]. 7. Packetization Rules Please see section 6 of RFC3984 [RFC3984]. The single NAL unit mode SHALL NOT be used. 8. De-Packetization Process (Informative) Please see section 7 of RFC3984 [RFC3984]. The single NAL unit mode SHALL NOT be used. 9. Payload Format Parameters Edt. note: this section 9 and its subsections will be updated according to the changes listed below, a little later in the process. For now, we just list the adjustments necessary, so not to bury any new information in the RFC 3984 text. Section 8 of [RFC3984] applies with the following modification. The sentence "The parameters are specified here as part of the MIME subtype registration for the ITU-T H.264 | ISO/IEC 14496-10 codec." is replaced with "The parameters are specified here as part of the MIME subtype registration for the SVC codec." 9.1. MIME Registration The MIME subtype for the SVC codec is allocated from the IETF tree. The receiver MUST ignore any unspecified parameter. Media Type name: video Media subtype name: H.264-SVC Required parameters: none OPTIONAL parameters: The optional MIME parameters specified in [RFC3984] apply, in addition to the following. sprop-scalability-info: This parameter MAY be used to convey the NAL unit containing the scalability information SEI message that MUST precede any other NAL units in decoding order. The parameter MUST NOT be used to indicate Wenger, Wang Standards Track [page 9] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 codec capability in any capability exchange procedure. The value of the parameter is the base64 representation of the NAL unit containing the scalability information SEI message as specified in [RFC3984]. Encoding considerations: This type is only defined for transfer via RTP (RFC 3550). Security considerations: See section 9 of this specification. Public specification: Please refer to section 15 of this specification. Additional information: None File extensions: none Macintosh file type code: none Object identifier or OID: none Person & email address to contact for further information: Intended usage: COMMON Author: Change controller: IETF Audio/Video Transport working group delegated from the IESG. SDP Parameters 9.1.1. Mapping of MIME Parameters to SDP The MIME media type video/SVC string is mapped to fields in the Session Description Protocol (SDP) as follows: * The media name in the "m=" line of SDP MUST be video. * The encoding name in the "a=rtpmap" line of SDP MUST be SVC (the MIME subtype). * The clock rate in the "a=rtpmap" line MUST be 90000. * The OPTIONAL parameters "profile-level-id", "max-mbps", "max-fs", "max-cpb", "max-dpb", "max-br", "redundant-pic-cap", "sprop- parameter-sets", "parameter-add", "packetization-mode", "sprop- interleaving-depth", "deint-buf-cap", "sprop-deint-buf-req", "sprop-init-buf-time", "sprop-max-don-diff", "max-rcmd-nalu- size", and "sprop-scalability-info", when present, MUST be included in the "a=fmtp" line of SDP. These parameters are expressed as a MIME media type string, in the form of a semicolon separated list of parameter=value pairs. Wenger, Wang Standards Track [page 10] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 9.1.2. Usage with the SDP Offer/Answer Model TBD. 9.1.3. Usage in Declarative Session Descriptions TBD. 10. Examples TBD. 11. Parameter Set Considerations Please see section 10 of RFC3984 [RFC3984]. 12. Security Considerations Please see section 11 of RFC3984 [RFC3984]. 13. Congestion Control Within any given RTP session carrying payload according to this specification, the provisions of section 12 of RFC3984 [RFC3984] apply. On key motivation for the introduction of a scalable codec has been the problem of network congestion as a whole. While scalability cannot reduce congestion for the transport path of a given RTP session, MANEs and layered multicast technologies can be used to alleviate network-wide congestion. MANEs MAY alleviate congestion on their outgoing network path by a) removing the NAL units belonging to hierarchically "highest" enhancement layer (or set of enhancement layers) from an RTP stream carrying base and enhancement layers. b) removing some or all bits of a given FGS NAL unit. In both cases, the incoming RTP session is terminated in the MANE, and a second RTP session originates at the MANE. The MANE acts as an RTP translator. The concept of scalability keeps the implementation and computational effort within the MANE low, and avoids expensive and delay-intensive full transcoding (in the sense of reconstruction and re-encoding). When scalable layers are transported in their own RTP sessions, an RTP receiver SHOULD unsubscribe to one or more enhancement layers when it senses congestion, similar to what has been described in [McCanne/Vetterli]. This behavior could perhaps be sufficient to ease the network load to an acceptable level of congestion. Nevertheless, it MUST follow the mechanisms described in section 12 of [RFC3984]. Wenger, Wang Standards Track [page 11] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 14. IANA Consideration Edt. note: A new MIME type should be registered from IANA. 15. Acknowledgements Funding for the RFC Editor function is currently provided by the Internet Society. 16. References 16.1. Normative References [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [MPEG4-10] ISO/IEC International Standard 14496-10:2003. [H.264] ITU-T Recommendation H.264, "Advanced video coding for generic audiovisual services", May 2003. [SVC] Joint Video Team, "Joint Scalable Video Model JSVM-3 Annex S", available from http://ftp3.itu.ch/av-arch/jvt-site/ 2005_07_Poznan/JVT-P202r1.zip., July 2007 [RFC3984] Wenger, S. Hannuksela, M, Stockhammer, T, Westerlund, M, Singer, D, "RTP Payload Format for H.264 Video", RFC 3984, February 2005 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 16.2. Informative References [McCanne/Vetterli] V. Jacobson S. McCanne and M. Vetterli. Receiver- driven layered multicast. In Proc. of ACM SIGCOMM'96, pages 117--130, Stanford, CA, August 1996. 17. Author's Addresses Stephan Wenger Phone: +358-50-486-0637 Nokia Research Center Email: stewe@stewe.org P.O. Box 100 FIN-33721 Tampere Finland Ye-Kui Wang Phone: +358-50-486-7004 Nokia Research Center Email: ye-kui.wang@nokia.com P.O. Box 100 FIN-33721 Tampere Finland 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 Wenger, Wang Standards Track [page 12] INTERNET-DRAFT Scalable Video Codec RTP Payload Format October 2005 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. 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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. Copyright Statement Copyright (C) The Internet Society (2005). 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. RFC Editor Considerations none Open Issues 18. Changes Log Wenger, Wang Standards Track [page 13]