Network Working Group S. Wenger Internet Draft Y.-K. Wang Document: draft-wenger-avt-rtp-svc-02.txt T. Schierl Expires: December 2006 June 2006 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 September 5, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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. Wenger, Wang, Schierl Standards Track [page 2] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 Table of Content RTP Payload Format for SVC Video...................................1 1. Introduction.................................................5 1.1. SVC - the scalable extensions of H.264/AVC..................5 2. Conventions..................................................5 3. The SVC Codec................................................6 3.1. Overview....................................................6 3.2. Parameter Set Concept.......................................6 3.3. Network Abstraction Layer Unit Header.......................7 4. Scope........................................................9 5. Definitions and Abbreviations...............................10 5.1. Definitions................................................10 5.2. Abbreviations..............................................11 6. RTP Payload Format..........................................11 6.1. Design Principles..........................................11 6.2. RTP Header Usage...........................................12 6.3. Common Structure of the RTP Payload Format.................12 6.4. NAL Unit Header Usage......................................12 6.5. Packetization Modes........................................13 6.6. Decoding Order Number (DON)................................13 6.7. Single NAL Unit Packet.....................................13 6.8. Aggregation Packets........................................13 6.9. Fragmentation Units (FUs)..................................13 6.10. Payload Content Scalability Information (PACSI) NAL Unit...14 7. Packetization Rules.........................................15 8. De-Packetization Process (Informative)......................16 9. Payload Format Parameters...................................16 9.1. MIME Registration..........................................16 9.2. SDP Parameters.............................................18 9.2.1. Mapping of MIME Parameters to SDP........................18 9.2.2. Usage with the SDP Offer/Answer Model....................18 9.2.3. Usage in Declarative Session Descriptions................18 9.3. Examples...................................................18 9.4. Parameter Set Considerations...............................18 10. Security Considerations.....................................19 11. Congestion Control..........................................19 12. IANA Consideration..........................................20 13. NAL Unit Reordering for Layered Multicast...................20 14. Informative Appendix: Application Examples..................20 14.1. Introduction...............................................20 14.2. Layered Multicast..........................................21 14.3. Streaming of an SVC scalable stream........................22 14.4. Multicast to MANE, SVC scalable stream to endpoint.........22 Wenger, Wang, Schierl Standards Track [page 3] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 14.5. Scenarios currently not considered for complexity reasons..24 14.6. Scenarios currently not considered for being unaligned with IP philosophy........................................................24 15. Acknowledgements............................................26 16. References..................................................26 14.7. Normative References.......................................26 14.8. Informative References.....................................26 15. Author's Addresses..........................................27 16. Intellectual Property Statement.............................27 17. Disclaimer of Validity......................................28 18. Copyright Statement.........................................28 19. RFC Editor Considerations...................................28 20. Open Issues.................................................28 21. Changes Log.................................................28 Wenger, Wang, Schierl Standards Track [page 4] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 1. Introduction 1.1. SVC - the scalable extensions 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. 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 backward compatible enhancement philosophy similar to what the video coding standardization committees implement, 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). Wenger, Wang, Schierl Standards Track [page 5] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 3. The SVC Codec 3.1. Overview SVC provides scalable video bitstreams. A scalable video bitstream contains a 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. 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 filtering and 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. SVC specifies the decoding order of these NAL units. The term "Layer" in Video Coding Layer and Network Abstraction Layer refers to a conceptual distinction, and is closely related to syntax layers (block, macroblock, slice, ... layers). It should not be confused with base and enhancement layers. 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 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 Wenger, Wang, Schierl Standards Track [page 6] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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 in the layer in which the active sequence parameter set is referred to. The 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 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. 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 (the bit fields are the same as in H.264/AVC and RFC 3984, while the semantics are slightly different, in a backward compatible way): +---------------+ |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 Wenger, Wang, Schierl Standards Track [page 7] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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 two more bytes as shown below. +---------------+---------------+ |0|1|2|3|4|5|6|7|0|1|2|3|4|5|6|7| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PRID |D| TL | DID | QL| +---------------+---------------+ PRID: 7 bits simple_priority_id. This component specifies a priority identifier for the NAL unit. When 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 with dependency_id equal to currDependencyId is not used in the decoding process of NAL units with dependency_id larger than 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. TL: 3 bits Wenger, Wang, Schierl Standards Track [page 8] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 temporal_level indicates the temporal layer (or frame rate) hierarchy. A layer consisted of pictures of a smaller temporal_level value has a smaller frame rate. DID: 3 bits dependency_id denotes 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 of a larger dependency_id value, while a picture of a larger dependency_id value is disallowed to be used for inter-layer prediction for coding of a picture of a smaller dependency_id value. QL: 2 bits quality_level designates the quality level hierarchy of a progressive refinement slice. At any temporal location and with identical dependency_id value, a quality enhancement of a picture with quality_level value equal to ql uses the quality enhancement or base quality information (the non-quality enhancement information of the slice when ql = 1) of the slice with quality_level value equal to ql- 1 for inter-layer prediction. When quality_level is larger than 0, the NAL unit contains a progressive refinement slice or part thereof. This memo introduces new NAL unit types, which are presented in section6.3. 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 6.4. 4. Scope This payload specification can only be used to carry the "naked" SVC NAL unit stream over RTP, and not the bitstream format according to 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. This specification allows, in a given RTP session, to encapsulate NAL units belong to o the base layer, or o one or more enhancement layers, or o the base layer and one or more enhancement layers Wenger, Wang, Schierl Standards Track [page 9] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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. base layer: The base layer is typically representing the minimal temporal and, or spatial resolution and, or minimal quality of an SVC bitstream. The base layer may be fully complying with [H.264]. The base layer is independently decodable without the requirement of using any other layer of the SVC bitstream. If the base layer contains NAL units fully conforming to [H.264] only, the layer is called H.264/AVC base layer. For such a layer the ability of signaling transport priority (simple_priority_id or temporal_level, dependency_id and quality_level) per NAL unit may not be given. operation point: A operation of a SVC bitstream represents a certain level of temporal, spatial and quality scalability. An operation point contains all NAL units required for successfully decoding a certain SVC enhancement layer, which represents the highest value of temporal and, or spatial and, or quality of the operation point. scalable enhancement layer: an SVC enhancement layer is identified by a certain NAL unit header value (transport priority) of simple_priority_id or, if present, by a combination of temporal_level, dependency_id, quality_level as defined in [SVC] and summarized in section 3.3. 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. Wenger, Wang, Schierl Standards Track [page 10] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 IDR access unit: An access unit in which all the primary coded pictures are IDR pictures. [Edt. note: This needs to be updated according to the new adoption of the enhancement-layer IDR (EIDR) concept in January 2006. At the time of writing, the SVC spec update for the January JVT meeting has not yet been available.] 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. [Edt. note: This needs to be updated according to the new adoption of the enhancement-layer IDR (EIDR) concept in January 2006. At the time of writing, the SVC spec update for the January JVT meeting has not yet been available.] progressive refinement slice: A progressive refinement slice [SVC] is contained in an SVC NAL unit and may be signaled, if extension_flag equal to one, by a quality_level not equal to zero. Such slices can be truncated byte-wise from the end in NAL unit payload byte-string order for bit-rate and quality reduction. This ability is also known as Fine Granularity Scalability (FGS). 5.2. Abbreviations In addition to the abbreviations defined in [RFC3984], the following ones are defined. CGS: Coarse Granularity Scalability FGS: Fine Granularity Scalability 6. RTP Payload Format 6.1. Design Principles The authors tried to follow design principles as follows: o Backward compatibility with RFC 3984 wherever possible. o As we expect the SVC base layer to be H.264/AVC compatible, we assume the base layer (when transmitted in its own session) to be Wenger, Wang, Schierl Standards Track [page 11] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 encapsulated using RFC 3984. Requiring this has the desirable side effect that it can be used by RFC 3984 legacy devices. o MANEs are signaling aware and rely on signaling information. In other words, MANEs have state. o MANEs terminate RTP sessions, and create different RTP sessions with perhaps modified content. [Edt. Note: need to clarify this wrt. Translators and Mixers.] o MANEs are within the security context of the RTP session. o Packet integrity needs to be preserved end-to-end (whereby end-to-end can mean endpoint to endpoint but also endpoint to MANE). o others? 6.2. RTP Header Usage Please see section 5.1 of RFC 3984 [RFC3984]. 6.3. Common Structure of the RTP Payload Format Please see section 5.2 of RFC 3984 [RFC3984]. 6.4. 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 sender, which is typically increasing from a layer of lower to a layer of higher importance. MANEs implementing unequal error protection can use this Wenger, Wang, Schierl Standards Track [page 12] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 information to protect more important NAL units better than less important ones, for example by including only the more important NAL units in a FEC protection mechanism. 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 NAL units with D equal to 1. Furthermore a MANE can determine whether the transmission of a NAL unit is required for successfully decoding a certain operation point of the SVC bitstream. 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 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 PRID, D, TL, DID and QL in combination may better indicate the relative transport priority. [Edt. note: such examples may be provided in Informative Appendix 13 in future versions.] 6.5. Packetization Modes Please see section 5.4 of RFC 3984 [RFC3984]. The single NAL unit mode SHALL NOT be used. 6.6. Decoding Order Number (DON) Please see section 5.5 of RFC 3984 [RFC3984]. 6.7. Single NAL Unit Packet Please see section 5.6 of RFC 3984 [RFC3984]. 6.8. Aggregation Packets Please see section 5.7 of RFC 3984 [RFC3984]. 6.9. Fragmentation Units (FUs) Please see section 5.8 of RFC 3984 [RFC3984]. Wenger, Wang, Schierl Standards Track [page 13] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 6.10. Payload Content Scalability Information (PACSI) NAL Unit A new NAL unit type, referred to as payload content scalability information (PACSI) NAL unit, is specified. The PACSI NAL unit, if present, MUST be the first NAL unit in an aggregation packet, and it MUST NOT be present in other types of packets. The PACSI NAL unit indicates scalability characteristics that are common for all the remaining NAL units in the payload, thus making it easier for MANEs to decide whether to forward or discard the packet. Senders MAY create PACSI NAL units and receivers can ignore them. Informative note: The NAL unit type for the PACSI NAL unit is selected among those values that are unspecified in the H.264/AVC specification and in RFC 3984. Thus, SVC streams having H.264/AVC base layer and including PACSI NAL units can be processed with RFC 3984 receivers and H.264/AVC decoders. When the first aggregation unit of an aggregation packet contains a PACSI NAL unit, there MUST be at least one additional aggregation unit present in the same packet. The RTP header fields are set according to the remaining NAL units in the aggregation packet. When a PACSI NAL unit is included in a multi-time aggregation packet, the decoding order number for the PACSI NAL unit MUST set to indicate that the PACSI NAL unit is the first NAL unit in decoding order among the NAL units in the aggregation packet or the PACSI NAL unit has an identical decoding order number to the first NAL unit in decoding order among the remaining NAL units in the aggregation packet. The structure of PACSI NAL unit is specified in Figure 1. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F|NRI| Type | PRID |D| TL | DID | QL| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1. Structure for payload content scalability information (PACSI) NAL unit. Wenger, Wang, Schierl Standards Track [page 14] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 The semantics of the fields in the PACSI NAL unit structure are specified in section 3.3. The values of the fields in PACSI NAL unit MUST be set as follows. o The F bit MUST be set to 1 if the F bit in any remaining NAL unit in the payload is equal to 1. Otherwise, the F bit MUST be set to 0. o The NRI field MUST be set to the highest value of NRI field among the remaining NAL units in the payload. o The Type field MUST be set to 30. o The PRID field MUST be set to the lowest value of the PRID field among the remaining NAL units in the payload. If the PRID field is not present in one of the remaining NAL units in the payload, the PRID field in the PACSI NAL unit must be set to 0. o The D bit MUST be set to 0 if the D bit in any remaining NAL unit in the payload is equal to 0. Otherwise, the D bit MUST be set to 1. o The TL field MUST be set to the lowest value of the TL field among the remaining NAL units in the payload. o The DID field MUST be set to the lowest value of the DID field among the remaining NAL units in the payload. o The QL field MUST be set to the lowest value of the QL field among the remaining NAL units in the payload. 7. Packetization Rules Please see section 6 of RFC 3984 [RFC3984]. The following rules apply in addition. The single NAL unit mode SHALL NOT be used. In an RTP session, the first NAL unit of an aggregation packet SHALL have a two- or three-byte NAL unit header containing the transport priority indicator, as described in section 3.3. Non-VCL NAL units SHALL be transmitted out-of-band or in a separate session for the current state of this specification. If aggregating NAL units of different layers within one aggregation packet, the first NAL unit of Wenger, Wang, Schierl Standards Track [page 15] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 the packet MUST have the highest transport priority of all NAL units contained in the packet. The order of NAL units within a packet is the same as the decoding order. 8. De-Packetization Process (Informative) Please see section 7 of RFC 3984 [RFC3984]. The following rules apply in addition. The single NAL unit mode SHALL NOT be used. Layered multicast is supported by this specification. An informative appendix on recovering NAL unit decoding order in layered multicast can be found in section 14. 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 Wenger, Wang, Schierl Standards Track [page 16] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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 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 [SVC]. sprop-transport-priority: This parameter MAY be used to signal the transport priority indicator value(s) in terms of the one or two byte SVC NAL unit header extension of one or more SVC layer(s) of one RTP session. A transport priority indicator is base64 coded. If more than one layer is transmitted within one RTP session, the transport priority indicator value of each layer MUST be itemized with decreasing importance for decoding and MUST be comma-separated. If a H.264/AVC base layer is part of the RTP session, this parameter SHALL not be used. 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: Wenger, Wang, Schierl Standards Track [page 17] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 Intended usage: COMMON Author: Change controller: IETF Audio/Video Transport working group delegated from the IESG. 9.2. SDP Parameters 9.2.1. 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", "sprop-transport-priority", 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. 9.2.2. Usage with the SDP Offer/Answer Model TBD. 9.2.3. Usage in Declarative Session Descriptions TBD. 9.3. Examples TBD. 9.4. Parameter Set Considerations Wenger, Wang, Schierl Standards Track [page 18] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 Please see section 10 of RFC 3984 [RFC3984]. 10. Security Considerations Please see section 11 of RFC 3984 [RFC3984]. 11. Congestion Control Within any given RTP session carrying payload according to this specification, the provisions of section 12 of RFC 3984 [RFC3984] apply. One key motivation for the recent attention to scalable codecs has been the increasing awareness of media codec designers to network congestion. While CGS scalability cannot reduce congestion for the transport path of a given RTP session, MANEs and layered multicast technologies can be used to alleviate congestion on a larger scale. FGS scalability can be helpful to reduce session bandwidth both end- to-end (with pre-coded content) and in network segments, again assuming the use of MANEs. 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 as long as the remaining bits still form a conforming SVC NAL unit. Edt. note: In the following paragraph, "translator" and "mixer" are not used consistently with RFC 3550. What we think we would need is a "mixer" that mixes only a single input in a single output (as a mixer terminates sessions). A "Translator" (that does not terminate the RTP session) carries certain unnecessary baggage which appears to make it undesirable for MANEs. The following paragraph can either be fixed into RFC 3550 style and logic (thereby removing an operation point we consider desirable), or we would need to explain in detail what we want to do (not really congestion control related and long). Perhaps we refer to the detailed discussions in the CCM draft... Added to open issues. 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 Wenger, Wang, Schierl Standards Track [page 19] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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]. 12. IANA Consideration [Edt. note: A new MIME type should be registered from IANA.] 13. NAL Unit Reordering for Layered Multicast Layered multicast is described in 14.2. In layered multicast, the base layer, one or more enhancement layers, or the base layer and one or more enhancement layers may be transmitted within separate RTP sessions, i.e. the NAL units required for decoding an access unit of a certain operation point of the scalable bitstream may be distributed in different RTP sessions. After receiving NAL units from different RTP sessions, recovering of the NAL unit decoding order is required. To enable a simple NAL unit decoder order recovering process, the following constraints shall be fulfilled in layered multicast: * The interleaved packetization mode must be used. * The DON values of all the NAL units, as specified section 5.5 of RFC 3984, shall indicate the correct NAL unit decoding order over all the RTP sessions. 14. Informative Appendix: Application Examples 14.1. Introduction Scalable video coding is a concept that has been around at least since MPEG-2 [MPEG2], which goes back as early as 1993. Nevertheless, it has never gained wide acceptance; perhaps partly Wenger, Wang, Schierl Standards Track [page 20] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 because applications didn't materialize in the form envisioned during standardization. MPEG and JVT, respectively, performed a requirement analysis before the SVC project was launched. Dozens of scenarios have been studied. While some of the scenarios appear not to follow the most basic design principles of the Internet -- and are therefore not appropriate for IETF standardization -- others are clearly in the scope of IETF work. Of these, this draft chooses the following subset for immediate consideration. Note that we do not reference the MPEG and JVT documents directly; partly, because at least the MPEG documents have a limited lifespan and are not publicly available, and partly because the language used in these documents is inappropriately video centric and imprecise, when it comes to protocol matters. With these remarks, we now introduce three main application scenarios that we consider as relevant, and that are implementable with this specification. 14.2. Layered Multicast This well-understood form of the use of layered coding [McCanne/Vetterli] implies that all layers are individually conveyed in their own RTP session using their own IP multicast address. Receivers "tune" into the layers by subscribing to the IP multicast, normally by using IGMP [IGMP]. Optimization forms could be envisioned in which a number of layers are sent combined in a single RTP session; but these optimizations are currently not considered in this document. Layered Multicast has the great advantage of simplicity and easy implementation. However, it has also the great disadvantage of utilizing many different ports. While we consider this not to be a major problem for a professionally maintained content server, receiving client endpoints need to open many ports to IP multicast addresses in their firewalls. This is a practical problem from a firewall/NAT viewpoint. Furthermore, even today IP multicast is not as widely deployed as many wish. We consider layered multicast an important application scenario for three reasons. First, it is well understood and the implementation constraints are well known. There may well by large scale IP networks outside the immediate Internet context that may wish to Wenger, Wang, Schierl Standards Track [page 21] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 employ layered multicast in the future. One possible example could be a combination of content creation and core-network distribution for the various mobile TV services, e.g. those being developed by 3GPP (MBMS) [MBMS] and DVB (DVB-H) [DVB-H]. Finally, when one base and one enhancement layer is in use and are being conveyed separately, that represents one operation point of layered multicast. 14.3. Streaming of an SVC scalable stream In this scenario, a streaming server has a repository of stored SVC coded layers for a given content. At the time of streaming, and according to the capabilities and connectivity of the client(s), the streaming server generates a scalable stream. This scalable stream is served to the client(s). Both unicast and multicast serving is possible. At the same time, the streaming server may use the same repository of stored layers to compose different streams (with a different set of layers) intended for different audiences. As every endpoint receives only a single SVC RTP session, the number of firewall pinholes can be optimized. In fact, only a single firewall pinhole is required. The main difference between this scenario and straightforward simulcasting lies in the architecture and the requirements of the streaming server, and is therefore out of the scope of IETF standardization. However, compelling arguments can be made why such a streaming server design makes sense. One possible argument is related to storage space and channel bandwidth. Another is bandwidth adaptivity without transcoding -- a considerable advantage in a congestion controlled network. When the streaming server learns about congestion, it can reduce sending bitrate by choosing fewer layers when composing the layered stream. SVC is designed to gracefully support both bandwidth rampdown and bandwidth rampup with a considerable dynamic range. This payload format is designed to allow for bandwidth flexibility in the mentioned sense, both for CGS and FGS layers. While, in theory, a transcoding step could achieve a similar dynamic range, the computational demands are impractically high and video quality is typically lowered -- therefore, few (if any) streaming servers implement full transcoding. 14.4. Multicast to MANE, SVC scalable stream to endpoint This final scenario is a bit more complex, and designed to optimize the network traffic in a core network, while still requiring only a Wenger, Wang, Schierl Standards Track [page 22] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 single pinhole in the endpoint's firewall. One of its key applications is the mobile TV market. Consider a large IP network, e.g. the core network of 3GPP. Streaming servers within this core network can be assumed to be professionally maintained. We assume that these servers can have many ports open to the network and that layered multicast is a real option. Therefore, we assume that the streaming server multicasts SVC scalable layers, instead of simulcasting different representations of the same content at different bit rates. Also consider many endpoints of different classes. Some of these endpoints may not have the processing power or the display size to meaningfully decode all layers; other may have these capabilities. Users of some endpoints may not wish to pay for high quality and are happy with a base service, which may be cheaper or even free. Other users are willing to pay for high quality. Finally, some connected users may have a bandwidth problem in that they can't receive the bandwidth they would want to receive -- be it through congestion, change of service quality, or for whatever other reasons. However, all these users have in common that they don't want to be exposed too much, and therefore the number of firewall pinholes need to be small. This situation can be handled best by introducing middleboxes close to the edge of the core network, which receive the layered multicast streams and compose the single SVC scalable bit stream according to the needs of the endpoint connected. These middleboxes are called MANEs throughout this specification. In practice, we envision the MANE to be part of (or at least physically and topologically close to) the base station of a mobile network, where all the signaling and media traffic necessarily are multiplexed on the same physical link. This is why we do not worry too much about decomposition aspects of the MANE as such. Edt. note: In the following paragraph, Mixers and Translators need to be clarified. MANEs necessarily need to be fairly complex devices. They certainly need to understand the signaling, so, for example, to associate the PT octet in the RTP header with the SVC payload type. Furthermore, they terminate the multicasted layered RTP sessions coming in from the core network side, and create new RTP sessions (perhaps even multicast sessions) to the endpoints connected to them. In RTP terminology, it appears that MANEs necessarily are mixers AND Wenger, Wang, Schierl Standards Track [page 23] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 translators; a MANE first mixes the content of one or more incoming RTP streams, and then "translates" it into the outgoing stream (which may involve pruning FGS coded NAL units and similar tasks). While the implementation complexity of a MANE, as discussed above, is fairly high, the computational demands are comparatively low. In particular, SVC and/or this specification contain means to easily generate the correct inter-layer decoding order of NAL units. It is also simple to identify the fine granularity scalable bits in a given NAL unit. No serious bit-oriented processing is required and no significant state information (beyond that of the signaling and perhaps the SVC sequence parameter sets) need to be kept. Finally, another scenario with very similar properties could be implemented in which the streaming server would send a single SVC scalable stream (containing basically all available scalable layers) to the MANE, and the MANE de-layers this scalable bit stream into its individual layers, before further processing. 14.5. Scenarios currently not considered for complexity reasons -- vacat -- 14.6. Scenarios currently not considered for being unaligned with IP philosophy Remarks have been made that the current draft does not take into consideration at least one application scenario which some JVT folks consider important. In particular, their idea is to make the RTP payload format (or the media stream itself) self-contained enough that a stateless, non signaling aware device can "thin" an RTP session to meet the bandwidth demands of the endpoint. They call this device a "Router" or "Gateway", and sometimes a MANE. Obviously, it's not a Router or Gateway in the IETF sense. To distinguish it from a MANE as defined in RFC 3984 and in this specification, let's call it a MDfH (Magic Device from Heaven). To simplify discussions, let's assume point-to-point traffic only. The endpoint has a signaling relationship with the streaming server, but it is known that the MDfH is somewhere in the media path (e.g. because the physical network topology ensures this). It has been requested, at least implicitly through MPEG's and JVT's requirements document, that the MDfH should be capable to intercept the SVC Wenger, Wang, Schierl Standards Track [page 24] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 scalable bit stream, modify it by dropping packets or parts thereof, and forwarding the resulting packet stream to the receiving endpoint. It has been requested that this payload specification contains protocol elements facilitating such an operation, and the argument has been made that the NRI field of RFC 3984 serves exactly the same purpose. The authors of this I-D do not consider the scenario above to be aligned with the most basic design philosophies the IETF follows, and therefore have not addressed the comments made (except through this section). In particular, we see the following problems with the MDfH approach): - As the very minimum, the MDfH would need to know which RTP streams are carrying SVC. We don't see how this could be accomplished but by using a static payload type. None of the IETF defined RTP profiles envision static payload types for SVC, and even the de- facto profiles developed by some application standard organizations (3GPP for example) do not use this outdated concept. Therefore, the MDfH necessarily needs to be at least "listening" to the signaling. - If the RTP packet payload were encrypted, it would be impossible to interpret the payload header and/or the first bytes of the media stream. We understand that there are crypto schemes under discussion that encrypt only the last n bytes of an RTP payload, but we are more than unsure that this is fully in line with the IETF's security vision. Even if the above two problems would have been overcome through standardization outside of the IETF, we still foresee serious design flaws: - An MDfH can't simply dump RTP packets it doesn't want to forward. It either needs to act as a full RTP Translator (implying that it patches RTCP RRs and such), or it needs to patch the RTP sequence numbers to fulfill the RTP specification. Not doing either would, for the receiver, look like the gaps in the sequence numbers occurred due to unintentional erasures, which has interesting effects on congestion control (if implemented), will break pretty much every meta-payload ever developed, and so on. (Many more points could be made here). - An MDfH also can't "prune" FGS packets. Again, doing so would not be compatible with meta payloads, and would mess up RTCP RRs and congestion control (if the congestion control is based on octet Wenger, Wang, Schierl Standards Track [page 25] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 count and not on packet count; there are discussions related to the former at least in the context of TFRC). In summary, based on our current knowledge we are not willing to specify protocol mechanisms that support an operation point that has so little in common with classic RTP use. 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-4 Annex G", available from http://ftp3.itu.ch/av-arch/jvt-site/ 2005_10_Nice/JVT-Q202.zip., October 2005 [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 [DVB-H] DVB - Digital Video Broadcasting (DVB); DVB-H Implementation Guidelines, ETSI TR 102 377, 2005 [IGMP] Cain, B., Deering S., Kovenlas, I., Fenner, B. and Thyagarajan, A., "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. Wenger, Wang, Schierl Standards Track [page 26] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 [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. [MBMS] 3GPP - Technical Specification Group Services and System Aspects; Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs (Release 6), December 2005 [MPEG2] ISO/IEC International Standard 13818-2:1993. 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 Thomas Schierl Phone: +49-30-31002-227 Fraunhofer HHI Email: schierl@hhi.fhg.de Einsteinufer 37 D-10587 Berlin Germany 18. 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 Wenger, Wang, Schierl Standards Track [page 27] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 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. 19. 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. 20. 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. 21. RFC Editor Considerations none 22. Open Issues 3. Need to clarify MANE, Mixers, and Translators throughout the document (consistently with RFC 3550). 4. Packetization rules need work oncs 3) is addressed 5. Alignment with JVT spec (ongoing) 23. Changes Log From -00 to -01 - 04.02.2006, StW: Added details to scope Wenger, Wang, Schierl Standards Track [page 28] INTERNET-DRAFT Scalable Video Codec RTP Payload Format February 2006 - 04.02.2006, StW: Added short subsection 6.1 "Design Principles" - 04.02.2006, StW: Added section 15, "Application Examples" - 06.02 - 03.03.2006, YkW: Various modifications throughout the document - 13.02.2006 - 03.03.2006 , ThS: Added definitions and additional information to section 3.3, 5.1, 7 and 8, parameters in section 9.1 and added section 14 for NAL unit re-ordering for layered multicast. Further modifications throughout the document From -01 to -02 - 06.03.2006, StW: Editorial improvements - 26.05.2006, YkW: Updated NAL unit header syntax and semantics according to the latest draft SVC spec - 20.06.2006, Miska/YkW: Added section 6.10 "Payload Content Scalability Information (PACSI) NAL Unit" - 20.06.2006, YkW: Updated the NAL unit reordering process for layered multicast (removed the old section 14 "Informative Appendix: NAL Unit Re-ordering for Layered Multicast" and added the new section 13 "NAL Unit Reordering for Layered Multicast") Wenger, Wang, Schierl Standards Track [page 29]