Network Working Group Phil Kerr Internet-Draft Ogg Vorbis Community / February 20, 2003 OpenDrama Expires: August 20, 2003 RTP Payload Format for Vorbis Encoded Audio Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. 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 RFC 2119 [1]. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document describes a RTP payload format for transporting Vorbis encoded audio. It details the encapsulation mechanism for raw Vorbis data and details the delivery mechanism for the decoder probability model, referred to as a codebook, and other decoder setup information. Kerr Expires August 20, 2003 [Page 1] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 Table of Contents 1. Introduction ........................................ 2 2. Payload Format ...................................... 3 2.1 RTP Header .......................................... 3 2.2 Payload Header ...................................... 4 2.3 Payload Data ........................................ 4 2.4 Example RTP Packet .................................. 5 3. Frame Packetizing ................................... 5 3.1 Example Fragmented Vorbis Packet .................... 6 4. IANA Considerations ................................. 7 5. Configuration headers ............................... 7 6. Session Description ................................. 10 7. Congestion Control .................................. 10 8. Security Considerations ............................. 10 9. Acknowledgments ..................................... 10 10. References .......................................... 11 11. Full Copyright Statement ............................ 11 12. Authors Address ..................................... 12 1 Introduction The Xiph.org Foundation creates and defines codecs for use in multimedia that are not encumbered by patents and thus may be freely implemented by any individual or organization. Vorbis is the general purpose multi-channel audio codec created by the Xiph.org Foundation. Vorbis encoded audio is generally encapsulated within an Ogg format bitstream, which provides framing and synchronization. For the purposes of RTP transport, this layer is unnecessary, and so raw Vorbis packets are used in the payload. Vorbis packets are unbounded in length currently. At some future point there will likely be a practical limit placed on packet length. Typical Vorbis packet sizes are from very small (2-3 bytes) to quite large (8-12 kilobytes). The reference implementation [2] typically produces packets less than ~800 bytes, except for the header packets which are ~4-12 kilobytes. Within a RTP context the maximum Vorbis packet SHOULD be kept below the MTU size of 1500 octets, including the RTP and payload headers, to avoid fragmentation. For the delivery of Vorbis audio using RTP the maximum size of the header block is limited to 64K. Kerr Expires August 20, 2003 [Page 2] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 2 Payload Format The standard RTP header is followed by an 8 bit payload header, then the payload data. 2.1 RTP Header 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The RTP header begins with an octet of fields (V, P, X, and CC) to support specialized RTP uses (see [4] and [5] for details). For Vorbis RTP applications, V is set to 2, and the P, X, and CC fields are set to 0. Marker (M): 1 bit Set to zero. Audio silence suppression not used. This conforms to section 4.1 of [6]. Payload Type (PT): 7 bits An RTP profile for a class of applications is expected to assign a payload type for this format, or a dynamically allocated payload type should be chosen which designates the payload as Vorbis. Sequence number: 16 bits The sequence number increments by one for each RTP data packet sent, and may be used by the receiver to detect packet loss and to restore packet sequence. This field is detailed further in [3]. Timestamp: 32 bits A timestamp representing the sampling time of the first sample of the first Vorbis packet in the RTP packet. The clock frequency MUST be set to the sample rate of the encoded audio data and is conveyed out-of-band. SSRC/CSRC identifiers: These two fields, 32 bits each with one SSRC field and a maximum of 16 CSRC field, are as defined in [3]. Kerr Expires August 20, 2003 [Page 3] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 2.2 Payload Header The first octet of the payload data is the payload header: 1 2 3 4 5 6 7 8 +---+---+---+---+---+---+---+---+ | C | F | R | # of packets | +---+---+---+---+---+---+---+---+ C: 1 bit Set to one if this is a continuation of a fragmented packet. F: 1 bit Set to one if the payload contains complete packets or if it contains the last fragment of a fragmented packet. R: 1 bit Reserved, must be set to zero by senders, and ignored by receivers. The last 5 bits are the number of complete packets in this payload. This provides for a maximum number of 32 Vorbis packets in the payload. If C is set to one, this number should be 0. 2.3 Payload Data If the payload contains a single Vorbis packet or a Vorbis packet fragment, the Vorbis packet data follows the payload header. For payloads which consist of multiple Vorbis packets, payload data consists of one octet representing the packet length followed by the packet data for each of the Vorbis packets in the payload. The Vorbis packet length octet is the length of the data block minus one. The payload packing of the Vorbis data packets SHOULD follow the guidelines set-out in section 4.4 of [5] where the oldest packet occurs immediately after the RTP packet header. Channel mapping of the audio is in accordance with BS. 775-1 ITU-R. Kerr Expires August 20, 2003 [Page 4] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 2.4 Example RTP Packet Here is an example RTP packet containing two Vorbis packets. RTP Packet Header: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 |0|0| 0 |0| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp (in sample rate units) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | contributing source (CSRC) identifiers | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Payload Data: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|1|0| # pks: 2| len | vorbis data ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...vorbis data... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | len | next vorbis packet data... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3 Frame Packetizing Each RTP packet contains either one complete Vorbis packet, one Vorbis packet fragment, or an integer number of complete Vorbis packets (upto a max of 32 packets, since the number of packets is defined by a 5 bit value). Any Vorbis packet that is larger than 256 octets and less than the path-MTU should be placed in a RTP packet by itself. Any Vorbis packet that is 256 bytes or less should be bundled in the RTP packet with as many Vorbis packets as will fit, up to a maximum of 32. If a Vorbis packet will not fit into the RTP packet, it must be fragmented. A fragmented packet has a zero in the last five bits of the payload header. Each fragment after the first will also set the Continued (C) bit to one in the payload header. The RTP packet containing the last fragment of the Vorbis packet will have the Marker (F) bit set to one. Kerr Expires August 20, 2003 [Page 5] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 3.1 Example Fragmented Vorbis Packet Here is an example fragmented Vorbis packet split over three RTP packets. RTP packet header details have been excluded from this example. Packet 1: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0|0|0| 0| len | vorbis data ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..vorbis data.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The number of packets field is set to 0. Packet 2: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|0|0| 0| len | vorbis data ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..vorbis data.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The C bit is set to 1 and the number of packets field is set to 0. For large Vorbis fragments there can be several of these type of payload packets. The maximum packet size should be no greater than the MTU of 1500 octets, including all RTP and payload headers. Packet 3: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|1|0| 0| len | vorbis data .. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..vorbis data.. | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This is the last Vorbis fragment packet. The C and F bits are set and the packet count remains set to 0. Kerr Expires August 20, 2003 [Page 6] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 4 IANA Considerations Media MIME type name: audio Media MIME subtype name: vorbis Required Parameters: none Optional Parameters: none 5 Configuration headers To decode a Vorbis stream three configuration header information blocks are needed. This data is sent out-of-band and is defined below as an APP defined RTCP message with the 4 octet name field set to VORB. On joining a session the first packet sent back to the client MUST be a Vorbis message containing the codec setup and codebook data. VORB RTCP packets MUST set the padding (P) flag and add the appropriate padding octets needed to conform with section 6.6 of [3]. Synchronising the configuration headers to the RTP stream is critical. A 32 bit timestamp field is used to indicate the timepoint when a VORB header MUST be applied to the RTP stream. VORB RTCP packets MUST be sent just ahead of the change in the RTP stream. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| subtype | PT=APP=204 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC/CSRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VORB | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp (in sample rate units) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vorbis Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Audio Sample Rate | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Bitrate Maximum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Bitrate Nominal | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Bitrate Minimum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | bsz 0 | bsz 1 | Num Audio Channels |c|m|o|x|x|x|x|x| +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ Kerr Expires August 20, 2003 [Page 7] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Codebook length | Codebook checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .. Codebook | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Vendor string length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor string .. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | User comments list length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .. User comment length / User comment | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ .. URI string | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The first Vorbis config header defines the Vorbis stream attributes. The Vorbis version MUST be set to zero to comply with this document. The fields Sample Rate up to Num Audio Channels are set in accordance with [6] with the bsz fields above referring to the blocksize parameters. The framing bit is not used for RTP transportation and so applications constructing Vorbis files MUST take care to set this if required. The next 8 bits are used to indicate the presence of the two other Vorbis stream config headers and the size overflow header. The c flag indicates the presence of a Codebook header block, the m flag indicates the presence of a comment metadata block. The o flag indicates if the size of either of the c and m headers would make the VORB packet greater than that allowed for a RTCP message. The remaining five bits, indicated with an x, are reserved/unused and MUST be set to 0. If the c flag is set then the next header block will contain the codebook configuration data. Unlike other mainstream audio codecs Vorbis has no statically configured probability model instead it packs all entropy decoding configuration, VQ and Huffman models into a self-contained codebook. This codebook block also requires additional identification information detailing the number of audio channels, bit rates and other information used to initalise the Vorbis stream. This setup information MUST be completely intact and a client can not decode a stream with an incomplete or corrupted codebook set. Kerr Expires August 20, 2003 [Page 8] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 A 16 bit codebook length field and a 16 bit 1's complement checksum of the codebook precedes the codebook datablock. The length field allows for codebooks to be upto 64k in size. The checksum is used to detect a corrupted codebook. If a checksum failure is detected then a new config header file SHOULD be obtained from SDP. If no SDP value is set and no other method for obtaining the config headers exists then this is considered to be a failure and should be reported to the client application. If the m flag is set then the next header block will contain the comment metadata, such as artist name, track title and so on. These metadata messages are not intended to be fully descriptive but to offer basic track/song information. This message MUST be sent at the start of the stream, together with the setup and codebook headers, even if it contains no information. During a session the metadata associated with the stream may change from that specified at the start, eg. a live concert broadcast changing acts/scenes, so clients MUST have the ability to receive m header blocks. Details on the format of the comments can be found in the Vorbis documentation [7]. The format for the data takes the form of a 32 bit codec vendors name length field followed by the name encoded in UTF-8. The next field denotes the number of user comments and then the user comments length and text field pairs upto the number indicated by the user comment list length. The framing bit is not used for RTP transportation and so applications constructing Vorbis files MUST take care to set this if required. If the o, overflow, bit is set then the URI of a whole header block is specified in an overflow URI field, which is a null terminated UTF-8 string. The header file specified at the URI MUST NOT have the overflow flag set, otherwise a loop condition will occur. If SDP information is available then the URI value set there MUST take precedent. Kerr Expires August 20, 2003 [Page 9] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 6 Session Description for Vorbis RTP Streams Session description information concerning the Vorbis stream SHOULD be provided if possible and must be in accordance with [8]. The contents of the Vorbis Header file referred to in the u attribute must contain all three of the config header blocks as specified above. The overflow bit of the header packet must not be set. u= m=audio RTP/AVP 98 c=IN IP4 a=rtpmap:98 vorbis/ The port value is specified by the server application bound to the URI specified in the c attribute. The bitrate value specified in the a attribute MUST match the Vorbis sample rate value. 7 Congestion Control Vorbis clients SHOULD send regular receiver reports detailing congestion. A mechanism for dynamically downgrading the stream, known as bitrate peeling, will allow for a graceful backing off of the stream bitrate. This feature is not available at present so an alternative would be to redirect the client to a lower bitrate stream if one is available. 8 Security Considerations RTP packets using this payload format are subject to the security considerations discussed in the RTP specification [3]. This implies that the confidentiality of the media stream is achieved by using encryption. Because the data compression used with this payload format is applied end-to-end, encryption may be performed on the compressed data. Where the size of a data block is set care must be taken to prevent buffer overflows in the client applications. 9 Acknowledgments This I-D is a draft-moffitt-vorbis-rtp-00.txt. The MIME type section is a continuation of draft-short-avt-rtp-vorbis-mime-00.txt Thanks to the AVT, Ogg Vorbis Communities / Xiph.org team including Steve Casner, Ralph Jiles, Tor-Einar Jarnbjo, John Lazzaro, Jack Moffitt, Colin Perkins, Barry Short, Mike Smith. Kerr Expires August 20, 2003 [Page 10] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 10 References 1. Key words for use in RFCs to Indicate Requirement Levels (RFC 2119), S. Bradner. 2. libvorbis: Available from the Xiph website, http://www.xiph.org 3. RTP: A Transport Protocol for Real-Time Applications (RFC 1889), Schulzrinne, et al. 4. RTP: A transport protocol for real-time applications. Work in progress, draft-ietf-avt-rtp-new-11.txt. 5. RTP Profile for Audio and Video Conferences with Minimal Control. Work in progress, draft-ietf-avt-profile-new-12.txt. 6. Ogg Vorbis I spec: Codec setup and packet decode. http://www.xiph.org/ogg/vorbis/doc/vorbis-spec-ref.html 7. Ogg Vorbis I spec: Comment field and header specification. http://www.xiph.org/ogg/vorbis/doc/v-comment.html 8. SDP: Session Description Protocol (RFC 2327), Handley, M. and V. Jacobson. 11 Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. Kerr Expires August 20, 2003 [Page 11] Internet Draft draft-kerr-avt-rtp-vorbis-01.txt February 20, 2003 This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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. 12 Authors Address Phil Kerr Centre for Music Technology University of Glasgow Glasgow, Scotland UK, G12 8LT Phone: +44 141 330 5740 Email: philkerr@elec.gla.ac.uk WWW: http://www.xiph.org/ Kerr Expires August 20, 2003 [Page 12]