AVT D. Singer Internet-Draft Apple Computer Inc. Expires: July 5, 2006 January 2006 Associating SMPTE time-codes with RTP streams draft-ietf-avt-smpte-rtp-01.txt 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 July 5, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document describes a mechanism for associating SMPTE time-codes with media streams, in a way that is independent of the RTP payload format of the media stream itself. Singer Expires July 5, 2006 [Page 1] Internet-Draft RTP SMPTE Time-codes January 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Signaling (setup) information . . . . . . . . . . . . . . . . 6 5. Implementation Note . . . . . . . . . . . . . . . . . . . . . 8 6. In-stream information . . . . . . . . . . . . . . . . . . . . 9 6.1. Format of the Time-code . . . . . . . . . . . . . . . . . 9 6.2. Associations in RTCP . . . . . . . . . . . . . . . . . . . 9 6.3. Associations in RTP . . . . . . . . . . . . . . . . . . . 10 7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 10. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 14 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 15 Appendix B. Change History . . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 Intellectual Property and Copyright Statements . . . . . . . . . . 18 Singer Expires July 5, 2006 [Page 2] Internet-Draft RTP SMPTE Time-codes January 2006 1. Introduction First a brief background on SMPTE time-codes [SMPTE-12M]. SMPTE time-codes count frames. There are two common forms of display: either a simple counter, or what looks like a normal clock value (hh:mm:ss.frame). When the frame rate is truly integer, then this can be a normal clock value, in that seconds tick by at the same rate as the seconds we know and love. However, NTSC video infamously runs slightly slower than 30 frames/ second. Some people call it 29.97 (which isn't quite right) and some say that a frame takes 1001 ticks of a 30000 tick/second clock (which is closer). Be that as it may, SMPTE time codes count 30 of these frames and deem that to make a second. This causes a SMPTE time-code display to 'run slow' compared to real- time. To ameliorate this, sometimes a format called drop-frame is used. Some of the frame numbers are skipped, so that the counter periodically 'catches up' (so some time-code-seconds actually only have 28 frames in them). It is worth noting that in neither case is the SMPTE time-code an accurate clock; in the first case, it runs slow, and in the second, the adjustments are abrupt and periodic - and still not quite accurate. Hence in the rest of this document I try to be clear when referring to a second in a time-code as a 'time-code second'. However, SMPTE time-codes do run in real-time when used with systems with integral frames/second (e.g. film content at 24 frames/second, or PAL video). The 'drift' issue is (I believe) unique to NTSC video. Singer Expires July 5, 2006 [Page 3] Internet-Draft RTP SMPTE Time-codes January 2006 2. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Singer Expires July 5, 2006 [Page 4] Internet-Draft RTP SMPTE Time-codes January 2006 3. Design Goals What we desire is a system that allows us to associate a SMPTE time- code with some media in an RTP [RFC3550] stream. Since in RTP all media has a clock already, we can often leverage that fact. If we treat the media as having 'segments' of time in which the time-code is simply counting up, then the time-code anywhere within a segment can be calculated if you know: o the RTP timestamp of the start of the segment; o the time-code of the start of the segment; o the counting rate and other parameters of the time-code; o the RTP timestamp where you want to know the time-code. There are two cases to consider: 1. the time-codes are piece-wise continuous with only occasional discontinuities; 2. the continuity of the time-codes is not certain (or not known). The first can be handled by providing details of the time-code axis and an initial mapping from RTP time to time-code time, and periodic mappings in RTCP packets. The second requires in-band signaling within the RTP packets themselves. Both cases are covered by this specification. Singer Expires July 5, 2006 [Page 5] Internet-Draft RTP SMPTE Time-codes January 2006 4. Signaling (setup) information If the recipient must ever calculate time-codes based on the RTP times, then some setup information is needed. This is sent out-of- band. The setup information includes: 1. the duration, in the RTP timescale, timescale, of a single frame- count in the 'frames' portion of the time-code (frame-duration) 2. the number of those frames that make a time-code-second (frames- per-tc-second) 3. is-NTSC-drop-frame: should the usual 'left out numbers' of drop- frame be applied or not? Note that other information we need to do the calculation (e.g. the clock rate of the RTP timestamp) is supplied already and assumed to be available. For example, if associated with a video track with the common time- scale of 90000, then frame-duration of 3003 and frames-per-tc-second of 30 would yield a 'normal' SMPTE time-code for NTSC video. Similarly values of 3750 and 24 yield a time-code for 24 fps film content, and so on. Note also that we supply explicitly the frame duration and frames/ second, even though they are obviously closely related. These removes any ambiguity of what the counter values should be in the case of drop-frame counting. These three values MUST correspond with each other. When SDP is used, these three parameters are transmitted as extensionattributes, with the following syntax. The form of the extension attributes is 'owned' by the extension name (one of the forms below, either the 'implicit' or 'explicit' name. These parameters to the extension do not need registration action beyond their documentation here. Singer Expires July 5, 2006 [Page 6] Internet-Draft RTP SMPTE Time-codes January 2006 digit = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9" integer = digit *(digit) frameduration = integer framespersecond = integer drop = "/drop-frame" | extensionattributes = frameduration "/" framespersecond drop If '/drop' is specified, then the first two frame numbers are omitted from the count of each minute,, except for minutes 00, 10, 20, 30, 40, 50, as documented in SMPTE specification [SMPTE-12M] section 4.2.2. Open question: should we normally compute framespersecond from frameduration? Singer Expires July 5, 2006 [Page 7] Internet-Draft RTP SMPTE Time-codes January 2006 5. Implementation Note This section contains a suggestion on how to calculate a time-code for a time T2, given an initial code at time T1, and the frame duration It might seem that when drop-frame is used, there is a 'fence post' problem: how many minutes in which frame-numbers are dropped have past since the initial time-code? However, this can be avoided if all calculations are 'zero-based'; then the number of 'fence posts' is known. framesSinceTCzero := TimeCodeToFrameCount( initialTimeCode ); framesSinceMapping := floor( (T2-T1)/frameDuration ); totalFrames := framesSinceTCzero + framesSinceMapping; timeCode := FrameCountToTimeCode( totalFrames ); Singer Expires July 5, 2006 [Page 8] Internet-Draft RTP SMPTE Time-codes January 2006 6. In-stream information 6.1. Format of the Time-code A compact binary SMPTE time-code in this design occupies 24 bits. It is NOT formatted in the BCD system, but uses binary fixed-width fields. It has the following structure: sign(1) -- 1 for negative, 0 for positive hours (5 bits) -- 0 to 23; the values 24-31 are reserved minutes (6 bits) -- 0 to 59; 60-63 are reserved seconds (6 bits) -- 0 to 59; 60-63 are reserved frames(6 bits) -- 0 to (frames-per-tc-second - 1) Note that these fields are larger than the provision in SMPTE 12M where binary-coded decimal is used (and notably, where only two bits are provided for the tens digit of the frame count, so frame numbers above 39 cannot be represented). Open question: should we allow for a full 8-byte SMPTE time-code formatted exactly as in SMPTE 12M? We are currently missing the 6 flag bits and the 8 4-bit binary groups. 6.2. Associations in RTCP When the time-codes are piece-wise continuous, we then supply in RTCP packets an RTP timestamp and an SMPTE time-code, for the start of each run of calculable time-codes. This establishes the time-code for all RTP times greater than or equal to the one given, until a subsequent RTCP packet reestablishes the mapping. Note that the RTP time-stamp in the RTCP mapping may not match the time-stamp of any frame in the media stream. For video, it normally would; but a time-stamp transition may happen part-way through a decoded audio frame. Since they share the same clock, the timing of that transition and the timing of the audio stream itself have the same accuracy. The association is a new RTCP Control Packet Type, using the value 194 (to be registered). This control packet has the following form: Singer Expires July 5, 2006 [Page 9] Internet-Draft RTP SMPTE Time-codes January 2006 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| SC |PT=SMPTETC=194 | length | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | SSRC of packet sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | RTP timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |S| hours | minutes | seconds | frames | reserved=0 | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ The fields S (sign), hours, minutes, seconds, and frames, are defined above. The length takes the fixed value 3, indicating a control packet of 4 32-bit words. 6.3. Associations in RTP When the time-codes are not known to be piece-wise continuous, or absolute surety of mapping is desired, then the mapping can be placed into some or all of the RTP packets. This is a less desirable route; it uses the RTP header extension, which some terminals may find problematic. And clearly placing mapping information in every packet uses more bandwidth. In as many RTP packets as needed (possibly all), a named header extension is used to associate an RTP time to a SMPTE time-code. (See related specification of named header extensions for RTP). There are two forms of this header extension. The first ('implicit') form associates the time-code with the RTP timestamp of the packet. The second ('explicit') form allows associates the time-code with a timestamp offset from the RTP timestamp of the packet. The implicit form has the name "org.ietf.avt.smpte12M.implicit/ 082005", and contains solely a 24-bit time-code as defined above. The explicit form has the name "org.ietf.avt.smpte12M.explicit/ 082005", and contains the 24-bit time-code as defined above, followed by a signed 32-bit offset D from the RTP timestamp. If the packet has timestamp T, this establishes an RTP to time-code association for the RTP time T+D. Singer Expires July 5, 2006 [Page 10] Internet-Draft RTP SMPTE Time-codes January 2006 7. Discussion This design has the advantage of not requiring the introduction of new IP packets into the sessions or new data into the main data channel, using low-bandwidth (vanishingly low in the case of streams with no discontinuities), and is independent of the design of the RTP packets themselves: the RTP profile (including possibly encryption) and the RTP payload format. SMPTE time-codes can be associated with any RTP stream, including those with existing payload formats. It might be argued that we could set the initial mapping also in the SDP, since RTCP packets might get lost. But this means that the SDP now has to have knowledge of the RTP random offset, which is nasty; and if one puts this APP packet into all sender reports, it's probably good enough. Then if you don't have time-codes, you don't have audio-video-sync either. This associates the time-code with a particular media stream. An alternative would be to make it an RTP stream in its own right; but the data rate is so low, this seems egregious. And by packing it inline, we can do this backwards-compatible for gateways etc. that already handle dual-stream. The RTCP packets (or the in-band codes) need not use the same RTP timestamp as the sender report (or transmission time) in the same RTCP packet. They can be sent 'ahead of need' if possible (e.g. for stored content, when the server can look-ahead) or just-in-time - send an RTCP immediately a discontinuity in the time-code is detected, and allow media-buffering in the client the chance to 'catch' the RTCP before the matching RTP packet is processed and displayed. There is no way in this draft to detect that an RTCP packet has been lost, and that a mapping may be being used outside its intended range. The likelihood of this happening could be reduced, however, by permitting a pair of RTP times in the mapping, and defining that the mapping is only valid between those times. This only works for stored media, when look-ahead is possible, of course. It is a discussion item whether it is worthwhile. The design assumes that clients will hold mappings until they are superseded, and that a client may need to buffer some number of upcoming mappings. It may be necessary to introduce explicit statements about the amount of buffering needed. For trick modes, it may be desirable to signal that a given section of media has the time-code running in reverse; this would require a new sign bit in the mapping record. Singer Expires July 5, 2006 [Page 11] Internet-Draft RTP SMPTE Time-codes January 2006 8. Security Considerations SMPTE time-codes are only informative and it is hard to see security considerations from associating them with media streams. Singer Expires July 5, 2006 [Page 12] Internet-Draft RTP SMPTE Time-codes January 2006 9. IANA Considerations The RTCP packet type used for SMPTE time-code needs to be registered. The abbreviation is "SMPTETC", the full name is "SMPTE time-code mapping", the suggested value is 194, and the specification is this document. The names used for the EXTMAP attribute need to be registered. Those names are "org.ietf.avt.smpte12M.implicit/082005" and "org.ietf.avt.smpte12M.explicit/082005", and they should be associated with this document. Singer Expires July 5, 2006 [Page 13] Internet-Draft RTP SMPTE Time-codes January 2006 10. RFC Editor Considerations None. Singer Expires July 5, 2006 [Page 14] Internet-Draft RTP SMPTE Time-codes January 2006 Appendix A. Acknowledgments Both Brian Link and John Lazzaro provided helpful comments on an initial draft. Colin Perkins was helpful in reviewing and dealing with the details. Singer Expires July 5, 2006 [Page 15] Internet-Draft RTP SMPTE Time-codes January 2006 Appendix B. Change History o August 2005: 00 Draft-avt-smpte-rtp made from draft-singer-smpte-rtp; added question on full time-code option o January 2006: 01 Updated to XML2RFC; inserted BNF, documented SDP usage, changed the name to org.ietf..., other cleanup 11. References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, STD 0064, July 2003. [SMPTE-12M] Society of Motion Picture and Television Engineers, "Television, Audio and Film - Time and Control Code", SMPTE 12M-1999. Singer Expires July 5, 2006 [Page 16] Internet-Draft RTP SMPTE Time-codes January 2006 Author's Address David Singer Apple Computer Inc. 1 Infinite Loop Cupertino, CA 95014 US Phone: +1 408 996 1010 Email: singer@apple.com Singer Expires July 5, 2006 [Page 17] Internet-Draft RTP SMPTE Time-codes January 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. 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 (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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Singer Expires July 5, 2006 [Page 18]