AVT D. Singer
Internet-Draft Apple Computer Inc.
Expires: June 8, 2007 December 5, 2006
Associating Time-codes with RTP streams
draft-ietf-avt-smpte-rtp-06.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 June 8, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a mechanism for associating time-codes, as
defined by the Society of Motion Picture and Television Engineers
(SMPTE), with media streams, in a way that is independent of the RTP
payload format of the media stream itself.
Singer Expires June 8, 2007 [Page 1]
Internet-Draft RTP SMPTE Time-codes December 2006
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Signaling (setup) information . . . . . . . . . . . . . . . . 7
5. In-stream information . . . . . . . . . . . . . . . . . . . . 9
5.1. Compact Format of the Time-code . . . . . . . . . . . . . 9
5.2. Full Format of the Time-code . . . . . . . . . . . . . . . 9
5.3. Associations in RTCP . . . . . . . . . . . . . . . . . . . 9
5.4. Associations in RTP . . . . . . . . . . . . . . . . . . . 10
6. Implementation Note . . . . . . . . . . . . . . . . . . . . . 12
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
10. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 16
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 17
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 18
11. Normative References . . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
Singer Expires June 8, 2007 [Page 2]
Internet-Draft RTP SMPTE Time-codes December 2006
1. Introduction
First a brief background on Time-codes [SMPTE-12M].
The time-code system in common use is defined by the Society of
Motion Picture and Television Engineers (SMPTE), and in it, 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), but
correctly a frame takes 1001 ticks of a 30000 tick/second clock. 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 the rest of this document tries 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).
This specification enables the carriage of time-codes in RTP and
RTCP, associating them with a media stream, and synchronizing the
time-codes with the RTP time-stamps. It uses the general RTP header
extension mechanism [hdrext].
Singer Expires June 8, 2007 [Page 3]
Internet-Draft RTP SMPTE Time-codes December 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 June 8, 2007 [Page 4]
Internet-Draft RTP SMPTE Time-codes December 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.
There are applications where the transport of all 8 bytes of the
SMPTE 12M timecode are important (e.g. when the date of the time-code
must be known, or when the RTP transport is used as a transparent
pipe). On the other hand, there are cases (e.g. when timecodes are
used with compressed audio) when bandwidth is also important. To
support both use cases, provision is made for both compact and full
forms of the time-code.
Receivers MUST support timecodes in both RTCP and RTP, and both forms
(compact and full) of the time-code. Senders, of course, are free to
choose.
Note that the compact form allows frame numbers greater than the full
form (a field of 6 bits vs. a full BCD digit and a 2-bit BCD digit,
which gives a maximum transmitted value of 29). In some cases, the
Singer Expires June 8, 2007 [Page 5]
Internet-Draft RTP SMPTE Time-codes December 2006
color frame flag (bit 11) is used to 'extend' the tens of frames
field from 2 to 3 bits; however, such practices are outside the scope
of this specification.
Singer Expires June 8, 2007 [Page 6]
Internet-Draft RTP SMPTE Time-codes December 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 MUST be sent out-
of-band, for example in a SIP offer/answer exchange. Since this is a
general header extension [hdrext], appropriate signaling for those
header extensions should be used.
The setup information should include:
1. the duration, in the RTP 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); framecounter values may be between 0 and
(frames_per_tc_second - 1)
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, as defined in the header extension specification
[hdrext], with the following syntax. The form of the extension
attributes is 'owned' by the extension name. These parameters to the
extension do not need registration action beyond their documentation
here. Note that the parameters are supplied as extension attributes,
suitable for in-line use in RTP, even if in a given stream only the
RTCP mapping is used.
Singer Expires June 8, 2007 [Page 7]
Internet-Draft RTP SMPTE Time-codes December 2006
digit = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
integer = digit *(digit)
frameduration = integer
framespersecond = integer
drop = "/drop" |
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.
The URI used for the signaling is
"urn:ietf:params:rtp-hdrext:smpte-tc". This URI signals the possible
presence of associations in RTCP or RTP, as defined below.
An example in SDP, on a stream with a timescale of 600, might be
a=extmap:4 urn:ietf:params:rtp-hdrext:smpte-tc 20/30
Singer Expires June 8, 2007 [Page 8]
Internet-Draft RTP SMPTE Time-codes December 2006
5. In-stream information
5.1. Compact 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).
5.2. Full Format of the Time-code
A full binary SMPTE time-code occupies 64 bits. It is formatted
exactly as in SMPTE 12M [SMPTE-12M].
5.3. 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 (see section 9). This control packet has one of the two
following forms, differentiated by its length:
Singer Expires June 8, 2007 [Page 9]
Internet-Draft RTP SMPTE Time-codes December 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=3 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| 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.
For this short form, the length takes the fixed value 3, indicating a
control packet of 4 32-bit words.
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=4 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| RTP timestamp |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Full 8-byte |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SMPTE 12M timecode |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
For this full time-code, long form, the length takes the fixed value
4, indicating a control packet of 5 32-bit words.
5.4. 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 RTP header
Singer Expires June 8, 2007 [Page 10]
Internet-Draft RTP SMPTE Time-codes December 2006
extension is used [hdrext] to associate an RTP time to a SMPTE time-
code.
There are two forms of this header extension, again differentiated by
their length. The short form associates a compact time-code with the
RTP timestamp of the packet. The long form allows associates a full
time-code with a timestamp offset from the RTP timestamp of the
packet.
The short form has a length of 3 bytes (24 bits). The long form has
a length of 12 bytes (96 bits), and consists of a full SMPTE 12M
time-code, 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 June 8, 2007 [Page 11]
Internet-Draft RTP SMPTE Time-codes December 2006
6. 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 );
The SMPTE engineering guideline [smpte-eg40] contains all the
appropriate equations, constants etc. for performing these and other
conversions.
Singer Expires June 8, 2007 [Page 12]
Internet-Draft RTP SMPTE Time-codes December 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 RTCP 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 -
sent as early feedback packets, for example following the rules in
[feedback], after 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 design assumes that clients will hold mappings until they are
superseded, and that a client may need to buffer some number of
upcoming mappings.
Singer Expires June 8, 2007 [Page 13]
Internet-Draft RTP SMPTE Time-codes December 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 June 8, 2007 [Page 14]
Internet-Draft RTP SMPTE Time-codes December 2006
9. IANA Considerations
The RTCP packet type used for SMPTE time-code needs to be registered,
in accordance with section 15 of [RFC3550]. The abbreviation is
"SMPTETC", the full name is "SMPTE time-code mapping", the suggested
value is 194, and the specification is this document.
The name smpte-tc needs to be registered into the rtp-hdrext section
of the urn:ietf: namespace, referring to this specification.
Singer Expires June 8, 2007 [Page 15]
Internet-Draft RTP SMPTE Time-codes December 2006
10. RFC Editor Considerations
The reference to an Internet Draft needs to be updated to the RFC
when it is published (which should be before this draft).
Singer Expires June 8, 2007 [Page 16]
Internet-Draft RTP SMPTE Time-codes December 2006
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 June 8, 2007 [Page 17]
Internet-Draft RTP SMPTE Time-codes December 2006
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
o June 2006: 02 Allowed both compact and full time-codes in both
RTCP and inline
o June 2006: 03 Minor clarifications, provided signalling example,
clarified that the same signalling is used for both RTCP and/or
RTP use
o August 2006: 04 many cleanups, thanks to Colin for the careful
read
o October 2006: 05 Changed to URI signalling
o December 2006: 06 Added EG40, changed the URI
11. Normative 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.
[feedback]
Singer, D., "A general mechanism for RTP Header
Extensions", ID draft-ietf-avt-rtcp-feedback-11,
August 2006.
[hdrext] Singer, D., "A general mechanism for RTP Header
Extensions", ID draft-ietf-avt-rtp-hdrext-06,
October 2006.
[smpte-eg40]
SMPTE, "Conversion of Time Values Between SMPTE 12M Time
Singer Expires June 8, 2007 [Page 18]
Internet-Draft RTP SMPTE Time-codes December 2006
Code, MPEG-2 PCR Time Base and Absolute Time",
SMPTE EG40-2002, August 2002.
Singer Expires June 8, 2007 [Page 19]
Internet-Draft RTP SMPTE Time-codes December 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 June 8, 2007 [Page 20]
Internet-Draft RTP SMPTE Time-codes December 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 June 8, 2007 [Page 21]
