Network Working Group D. Singer
Request for Comments: 5484 Apple Computer Inc.
Category: Standards Track March 2009
Associating Time-Codes with RTP Streams
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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document authors. All rights reserved.
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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.
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Table of Contents
1. Introduction ....................................................2
2. Requirements Notation ...........................................3
3. Design Goals ....................................................3
4. Requirements and Constraints ....................................4
5. Signaling Information ...........................................4
6. In-Stream Information ...........................................6
6.1. Compact Format of the Time-Code ............................6
6.2. Full Format of the Time-Code ...............................7
6.3. Associations in RTCP .......................................8
6.4. Associations in RTP ........................................9
7. Implementation Note (Informative) ..............................10
8. Discussion (Informative) .......................................10
9. Security Considerations ........................................11
10. IANA Considerations ...........................................11
11. Acknowledgments ...............................................12
12. References ....................................................12
12.1. Normative References .....................................12
12.2. Informative References ...................................12
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); in it, time-codes
count frames. A common form of the display looks like a normal clock
value (hh:mm:ss.frame). When the frame rate is truly integral, 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
per second (fps). Some people call it 29.97, which isn't quite
right; to be accurate, 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 an 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).
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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 fps (e.g., film content at 24 fps or PAL video).
This specification defines how to carry time-codes in RTP and RTCP
(RTP Control Protocol), associate them with a media stream, and
synchronize them with the RTP timestamps. It uses the general RTP
header extension mechanism [RFC5285].
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].
3. Design Goals
What we desire is a system that allows us to associate an 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 as well as
periodic mappings in RTCP packets. This is defined in Section 6.3.
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The second requires in-band signaling within the RTP packets
themselves. This is defined in Section 6.4.
There are applications where the transport of all 8 bytes of the
SMPTE 12M time-code 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 time-codes 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.
4. Requirements and Constraints
Receivers MUST support time-codes in both RTCP and RTP as well as
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 binary-coded decimal (BCD) digit
and a 2-bit BCD digit, which gives a maximum transmitted value of
29). In some cases, the 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.
In the case that a presentation contains more than one stream,
senders MUST continue to send the standard RTP synchronization
information in RTCP, even if the streams carry SMPTE time-codes that
could be used for synchronization. In fact, when time-codes are
carried by more than one stream, this document does not constrain the
time-codes: at a given point in time, they may be the same, or they
may differ (e.g., if they carry the original time-codes of different
source material that was edited together).
5. Signaling 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 [RFC3264].
Since this specification is a general header extension [RFC5285],
when the Session Description Protocol (SDP) is used, the 'extmap'
attribute defined by the extension mechanism is also 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)
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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. the drop-frame indication, is-NTSC-drop-frame, which indicates
whether the usual drop-frame behavior should 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 stream with the common time-
scale of 90000 ticks per second, then a 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 fps, even
though they are obviously closely related. This 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 the SDP is used, these three parameters are transmitted as
extensionattributes, as defined in the header extension specification
[RFC5285], with the following ABNF syntax [RFC5234]. 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.
digit = "0"/"1"/"2"/"3"/"4"/"5"/"6"/"7"/"8"/"9"
integer = 1*digit
frame-duration-length = integer
timestamp-rate = integer
frame-duration = frame-duration-length "@" timestamp-rate
frames-per-tc-second = integer
drop = "/drop"
extensionattributes = frame-duration "/" frames-per-tc-second [drop]
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The frame duration is specified as a count of ticks of a clock that
has timestamp-rate ticks per second. It is recommended that the
timestamp-rate be the same as the clock rate of the RTP stream in
which the extension is embedded, to avoid the loss of accuracy in
conversion of timestamps. If the payload type changes during a
stream, especially between payloads with different clock rates, it is
strongly recommended that the header extension be included on the
first packet(s) of the new payload, to set the mapping for the new
clock rate explicitly.
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,
and 50, as documented in Section 4.2.2 of SMPTE specification
[SMPTE-12M]. (Note that this usually only applies to NTSC video.)
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 the SDP, for film material, on a stream with a
timescale of 600, might be:
a=extmap:4 urn:ietf:params:rtp-hdrext:smpte-tc 25@600/24
Another example, for drop-frame NTSC, on a stream with a timescale of
600, might be:
a=extmap:4 urn:ietf:params:rtp-hdrext:smpte-tc 20@600/30/drop
6. In-Stream Information
6.1. 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
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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 BCD (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).
6.2. Full Format of the Time-Code
A full SMPTE time-code occupies 64 bits. It is formatted exactly as
defined in Sections 7 and 8 of SMPTE 12M [SMPTE-12M], without the
16-bit syncword. The value of the "drop frame flag" MUST agree with
the use of the "drop" indicator in the signaling.
Here are the bit assignments from SMPTE 12M, for information:
0--3 Units of frames
4--7 First binary group
8--9 Tens of frames
10 Drop frame flag
11 Color frame flag
12--15 Second binary group
16--19 Units of seconds
20--23 Third binary group
24--26 Tens of seconds
27 Polarity correction
28--31 Fourth binary group
32--35 Units of minutes
36--39 Fifth binary group
40--42 Tens of minutes
43 Binary group flag BGF0
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RFC 5484 RTP SMPTE Time-Codes March 2009
44--47 Sixth binary group
48--51 Units of hours
52--55 Seventh binary group
56--57 Tens of hours
58 Binary group flag BGF1
59 Binary group flag BGF2
60--63 Eighth binary group
6.3. 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 timestamp in the RTCP mapping may not match the
timestamp of any frame in the media stream. For video, it normally
would; but a timestamp 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 RTCP packets 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'. For example, packets sent
'just-in-time' may be sent as early feedback packets, following the
rules in [RFC4585], after a discontinuity in the time-code is
detected. Such packets allow media-buffering in the client the
chance to 'catch' the RTCP before the matching RTP packet is
processed and displayed.
The association is a new RTCP Control Packet Type, using the value
194 (see Section 10). This control packet has one of the two
following forms, differentiated by its length.
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RFC 5484 RTP SMPTE Time-Codes March 2009
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 |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 1: RTCP Short Form Packet
The fields S (sign), hours, minutes, seconds, and frames are defined
in Section 6.1.
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 time-code |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 2: RTCP Full Form Packet
For this full time-code (long form), the length takes the fixed value
4, indicating a control packet of 5 32-bit words.
6.4. 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 [RFC5285], which some terminals may
find problematic. And clearly placing mapping information in every
packet uses more bandwidth.
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In as many RTP packets as needed (possibly all), an RTP header
extension is used [RFC5285] to associate an RTP time to an 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.
7. Implementation Note (Informative)
This section contains a suggestion on how to calculate both 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
passed 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.
8. Discussion (Informative)
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 by using low-bandwidth (vanishingly low in the case of
streams with no discontinuities), and it 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.
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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;
also, if one puts this RTCP packet into all sender reports, that's
probably good enough. Then if you don't have time-codes, you don't
have audio-video-sync either.
This specification associates the time-code with a particular media
stream. An alternative would be to make it an RTP stream in its own
right; however, the data rate is so low, this seems egregious. By
packing it inline, we can do this backwards-compatible for gateways,
etc., that already handle dual-stream.
There is no way described in this document 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.
9. Security Considerations
SMPTE time-codes are only informative and there are no known security
considerations from associating them with media streams.
10. IANA Considerations
The RTCP packet type used for SMPTE time-codes has been registered,
in accordance with Section 15 of [RFC3550]. IANA has added a new
value to the RTCP Control Packet types sub-registry of the Real-Time
Transport Protocol (RTP) Parameters registry, according to the
following data:
abbrev. name value Reference
--------- ----------------------- ------ ---------
SMPTETC SMPTE time-code mapping 194 RFC 5484
Additionally, IANA has registered a new extension URI to the RTP
Compact Header Extensions sub-registry of the Real-Time Transport
Protocol (RTP) Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:smpte-tc
Description: SMPTE time-code mapping
Contact: singer@apple.com
Reference: RFC 5484
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11. 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. Ladan Gharai provided a thoughtful final review.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with Session Description Protocol (SDP)",
RFC 3264, June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J.
Rey, "Extended RTP Profile for Real-time Transport
Control Protocol (RTCP)-Based Feedback (RTP/AVPF)",
RFC 4585, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for
RTP Header Extensions", RFC 5285, July 2008.
12.2. Informative References
[SMPTE-12M] Society of Motion Picture and Television Engineers,
"SMPTE Standard for Television -- Time and Control
Code", SMPTE 12M-1-2008.
[SMPTE-EG40] SMPTE, "Conversion of Time Values Between SMPTE 12M
Time Code, MPEG-2 PCR Time Base and Absolute Time",
SMPTE EG40-2002, August 2002.
Singer Standards Track [Page 12]RFC 5484 RTP SMPTE Time-Codes March 2009
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 Standards Track [Page 13]
