- From: <Silvia.Pfeiffer@csiro.au>
- Date: Sat, 07 Jun 2003 23:46:02 +1000
- To: uri@w3.org
- Message-ID: <3EE1EC9A.7090609@csiro.au>
Dear all,
I need to pick up the issue of temporal URI fragments again. Attached is
another revision that addresses some of the previously stated issues.
Now that we have sorted through all the other open issues, it is high
time to resume the discussion on the one issue of temporal URI fragments
that does not conform with the current URI standard specification. I
would like to get this issue added to the issues list and sorted out
before declaring the revision finished.
I am still hopeful that we can come to a future-proof expert agreement
on how to standardise uniform addressing of temporal and named offsets
into time-continuous data through URIs. It will have large implications
on the usefulness of the Web for digital online radio & TV stations as
well for the Web access of large volume (and I'm talking about terabytes
of data) archives.
Let me explain the issue in a sequence of statements:
1.) There is a need to standardise a specification for temporal URIs:
- I have received comments from many different standards bodies (SMPTE,
MPEG, Pro-MPEG) that supported this temporal addressing scheme
- in MPEG-21, several different ways of specifying temporal references
into resources are being proposed, such as #media_time(17:30) or
#01:00:00:00; our suggestion is another one
- all temporal Web resources share a need to identify temporal offsets
and users should not have to learn and get confused by many different
ways of specifying them in URIs when we can prescribe one standard way
of doing so.
2.) Direct addressing of named highlights in time-continuous resources
(such as audio & video) as fragment identifiers is desirable:
- time-continuous media on the Web is non-attractive because it is
awkward and inefficient to use - one main reason being its inherent
inaccessability: bits of interest cannot be addressed directly
- just as it is useful and sensible for html resources to be able to
address named anchors (i.e. named objects or offsets) through fragments,
it is also useful and sensible for time-continuous resources to be able
to address named objects through fragments.
- many file formats of time-continuous data already have a way of
specifying special objects/highlights by names (e.g. QuickTime chapters,
WindowsMedia markers)
- direct access to named highlights through use of the name in fragments
is desirable
3.) There is a requirement for time-continuous resources to perform the
retrieval action of subparts of media server-side:
- time-continuous media has large volumes; just a 1 hour video will
require 25 GB in DV format
- a user who requests a interval of or a time offset into such a
resource should not need to have to be made to wait while data that he
has not requested is getting delivered
4.) The problem of using fragments for addressing offsets into
time-continuous resources:
- fragment identifiers are a natural and the prefered way of
specification of temporal and named URI references by at least MPEG-21
and ourselves because fragments imply that the original resource is
bascially unchanged and that only a subpart is referenced (as said in
the new I-D: "identifying information that is selective within that
resource")
- forcing URI fragments into time-continuous resources to be interpreted
client-side makes them nearly useless (see 3 above)
- the proposal to use the query ("?") component instead of the fragment
identifier for the specification of temporal URI references may be a
solution for temporal URIs, but fails for named fragments and fails to
address the point. If fragments are useful for anything more than
small-size resources (such as html and pdf), then addressing of subparts
of resources also needs to result in resources that represent that subpart.
- while the current URI standard says that "fragment identifiers have a
special role in information systems as the primary form of client-side
indirect referencing", this is a prescription that is not technically
necessary and only mirrors current use (as in html or pdf). Without
limiting this current use, we can easily add the possibility to perform
referencing on the server where needed.
==
The simple summary to my rant is that people will build systems that use
temporal URI fragment identifiers into time-continuous media no matter
whether we support it or not. And they will not want to be restricted by
having to perform the offset action client-side. RTP/RTSP already offer
a way around this problem by specifying the temporal offset or interval
in a protocol parameter. If a temporal URI fragment is used over RTSP,
the effect will be that the user agent strips off the fragment and puts
it into that protocol parameter and then retrieves for its user the
subpart of the resource that was requested. This effectively contradicts
the prescription of the current version of the URI standard of having to
perform the offset action on the client side.
We currently have the chance to foresee this usage and allow it under
controlled conditions. I firmly believe that the alternative is that we
will get back to another revision of this standard a few years down the
track where we will have to incorporate this change under more
uncontrolled conditions with many non-interoperable implementations
around. Therefore, let's resolve it now.
My proposal to resolve this problem is based on the fact that the
"retrieved representation" of the resource on which the fragment offset
gets performed is the same on the client as the one that is available at
the server's side just before sending it off. I'd therefore like to
argue that we could as well allow the application of the fragment offset
on the server side as the very final step, in cases where this is more
appropriate to the MIME type and the user's requirements.
Best Regards,
Silvia Pfeiffer.
Network Working Group S. Pfeiffer
Internet-Draft C. Parker
Expires: December 6, 2003 CSIRO
June 7, 2003
Syntax of temporal URI fragment specifications
draft-pfeiffer-temporal-fragments-01
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
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This Internet-Draft will expire on December 6, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies a syntax for temporal offsets and intervals
as URI fragments. Such fragment identifiers are useful to directly
access temporal offset points and intervals in time-continuous
resources such as audio and video. The URI fragment syntax specified
in this document is comformant to the Generic URI Syntax as specified
in RFC 2396 [2].
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].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Temporal fragment specification . . . . . . . . . . . . . . . 4
3. Time schemes . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Intended usage . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Security considerations . . . . . . . . . . . . . . . . . . . 9
6. Disucssion points to be resolved . . . . . . . . . . . . . . . 10
6.1 Interpretation of temporal fragments on server . . . . . . . . 10
7. ChangeLog . . . . . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
Many resources on the Internet are time-continuous data such as audio
or video files or streams. This document describes a standard way of
addressing temporal offsets into and temporal intervals of such
resources through a temporal URI fragment syntax. In this way,
points of interest and intervals in time-continuous files or streams
can be directly accessed. The aim is to make it simple to
incorporate infrastructure into the Web which supports the browsing
and searching of time-continuous media. The interpretation of the
temporal fragment is however dependent on the URI scheme in use and
the content type of the resource referenced.
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2. Temporal fragment specification
Fragments are generally specified in a URI after the crosshatch ("#")
character. The format of fragment identifiers specified in this
document is conformant to the URI RFC 2396 [2] for fragment
identifiers, but retricts their use to an interpretation on the user
agent only. See section 5 for a proposed alternative usage.
Temporal fragments start with the reserved character "@",
representing the time-continuous resource "at" a certain temporal
offset. The "@" character is reserved and this specification is
giving it a reserved purpose. Having the "@" character at the start
simplifies parsing of a temporal fragment specification, helping to
e.g. distinguish between a fragment given by name as "#smpte-25" and
a fragment given as a temporal offset as "#@smpte-25=01:01:01:01".
The specification of a temporal fragment offset itself is given as a
name-value pair, where the name specifies a time scheme to use and
the value is the time specification itself. The syntax is closely
related to the specification of relative timestamps of the RTSP
protocol parameters as given in RFC 2326 [3].
Temporal intervals can be specified as well. This is achieved by
adding the reserved character "-" and another time specification that
adheres to the time scheme used for the specification of the first
time point.
The BNF for temporal fragment offsets and temporal intervals is:
temporal-fragment = "@" [ timescheme "=" ] timespec ["-" timespec]
timescheme = *unreserved
timespec = *uric
There are several time schemes that can be used. The default time
scheme is "npt" (normal play time). The available time schemes and
their specifications are described in the next section.
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3. Time schemes
A time scheme is a short identifier for a type of time specification.
The following general time schemes are specified in this document.
Further time schemes are expected to emerge and should probably be
registered through IANA (XXX this needs to be discussed XXX).
o "npt" (Normal Play Time; base of seconds as used in the RTSTP
standard [3])
o "smpte" (Society of Motion Picture and Television Engineers time
codes as specified in the SMPTE time and control code standard
[4])
o "utc" (Universal Time Code giving wall-clock time as specified in
the ISO 8601 standard [5]).
Thus, the available time schemes are:
NPT time has a second or subsecond resolution. It is specified as
H:M:S.h (npt-hhmmss) or S.h (npt-sec), where H=hours, M=minutes,
S=second and h=fractions of a second. Negative values are not
allowed.
Specification as BNF:
npt-spec = "npt=" npt-time
npt-time = npt-sec | npt-hhmmss
npt-sec = 1*DIGIT [ "." *DIGIT ]
npt-hhmmss = npt-hh ":" npt-mm ":" npt-ss [ "." *DIGIT ]
npt-hh = 1*DIGIT
npt-mm = 1*2DIGIT
npt-ss = 1*2DIGIT
SMPTE time codes [4] are optimized for frame level accuracy.
SMPTE is specified as HH:MM:SS:FF, where HH=hours, MM=minutes,
SS=second, FF=frames. The drop-frame algorithms for calculating
the exact times can be found in the references SMPTE standard.
Negative values are not allowed.
"smpte-24=" SMPTE time with a 24 fps basis
"smpte-24-drop=" SMPTE time with a 24/1.001 fps basis
"smpte-25=" SMPTE time with a 25 fps basis
"smpte-30=" SMPTE time with a 30 fps basis
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"smpte-30-drop=" SMPTE time with a 30/1.001 fps basis
"smpte-50=" SMPTE time with a 50 fps basis
"smpte-60=" SMPTE time with a 60 fps basis
"smpte-60-drop=" SMPTE time with a 60/1.001 fps basis
Specification as BNF:
smpte-spec = smpte-type "=" smpte-time
smpte-type = "smpte-24" | "smpte-24-drop" | "smpte-25" |
"smpte-30" | "smpte-30-drop" | "smpte-50" |
"smpte-60" | "smpte-60-drop"
smpte-time = smpte-hh ":" smpte-mm ":" smpte-ss [ ":" *2DIGIT ]
smpte-hh = 1*2DIGIT
smpte-mm = 1*2DIGIT
smpte-ss = 1*2DIGIT
UTC time has a second or subsecond resolution. It is given as
YYYYMMDDTHHmmss.hhZ, where Y=year, M=month, D=day, H=hour,
m=minute, s=second, h=subseconds (one-hundredth of a second).
Specification as BNF:
utc-spec = "clock=" utc-time
utc-time = utc-date "T" utc-hhmmss "Z"
utc-date = 8DIGIT
utc-hhmmss = 6DIGIT [ "." *DIGIT ]
Examples for specifications of temporal fragment offsets are:
http://www.foo.bar/matrix.au#@smpte-25=10:07:33:06
http://www.foo.bar/matrix.au#@npt=10:7:33.25
http://www.foo.bar/matrix.au#@10:7:33.25
http://www.foo.bar/matrix.au#@npt=36453.25
(all four specify the same time point)
rtp://www.foo.bar/matrix.mpg#@clock=20021107T173045.25Z
(for Thu Jul 11 05:30:45 UTC 2002 and a quarter seconds)
The semantic interpretation of time specifications given with any of
the schemes depends on the resource. With every resource there are
two associated timebases: a UTC timebase which may e.g. specify the
creation time of the resource, and a playback timebase used for
display in a user agent while viewing the resource.
The playback timebase of a resource defaults to 0 seconds if the
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resource has no other timebase associated with it. For example, with
professional video production, the first frame of video of a program
normally refers to a SMPTE timebase of 01:00:00:00, not 00:00:00:00.
This practice arose from the requirements of program production and
analog videotape recording technology, and it has subsequently become
a uniform, almost ironclad practice worldwide. Associating such a
practice to a digital video resource requires a way to store that
timebase with the resource, which may or may not be possible,
depending on the content type of the resource.
Examples: If a resource has an associated timebase of 3600 seconds,
and the given temporal fragment offset is 4000 sec, a seek time 400
sec into the resource is requested. If the timebase is given as
clock time 20001010T142211.23Z and the temporal offset specified is
20001010T145511.23Z, the time 33 minutes into the resource is
requested.
The UTC timebase of a resource defaults to non-specified.
Associating such a UTC timebase with a resource requires a way to
store that timebase with the resource. For example, for a resource
that is a file on a server, it may be chosen to be the time of
storage of that resource.
Examples for specifications of temporal intervals are:
http://www.foo.bar/matrix.au#@smpte-25=10:07:33:05-10:07:37:21
http://www.foo.bar/matrix.au#@npt=10:7:33.25-10:7:37.8
http://www.foo.bar/matrix.au#@10:7:33.25-10:7:37.8
http://www.foo.bar/matrix.au#@npt=36453.25-36457.8
(all four specify the same temporal interval)
rtp://www.foo.bar/matrix.mpg#@clock=20021107T173045.25Z-20021107T173049.80Z
The semantic interpretation of these temporal intervals depends on
the time scheme. Unless specified differently, the temporal
intervals given are closed intervals, i.e. they start at the first
time point and finish at the second time point: [time_from;time_to].
For SMPTE timecodes, however, it is conventional to express such
temporal intervals as IN and OUT times for editing. Thus, the IN
time specifies the first frame that is included in the interval and
the OUT time specifies the first frame that is not included in the
interval. Therefore, a SMPTE interval is specified as
[time_from;time_to[, which explains the additional frame in the above
example.
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4. Intended usage
The temporal fragment specification scheme is intended to be used on
time-continuous resources. An example of such resources are all
resources of MIME types "audio/*" and "video/*". The protocol
through which these resources are accessed are expected to be mainly
http or rtp/rtsp, which are especially suited for such resources.
It is RECOMMENDED that user agents do not strip off the temporal
fragment from a given URI before forwarding it to a server. [XXX:
This is contrary to the current prescription in the URI standard and
needs to get resolved -> see Section 6.]
A retrieval action on a URI that includes a temporal fragment SHOULD
result in a time-continuous resource that starts at the given
temporal offset. As time-continuous resources often come with high
bandwidth requirements, this avoids unnecessary network load. For
example, a 1 hour Digital Video (DV format) requires about 25 GB
(MPEG-2 reduces that to about 3 GB, but this format must be prepared
for addressing high-quality, high-resolution time-continuous
bitstreams of the future). Serving out only the requested interval
of a resource also significantly reduces the delay for the user agent
for receiving relevant data. Alternatively, the user agent MAY wait
until the retrieval action has failed, then resend the URI with the
fragment stripped off and perform the offset action locally on the
retrieved resource.
Servers that support the temporal fragment offset MUST implement a
retrieval action of time-continuous resources with such fragment
specifications by serving the requested resource from the temporal
offset onwards. For many time-continuous resources - especially when
in compressed format - this means that the server has to parse the
structure of the resource and construct another valid resource from
the original resource's header information and data frames. If a
server cannot perform the fragment offset, it MUST return an error as
otherwise the user agent cannot identify if the offset action was
performed or not.
It is expected that over time more servers and client applications
understand and handle the temporal fragment offset and thus enable
direct networked access to content in time-continuous resources.
Also network proxies may begin to understand such temporal offsets
and can exploit them for caching.
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5. Security considerations
This specification does not create any new security issues beyond the
ones already specified for URIs in RFC 2396 [2].
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6. Disucssion points to be resolved
6.1 Interpretation of temporal fragments on server
In the current version of the URI standard [2], it is prescribed that
fragment specifications get interpreted by the user agent.
Therefore, the intended use of fragments to retrieve only a temporal
interval of the time-continuous data or the data from a certain
offset point onwards is not generally allowed. However, there are
ways to effectively get around this restriction by using (mis-using?)
communication protocols.
When using http, we can invent a new protocol parameter that gets
filled by the user agent with the temporal fragment specification and
that gets interpreted by the server. If we don't do that, we
seriously defect http from being usable for time-continuous media in
the future.
When using rtp/rtsp, a client can strip off the fragment
specification and map it onto the Range header field of the rtsp
protocol, which will then tell the server which subpart of the
time-continuous data bistream to serve out.
A much cleaner way to resolve this problem would be to change the URI
standard to allow for a server-side interpretation of fragment
offsets after all other actions have been performed on the resource.
This will make it independent of the protocol in use and it will
enable intermediate proxies to store and forward parts of a media
resource. It is then the user agent's choice whether or not to strip
off the fragment offset and interpret it locally after the retrieval
action or to forward it to the server with an expectation to receive
only that subpart of the resource. Servers are not forced to
implement that specification. It is however necessary to return an
error if they cannot handle temporal fragment specifications to avoid
a double offset action by both server and client.
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7. ChangeLog
draft-pfeiffer-temporal-fragments-01:
Extension of the number of available SMPTE time-schemes. Many
thanks to Bill Miller and Oliver Morgan of the SMPTE for their
input on these changes.
Deleted "start" and "now" as time specification values.
Extension of the temporal fragment addressing to also address
temporal intervals, not only time points.
Added section that includes some key points of discussion where
the existing URI standard contradicts the use of fragments for
time-continuous data.
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirements
Levels", RFC 2119, BCP 14, March 1997.
[2] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[3] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[4] The Society of Motion Picture and Television Engineers, "SMPTE
STANDARD for Television, Audio and Film - Time and Control
Code", ANSI 12M-1999, September 1999.
[5] ISO, TC154., "Data elements and interchange formats --
Information interchange -- Representation of dates and times",
ISO 8601, 2000.
Authors' Addresses
Silvia Pfeiffer
Commonwealth Scientific and Industrial Research Organisation CSIRO, Australia
Locked Bag 17
North Ryde, NSW 2113
Australia
Phone: +61 2 9325 3141
EMail: Silvia.Pfeiffer@csiro.au
URI: http://www.cmis.csiro.au/Silvia.Pfeiffer/
Conrad D. Parker
Commonwealth Scientific and Industrial Research Organisation CSIRO, Australia
Locked Bag 17
North Ryde, NSW 2113
Australia
Phone: +61 2 9325 3133
EMail: Conrad.Parker@csiro.au
URI: http://www.cmis.csiro.au/Conrad.Parker/
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Appendix A. Acknowledgements
The authors greatly acknowledge the contributions of Andre Pang and
Andrew Nesbit in developing this syntax. We also thank the SMPTE for
their contributions and the URI discussion group at the W3C
(uri@w3.org) for their many comments on this document.
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Received on Saturday, 7 June 2003 09:49:00 UTC