Network Working Group S. Pfeiffer Internet-Draft C. Parker Expires: August 25, 2003 CSIRO February 24, 2003 Syntax of temporal URI fragment specifications draft-pfeiffer-temporal-fragments-00 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. This Internet-Draft will expire on August 25, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document specifies a syntax for using time offsets as the fragment portion of a URI. Such fragment identifiers are useful to directly access points of interest 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 syntax is closely related to the specification of relative timestamps of the RTSP protocol parameters as given in RFC 2326 [3]. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this Pfeiffer & Parker Expires August 25, 2003 [Page 1] Internet-Draft Temporal URI Fragments February 2003 document are to be interpreted as described in RFC 2119 [1]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Temporal fragment specification . . . . . . . . . . . . . . . 4 3. Time schemes . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 NPT time codes . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2 SMPTE time codes . . . . . . . . . . . . . . . . . . . . . . . 6 3.3 UTC time codes . . . . . . . . . . . . . . . . . . . . . . . . 7 3.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.5 Time interpretation . . . . . . . . . . . . . . . . . . . . . 8 4. Intended usage . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Security considerations . . . . . . . . . . . . . . . . . . . 10 References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13 Pfeiffer & Parker Expires August 25, 2003 [Page 2] Internet-Draft Temporal URI Fragments February 2003 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 such resources through a temporal URI fragment syntax. In this way, points of interest 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 browsing and searching of time-continuous media. The interpretation of the temporal fragment is however dependent on the URI scheme in use and the type of resource referenced. Pfeiffer & Parker Expires August 25, 2003 [Page 3] Internet-Draft Temporal URI Fragments February 2003 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. This is the relevant extract of RFC 2396's URI description in BNF: URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ] fragment = *uric uric = reserved | unreserved | escaped reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | "$" | "," unreserved = alphanum | mark mark = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")" escaped = "%" hex hex hex = digit | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f" alphanum = alpha | digit alpha = lowalpha | upalpha lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" upalpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" Temporal fragments start with the reserved character "@", representing the time-continuous resource "at" a certain temporal offset. As it is a reserved character, it can only be used for delimiting the structure of a URI component. No other specification of the use of the "@" character in URI fragments is published. Therefore, reserving the "@" character for temporal URI fragments is possible without breaking interoperability with existing URI implementations. 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 Pfeiffer & Parker Expires August 25, 2003 [Page 4] Internet-Draft Temporal URI Fragments February 2003 protocol parameters as given in RFC 2326 [3]. The BNF for temporal URI fragments is: temporal-fragment = "@" [ timescheme "=" ] 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 are described in the next section. The specification format of the timespec value is dependent on the timescheme and therefore also given in the next section. There are however some special values that are available for every timescheme. Their interpretation is dependent on the retrieval context. The special values are: o start - specifying the beginning of a file/stream. o now - specifying the current stream offset. Pfeiffer & Parker Expires August 25, 2003 [Page 5] Internet-Draft Temporal URI Fragments February 2003 3. Time schemes A time scheme is a short identifier for a type of time specification. The following time schemes are available: o npt (Normal Play Time; base of seconds) o smpte (Society of Motion Picture and Television Engineers time code; base of 30 fps) o smpte-30-drop (SMPTE with base of 29.97 fps) o smpte-25 (SMPTE with base of 25 fps) o clock (Universal Time Code UTC base of clock time) 3.1 NPT time codes Normal Play Time (NPT) is the time a user agent displays for a time- continuous resource. NPT 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. The BNF for these timespecs is: npt-time = npt-sec | npt-hhmmss npt-sec = *digit [ "." *digit ] npt-hhmmss = npt-hh ":" npt-mm ":" npt-ss [ "." *digit ] npt-hh = *digit npt-mm = 2digit npt-ss = 2digit 2digit = ["0" | "1" | "2" | "3" | "4" | "5"] digit 3.2 SMPTE time codes The Society of Motion Picture and Television Engineers time code (SMPTE) is also the time a user agent displays for a time-continuous resource. This time code is optimized for frame level access accuracy. SMPTE is specified as HH:MM:SS:FF.sf, where HH=hours, MM=minutes, SS=second, FF=frames, and sf=subframes (one-hundredth of a frame). Negative values are not allowed. There are three different SMPTE timecode specifications depending on the framerate: "smpte-time" with a framerate of 30 fps, "smpte-25-time" with a Pfeiffer & Parker Expires August 25, 2003 [Page 6] Internet-Draft Temporal URI Fragments February 2003 framerate of 25 fps, and "smpte-drop-30-time" with a framerate of 29.97 fps. The BNF for the timespec is: smpte-time = smpte-hh ":" smpte-mm ":" smpte-ss [ ":" smpte-ff ] [ "." smpte-sf ] smpte-hh = *digit smpte-mm = 2DIGIT smpte-ss = 2DIGIT smpte-ff = "0" | "1" | "2" digit smpte-sf = digit digit 2DIGIT = "0" | "1" | "2" | "3" | "4" | "5" digit 3.3 UTC time codes Universal Time Codes (UTC) are specified in GMT format according to ISO 8601 [4], compressed form. This absolute time builds a relationship to a real-world clock time for a given resource, e.g. the creation or storage time of the resource. 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). The BNF for the clock specification is: utc-time = utc-date "T" utc-hhmmss "Z" utc-date = utc-year utc-month utc-day utc-hhmmss = utc-hh utc-mm utc-ss ["." utc-fraction] utc-year = digit digit digit digit utc-month = "01" | "02" | "03" | "04" | "05" | "06" | "07" | "08" | "09" | "10" | "11" | "12" utc-day = "01" | "02" | ... | "29" | "30" | "31" utc-hh = "00" | "01" | ... | "21" | "22" | "23" utc-mm = 2DIGIT utc-ss = 2DIGIT utc-fraction = *digit 2DIGIT = "0" | "1" | "2" | "3" | "4" | "5" digit 3.4 Examples Examples for these formats are: Pfeiffer & Parker Expires August 25, 2003 [Page 7] Internet-Draft Temporal URI Fragments February 2003 http://www.foo.bar/matrix.au#@smpte=10:07:33:24 http://www.foo.bar/matrix.au#@npt=10:7:33.8 http://www.foo.bar/matrix.au#@10:7:33.8 http://www.foo.bar/matrix.au#@npt=36453.8 (all four specify the same time) rtp://www.foo.bar/archive.mpg#@clock=20021107T173045.25Z (for Thu Jul 11 05:30:45 UTC 2002 and a quarter seconds) rtp://www.foo.bar/news.mpg#@now 3.5 Time interpretation The semantic interpretation of time specifications given with any of the schemes depends on the resource. It is RECOMMENDED that servers support all of the above time schemes and time specifications. The temporal offset given in npt or smpte* is by default calculated relative to a start time of 0 at the beginning of the resource. Therefore, a user agent MUST take into account the temporal fragment offset when displaying the playback time. The UTC time specification is a real world date and time associated with the resource. This may e.g. be the broadcast time, the creation time or the archival time of the resource. The context determines the meaning. Examples: If a resource is requested with a temporal fragment offset of 400 sec and is served out from that offset point, the user agent MUST start the display of that resource with a "400 sec" playtime. A resource that has a clock time of 20001010T142211.23Z and the requested temporal offset is 20001010T145511.23Z, then the time 33 minutes into the resource is requested. Pfeiffer & Parker Expires August 25, 2003 [Page 8] Internet-Draft Temporal URI Fragments February 2003 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 with which temporal offsets are used is not restricted, though the http or rtp protocols are especially suited. Protocol-specific error codes MUST be used on URIs with temporal fragments where retrieval is impossible (such as "fragment time is beyond end of file"). As time-continuous resources often have high bandwidth requirements, it is advantageous to avoid the unnecessary network load associated with delivering the portion of a resource prior to the desired temporal offset. Hence it is RECOMMENDED that user agents do not strip off the temporal fragment from a given URI before forwarding it to a server. Servers that receive a request for a resource with a temporal URI fragment MUST provide the requested resource from the temporal offset onwards, in a manner appropriate to the data format of the resource. For example, when serving a request for a compressed media file at a temporal offset, the file's initial headers may need to be repeated verbatim, followed by the media data from a calculated byte offset. If a server does not support temporal URI fragments, it MUST return a protocol-specific error code. It especially MUST NOT serve out the complete document as otherwise the user agent cannot distinguish between an offset and a non-offset document. If unable to retrieve a resource including a temporal URI fragment, user agents SHOULD retry the request with the URI stripped off by the fragment and perform the offset action locally. 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. Pfeiffer & Parker Expires August 25, 2003 [Page 9] Internet-Draft Temporal URI Fragments February 2003 5. Security considerations This specification does not create any new security issues beyond those already specified for URIs in RFC 2396 [2]. Pfeiffer & Parker Expires August 25, 2003 [Page 10] Internet-Draft Temporal URI Fragments February 2003 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] 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, 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, 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/ Pfeiffer & Parker Expires August 25, 2003 [Page 11] Internet-Draft Temporal URI Fragments February 2003 Appendix A. Acknowledgements The authors greatly acknowledge the contributions of Andre Pang and Andrew Nesbit in developing this syntax. Pfeiffer & Parker Expires August 25, 2003 [Page 12] Internet-Draft Temporal URI Fragments February 2003 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. 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