rfc2424.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                         G. Vaudreuil
Request for Comments: 2424                             Lucent Technologies
Category: Standards Track                                       G. Parsons
                                                          Northern Telecom
                                                            September 1998


                            Content Duration
                         MIME Header Definition

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.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Overview

   This document describes the MIME header Content-Duration that is
   intended for use with any timed media content (typically audio/* or
   video/*).

1.  Abstract

   This document describes the MIME header Content-Duration that is
   intended for use with any time varying media content (typically
   audio/* or video/*).  The length of time is represented in seconds
   without any units indication.

2. Content-Duration Header Field

   Time varying media contents, for example, a spoken voice message or a
   video clip, have an inherent time duration.  Many audio and video
   encodings may include their duration as header information or may
   allow accurate calculation based on the byte length of the data.
   However, it may be useful to present the time duration of the content
   in a MIME header to allow its simple determination without dealing
   with the actual content.







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RFC 2424                    Content-Duration              September 1998


   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 [REQ].

2.1 Syntax

   The Content-Duration field's value is a single number specifying the
   time duration in seconds of the content.  Formally:

   duration := "Content-Duration" ":" 1*10DIGIT

   Note that practically (though highly unlikely in MIME media), the
   upper bound on the numerical value of the time duration is (2^^31 -
   1) or 2147483647.

2.2 Semantics

   This field represents the time duration of the associated time
   varying media content.  The time duration is noted in seconds with no
   units tag.  The time value should be exact, however the exact value
   of the time duration cannot be known without opening the content and
   playing it.  If an exact value must be known, then the latter method
   should be used.  This mechanism simply allows placing a sender
   determined time duration value in the header for easy access.

   Though there are several ways to present this duration to the
   recipient (e.g. with the inbox headers, when audio attachment
   opened), the actual use of this field on reception is a local
   implementation issue.

2.3 Example

   In this example the content duration represents 33 seconds:

       Content-Duration: 33

3. VPIM Usage

   The Content-Duration header field for the audio/32KADPCM sub-type is
   a useful component of the VPIM specification [VPIM2].  All VPIM
   Messages MUST contain this sub-type to carry the audio of a voice
   message.  It may be useful in some instances (e.g. viewing on a
   simple MIME or non-MIME desktop) to have the time duration of the
   voice message available without having to open the audio content.







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RFC 2424                    Content-Duration              September 1998


4. Security Considerations

   This definition introduces the option of explicitly identifying the
   time duration of an audio/* or video/* content outside of the binary
   data that forms the content.  In some environments (though likely not
   the majority), the identification of the actual time duration in a
   header field may be a security issue and as a result should not be
   noted.  Reliance on the time indicated in this header field cannot be
   trusted for the purposes of determining the exact size of the data.
   The exact length of the data must be determined by examining the data
   itself.

5. Authors' Addresses

   Glenn W. Parsons
   Northern Telecom
   P.O. Box 3511, Station C
   Ottawa, ON  K1Y 4H7
   Canada

   Phone: [phone]
   Fax: [phone]
   EMail: [e-mail]


   Gregory M. Vaudreuil
   Lucent Technologies
   17080 Dallas Parkway
   Dallas, TX  75248-1905
   United States

   Phone/Fax: [phone]
   EMail: [e-mail]

6. References

   [MIME2] Freed, N., and N. Borenstein, "Multipurpose Internet Mail
           Extensions (MIME) Part Two: Media Types", RFC 2046, November
           1996.

   [VPIM2] Vaudreuil, G., and G. Parsons, "Voice Profile for Internet
           Mail - version 2", RFC 2421, September 1998.

   [REQ] Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.






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RFC 2424                    Content-Duration              September 1998


7.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  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.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.
























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