Content Duration MIME Header Definition
A MIME header that tells you how long audio or video plays.
Vaudreuil, G. and G. Parsons, "Content Duration MIME Header Definition", RFC 2424, DOI 10.17487/RFC2424, September 1998, <https://www.rfc-editor.org/info/rfc2424>.
@misc{rfc2424,
series = {Request for Comments},
number = 2424,
howpublished = {RFC 2424},
publisher = {RFC Editor},
doi = {10.17487/RFC2424},
url = {https://www.rfc-editor.org/info/rfc2424},
author = {G. Vaudreuil and G. Parsons},
title = {{Content Duration MIME Header Definition}},
pagetotal = 4,
year = 1998,
month = sep,
} RFC text
Contents
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.
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.
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: +1-613-763-7582
Fax: +1-613-763-4461
EMail: Glenn.Parsons@Nortel.ca
Gregory M. Vaudreuil
Lucent Technologies
17080 Dallas Parkway
Dallas, TX 75248-1905
United States
Phone/Fax: +1-972-733-2722
EMail: GregV@Lucent.Com
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.
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
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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.
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"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.
AI ELI5
On this page
§1What Content-Duration Is
RFC 2424 defines a simple MIME header field called Content-Duration intended for audio and video content. The header carries a single number: the length of the content in seconds.
Many audio and video formats store their own duration internally, but a sender may need to expose that duration in the email header itself. The header lets a mail client show the playing time before the user opens the attachment, useful when previewing voice messages or video clips inline.
It is a number in the message header that says how many seconds the audio or video will play.
§2The Header Field
The Content-Duration header is formatted as the keyword, followed by a colon and a number. That number is the duration in seconds with no units tag (just the digit).
The value is a string of up to 10 digits. The theoretical upper bound is 2 to the 31st power minus 1 (about 68 years). The RFC notes that an exact value can only be known by playing the content, so the header carries whatever the sender determines, but clients should not rely on it for calculations requiring precision.
It looks like Content-Duration: 45. That means 45 seconds. That is all.
§3Voicemail and VPIM
The RFC singles out VPIM (Voice Profile for Internet Mail) as a key use case. VPIM messages carry voice recordings and specify an audio subtype. Including Content-Duration lets a VPIM client display the length of a voice message when listing the inbox, or when viewing on a simple MIME reader that cannot decode audio.
Placing the duration in a header makes it accessible to mail systems that do not understand the audio format inside.
When you get a voice mail, the mail program can show you the length before you have to listen to it.
§4Trust and Limits
The header value is supplied by the sender and is not cryptographically verified. Clients must not use it to determine the actual size of the data; the real length must be discovered by reading the content itself.
In rare cases, exposing the duration in a header may pose a privacy or security concern (for example, a recording of a fixed length might leak information). Implementers should consider whether the header is appropriate for their use.
The number might not be exact, so do not trust it for anything important.