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RFC 4021

 
 
 

Registration of Mail and MIME Header Fields

Part 2 of 2, p. 26 to 54
Prev RFC Part

 


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2.1.40.  Header Field: Alternate-Recipient

   Description:
      Controls forwarding to alternate recipients

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Controls whether this message may be forwarded to an alternate
      recipient, such as a postmaster, if delivery to the intended
      recipient is not possible.  Default: Allowed.  RFC 2156 (MIXER),
      not for general use.

2.1.41.  Header Field: Original-Encoded-Information-Types

   Description:
      Body part types in message

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Which body part types occur in this message.  RFC 2156 (MIXER);
      not for general use.

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2.1.42.  Header Field: Content-Return

   Description:
      Return content on non-delivery?

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      Indicates whether the content of a message is to be returned with
      non-delivery notifications.  Introduced by RFC 1327 and
      subsequently changed by RFC 2156 to avoid confusion with MIME
      defined fields.

2.1.43.  Header Field: Generate-Delivery-Report

   Description:
      Request delivery report generation

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Indicates whether a delivery report is wanted at successful
      delivery.  Default is not to generate such a report.  RFC 2156
      (MIXER); not for general use.

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2.1.44.  Header Field: Prevent-NonDelivery-Report

   Description:
      Non-delivery report required?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Indicates whether a non-delivery report is wanted on delivery
      error.  Default is to generate such a report.  RFC 2156 (MIXER);
      not for general use.

2.1.45.  Header Field: Obsoletes

   Description:
      Reference message to be replaced

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      Reference to a previous message being corrected and replaced.
      Compare to 'Supersedes:',f used in Usenet News.  Introduced by RFC
      1327 and subsequently renamed by RFC 2156 to 'Supersedes'.

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2.1.46.  Header Field: Supersedes

   Description:
      Reference message to be replaced

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Reference to a previous message being corrected and replaced.
      Renamed version of obsolete 'Obsoletes' header field.  RFC 2156
      (MIXER); not for general use.

2.1.47.  Header Field: Content-Identifier

   Description:
      Message content identifier

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      A text string that identifies the content of a message.
      Introduced by RFC 1327 and subsequently changed by RFC 2156 to
      avoid confusion with MIME defined fields.  Gateways that reverse
      map may support the old field.

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2.1.48.  Header Field: Delivery-Date

   Description:
      Message delivery time

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      The time when a message was delivered to its recipient.  RFC 2156
      (MIXER); not for general use.

2.1.49.  Header Field: Expiry-Date

   Description:
      Message expiry time

   Applicable protocol: Mail [18]

   Status: obsolete

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1327 [3]

   Related information:
      Time at which a message loses its validity.  Introduced by RFC
      1327 and subsequently changed by RFC 2156 to 'Expires:'.

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2.1.50.  Header Field: Expires

   Description:
      Message expiry time

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Time at which a message loses its validity.  Renamed version of
      obsolete Expiry-Date header field.  RFC 2156 (MIXER), not for
      general use.

2.1.51.  Header Field: Reply-By

   Description:
      Time by which a reply is requested

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Latest time by which a reply is requested (not demanded).  RFC
      2156 (MIXER); not for general use.

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2.1.52.  Header Field: Importance

   Description:
      Message importance

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      A hint from the originator to the recipients about how important a
      message is.  Values: High, normal, or low.  Not used to control
      transmission speed.  Proposed for use with RFC 2156 (MIXER) [10]
      and RFC 3801 (VPIM) [14].

2.1.53.  Header Field: Incomplete-Copy

   Description:
      Body parts are missing

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Body parts are missing.  RFC 2156 (MIXER); not for general use.

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2.1.54.  Header Field: Priority

   Description:
      Message priority

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can be 'normal', 'urgent', or 'non-urgent' and can influence
      transmission speed and delivery.  RFC 2156 (MIXER); not for
      general use.

2.1.55.  Header Field: Sensitivity

   Description:
      Message content sensitivity

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      How sensitive it is to disclose this message to people other than
      the specified recipients.  Values: Personal, private, and company
      confidential.  The absence of this header field in messages
      gatewayed from X.400 indicates that the message is not sensitive.
      Proposed for use with RFC 2156 (MIXER) [10] and RFC 3801 (VPIM)
      [14].

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2.1.56.  Header Field: Language

   Description:
      X.400 message content language

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can include a code for the natural language used in a message;
      e.g., 'en' for English.  See also 'Content-Language'.  RFC 2156
      (MIXER); not for general use.

2.1.57.  Header Field: Conversion

   Description:
      Conversion allowed?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      The body of this message may not be converted from one character
      set to another.  Values: prohibited and allowed.  RFC 2156
      (MIXER); not for general use.

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2.1.58.  Header Field: Conversion-With-Loss

   Description:
      Lossy conversion allowed?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      The body of this message may not be converted from one character
      set to another if information will be lost.  Values: prohibited
      and allowed.  RFC 2156 (MIXER); not for general use.

2.1.59.  Header Field: Message-Type

   Description:
      Message type: delivery report?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Only used with the value 'Delivery Report' to indicate that this
      is a delivery report gatewayed from X.400.  RFC 2156 (MIXER); not
      for general use.

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2.1.60.  Header Field: Autosubmitted

   Description:
      Automatically submitted indicator

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Has been automatically submitted.  RFC 2156 (MIXER); not for
      general use.

2.1.61.  Header Field: Autoforwarded

   Description:
      Automatically forwarded indicator

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Has been automatically forwarded.  RFC 2156 (MIXER), not for
      general use.

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2.1.62.  Header Field: Discarded-X400-IPMS-Extensions

   Description:
      X.400 IPM extensions discarded

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can be used in Internet mail to indicate X.400 IPM extensions that
      could not be mapped to Internet mail format.  RFC 2156 (MIXER);
      not for general use.

2.1.63.  Header Field: Discarded-X400-MTS-Extensions

   Description:
      X.400 MTS extensions discarded

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Can be used in Internet mail to indicate X.400 MTS extensions that
      could not be mapped to Internet mail format.  RFC 2156 (MIXER);
      not for general use.

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2.1.64.  Header Field: Disclose-Recipients

   Description:
      Disclose names of other recipients?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Tells whether recipients are to be told the names of other
      recipients of the same message.  This is primarily an X.400
      facility.  In X.400, this is an envelope attribute and refers to
      disclosure of the envelope recipient list.  Disclosure of other
      recipients is done in Internet mail via the To:, cc:, and bcc:
      header fields.  Not for general use.

2.1.65.  Header Field: Deferred-Delivery

   Description:
      Deferred delivery information

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Provides information about deferred delivery service to the
      recipient.  RFC 2156 (MIXER); not for general use.

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2.1.66.  Header Field: Latest-Delivery-Time

   Description:
      Latest delivery time requested

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Provides the recipient with information about requested delivery
      but will not be acted on by the SMTP infrastructure.  RFC 2156
      (MIXER); not for general use.

2.1.67.  Header Field: Originator-Return-Address

   Description:
      Originator return address

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Originator return address.  RFC 2156 (MIXER); not for general use.

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2.1.68.  Header Field: X400-Content-Identifier

   Description:
      Message content identifier

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      A text string that identifies the content of a message.  Renamed
      version of obsolete Content-Identifier field.  RFC 2156 (MIXER);
      not for general use.

2.1.69.  Header Field: X400-Content-Return

   Description:
      Return content on non-delivery?

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      Indicates whether the content of a message is to be returned with
      non-delivery notifications.  Renamed version of obsolete Content-
      Return field.  RFC 2156 (MIXER); not for general use.

Top      Up      ToC       Page 41 
2.1.70.  Header Field: X400-Content-Type

   Description:
      X400 content type

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 content type.  RFC 2156 (MIXER); not for general use.

2.1.71.  Header Field: X400-MTS-Identifier

   Description:
      X400 MTS-Identifier

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 MTS-Identifier.  RFC 2156 (MIXER); not for general use.

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2.1.72.  Header Field: X400-Originator

   Description:
      X400 Originator

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Originator.  RFC 2156 (MIXER); not for general use.

2.1.73.  Header Field: X400-Received

   Description:
      X400 Received

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Received.  RFC 2156 (MIXER); not for general use.

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2.1.74.  Header Field: X400-Recipients

   Description:
      X400 Recipients

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Recipients.  RFC 2156 (MIXER); not for general use.

2.1.75.  Header Field: X400-Trace

   Description:
      X400 Trace

   Applicable protocol: Mail [18]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2156 [10]

   Related information:
      X400 Trace.  RFC 2156 (MIXER), not for general use.

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2.2.  Permanent MIME Header Field Registrations

      Header name         Protocol
      -----------         --------
      MIME-Version        MIME      MIME version number
      Content-ID          MIME      Identify content body part
      Content-Description MIME      Description of message body part
      Content-Transfer-Encoding
                          MIME      Content transfer encoding applied
      Content-Type        MIME      MIME content type
      Content-Base        MIME      Base to be used for resolving
                                    relative URIs within this content
                                    part
      Content-Location    MIME      URI for retrieving a body part
      Content-features    MIME      Indicates content features of a
                                    MIME body part
      Content-Disposition MIME      Intended content disposition and
                                    file name
      Content-Language    MIME      Language of message content
      Content-Alternative MIME      Alternative content available
      Content-MD5         MIME      MD5 checksum of content
      Content-Duration    MIME      Time duration of content

2.2.1.  Header Field: MIME-Version

   Description:
      MIME version number

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 4)

   Related information:
      An indicator that this message is formatted according to the MIME
      standard, and an indication of which version of MIME is used.

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2.2.2.  Header Field: Content-ID

   Description:
      Identify content body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 7)

   Related information:
      Specifies a Unique ID for one MIME body part of the content of a
      message.

2.2.3.  Header Field: Content-Description

   Description:
      Description of message body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 8)

   Related information:
      Description of a particular body part of a message; for example, a
      caption for an image body part.

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2.2.4.  Header Field: Content-Transfer-Encoding

   Description:
      Content transfer encoding applied

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 6)

   Related information:
      Coding method used in a MIME message body part.

2.2.5.  Header Field: Content-Type

   Description:
      MIME content type

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2045 [7] (section 5)

   Related information:
      Format of content (character set, etc.)  Note that the values for
      this header field are defined in different ways in RFC 1049 and in
      MIME (RFC 2045).  The 'MIME-version' header field will show
      whether Content-Type is to be interpreted according to RFC 1049 or
      according to MIME.  The MIME definition should be used in
      generating mail.  RFC 1049 has 'historic' status.  RFC 1766 [5]
      defines a parameter 'difference' to this header field.  Various
      other Content-Type define various additional parameters.  For
      example, the parameter 'charset' is mandatory for all textual
      Content-Types.  See also RFC 1049, RFC 1123: 5.2.13, and RFC 1766:
      4.1.

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2.2.6.  Header Field: Content-Base

   Description:
      Base to be used for resolving relative URIs within this content
      part.

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2110 [8]

   Related information:
      Base to be used for resolving relative URIs within this content
      part.  See also Content-Location.  This header was included in the
      first version of MHTML and HTTP 1.1 but removed in the second
      version (RFC 2557).

2.2.7.  Header Field: Content-Location

   Description:
      URI for retrieving a body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2557 [16]

   Related information:
      URI using which the content of this body-part part was retrieved,
      might be retrievable, or which otherwise gives a globally unique
      identification of the content.

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2.2.8.  Header Field: Content-features

   Description:
      Indicates content features of a MIME body part

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2912 [19] (section 3)

   Related information:
      The 'Content-features:' header can be used to annotate a MIME body
      part with a media feature expression, to indicate features of the
      body part content.  See also RFC 2533, RFC 2506, and RFC 2045.

2.2.9.  Header Field: Content-Disposition

   Description:
      Intended content disposition and file name

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2183 [11]

   Related information:
      Indicates whether a MIME body part is to be shown inline or is an
      attachment; can also indicate a suggested filename for use when
      saving an attachment to a file.

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2.2.10.  Header Field: Content-Language

   Description:
      Language of message content

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3282 [21]

   Related information:
      Can include a code for the natural language used in a message;
      e.g., 'en' for English.  Can also contain a list of languages for
      a message containing more than one language.

2.2.11.  Header Field: Content-Alternative

   Description:
      Alternative content available

   Applicable protocol: MIME [7]

   Status: work-in-progress

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 3297 [22]

   Related information:
      Information about the media features of alternative content
      formats available for the current message.

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2.2.12.  Header Field: Content-MD5

   Description:
      MD5 checksum of content

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 1864 [6]

   Related information:
      Checksum of content to ensure that it has not been modified.

2.2.13.  Header Field: Content-Duration

   Description:
      Time duration of content

   Applicable protocol: MIME [7]

   Status: standards-track

   Author/change controller:
      IETF  (mailto:iesg@ietf.org)
      Internet Engineering Task Force

   Specification document(s):
      RFC 2424 [15]

   Related information:
      Time duration of body part content, in seconds (e.g., for audio
      message).

3.  IANA Considerations

   Section 2 of this specification provides initial registrations of
   mail and MIME header fields in the "Permanent Message Header Field
   Registry", defined by registration procedures for message header
   fields [1].

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4.  Security Considerations

   No security considerations are introduced by this registration
   document beyond those already inherent in use of the mail message
   header fields referenced.

5.  Acknowledgements

   Most of the information in this document has been derived from Jacob
   Palme's work in RFC 2076 [25] and subsequent updates [26].  The
   authors also gratefully acknowledge contributions and constructive
   input from Mark Nottingham, Bruce Lilly, Keith Moore, and Charles
   Lindsey (the mention of whom is not intended to imply their
   unqualified support for material herein).

6.  References

6.1.  Normative References

   [1]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
        Procedures for Message Header Fields", BCP 90, RFC 3864,
        September 2004.

   [2]  Crocker, D., "Standard for the format of ARPA Internet text
        messages", STD 11, RFC 822, August 1982.

   [3]  Hardcastle-Kille, S., "Mapping between X.400(1988) / ISO 10021
        and RFC 822", RFC 1327, May 1992.

   [4]  Costanzo, A., Robinson, D., and R. Ullmann, "Encoding Header
        Field for Internet Messages", RFC 1505, August 1993.

   [5]  Alvestrand, H., "Tags for the Identification of Languages", RFC
        1766, March 1995.

   [6]  Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864,
        October 1995.

   [7]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

   [8]  Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
        Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
        1997.

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

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   [10] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
        between X.400 and RFC 822/MIME", RFC 2156, January 1998.

   [11] Troost, R., Dorner, S., and K. Moore, "Communicating
        Presentation Information in Internet Messages: The Content-
        Disposition Header Field", RFC 2183, August 1997.

   [12] Hansen, T. and G. Vaudreuil, "Message Disposition Notification",
        RFC 3798, May 2004.

   [13] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax for
        Core Mail List Commands and their Transport through Message
        Header Fields", RFC 2369, July 1998.

   [14] Vaudreuil, G. and G. Parsons, "Voice Profile for Internet Mail -
        version 2 (VPIMv2)", RFC 3801, June 2004.

   [15] Vaudreuil, G. and G. Parsons, "Content Duration MIME Header
        Definition", RFC 3803, June 2004.

   [16] Palme, J., Hopmann, A., and N. Shelness, "MIME Encapsulation of
        Aggregate Documents, such as HTML (MHTML)", RFC 2557, March
        1999.

   [17] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
        2001.

   [18] Resnick, P., "Internet Message Format", RFC 2822, April 2001.

   [19] Klyne, G., "Indicating Media Features for MIME Content", RFC
        2912, September 2000.

   [20] Chandhok, R. and G. Wenger, "List-Id: A Structured Field and
        Namespace for the Identification of Mailing Lists", RFC 2919,
        March 2001.

   [21] Alvestrand, H., "Content Language Headers", RFC 3282, May 2002.

   [22] Klyne, G., Iwazaki, R., and D. Crocker, "Content Negotiation for
        Messaging Services based on Email", RFC 3297, July 2002.

   [23] Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
        Context for Internet Mail", RFC 3458, January 2003.

   [24] Miller, J., Krauskopf, T., Resnick, P. and W. Treese, "PICS
        Label Distribution Label Syntax and Communication Protocols",
        W3C Recommendation REC-PICS-labels, October 1996,
        <http://www.w3.org/TR/REC-PICS-labels>.

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6.2.  Informative References

   [25] Palme, J., "Common Internet Message Headers", RFC 2076, February
        1997.

   [26] Palme, J., "Common Internet Message Header Fields", Work in
        Progress.

URIs

   [27] <http://www.ninebynine.org/IETF/Messaging/HdrRegistry/
        Intro.html>

Authors' Addresses

   Graham Klyne
   Image Bioinformatics Research Group
   Department of Zoology, University of Oxford
   South Parks Road, Oxford OX1 3PS, UK

   Phone: +44-(0)1865-281991
   Fax:   +44-(0)1865-310447
   EMail: GK-IETF@ninebynine.org


   Jacob Palme
   Stockholm University/KTH
   Forum 100
   Kista  S-164 40
   Sweden

   Phone: +46-8-16 16 67
   Fax:   +46-8-783 08 29
   EMail: jpalme@dsv.su.se

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