MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text
RFC 2047

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Network Working Group                                           K. Moore
Request for Comments: 2047                       University of Tennessee
Obsoletes: 1521, 1522, 1590                                November 1996
Category: Standards Track

        MIME (Multipurpose Internet Mail Extensions) Part Three:
              Message Header Extensions for Non-ASCII Text

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.

Abstract

   STD 11, RFC 822, defines a message representation protocol specifying
   considerable detail about US-ASCII message headers, and leaves the
   message content, or message body, as flat US-ASCII text.  This set of
   documents, collectively called the Multipurpose Internet Mail
   Extensions, or MIME, redefines the format of messages to allow for

   (1) textual message bodies in character sets other than US-ASCII,

   (2) an extensible set of different formats for non-textual message
       bodies,

   (3) multi-part message bodies, and

   (4) textual header information in character sets other than US-ASCII.

   These documents are based on earlier work documented in RFC 934, STD
   11, and RFC 1049, but extends and revises them.  Because RFC 822 said
   so little about message bodies, these documents are largely
   orthogonal to (rather than a revision of) RFC 822.

   This particular document is the third document in the series.  It
   describes extensions to RFC 822 to allow non-US-ASCII text data in
   Internet mail header fields.

Moore                       Standards Track                     [Page 1]
RFC 2047               Message Header Extensions           November 1996

   Other documents in this series include:

   + RFC 2045, which specifies the various headers used to describe
     the structure of MIME messages.

   + RFC 2046, which defines the general structure of the MIME media
     typing system and defines an initial set of media types,

   + RFC 2048, which specifies various IANA registration procedures
     for MIME-related facilities, and

   + RFC 2049, which describes MIME conformance criteria and
     provides some illustrative examples of MIME message formats,
     acknowledgements, and the bibliography.

   These documents are revisions of RFCs 1521, 1522, and 1590, which
   themselves were revisions of RFCs 1341 and 1342.  An appendix in RFC
   2049 describes differences and changes from previous versions.

1. Introduction

   RFC 2045 describes a mechanism for denoting textual body parts which
   are coded in various character sets, as well as methods for encoding
   such body parts as sequences of printable US-ASCII characters.  This
   memo describes similar techniques to allow the encoding of non-ASCII
   text in various portions of a RFC 822 [2] message header, in a manner
   which is unlikely to confuse existing message handling software.

   Like the encoding techniques described in RFC 2045, the techniques
   outlined here were designed to allow the use of non-ASCII characters
   in message headers in a way which is unlikely to be disturbed by the
   quirks of existing Internet mail handling programs.  In particular,
   some mail relaying programs are known to (a) delete some message
   header fields while retaining others, (b) rearrange the order of
   addresses in To or Cc fields, (c) rearrange the (vertical) order of
   header fields, and/or (d) "wrap" message headers at different places
   than those in the original message.  In addition, some mail reading
   programs are known to have difficulty correctly parsing message
   headers which, while legal according to RFC 822, make use of
   backslash-quoting to "hide" special characters such as "<", ",", or
   ":", or which exploit other infrequently-used features of that
   specification.

   While it is unfortunate that these programs do not correctly
   interpret RFC 822 headers, to "break" these programs would cause
   severe operational problems for the Internet mail system.  The
   extensions described in this memo therefore do not rely on little-
   used features of RFC 822.

Moore                       Standards Track                     [Page 2]
RFC 2047               Message Header Extensions           November 1996

   Instead, certain sequences of "ordinary" printable ASCII characters
   (known as "encoded-words") are reserved for use as encoded data.  The
   syntax of encoded-words is such that they are unlikely to
   "accidentally" appear as normal text in message headers.
   Furthermore, the characters used in encoded-words are restricted to
   those which do not have special meanings in the context in which the
   encoded-word appears.

   Generally, an "encoded-word" is a sequence of printable ASCII