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2. MIME Conformance The mechanisms
2. MIME Conformance
The mechanisms described in these documents are open-ended. It is
definitely not expected that all implementations will support all
available media types, nor that they will all share the same
extensions. In order to promote interoperability, however, it is
useful to define the concept of "MIME-conformance" to define a
certain level of implementation that allows the useful interworking
of messages with content that differs from US-ASCII text. In this
section, we specify the requirements for such conformance.
Freed & Borenstein Standards Track [Page 2]
RFC 2049 MIME Conformance November 1996
A mail user agent that is MIME-conformant MUST:
(1) Always generate a "MIME-Version: 1.0" header field in
any message it creates.
(2) Recognize the Content-Transfer-Encoding header field
and decode all received data encoded by either quoted-
printable or base64 implementations. The identity
transformations 7bit, 8bit, and binary must also be
recognized.
Any non-7bit data that is sent without encoding must be
properly labelled with a content-transfer-encoding of
8bit or binary, as appropriate. If the underlying
transport does not support 8bit or binary (as SMTP
[RFC-821] does not), the sender is required to both
encode and label data using an appropriate Content-
Transfer-Encoding such as quoted-printable or base64.
(3) Must treat any unrecognized Content-Transfer-Encoding
as if it had a Content-Type of "application/octet-
stream", regardless of whether or not the actual
Content-Type is recognized.
(4) Recognize and interpret the Content-Type header field,
and avoid showing users raw data with a Content-Type
field other than text. Implementations must be able
to send at least text/plain messages, with the
character set specified with the charset parameter if
it is not US-ASCII.
(5) Ignore any content type parameters whose names they do
not recognize.
(6) Explicitly handle the following media type values, to
at least the following extents:
Text:
-- Recognize and display "text" mail with the
character set "US-ASCII."
-- Recognize other character sets at least to the
extent of being able to inform the user about what
character set the message uses.
Freed & Borenstein Standards Track [Page 3]
RFC 2049 MIME Conformance November 1996
-- Recognize the "ISO-8859-*" character sets to the
extent of being able to display those characters that
are common to ISO-8859-* and US-ASCII, namely all
characters represented by octet values 1-127.
-- For unrecognized subtypes in a known character
set, show or offer to show the user the "raw" version
of the data after conversion of the content from
canonical form to local form.
-- Treat material in an unknown character set as if
it were "application/octet-stream".
Image, audio, and video:
-- At a minumum provide facilities to treat any
unrecognized subtypes as if they were
"application/octet-stream".
Application:
-- Offer the ability to remove either of the quoted-
printable or base64 encodings defined in this
document if they were used and put the resulting
information in a user file.
Multipart:
-- Recognize the mixed subtype. Display all relevant
information on the message level and the body part
header level and then display or offer to display
each of the body parts individually.
-- Recognize the "alternative" subtype, and avoid
showing the user redundant parts of
multipart/alternative mail.
-- Recognize the "multipart/digest" subtype,
specifically using "message/rfc822" rather than
"text/plain" as the default media type for body parts
inside "multipart/digest" entities.
-- Treat any unrecognized subtypes as if they were
"mixed".
Freed & Borenstein Standards Track [Page 4]
RFC 2049 MIME Conformance November 1996
Message:
-- Recognize and display at least the RFC822 message
encapsulation (message/rfc822) in such a way as to
preserve any recursive structure, that is, displaying
or offering to display the encapsulated data in
accordance with its media type.
-- Treat any unrecognized subtypes as if they were
"application/octet-stream".
(7) Upon encountering any unrecognized Content-Type field,
an implementation must treat it as if it had a media
type of "application/octet-stream" with no parameter
sub-arguments. How such data are handled is up to an
implementation, but likely options for handling such
unrecognized data include offering the user to write it
into a file (decoded from its mail transport format) or
offering the user to name a program to which the
decoded data should be passed as input.
(8) Conformant user agents are required, if they provide
non-standard support for non-MIME messages employing
character sets other than US-ASCII, to do so on
received messages only. Conforming user agents must not
send non-MIME messages containing anything other than
US-ASCII text.
In particular, the use of non-US-ASCII text in mail
messages without a MIME-Version field is strongly
discouraged as it impedes interoperability when sending
messages between regions with different localization
conventions. Conforming user agents MUST include proper
MIME labelling when sending anything other than plain
text in the US-ASCII character set.
In addition, non-MIME user agents should be upgraded if
at all possible to include appropriate MIME header
information in the messages they send even if nothing
else in MIME is supported. This upgrade will have
little, if any, effect on non-MIME recipients and will
aid MIME in correctly displaying such messages. It
also provides a smooth transition path to eventual
adoption of other MIME capabilities.
(9) Conforming user agents must ensure that any string of
non-white-space printable US-ASCII characters within a
"*text" or "*ctext" that begins with "=?" and ends with
Freed & Borenstein Standards Track [Page 5]
RFC 2049 MIME Conformance November 1996
"?=" be a valid encoded-word. ("begins" means: At the
start of the field-body or immediately following
linear-white-space; "ends" means: At the end of the
field-body or immediately preceding linear-white-
space.) In addition, any "word" within a "phrase" that
begins with "=?" and ends with "?=" must be a valid
encoded-word.
(10) Conforming user agents must be able to distinguish
encoded-words from "text", "ctext", or "word"s,
according to the rules in section 4, anytime they
appear in appropriate places in message headers. It
must support both the "B" and "Q" encodings for any
character set which it supports. The program must be
able to display the unencoded text if the character set
is "US-ASCII". For the ISO-8859-* character sets, the
mail reading program must at least be able to display
the characters which are also in the US-ASCII set.
A user agent that meets the above conditions is said to be MIME-
conformant. The meaning of this phrase is that it is assumed to be
"safe" to send virtually any kind of properly-marked data to users of
such mail systems, because such systems will at least be able to
treat the data as undifferentiated binary, and will not simply splash
it onto the screen of unsuspecting users.
There is another sense in which it is always "safe" to send data in a
format that is MIME-conformant, which is that such data will not
break or be broken by any known systems that are conformant with RFC
821 and RFC 822. User agents that are MIME-conformant have the
additional guarantee that the user will not be shown data that were
never intended to be viewed as text.
3. Guidelines for Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become
corrupted at several stages in its travel to a final destination.
Specifically, email sent throughout the Internet may travel across
many networking technologies. Many networking and mail technologies
do not support the full functionality possible in the SMTP transport
environment. Mail traversing these systems is likely to be modified
in order that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet.
These MTAs, speaking the SMTP protocol, alter messages on the fly to
take advantage of the internal data structure of the hosts they are
implemented on, or are just plain broken.
Freed & Borenstein Standards Track [Page 6]
RFC 2049 MIME Conforman
The mechanisms described in these documents are open-ended. It is
definitely not expected that all implementations will support all
available media types, nor that they will all share the same
extensions. In order to promote interoperability, however, it is
useful to define the concept of "MIME-conformance" to define a
certain level of implementation that allows the useful interworking
of messages with content that differs from US-ASCII text. In this
section, we specify the requirements for such conformance.
Freed & Borenstein Standards Track [Page 2]
RFC 2049 MIME Conformance November 1996
A mail user agent that is MIME-conformant MUST:
(1) Always generate a "MIME-Version: 1.0" header field in
any message it creates.
(2) Recognize the Content-Transfer-Encoding header field
and decode all received data encoded by either quoted-
printable or base64 implementations. The identity
transformations 7bit, 8bit, and binary must also be
recognized.
Any non-7bit data that is sent without encoding must be
properly labelled with a content-transfer-encoding of
8bit or binary, as appropriate. If the underlying
transport does not support 8bit or binary (as SMTP
[RFC-821] does not), the sender is required to both
encode and label data using an appropriate Content-
Transfer-Encoding such as quoted-printable or base64.
(3) Must treat any unrecognized Content-Transfer-Encoding
as if it had a Content-Type of "application/octet-
stream", regardless of whether or not the actual
Content-Type is recognized.
(4) Recognize and interpret the Content-Type header field,
and avoid showing users raw data with a Content-Type
field other than text. Implementations must be able
to send at least text/plain messages, with the
character set specified with the charset parameter if
it is not US-ASCII.
(5) Ignore any content type parameters whose names they do
not recognize.
(6) Explicitly handle the following media type values, to
at least the following extents:
Text:
-- Recognize and display "text" mail with the
character set "US-ASCII."
-- Recognize other character sets at least to the
extent of being able to inform the user about what
character set the message uses.
Freed & Borenstein Standards Track [Page 3]
RFC 2049 MIME Conformance November 1996
-- Recognize the "ISO-8859-*" character sets to the
extent of being able to display those characters that
are common to ISO-8859-* and US-ASCII, namely all
characters represented by octet values 1-127.
-- For unrecognized subtypes in a known character
set, show or offer to show the user the "raw" version
of the data after conversion of the content from
canonical form to local form.
-- Treat material in an unknown character set as if
it were "application/octet-stream".
Image, audio, and video:
-- At a minumum provide facilities to treat any
unrecognized subtypes as if they were
"application/octet-stream".
Application:
-- Offer the ability to remove either of the quoted-
printable or base64 encodings defined in this
document if they were used and put the resulting
information in a user file.
Multipart:
-- Recognize the mixed subtype. Display all relevant
information on the message level and the body part
header level and then display or offer to display
each of the body parts individually.
-- Recognize the "alternative" subtype, and avoid
showing the user redundant parts of
multipart/alternative mail.
-- Recognize the "multipart/digest" subtype,
specifically using "message/rfc822" rather than
"text/plain" as the default media type for body parts
inside "multipart/digest" entities.
-- Treat any unrecognized subtypes as if they were
"mixed".
Freed & Borenstein Standards Track [Page 4]
RFC 2049 MIME Conformance November 1996
Message:
-- Recognize and display at least the RFC822 message
encapsulation (message/rfc822) in such a way as to
preserve any recursive structure, that is, displaying
or offering to display the encapsulated data in
accordance with its media type.
-- Treat any unrecognized subtypes as if they were
"application/octet-stream".
(7) Upon encountering any unrecognized Content-Type field,
an implementation must treat it as if it had a media
type of "application/octet-stream" with no parameter
sub-arguments. How such data are handled is up to an
implementation, but likely options for handling such
unrecognized data include offering the user to write it
into a file (decoded from its mail transport format) or
offering the user to name a program to which the
decoded data should be passed as input.
(8) Conformant user agents are required, if they provide
non-standard support for non-MIME messages employing
character sets other than US-ASCII, to do so on
received messages only. Conforming user agents must not
send non-MIME messages containing anything other than
US-ASCII text.
In particular, the use of non-US-ASCII text in mail
messages without a MIME-Version field is strongly
discouraged as it impedes interoperability when sending
messages between regions with different localization
conventions. Conforming user agents MUST include proper
MIME labelling when sending anything other than plain
text in the US-ASCII character set.
In addition, non-MIME user agents should be upgraded if
at all possible to include appropriate MIME header
information in the messages they send even if nothing
else in MIME is supported. This upgrade will have
little, if any, effect on non-MIME recipients and will
aid MIME in correctly displaying such messages. It
also provides a smooth transition path to eventual
adoption of other MIME capabilities.
(9) Conforming user agents must ensure that any string of
non-white-space printable US-ASCII characters within a
"*text" or "*ctext" that begins with "=?" and ends with
Freed & Borenstein Standards Track [Page 5]
RFC 2049 MIME Conformance November 1996
"?=" be a valid encoded-word. ("begins" means: At the
start of the field-body or immediately following
linear-white-space; "ends" means: At the end of the
field-body or immediately preceding linear-white-
space.) In addition, any "word" within a "phrase" that
begins with "=?" and ends with "?=" must be a valid
encoded-word.
(10) Conforming user agents must be able to distinguish
encoded-words from "text", "ctext", or "word"s,
according to the rules in section 4, anytime they
appear in appropriate places in message headers. It
must support both the "B" and "Q" encodings for any
character set which it supports. The program must be
able to display the unencoded text if the character set
is "US-ASCII". For the ISO-8859-* character sets, the
mail reading program must at least be able to display
the characters which are also in the US-ASCII set.
A user agent that meets the above conditions is said to be MIME-
conformant. The meaning of this phrase is that it is assumed to be
"safe" to send virtually any kind of properly-marked data to users of
such mail systems, because such systems will at least be able to
treat the data as undifferentiated binary, and will not simply splash
it onto the screen of unsuspecting users.
There is another sense in which it is always "safe" to send data in a
format that is MIME-conformant, which is that such data will not
break or be broken by any known systems that are conformant with RFC
821 and RFC 822. User agents that are MIME-conformant have the
additional guarantee that the user will not be shown data that were
never intended to be viewed as text.
3. Guidelines for Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become
corrupted at several stages in its travel to a final destination.
Specifically, email sent throughout the Internet may travel across
many networking technologies. Many networking and mail technologies
do not support the full functionality possible in the SMTP transport
environment. Mail traversing these systems is likely to be modified
in order that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet.
These MTAs, speaking the SMTP protocol, alter messages on the fly to
take advantage of the internal data structure of the hosts they are
implemented on, or are just plain broken.
Freed & Borenstein Standards Track [Page 6]
RFC 2049 MIME Conforman
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2. MIME phù hợp Các cơ chế được mô tả trong các tài liệu được mở. Nó là chắc chắn không dự kiến rằng tất cả việc triển khai sẽ hỗ trợ tất cả Các loại phương tiện truyền thông có sẵn, cũng không họ sẽ tất cả chia sẻ tương tự Tiện ích mở rộng. Để thúc đẩy khả năng tương tác, Tuy nhiên, nó là hữu ích để xác định các khái niệm về "MIME-phù hợp" để xác định một một số mức độ thực hiện đã cho phép liên hữu ích của tin nhắn với nội dung khác với văn bản US-ASCII. Trong điều này phần, chúng tôi xác định các yêu cầu cho phù hợp như vậy.Giải phóng & theo dõi tiêu chuẩn Borenstein [Trang 2] RFC 2049 MIME phù hợp tháng 11 năm 1996 Một thư người sử dụng đại lý đó là phải MIME-conformant: (1) luôn luôn tạo ra một "MIME-Phiên bản: 1.0" tiêu đề trường trong bất kỳ tin nhắn nó tạo ra. (2) nhận ra lĩnh vực nội dung-Transfer-Encoding header và giải mã dữ liệu tất cả nhận được mã hóa theo một trong hai trích dẫn- in hoặc base64 hiện thực. Danh tính biến đổi 7 bit, 8 bit, và nhị phân cũng phải được công nhận. Bất kỳ không - 7 bit dữ liệu được gửi mà không có mã hóa phải đúng với nhãn hiệu với một nội dung-chuyển-mã hóa của 8 bit hoặc nhị phân, nếu thích hợp. Nếu các cơ bản giao thông vận tải không hỗ trợ 8 bit hoặc nhị phân (như SMTP [RFC-821] không), người gửi phải có cho cả hai mã hóa và nhãn dữ liệu bằng cách sử dụng một nội dung thích hợp- Transfer-Encoding chẳng hạn như trích dẫn-in hoặc base64. (3) phải điều trị bất kỳ không được công nhận nội dung-chuyển-mã hóa Nếu như nó có kiểu nội dung của "ứng dụng/octet- dòng", bất kể có hay không thực tế Loại nội dung được công nhận. (4) công nhận và giải thích trường tiêu đề Content-Type, và tránh hiển thị người dùng các dữ liệu thô với kiểu nội dung lĩnh vực khác hơn so với văn bản. Triển khai phải có khả năng gửi tin nhắn, ít văn bản/đồng bằng với các ký tự được chỉ định với tham số charset nếu nó không phải là U.S.-ASCII. (5) bỏ qua bất kỳ tham số loại nội dung mà tên họ làm không nhận ra. (6) một cách rõ ràng xử lý các giá trị loại sau phương tiện truyền thông, để ít nhất là sau extents: Văn bản: -Nhận ra và hiển thị "văn bản" thư với các ký tự đặt "US-ASCII." -Nhận ra ký tự khác đặt ít đến các phạm vi của việc có thể để thông báo cho người dùng về những gì ký tự đặt sử dụng tin nhắn.Giải phóng & theo dõi tiêu chuẩn Borenstein [Trang 3] RFC 2049 MIME phù hợp tháng 11 năm 1996 -Nhận ra những "ISO - 8859-*" ký tự đặt cho các phạm vi của việc có thể để hiển thị những ký tự mà phổ biến để ISO - 8859-* và U.S.-ASCII, cụ thể là tất cả ký tự đại diện bởi octet giá trị 1-127. --Cho các phân nhóm không được công nhận trong một nhân vật nổi tiếng thiết lập, Hiển thị hoặc cung cấp để hiển thị người dùng các phiên bản "nguyên" dữ liệu sau khi chuyển đổi nội dung từ kinh điển hình thức để tạo địa phương. -Điều trị liệu trong một bộ không rõ như thể nó đã được "ứng dụng/octet-dòng". Hình ảnh, âm thanh, và video: -Tại một minumum cung cấp các tiện nghi để điều trị bất kỳ không nhận dạng được phân nhóm như thể họ là "ứng dụng/octet-dòng". Ứng dụng: -Cung cấp khả năng để loại bỏ một trong những trích dẫn- in hoặc base64 mã hóa được định nghĩa trong điều này tài liệu nếu họ đã được sử dụng và đưa các kết quả thông tin trong một tập tin người sử dụng. Multipart: -Nhận ra phiên bản hỗn hợp. Hiển thị tất cả có liên quan thông tin về mức độ tin nhắn và phụ tùng thay thế tiêu đề cấp và sau đó hiển thị hoặc cung cấp để hiển thị mỗi cơ thể thành phần riêng lẻ. -Nhận ra phiên bản "thay thế", và tránh Đang hiển thị cho người dùng các bộ phận dư thừa của thư nhiều phần/thay thế. -Nhận ra phiên bản "phần/tiêu hóa", cụ thể bằng cách sử dụng "thư/rfc822" thay vì "văn bản/đồng bằng" như là phương tiện mặc định loại cho các bộ phận cơ thể bên trong "phần/tiêu hóa" thực thể. -Điều trị bất kỳ phân nhóm không được công nhận như thể họ là "hỗn hợp".Giải phóng & theo dõi tiêu chuẩn Borenstein [trang 4] RFC 2049 MIME phù hợp tháng 11 năm 1996 Thông báo: -Nhận ra và hiển thị ít thư RFC822 đóng gói (thư/rfc822) như vậy là để bảo vệ bất kỳ cấu trúc đệ quy, trong đó, Hiển thị hoặc cung cấp để hiển thị dữ liệu đóng gói trong phù hợp với loại phương tiện truyền thông. -Điều trị bất kỳ phân nhóm không được công nhận như thể họ là "ứng dụng/octet-dòng". (7) sau khi gặp phải bất kỳ lĩnh vực kiểu nội dung không được chấp nhận, thực hiện một phải đối xử với nó như thể nó đã có một phương tiện truyền thông loại "ứng dụng/octet-dòng" với tham số không tiểu luận cứ. Làm thế nào dữ liệu đó được xử lý là lên đến một thực hiện, nhưng các tùy chọn có khả năng để xử lý như vậy không được công nhận dữ liệu bao gồm cung cấp cho người sử dụng để viết nó vào một tập tin (giải mã từ dạng vận tải thư) hoặc cung cấp cho người dùng để đặt tên một chương trình mà các decoded dữ liệu nên được thông qua như đầu vào. (8) Conformant người sử dụng đại lý được yêu cầu, nếu họ cung cấp Phi tiêu chuẩn hỗ trợ cho các thư MIME sử dụng bộ ký tự khác hơn so với U.S.-ASCII, để vv. tin nhắn nhận được chỉ. Phù hợp người sử dụng đại lý phải không gửi tin nhắn MIME có chứa bất cứ điều gì khác hơn so với Văn bản US-ASCII. Đặc biệt, việc sử dụng không U.S. phải ASCII văn bản trong thư Các tin nhắn mà không có một lĩnh vực MIME-Phiên bản là mạnh mẽ khuyến khích như nó cản trở khả năng tương tác khi gửi tin nhắn giữa khu vực với địa phương hoá khác nhau Công ước. Phù hợp các đại lý người dùng phải bao gồm phù hợp MIME ghi nhãn khi gửi bất cứ điều gì khác hơn so với đồng bằng văn bản trong bộ ký tự US-ASCII. Ngoài ra, người sử dụng MIME đại lý nên được nâng cấp nếu ở tất cả có thể bao gồm tiêu đề MIME thích hợp thông tin trong thư họ gửi thậm chí nếu không có gì khác trong MIME được hỗ trợ. Nâng cấp này sẽ có ít, nếu có, có hiệu lực ngày-MIME người nhận và sẽ hỗ trợ MIME trong một cách chính xác Hiển thị các tin nhắn. Nó cũng cung cấp một con đường chuyển tiếp êm ả để cuối cùng nhận con nuôi của các khả năng MIME. Conforming (9) các đại lý người sử dụng phải đảm bảo rằng bất kỳ Chuỗi ký tự US-ASCII in phòng không trắng không gian trong vòng một "*text" or "*ctext" that begins with "=?" and ends withFreed & Borenstein Standards Track [Page 5] RFC 2049 MIME Conformance November 1996 "?=" be a valid encoded-word. ("begins" means: At the start of the field-body or immediately following linear-white-space; "ends" means: At the end of the field-body or immediately preceding linear-white- space.) In addition, any "word" within a "phrase" that begins with "=?" and ends with "?=" must be a valid encoded-word. (10) Conforming user agents must be able to distinguish encoded-words from "text", "ctext", or "word"s, according to the rules in section 4, anytime they appear in appropriate places in message headers. It must support both the "B" and "Q" encodings for any character set which it supports. The program must be able to display the unencoded text if the character set is "US-ASCII". For the ISO-8859-* character sets, the mail reading program must at least be able to display the characters which are also in the US-ASCII set. A user agent that meets the above conditions is said to be MIME- conformant. The meaning of this phrase is that it is assumed to be "safe" to send virtually any kind of properly-marked data to users of such mail systems, because such systems will at least be able to treat the data as undifferentiated binary, and will not simply splash it onto the screen of unsuspecting users. There is another sense in which it is always "safe" to send data in a format that is MIME-conformant, which is that such data will not
break or be broken by any known systems that are conformant with RFC
821 and RFC 822. User agents that are MIME-conformant have the
additional guarantee that the user will not be shown data that were
never intended to be viewed as text.
3. Guidelines for Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become
corrupted at several stages in its travel to a final destination.
Specifically, email sent throughout the Internet may travel across
many networking technologies. Many networking and mail technologies
do not support the full functionality possible in the SMTP transport
environment. Mail traversing these systems is likely to be modified
in order that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet.
These MTAs, speaking the SMTP protocol, alter messages on the fly to
take advantage of the internal data structure of the hosts they are
implemented on, or are just plain broken.
Freed & Borenstein Standards Track [Page 6]
RFC 2049 MIME Conforman
break or be broken by any known systems that are conformant with RFC
821 and RFC 822. User agents that are MIME-conformant have the
additional guarantee that the user will not be shown data that were
never intended to be viewed as text.
3. Guidelines for Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become
corrupted at several stages in its travel to a final destination.
Specifically, email sent throughout the Internet may travel across
many networking technologies. Many networking and mail technologies
do not support the full functionality possible in the SMTP transport
environment. Mail traversing these systems is likely to be modified
in order that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet.
These MTAs, speaking the SMTP protocol, alter messages on the fly to
take advantage of the internal data structure of the hosts they are
implemented on, or are just plain broken.
Freed & Borenstein Standards Track [Page 6]
RFC 2049 MIME Conforman
đang được dịch, vui lòng đợi..
2. MIME Conformance
The mechanisms described in these documents are open-ended. It is
definitely not expected that all implementations will support all
available media types, nor that they will all share the same
extensions. In order to promote interoperability, however, it is
useful to define the concept of "MIME-conformance" to define a
certain level of implementation that allows the useful interworking
of messages with content that differs from US-ASCII text. In this
section, we specify the requirements for such conformance.
Freed & Borenstein Standards Track [Page 2]
RFC 2049 MIME Conformance November 1996
A mail user agent that is MIME-conformant MUST:
(1) Always generate a "MIME-Version: 1.0" header field in
any message it creates.
(2) Recognize the Content-Transfer-Encoding header field
and decode all received data encoded by either quoted-
printable or base64 implementations. The identity
transformations 7bit, 8bit, and binary must also be
recognized.
Any non-7bit data that is sent without encoding must be
properly labelled with a content-transfer-encoding of
8bit or binary, as appropriate. If the underlying
transport does not support 8bit or binary (as SMTP
[RFC-821] does not), the sender is required to both
encode and label data using an appropriate Content-
Transfer-Encoding such as quoted-printable or base64.
(3) Must treat any unrecognized Content-Transfer-Encoding
as if it had a Content-Type of "application/octet-
stream", regardless of whether or not the actual
Content-Type is recognized.
(4) Recognize and interpret the Content-Type header field,
and avoid showing users raw data with a Content-Type
field other than text. Implementations must be able
to send at least text/plain messages, with the
character set specified with the charset parameter if
it is not US-ASCII.
(5) Ignore any content type parameters whose names they do
not recognize.
(6) Explicitly handle the following media type values, to
at least the following extents:
Text:
-- Recognize and display "text" mail with the
character set "US-ASCII."
-- Recognize other character sets at least to the
extent of being able to inform the user about what
character set the message uses.
Freed & Borenstein Standards Track [Page 3]
RFC 2049 MIME Conformance November 1996
-- Recognize the "ISO-8859-*" character sets to the
extent of being able to display those characters that
are common to ISO-8859-* and US-ASCII, namely all
characters represented by octet values 1-127.
-- For unrecognized subtypes in a known character
set, show or offer to show the user the "raw" version
of the data after conversion of the content from
canonical form to local form.
-- Treat material in an unknown character set as if
it were "application/octet-stream".
Image, audio, and video:
-- At a minumum provide facilities to treat any
unrecognized subtypes as if they were
"application/octet-stream".
Application:
-- Offer the ability to remove either of the quoted-
printable or base64 encodings defined in this
document if they were used and put the resulting
information in a user file.
Multipart:
-- Recognize the mixed subtype. Display all relevant
information on the message level and the body part
header level and then display or offer to display
each of the body parts individually.
-- Recognize the "alternative" subtype, and avoid
showing the user redundant parts of
multipart/alternative mail.
-- Recognize the "multipart/digest" subtype,
specifically using "message/rfc822" rather than
"text/plain" as the default media type for body parts
inside "multipart/digest" entities.
-- Treat any unrecognized subtypes as if they were
"mixed".
Freed & Borenstein Standards Track [Page 4]
RFC 2049 MIME Conformance November 1996
Message:
-- Recognize and display at least the RFC822 message
encapsulation (message/rfc822) in such a way as to
preserve any recursive structure, that is, displaying
or offering to display the encapsulated data in
accordance with its media type.
-- Treat any unrecognized subtypes as if they were
"application/octet-stream".
(7) Upon encountering any unrecognized Content-Type field,
an implementation must treat it as if it had a media
type of "application/octet-stream" with no parameter
sub-arguments. How such data are handled is up to an
implementation, but likely options for handling such
unrecognized data include offering the user to write it
into a file (decoded from its mail transport format) or
offering the user to name a program to which the
decoded data should be passed as input.
(8) Conformant user agents are required, if they provide
non-standard support for non-MIME messages employing
character sets other than US-ASCII, to do so on
received messages only. Conforming user agents must not
send non-MIME messages containing anything other than
US-ASCII text.
In particular, the use of non-US-ASCII text in mail
messages without a MIME-Version field is strongly
discouraged as it impedes interoperability when sending
messages between regions with different localization
conventions. Conforming user agents MUST include proper
MIME labelling when sending anything other than plain
text in the US-ASCII character set.
In addition, non-MIME user agents should be upgraded if
at all possible to include appropriate MIME header
information in the messages they send even if nothing
else in MIME is supported. This upgrade will have
little, if any, effect on non-MIME recipients and will
aid MIME in correctly displaying such messages. It
also provides a smooth transition path to eventual
adoption of other MIME capabilities.
(9) Conforming user agents must ensure that any string of
non-white-space printable US-ASCII characters within a
"*text" or "*ctext" that begins with "=?" and ends with
Freed & Borenstein Standards Track [Page 5]
RFC 2049 MIME Conformance November 1996
"?=" be a valid encoded-word. ("begins" means: At the
start of the field-body or immediately following
linear-white-space; "ends" means: At the end of the
field-body or immediately preceding linear-white-
space.) In addition, any "word" within a "phrase" that
begins with "=?" and ends with "?=" must be a valid
encoded-word.
(10) Conforming user agents must be able to distinguish
encoded-words from "text", "ctext", or "word"s,
according to the rules in section 4, anytime they
appear in appropriate places in message headers. It
must support both the "B" and "Q" encodings for any
character set which it supports. The program must be
able to display the unencoded text if the character set
is "US-ASCII". For the ISO-8859-* character sets, the
mail reading program must at least be able to display
the characters which are also in the US-ASCII set.
A user agent that meets the above conditions is said to be MIME-
conformant. The meaning of this phrase is that it is assumed to be
"safe" to send virtually any kind of properly-marked data to users of
such mail systems, because such systems will at least be able to
treat the data as undifferentiated binary, and will not simply splash
it onto the screen of unsuspecting users.
There is another sense in which it is always "safe" to send data in a
format that is MIME-conformant, which is that such data will not
break or be broken by any known systems that are conformant with RFC
821 and RFC 822. User agents that are MIME-conformant have the
additional guarantee that the user will not be shown data that were
never intended to be viewed as text.
3. Guidelines for Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become
corrupted at several stages in its travel to a final destination.
Specifically, email sent throughout the Internet may travel across
many networking technologies. Many networking and mail technologies
do not support the full functionality possible in the SMTP transport
environment. Mail traversing these systems is likely to be modified
in order that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet.
These MTAs, speaking the SMTP protocol, alter messages on the fly to
take advantage of the internal data structure of the hosts they are
implemented on, or are just plain broken.
Freed & Borenstein Standards Track [Page 6]
RFC 2049 MIME Conforman
The mechanisms described in these documents are open-ended. It is
definitely not expected that all implementations will support all
available media types, nor that they will all share the same
extensions. In order to promote interoperability, however, it is
useful to define the concept of "MIME-conformance" to define a
certain level of implementation that allows the useful interworking
of messages with content that differs from US-ASCII text. In this
section, we specify the requirements for such conformance.
Freed & Borenstein Standards Track [Page 2]
RFC 2049 MIME Conformance November 1996
A mail user agent that is MIME-conformant MUST:
(1) Always generate a "MIME-Version: 1.0" header field in
any message it creates.
(2) Recognize the Content-Transfer-Encoding header field
and decode all received data encoded by either quoted-
printable or base64 implementations. The identity
transformations 7bit, 8bit, and binary must also be
recognized.
Any non-7bit data that is sent without encoding must be
properly labelled with a content-transfer-encoding of
8bit or binary, as appropriate. If the underlying
transport does not support 8bit or binary (as SMTP
[RFC-821] does not), the sender is required to both
encode and label data using an appropriate Content-
Transfer-Encoding such as quoted-printable or base64.
(3) Must treat any unrecognized Content-Transfer-Encoding
as if it had a Content-Type of "application/octet-
stream", regardless of whether or not the actual
Content-Type is recognized.
(4) Recognize and interpret the Content-Type header field,
and avoid showing users raw data with a Content-Type
field other than text. Implementations must be able
to send at least text/plain messages, with the
character set specified with the charset parameter if
it is not US-ASCII.
(5) Ignore any content type parameters whose names they do
not recognize.
(6) Explicitly handle the following media type values, to
at least the following extents:
Text:
-- Recognize and display "text" mail with the
character set "US-ASCII."
-- Recognize other character sets at least to the
extent of being able to inform the user about what
character set the message uses.
Freed & Borenstein Standards Track [Page 3]
RFC 2049 MIME Conformance November 1996
-- Recognize the "ISO-8859-*" character sets to the
extent of being able to display those characters that
are common to ISO-8859-* and US-ASCII, namely all
characters represented by octet values 1-127.
-- For unrecognized subtypes in a known character
set, show or offer to show the user the "raw" version
of the data after conversion of the content from
canonical form to local form.
-- Treat material in an unknown character set as if
it were "application/octet-stream".
Image, audio, and video:
-- At a minumum provide facilities to treat any
unrecognized subtypes as if they were
"application/octet-stream".
Application:
-- Offer the ability to remove either of the quoted-
printable or base64 encodings defined in this
document if they were used and put the resulting
information in a user file.
Multipart:
-- Recognize the mixed subtype. Display all relevant
information on the message level and the body part
header level and then display or offer to display
each of the body parts individually.
-- Recognize the "alternative" subtype, and avoid
showing the user redundant parts of
multipart/alternative mail.
-- Recognize the "multipart/digest" subtype,
specifically using "message/rfc822" rather than
"text/plain" as the default media type for body parts
inside "multipart/digest" entities.
-- Treat any unrecognized subtypes as if they were
"mixed".
Freed & Borenstein Standards Track [Page 4]
RFC 2049 MIME Conformance November 1996
Message:
-- Recognize and display at least the RFC822 message
encapsulation (message/rfc822) in such a way as to
preserve any recursive structure, that is, displaying
or offering to display the encapsulated data in
accordance with its media type.
-- Treat any unrecognized subtypes as if they were
"application/octet-stream".
(7) Upon encountering any unrecognized Content-Type field,
an implementation must treat it as if it had a media
type of "application/octet-stream" with no parameter
sub-arguments. How such data are handled is up to an
implementation, but likely options for handling such
unrecognized data include offering the user to write it
into a file (decoded from its mail transport format) or
offering the user to name a program to which the
decoded data should be passed as input.
(8) Conformant user agents are required, if they provide
non-standard support for non-MIME messages employing
character sets other than US-ASCII, to do so on
received messages only. Conforming user agents must not
send non-MIME messages containing anything other than
US-ASCII text.
In particular, the use of non-US-ASCII text in mail
messages without a MIME-Version field is strongly
discouraged as it impedes interoperability when sending
messages between regions with different localization
conventions. Conforming user agents MUST include proper
MIME labelling when sending anything other than plain
text in the US-ASCII character set.
In addition, non-MIME user agents should be upgraded if
at all possible to include appropriate MIME header
information in the messages they send even if nothing
else in MIME is supported. This upgrade will have
little, if any, effect on non-MIME recipients and will
aid MIME in correctly displaying such messages. It
also provides a smooth transition path to eventual
adoption of other MIME capabilities.
(9) Conforming user agents must ensure that any string of
non-white-space printable US-ASCII characters within a
"*text" or "*ctext" that begins with "=?" and ends with
Freed & Borenstein Standards Track [Page 5]
RFC 2049 MIME Conformance November 1996
"?=" be a valid encoded-word. ("begins" means: At the
start of the field-body or immediately following
linear-white-space; "ends" means: At the end of the
field-body or immediately preceding linear-white-
space.) In addition, any "word" within a "phrase" that
begins with "=?" and ends with "?=" must be a valid
encoded-word.
(10) Conforming user agents must be able to distinguish
encoded-words from "text", "ctext", or "word"s,
according to the rules in section 4, anytime they
appear in appropriate places in message headers. It
must support both the "B" and "Q" encodings for any
character set which it supports. The program must be
able to display the unencoded text if the character set
is "US-ASCII". For the ISO-8859-* character sets, the
mail reading program must at least be able to display
the characters which are also in the US-ASCII set.
A user agent that meets the above conditions is said to be MIME-
conformant. The meaning of this phrase is that it is assumed to be
"safe" to send virtually any kind of properly-marked data to users of
such mail systems, because such systems will at least be able to
treat the data as undifferentiated binary, and will not simply splash
it onto the screen of unsuspecting users.
There is another sense in which it is always "safe" to send data in a
format that is MIME-conformant, which is that such data will not
break or be broken by any known systems that are conformant with RFC
821 and RFC 822. User agents that are MIME-conformant have the
additional guarantee that the user will not be shown data that were
never intended to be viewed as text.
3. Guidelines for Sending Email Data
Internet email is not a perfect, homogeneous system. Mail may become
corrupted at several stages in its travel to a final destination.
Specifically, email sent throughout the Internet may travel across
many networking technologies. Many networking and mail technologies
do not support the full functionality possible in the SMTP transport
environment. Mail traversing these systems is likely to be modified
in order that it can be transported.
There exist many widely-deployed non-conformant MTAs in the Internet.
These MTAs, speaking the SMTP protocol, alter messages on the fly to
take advantage of the internal data structure of the hosts they are
implemented on, or are just plain broken.
Freed & Borenstein Standards Track [Page 6]
RFC 2049 MIME Conforman
đang được dịch, vui lòng đợi..
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- liên lạc với bạn bè
