※ [本文轉錄自 CSIE_R221 看板]
作者: Psycap (siraPcitsatnaF) 看板: CSIE_R221
標題: 實驗三相關問題--信件內容編碼問題
時間: Mon Apr 28 10:42:32 2008
參考以下網址
http://www.free-newslettertemplates.com/smtp.html
A character set is simply a mapping of byte values to characters.
The most common character set is US-ASCII, which has 32 (non-printable)
control characters and 96 (mostly printable) other characters, for a total of
128. These 128 characters can be encoded in 7 bits of data, so each 8-bit
byte representing one of these characters has the lower 7 bits set to the
appropriate value for the given character and the 8th (high) bit set to zero.
US-ASCII is therefore considered a single-byte 7-bit character set.
Many European languages have accentuated characters (like the German ü, the
French ç and é, the Danish ø and the Spanish ñ). Such languages are
commonly represented by characters sets whose lower half (i.e., values 0 -
127) are identical to those of US-ASCII, and whose upper half (i.e., values
128 - 255) represent these accentuated characters.
These are therefore considered single-byte 8-bit characters sets; an example
is ISO-8859-1.
Many Asian languages have so many characters that they need multiple bytes to
represent them all.
They are therefore considered multiple-byte character sets. Do note that not
only the sending mail program must be able to configure this correctly. The
receiving program or product must also support the character set used.
Otherwise an error will occur and the most common problem is badly formated
email content.
Headers & Bodies
Each message consists of two parts. The headers contain information about who
authored the message, the intended recipients, the time of creation, the
subject of the message, delivery stamps, ... Each header is of the form
"keyword: value", where keyword is a special word (like From or Date)
identifying the type of information contained in that header, and value is
the information itself.
A blank line always separates the headers from the body. The body contains
the information the sender is trying to communicate. The "message" as most
people think of it is really the body of the message.
MIME
For many years, most messages were plain text in the US-ASCII character set,
so no structure was needed for message bodies. The explosion of messaging in
Europe and Asia in the mid 1990s and that of transmission of multi-media
messages in the late 1990s brought about such a need.
Content-Type: text/plain; charset=us-ascii
indicates that the message consists of plain text in the US-ASCII character
set. MIME also specifies how to encode data when necessary (more on this
below). It is the responsibility of the receiving user agent to use this
information to display the message in a form that will be understood by the
user.
Transfer Protocols
The language spoken between transfer agents is known as a transfer protocol.
There are many in existence; the most common is Simple Mail Transfer Protocol.
Envelopes and Bodies
SMTP uses the concept of an envelope to transfer messages; this merely
contains information about from whom the message originated and to whom it is
destined. The originator address is important: in case there is a problem
transferring or delivering the message, the originator can be notified.
The SMTP body is the entire message as defined above in Headers & Bodies. So
the message headers plus the message body equals the SMTP body. The term SMTP
body is not used that commonly, but it is important to distinguish it from
the message body.
7-bit data vs. 8-bit data
For historical reasons relating to the US-ASCII character set, SMTP is a
7-bit protocol, which means it limits bytes of data sent to use only the
low-order 7-bits. If the 8th (high) bit of a byte is set, SMTP dictates that
the bit must be zeroed out.
In order for a message containing 8-bit data to be transferred without data
loss, the message must first be encoded into 7-bit data. As most early e-mail
users spoke English, however, and most computers used the 7-bit US-ASCII
character set, this was not a problem.
By the 1990s, however, several factors had increased the need for 8-bit
message transfer. As mentioned above, European languages often use 8-bit
character sets, and Asian language character sets often require multiple
bytes; their transmission is greatly simplified if all 8 bits can be
transferred unaltered. Finally, the explosion of multi-media messages like
audio and video clips have brought about a two-fold need for 8-bit message
transfer: encoding messages into 7-bit data is not only cumbersome, but the
resultant encoded message is significantly (typically 33%) larger than the
original message.
To meet this need, SMTP has been extended to allow 8-bit data to be properly
transferred between consenting transfer agents. The negotiating process used
to verify consent is specified in RFC 1869, which describes the general
extension mechanism to SMTP (called ESMTP), and RFC 1652, which describes the
specific extension to allow 8-bit data transfer, called 8BITMIME. If a
transfer agent has a message containing 8-bit data and it cannot negotiate
the proper transfer of that data, it must either encode the message into
7-bit data using MIME, or return the message to the sender indicating the
reason for the return.
It is no coincidence that MIME and ESMTP have common rationales and goals;
they were developed in conjunction with each other towards the same end.
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