Previous: 4. The Content-Type Header Field
Next: 6. Additional Content-Header Fields
It is necessary, therefore, to define a standard mechanism for re- encoding such data into a 7-bit short-line format. This document specifies that such encodings will be indicated by a new "Content-Transfer-Encoding" header field. The Content-Transfer-Encoding field is used to indicate the type of transformation that has been used in order to represent the body in an acceptable manner for transport.
Unlike Content-Types, a proliferation of Content-Transfer-Encoding values is undesirable and unnecessary. However, establishing only a single Content-Transfer-Encoding mechanism does not seem possible. There is a tradeoff between the desire for a compact and efficient encoding of largely-binary data and the desire for a readable encoding of data that is mostly, but not entirely, 7-bit data. For this reason, at least two encoding mechanisms are necessary: a "readable" encoding and a "dense" encoding.
The Content-Transfer-Encoding field is designed to specify an invertible mapping between the "native" representation of a type of data and a representation that can be readily exchanged using 7 bit mail transport protocols, such as those defined by RFC 821 (SMTP). This field has not been defined by any previous standard. The field's value is a single token specifying the type of encoding, as enumerated below. Formally:
encoding := "Content-Transfer-Encoding" ":" mechanism mechanism := "7bit" ; case-insensitive / "quoted-printable" / "base64" / "8bit" / "binary" / x-token
These values are not case sensitive. That is, Base64 and BASE64 and bAsE64 are all equivalent. An encoding type of 7BIT requires that the body is already in a seven-bit mail-ready representation. This is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the Content-Transfer-Encoding header field is not present.
The values "8bit", "7bit", and "binary" all mean that NO encoding has been performed. However, they are potentially useful as indications of the kind of data contained in the object, and therefore of the kind of encoding that might need to be performed for transmission in a given transport system. In particular:
Mail transport for unencoded 8-bit data is defined in RFC-1426 [RFC-1426]. As of the publication of this document, there are no standardized Internet mail transports for which it is legitimate to include unencoded binary data in mail bodies. Thus there are no circumstances in which the "binary" Content-Transfer-Encoding is actually legal on the Internet. However, in the event that binary mail transport becomes a reality in Internet mail, or when this document is used in conjunction with any other binary-capable transport mechanism, binary bodies should be labeled as such using this mechanism.
Implementors may, if necessary, define new Content-Transfer-Encoding values, but must use an x-token, which is a name prefixed by "X-" to indicate its non-standard status, e.g., "Content-Transfer-Encoding: x-my-new-encoding". However, unlike Content-Types and subtypes, the creation of new Content-Transfer-Encoding values is explicitly and strongly discouraged, as it seems likely to hinder interoperability with little potential benefit. Their use is allowed only as the result of an agreement between cooperating user agents.
If a Content-Transfer-Encoding header field appears as part of a message header, it applies to the entire body of that message. If a Content-Transfer-Encoding header field appears as part of a body part's headers, it applies only to the body of that body part. If an entity is of type "multipart" or "message", the Content-Transfer-Encoding is not permitted to have any value other than a bit width (e.g., "7bit", "8bit", etc.) or "binary".
It should be noted that email is character-oriented, so that the mechanisms described here are mechanisms for encoding arbitrary octet streams, not bit streams. If a bit stream is to be encoded via one of these mechanisms, it must first be converted to an 8-bit byte stream using the network standard bit order ("big-endian"), in which the earlier bits in a stream become the higher-order bits in a byte. A bit stream not ending at an 8-bit boundary must be padded with zeroes. This document provides a mechanism for noting the addition of such padding in the case of the application Content-Type, which has a "padding" parameter.
The encoding mechanisms defined here explicitly encode all data in ASCII. Thus, for example, suppose an entity has header fields such as:
Content-Type: text/plain; charset=ISO-8859-1 Content-transfer-encoding: base64
This must be interpreted to mean that the body is a base64 ASCII encoding of data that was originally in ISO-8859-1, and will be in that character set again after decoding.
The following sections will define the two standard encoding mechanisms. The definition of new content-transfer-encodings is explicitly discouraged and should only occur when absolutely necessary. All content-transfer-encoding namespace except that beginning with "X-" is explicitly reserved to the IANA for future use. Private agreements about content-transfer-encodings are also explicitly discouraged.
Certain Content-Transfer-Encoding values may only be used on certain Content-Types. In particular, it is expressly forbidden to use any encodings other than "7bit", "8bit", or "binary" with any Content-Type that recursively includes other Content-Type fields, notably the "multipart" and "message" Content-Types. All encodings that are desired for bodies of type multipart or message must be done at the innermost level, by encoding the actual body that needs to be encoded.
In this encoding, octets are to be represented as determined by the following rules:
Note that many implementations may elect to encode the local representation of various content types directly, as described in Appendix G. In particular, this may apply to plain text material on systems that use newline conventions other than CRLF delimiters. Such an implementation is permissible, but the generation of line breaks must be generalized to account for the case where alternate representations of newline sequences are used.
Now's the time for all folk to come to the aid of their country.
This can be represented, in the Quoted-Printable encoding, as
Now's the time = for all folk to come= to the aid of their country.
This provides a mechanism with which long lines are encoded in such a way as to be restored by the user agent. The 76 character limit does not count the trailing CRLF, but counts all other characters, including any equal signs.
Since the hyphen character ("-") is represented as itself in the Quoted-Printable encoding, care must be taken, when encapsulating a quoted-printable encoded body in a multipart entity, to ensure that the encapsulation boundary does not appear anywhere in the encoded body. (A good strategy is to choose a boundary that includes a character sequence such as "=_" which can never appear in a quoted-printable body. See the definition of multipart messages later in this document.)
according to rule #1. See Appendix B for more information.
Because quoted-printable data is generally assumed to be line-oriented, it is to be expected that the representation of the breaks between the lines of quoted printable data may be altered in transport, in the same manner that plain text mail has always been altered in Internet mail when passing between systems with differing newline conventions. If such alterations are likely to constitute a corruption of the data, it is probably more sensible to use the base64 encoding rather than the quoted-printable encoding.
For formalists, the syntax of quoted-printable data is described by the following grammar:
quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF) ; Maximum line length of 76 characters excluding CRLF ptext := octet /<any ASCII character except "=", SPACE, or TAB> ; characters not listed as "mail-safe" in Appendix B ; are also not recommended. octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") ; octet must be used for characters > 127, =, SPACE, or TAB, ; and is recommended for any characters not listed in ; Appendix B as "mail-safe".
A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.)
The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right, a 24-bit input group is formed by concatenating 3 8-bit input groups. These 24 bits are then treated as 4 concatenated 6-bit groups, each of which is translated into a single digit in the base64 alphabet. When encoding a bit stream via the base64 encoding, the bit stream must be presumed to be ordered with the most-significant-bit first. That is, the first bit in the stream will be the high-order bit in the first byte, and the eighth bit will be the low-order bit in the first byte, and so on.
Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. These characters, identified in Table 1, below, are selected so as to be universally representable, and the set excludes characters with particular significance to SMTP (e.g., ".", CR, LF) and to the encapsulation boundaries defined in this document (e.g., "-").
Value Encoding Value Encoding Value Encoding Value Encoding 0 A 17 R 34 i 51 z 1 B 18 S 35 j 52 0 2 C 19 T 36 k 53 1 3 D 20 U 37 l 54 2 4 E 21 V 38 m 55 3 5 F 22 W 39 n 56 4 6 G 23 X 40 o 57 5 7 H 24 Y 41 p 58 6 8 I 25 Z 42 q 59 7 9 J 26 a 43 r 60 8 10 K 27 b 44 s 61 9 11 L 28 c 45 t 62 + 12 M 29 d 46 u 63 / 13 N 30 e 47 v 14 O 31 f 48 w (pad) = 15 P 32 g 49 x 16 Q 33 h 50 y
The output stream (encoded bytes) must be represented in lines of no more than 76 characters each. All line breaks or other characters not found in Table 1 must be ignored by decoding software. In base64 data, characters other than those in Table 1, line breaks, and other white space probably indicate a transmission error, about which a warning message or even a message rejection might be appropriate under some circumstances.
Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. A full encoding quantum is always completed at the end of a body. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Padding at the end of the data is performed using the '=' character. Since all base64 input is an integral number of octets, only the following cases can arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output will be two characters followed by two "=" padding characters, or (3) the final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output will be three characters followed by one "=" padding character.
Because it is used only for padding at the end of the data, the occurrence of any '=' characters may be taken as evidence that the end of the data has been reached (without truncation in transit). No such assurance is possible, however, when the number of octets transmitted was a multiple of three.
Any characters outside of the base64 alphabet are to be ignored in base64-encoded data. The same applies to any illegal sequence of characters in the base64 encoding, such as "====="
Care must be taken to use the proper octets for line breaks if base64 encoding is applied directly to text material that has not been converted to canonical form. In particular, text line breaks must be converted into CRLF sequences prior to base64 encoding. The important thing to note is that this may be done directly by the encoder rather than in a prior canonicalization step in some implementations.
Previous: 4. The Content-Type Header Field
Next: 6. Additional Content-Header Fields