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TCP/IP Tutorial and Technical Overview phần 7

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những người bắt buộc, và một số có cả hai. Tham số kiểu phụ có thể không được bỏ qua, nhưng toàn bộ lĩnh vực có thể, trong trường hợp giá trị mặc định là văn bản / đồng bằng. Có bảy nội dung tiêu chuẩn-loại: văn bản kiểu phụ duy nhất, đồng bằng, | mandatory ones and some have both. The subtype parameter cannot be omitted but the whole field can in which case the default value is text plain. There are seven standard content-types Text A single subtype plain is defined for the text type specifying unformatted text. A parameter can optionally be included with this type subtype pair in order to specify the character set of the text. The following values are permitted for this parameter - us-ascii The text consists of ASCII characters in the range 0 to 127 decimal . This is the default for compatibility with RFC 2822 . - iso-8859-x Where x is in the range 1 to 9 for the different parts of the ISO-8859 standard. The text consists of ISO characters in the range 0 to 255 decimal . All of the ISO-8859 character sets are ASCII-based with national language characters and other special characters in the range 128 to 255. Note that if the text contains no characters with values above 127 the character set is specified as us-ascii because it can be adequately represented in that character set. - Format The data consists of fixed or flowed text. When specified an additional parameter of fixed or flowed can be specified to indicate the exact nature of the text. If no parameter is included fixed is assumed. - DelSp Indicates if trailing whitepace in fixed or flowed text should be preserved or deleted. Its values are yes or no and if nothing is specified no is assumed. Noted that us-ascii and iso-8859-x were initially defined for MIME in RFC 2046 while Format and DelSp were added by RFC 3676. Further subtypes can be added to describe other readable text formats such as word processor formats that contain formatting information for an application to enhance the appearance of the text. Multipart The message body can contain multiple objects of independent data types. In each case the body is divided into parts by lines called encapsulation boundaries. The contents of the boundary are defined with a parameter in the content-type