Multipart

Why use multipart?

• Multipart solves the problem of efficient file upload.
• Before Multipart, the standard for uploading files was "application/x-www-form-urlencoded", which required clients to URL-encode the file before uploading it.
• URL-encoding is efficient if the file is mostly ASCII text, but if it's mostly binary data, then you have to URL-escape almost every byte, which is very inefficient. （即纯文本的上传，用urlencoded效率也是挺高的
• If you wanted to upload multiple files without encoding them, you could send multiple HTTP requests.
  • But that has more latency than sending them all in one request.
• So in 1998, RFC 2388 proposed a new standard, "multipart/form-data", which lets you send many files in one HTTP body without encoding them.
  • No encoding means you save a lot of CPU cycles and keeps the total body size small.
  • You can use it to upload files from any HTTP client to any HTTP server. (not only in <form>)
  • The server can stream each part separately

What is multipart?

• message/几乎不用
• multipart/的实例：multipart/form-data

How is multipart implemented?

• The HTTP body contains multiple parts
• Each part is separated by a "boundary delimiter"
  • The root HTTP message has a header which defines the boundary delimiter
• 其它header:
  • Content-Disposition : defines each part's filename or the name of the form field that contained it.
  • Content-Type: defines each part's MIME type.
    • It defaults to text/plain.
    • Unstructured binary data should use application/octet-stream
    • but if you know the type you should use it, e.g. application/zip, application/pdf, etc.
  • No other headers can be used!

RFC 7578: "The multipart/form-data media type does not support any MIME header fields in parts other than Content-Type, Content-Disposition, and (in limited circumstances) Content-Transfer-Encoding. Other header fields MUST NOT be included and MUST be ignored.".

a HTTP multipart body with 3 FIFs:

POST /cgi-bin/qtest HTTP/1.1
Content-Type: multipart/form-data; boundary=2a8ae6ad-f4ad-4d9a-a92c-6d217011fe0f
Content-Length: 514

--2a8ae6ad-f4ad-4d9a-a92c-6d217011fe0f
Content-Disposition: form-data; name="datafile1"; filename="r.gif"
Content-Type: image/gif

GIF87a.............,...........D..;
--2a8ae6ad-f4ad-4d9a-a92c-6d217011fe0f
Content-Disposition: form-data; name="datafile2"; filename="g.gif"
Content-Type: image/gif

GIF87a.............,...........D..;
--2a8ae6ad-f4ad-4d9a-a92c-6d217011fe0f
Content-Disposition: form-data; name="datafile3"; filename="b.gif"
Content-Type: image/gif

GIF87a.............,...........D..;
--2a8ae6ad-f4ad-4d9a-a92c-6d217011fe0f--

Compression

• You can gzip the entire Multipart response(不能部分gzip)