What is the boundary parameter in an HTTP multi-part (POST) Request?

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To quote from the RFC 1341, section 7.2.1, what I consider to be the relevant bits on the boundary parameter of the Content-Type header (for MIME):

All subtypes of “multipart” share a common syntax …

The Content-Type field for multipart entities requires one parameter, “boundary”, which is used to specify the encapsulation boundary. The encapsulation boundary is defined as a line consisting entirely of two hyphen characters (“-“, decimal code 45) followed by the boundary parameter value from the Content-Type header field.

and then clarifies:

Thus, a typical multipart Content-Type header field might look like this:

 Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08jU534c0p

This indicates that the entity consists of several parts, each itself with a structure that is syntactically identical to an RFC 822 message, except that the header area might be completely empty, and that the parts are each preceded by the line
–gc0p4Jq0M2Yt08jU534c0p

Things to Note:

  1. The encapsulation boundary must occur at the beginning of a line, i.e., following a CRLF (Carriage Return-Line Feed)
  2. The boundary must be followed immediately either by another CRLF and the header fields for the next part, or by two CRLFs, in which case there are no header fields for the next part (and it is therefore assumed to be of Content-Type text/plain).
  3. Encapsulation boundaries must not appear within the encapsulations, and must be no longer than 70 characters, not counting the two leading hyphens.

Last but not least:

The encapsulation boundary following the last body part is a distinguished delimiter that indicates that no further body parts will follow. Such a delimiter is identical to the previous delimiters, with the addition of two more hyphens at the end of the line:

 --gc0p4Jq0M2Yt08jU534c0p--

I hope this helps someone else in the future, as I had to roam for a while before getting the full picture (please ensure to read the necessary RFCs to get the deepest understanding).

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