multipart

Multipart is a term used to describe a message or data carrier that combines several discrete parts into a single transport unit. In networking and email, multipart messages rely on the MIME (Multipurpose Internet Mail Extensions) standard to allow attachments, alternative representations, and related data to be bundled together within one envelope.

A multipart message uses a boundary parameter in the Content-Type header to separate its parts. Each part begins with headers such as Content-Type, Content-Disposition, or Content-Transfer-Encoding, followed by a blank line and then the part’s data. The boundary line is typically formatted as --boundary, and the final boundary ends the content with --boundary--. The boundary must be chosen so it does not appear in the actual data to prevent misparsing.

Common multipart types include multipart/mixed for combining different content types (such as text and attachments), multipart/related for a set of interdependent parts (such as an HTML document with embedded images), and multipart/alternative for presenting alternate representations of the same content (like plain text and HTML). A widely used subtype is multipart/form-data, which encodes form fields and file uploads in HTTP requests.

Applications span email and web technologies. In email, multipart enables attachments and compound messages; in web contexts, multipart/form-data supports file uploads and complex form submissions. The multipart family is defined in MIME (RFC 2045 through RFC 2049) with HTTP-related extensions described in related RFCs, such as RFCs addressing form data.

Processing multipart data requires careful parsing to honor boundaries, correctly interpret each part’s headers, and manage resource usage for large payloads, including security considerations to prevent boundary collisions and data leakage.