Opt in at the field definition
01An HTTP field definition can adopt this encoding for a specific text parameter. An asterisk on an unfamiliar parameter name does not automatically give it these semantics.
This profile supports character encoding and language information. It does not include continuations, encoded-words or encoding of multipart message bodies.
Source for this drawing: RFC 8187 · Read RFC section 4 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
- HTTP field
- A named part of an HTTP message, often called a header field.
- Parameter
- A name=value item inside a field value, interpreted under that field’s definition.
- ext-value
- The RFC 8187 grammar for the extended parameter value.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Read the field definition before interpreting an extended parameter.
- A field specification ought to explicitly reference this encoding and use ext-value for the relevant parameter. Ending the name in * is a convention, not automatic permission to reinterpret every such name.
- RFC 8187 §4 uses a fictional foo field with ordinary title and extended title*. This guide uses that example rather than inventing a deployed header or a universal parameter grammar.
- The extension ought to be used for human-readable text when its language is known, and when a value needs non-ASCII characters. It does not apply to HTTP message payloads such as multipart/form-data.
- MIME parameter continuations such as title*0* and title*1* are not part of this encoding. RFC 2047 encoded-words and their language extension are not included either.
The three states are separate scope examples. RFC 8187 is a simplified HTTP profile of MIME-related encoding, not permission to copy every MIME feature into HTTP. It does not redefine one generic parameter grammar; whitespace and duplicates belong to the adopting field definition.
Read the RFC section ↗