Choose the sender’s charset label
04The MIME charset label tells a receiver how the octets are serialized. RFC2781 distinguishes known byte order, unknown byte order and formats that mandate a signature.
Separate format rule: if the format mandates a BOM, RFC2781 identifies an exception to the known-order label SHOULD. Use UTF-16 for that case; the format rule is not a universal UTF-16 requirement.
Source for this drawing: RFC 2781 · Read RFC source: § 3.3 ↗
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.
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Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
When explicitly labelling text of known order, applications SHOULD use UTF-16BE or UTF-16LE as appropriate. Matching serialization is mandatory.
- When explicitly labelling text of known order, applications SHOULD use UTF-16BE or UTF-16LE as appropriate. Matching serialization is mandatory.
- Systems labelling UTF-16BE or UTF-16LE MUST NOT prepend a byte-order mark (BOM) to the text. A genuine text character is a different matter.
- An explicitly labelling application that does not know the order MUST use UTF-16 and SHOULD ensure an initial FEFF signature. Formats mandating a BOM are an exception to the known-order label recommendation.
MIME means Multipurpose Internet Mail Extensions; charset labels identify an encoding/serialization. These are the RFC’s requirements/recommendations, not a production label detector or permission for every media type to carry UTF-16.
Read the RFC section ↗