Critical Content Multi-purpose Internet Mail Extensions (MIME) Parameter

Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract This document describes the use of a mechanism for identifying body parts that a sender deems critical in a multi-part Internet mail message. The mechanism described is a parameter to Content-Disposition, as described by RFC 3204. By knowing what parts of a message the sender deems critical, a content gateway can intelligently handle multi-part messages when providing gateway services to systems of lesser capability. Critical content can help a content gateway to decide what parts to forward. It can indicate how hard a gateway should try to deliver a body part. It can help the gateway to pick body parts that are safe to silently delete when a system of lesser capability receives a message. In addition, critical content can help the gateway chose the notification strategy for the receiving system. Likewise, if the sender expects the destination to do some processing on a body part, critical content allows the sender to mark body parts that the receiver must process. This document refers generically to the sender of a message in the masculine (he/him/his) and the recipient of the message in the feminine (she/her/hers). This convention is purely for convenience and makes no assumption about the gender of a message sender or recipient. The key words "MUST", "MUST NOT", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [2]. The word "REQUIRED" in this document does not follow the definition found in RFC 2119. This is because this document defines a parameter named "REQUIRED". There is no requirement in this document that is "REQUIRED", so there is no confusion. In this document, the "sending agent" is the originator of the message. It could be a mail user agent (MUA) for an Internet message, or a SIP User Agent Client (UAC) for a SIP [3] message. The "endpoint" is the receiving device, of lesser capability than the sending agent. NOTE: Notes, such as this one, provide additional nonessential information that the reader may skip without missing anything essential. The primary purpose of these …