Content-ID and Message-ID Uniform Resource Locators (RFC2111)
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow references to messages and the body parts of messages. For example, within a single multipart message, one HTML body part might include embedded references to other parts of the same message. [STANDARDS-TRACK]
Network Working Group E. Levinson Request for Comments: 2111 XIson, Inc. Category: Standards Track March 1997
Content-ID and Message-ID Uniform Resource Locators
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.
The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow references to messages and the body parts of messages. For example, within a single multipart message, one HTML body part might include embedded references to other parts of the same message.
The use of [MIME] within email to convey Web pages and their associated images requires a URL scheme to permit the HTML to refer to the images or other data included in the message. The Content-ID Uniform Resource Locator, "cid:", serves that purpose.
Similarly Net News readers use Message-IDs to link related messages together. The Message-ID URL provides a scheme, "mid:", to refer to such messages as a "resource".
The "mid" (Message-ID) and "cid" (Content-ID) URL schemes provide identifiers for messages and their body parts. The "mid" scheme uses (a part of) the message-id of an email message to refer to a specific message. The "cid" scheme refers to a specific body part of a message; its use is generally limited to references to other body parts in the same message as the referring body part. The "mid" scheme may also refer to a specific body part within a designated message, by including the content-ID’s address.
A note on terminology. The terms "body part" and "MIME entity" are used interchangeably. They refer to the headers and body of a MIME message, either the message itself or one of the body parts contained in a Multipart message.
Levinson Standards Track [Page 1]
RFC 2111 CID and MID URLs March 1997
2. The MID and CID URL Schemes
RFC1738 [URL] reserves the "mid" and "cid" schemes for Message-ID and Content-ID respectively. This memorandum defines the syntax for those URLs. Because they use the same syntactic elements they are presented together.
The URLs take the form
content-id = url-addr-spec
message-id = url-addr-spec
url-addr-spec = addr-spec ; URL encoding of RFC 822 addr-spec
cid-url = "cid" ":" content-id
mid-url = "mid" ":" message-id [ "/" content-id ]
Note: in Internet mail messages, the addr-spec in a Content-ID [MIME] or Message-ID  header are enclosed in angle brackets (<>). Since addr-spec in a Message-ID or Content-ID might contain characters not allowed within a URL; any such character (including "/", which is reserved within the "mid" scheme) must be hex- encoded using the %hh escape mechanism in [URL].
A "mid" URL with only a "message-id" refers to an entire message. With the appended "content-id", it refers to a body part...