Content-ID and Message-ID Uniform Resource Locators (RFC2111)
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2000-Sep-13
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.
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.
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
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" ":"...