RFC 8262 PROPOSED STANDARD SIP · HEADERS · MIME
SIP Content-ID: Content-ID Header Field in the Session Initiation Protocol (SIP)
A SIP header field that names a whole message body, so other headers can point to it.
RFC EDITOR
Holmberg, C. and I. Sedlacek, "Content-ID Header Field in the Session Initiation Protocol (SIP)", RFC 8262, DOI 10.17487/RFC8262, October 2017, <https://www.rfc-editor.org/info/rfc8262>.
BIBTEX
@misc{rfc8262,
series = {Request for Comments},
number = 8262,
howpublished = {RFC 8262},
publisher = {RFC Editor},
doi = {10.17487/RFC8262},
url = {https://www.rfc-editor.org/info/rfc8262},
author = {C. Holmberg and I. Sedlacek},
title = {{Content-ID Header Field in the Session Initiation Protocol (SIP)}},
pagetotal = 14,
year = 2017,
month = oct,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Holmberg
Request for Comments: 8262 I. Sedlacek
Updates: 5368, 5621, 6442 Ericsson
Category: Standards Track October 2017
ISSN: 2070-1721
Content-ID Header Field in the Session Initiation Protocol (SIP)
Abstract
This document specifies the Content-ID header field for usage in the
Session Initiation Protocol (SIP). This document also updates RFC
5621, which only allows a Content-ID URL to reference a body part
that is part of a multipart message-body. This update enables a
Content-ID URL to reference a complete message-body and metadata
provided by some additional SIP header fields.
This document updates RFC 5368 and RFC 6442 by clarifying their usage
of the SIP Content-ID header field.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8262.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Identifying a Body Part . . . . . . . . . . . . . . . . . 3
1.2. Referencing a Body Part . . . . . . . . . . . . . . . . . 3
1.3. Problem Statement . . . . . . . . . . . . . . . . . . . . 4
1.4. Consequences . . . . . . . . . . . . . . . . . . . . . . 4
1.4.1. Example 1: SIP INVITE . . . . . . . . . . . . . . . . 4
1.4.2. Example 2: SIP REFER . . . . . . . . . . . . . . . . 6
1.5. Solution . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6. Backward Compatibility . . . . . . . . . . . . . . . . . 7
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Content-ID Header Field . . . . . . . . . . . . . . . . . . . 8
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Procedures . . . . . . . . . . . . . . . . . . . . . . . 9
3.4.1. User Agent (UA) Procedures . . . . . . . . . . . . . 9
3.4.2. Proxy Procedures . . . . . . . . . . . . . . . . . . 9
3.4.3. Example: Referencing the Message-Body of a SIP
Message . . . . . . . . . . . . . . . . . . . . . . . 9
4. Update to RFC 5368 . . . . . . . . . . . . . . . . . . . . . 10
5. Update to RFC 5621 . . . . . . . . . . . . . . . . . . . . . 11
6. Update to RFC 6442 . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8.1. Header Field . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
1.1. Identifying a Body Part
A SIP message consists of a start-line, one or more header fields, an
empty line indicating the end of the header fields, and an optional
message-body as specified in [RFC3261].
The message-body can be a non-multipart message-body or a multipart
message-body as specified in [RFC3261].
[RFC5621] defines generic handling of a multipart message-body in a
SIP message.
A multipart message-body contains zero, one, or several body parts
encoded using the format define in [RFC2045].
A body part in the multipart message-body is described using header
fields such as Content-Disposition, Content-Encoding, and Content-
Type, which provide information on the content of the body part as
specified in [RFC5621]. A body part in the multipart message-body
can also contain a Content-ID header field with an ID value uniquely
identifying the body part as specified in [RFC2045].
1.2. Referencing a Body Part
A SIP header field can reference a body part using a Content-ID URL
as specified in [RFC5621].
The Content-ID URL is specified in [RFC2392]. [RFC2392] specifies
how to identify the body part referenced by a Content-ID URL. The
Content-ID URL value is included in the Content-ID header field of
the body part.
Examples of SIP header fields referencing a body part using a
Content-ID URL are:
o [RFC6442] specifies how a Geolocation header field references a
body part using a Content-ID URL for providing location
information.
o [RFC5368] specifies how a Refer-To header field references a body
part using a Content-ID URL to provide a list of targets.
1.3. Problem Statement
How to uniquely identify a complete message-body of a SIP message
using a Content-ID header field and how to reference a complete
message-body using a Content-ID URL are not currently specified.
Note: In [RFC5621], the Content-ID URL references a specific body
part only.
Some existing specifications, such as [RFC5368], contain examples
that show usage of a SIP Content-ID header field referencing a
complete message-body, even though such usage has never been
specified. Many implementors have interpreted these examples to
indicate that such usage is allowed by the corresponding
specification, despite the absence of language allowing it. This
document updates the normative language in the affected documents to
explicitly allow such usage.
1.4. Consequences
The examples below show the consequences of the problem described
above.
1.4.1. Example 1: SIP INVITE
If a User Agent Client (UAC) sends an INVITE request that conveys
location by value (as specified in [RFC6442]) and decides not to
include a Session Description Protocol (SDP) offer, then the UAC
needs to include only one MIME entity in the INVITE request. This
MIME entity can be, for example, of the 'application/pidf+xml' MIME
type.
However, due to [RFC6442] requiring inclusion of a Geolocation header
field referencing the body part with the location information, the
UAC includes a multipart message-body with a single body part in the
INVITE request, and includes the location information of
'application/pidf+xml' MIME type and an associated Content-ID header
field in the body part.
Example message (SIP INVITE):
INVITE sips:bob@biloxi.example.com SIP/2.0
Via: SIPS/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK74bf9
Max-Forwards: 70
To: Bob <sips:bob@biloxi.example.com>
From: Alice <sips:alice@atlanta.example.com>;tag=9fxced76sl
Call-ID: 3848276298220188511@atlanta.example.com
Geolocation: <cid:target123@atlanta.example.com>
Geolocation-Routing: no
Accept: application/sdp, application/pidf+xml
CSeq: 31862 INVITE
Contact: <sips:alice@atlanta.example.com>
Content-Type: multipart/mixed; boundary=boundary1
Content-Length: ...
--boundary1
Content-Type: application/pidf+xml
Content-ID: <target123@atlanta.example.com>
<?xml version="1.0" encoding="UTF-8"?>
<presence
xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:gp="urn:ietf:params:xml:ns:pidf:geopriv10"
xmlns:gbp="urn:ietf:params:xml:ns:pidf:geopriv10:basicPolicy"
xmlns:cl="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr"
xmlns:gml="http://www.opengis.net/gml"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
entity="pres:alice@atlanta.example.com"
>
<dm:device id="target123-1">
<gp:geopriv>
<gp:location-info>
<gml:location>
<gml:Point srsName="urn:ogc:def:crs:EPSG::4326">
<gml:pos>32.86726 -97.16054</gml:pos>
</gml:Point>
</gml:location>
</gp:location-info>
<gp:usage-rules>
<gbp:retransmission-allowed>no
</gbp:retransmission-allowed>
<gbp:retention-expiry>2010-11-14T20:00:00Z
</gbp:retention-expiry>
</gp:usage-rules>
<gp:method>802.11</gp:method>
</gp:geopriv>
<dm:deviceID>mac:1234567890ab</dm:deviceID>
<dm:timestamp>2010-11-04T20:57:29Z</dm:timestamp>
</dm:device>
</presence>
--boundary1--
1.4.2. Example 2: SIP REFER
If a UAC sends a REFER request including a list of targets as
specified in [RFC5368], then the UAC needs to include only one MIME
entity in the REFER request. This MIME entity is of the
'application/resource-lists+xml' MIME type.
However, due to [RFC5368] requiring inclusion of a Refer-To header
field referencing the body part containing the list of targets, the
UAC includes a multipart message-body with a single body part in the
REFER request and includes the list of targets of 'application/
resource-lists+xml' MIME type and an associated Content-ID header
field in the body part.
Example message (SIP REFER):
REFER sip:conf-123@example.com;gruu;opaque=hha9s8d-999a SIP/2.0
Via: SIP/2.0/TCP client.chicago.example.com;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70
To: "Conference 123" <sip:conf-123@example.com>
From: Carol <sip:carol@chicago.example.com>;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 2 REFER
Contact: <sip:carol@client.chicago.example.com>
Refer-To: <cid:cn35t8jf02@example.com>
Refer-Sub: false
Require: multiple-refer, norefersub
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Content-Type: multipart/mixed; boundary=boundary1
Content-Length: ...
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
Content-ID: <cn35t8jf02@example.com>
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists
xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
>
<list>
<entry uri="sip:bill@example.com?method=BYE"/>
<entry uri="sip:joe@example.org?method=BYE"/>
<entry uri="sip:ted@example.net?method=BYE"/>
</list>
</resource-lists>
--boundary1--
1.5. Solution
In order to solve the problems described above, this document:
o Specifies and registers the Content-ID header field as a SIP
header field.
o Specifies that, when used as a SIP header field, the Content-ID
header field identifies the complete message-body and the metadata
provided by some additional SIP header fields of the SIP message.
o Updates [RFC5621] to enable a Content-ID URL to reference a
complete message-body and the metadata provided by some additional
SIP header fields.
o Updates [RFC5368] and [RFC6442] by adding text that explicitly
states that a SIP Content-ID header field can be used.
1.6. Backward Compatibility
If an existing specification only defines the usage of a multipart
message-body to carry a single body part to be referenced by a
Content-ID URL, implementations MUST NOT carry the MIME entity in a
non-multipart message-body unless the specification is updated to
explicitly allow it.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Content-ID Header Field
3.1. Introduction
This section defines the usage of the Content-ID header field for
SIP.
3.2. Syntax
The ABNF [RFC5234] for the Content-ID header field is:
Content-ID = "Content-ID" HCOLON msg-id
msg-id = "<" id-left "@" id-right ">"
Note: id-left and id-right are specified in [RFC5322]. HCOLON is
defined in [RFC3261].
Note: When used in a SIP header field, the msg-id syntax has been
simplified, compared to the syntax in [RFC5322], to disallow the use
of comments and to adopt to the SIP usage of leading white space.
The value of the Content-ID header field value must be unique in the
context of a given SIP message, including any embedded MIME
Content-ID header field values. Note that the SIP Content-ID header
field value is not expected to be unique among all SIP messages; it
has no meaning outside of the message in which it is included.
3.3. Semantics
The Content-ID header field included in the header fields of a SIP
message identifies the message-body of the SIP message and the
metadata provided by:
o A MIME-Version header field, if included in the header fields of
the SIP message.
o Any 'Content-' prefixed header fields (including the Content-ID
header field itself) included in the header fields of the SIP
message.
The Content-ID header field can be included in any SIP message that
is allowed to contain a message-body.
Note: The message-body identified by the Content-ID header field can
be a non-multipart message-body or a multipart message-body.
3.4. Procedures
3.4.1. User Agent (UA) Procedures
A UA MAY include a Content-ID header field in any SIP message that is
allowed to contain a message-body.
A UA MUST NOT include a Content-ID header field in any SIP message
that is not allowed to contain a message-body.
A UA MUST set the value of the Content-ID header field to a value
that is unique in the context of the SIP message.
3.4.2. Proxy Procedures
A proxy MUST NOT add a Content-ID header field in a SIP message.
A proxy MUST NOT modify a Content-ID header field included in a SIP
message.
A proxy MUST NOT delete a Content-ID header field from a SIP message.
3.4.3. Example: Referencing the Message-Body of a SIP Message
The figure shows an example from [RFC5368], where the SIP Content-ID
header field is used to reference the message-body (non-multipart) of
a SIP message.
REFER sip:conf-123@example.com;gruu;opaque=hha9s8d-999a SIP/2.0
Via: SIP/2.0/TCP client.chicago.example.com
;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70
To: "Conference 123" <sip:conf-123@example.com>
From: Carol <sip:carol@chicago.example.com>;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 2 REFER
Contact: <sip:carol@client.chicago.example.com>
Refer-To: <cid:cn35t8jf02@example.com>
Refer-Sub: false
Require: multiple-refer, norefersub
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
Content-Length: 362
Content-ID: <cn35t8jf02@example.com>
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<list>
<entry uri="sip:bill@example.com?method=BYE" />
<entry uri="sip:joe@example.org?method=BYE" />
<entry uri="sip:ted@example.net?method=BYE" />
</list>
</resource-lists>
4. Update to RFC 5368
This section updates the second paragraph in Section 7 of [RFC5368]
by allowing usage of either a MIME Content-ID header field or a SIP
Content-ID header field to label the body part or the message-body
carrying the URI list.
OLD TEXT:
The Refer-To header field of a REFER request with multiple REFER-
Targets MUST contain a pointer (i.e., a Content-ID Uniform
Resource Locator (URL) as per RFC 2392 [RFC2392]) that points to
the body part that carries the URI list. The REFER-Issuer SHOULD
NOT include any particular URI more than once in the URI list.
NEW TEXT:
The Refer-To header field of a REFER request with multiple REFER-
Targets MUST contain a pointer (i.e., a Content-ID Uniform
Resource Locator (URL) as per RFC 2392 [RFC2392]) that points to
the body part or message-body that carries the URI list. The
REFER-Issuer SHOULD NOT include any particular URI more than once
in the URI list. The REFER request can use either a MIME Content-
ID header field [RFC4483] or a SIP Content-ID header field
[RFC8262] to label the body part or the message-body.
5. Update to RFC 5621
This section updates Section 9.1 of [RFC5621] by allowing a Content-
ID URL to reference a message-body and the related metadata
(Section 3.3) in addition to allowing a reference to a body part.
OLD TEXT:
Content-ID URLs allow creating references to body parts. A given
Content-ID URL [RFC2392], which can appear in a header field or
within a body part (e.g., in an SDP attribute), points to a
particular body part.
NEW TEXT:
Content-ID URLs allow the creation of references to body parts or
message-bodies (and the header fields describing the message-
bodies). A given Content-ID URL [RFC2392], which can appear in a
header field or within a body part (e.g., in an SDP attribute),
points to a particular body part or the message-body (and the
header fields describing the message-body).
6. Update to RFC 6442
This section updates the second paragraph in Section 3.1 of [RFC6442]
by allowing usage of either a MIME Content-ID header field or a SIP
Content-ID header field to label the body part or the message-body
carrying the location data.
OLD TEXT:
In Figure 1, Alice is both the Target and the LS that is conveying
her location directly to Bob, who acts as an LR. This conveyance
is point-to-point: it does not pass through any SIP-layer
intermediary. A Location Object appears by-value in the initial
SIP request as a MIME body, and Bob responds to that SIP request
as appropriate. There is a 'Bad Location Information' response
code introduced within this document to specifically inform Alice
if she conveys bad location information to Bob (e.g., Bob "cannot
parse the location provided", or "there is not enough location
information to determine where Alice is").
NEW TEXT:
In Figure 1, Alice is both the Target and the LS that is conveying
her location directly to Bob, who acts as an LR. This conveyance
is point-to-point: it does not pass through any SIP-layer
intermediary. A Location Object appears by-value in the initial
SIP request as a MIME body, and Bob responds to that SIP request
as appropriate. Either a MIME Content-ID header field [RFC4483]
or the SIP Content-ID header field [RFC8262] MUST be used to label
the location information. There is a 'Bad Location Information'
response code introduced within this document to specifically
inform Alice if she conveys bad location information to Bob (e.g.,
Bob "cannot parse the location provided", or "there is not enough
location information to determine where Alice is").
7. Security Considerations
The Content-ID header field value MUST NOT reveal sensitive user
information.
If the message-body associated with the Content-ID header field is an
encrypted body, it MUST NOT be possible to derive a key that can be
used to decrypt the body from the Content-ID header field value.
8. IANA Considerations
This specification registers a new SIP header field according to the
procedures defined in [RFC3261].
8.1. Header Field
The header field described in Section 3 has been registered in the
"Header Fields" sub-registry of the "Session Initiation Protocol
(SIP) Parameters" registry by adding a row with these values:
Header Name: Content-ID
compact:
Reference: RFC 8262
9. References
9.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
<https://www.rfc-editor.org/info/rfc2392>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC4483] Burger, E., Ed., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
DOI 10.17487/RFC4483, May 2006,
<https://www.rfc-editor.org/info/rfc4483>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5621] Camarillo, G., "Message Body Handling in the Session
Initiation Protocol (SIP)", RFC 5621,
DOI 10.17487/RFC5621, September 2009,
<https://www.rfc-editor.org/info/rfc5621>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC5368] Camarillo, G., Niemi, A., Isomaki, M., Garcia-Martin, M.,
and H. Khartabil, "Referring to Multiple Resources in the
Session Initiation Protocol (SIP)", RFC 5368,
DOI 10.17487/RFC5368, October 2008,
<https://www.rfc-editor.org/info/rfc5368>.
[RFC6442] Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
for the Session Initiation Protocol", RFC 6442,
DOI 10.17487/RFC6442, December 2011,
<https://www.rfc-editor.org/info/rfc6442>.
Authors' Addresses
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
Ivo Sedlacek
Ericsson
Sokolovska 79
Praha 18600
Czech Republic
Email: ivo.sedlacek@ericsson.com
AI ELI5
On this page
PROBLEMA Content-ID URL could only point to one body part inside a multipart SIP body, so a sender with a single piece of content still had to wrap it in a multipart body just to give it a reference.
MECHANISMRFC 8262 defines Content-ID as a SIP header field that identifies the complete message-body and its metadata, and updates RFC 5621, RFC 5368 and RFC 6442 to allow referencing a whole message-body.
WHY IT MATTERSA SIP message can carry one non-multipart body and still be referenced by a Content-ID URL. The RFC is a Proposed Standard and is not obsoleted.
§1Body Parts and Content-ID URLs
A SIP message has a start-line, header fields, an empty line and an optional message-body (RFC 3261). The body can be multipart or not. A multipart body contains body parts in the MIME format, and each part may carry a Content-ID header field with a unique ID.
A SIP header field can point at a body part with a Content-ID URL (RFC 2392). For example, RFC 6442 has a Geolocation header field point at a part holding location information, and RFC 5368 has a Refer-To header field point at a part holding a list of targets.
A body part is like a sticker-labelled envelope inside a bigger package, and a Content-ID URL says which labelled envelope you mean.
§2The Problem
How to identify a complete message-body with a Content-ID header field, and how to reference it with a Content-ID URL, was never specified. In RFC 5621 the URL references a specific body part only.
The consequence is wasteful. A UAC that sends an INVITE with location by value and no SDP offer has only one MIME entity to send, but because the Geolocation header must reference a body part, it wraps that entity in a multipart body with a single part. The same happens with a REFER that carries a list of targets.
Some existing specifications, such as RFC 5368, contain examples that show a SIP Content-ID header field referencing a complete message-body even though that was never specified. Many implementors read the examples as allowing it. This document makes the normative language say so explicitly.
It is like having to put one letter inside a box just so it can have a label. This RFC lets you label the letter itself.
§3The Content-ID Header Field
The syntax is Content-ID = "Content-ID" HCOLON msg-id, where msg-id = "<" id-left "@" id-right ">". The syntax is simplified compared to RFC 5322 to disallow comments. The value must be unique within a given SIP message, including any embedded MIME Content-ID values. It need not be unique across messages and has no meaning outside the message that carries it.
Used as a SIP header field, Content-ID identifies the message-body together with the metadata provided by a MIME-Version header field (if present) and by any header fields with a Content- prefix, including Content-ID itself. The body it identifies can be multipart or non-multipart. It can appear in any SIP message that is allowed to contain a message-body.
The tag has to be different from every other tag in the same message, but it only matters inside that one message.
§4User Agent and Proxy Procedures
A UA MAY include a Content-ID header field in any SIP message that is allowed to contain a message-body, and MUST NOT include one in a message that is not. The UA MUST set the value to something unique within that message.
A proxy MUST NOT add, modify or delete a Content-ID header field.
For backward compatibility, if an existing specification only defines carrying a single body part in a multipart body for a Content-ID URL to reference, implementations MUST NOT put the MIME entity in a non-multipart body unless that specification is updated to explicitly allow it.
The sender writes the name tag. The people who pass the message along must leave it exactly as it is.
Content-ID naming a non-multipart body
Refer-To: <cid:cn35t8jf02@example.com>
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
Content-Length: 362
Content-ID: <cn35t8jf02@example.com>§5Updates to Other RFCs
Three RFCs are updated with OLD and NEW text.
- RFC 5621, Section 9.1: a Content-ID URL may now point to a body part or to the message-body and the header fields describing it.
- RFC 5368, Section 7: the Refer-To pointer may target the body part or the message-body carrying the URI list, and either a MIME Content-ID header field (RFC 4483) or a SIP Content-ID header field can label it.
- RFC 6442, Section 3.1: either a MIME Content-ID or the SIP Content-ID MUST be used to label the location information.
Older rulebooks said only envelopes could have labels. This RFC adds a note to each rulebook saying the whole package can have one too.
§6Security and IANA
The Content-ID value MUST NOT reveal sensitive user information. If the associated message-body is encrypted, it MUST NOT be possible to derive a decryption key from the Content-ID value.
IANA registered Content-ID in the Header Fields sub-registry of the Session Initiation Protocol (SIP) Parameters registry, with the reference RFC 8262 and no compact form.
The name tag must not give away secrets, and it must never hold a hint for opening a locked box.
§7References
[RFC2392]RFC 2392Levinson, E., "Content-ID and Message-ID Uniform Resource Locators", RFC 2392, August 1998.August 1998[RFC3261]RFC 3261Rosenberg, J., et al., "SIP: Session Initiation Protocol", RFC 3261, June 2002.July 2002[RFC5621]RFC 5621Camarillo, G., "Message Body Handling in the Session Initiation Protocol (SIP)", RFC 5621, September 2009.September 2009[RFC5368]RFC 5368Camarillo, G., et al., "Referring to Multiple Resources in the Session Initiation Protocol (SIP)", RFC 5368, October 2008.October 2008[RFC6442]RFC 6442Polk, J., Rosen, B., and J. Peterson, "Location Conveyance for the Session Initiation Protocol", RFC 6442, December 2011.December 2011[RFC4483]RFC 4483Burger, E., Ed., "A Mechanism for Content Indirection in Session Initiation Protocol (SIP) Messages", RFC 4483, May 2006.May 2006