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Keywords: [--------], Message body, MIME, SIP
Network Working Group G. Camarillo
Request for Comments: 5621 Ericsson
Updates: 3204, 3261, 3459 September 2009
Category: Standards Track
Message Body Handling in the Session Initiation Protocol (SIP)
Abstract
This document specifies how message bodies are handled in SIP.
Additionally, this document specifies SIP user agent support for MIME
(Multipurpose Internet Mail Extensions) in message bodies.
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.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Camarillo Standards Track [Page 1]
RFC 5621 Message Body Handling in SIP September 2009
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Message Body Encoding . . . . . . . . . . . . . . . . . . . . 3
3.1. Background on Message Body Encoding . . . . . . . . . . . 3
3.2. UA Behavior to Encode Binary Message Bodies . . . . . . . 5
4. 'multipart' Message Bodies . . . . . . . . . . . . . . . . . . 6
4.1. Background on 'multipart' Message Bodies . . . . . . . . . 6
4.2. Mandatory Support for 'multipart' Message Bodies . . . . . 7
4.3. UA Behavior to Generate 'multipart' Message Bodies . . . . 7
5. 'multipart/mixed' Message Bodies . . . . . . . . . . . . . . . 7
6. 'multipart/alternative' Message Bodies . . . . . . . . . . . . 8
6.1. Background on 'multipart/alternative' Message Bodies . . . 8
6.2. UA Behavior to Generate 'multipart/alternative'
Message Bodies . . . . . . . . . . . . . . . . . . . . . . 8
6.3. UA Behavior to Process 'multipart/alternative' Message
Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. 'multipart/related' Message Bodies . . . . . . . . . . . . . . 9
7.1. Background on 'multipart/related' Message Bodies . . . . . 9
7.2. UA Behavior to Generate 'multipart/related' Message
Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.3. UA Behavior to Process 'multipart/related' Message
Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Disposition Types . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Background on Content and Disposition Types in SIP . . . . 10
8.2. UA Behavior to Set the 'handling' Parameter . . . . . . . 12
8.3. UA Behavior to Process 'multipart/alternative' . . . . . . 13
8.4. UAS Behavior to Report Unsupported Message Bodies . . . . 13
9. Message Body Processing . . . . . . . . . . . . . . . . . . . 14
9.1. Background on References to Message Body Parts . . . . . . 14
9.2. UA Behavior to Generate References to Message Bodies . . . 14
9.3. UA Behavior to Process Message Bodies . . . . . . . . . . 14
9.4. The 'by-reference' Disposition Type . . . . . . . . . . . 15
10. Guidelines to Authors of SIP Extensions . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12.1. Registration of the 'by-reference' Disposition Type . . . 17
12.2. Update of the 'handling' Parameter Registration . . . . . 17
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.1. Normative References . . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . . 18
Camarillo Standards Track [Page 2]
RFC 5621 Message Body Handling in SIP September 2009
1. Introduction
Message body handling in SIP was originally specified in [RFC3261],
which relied on earlier specifications (e.g., MIME) to describe some
areas. This document contains background material on how bodies are
handled in SIP and normative material on areas that had not been
specified before or whose specifications needed to be completed.
Sections containing background material are clearly identified as
such by their titles. The material on the normative sections is
based on experience gained since [RFC3261] was written. Implementers
need to implement what is specified in [RFC3261] (and its references)
in addition to what is specified in this document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The following abbreviations are used in this document.
UA: User Agent
UAC: User Agent Client
UAS: User Agent Server
URL: Uniform Resource Locator
3. Message Body Encoding
This section deals with the encoding of message bodies in SIP.
3.1. Background on Message Body Encoding
SIP [RFC3261] messages consist of an initial line (request line in
requests and status line in responses), a set of header fields, and
an optional message body. The message body is described using header
fields such as Content-Disposition, Content-Encoding, and Content-
Type, which provide information on its contents. Figure 1 shows a
SIP message that carries a body. Some of the header fields are not
shown for simplicity:
Camarillo Standards Track [Page 3]
RFC 5621 Message Body Handling in SIP September 2009
INVITE sip:conf-fact@example.com SIP/2.0
Content-Type: application/sdp
Content-Length: 192
v=0
o=alice 2890844526 2890842807 IN IP4 atlanta.example.com
s=-
c=IN IP4 192.0.2.1
t=0 0
m=audio 20000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 20002 RTP/AVP 31
a=rtpmap:31 H261/90000
Figure 1: SIP message carrying a body
The message body of a SIP message can be divided into various body
parts. Multipart message bodies are encoded using the MIME
(Multipurpose Internet Mail Extensions) [RFC2045] format. Body parts
are also described using header fields such as Content-Disposition,
Content-Encoding, and Content-Type, which provide information on the
contents of a particular body part. Figure 2 shows a SIP message
that carries two body parts. Some of the header fields are not shown
for simplicity:
Camarillo Standards Track [Page 4]
RFC 5621 Message Body Handling in SIP September 2009
INVITE sip:conf-fact@example.com SIP/2.0
Content-Type: multipart/mixed;boundary="boundary1"
Content-Length: 619
--boundary1
Content-Type: application/sdp
v=0
o=alice 2890844526 2890842807 IN IP4 atlanta.example.com
s=-
c=IN IP4 192.0.2.1
t=0 0
m=audio 20000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 20002 RTP/AVP 31
a=rtpmap:31 H261/90000
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="sip:bill@example.com"/>
<entry uri="sip:randy@example.net"/>
<entry uri="sip:joe@example.org"/>
</list>
</resource-lists>
--boundary1--
Figure 2: SIP message carrying a body
SIP uses S/MIME [RFC3851] to protect message bodies. As specified in
[RFC3261], UASs that cannot decrypt a message body or a body part can
use the 493 (Undecipherable) response to report the error.
3.2. UA Behavior to Encode Binary Message Bodies
SIP messages can carry binary message bodies such as legacy
signalling objects [RFC3204]. SIP proxy servers are 8-bit safe.
That is, they are able to handle binary bodies. Therefore, there is
no need to use encodings such as base64 to transport binary bodies in
SIP messages. Consequently, UAs SHOULD use the binary transfer
encoding [RFC4289] for all payloads in SIP, including binary
payloads. The only case where a UA MAY use a different encoding is
when transferring application data between applications that only
handle a different encoding (e.g., base64).
Camarillo Standards Track [Page 5]
RFC 5621 Message Body Handling in SIP September 2009
4. 'multipart' Message Bodies
This section deals with 'multipart' message bodies and their
handling.
4.1. Background on 'multipart' Message Bodies
[RFC3261] did not mandate support for 'multipart' message bodies in
MIME format [RFC2046]. However, since [RFC3261] was written, many
SIP extensions rely on them.
The use of 'multipart/mixed' MIME bodies is a useful tool to build
SIP extensions. An example of such an extension could be the
inclusion of location information in an INVITE request. Such an
INVITE request would use the 'multipart/mixed' MIME type [RFC2046] to
carry two body parts: a session description and a location object.
An example of an existing extension that uses 'multipart/mixed' to
send a session description and a legacy-signalling object is defined
in [RFC3204].
Another MIME type that is useful to build SIP extensions is
'multipart/alternative' [RFC2046]. Each body part within a
'multipart/alternative' carries an alternative version of the same
information.
The transition from SDP to new session description protocols could be
implemented using 'multipart/alternative' bodies. SIP messages
(e.g., INVITE requests) could carry a 'multipart/alternative' body
with two body parts: a session description written in SDP and a
session description written in a newer session description format.
Legacy recipient UAs would use the session description written in
SDP. New recipient UAs would use the one written in the newer
format.
Nested MIME bodies are yet another useful tool to build and combine
SIP extensions. Using the extensions in the previous examples, a UA
that supported a new session description format and that needed to
include a location object in an INVITE request would include a
'multipart/mixed' body with two body parts: a location object and a
'multipart/alternative'. The 'multipart/alternative' body part
would, in turn, have two body parts: a session description written in
SDP and a session description written in the newer session
description format.
Camarillo Standards Track [Page 6]
RFC 5621 Message Body Handling in SIP September 2009
4.2. Mandatory Support for 'multipart' Message Bodies
For all MIME-based extensions to work, the recipient needs to be able
to decode the multipart bodies. Therefore, SIP UAs MUST support
parsing 'multipart' MIME bodies, including nested body parts.
Additionally, UAs MUST support the 'multipart/mixed' and 'multipart/
alternative' MIME types. Support for other MIME types such as
'multipart/related' is OPTIONAL.
Note that, by default, unknown 'multipart' subtypes are treated as
'multipart/mixed'. Also note that SIP extensions can also include
'multipart' MIME bodies in responses. That is why both UACs and
UASs need to support 'multipart' bodies.
Legacy SIP UAs without support for 'multipart' bodies generate a 415
(Unsupported Media Type) response when they receive a 'multipart'
body in a request. A UAC sending a 'multipart' body can receive such
an error response when communicating with a legacy SIP UA that
predates this specification.
It has been observed in the field that a number of legacy SIP UAs
without support for 'multipart' bodies simply ignored those bodies
when they were received. These UAs did not return any error
response. Unsurprisingly, SIP UAs not being able to report this
type of error have caused serious interoperability problems in the
past.
4.3. UA Behavior to Generate 'multipart' Message Bodies
UAs SHOULD avoid unnecessarily nesting body parts because doing so
would, unnecessarily, make processing the body more laborious for the
receiver. However, [RFC2046] states that a 'multipart' media type
with a single body part is useful in some circumstances (e.g., for
sending non-text media types). In any case, UAs SHOULD NOT nest one
'multipart/mixed' within another unless there is a need to reference
the nested one (i.e., using the Content ID of the nested body part).
Additionally, UAs SHOULD NOT nest one 'multipart/alternative' within
another.
Note that UAs receiving unnecessarily nested body parts treat them
as if they were not nested.
5. 'multipart/mixed' Message Bodies
This section does not specify any additional behavior regarding how
to generate and process 'multipart/mixed' bodies. This section is
simply included for completeness.
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RFC 5621 Message Body Handling in SIP September 2009
6. 'multipart/alternative' Message Bodies
This section deals with 'multipart/alternative' message bodies and
their handling.
6.1. Background on 'multipart/alternative' Message Bodies
Each body part within a 'multipart/alternative' carries an
alternative version of the same information. The body parts are
ordered so that the last one is the richest representation of the
information. The recipient of a 'multipart/alternative' body chooses
the last body part it understands.
Note that within a body part encoded in a given format (i.e., of a
given content type), there can be optional elements that can
provide richer information to the recipient in case the recipient
supports them. For example, in SDP (Session Description Protocol)
[RFC4566], those optional elements are encoded in 'a' lines.
These types of optional elements are internal to a body part and
are not visible at the MIME level. That is, a body part is
understood if the recipient understands its content type,
regardless of whether or not the body part's optional elements are
understood.
Note as well that each part of a 'multipart/alternative' body
represents the same data, but the mapping between any two parts is
not necessarily without information loss. For example,
information can be lost when translating 'text/html' to 'text/
plain'. [RFC2046] recommends that each part should have a
different Content-ID value in the case where the information
content of the two parts is not identical.
6.2. UA Behavior to Generate 'multipart/alternative' Message Bodies
Section 8.2 mandates all the top-level body parts within a
'multipart/alternative' to have the same disposition type.
The 'session' and 'early-session' [RFC3959] disposition types require
that all the body parts of a 'multipart/alternative' body have
different content types. Consequently, for the 'session' and 'early-
session' disposition types, UAs MUST NOT place more than one body
part with a given content type in a 'multipart/alternative' body.
That is, for 'session' and 'early-session', no body part within a
'multipart/alternative' can have the same content type as another
body part within the same 'multipart/alternative'.
Camarillo Standards Track [Page 8]
RFC 5621 Message Body Handling in SIP September 2009
6.3. UA Behavior to Process 'multipart/alternative' Message Bodies
This section does not specify any additional behavior regarding how
to process 'multipart/alternative' bodies. This section is simply
included for completeness.
7. 'multipart/related' Message Bodies
This section deals with 'multipart/related' message bodies and their
handling.
7.1. Background on 'multipart/related' Message Bodies
Compound objects in MIME are represented using the 'multipart/
related' content type [RFC2387]. The body parts within a particular
'multipart/related' body are all part of a compound object and are
processed as such. The body part within a 'multipart/related' body
that needs to be processed first is referred to as the 'root' body
part. The root body part of a 'multipart/related' body is identified
by the 'start' parameter, which is a Content-Type header field
parameter and contains a Content-ID URL pointing to the root body
part. If the start parameter is not present, the root body part is,
by default, the first body part of the 'multipart/related'. An
example of a compound object is a web page that contains images. The
html body part would be the root. The remaining body parts would
contain the images. An example of a SIP extension using 'multipart/
related' is specified in [RFC4662].
7.2. UA Behavior to Generate 'multipart/related' Message Bodies
This section does not specify any additional behavior regarding how
to generate 'multipart/related' bodies. This section is simply
included for completeness.
7.3. UA Behavior to Process 'multipart/related' Message Bodies
Per [RFC2387], a UA processing a 'multipart/related' body processes
the body as a compound object ignoring the disposition types of the
body parts within it. Ignoring the disposition types of the
individual body parts makes sense in the context in which 'multipart/
related' was originally specified. For instance, in the example of
the web page, the implicit disposition type for the images would be
'inline', since the images are displayed as indicated by the root
html file. However, in SIP, the disposition types of the individual
body parts within a 'multipart/related' play an important role and,
thus, need to be considered by the UA processing the 'multipart/
related'. Different SIP extensions that use the same disposition
type for the 'multipart/related' body can be distinguished by the
Camarillo Standards Track [Page 9]
RFC 5621 Message Body Handling in SIP September 2009
disposition types of the individual body parts within the 'multipart/
related'. Consequently, SIP UAs processing a 'multipart/related'
body with a given disposition type MUST process the disposition types
of the body parts within it according to the SIP extension making use
the disposition type of the 'multipart/related'.
Note that UAs that do not understand 'multipart/related' will
treat 'multipart/related' bodies as 'multipart/mixed' bodies.
These UAs will not be able to process a given body as a compound
object. Instead, they will process the body parts according to
their disposition type as if each body part was independent from
each other.
8. Disposition Types
This section deals with disposition types in message bodies.
8.1. Background on Content and Disposition Types in SIP
The Content-Disposition header field, defined in [RFC2183] and
extended by [RFC3261], describes how to handle a SIP message's body
or an individual body part. Examples of disposition types used in
SIP in the Content-Disposition header field are 'session' and
'render'.
[RFC3204] and [RFC3459] define the 'handling' parameter for the
Content-Disposition header field. This parameter describes how a UAS
reacts if it receives a message body whose content type or
disposition type it does not understand. If the parameter has the
value 'optional', the UAS ignores the message body; if the parameter
has the value 'required', the UAS returns a 415 (Unsupported Media
Type) response. The default value for the 'handling' parameter is
'required'. The following is an example of a Content-Disposition
header field:
Content-Disposition: signal; handling=optional
[RFC3204] identifies two situations where a UAS (User Agent Server)
needs to reject a request with a body part whose handling is
required:
1. if it has an unknown content type.
2. if it has an unknown disposition type.
If the UAS did not understand the content type of the body part, the
UAS can add an Accept header field to its 415 (Unsupported Media
Type) response listing the content types that the UAS does
Camarillo Standards Track [Page 10]
RFC 5621 Message Body Handling in SIP September 2009
understand. Nevertheless, there is no mechanism for a UAS that does
not understand the disposition type of a body part to inform the UAC
about which disposition type was not understood or about the
disposition types that are understood by the UAS.
The reason for not having such a mechanism is that disposition types
are typically supported within a context. Outside that context, a UA
need not support the disposition type. For example, a UA can support
the 'session' disposition type for body parts in INVITE and UPDATE
requests and their responses. However, the same UA would not support
the 'session' disposition type in MESSAGE requests.
In another example, a UA can support the 'render' disposition type
for 'text/plain' and 'text/html' body parts in MESSAGE requests.
Additionally, the UA can support the 'session' disposition type for
'application/sdp' body parts in INVITE and UPDATE requests and their
responses. However, the UA might not support the 'render'
disposition type for 'application/sdp' body parts in MESSAGE
requests, even if, in different contexts, the UA supported all of the
following: the 'render' disposition type, the 'application/sdp'
content type, and the MESSAGE method.
A given context is generally (but not necessarily) defined by a
method, a disposition type, and a content type. Support for a
specific context is usually defined within an extension. For
example, the extension for instant messaging in SIP [RFC3428]
mandates support for the MESSAGE method, the 'render' disposition
type, and the 'text/plain' content type.
Note that, effectively, content types are also supported within a
context. Therefore, the use of the Accept header field in a 415
(Unsupported Media Type) response is not enough to describe in
which contexts a particular content type is supported.
Therefore, support for a particular disposition type within a given
context is typically signalled by the use of a particular method or
an option-tag in a Supported or a Require header field. When support
for a particular disposition type within a context is mandated,
support for a default content type is also mandated (e.g., a UA that
supports the 'session' disposition type in an INVITE request needs to
support the 'application/sdp' content type).
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8.2. UA Behavior to Set the 'handling' Parameter
As stated earlier, the 'handling' Content-Disposition parameter can
take two values: 'required' or 'optional'. While it is typically
easy for a UA to decide which type of handling an individual body
part requires, setting the 'handling' parameter of 'multipart' bodies
requires extra considerations.
If the handling of a 'multipart/mixed' body as a whole is required
for processing its enclosing body part or message, the UA MUST set
the 'handling' parameter of the 'multipart/mixed' body to 'required'.
Otherwise, the UA MUST set it to 'optional'. The 'handling'
parameters of the top-level body parts within the 'multipart/mixed'
body are set independently from the 'handling' parameter of the
'multipart/mixed' body. If the handling of a particular top-level
body part is required, the UA MUST set the 'handling' parameter of
that body part 'required'. Otherwise, the UA MUST set it to
'optional'.
Per the previous rules, a 'multipart/mixed' body whose handling is
optional can contain body parts whose handling is required. In
such case, the receiver is required to process the body parts
whose handling is required if and only if the receiver decides to
process the optional 'multipart/mixed' body.
Also per the previous rules, a 'multipart/mixed' body whose
handling is required can contain only body parts whose handling is
optional. In such case, the receiver is required to process the
body as a whole but, when processing it, the receiver may decide
(based on its local policy) not to process any of the body parts.
The 'handling' parameter is a Content-Disposition parameter.
Therefore, in order to set this parameter, it is necessary to provide
the 'multipart/mixed' body with a disposition type. Per [RFC3261],
the default disposition type for 'application/sdp' is 'session' and
for other bodies is 'render'. UAs SHOULD assign 'multipart/mixed'
bodies a disposition type of 'render'.
Note that the fact that 'multipart/mixed' bodies have a default
disposition type of 'render' does not imply that they will be
rendered to the user. The way the body parts within the
'multipart/mixed' are handled depends on the disposition types of
the individual body parts. The actual disposition type of the
whole 'multipart/mixed' is irrelevant. The 'render' disposition
type has been chosen for 'multipart/mixed' bodies simply because
'render' is the default disposition type in SIP.
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If the handling of a 'multipart/alternative' body as a whole is
required for processing its enclosing body part or message, the UA
MUST set the 'handling' parameter of the 'multipart/alternative' body
to 'required'. Otherwise, the UA MUST set it to 'optional'. The UA
SHOULD also set the 'handling' parameter of all the top-level body
part within the 'multipart/alternative' to 'optional'.
The receiver will process the body parts based on the handling
parameter of the 'multipart/alternative' body. The receiver will
ignore the handling parameters of the body parts. That is why
setting them to 'optional' is at the "SHOULD" level and not at the
"MUST" level -- their value is irrelevant.
The UA MUST use the same disposition type for the 'multipart/
alternative' body and all its top-level body parts.
If the handling of a 'multipart/related' body as a whole is required
for processing its enclosing body part or message, the UA MUST set
the 'handling' parameter of the 'multipart/related' body to
'required'. Otherwise, the UA MUST set it to 'optional'. The
'handling' parameters of the top-level body parts within the
'multipart/related' body are set independently from the 'handling'
parameter of the 'multipart/related' body. If the handling of a
particular top-level body part is required, the UA MUST set the
'handling' parameter of that body part to 'required'. Otherwise, the
UA MUST set it to 'optional'. If at least one top-level body part
within a 'multipart/related' body has a 'handling' parameter of
'required', the UA SHOULD set the 'handling' parameter of the root
body part to 'required'.
8.3. UA Behavior to Process 'multipart/alternative'
The receiver of a 'multipart/alternative' body MUST process the body
based on its handling parameter. The receiver SHOULD ignore the
handling parameters of the body parts within the 'multipart/
alternative'.
8.4. UAS Behavior to Report Unsupported Message Bodies
If a UAS cannot process a request because, in the given context, the
UAS does not support the content type or the disposition type of a
body part whose handling is required, the UAS SHOULD return a 415
(Unsupported Media Type) response even if the UAS supported the
content type, the disposition type, or both in a different context.
Consequently, it is possible to receive a 415 (Unsupported Media
Type) response with an Accept header field containing all the
content types used in the request.
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If a UAS receives a request with a body part whose disposition type
is not compatible with the way the body part is supposed to be
handled according to other parts of the SIP message (e.g., a Refer-To
header field with a Content-ID URL pointing to a body part whose
disposition type is 'session'), the UAS SHOULD return a 415
(Unsupported Media Type) response.
9. Message Body Processing
This section deals with the processing of message bodies and how that
processing is influenced by the presence of references to them.
9.1. Background on References to Message Body Parts
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. The way to handle that body part is defined by
the field the Content-ID URL appears. For example, the extension to
refer to multiple resources in SIP [RFC5368] places a Content-ID URL
in a Refer-To header field. Such a Content-ID URL points to a body
part that carries a URI list. In another example, the extension for
file transfer in SDP [RFC5547] places a Content-ID URL in a 'file-
icon' SDP attribute. This Content-ID URL points to a body part that
carries a (typically small) picture.
9.2. UA Behavior to Generate References to Message Bodies
UAs MUST only include forward references in the SIP messages they
generate. That is, an element in a SIP message can reference a body
part only if the body part appears after the element. Consequently,
a given body part can only be referenced by another body part that
appears before it or by a header field. Having only forward
references allows recipients to process body parts as they parse
them. They do not need to parse the remainder of the message in
order to process a body part.
It was considered to only allow (forward) references among body
parts that belonged to the same 'multipart/related' [RFC2387]
wrapper. However, it was finally decided that this extra
constraint was not necessary.
9.3. UA Behavior to Process Message Bodies
In order to process a message body or a body part, a UA needs to know
whether a SIP header field or another body part contains a reference
to the message body or body part (e.g., a Content-ID URL pointing to
it). If the body part is not referenced in any way (e.g., there are
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no header fields or other body parts with a Content-ID URL pointing
to it), the UA processes the body part as indicated by its
disposition type and the context in which the body part was received.
If the SIP message contains a reference to the body part, the UA
processes the body part according to the reference. If the SIP
message contains more than one reference to the body part (e.g., two
header fields contain Content-ID URLs pointing to the body part), the
UA processes the body part as many times as references are.
Note that, following the rules in [RFC3204], if a UA does not
understand a body part whose handling is optional, the UA ignores
it. Also note that the content indirection mechanism in SIP
[RFC4483] allows UAs to point to external bodies. Therefore, a UA
receiving a SIP message that uses content indirection could need
to fetch a body part (e.g., using HTTP [RFC2616]) in order to
process it.
9.4. The 'by-reference' Disposition Type
Per the rules in Section 9.3, if a SIP message contains a reference
to a body part, the UA processes the body part according to the
reference. Since the reference provides the context in which the
body part needs to be processed, the disposition type of the body
part is irrelevant. However, a UA that missed a reference to a body
part (e.g., because the reference was in a header field the UA did
not support) would attempt to process the body part according to its
disposition type alone. To keep this from happening, we define a new
disposition type for the Content-Disposition header field: by-
reference.
A body part whose disposition type is 'by-reference' needs to be
handled according to a reference to the body part that is located in
the same SIP message as the body part (given that SIP only allows
forward references, the reference will appear in the same SIP message
before the body part). A recipient of a body part whose disposition
type is 'by-reference' that cannot find any reference to the body
part (e.g., the reference was in a header field the recipient does
not support and, thus, did not process) MUST NOT process the body
part. Consequently, if the handling of the body part was required,
the UA needs to report an error.
Note that extensions that predate this specification use
references to body parts whose disposition type is not 'by-
reference'. Those extensions use option-tags to make sure the
recipient understands the whole extension and, thus, cannot miss
the reference and attempt to process the body part according to
its disposition type alone.
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10. Guidelines to Authors of SIP Extensions
These guidelines are intended for authors of SIP extensions that
involve, in some way, message bodies or body parts. These guidelines
discuss aspects that authors of such extensions need to consider when
designing them.
This specification mandates support for 'multipart/mixed' and
'multipart/alternative'. At present, there are no SIP extensions
that use different 'multipart' subtypes such as parallel [RFC2046] or
digest [RFC2046]. If such extensions were to be defined in the
future, their authors would need to make sure (e.g., by using an
option-tag or by other means) that entities receiving those
'multipart' subtypes were able to process them. As stated earlier,
UAs treat unknown 'multipart' subtypes as 'multipart/mixed'.
Authors of SIP extensions making use of 'multipart/related' bodies
have to explicitly address the handling of the disposition types of
the body parts within the 'multipart/related' body. Authors wishing
to make use of 'multipart/related' bodies should keep in mind that
UAs that do not understand 'multipart/related' will treat it as
'multipart/mixed'. If such treatment by a recipient is not
acceptable for a particular extension, the authors of such extension
would need to make sure (e.g., by using an option-tag or by other
means) that entities receiving the 'multipart/related' body were able
to correctly process them.
As stated earlier, SIP extensions can also include 'multipart' MIME
bodies in responses. Hence, a response can be extremely complex and
the UAC receiving the response might not be able to process it
correctly. Because UACs receiving a response cannot report errors to
the UAS that generated the response (i.e., error responses can only
be generated for requests), authors of SIP extensions need to make
sure that requests clearly indicate (e.g., by using an option-tag or
by other means) the capabilities of the UAC so that UASs can decide
what to include in their responses.
11. Security Considerations
This document specifies how SIP entities handle message bodies.
[RFC3261] discusses what type of information is encoded in SIP
message bodies and how SIP entities can protect that information. In
addition to the hop-by-hop security SIP can provide, SIP can also
secure information in an end-to-end fashion. SIP message bodies can
be end-to-end encrypted and integrity protected using S/MIME
[RFC3851], as described in [RFC3261].
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12. IANA Considerations
This document contains two actions that have been completed by IANA.
12.1. Registration of the 'by-reference' Disposition Type
This document defines a new Content-Disposition header field
disposition type (by-reference) Section 9.4. This value has been
registered in the IANA registry for Mail Content Disposition Values
with the following description:
by-reference The body needs to be handled according to a
reference to the body that is located in
the same SIP message as the body.
12.2. Update of the 'handling' Parameter Registration
References to this specification, to [RFC3204], and to [RFC3459] have
been added to the entry for the Content-Disposition 'handling'
parameter in the Header Field Parameters and Parameter Values
registry. The following is the resulting entry.
Predefined
Header Field Parameter Name Values Reference
------------------- --------------- --------- -------------------
Content-Disposition handling Yes [RFC3204] [RFC3261]
[RFC3459] [RFC5621]
13. Acknowledgements
The ideas in this document were originally discussed with Paul
Kyzivat. Christer Holmberg, Francois Audet, Dan Wing, Adam Roach,
Keith Drage, and Dale Worley provided comments on it. Dave Crocker
performed a thorough review on the whole document.
14. References
14.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.
[RFC2387] Levinson, E., "The MIME Multipart/Related Content-type",
RFC 2387, August 1998.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[RFC3204] Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet,
F., Watson, M., and M. Zonoun, "MIME media types for ISUP
and QSIG Objects", RFC 3204, December 2001.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3459] Burger, E., "Critical Content Multi-purpose Internet Mail
Extensions (MIME) Parameter", RFC 3459, January 2003.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[RFC3959] Camarillo, G., "The Early Session Disposition Type for the
Session Initiation Protocol (SIP)", RFC 3959,
December 2004.
[RFC4483] Burger, E., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
May 2006.
14.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
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[RFC4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, December 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.
[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,
October 2008.
[RFC5547] Garcia-Martin, M., Isomaki, M., Camarillo, G., Loreto, S.,
and P. Kyzivat, "A Session Description Protocol (SDP)
Offer/Answer Mechanism to Enable File Transfer", RFC 5547,
May 2009.
Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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