SIP Working Group G. Camarillo
Internet-Draft Ericsson
Expires: March 1, 2008 August 29, 2007
Message Body Handling in the Session Initiation Protocol (SIP)
draft-ietf-sip-body-handling-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document clarifies how message bodies are handled in SIP.
Additionally, it discusses to which degree SIP user agents need to
support MIME (Multipurpose Internet Mail Extensions)-encoding of body
parts.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Multipart Message Bodies . . . . . . . . . . . . . . . . . . . 3
3.1. General Considerations . . . . . . . . . . . . . . . . . . 4
3.2. Body Generation . . . . . . . . . . . . . . . . . . . . . 5
3.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 5
4. Message-body and Body-part Disposition . . . . . . . . . . . . 6
4.1. Body Generation . . . . . . . . . . . . . . . . . . . . . 7
4.2. Body Processing . . . . . . . . . . . . . . . . . . . . . 8
4.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
5. Guidelines to Authors of SIP Extensions . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informational References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
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.
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.
Section 3 discusses issues related to the handling of multipart
message bodies in SIP. Section 4 discusses issues related to the
disposition of message bodies and body parts in SIP.
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].
3. 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. Therefore, this specification updates
[RFC3261]'s recommendation regarding support for multipart MIME
bodies.
It is expected that most SIP UAs will implement extensions that
require them to generate 'multipart/mixed' MIME bodies. An example
of such an extension would be the inclusion of location information
in an INVITE request. Such an INVITE request would use the
'multipart/mixed' MIME type 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 a number of SIP UAs will need to generate is
'multipart/alternative'. 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
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representation of the information. This way, 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 may be optional elements that may
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 may be lost when translating 'text/html' to 'text/
plain'.
It is expected that the transition from SDP to new session
description protocols is implemented using 'multipart/alternative'
bodies. SIP messages (e.g., INVITE requests) would 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.
A number of SIP UAs will also need to generate nested MIME bodies.
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.
3.1. General Considerations
For all MIME-based extensions to work, the recipient needs to be able
to decode the multipart bodies. Therefore, SIP UAs MUST be able to
parse 'multipart' MIME bodies, including nested body parts. In
particular, UAs MUST support the 'multipart/mixed' and 'multipart/
alternative' MIME types. Note that, by default, unknown 'multipart'
subtypes are treated as 'multipart/mixed'.
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Note that SIP extensions may also include 'multipart' MIME bodies
in responses. That is why both UACs and UASs need to support
'multipart' bodies.
3.2. Body Generation
UAs should avoid unnecessarily nesting body parts. 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.
All the body parts within a 'multipart/alternative' have the same
disposition type (see Section 4.1). Some disposition types require
that all the body parts of a 'multipart/alternative' body have
different content types. In particular, for the 'session' and
'early-session' [RFC3959] 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'.
As stated earlier, the mapping between two body parts within a
'multipart/alternative' body may imply information loss. [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.
A body part can only reference another body part if both are within
the same 'multipart/related' wrapper. Therefore, UAs MUST ensure
that any given body part only references body parts within its
'multipart/related' wrapper.
UAs MUST use the binary transfer encoding for binary payloads in SIP.
3.3. UAS Behavior
Section 3.1 mandates that all UAs support 'multipart' bodies.
However, if a particular UAS does not support 'multipart' bodies and
receives one, the UAS SHOULD return a 415 (Unsupported Media Type)
response.
Note that it is essential that UASs without MIME support are at
least able to return an error response when receiving a
'multipart' body. Not being able to signal this type of error
could cause serious interoperability problems. Legacy UASs
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without MIME support that, for some reason, cannot be immediately
upgraded to support MIME, should at least be upgraded to be able
to report this error.
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.
4. Message-body and Body-part Disposition
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
should react 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 it has the
value 'required', the UAS returns a 415 (Unsupported Media Type)
response. The default value for the 'handling' parameter is
'required'.
[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 (User Agent Server) did not understand the content type of
the body part, it can add an Accept header field to its 415
(Unsupported Media Type) response listing the content types that the
UAS does understand. Nevertheless, there is no mechanism for a UAS
that does not understand the disposition type of a body part to
inform the UAC (User Agent Client) 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
(User Agent) may not support the disposition type. For example, a UA
may 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.
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In another example, a UA may support the 'render' disposition type
for 'text/plain' and 'text/html' body parts in MESSAGE requests.
Additionally, the UA may support the 'session' disposition type for
'application/sdp' body parts in INVITE and UPDATE requests and their
responses. However, the UA may not support the 'render' disposition
type for 'application/sdp' body parts in MESSAGE requests, even if,
in different contexts, the UA supported all 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).
Content-ID URLs (Uniform Resource Locators) are another tool to
describe how a body part should be handled. Some extensions use a
Content-ID URL [RFC2392], which can appear in a header field or
within a body part (e.g., in an SDP attribute), that points to a body
part. The way to handle that body part is defined by the field the
Content-ID URL appears in and by the disposition type of the body
part. For example, the extension to refer to multiple resources in
SIP [I-D.ietf-sip-multiple-refer] places a Content-ID URL in a
Refer-To header field. Such a Content-ID URL points to a body part
whose disposition type is supposed to be 'recipient-list'. In
another example, the extension for file transfer in SDP
[I-D.ietf-mmusic-file-transfer-mech] places a Content-ID URL in a
'file-icon' SDP attribute. This Content-ID URL points to a body part
whose disposition type is supposed to be 'icon'.
4.1. Body Generation
As stated earlier, the 'handling' Content-Disposition parameter can
take two values: 'required' or 'optional'. While it is typically
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easy for a UA to decide which type of handling an individual body
part requires, setting the 'handing' parameter of 'multipart' bodies
requires extra considerations.
If at least one of the body parts within a 'multipart/mixed' body has
a 'handling' value of 'required', the UA MUST set the 'handling'
parameter of the 'multipart/mixed' body to 'required'. If all the
body parts within a 'multipart/mixed' body have a 'handling' value of
'optional', the UA MUST set the 'handling' parameter of the
'multipart/mixed' body to 'optional'.
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
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
it is the default disposition type in SIP.
If the handling of a 'multipart/alternative' body is required, the UA
MUST set the 'handling' parameter of the 'multipart/alternative' body
and to the last body part within the 'multipart/alternative' to
'required'. Additionally, the UA MUST set the 'handling' parameter
of all body parts within the 'multipart/alternative' except the last
one to 'optional'. The UA MUST use the same disposition type for the
'multipart/alternative' body and all its body parts.
4.2. Body Processing
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 it (e.g., a Content-ID URL pointing to it). If the body part is
not referenced in any way (e.g., there are 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 and the
disposition type of the body part. If the SIP message contains more
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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 there are.
A UA looking for references to a body part starts by parsing the SIP
message's header fields. Additionally, if the body part is within a
'multipart/related' [RFC2387] wrapper, the body parts within the
'multipart/related' wrapper may reference each other. Therefore, the
UA processes the body parts in the 'multipart/related', starting with
its 'root', looking for references to the body part.
Note that, per [RFC2387], a UA processing a 'multipart/related'
body processes it as a compound object ignoring the disposition
types of the body parts within it.
Following the rules in [RFC3204], if a UA does not understand a body
part whose handling is optional, it ignores it.
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 may need to fetch a
body part (e.g., using HTTP [RFC2616]) in order to process it.
4.3. UAS Behavior
If a UAS cannot process a request because, in the given context, it
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.
If a UAS receives a request with a body part whose disposition type
is not compatible with the way the body part should 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.
5. 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.
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This specification mandates support for 'multipart/mixed' and
'multipart/alternative' and describes how to handle 'multipart/
related' [RFC2387] bodies. 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'.
Body parts within a 'multipart/related' wrapper can reference each
other. Per [RFC2387], a UA processing a 'multipart/related' body
processes it as a compound object ignoring the disposition types of
the body parts within it. However, UAs that do not understand
'multipart/related' will treat it as 'multipart/mixed'. These UAs
will not be able to process the references among the body parts and
will process the body parts according to their disposition type.
When a SIP UA receives a header field or an optional body part it
does not understand, the UA ignores it. A header field or a body
part carrying a reference to another body part (e.g., a Content-ID
URL) can influence the way that body part is handled. If a header
field or a body part carrying a reference to a body part is not
understood and, thus, ignored by its recipient, the body part could
be handled in an unintended way. Therefore, authors of SIP
extensions that involve references to body parts need to make sure
(e.g., by using an option-tag or by other means) that entities
processing those extensions do not behave in unintended ways.
Additionally, authors of such extensions need to specify the
acceptable disposition types of the referenced body part and a
default, mandatory to support, content type per disposition type.
As stated earlier, SIP extensions may also include 'multipart' MIME
bodies in responses. However, UACs receiving a response cannot
report errors to the UAS that generated the response (i.e., error
responses can only be generated for requests). Therefore, 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.
6. Security Considerations
TBD.
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7. Acknowledgements
The ideas in this document were discussed with Paul Kyzivat.
Christer Holmberg, Francois Audet, and Dan Wing provided comments on
this document.
8. IANA Considerations
This document does not contain any IANA actions.
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, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[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.
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[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.
9.2. Informational 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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[I-D.ietf-sip-multiple-refer]
Camarillo, G., "Referring to Multiple Resources in the
Session Initiation Protocol (SIP)",
draft-ietf-sip-multiple-refer-01 (work in progress),
January 2007.
[I-D.ietf-mmusic-file-transfer-mech]
Garcia-Martin, M., "A Session Description Protocol (SDP)
Offer/Answer Mechanism to Enable File Transfer",
draft-ietf-mmusic-file-transfer-mech-03 (work in
progress), June 2007.
Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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Full Copyright Statement
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