Internet DRAFT - draft-rajeshkumar-mmusic-gpmd
draft-rajeshkumar-mmusic-gpmd
MMUSIC Working Group R. Kumar
Internet-Draft F. Andreasen
Document: draft-rajeshkumar-mmusic-gpmd-03.txt Cisco Systems
Expires: November 2003 May 2003
SDP attribute for Qualifying Media Formats with Generic Parameters
Status of this Memo
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Abstract
This document defines a new SDP attribute called general-purpose
media descriptor (gpmd). The gpmd attribute allows the use of new
informative parameters, gpmd parameters, to qualify existing media
formats. These gpmd parameters are not part of the standard (e.g.,
MIME) definition of the media format and support for them with a
given media format can not be assumed. Their use is therefore
limited to cases where they provide information that may be of use to
the other party in a session but is not critical to the use of the
particular media format. This document also defines a specific gpmd
parameter, voice-band data, which can be used to describe a media
format as carrying voice-band data. This enables the receiver to
optimize its handling of the media received.
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Conventions used in this document
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 RFC-2119 [1].
Table of Contents
1. Introduction...................................................2
2. Definition of the "gpmd" Attribute.............................3
2.1 General Semantics..........................................4
2.2 Offer/Answer Support.......................................5
2.3 ABNF Grammar for "gpmd" Attribute..........................5
2.4 Defining gpmd Parameters...................................6
3. The Voice-Band Data gpmd Parameter.............................6
4. Contrasting fmtp with gpmd.....................................8
5. Security Considerations........................................8
6. IANA considerations............................................9
6.1 Registration of the SDP attribute "gpmd"...................9
6.2 Creation of New SDP Sub-Registry for "gpmd" Parameters.....9
6.3 IANA Registration of the "gpmd" Parameter "vbd"............9
7. Normative References..........................................10
8. Informative References........................................10
Acknowledgments..................................................10
Authors' Addresses...............................................10
Intellectual Property Statement..................................11
Full Copyright Statement.........................................11
Acknowledgement..................................................12
1. Introduction
When a media format is defined and registered as a MIME type, part of
the definition includes a list of required and optional parameters
that can be supplied for the particular media format. For example,
the registration of the "audio/PCMU" MIME type in [6] defines "rate"
as a required parameter and "channels", "ptime", and "maxptime" as
optional parameters that qualify the operation of the PCMU encoding
method. Some media parameters may be common among different encoding
methods while others may be specific to a particular encoding method.
In either case, the parameters in question must be defined as part of
the definition and registration of the particular media format; new
parameters cannot be used without providing a new definition and
registration of the media format.
However, some applications can benefit from qualifying existing well-
defined media formats with new informative generic parameters that
are not critical to the use of that media format, but nevertheless
would be beneficial for the other side to know. For example, an
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endpoint might benefit from knowing that an RTP payload type will be
treated, for the duration of a session, as containing a modulated
data signal. Other such cases might arise in the future.
When using the Session Description Protocol (SDP) [3] to describe
media streams, media format specific parameters, that are not already
defined in SDP, are signaled through the use of the "fmtp" attribute.
The use of the "fmtp" attribute parameter however is limited to media
format parameters that are defined as part of the media format
registration; new informative generic parameters cannot be provided
with the "fmtp" parameter. We therefore need to provide the
following:
1. An SDP mechanism to bind, on a per-session basis, values of one
or more informative generic parameters to a media format.
2. A mechanism for defining and registering such parameters.
In this document, we define a new SDP attribute called general-
purpose media descriptor (gpmd) that can provide one or more of these
informative generic parameters for a media format. An endpoint
chooses which generic parameters to advertise using this general-
purpose media descriptor.
In Section 2, we define the "gpmd" attribute as well as general
requirements and procedures for gpmd parameters. In Section 3, we
define a specific voice-band data gpmd parameter. Section 4 explains
the difference between the "gpmd" parameter and the existing "fmtp"
parameter and in Section 5 we provide security considerations. In
Section 6, we provide the IANA registration procedures for gpmd
parameters followed by IANA registration of the voice-band data gpmd
parameter.
2. Definition of the "gpmd" Attribute
The "gpmd" attribute is a media-level attribute; it MUST NOT be
provided at the session-level. The attribute is used when an
endpoint wants to provide one or more informative generic parameters
for a given media format in a media stream, a.k.a. a general-purpose
media descriptor (gpmd).
The attribute is defined as:
a=gpmd:<format> <generic parameter list>
where <generic parameter list> is represented as a semicolon-
separated list of case-sensitive <type>=<value> pairs (a formal ABNF
grammar for the "gpmd" attribute is provided in Section 2.3). For
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example, the generic parameter "vbd" (see Section 3) is a boolean
which may be associated with any RTP payload type. Thus:
m=audio 3456 RTP/AVP 0 15
a=gpmd:0 vbd=yes
enables voice-band data treatment for the static RTP payload type of
0 (PCMU).
Note that <format> need not be an RTP payload type. It is
permissible to use any value of <format> that can appear in an SDP
"m=" line, as long as the definition of each generic parameter allows
it to be used to qualify that media format. Thus, a hypothetical
generic parameter, "someParameter", may be enclosed in a "gpmd"
attribute associated with the format "wb" (whiteboard) as follows:
m=application 3456 udp wb
a=gpmd:wb someParameter=someValue
The "gpmd" attribute is charset-independent. This property is
inherited by all parameters enclosed within the "gpmd" attribute.
2.1 General Semantics
A general-purpose media descriptor contains one or more gpmd
parameters. A gpmd parameter applies to one media format in one
media stream, just like an "fmtp" parameter. In the following, all
statements regarding gpmd parameters are to be taken in the context
of a given media format in a given media stream unless otherwise
noted. A gpmd parameter MUST by definition be informative and
support for it by any party in a session MUST be optional. Correct
operation of a given media stream MUST NOT depend on one or more
participants either supporting or not supporting a given gpmd
parameter. A gpmd parameter can be thought of as providing an
optimization hint to the other party. If the other party supports
it, improved operation may result, but if the other party does not
support it, the operation MUST NOT be any worse, than if the gpmd
parameter(s) had not been indicated in the first place. A gpmd
parameter can by default be used with any media format, however the
definition of particular gpmd parameter MAY limit its scope to a
subset of media formats. If the parameter is used with an
unsupported media format, the parameter SHOULD be ignored.
Like other new SDP attributes, support for the "gpmd" attribute is
optional. SDP parsers that do not understand it MUST ignore it. The
same is true for each generic parameter enclosed within the "gpmd"
attribute; SDP parsers MUST ignore unsupported and erroneous gpmd
parameters.
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2.2 Offer/Answer Support
A gpmd parameter can be either "unilateral" or "bilateral" as
explained below. Support for and use of the gpmd attribute and gpmd
parameters is specified in terms of the offer/answer model [4]:
A bilateral gpmd parameter can always be present in an offer, however
it MUST only be present in an answer, if it was present in the
corresponding offer. If an offer contains a bilateral gpmd parameter
which the answerer does not want to support, the answer MUST NOT
include this gpmd parameter. If the answerer does not want to
support any of the gpmd parameters offered, the answer MUST NOT
include the gpmd attribute. The operation associated with a
bilateral gpmd parameter MUST only be performed if both the offer and
the answer indicated support for this parameter, i.e. the two sides
have agreed to use it. In all other cases, operation MUST be as if
the gpmd parameter had not been included in the first place. The
only exception to this rule is in the period between the offer being
issued and the answer being received; during that time, the offerer
MAY use the operation associated with the offered gpmd parameter for
any media received for that offer.
A unilateral gpmd parameter can always be present in either an offer
or an answer. Unilateral parameters can be thought of as being
advertised rather than negotiated. They convey information that may
be of interest to the other party, but there is no need for the two
sides to negotiate or agree on this information. Thus, if an offer
contains a unilateral gpmd parameter, the answerer MAY choose to
adjust its local operation in accordance with that parameter. The
offerer however can neither depend on nor assume that the answerer
will do so, irrespective of whether the parameter is included in the
answer or not. If the answer does include the parameter, it is
completely independent of the parameter provided in the offer, i.e.,
the parameter is truly unilateral. The answer operation is similar
to the offer operation. Note that unilateral gpmd parameters can be
provided in an answer that did not contain a corresponding gpmd
parameter in the offer, and that any such parameters are
unidirectional, i.e., they apply only to the party that sends it and
are not negotiated between the two parties.
2.3 ABNF Grammar for "gpmd" Attribute
This section provides an Augmented BNF (ABNF) grammar [2] for the
"gpmd" attribute. Here, generic parameters that may be enclosed
within the "gpmd" attribute are referred to as "subparameters". The
"subparameters" are case-sensitive.
RFC 2327 [3] generically defines SDP attributes as follows:
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attribute-fields = *("a=" attribute CRLF)
attribute = (att-field ":" att-value) | att-field
For the "gpmd" attribute, a special case of this ABNF syntax may be
stated as follows:
gpmd-attribute-field = "a=" gpmd-attribute CRLF
gpmd-attribute = "gpmd" ":" gpmd-format SP gpmd-value
gpmd-format = token ; for audio media, this is
; typically an RTP payload type
gpmd-value = gpmd-subparameter *(";" gpmd-subparameter)
gpmd-subparameter = gpmd-subpar-name "=" gpmd-subpar-value
gpmd-subpar-name = 1*(alpha-numeric)
gpmd-subpar-value = 1*(gpmd-val-char)
alpha-numeric = ALPHA / DIGIT
; ALPHA and DIGIT from RFC 2234 [2]
gpmd-val-char = %x21-3A / %x3C-7E
; VCHAR [2], except semi-colon (";")
2.4 Defining gpmd Parameters
When defining a gpmd parameter, the following MUST be specified:
* A descriptive name and a general description of what the gpmd
parameter conveys MUST be provided. In so doing, it MUST be
ensured, that the gpmd parameter does not violate the semantic
requirements specified in Section 2.1. In particular, the
requirement that correct operation of a given media stream does not
depend on either party supporting the parameter MUST be honored.
* The actual gpmd parameter name used in the gpmd attribute and the
possible values MUST be specified according to the grammar defined
in Section 2.3.
* The offer/answer model, i.e., unilateral or bilateral (as specified
in Section 2.2) to be used for this gpmd parameter SHOULD be
specified. If it is not specified, it is assumed that the
bilateral model is used.
* The definition MAY include a description of the media formats the
gpmd parameter is expected to be used with, e.g. all audio codecs,
or one or more specific codecs. Use of the gpmd parameter with
other media formats does not constitute an error, but it may not
result in any meaningful optimization either.
3. The Voice-Band Data gpmd Parameter
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This section defines a new gpmd parameter called Voice-Band Data
("vbd"). The registration of this parameter is addressed in Section
6.3.
The "vbd" gpmd parameter indicates voice-band data. It is a boolean
that may be assigned the value "yes" or "no" (without the quotes).
Using this generic parameter, any RTP audio payload type may be
declared to carry voice-band data. In practice, most applications
are currently likely to use one of the following audio media formats
for the carriage of voice band data signals: PCMU, PCMA, G726-40 and
G726-32. The "vbd" parameter is a bilateral gpmd parameter, and
hence support for it MUST be negotiated in order to use any
optimizations associated with it in either direction. The exact
optimizations to be used are implementation dependent, however one
example is to increase the size of the jitter buffer when voice-band
data is present.
When use of a "vbd" media format has been negotiated, voice-band data
SHOULD be transported using either a "vbd" media format or another
negotiated media format suitable for transporting the voice-band data
in question. Examples of such media formats include T.38 fax relay
and V.150 modem relay. Non-examples include G.723.1, G.729, and
PCMA/PCMU without an accompanying "vbd" gpmd parameter. Note that
selection of specific suitable media formats is typically done on the
basis of stimuli, such as the detection of a calling or an answer
tone, a V.21 preamble with fax flags or a modem CM signal, which may
occur at different times. In any case RFC 2833 telephone events [5]
may be used to relay relevant data modem and fax events.
Note that in a given session, a single RTP format (e.g., PCMU) may be
associated with multiple payload types. For a given RTP format, one
of these may be a static payload type, while the others are
dynamically assigned payload types. Regardless of whether a given
payload type is static or dynamic, it may be declared to carry voice-
band data. Without the parameter "vbd", the payload type, whether
static or dynamic, is insufficient to determine whether a modem
signal is being encoded or not. Instead, the contents of the
incoming media stream would have to be analyzed.
The following example shows a static and a dynamic payload type
associated with the PCMU format. Only the dynamic payload type, 98,
is earmarked for voice-band data treatment.
m=audio 3456 RTP/AVP 0 15 98
a=rtpmap:98 PCMU/8000
a=gpmd:98 vbd=yes
The following example reverses the association of payload type with
voice-band data. Of the static and dynamic payload types associated
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with PCMU, only the static payload type, 0, is earmarked for voice-
band data treatment.
m=audio 3456 RTP/AVP 0 15 98
a=gpmd:0 vbd=yes
a=rtpmap:98 PCMU/8000
4. Contrasting fmtp with gpmd
The SDP "fmtp" parameter and the "gpmd" parameter defined in this
document may seem very similar. Although we have explained the
rationale behind the gpmd parameter in Section 1, we here provide a
quick overview of the difference between the fmtp and the gpmd
parameters for easy reference:
Parameters provided within the "fmtp" attribute must be provided in
the standard definition (e.g. MIME registration) of that media
format. Some of these parameters are mandatory, while others are
optional. It is only parameters provided in this definition that can
be used with the "fmtp" parameter. The reason for this is, that the
"fmtp" parameter is intended to provide codec specific parameters
that the SDP parser and higher level application may not fully
understand, however if the codec in question is supported, it is
guaranteed that any such parameters will be supported as well.
By contrast, the parameters that may be provided within a "gpmd" line
are not part of the standard definition (e.g. MIME registration) of
the media format. These parameters are maintained in a separate IANA
registry (Section 6.2). The definition of a "gpmd" parameter may
indicate the range of media formats which may be qualified by it (all
media formats by default). All "gpmd" enclosed parameters are
optional at the SDP parser level, though some applications may
negotiate particular parameters. When a given codec is supported,
gpmd parameters for that codec may or may not be supported.
5. Security Considerations
The general purpose media descriptor defined in this document is
informative in nature and furthermore not critical to the use of any
particular media format. Presence or absence of a gpmd parameter may
however lead to use of or lack of certain optimizations on either the
sending or receiving side. An attacker altering the presence or
absence of one or more gpmd parameters in a session description may
therefore negatively influence the resulting media stream.
It is possible to protect against such attacks by employing integrity
protection mechanism on the protocol(s) used to exchange the session
description or by use of integrity protection mechanism on the
session description itself.
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6. IANA considerations
There are three IANA actions in this document:
* Registration of "gpmd" as a new SDP attribute (Section 6.1).
* Creation of a new SDP sub-registry for gpmd parameters (Section
6.2).
* Registration of "vbd" as a generic parameter within the SDP sub-
registry for "gpmd" (Section 6.3).
6.1 Registration of the SDP attribute "gpmd"
This document defines a new SDP attribute, which the IANA is hereby
requested to register as follows:
Attribute name: gpmd
Long form name: General-purpose media descriptor
Attribute type: Media-level
Subject to charset: No
Purpose: Informative generic parameters for media formats
Attribute values: As defined in Section 2 of this document
6.2 Creation of New SDP Sub-Registry for "gpmd" Parameters
The IANA is hereby requested to create a new SDP sub-registry for
gpmd parameters. Each entry in this registry shall contain the
following:
* A descriptive name for the gpmd parameter.
* An alphanumeric character string defining the token for the
parameter name in a gpmd attribute (as specified in Section 2).
* A reference to the document defining the gpmd parameter.
In order for the IANA to register a gpmd parameter defined outside of
this document, the parameter MUST be defined in a Standards Track RFC
in accordance with the requirements defined in Section 2.4.
6.3 IANA Registration of the "gpmd" Parameter "vbd"
The IANA is hereby requested to register a new SDP gpmd parameter as
follows:
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* Descriptive Name: Voice-band data
* Token: vbd
7. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, Internet Mail Consortium and
Demon Internet Ltd., November 1997.
[3] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[4] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] H. Schulzrinne, and S. Petrack, RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals.
8. Informative References
[6] Casner, S., and P. Hoschka, "MIME Type Registration of RTP
Payload Formats", Work in progress.
Acknowledgments
Henning Schulzrinne provided the original inspiration for this
document by pointing out the inherent simplicity and self-
synchronizing nature of changing payload types. Steven Casner,
Henning Schulzrinne and Colin Perkins all helped refine the idea of
this new, generic SDP attribute. Colin Perkins suggested the IANA
registry mechanism needed with the "gpmd" attribute. Bill Foster
pursued the logical antecedents of this draft. Magnus Westerlund
reviewed and corrected an earlier version of the ABNF grammar.
Ruediger Kreuter refined the text on transporting voice band data
with non-"vbd" media formats.
Authors' Addresses
Rajesh Kumar
Cisco Systems
170 West Tasman Dr
San Jose, CA
Email: rkumar@cisco.com
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Flemming Andreasen
Cisco Systems
499 Thornall Street, 8th Floor
Edison, NJ
Email: fandreas@cisco.com
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