MMUSIC WG M. Garcia-Martin
Internet-Draft Ericsson
Intended status: Standards Track S. Veikkolainen
Expires: August 6, 2011 Nokia
R. Gilman
February 2, 2011
Miscellaneous Capabilities Negotiation in the Session Description
Protocol (SDP)
draft-garcia-mmusic-sdp-icap-bcap-01
Abstract
SDP has been extended with a capability negotiation mechanism
framework that allows the endpoints to negotiate transport protocols
and attributes. This framework has been extended with a media
capabilities negotiation mechanism that allows endpoints to negotiate
additional media-related capabilities. This negotiation is embedded
into the widely-used SDP offer/answer procedures.
This memo extends the SDP capability negotiation framework to allow
endpoints to negotiate two additional SDP capabilities. In
particular, this memo provides a mechanism to negotiate media titles
("i=" line for each media) and media bandwidth ("b=" line).
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on August 6, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5
3.1. Extensions to SDP . . . . . . . . . . . . . . . . . . . . 5
3.1.1. Bandwidth Capability . . . . . . . . . . . . . . . . . 6
3.1.2. Information Capability . . . . . . . . . . . . . . . . 8
3.2. Session Level versus Media Level . . . . . . . . . . . . . 10
4. Field Replacement Rules . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. New SDP Attributes . . . . . . . . . . . . . . . . . . . . 11
5.2. New Option Tags . . . . . . . . . . . . . . . . . . . . . 11
5.3. New SDP Capability Negotiation Configuration Parameters . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The Session Description Protocol (SDP) [RFC4566] is intended for
describing multimedia sessions for the purposes of session
announcement, session invitation, and other forms of multimedia
session initiation. SDP has been extended with a capability
negotiation mechanism framework [RFC5939] which allows the endpoints
to negotiate capabilities, such as support for Realtime Transport
Protocol (RTP) [RFC3550] and Secure Realtime Transport Protocol
(SRTP) [RFC3711]. The SDP media capabilities [RFC5939] provides
negotiation capabilities to media lines as well.
The capability negotiation is embedded into the widely used SDP
offer/answer procedure [RFC3264]. This memo provides the means to
negotiate further capabilities than those specified in the SDP
capability negotiation mechanism framework [RFC5939] and the SDP
media capabilities negotiation
[I-D.ietf-mmusic-sdp-media-capabilities]. In particular, this memo
provides a mechanism to negotiate media titles ("i=") and media
bandwidth ("b=").
It would have been possible to define a mechanism to negotiate media
encryption keys ("k="). However, the usage of the media encryption
keys ("k=") is highly discouraged in favour of other existing more
sophisticated mechanisms. Therefore, we are not providing a
mechanism to provide capabilities for media encryption keys ("k=") at
this stage.
Since the two added capabilities are highly unconnected, it is not
expected that implementations will support both at the same time.
Instead, it is expected that applications will choose their needed
capability for their specific purpose. Due to this, we are writing
the normative part pertaining to both capabilities in a self-
contained section: Section 3.1.1 describes the bandwidth capability
extension, and Section 3.1.2 describes the information capability
extension. Separate option tags are defined for both capabilities.
2. 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 BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant
implementations.
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3. Protocol Description
3.1. Extensions to SDP
The SDP Capability Negotiation Framework [RFC5939] and the SDP media
capabilities negotiation [I-D.ietf-mmusic-sdp-media-capabilities]
specify attributes for negotiating SDP capabilities. These documents
specify new attributes (e.g., 'acap', 'tcap', 'mcap') for achieving
their purpose. In this document we define two new additional
capability attributes for SDP lines of the the general form:
type=value
for types "i" and "b". The corresponding capability attributes are
defined as "icap" and "bcap", respectively.
From the sub-rules of "a=" line in SDP [RFC4566], SDP attributes are
of the form:
attribute = (att-field ":" att-value) / att-field
att-field = token
att-value = byte-string
Capability attributes use only the 'att-field:att-value' form.
The new attributes may be referenced in potential configurations
("a=pcfg") or in latent configurations ("a=lcfg"), as productions
conforming to the extension-config-list as defined in [RFC5939].
extension-config-list = ["+"] ext-cap-name "="
ext-cap-list
ext-cap-name = 1*(ALPHA / DIGIT)
ext-cap-list = 1*VCHAR ; defined in [RFC5234]
The optional "+" is used to indicate that the entire configuration,
not just the parameter, must be ignored if the parameter is not
supported. The attributes may be referenced in actual configurations
as productions conforming to the sel-extension-config defined in
[RFC5939].
sel-extension-config = ext-cap-name "=" 1*VCHAR
The specific parameters are defined in the individual description of
each capability, below.
It is not the intention of this work to negotiate these new
capabilities at the session level, rather only at the media level.
Therefore, capabilities referenced by any configuration attribute
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MUST appear at the media level when a configuration is "converted" to
a corresponding media block. For this reason, the "icap" attribute
is called the "media information capability". Specific values for
each new attribute are described below.
3.1.1. Bandwidth Capability
According to RFC 4566 [RFC4566] the bandwidth field denotes the
proposed bandwidth to be used by the session or media. In this memo,
we specify the bandwidth at the media level. The bandwidth field is
specified in RFC 4566 [RFC4566] with the following syntax:
b=<bwtype>:<bandwidth>
where <bwtype> is an alphanumeric modifier giving the meaning of the
<bandwidth> figure.
In this document, we define a new capability attribute: the bandwidth
capability attribute "bcap". This attribute lists bandwidth as
capabilities according to the following definition:
"a=bcap:" bw-cap-num 1*WSP bwtype ":" bandwidth CRLF
where <bw-cap-num> is a unique ordinal identifier of the bandwidth
capability, and the other elements are as defined for the "b=" field
in [RFC4566].
This format satisfies the general attribute production rules in
[RFC4566] according to the following Augmented Backus-Naur Form
(ABNF) [RFC5234] syntax:
att-field = "bcap"
att-value = bw-cap-num 1*WSP bwtype ":" bandwidth
bw-cap-num = 1*DIGIT ; integer between 1 and 2^31-1, inclusive
Negotiation of bandwidth per media stream can be useful when
negotiating media encoding capabilities with different bandwidths.
3.1.1.1. Configuration Parameters
The SDP capability negotiation framework [RFC5939] provides for the
existence of the "pcfg" and "acfg" attributes, which can carry one or
more potential configurations to be negotiated. The concept is
extended by the the SDP media capabilities negotiation
[I-D.ietf-mmusic-sdp-media-capabilities] with an "lcfg" attribute
that conveys latent configurations.
Extensions to the "pcfg" and "lcfg" attributes are defined through
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<extension-config-list>, and extensions to the "acfg" attribute are
defined through the <sel-extension-config> as defined in [RFC5939].
In this document we extend the <extension-config-list> field to be
able to convey lists of bandwidth capabilities in latent or potential
configurations, according to the following Augmented Backus-Naur Form
(ABNF) [RFC5234] syntax:
extension-config-list = bandwidth-config-list
bandwidth-config-list = ["+"] "b=" bw-cap-list *(BAR b-cap-list)
bw-cap-list = bw-cap-num *("," b-cap-num)
bw-cap-num = 1*DIGIT ; 1 to 2^32-1 inclusive
Figure 1: Syntax of the bandwidth parameter in lcfg and pcfg
attributes
Each bandwidth capability configuration is a comma-separated list of
bandwidth capability attribute numbers where 'b-cap-num' refers to
the bw-cap-num bandwidth capability numbers defined explicitly
earlier in this document, and hence must be between 1 and 2^31-1
(both included). Alternative bandwidth configurations are separated
by a vertical bar ("|").
The bandwidth parameter to the actual configuration attribute
("a=acfg") is formulated as a sel-extension-config with
ext-cap-name = "b"
hence
sel-extension-config = sel-bandwidth-config
sel-bandwidth-config = "b=" bw-cap-list ; bw-cap-list as above.
Figure 2: Syntax of the bandwidth parameter in acfg attributes
3.1.1.2. Option tag
The SDP capability negotiation framework [RFC5939] allows for
capability negotiation extensions to be defined. Associated with
each such extension is an option tag that identifies the extension in
question. Hereby, we define a new option tag "bcap-v0" that
identifies support for the bandwidth capability. This option tag
SHOULD be added to other existing option tags present in the "csup"
and "creq" attributes in SDP, according to the procedures defined in
the SDP Capability Negotiation Framework [RFC5939].
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3.1.2. Information Capability
RFC 4566 [RFC4566] provides for the existence of an information field
expressed in the format of the "i=" line, which can appear either at
the session level or at the media level. An "i=" line that is
present at the session level is known as the "session name", and its
purpose is to convey a human-readable textual information about the
session. We don't see much usage of capabilities related to the "i="
line at the session level.
The "i=" line in SDP can also appear at the media level, in which
case it is used to provide human-readable information about the media
stream to which it is related, e.g., it may indicate the purpose of
the media stream. The information field is not to be confused with
the label attribute ("a=label:"), [RFC4574]) which provides a
machine-readable tag. It is foreseen that applications declaring
capabilities related to different configurations of a media stream
may need to provide different identifying information for each of
those configurations. That is, a party might offer alternative media
configurations for a stream, each of which represents a different
presentation of the same or similar information. For example, an
audio stream might offer English or Spanish configurations, or a
video stream might offer a choice of video source such as speaker
camera, group camera, or document viewer. The information capability
is needed to inform the answering user in order to select the proper
choice, and the label is used to inform the offering machine which
choice the answerer has selected. Hence, there is value in defining
a mechanism to provide information of media streams as capabilities.
According to SDP [RFC4566], the media label has the following syntax:
"i="text
where "text" represents a human-readable text indicating the purpose
of the media stream.
In this document we define a new capability attribute: the
information media capability, "icap". This attribute lists
information media labels as capabilities, according to the following
definition:
"a=icap:" info-cap-num 1*WSP text
where <info-cap-num> is the unique ordinal identifier of the
particular media information capability and <text> is a human-
readable text that indicates the purpose of the media stream it is
supposed to characterize.
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As an example, one might use:
a=icap:1 Document Camera
to represent a purpose of a media stream identified with the
capability number 1.
The media information capability attribute satisfies the general
attribute production rules in [RFC4566] according to the following
Augmented Backus-Naur Form (ABNF) [RFC5234] syntax:
att-field = "icap"
att-value = info-cap-num 1*WSP text
; text is defined in RFC 4566
info-cap-num = 1*DIGIT ; integer between 1 and 2^31-1
3.1.2.1. Configuration Parameters
The SDP Capability Negotiation Framework [RFC5939] provides for the
existence of the "pcfg" and "acfg" attributes, which can carry one or
more potential configurations to be negotiated. The concept is
extended by the the SDP media capabilities negotiation
[I-D.ietf-mmusic-sdp-media-capabilities] with an "lcfg" attribute
that conveys latent configurations.
In this document, we define an <info-config-list> parameter to be
used to convey information capabilities in a potential or latent
configuration. This parameter is defined as an <extension-config-
list> with the following associations:
ext-cap-name = "i"
ext-cap-list = info-cap-list
This leads to the following definition for the information capability
parameter:
extension-config-list = info-config-list
info-config-list = "i=" info-cap-list
info-cap-list = info-cap-num *(BAR info-cap-num); BAR defined in RFC5939
info-cap-num = 1*DIGIT ; 1 to 2^32-1 inclusive
Figure 3: Syntax of the information capability parameter in lcfg and
pcfg attributes
Each potential capability configuration contains a single information
capability attribute number where 'info-cap-num' is the information
capability number defined explicitly earlier in this document, and
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hence must be between 1 and 2^31-1 (both included). The information
capability allows the expression of only a single capability in each
alternative, since no more than a single information field is
permitted per media block. Nevertheless, it is still allowed to
express alternative potential information configurations separated by
a vertical bar ("|").
3.1.2.2. Option Tag
At present, it is difficult to envision a scenario in which the
'icap' attribute must be supported or the offer must be rejected. In
most cases, if the icap attribute or its contents were to be ignored,
an offered configuration could still be chosen based on other
criteria such as configuration numbering. However, one might imagine
an SDP offer that contained English and Spanish potential
configurations for an audio stream. The session might be
unintelligible if the choice is based on configuration numbering,
rather than informed user selection. Based on such considerations,
it may well prove useful to announce the ability to use the icap
attribute and its contents to select media configurations, or to
inform the user about the selected configuration(s). Therefore, we
define a new option tag of "icap-v0" that identifies support for the
media information capability. This option tag SHOULD be added to
other existing option tags present in the "csup" and/or "creq"
attributes in SDP, according to the procedures defined in the SDP
Capability Negotiation Framework [RFC5939]. The discusion above
suggests that "icap-v0" will typically appear in a "csup" attribute,
but rarely in a "creq" attribute.
3.2. Session Level versus Media Level
The icap and bcap attributes can appear at the session level and/or
at the media level, but MUST be interpreted as a media-level
capability. To avoid confusion, the <type-attr-num> for each line
must be unique across all capability attributes of the same type
within the entire session description. As described below, these
capability attributes may be referenced by acfg, pcfg and/or lcfg
attributes.
4. Field Replacement Rules
To simplify the construction of SDP records, given the need to
include fields at the base level for endpoints that do not support
capabilities negotiation, we define some simple field-replacement
rules for those fields invoked by potential or latent configurations.
In particular, any i-field invoked by a configuration MUST replace
the corresponding field, if present at the base media level. Any
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b-field invoked by a configuration MUST replace any b-field of the
same bandwidth type at the media level.
5. IANA Considerations
5.1. New SDP Attributes
IANA is hereby requested to register the following new SDP
attributes:
Attribute name: icap
Long form name: Information Capability
Type of attribute: Media-level
Subject to charset: Yes
Purpose: Negotiate human-readable media information
Appropriate values: See Section 3.1.2
Attribute name: bcap
Long form name: Bandwidth Capability
Type of attribute: Media-level
Subject to charset: No
Purpose: Negotiate media-level bandwidths
Appropriate values: See Section 3.1.1
5.2. New Option Tags
IANA is hereby requested to add the new option tags "bcap-v0" and
"icap-v0", defined herein, to the SDP Capability Negotiation Option
Tag Registry.
5.3. New SDP Capability Negotiation Configuration Parameters
IANA is hereby requested to add the new parameter identifiers "i" for
"information" and "b" for "bandwidth" to the Potential Configuration
Parameter Registry. These parameters are permitted in 'lcfg',
'acfg', and 'pcfg' attributes.
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6. Security Considerations
This document provides an extension on top of RFC 4566 [RFC4566], RFC
3264 [RFC3264], SDP Capability Negotiation Framework [RFC5939], and
SDP media capabilities negotiation
[I-D.ietf-mmusic-sdp-media-capabilities]. As such, the security
considerations of those documents apply.
7. Acknowledgments
Thanks to Christer Holmberg, Alf Heidermark, and Ingemar Johansson
for arguing for the existence of this document and early reviewing
it.
8. References
8.1. Normative References
[I-D.ietf-mmusic-sdp-media-capabilities]
Gilman, R., Even, R., and F. Andreasen, "SDP media
capabilities Negotiation",
draft-ietf-mmusic-sdp-media-capabilities-10 (work in
progress), July 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.
8.2. Informative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4574] Levin, O. and G. Camarillo, "The Session Description
Protocol (SDP) Label Attribute", RFC 4574, August 2006.
Authors' Addresses
Miguel A. Garcia-Martin
Ericsson
Calle Via de los Poblados 13
Madrid, 28033
Spain
Phone: +34 91 339 1000
Email: miguel.a.garcia@ericsson.com
Simo Veikkolainen
Nokia
P.O. Box 407
NOKIA GROUP, FI 00045
Finland
Phone: +358 50 486 4463
Email: simo.veikkolainen@nokia.com
Robert R. Gilman
3243 W. 11th Ave. Dr.
Broomfield, Colorado 80020
U.S.A.
Phone: +1 303 898 9780
Email: bob_gilman@comcast.net
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