MMUSIC Working Group F. Andreasen
Internet Draft Cisco Systems
Expires: July 2007 January 28, 2007
SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-01.txt
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Abstract
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. SDP was
not intended to provide capability indication or capability
negotiation, however over the years, SDP has seen widespread adoption
and as a result it has been gradually extended to provide limited
support for these. SDP and its current extensions however do not have
the ability to negotiate one or more alternative transport protocols
(e.g. RTP profiles) which makes it particularly difficult to deploy
new RTP profiles such as secure RTP or RTP with RTCP-based feedback.
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The purpose of this document is to address that and other real-life
limitations by extending SDP with capability negotiation parameters
and associated offer/answer procedures to use those parameters in a
backwards compatible manner.
The solution provided in this document provides a general SDP
capability negotiation framework. It also defines specifically how to
provide attributes and transport protocols as capabilities and
negotiate them using the framework. Extensions for other types of
capabilities (e.g. media types and formats) may be provided in other
documents.
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 [RFC2119].
Table of Contents
1. Introduction...................................................3
2. SDP Capability Negotiation Solution............................5
2.1. Solution Overview.........................................5
2.2. Version and Extension Indication Attributes...............8
2.2.1. Supported Capability Negotiation Extensions Attribute8
2.2.2. Required Capability Negotiation Extension Attribute.10
2.3. Capability Attributes....................................11
2.3.1. Attribute Capability Attribute......................11
2.3.2. Transport Protocol Capability Attribute.............13
2.4. Configuration Attributes.................................13
2.4.1. Potential Configuration Attribute...................13
2.4.2. Actual Configuration Attribute......................17
2.5. Offer/Answer Model Extensions............................18
2.5.1. Generating the Initial Offer........................18
2.5.2. Generating the Answer...............................19
2.5.3. Offerer Processing of the Answer....................20
2.5.4. Modifying the Session...............................20
3. Examples......................................................21
3.1. Best-Effort Secure RTP...................................21
4. Security Considerations.......................................23
5. IANA Considerations...........................................23
6. To Do and Open Issues.........................................23
7. Acknowledgments...............................................23
8. Change Log....................................................24
8.1. draft-ietf-mmusic-sdp-capability-negotiation-01..........24
8.2. draft-ietf-mmusic-sdp-capability-negotiation-00..........24
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9. References....................................................26
9.1. Normative References.....................................26
9.2. Informative References...................................26
Author's Addresses...............................................28
Intellectual Property Statement..................................28
Disclaimer of Validity...........................................29
Copyright Statement..............................................29
Acknowledgment...................................................29
1. Introduction
The Session Description Protocol (SDP) was intended for describing
multimedia sessions for the purposes of session announcement, session
invitation, and other forms of multimedia session initiation. The SDP
contains one or more media stream descriptions with information such
as IP-address and port, type of media stream (e.g. audio or video),
transport protocol (possibly including profile information, e.g.
RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other
session and media stream parameters that define the session.
Simply providing media stream descriptions is sufficient for session
announcements for a broadcast application, where the media stream
parameters are fixed for all participants. When a participant wants
to join the session, he obtains the session announcement and uses the
media descriptions provided, e.g., joins a multicast group and
receives media packets in the encoding format specified. If the
media stream description is not supported by the participant, he is
unable to receive the media.
Such restrictions are not generally acceptable to multimedia session
invitations, where two or more entities attempt to establish a media
session that uses a set of media stream parameters acceptable to all
participants. First of all, each entity must inform the other of its
receive address, and secondly, the entities need to agree on the
media stream parameters to use for the session, e.g. transport
protocols and codecs. We here make a distinction between the
capabilities supported by each participant, the way in which those
capabilities can be supported and the parameters that can actually be
used for the session. More generally, we can say that we have the
following:
o A set of capabilities for the session and its associated media
stream components, supported by each side.
o A set of potential configurations indicating which of those
capabilities can be used for the session and its associated media
stream components.
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o A set of actual configurations for the session and its associated
media stream components, which specifies which session parameters
to use and which media stream components to use and with what
parameters.
o A negotiation process that takes the set of potential
configurations (lists of capabilities) as input and provides the
actual configurations as output.
SDP by itself was designed to provide only one of these, namely the
actual configurations, however over the years, use of SDP has been
extended beyond its original scope. Session negotiation semantics
were defined by the offer/answer model in RFC 3264. It defines how
two entities, an offerer and an answerer, exchange session
descriptions to negotiate a session. The offerer can include one or
more media formats (codecs) per media stream, and the answerer then
selects one or more of those offered and returns them in an answer.
Both the offer and the answer contain actual configurations -
capabilities and potential configurations are not supported. The
answer however may reduce the set of actual configurations from the
offer. The answer may also extend the set of actual configurations
that can be used to receive media by the answerer.
Other relevant extensions have been defined. Simple capability
declarations, which define how to provide a simple and limited set of
capability descriptions in SDP was defined in RFC 3407. Grouping of
media lines, which defines how media lines in SDP can have other
semantics than the traditional "simultaneous media streams"
semantics, was defined in RFC 3388, etc.
Each of these extensions was designed to solve a specific limitation
of SDP. Since SDP had already been stretched beyond its original
intent, a more comprehensive capability declaration and negotiation
process was intentionally not defined. Instead, work on a "next
generation" of a protocol to provide session description and
capability negotiation was initiated [SDPng]. SDPng however has not
gained traction and has remained as work in progress for an extended
period of time. Existing real-time multimedia communication
protocols such as SIP, RTSP, Megaco, and MGCP continue to use SDP.
SDP and its current extensions however do not address an increasingly
important problem: the ability to negotiate one or more alternative
transport protocols (e.g., RTP profiles). This makes it difficult to
deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with
RTCP-Based Feedback [AVPF], etc. This particular problem is
exacerbated by the fact that RTP profiles are defined independently.
When a new profile is defined and N other profiles already exist,
there is a potential need for defining N additional profiles, since
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profiles cannot be combined automatically. For example, in order to
support the plain and secure RTP version of RTP with and without
RTCP-based feedback, four separate profiles (and hence profile
definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP [SRTP], RTP/AVPF
[AVPF], and RTP/SAVPF [SAVPF]. In addition to the pressing profile
negotiation problem, other important real-life constraints have been
found as well.
The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating capabilities and their
associated potential configurations and negotiate the use of those
potential configurations as actual configurations. It is not the
intent to provide a full-fledged capability indication and
negotiation mechanism along the lines of SDPng or ITU-T H.245.
Instead, the focus is on addressing a set of well-known real-life
limitations. More specifically, the solution provided in this
document provides a general SDP capability negotiation framework. It
also defines specifically how to provide attributes and transport
protocols as capabilities and negotiate them using the framework.
Extensions for other types of capabilities (e.g. media types and
formats) may be provided in other documents.
As mentioned above, SDP is used by several protocols, and hence the
mechanism should be usable by all of these. One particularly
important protocol for this problem however is the Session Initiation
Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is
not specific to SIP) to negotiate sessions and hence any mechanism
must at least consider how it either interacts with offer/answer, or
how it should extend it.
The rest of the document is structured as follows. In Section 2. we
present our SDP capability negotiation solution followed by examples
in Section 3. and security considerations in Section 4.
2. SDP Capability Negotiation Solution
In this section we first provide an overview of the SDP Capability
negotiation solution. This is followed by definitions of new SDP
attributes for the solution and its associated updated offer/answer
procedures.
2.1. Solution Overview
The solution consists of the following:
o Two new attributes to support versioning and extensions to the
framework itself as follows:
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o A new attribute ("a=csup") that lists the supported base and
extension options to the framework.
o A new attribute ("a=creq") that lists the base and or
extensions to the framework that are required to be supported
by the entity receiving the SDP.
o Two new attributes used to express capabilities as follows
(additional attributes can be defined as extensions):
o A new attribute ("a=capar") that defines how to list attribute
parameter values ("a=" values) as capabilities.
o A new attribute ("a=ctrpr") that defines how to list transport
protocols (e.g. "RTP/AVP") as capabilities.
o Two new attributes to negotiate configurations as follows:
o A new attribute ("a=pcfg") that lists the potential
configurations supported. This is done by reference to the
capabilities from the SDP in question. Multiple potential
configurations have an explicitly indicated ordering
associated with them. Extension capabilities can be defined
and included in the potential configurations.
o A new attribute ("a=acfg") to be used in an answer SDP. The
attribute identifies which of the potential configurations
from an offer SDP were used as actual configurations to form
the answer SDP. Extension capabilities can included.
o Extensions to the offer/answer model that allow for capabilities
and potential configurations to be included in an offer. When
included at the session level, they constitute latent capabilities
that may be used to guide a subsequent offer. When included at the
media level, they constitute offers that may be accepted by the
answerer instead of the actual configuration(s) included in the
"m=" line(s). The answerer indicates which (if any) of the
potential configurations it used to form the answer by including
the actual configuration attribute ("a=acfg") in the answer.
Capabilities and potential configurations may be included in
answers as well, where they can aid in guiding a subsequent new
offer.
The mechanism is illustrated by the offer/answer exchange below,
where Alice sends an offer to Bob:
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Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (RTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=creq: v0
a=ctrpr:1 RTP/SAVP
a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg:1 t=1 a=1
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The required base and extensions are provided by the
"a=creq" attribute, which indicates that the option tag "v0", which
indicates the base framework defined here, must be supported. The
capabilities are provided by the "a=ctrpr" and "a=capar" attributes.
The capabilities indicate that secure RTP under the AVP profile
("RTP/SAVP") is supported with an associated transport capability
handle of 1. The "capar" attribute provides an attribute capability
with a handle of 1. The attribute capability is a "crypto" attribute,
which provides the keying material for SRTP using SDP security
descriptions [SDES]. The "a=pcfg" attribute provides the potential
configuration included in the offer by reference to the capability
parameters. One alternatives is provided; it has a preference of 1
and it consists of transport protocol capability 1 (i.e. the RTP/SAVP
profile - secure RTP), and the attribute capability 1, i.e. the
crypto attribute provided. Potential configurations are always
preferred over actual configurations, and hence Alice is expressing a
preference for using secure RTP.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation framework, and hence he accepts the
(preferred) potential configuration for Secure RTP provided by Alice:
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v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=acfg: t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute. If Bob supported one or more extensions to the
capability negotiation framework, he would have included those in the
answer as well.
Note that in this particular example, the answerer supported the
capability negotiation extensions defined here, however had he not,
the answerer would simply have ignored the new attributes and
accepted the offer to use normal RTP. In that case, the following
answer would have been generated instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/AVP 0 18
2.2. Version and Extension Indication Attributes
In this section, we present the new attributes associated with
indicating the SDP capability negotiation version and extensions
supported and required.
2.2.1. Supported Capability Negotiation Extensions Attribute
The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined
in Section 5.
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The Supported Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions supported by the entity that
generated the SDP. The attribute is defined as follows:
a=csup: <option-tag-list>
where <option-tag-list> is defined by the following ABNF:
option-tag-list = option-tag *(COMMA option-tag)
option-tag = token ; defined in [SDP]
COMMA = *WSP "," *WSP ; defined in [RFC4234]
White-space is permitted before the <option-tag-list>.
Implementers familiar with SIP should note that the above
definition of COMMA differs from the one in [RFC3261].
A special base option tag with a value of "v0" is defined for the
basic SDP capability negotiation framework specified in this
document. Entities can use this option tag with the "a=csup"
attribute to indicate support for the SDP capability negotiation
framework specified in this document.
The following examples illustrates the use of the "a=csup" attribute
with two hypothetical option tags, "foo" and "bar":
a=csup: foo
a=csup: bar
a=csup: foo, bar
The "a=csup" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media-stream
in question only (option-tags provided at the session level apply as
well). There can be one or more "a=csup" attributes at both the
session and media-level (one or more per media stream in the latter
case).
Whenever an entity that supports one or more extensions to the SDP
Capability Negotiation framework generates an SDP, it SHOULD include
the "a=csup" attribute with the option tags for the extensions it
supports at the session and/or media-level, unless those option tags
are already provided in one or more "a=creq" attribute (see Section
2.2.2. ) at the relevant levels. The base option tag MAY be included.
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2.2.2. Required Capability Negotiation Extension Attribute
The SDP Capability negotiation solution allows for capability
negotiation extensions to be defined. Associated with each such
extension is an option tag that identifies the extension in question.
Option-tags MUST be registered with IANA per the procedures defined
in Section 5.
The Required Capability Negotiation Extensions attribute ("a=csup")
contains a comma-separated list of option tags identifying the SDP
Capability negotiation extensions that MUST be supported by the
entity receiving the SDP in order for that entity to properly process
the SDP Capability negotiation. The attribute is defined as follows:
a=creq: <option-tag-list>
where <option-tag-list> is defined in Section 2.2.1.
White-space is permitted before the <option-tag-list>.
The following examples illustrate the use of the "a=creq" attribute
with two hypothetical option tags, "foo" and "bar":
a=creq: foo
a=creq: bar
a=creq: foo, bar
The "a=creq" attribute can be provided at the session and the media-
level. When provided at the session-level, it applies to the entire
SDP. When provided at the media-level, it applies to the media-stream
in question only (required option tags provided at the session level
apply as well). There can be one or more "a=creq" attributes at both
the session and media-level (one or more per media stream in the
latter case).
When an entity generates an SDP and it requires the recipient of that
SDP to support one or more SDP capability negotiation extensions in
order to properly process the SDP Capability negotiation, the
"a=creq" attribute MUST be included with option-tags that identify
the required extensions at the session and/or media level, unless it
is already known that the receiving entity supports those option-tags
at the relevant levels (in which case their inclusion is OPTIONAL).
An example of this is when generating an answer to an offer. If the
answerer supports the required option-tags from the offer, and the
answerer does not require any additional option-tags beyond what
was listed in either the required ("creq") or supported ("csup")
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attributes from the offer, then the answerer is not required to
include a required ("creq") attribute with any option-tags that may
need to be supported (such as the base option tag - "v0").
A recipient that receives an SDP and does not support one or more of
the required extensions listed in a "creq" attribute, MUST NOT
perform the SDP capability negotiation defined in this document. For
non-supported extensions provided at the session-level, this implies
that SDP capability negotiation MUST NOT be performed at all. For
non-supported extensions at the media-level, this implies that SDP
capability negotiation MUST NOT be performed for the media stream in
question.
When an entity does not support one or more required SDP capability
negotiation extensions, the entity SHOULD proceed as if the SDP
capability negotiation attributes were not included in the first
place, i.e. all the capability negotiation attributes should be
ignored.
This ensures that introduction of the SDP capability negotiation
mechanism does not introduce any new failure scenarios.
The above rules apply to the base option tag as well. Thus, entities
compliant to this specification MUST include a "creq" attribute (at
least in an offer) that includes the option tag "v0" as illustrated
below:
a=creq: v0
2.3. Capability Attributes
In this section, we present the new attributes associated with
indicating the capabilities for use by the SDP Capability
negotiation.
2.3.1. Attribute Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new
attribute capability attribute ("a=capar"), which is defined as
follows:
a=capar: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 2^32-1 (both
included) used to number the attribute capability and <att-par> is an
attribute ("a=") in its full '<type>=<value>' form (see [SDP]).
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The "capar" attribute can be provided at the session level for
session-level attributes and the media level for media-level
attributes. The "capar" attribute MUST NOT be used to provide a
media-level attribute at the session-level or vice versa.
Each occurrence of the "capar" attribute in the entire session
description MUST use a different value of <app-cap-num>.
There is a need to be able to reference both session-level and
media-level attributes in potential configurations at the media
level, and this provides for a simple solution to avoiding overlap
between the handle references.
The <att-cap-num> values provided are independent of similar <cap-
num> values provided for other attributes, i.e., they form a separate
name-space for attribute parameter capabilities.
The following examples illustrate use of the "capar" attribute:
a=capar: 1 a=ptime:20
a=capar: 2 a=ptime:30
a=capar: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA
AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0
JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO
SrzKTAv9zV
a=capar: 4 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
The first two provide attribute values for the ptime attribute. The
third one provides SRTP parameters by using MIKEY with the key-mgmt
attribute [KMGMT]. The fourth one provides SRTP parameters by use of
security descriptions with the crypto attribute [SDES].
Readers familiar with RFC 3407 may notice the similarity between
the RFC 3407 "cpar" attribute and the above. There are however a
couple of important differences, most notably that the "capar"
attribute contains a handle that enables referencing it and it
furthermore supports attributes only (the "cpar" attribute defined
in RFC 3407 supports bandwidth information as well). The "capar"
attribute also is not automatically associated with any particular
capabilities.
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2.3.2. Transport Protocol Capability Attribute
Transport Protocols can be expressed as capabilities by use of a new
Transport Protocol Capability attribute ("a=ctrpr") defined as
follows:
a=ctrpr: <trpr-cap-num> <proto-list>
where <trpr-cap-num> is an integer between 1 and 2^32-1 (both
included) used to number the transport address capability for later
reference, and <proto-list> is one or more <proto>, separated by
white space, as defined in the SDP "m=" line.
The "ctrpr" attribute can be provided at the session- and media-
level. Each occurrence of the "ctrpr" attribute in the entire session
description MUST use a different value of <trpr-cap-num>. When
multiple <proto> values are provided, the first one is associated
with the value <trpr-cap-num>, the second one with the value one
higher, etc. The <trpr-cap-num> values provided are independent of
similar <cap-num> values provided for other attributes, i.e., they
form a separate name-space for transport protocol capabilities.
Below, we provide examples of the "a=ctrpr" attribute:
a=ctrpr: 1 RTP/AVP
a=ctrpr: 2 RTP/AVPF
a=ctrpr: 3 RTP/SAVP RTP/SAVPF
The first one provides a capability for the "RTP/AVP" profile defined
in [RFC3551] and the second one provides a capability for the RTP
with RTCP-Based Feedback profile defined in [AVPF]. The third one
provides capabilities for the "RTP/SAVP" and "RTP/SAVPF" profiles.
2.4. Configuration Attributes
2.4.1. Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <preference> <pot-cfg-list>
where <preference> is an integer between 1 and 2^32-1 (both included)
and <pot-cfg-list> is defined as
pot-cfg-list = pot-config *(1*WSP pot-config)
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pot-config = pot-attribute-parameter-config |
pot-transport-protocol-config |
pot-extension-config
The potential configuration attribute includes a preference
indication (lowest number is most preferred) followed by one or more
of potential attribute parameter configuration and transport protocol
configuration. Each of these MUST NOT be present more than once in a
particular potential configuration attribute. Potential extension
configurations can be included as well. There can be more than one
potential extension configuration, however each particular potential
extension configuration MUST NOT be present more than once in a given
potential configuration attribute. Together, these values define a
potential configuration.
There can be multiple potential configurations provided at the
session-level as well as the media-level. The semantics for each of
these levels differ. A potential configuration at the session level
provides a set of latent capabilities. A latent capability is merely
an indication that the potential configuration could be supported,
however it does not represent a willingness to do so at the current
time. A potential configuration at the media level on the other hand
indicates not only a willingness, but in fact a desire to use the
potential configuration.
In the case of offer/answer, this implies that a potential
configuration at the session level does not constitute an
alternative offer whereas it does at the media level.
Associated with each potential configuration is a preference
indication, which is an integer between 1 and 2^32-1 (both included)
to indicate the relative preference of potential configurations. The
scope of the preference (and in fact each occurrence of a potential
configuration attribute) is the session-level, when provided there,
or the particular media stream it is provided at.
Attribute capabilities are included in a potential configuration by
use of the pot-attribute-parameter-config parameter, which is defined
by the following ABNF:
pot-attribute-parameter-config
= "a=" capar-cap-list *(BAR capar-cap-list)
capar-cap-list = att-cap-num *(COMMA att-cap-num)
att-cap-num = 1*DIGIT ;defined in [RFC4234]
Each potential attribute parameter configuration list is a comma-
separated list of attribute capability numbers where att-cap-num
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refers to attribute capability numbers defined above and hence MUST
be between 1 and 2^32-1 (both included). Alternative potential
attribute parameter configurations are separated by a vertical bar
("|"), the scope of which extends to the next alternative (i.e. ","
has higher precedence than "|"). The alternatives are ordered by
preference.
Transport protocol capabilities are included in a potential
configuration by use of the pot-transport-protocol-config parameter,
which is defined by the following ABNF:
pot-transport-protocol-config =
"t=" trpr-cap-num *(BAR trpr-cap-num)
trpr-cap-num = 1*DIGIT ; defined in [RFC4234]
The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 2^32-1 (both included).
Alternative potential transport protocol configurations are separated
by a vertical bar ("|"). The alternatives are ordered by preference.
When transport protocol capabilities are not included in a potential
configuration at the media level, the transport protocol information
from the associated "m=" line will be used. At the session-level,
lack of a transport protocol capability indication simply implies
that no such information is provided.
Extension capabilities can be included in a potential configuration
as well. Such extensions MUST adhere to the following ABNF:
pot-extension-config = ext-cap-name "="
ext-cap-list *(BAR ext-cap-list)
ext-cap-name = token ; defined in [SDP]
ext-cap-list = ext-cap-num *(COMMA ext-cap-num)
ext-cap-num = 1*DIGIT ; defined in [RFC4234]
The ext-cap-name refers to the type of extension capability and the
ext-cap-num refers to a capability number associated with that
particular type of extension capability. The number MUST be between
1 and 2^32-1 (both included). Alternative potential extension
configurations for a particular extension are separated by a vertical
bar ("|"),the scope of which extends to the next alternative (i.e.
"," has higher precedence than "|"). Unsupported or unknown
potential extension configs MUST be ignored.
The "creq" attribute and its associated rules can be used to ensure
that required extensions are supported in the first place.
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Potential configurations can be provided at the session level and the
media level and in either case, it is syntactically possible to
reference attribute capabilities provided at either the session or
the media level. There are however semantic rules and limitations
associated with this: At the session-level, a potential configuration
MUST NOT reference any attribute capabilities provided at the media-
level. The converse however is permitted, i.e. a media-level
potential configuration can reference a session-level attribute
capability. The semantics of doing so (should that potential
configuration be chosen), is that the corresponding attribute
(provided within that attribute capability) will be considered part
of the active configuration at the *session* level. In other words,
it will be as-if that attribute was simply provided at the session-
level in the first place. Note that individual media streams perform
capability negotiation individually, and hence it is possible that
another media stream (where the attribute was part of a potential
configuration) chose a configuration without that session level
attribute. The session-level attribute however remains "active" and
hence applies to the entire session. It is up to the entity that
generates the SDP to ensure that the resulting active configuration
SDP is still meaningful.
[EDITOR'S NOTE: There are too many subtle differences between
potential configurations at the session and media level. I'm
inclined to have two similarly looking (but different) attributes
instead as that will make it more straightforward and intuitive.
That also leaves the door open to have more than latent
capabilities at the session level in case that is needed later]
Below, we provide an example of the "a=pcfg" attribute in a complete
media description in order to properly indicate the supporting
attributes:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVPF 0 18
a=creq: v0
a=capar:1 crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=ctrpr: 1 RTP/AVPF RTP/AVP
a=ctrpr: 3 RTP/SAVPF RTP/SAVP
a=pcfg:1 t=3|4 a=1
a=pcfg:8 t=1|2
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We have two potential configurations listed here. The first one (and
most preferred, since it's preference is "1") indicates that either
of the profiles RTP/SAVPF or RTP/SAVP (specified by the transport
protocol capability numbers 3 and 4) can be supported with attribute
capability 1 (the "crypto" attribute); RTP/SAVPF is preferred since
it is listed first. The second potential configuration indicates that
the RTP/AVPF of RTP/AVP profile can be used, with RTP/AVPF being the
preferred one. This non secure RTP alternative is the less preferred
one since it's preference is "8".
2.4.2. Actual Configuration Attribute
The actual configuration attribute identifies which of the potential
configurations from an offer SDP were used as actual configurations
in an answer SDP. This is done by reference to the attribute
capabilities and transport protocol capabilities from the offer that
were actually used by the answerer in his offer/answer procedure. If
extension capabilities were used, those will be included by reference
as well.
The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <act-cfg-list>
where <act-cfg-list> is defined as
act-cfg-list = capability *(1*WSP capability)
capability = act-attribute-parameter-config |
act-transport-protocol-config |
act-extension-config
act-attribute-parameter-config is defined by the following ABNF:
act-attribute-parameter-config = "a=" capar-cap-list
where capar-cap-list is as defined in Section 2.4.1.
act-transport-protocol-config is defined by the following ABNF:
act-transport-protocol-config = "t=" trpr-cap-num
where trpr-cap-num is as defined in Section 2.4.1.
trpr-cap-num = 1*3DIGIT ; defined in [RFC4234]
act-extension-config is defined by the following ABNF:
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act-extension-config = ext-cap-name "=" ext-cap-list
where ext-cap-name and ext-cap-list are as defined in Section 2.4.1.
The actual configuration ("a=acfg") attribute can be provided at the
media-level only. There MUST NOT be more than one occurrence of an
actual configuration attribute within a given media description.
Below, we provide an example of the "a=acfg" attribute (building on
the previous example with the potential configuration attribute):
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVPF 0
a=creq: 0
a=acfg: t=3 a=1
It indicates that the answerer used an offer consisting of transport
protocol capability 2 from the offer (RTP/SAVPF) and attribute
capability 1 (the "crypto" attribute.
2.5. Offer/Answer Model Extensions
In this section, we define extensions to the offer/answer model
defined in [RFC3264] to allow for potential configurations to be
included in an offer, where they constitute offers that may be
accepted by the answerer instead of the actual configuration(s)
included in the "m=" line(s).
[EDITOR'S NOTE: Multicast considerations have been omitted for
now.]
TO DO: Elaborate and firm up offer/answer procedures.
2.5.1. Generating the Initial Offer
An offerer that wants to use the SDP capability negotiation
extensions defined in this document MUST include the following in the
offer:
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o an SDP capability negotiation required extensions attribute ("a-
creq") that contains the option tag "v0". It must either be
provided at the session-level or for each individual media stream.
Option tags for any other required extensions MUST be included as
well (in accordance with Section 2.2.2. )
o one or more attribute capability attributes (as defined in Section
2.3.1. ) if alternative attribute parameter values are to be
indicated as offerer capabilities or be negotiated.
o one or more transport protocol capability attributes (as defined
in Section 2.3.2. ) if alternative transport protocols are to be
to be indicated as offerer capabilities or be negotiated.
o one or more potential configuration attributes (as defined in
Section 2.4. ) if alternative potential configurations are to be
negotiated.
o one or more required capability negotiation extension attributes
(as defined in Section 2.2.2. ), if the answerer is required to
support one or more SDP capability negotiation extensions.
The offerer SHOULD furthermore include the following:
o one or more supported capability negotiation extension attributes
("a=csup" as defined in Section 2.2.1. ), if the offerer supports
one or more SDP capability negotiation extensions that have not
been included in one or more "a=creq" attributes at the relevant
session and media level(s).
The capabilities provided merely indicate what the offerer is capable
of doing. They do not constitute a commitment or even an indication
to actually use them. This applies to potential configurations listed
at the session level as well. Conversely, each of the potential
configurations listed at the media level constitutes an alternative
offer which may be used to negotiate and establish the session.
The current actual configuration is included in the "m=" line (as
defined by [RFC3264]).
2.5.2. Generating the Answer
When the answerer receives an offer with valid SDP capability
negotiation information in it and in particular with one or more
valid potential configuration information attributes present, it may
use any of the potential configurations as an alternative offer. A
potential configuration information attribute is valid if all of the
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capabilities (attribute capabilities, transport protocol capabilities
and any extension capabilities) it references are present and valid
themselves.
The actual configuration is contained in the media description's "m="
line. The answerer can send media to the offerer in accordance with
the actual configuration, however if it chooses to use one of the
alternative potential configurations, media sent to the offerer may
be discarded by the offerer until the answer is received.
If the answerer chooses to accept one of the alternative potential
configurations instead of the actual configuration, the answerer MUST
generate an answer as if the offer contained that potential
configuration instead of the actual configuration included. The
answerer MUST also include an actual configuration attribute in the
answer that identifies the potential configuration from the offer
used by the answerer. The actual configuration attribute in the
answer MUST include information about the attribute capabilities,
transport protocol parameters, and extension capabilities from the
potential configuration that were used to generate the answer.
2.5.3. Offerer Processing of the Answer
When the offerer included potential configurations for a media
stream, it MUST examine the answer for the presence of an actual
configuration attribute for each such media stream. If the attribute
is missing, offerer processing of the answer MUST proceed as defined
by [RFC3264]. If the attribute is present, processing continues as
follows:
The actual configuration attribute specifies which of the potential
configurations were used by the answerer to generate the answer. This
includes all the types of capabilities from the potential
configuration offered, i.e. the attribute capabilities ("a=capar"),
transport protocol capabilities ("a=ctrpr"), and any extension
capability parameters included.
The offerer MUST now process the answer as if the offer had contained
the potential configuration as the actual configuration in the media
description ("m=" line) and relevant attributes in the offer.
2.5.4. Modifying the Session
Potential configurations may be included in subsequent offers as
defined in [RFC3264, Section 8]. The procedure for doing so is
similar to that described above with the answer including an
indication of the actual configuration used by the answerer.
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3. Examples
In this section, we provide examples showing how to use the SDP
Capability Negotiation.
3.1. Best-Effort Secure RTP
The following example illustrates how to use the SDP Capability
negotiation extensions to support so-called Best-Effort Secure RTP.
In that scenario, the offerer supports both RTP and Secure RTP. If
the answerer does not support secure RTP (or the SDP capability
negotiation extensions), an RTP session will be established. However,
if the answerer supports Secure RTP and the SDP Capability
Negotiation extensions, a Secure RTP session will be established.
The best-effort Secure RTP negotiation is illustrated by the
offer/answer exchange below, where Alice sends an offer to Bob:
Alice Bob
| (1) Offer (SRTP and RTP) |
|--------------------------------->|
| |
| (2) Answer (RTP) |
|<---------------------------------|
| |
Alice's offer includes RTP and SRTP as alternatives. RTP is the
default, but SRTP is the preferred one:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=creq: v0
a=ctrpr:1 RTP/SAVP RTP/AVP
a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
FEC_ORDER=FEC_SRTP
a=pcfg:5 t=1 a=1
a=pcfg:10 t=2
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. Alice indicates that support for the base protocol
defined here is required by including the "a=creq" attribute
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containing the value "v0". The capabilities are provided by the
"a=ctrpr" and "a=capar" attributes. The capabilities indicate that
both Secure RTP and normal RTP are supported. The "capar" attribute
provides a capability parameter with a handle of 1. The capability
parameter is a "crypto" attribute in the capability set, which
provides the keying material for SRTP using SDP security descriptions
[SDES]. The "a=pcfg" attribute provides the potential configurations
included in the offer by reference to the capabilities. Two
alternatives are provided; the first one with preference "5" (and
hence the preferred one since the preference on the second one is
"10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP)
together with the attribute capability 1, i.e. the crypto attribute
provided. The second one is using transport protocol capability 2.
Note that we could have omitted the second potential configuration
since it equals the actual configuration (which is always the least
preferred configuration).
Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP
Capability Negotiation extensions, and hence he accepts the potential
configuration for Secure RTP provided by Alice:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/SAVP 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_80
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4
a=csup: foo
a=acfg: t=1 a=1
Bob includes the "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with transport protocol capability 1 and attribute
capability 1 from the offer SDP (i.e. the RTP/SAVP profile using the
keying material provided). Bob also includes his keying material in
a crypto attribute. Finally, Bob supports an SDP capability
negotiation extension with the option tag "foo" and hence he includes
the "a=csup" parameter containing value "foo" in the answer.
Note that in this particular example, the answerer supported the
capability extensions defined here, however had he not, the answerer
would simply have ignored the new attributes and accepted the offer
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to use normal RTP. In that case, the following answer would have been
generated instead:
v=0
o=- 24351 621814 IN IP4 128.96.41.2
s=
c=IN IP4 128.96.41.2
t=0 0
m=audio 4567 RTP/AVP 0 18
4. Security Considerations
TBD.
5. IANA Considerations
TBD.
[EDITOR'S NOTE: Need to define registry and procedures for option
tags]
[EIDTOR'S NOTE: Need to define registry and procedures for extension
capabilities]
6. To Do and Open Issues
o Capability descriptions, potential configurations and actual
configurations can be provided at both the session level and media
level. It needs to be decided what the relationship between the
session level and media level parameters are.
o Look for "EDITOR'S NOTE" throughout the document.
7. Acknowledgments
This document is heavily influenced by the discussions and work done
by the SDP Capability Negotiation Design team. The following people
in particular provided useful comments and suggestions to either the
document itself or the overall direction of the solution defined in
here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg
Ott, Colin Perkins, and Thomas Stach.
Francois Audet and Dan Wing provided useful comments on earlier
versions of this document.
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8. Change Log
8.1. draft-ietf-mmusic-sdp-capability-negotiation-01
The following are the major changes compared to version -00:
o Media capabilities are no longer considered a core capability and
hence have been removed. This leaves transport protocols and
attributes as the only capabilities defined by the core.
o Version attribute has been removed and an option tag to indicate
the actual version has been defined instead.
o Clarified rules for session-level and media level attributes
provided at either level as well how they can be used in potential
configurations.
o Potential configuration parameters no longer have implicit
ordering; an explicit preference indicator is now included.
o The parameter name for transport protocols in the potential and
actual configuration attributes have been changed "p" to "t".
o Clarified operator precedence within potential and actual
configuration attributes.
o Potential configurations at the session level now limited to
indicate latent capability configurations. Consequently, an actual
configuration attribute can no longer be provided at the session
level.
o Cleaned up capability and potential configuration terminology -
they are now two clearly different things.
8.2. draft-ietf-mmusic-sdp-capability-negotiation-00
Version 00 is the initial version. The solution provided in this
initial version is based on an earlier (individual submission)
version of [SDPCapNeg]. The following are the major changes compared
to that document:
o Solution no longer based on RFC 3407, but defines a set of similar
attributes (with some differences).
o Various minor changes to the previously defined attributes.
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o Multiple transport capabilities can be included in a single
"ctrpr" attribute
o A version attribute is now included.
o Extensions to the framework are formally supported.
o Option tags and the ability to list supported and required
extensions are supported.
o A best-effort SRTP example use case has been added.
o Some terminology change throughout to more clearly indicate what
constitutes capabilities and what constitutes configurations.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for
Syntax Specifications: ABNF", RFC 2234, Internet Mail
Consortium and Demon Internet Ltd., November 1997.
[RFC3264] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple
Capability Declaration", RFC 3407, October 2002.
[RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October
2003.
[RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
9.2. Informative References
[RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 2327, April 1998.
[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.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
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[RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3551, July
2003.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851, July
2004.
[RFC4091] Camarillo, G., and J. Rosenberg, The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework, RFC 4091, June 2005.
[AVPF] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF)",
Work in Progress, August 2004.
[I-D.jennings-sipping-multipart] Wing, D., and C. Jennings, "Session
Initiation Protocol (SIP) Offer/Answer with Multipart
Alternative", Work in Progress, March 2006.
[SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for
RTCP-based Feedback (RTP/SAVPF)", Work in Progress,
December 2005.
[SDES] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol Security Descriptions for Media
Streams", RFC 4568, July 2006.
[SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session Description
and Capability Negotiation", Work in Progress, February
2005.
[BESRTP] Kaplan, H., and F. Audet, "Session Description Protocol
(SDP) Offer/Answer Negotiation for Best-Effort Secure Real-
Time Transport Protocol, Work in progress, August 2006.
[KMGMT] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
RFC 4567, July 2006.
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[SDPCapNegRqts] Andreasen, F. "SDP Capability Negotiation:
Requirementes and Review of Existing Work", work in
progress, December 2006.
[SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in
progress, December 2006.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
Author's Addresses
Flemming Andreasen
Cisco Systems
Edison, NJ
Email: fandreas@cisco.com
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Andreasen Expires July 28, 2007 [Page 28]
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Disclaimer of Validity
This document and the information contained herein are provided on an
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Andreasen Expires July 28, 2007 [Page 29]
