One document matched: draft-ietf-mmusic-sdp-capability-negotiation-00.txt
MMUSIC Working Group F. Andreasen
Internet Draft Cisco Systems
Expires: July 2007 January 2, 2007
SDP Capability Negotiation
draft-ietf-mmusic-sdp-capability-negotiation-00.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.
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. SDP Capability Negotiation Version Attribute.........8
2.2.2. Supported Capability Negotiation Extensions Attribute9
2.2.3. Required Capability Negotiation Extension Attribute.10
2.3. Capability Attributes....................................11
2.3.1. Media Type and Format Capability Attribute..........11
2.3.2. Attribute Parameter Capability Attribute............14
2.3.3. Transport Protocol Capability Attribute.............15
2.4. Configuration Attributes.................................16
2.4.1. Potential Configuration Attribute...................16
2.4.2. Actual Configuration Attribute......................19
2.5. Offer/Answer Model Extensions............................20
2.5.1. Generating the Initial Offer........................21
2.5.2. Generating the Answer...............................22
2.5.3. Offerer Processing of the Answer....................22
2.5.4. Modifying the Session...............................23
3. Examples......................................................23
3.1. Best-Effort Secure RTP...................................23
4. Security Considerations.......................................25
5. IANA Considerations...........................................25
6. To Do and Open Issues.........................................26
7. Acknowledgments...............................................26
8. Change Log....................................................26
8.1. draft-ietf-mmusic-sdp-capability-negotiation-00..........26
9. References....................................................27
9.1. Normative References.....................................27
9.2. Informative References...................................27
Author's Addresses...............................................29
Intellectual Property Statement..................................29
Disclaimer of Validity...........................................30
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Copyright Statement..............................................30
Acknowledgment...................................................30
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 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 and potential configurations of the media
stream components, supported by each side.
o A set of actual configurations of the media stream components,
which specifies which media stream components to use and with what
parameters.
o A negotiation process that takes the set of potential
configurations (capabilities) as input and provides the actual
configurations as output.
SDP by itself was designed to provide only the second of these, i.e.,
the actual configurations, however over the years, use of SDP has
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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 -
potential configurations are not supported. The answer however may
reduce the set of actual configurations from the offer.
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
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
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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.
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 0we
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 Three new attributes to support versioning and extensions to the
framework itself as follows:
o A new attribute ("a=cver") that lists the version of the SDP
capability negotiation framework being used.
o A new attribute ("a=csup") that lists the supported extensions
to the framework.
o A new attribute ("a=creq") that lists the extensions to the
framework that are required to be supported by the entity
receiving the SDP.
o Three new attributes used to express capabilities as follows
(additional attributes can be defined as extensions):
o A new attribute ("a=cmed") that defines how to list the media
types and media formats supported as capabilities.
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o A new attribute ("a=capar") that defines how to list the
attribute parameter values as capabilities.
o A new attribute ("a=ctrpr") that defines how to list transport
protocols 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
above capabilities from the SDP in question. The potential
configurations are listed in order of preference. Extensions
can be defined as well 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.
o Extensions to the offer/answer model that allow for capabilities
and 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). The answerer indicates which (if any) of the potential
configurations it used to form the answer by including the actual
configuration attribute ("a=cfg") 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:
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
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s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=cver:0
a=cmed:1 audio RTP/AVP 0 18 96
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=capar:2 a=rtpmap:96 iLBC/8000
a=pcfg: m=1,2|3 p=1 a=1,2
a=pcfg: m=1,2|3 a=2
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capabilities are provided by the "a=cver",
"a=cmed", "a=ctrpr" and "a=capar" attributes. The capabilities
indicate that PCMU, G.729 and iLBC are supported with either RTP or
secure RTP. The first "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 second "capar"
attribute provides the "rtpmap" for the dynamic payload type 96,
which is mapped to the iLBC codec. The "a=pcfg" attribute provides
the potential configurations included in the offer by reference to
the capability declarations. Two alternatives are provided; the
first one, and hence the preferred one is using media capabilities 1
and 2, i.e. PCMU and G.729, or media capability 3, i.e. iLBC.
Furthermore, transport protocol capability 1 (i.e. the RTP/SAVP
profile - secure RTP), and the attribute capability parameter 1, i.e.
the crypto attribute provided, and the attribute capability parameter
2, i.e. the rtpmap for iLBC is included. The second one is simply
using media capabilities 1 and 2, i.e. PCMU and G.729, or media
capability 3, i.e. iLBC under the RTP/AVP profile as listed in the
"m=" line. The "capar" parameter is still needed to provide the
rtpmap for iLBC.
Bob receives the SDP offer from Alice. Bob supports RTP, but not
SRTP, and hence he accepts the potential configuration for RTP
provided by Alice. Furthermore, Bob wants to use the iLBC codec and
hence generates the following answer:
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 96
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a=rtpmap:96 iLBC/8000
a=cver: 0
a=acfg: m=3 a=2
Bob includes the "a=cver" and "a=acfg" attribute in the answer to
inform Alice that he based his answer on an offer containing the
potential configuration with media capability 3 from the offer SDP
(i.e. iLBC under the RTP/AVP profile) and the attribute capability
parameter 2, i.e. the associated rtpmap. Note that in this
particular example, the answerer supported the capability extensions
defined here, however had he not, he would simply have processed the
offer based on the offered PCMU and G.729 codecs under the RTP/AVP
profile only. Consequently, the answer would have omitted the
"a=cver" and "a=acfg" attribute line and chosen one or both of the
PCMU and G.729 codecs instead.
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. SDP Capability Negotiation Version Attribute
The SDP Capability Negotiation Version attribute ("a=cver") lists the
version of the SDP Capability Negotiation supported by the entity
that generated the SDP. The attribute is defined as follows:
a=cver: <version>
where <version> is a non-zero positive integer. White space is
permitted, but not required, before <version>. The value of <version>
defined by this document is 0 as illustrated by the following
example:
a=cver: 0
The SDP Capability Negotiation version attribute MUST be present in
each SDP that uses the SDP Capability negotiation solution defined in
this document. The attribute can be provided at either the session-
or media-level, however there MUST NOT be more than one occurrence of
it. Furthermore, the attribute SHOULD be the first of the SDP
capability negotiation attributes provided.
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2.2.2. 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.
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 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].
[EDITOR'S NOTE: There's nothing specific to the SDP Capability
Negotiation Solution for this parameter. Should consider
generalizing and/or providing in a separate 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. 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
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the "a=csup" attribute with the option tags for the extensions it
supports.
2.2.3. 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 2.2.2.
White-space is permitted before the <option-tag-list>.
[EDITOR'S NOTE: There's nothing specific to the SDP Capability
Negotiation Solution for this parameter. Should consider
generalizing and/or providing in a separate document.]
The following examples illustrates 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. 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).
Whenever 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.
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A recipient that receives such an SDP and does not support one or
more of the required extensions, 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 MSUT 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.
This ensures that introduction of the SDP capability negotiation
mechanism does not introduce any new failure scenarios.
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. Media Type and Format Capability Attribute
Media types and media formats can be expressed as capabilities by use
of the "a=cmed" attribute, which is defined as follows:
a=cmed: <med-cap-num> <media> [<proto> <fmt list>]
where <med-cap-num> is an integer between 1 and 2^32-1 (both
included) used to number the media capabilities, and <media>,
<proto>, and <fmt list> are defined as in the SDP "m=" line. The <fmt
list> may contain multiple media formats. In that case, the media
format capability number associated with the first one provided is
the value of <med-cap-num>, the number associated with the second one
is one higher, etc. Each occurrence of the attribute MUST use a
different value of <med-cap-num>. Furthermore, when a "cmed"
attribute indicates more than one media format, the capability
numbers implied MUST NOT be used by any other "cmed" attribute in the
session description (explicitly or implicitly). When <proto> and
<fmt list> are omitted, the media capability merely indicates support
for the <media> type in question, without any details as to what kind
of transport protocol and media formats are supported. This can for
example be used to indicate support for additional types of media
than those included as actual configurations in an offer or answer.
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A media capability merely indicates possible support for the media
type and media format(s) in question. In order to actually use a
media capability in an offer/answer exchange, it must be referenced
in a potential configuration (see Section 2.4.1.
Media capabilities can be provided at the session-level and the
media-level. Media capabilities provided at the session level apply
to the session description in general, whereas media capabilities
provided at the media level apply to that media stream only. In
either case, the scope of the <med-cap-num> is the entire session
description. This enables each media capability to be referenced
across the entire session description (e.g. in a potential
configuration - see Section 2.4.1.
[EDITOR'S NOTE: This is clear as mud. If a media capability applies
to a media-stream only, then why can it still be referenced and
hence used as capabilities in other media streams (by the "a=pcfg")
attribute. The motivation is message size efficiency, but the means
are not clean. Session versus media-level syntax and semantics need
further consideration]
The <proto> parameter indicates the default transport protocol
associated with the media capability. As described in [RFC4566], the
value of the <proto> parameter guides the interpretation of the <fmt
list>, which is why it is included here. Note that <proto> is also a
capability that can be negotiated separately (see Section 2.3.3.
The <fmt list> contains one or more media formats supported, the
interpretation of which depends on the value of <proto>. The rules
that apply to "m=" lines (as defined in [SDP]) for interpretation of
these apply here as well. For RTP-based transports, this implies
that the <fmt list> contains one or more RTP payload type numbers.
When those payload type numbers are dynamic, SDP requires an
"a=rtpmap" attribute to determine the actual codec. In the case of
SDP capability negotiation, such additional attribute parameters MUST
be provided in conjunction with the media capability. There are two
different cases to consider for this:
o The media capability is provided at the session level: In this
case, the required parameters MUST be provided in one or more
attribute parameter capabilities (see Section 2.3.2. listed
before the first "m=" line as well as before any other media
capability attributes ("a=cmed").
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o The media capability is provided at the media stream level: In
this case, when the payload type numbers are part of the "m=" line
itself, this is done by use of the "a=rtpmap" attribute as usual.
In all other cases, the required parameters MUST be provided in
one or more attribute parameter capabilities (see Section 2.3.2.
within the media stream description (i.e. before the next "m="
line).
[EDITOR'S NOTE: The above assumes that intermediaries will not
reorder session-level attributes. It would be safer to explicitly
link the two, but that will require yet another attribute. Also, it
sends us down the path of building more complicated capabilities
(made up of multiple parameters). Another issue here is that
payload type numbers, which really have only media-level scope, are
ill-suited to be used at the session-level or across multiple media
streams (as is being done here). However, an alternative (and more
proper) solution seems to involve significantly more work and
deviations from the current SDP framework. This in turn makes it
more difficult to automatically use new media types, formats,
protocols, etc. defined elsewhere within this framework, and that
is a major disadvantage. Another option is to forgo the session-
level media capabilities as well as the ability to reference across
media streams - it will make the solution less efficient though and
difficult to express latent capabilities for media streams not
included in the offer or answer.]
The following example illustrates the first case above:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
a=cmed: 1 audio RTP/AVP 96
a=capar: 1 a=rtpmap:96 G729/8000
m=audio...
The following example illustrates the second case above:
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 2345 RTP/AVP 96
a=rtpmap:96 G729/8000
a=cver:0
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a=cmed: 1 audio RTP/AVP 96 97
a=capar: 1 a=rtpmap:97 iLBC/8000
Note: Readers familiar with RFC 3407 may notice the similarity
between the "cmed" attribute defined above and the "cdsc" attribute
defined in RFC 3407. There are however a couple of important
differences, namely an increase in the capability numbering space
as well as a relaxation of certain requirements found in RFC 3407.
To simplify overall operation, the "cmed" parameter is limited to
media-level operation only as well.
2.3.2. Attribute Parameter Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new
attribute parameter 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 parameter capability and <att-
par> is an attribute ("a=") in its full '<type>=<value>' form (see
[SDP]).
The "capar" attribute can be provided at the session level and the
media level. Each occurrence of the attribute MUST use a different
value of <app-cap-num>. 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.
Attribute parameter capabilities are generally used for two things.
First of all, they may be necessary to interpret a media format
capability (e.g. by including an rtpmap), or they may provide
attribute value parameters that are referenced in potential
configurations (see Section 2.4.1. )
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
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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.
2.3.3. 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 255 (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 attribute 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
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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.
Note that the "cmed" attribute provides a similar functionality by
including <proto>, however having this as a separate capability
indication can provide significant message size reduction when
negotiating alternative profiles (of which there can be many). In
particular, there is no need to repeat supported payload types. Also,
use of this attribute combined with the potential configuration
attribute (see Section 2.4. ) provides for more expressive power.
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: <pot-cfg-list>
where <pot-cfg-list> is defined as
pot-cfg-list = pot-config *(1*WSP pot-config)
pot-config = pot-media-config |
pot-attribute-parameter-config |
pot-transport-protocol-config |
pot-extension-config
The potential configuration attribute includes one or more sets of
potential media configurations, attribute parameter configurations
and transport protocol configurations. 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 set of potential configurations.
There can be one or more potential configuration attributes provided
at the session-level as well as for each media stream. The attributes
are provided in order of preference.
[EDITOR'S NOTE: We run into another issue with session-level media
capabilities here. In the offer/answer model, potential
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configurations at the media-level constitute alternative offers,
however at the session-level, that would/should not be the case for
media capabilities. Other parameters however may be used as
alternative offers at the session level (e.g. key-mgmt attributes
at the session level)]
pot-media-config is defined by the following ABNF:
pot-media-config = "m=" med-cap-list *(BAR med-cap-list)
med-cap-list = med-cap-num *(COMMA med-cap-num)
med-cap-num = 1*DIGIT ; defined in [RFC4234]
BAR = *WSP "|" *WSP ; defined in [RFC4234]
Each potential media configuration is a comma-separated list of media
capability numbers where med-cap-num refers to media capability
numbers and hence MUST be between 1 and 2^32-1 (both included).
Alternative potential media configurations are separated by a
vertical bar ("|"). The alternatives are ordered by preference. When
media capabilities are not included in a potential configuration at
the media level, the media type and media format from the associated
"m=" line will be used.
pot-attribute-parameter-config 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 parameter numbers where att-
cap-num refers to attribute parameter 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 alternatives are ordered by
preference.
pot-transport-protocol-config is defined by the following ABNF:
pot-transport-protocol-config =
"p=" 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.
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When transport protocol capabilities are not included in a potential
configuration, the transport protocol information from an included
potential media configuration will be used. If a potential media
configuration is not included, the transport protocol from the media
description ("m=" line) will be used instead.
pot-extension-config is defined by 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 ("|"). Unsupported or unknown potential extension configs MUST
be ignored, unless an option tag showing the extension as being
required was included (see Section 2.2.3.
The potential configuration ("a=pcfg") attribute can be provided at
the session level and the media-level. Each occurrence of the
attribute within a given media description ("m=" line) defines a set
of potential configurations that can be used for that media
description.
TO DO: Need to decide on relationship between session-level and
media-level (how should conflicts, overlap, etc. be handled -
simplicity at the possible expense of expressive power is
preferable in the editor's opinion).
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/SAVPF 0 18
a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=cver: 0
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a=cmed: 1 audio RTP/SAVP 0 4 18
a=ctrpr: 1 RTP/AVP RTP/AVPF
a=ctrpr: 3 RTP/SAVP RTP/SAVPF
a=pcfg: m=1|3 p=1|2|3|4
a=pcfg: m=2 p=1
We have two potential configurations listed here. The first one
indicates that PCMU (payload type number 0 referenced by media
capability number 1) or G.729 (payload type number 18 referenced by
media capability number 3) can be supported with either of the
profiles RTP/AVP, RTP/AVPF, RTP/SAVP, or RTP/SAVPF (specified by the
transport protocol capability numbers 1, 2, 3 and 4). The second
potential configuration indicates that G.723 (payload type number 4
referenced by media capability number 2) can be supported with the
RTP/AVP profile only (transport protocol capability number 1).
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 media
capabilities, attribute parameter 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-media-config |
act-attribute-parameter-config |
act-transport-protocol-config |
act-extension-config
act-media-config is defined by the following ABNF:
act-media-config = "m=" med-cap-list
where med-cap-list is as defined in Section 2.4.1.
act-attribute-parameter-config is defined by the following ABNF:
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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 = "p=" 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:
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
session-level and the media-level. There MUST NOT be more than one
occurrence of an actual configuration attribute at the session level,
and 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/AVPF 0
a=cver: 0
a=acfg: m=1 p=2
It indicates that the answerer used an offer consisting of media
capability 1 from the offer (PCMU) and transport protocol capability
2 from the offer (RTP/AVPF).
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).
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[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:
o an SDP capability negotiation version attribute with the version
set to 0
o one or more media capabilities (as defined in Section 2.3.1. ), if
alternative media types and media formats are to be indicated as
offerer capabilities or be negotiated.
o one or more attribute parameter capability attributes (as defined
in Section 2.3.2. ) 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 2.3.3. ) 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 2.2.3. ), 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
(as defined in Section 2.2.2. ), if the offerer supports one or
more SDP capability negotiation extensions.
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. Conversely, each of the potential
configurations listed constitutes an alternative offer which may be
used to negotiate and establish the session.
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[EDITOR'S NOTE: This is only partially true for potential
configurations listed at the session level. The only thing we want
to offer up as alternative offers at the session level is
attributes - not media types or media formats, which should be
capabilities only at the session level]
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
capabilities (media, attribute capabilities, transport protocol 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 media capabilities,
attribute capability parameters, 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:
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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 media formats ("cmed" capabilities),
attribute capability parameters ("capar"), transport protocol
capabilities ("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.
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:
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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=cver:0
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: p=1 a=1
a=pcfg: p=2
The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The capability declaration is provided by the
"a=cver", "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, and
hence the preferred one is transport protocol capability 1 (RTP/SAVP,
i.e. secure RTP) together with the attribute capability parameter 1,
i.e. the crypto attribute provided. The second one is using transport
protocol capability 2. Since there are no media format capabilities
included, the media format parameters from the media description
itself is used.
Bob receives the SDP offer from Alice. Bob supports SRTP and the SCP
Capability Negotiation extensions, and hence he accepts the 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=cver: 0
a=acfg: p=1 a=1
Bob includes the "a=cver" and "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 parameter 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.
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
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]
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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
Thanks to Francois Audet and Dan Wing for comments on earlier
versions of this document.
8. Change Log
8.1. 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.
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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Disclaimer of Validity
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Acknowledgment
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Internet Society.
Andreasen Expires July 2, 2007 [Page 30]
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