MMUSIC Working Group F. Andreasen
Internet Draft Cisco Systems
Expires: April 2007 October 20, 2006
SDP Capability Negotiation
draft-andreasen-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 and RTP with RTCP-based feedback.
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The purpose of this document is to address that by identifying a set
of requirements, evaluate existing work in this area, and provide a
recommended solution for extending SDP with capability negotiation.
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. Requirements...................................................5
3. Review of Existing Work........................................7
3.1. Grouping of Media Lines...................................8
3.2. Session Description Protocol (SDP) Simple Capability
Declaration....................................................9
3.3. Session Description and Capability Negotiation (SDPng)...10
3.4. Multipart/alternative....................................12
3.5. Sharing Ports Between "m=" Lines.........................13
3.6. Opportunistic Encryption Using a Session Attribute.......14
3.7. Best-Effort Secure Real-Time Transport Protocol..........14
3.8. Opportunistic Encryption using Probing...................15
4. Proposed Solution.............................................15
4.1. Solution Overview........................................15
4.2. Extensions to Simcap.....................................17
4.3. Attribute Definitions....................................18
4.3.1. The Attribute Parameter Capability Attribute........18
4.3.2. The Transport Protocol Capability Attribute.........19
4.3.3. The Potential Configuration Attribute...............20
4.3.4. The Actual Configuration Attribute..................22
4.4. Offer/Answer Model Extensions............................24
4.4.1. Generating the Initial Offer........................24
4.4.2. Generating the Answer...............................24
4.4.3. Offerer Processing of the Answer....................25
4.4.4. Modifying the Session...............................25
5. Examples......................................................26
6. Security Considerations.......................................26
7. IANA Considerations...........................................26
8. To Do and Open Issues.........................................26
9. Acknowledgments...............................................26
10. Change Log...................................................26
10.1. Changes since -00.......................................26
11. References...................................................28
11.1. Normative References....................................28
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11.2. Informative References..................................28
Author's Addresses...............................................30
Intellectual Property Statement..................................30
Disclaimer of Validity...........................................30
Copyright Statement..............................................30
Acknowledgment...................................................31
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 using 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, or 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.
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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
been extended beyond its original scope. Session negotiation
semantics was defined by the offer/answer model in RFC 3264. It
defines how two entities, an offerer and an answerer, exchange SDPs
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 defines 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.
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The purpose of this document is to define a mechanism that enables
SDP to provide limited support for indicating potential
configurations (capabilities) 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.
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. We first provide a
set of requirements for the solution in Section 2. In Section 3. we
review relevant existing work in this area, how a solution based on
that might look, and the pros and cons associated with each. In
Section 4. we present our proposed solution in more detail followed
examples in Section 5. and security considerations in Section 6.
2. Requirements
REQ-10: It MUST be possible to indicate and negotiate alternative
media formats on a per media stream basis.
For example, many implementations support multiple codecs, but
only one at a time. Changes between codecs cannot be done on-
the-fly, e.g. when receiving a simple RTP payload type change.
REQ-20: It MUST be possible to indicate and negotiate alternative
attribute values ("a=") on a per media stream basis.
For example, T.38 defines new attributes that may need to be
conveyed as part of a capability.
REQ-25: It MUST be possible to indicate and negotiate alternative
attribute values ("a=") at the session level.
REQ-30: It MUST be possible to indicate and negotiate alternative
media format parameter values ("a=fmtp") per media format on a per
media stream basis.
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For example, a media format (codec) indicated as an alternative
capability may include fmtp parameters.
REQ-40: It MUST be possible to indicate and negotiate alternative
transport protocols, e.g. different RTP profiles, on a per media
stream basis.
For example, "RTP/AVP" and "RTP/SAVP" may be alternatives.
REQ-50: It MUST be possible to indicate and negotiate alternative
transport protocol and media type combinations on a per media stream
basis.
For example, an entity may support a fax call using either T.38
fax relay ("m=image <port> udptl t38") or PCMU ("m=audio <port>
RTP/AVP 0").
REQ-80: The mechanism MUST be backwards compatible for SIP. Ideally,
the mechanism should be completely transparent to entities that do
not support it, without the need for any further signaling.
REQ-90: The mechanism MUST either be backwards compatible for Megaco
and MGCP or it MUST be possible to interwork it with Megaco and MGCP
without any additional signaling between the MGC and its peer (e.g.
another SIP UA as opposed to a media gateway).
For example, if a media gateway controller (MGC) uses SIP to
communicate with peers, and the MGC uses Megaco or MGCP to
control a media gateway, it must be possible to translate between
the mechanism and normal SDP. Avoiding interworking requirements
in the MGC is desirable.
REQ-100: The mechanism MUST work within the context of the
offer/answer model [RFC3264]. Specifically, it MUST be possible to
negotiate alternatives within a single offer/answer exchange.
REQ-110: The offer/answer model requires the offerer to be able to
receive media for any media streams listed as either "recvonly" or
"sendrecv" in an offer, as soon as that offer is generated. The
mechanism MUST preserve this capability for all actual configurations
included in an offer.
Potential configurations do not have such a requirement.
REQ-120: The mechanism MUST enable inclusion of potential
configurations (alternative capabilities) in the offer - the answer
would then indicate which, if any of these potential configurations
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were accepted. The offerer is not required to process media for a
specific potential configuration until the offerer receives an answer
showing that potential configuration was accepted.
Note that this implies that it may not be possible for the
offerer to process early media generated using a potential
configuration (as opposed to the actual configuration) until the
answer has been received.
REQ-130: The mechanism MUST work in the presence of SIP forking.
REQ-140: The mechanism SHOULD be reasonably efficient in terms of
overall message size.
This is a relative requirement to evaluate alternative solutions
as opposed to an absolute and quantifiable requirement. Use of
compression techniques can help reduce the size of text-based
messages, however it is still considered important to try and
keep the message size reasonably small.
Above, we presented the requirements for the capability negotiation
mechanism. Below, we provide a set of features that were considered
and then explicitly deemed to be out-of-scope:
o Indication of mandatory and optional capabilities.
o Constraints on combinations of configurations, e.g. inability to
use a video codec together with a particular audio codec,
parameter X values that can only be used with parameter Y values,
etc.
o Support for negotiation of unicast and multicast addresses as
alternatives. It was suggested as a requirement initially, but
subsequent discussion led to its removal.
o Support for negotiation of IPv4 and IPv6 addresses as
alternatives. It was suggested as a requirement initially, but
subsequent discussion let to its removal.
3. Review of Existing Work
In this section, we provide an overview of existing relevant work
that has either been completed or is work in progress. For each
item, we outline how/if it can be used to address the requirements
provided and the pros and cons of doing so.
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3.1. Grouping of Media Lines
Grouping of Media Lines is defined in [RFC3388]. RFC 3388 defines a
framework that enables two or media lines to be grouped together for
different purposes. Each media line is assigned an identifier and one
or more group attributes then references two or more of those
identifiers. Associated with each group attribute is a semantics
indication. One semantic indication is the Alternative Network
Address Types ("ANAT") [RFC4091] which allows for IPv4 and IPv6
addresses to be specified as alternatives. The requirements presented
above go beyond that, however a new semantic to simply indicate
alternative media lines and associated negotiation rules could easily
be defined.
The main advantages of such an approach would be:
o Mechanism Reuse: Several semantics have already been defined
which increases the likelihood of an implementation supporting the
framework.
The disadvantages of such an approach would be:
o Backwards Compatibility: The mechanism is not transparently
backwards compatible. If an entity that does not support the
mechanism receives it, the entity may incorrectly interpret the
SDP as consisting of multiple media streams. While RFC 3388
defines procedures for recognizing and recover from this when
using offer/answer, it can still lead to unintended behavior with
endpoints that do not support the mechanism.
In practice, it is not clear how much of an issue this is, at
least for intelligent SIP endpoints. Most current
implementations generally accept only one media stream of a
given type (e.g. audio). Use of alternatives with different
media stream types (e.g. a fax call using "audio" for voice-
band data or "image" for T.38) makes it less clear though.
Also, Media Gateway Controllers and Media Gateways that do not
support grouping of media lines have been known to encounter
problems.
o Semantics Combination Issues: Multiple semantics may be provided
by use of grouping, however they may interact with each other
unintentionally. For example, the "FID" semantics defined in RFC
3388 forbids grouping of media lines with the same transport
address, however that would be needed for alternative
capabilities. Thus, using "FID" and alternative capabilities
together would require special consideration.
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o Some Combinatoric Explosion: The mechanism is not ideal to
indicate alternative capabilities for multiple parameters or media
formats within a particular media stream. For example, alternative
attribute values and media format parameters for several codecs
would lead to combinatoric explosion.
[Editor's note: In practice, it is not clear this is a huge issue
though.]
o Message Size: Each alternative requires full duplication of all
the relevant media stream parameters.
[Editor's note: In practice, it is not clear this is a huge issue
though.]
3.2. Session Description Protocol (SDP) Simple Capability Declaration
SDP Simple Capability Declaration (simcap) is defined in [RFC3407].
It defines a set of SDP attributes that enables capabilities to be
described at a session level or on a per media stream basis. RFC
3407 defines capability declaration only - actual negotiation
procedures taking advantage of such capabilities have not been
defined. Such rules however could easily be defined - the negotiation
part would extend the offer/answer model to examine alternative
configurations (capabilities). In conjunction with that, attributes
to indicate the alternative configurations accepted would likely be
needed as well.
The main advantages of this approach are:
o Satisfies all the requirements provided above. In particular, by
relying solely on SDP attributes, transparent backwards
compatibility is always ensured.
The disadvantages of this approach are:
o Offered Capabilities Hidden in Attributes: An offer may be
accepted by the answerer and a media stream established based on
SDP parameters contained in SDP attributes not known to
intermediaries. Such intermediaries may be back-to-back user
agents, or proxies that need to inspect the SDP, e.g., to
authorize Quality of Service, add transcoders, etc.
o Maximum of 255 alternative media formats per SDP: RFC 3407
currently allows a maximum of 255 alternative media formats
(codecs) per SDP. This may be too restrictive.
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3.3. Session Description and Capability Negotiation (SDPng)
The Session Description and Capability Negotiation protocol [SDPng]
was intended as a replacement for SDP [SDP]. SDPng includes a full
capability indication and negotiation framework that would address
the shortcomings of SDP and satisfy the requirements provided above.
However, SDPng has not gained traction, in large part due to existing
widespread adoption of SDP. As a consequence, SDPng has remained as
work in progress with limited progress for an extended period of
time.
SDPng consists of two things: an SDPng description, which is an XML
document that describes the actual and/or potential configurations as
well as an optional negotiation process (an offer/answer compliant
process is included as part of SDPng). The SDPng description consists
of up to five parts:
o Capabilities: An optional list of capabilities (potential
configurations) to be matched with the other parties'
capabilities.
o Definitions: An optional set of definitions of commonly used
parameters for later referencing.
o Configurations: A mandatory description of the conference
components, each of which can provide a list of alternative
configurations.
o Constraints: An optional set of constraints of combinations
of configurations. Constraints are not defined as part of the
base SDPng specification.
o Session Information: Optional meta information on the
conferences and individual components.
SDPng is application-agnostic with the base specification defining a
basic XML schema supporting the above. In order to actually use
SDPng, application-specific packages are needed. Packages define
things such as media types, codecs and their configuration
parameters, etc. The base SDPng specification includes a couple of
example packages to support audio, video, and RTP.
One approach to extending SDP with capability indication and
negotiation capabilities could be to adopt the mechanisms defined by
SDPng that are necessary to satisfy the requirements provided above.
Those areas could then be included within SDP itself, e.g. in the
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form of one or more SDP attributes ("a=") containing the actual SDPng
description. The areas to consider here include:
o Capabilities: This would be needed to describe alternative media
formats and media format parameters.
o Configurations: This would be needed to define alternative
configurations
The constraints and session information parts of SDPng would not be
used.
The main advantages of such an approach would be:
o SDPng was designed and intended to solve the general capability
negotiation problems faced by SDP. A considerable amount of work
has already gone into it and it was originally targeted as the
long-term direction (replacement for SDP).
The disadvantages of such an approach would be:
o Duplicate Encoding and Specification Work: SDPng uses a
different coding format than SDP and hence all SDP parameters
(incl. codecs and transports) that need to be provided will need
to have an equivalent SDPng definition. There is currently no
automatic process for translating all SDP parameters or values
into corresponding SDPng parameters or values; many existing SDP
parameters and values currently have no corresponding SDPng
definition.
o SDPng is Work in Progress: SDPng is currently work in progress but
has seen limited interest and progress for a while. Adoption of a
subset of its current definition may end up differing from the
final specification. Also, the current SDPng specification needs
further clarification and semantic tightening in a number of areas
that would be of relevance to this approach.
o Negotiation of Transport Parameters: SDPng currently does not
support negotiation of transport parameters as individual
capabilities. It is however still possible to negotiate different
transport parameters by providing alternative configurations.
o Verbose Encoding and Large Message Size: SDPng descriptions are
XML documents, which are fairly verbose and result in descriptions
that are substantially larger than existing SDP.
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3.4. Multipart/alternative
In [I-D.jenning-sipping-multipart], the use of multipart/alternative
MIME is proposed as a way to support multiple alternative offers.
Each alternative offer has an id associated with it by use of a new
MIME header field called Content-Answering-CID. The answerer chooses
one of the offers and performs normal offer/answer operation on that
offer, and then sends back a single answer which includes the
Content-Answering-CID value of the offer chosen.
The main advantages of this approach are:
o It allows for use of alternative encodings of the offer, e.g. SDP
and SDPng, as well as varying levels of confidentiality and
integrity by use of S/MIME [RFC3851].
Use of multipart/alternative to solve the SDP capability negotiation
problems however has several shortcomings:
o Backwards Compatibility: Neither SIP nor RTSP mandate support
for Multipart MIME. In the case of SIP, multipart/alternative is
generally incompatible with existing SIP proxies, firewalls,
Session Border Controllers, and endpoints.
o Heterogeneous Error Response Forking Problem (HERFP): When a SIP
proxy forks a request to multiple Contacts, each of which generate
a response, the proxy only forwards the "best" of these responses
to the request originator. If one or more of the Contacts do not
support multipart/alternative, the request originator may never
discover this. Instead, only a Contact that supports
multipart/alternative will be able to generate an answer that
reaches the request originator.
o Combinatoric Explosions: Use of multipart/alternative to convey
alternatives on a per media stream basis or even per media format
parameter basis quickly leads to combinatoric explosions.
o Message Size: Each alternative requires full duplication of all
the relevant SDP parameters (one complete SDP per alternative).
It should be noted, that use of multipart/alternative has been
discussed several times before and, in large part due to the problems
mentioned above as well as the semantics defined for
multipart/alternative [RFC2046], has met with opposition when it
comes to addressing the above types of requirements.
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3.5. Sharing Ports Between "m=" Lines
SDP [SDP] does not state whether two "m=" lines can share the same
transport address or not but rather leaves this explicitly undefined.
It has been suggested that alternative capabilities for a media
stream could be indicated by including multiple media stream
descriptions sharing the same transport address (i.e. using the same
port number in the "m=" line and sharing the same IP-address).
Such practice was not defined in [RFC2327], however it was
suggested in an Internet-Draft version of [SDP]. Following
discussion of the potential problems it introduced, it was removed.
The main advantages of this approach would be:
o May not require any additional extensions to SDP - only additional
semantics.
[Editor's note: It is somewhat unclear how it would work without
extensions if we allow for alternative attributes and media format
parameters and the offerer needs to always know which ones were
accepted]
The disadvantages of this approach would be:
o Backwards Compatibility Issues: Since sharing of transport
addresses between multiple streams was never specified as part of
SDP, backwards compatibility is likely to be an issue. Some
implementations may support it whereas others may not. The lack of
an explicit signaling indication to indicate the desired operation
may lead to ungraceful failure scenarios. Offer/answer semantics
would be unclear here as well.
o Some Combinatoric Explosion: The mechanism is not ideal to
indicate alternative capabilities for multiple parameters or media
formats within a particular media stream. For example, alternative
attribute values and media format parameters for several codecs
would lead to combinatoric explosion.
o Message Size: Each alternative requires full duplication of all
the relevant media stream parameters.
[Editor's note: In practice, it is not clear this is a huge issue
though.]
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3.6. Opportunistic Encryption Using a Session Attribute
This approach was suggested to address the specific scenario of
negotiating either RTP or SRTP. The endpoints signal their desire to
do SRTP by listing RTP (RTP/AVP) as the transport protocol in the
"m=" line in the offer together with an attribute ("a=") that
indicates whether SRTP is supported or not. If the answerer supports
SRTP and wants to use it, the answer then includes SRTP (RTP/SAVP) as
the transport protocol in the "m=" line.
The main advantages of this approach are:
o Compatible with non-SRTP-aware endpoints.
The disadvantages of this approach are:
o Does not allow the offerer to indicate alternatives other than
SRTP (including vanilla RTP as an alternative to SRTP).
o Addresses only a small subset of the requirements provided above.
3.7. Best-Effort Secure Real-Time Transport Protocol
This approach is documented in [BESRTP]. The approach is similar to
the one described above, except it does not actually include any
explicit signaling indication as to the transport protocols
supported. Instead, support for the Secure RTP profile [SRTP] is
inferred based on the presence of the crypto attribute defined in
[SDES] and/or the key-mgmt attribute defined in [KMGMT].
The main advantages of this approach are:
o Compatible with non-SRTP-aware endpoints.
The disadvantages of this approach are:
o Defines new semantics above and beyond those defined by RFC 3264,
RFC 4567, and RFC 4568 without any explicit signaling in the offer
to that effect. This in turn may lead to unintended side-effects.
Without explicit signaling indication, it is questionable to
infer that presence of e.g. a crypto parameter in the offer
indeed indicates that the offer wants to use the mechanism
defined by the proposal. Furthermore, Section 5.1.2 of [SDES]
defines generic operation where presence of a crypto attribute
without e.g. SRTP as the offered transport protocol could
result in the media stream being rejected.
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o Does not allow the offerer to indicate alternatives other than the
inferred SRTP (including vanilla RTP as an alternative to SRTP).
o Addresses only a small subset of the requirements provided above.
3.8. Opportunistic Encryption using Probing
This is another approach suggested to address the specific scenario
of negotiating either RTP or SRTP. In this case, the endpoints first
establish an RTP session using RTP (RTP/AVP). The endpoints send
probe messages, over the media path, to determine if the remote
endpoint supports their keying technique.
The main advantages of this approach are:
o Compatible with non-SRTP-aware endpoints.
The disadvantages of this approach are:
o Addresses only a small subset of the requirements provided above.
4. Proposed Solution
Based on the requirements provided in Section 2. and the alternatives
examined in Section 3. the solution based on the Session Description
Protocol (SDP) Simple Capability Declaration (simcap) [RFC3407] with
extensions as outlined in Section 3.2. is preferred. In this section
we present that solution in detail.
4.1. Solution Overview
The solution consists of the following:
o The capability declaration mechanism defined by simcap [RFC3407]
with a few extensions.
o A new attribute ("a=capar") similar to the "a=cpar" attribute
defined by simcap, except with a handle that enables referencing
individual attribute capabilities (and for attributes only).
o A new attribute ("a=ctrpr") that defines how to list transport
protocols as capabilities.
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o A new attribute ("a=pcfg") that lists the potential configurations
supported by the entity that generated the SDP. This is done by
reference to the extended simcap capabilities from the SDP in
question, and optionally one or more of the transport protocol
capabilities. The potential configurations are listed in order of
preference.
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. This is done by listing the potential configurations that
were used from the offer SDP.
o Extensions to the offer/answer model that 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).
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
s=
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 18
a=sqn: 0
a=cdsc: 1 audio RTP/AVP 0 18
a=cdsc: 3 audio RTP/SAVP 0 18
a=capar: 1 a=crypto:1 AES_CM_128_HMAC_SHA1_32
inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32
a=pcfg: c=3,4 a=1
a=pcfg: c=1,2
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The "m=" line indicates that Alice is offering to use plain RTP with
PCMU or G.729. The extended simcap capability declaration is
provided by the "a=sqn" and "a=cdsc" attributes as defined in
[RFC3407], and the new "a=capar" attribute defined in this document.
The capabilities indicate that PCMU and G.729 are supported with
either RTP or secure RTP. 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 new "a=pcfg"
attribute provides the potential configurations included in the offer
by reference to the simcap capability declarations. Two alternatives
are provided; the first one, and hence the preferred one is using
capabilities 3 and 4, i.e. PCMU and G.729 under the RTP/SAVP profile
(secure RTP) together with the attribute capability parameter 1, i.e.
the crypto attribute provided. The second one is using capabilities 1
and 2, i.e. PCMU and G.729 under the RTP/AVP profile.
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:
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
a=acfg: c=1,2
Bob includes the new "a=acfg" attribute in the answer to inform Alice
that he based his answer on an offer containing the potential
configuration with capabilities 1 and 2 from the offer SDP (i.e. PCMU
and G.729 under the RTP/AVP profile). Note that in this particular
example, the answerer supported the capability extensions defined
here, however had he not, everything would still have worked fine
since the actual configuration is what was being used. Consequently,
the answer would simply have omitted the "a=acfg" attribute line.
4.2. Extensions to Simcap
Simcap [RFC3407] defines capability descriptions to be on the form:
a=cdsc: <cap-num> <media> <transport> <fmt list>
where <cap-num> is an integer between 1 and 255 (both included) used
to number the capabilities, and <media>, <transport>, and <fmt list>
are defined as in the SDP "m=" line. We extend that definition here
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to allow for wild-carding of the <media>, <transport> and <fmt list>
parameters at the session level only. The wild-card character to use
is asterisk ("*"). This enables us to provide session level
capability parameters that are not specific to any particular media
stream, or applies only to certain types of media streams. Such
capability parameters apply to all media streams that match the
combined <media>, <transport> and <fmt list> provided.
An example use case is to allow for negotiation of MIKEY at the
session level outside of a specific simcap capability description
(and hence media type) by use of the key management framework
[KMGMT].
This is illustrated by the following examples:
a=cdsc: 1 * * *
a=cdsc: 2 audio * *
In the first example, the capability description applies to all media
stream. In the second example, the capability description applies to
media streams of type audio only.
Simcap capability descriptions start with a sequence number ("a=sqn")
and, as specified in [RFC3407], require that a capability description
as defined by simcap, i.e. an "a=cdsc" line, follows immediately
after the sequence number. We remove that requirement here. As a
result of that, we enable the new "a=capar" attribute (and other
parameters) to follow after the sequence number. There is however not
a requirement that it follows immediately after the sequence number.
4.3. Attribute Definitions
4.3.1. The Attribute Parameter Capability Attribute
Attributes can be expressed as negotiable parameters by use of a new
attribute parameter capability attribute ("a=capar") similar to the
"a=cpar" attribute defined by simcap, except with a handle that
enables referencing it and supporting attributes only (the "cpar"
attribute defined in RFC 3407 supports bandwidth information as
well). The attribute is defined as follows:
a=capar: <att-cap-num> <att-par>
where <att-cap-num> is an integer between 1 and 255 (both included)
used to number the attribute parameter 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 or the
media level. Each occurrence of the attribute MUST use a different
value of <app-cap-num>, with the first one being 1, the second one
being 2, etc. 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.
TO DO: There is a need to clarify the relationship between this
one, the simcap cpar values, and regular attributes (actual
configuration attributes). The basic idea is that attributes that
can only be used with certain potential configurations should be
provided here and then included by reference in those potential
configurations.
The following example illustrates 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 ones 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].
4.3.2. The 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>
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> is defined as in the SDP "m=" line.
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The "ctrpr" attribute can be provided at either the session or media-
level. Each occurrence of the attribute MUST use a different value of
<trpr-cap-num>, with the first one being 1, the second one being 2,
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
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].
Note that the simcap extensions already provide similar functionality
by inclusion in the "cdsc" attribute (as illustrated by the example
in Section 4.1. ), 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 4.3.3. ) provides for more expressive power.
4.3.3. The Potential Configuration Attribute
Potential Configurations can be expressed by use of a new Potential
Configuration Attribute ("a=pcfg") defined as follows:
a=pcfg: <simcap-capabilities>
[<attribute-parameter-capabilities>]
[<transport-protocol-capabilities>]
The potential configuration attribute includes one or more sets of
simcap-capabilities. A list of attribute parameter capabilities and a
list of transport protocol capabilities can optionally be included as
well. 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.
TO DO: Clean up capability and configuration terminology.
<simcap-capabilities> is defined by the following ABNF:
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simcap-capabilities = "c=" cap-list *("|" cap-list)
cap-list = cap-num *("," cap-num)
cap-num = 1*3DIGIT ; defined in [RFC4234]
Each capability list is a comma-separated list of simcap capability
numbers where cap-num refers to simcap capability numbers and hence
MUST be between 1 and 255 (both included). Alternative potential
simcap configurations are separated by a vertical bar ("|"). The
alternatives are ordered by preference.
<attribute-parameter-capabilities> is defined by the following ABNF:
attribute-parameter-capabilities
= "a=" capar-cap-list *("|" capar-cap-list)
capar-cap-list = att-cap-num *("," att-cap-num)
att-cap-num = 1*3DIGIT ;defined in [RFC4234]
Each attribute parameter capability 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 255 (both included). Alternative attribute parameter
capabilities are separated by a vertical bar ("|"). The alternatives
are ordered by preference.
<transport-protocol-capabilities> is defined by the following ABNF:
transport-protocol-capabilities =
"p=" trpr-cap-num *("|" trpr-cap-num)
trpr-cap-num = 1*3DIGIT ; defined in [RFC4234]
The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 255 (both included).
Alternative transport protocol capabilities are separated by a
vertical bar ("|"). When transport protocol capabilities are not
included, the transport protocol information from the media
description ("m=" line) will be used.
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).
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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=sqn: 0
a=cdsc: 1 audio RTP/SAVP 0 4 18
a=ctrpr: 1 RTP/AVP
a=ctrpr: 2 RTP/AVPF
a=ctrpr: 3 RTP/SAVP
a=ctrpr: 4 RTP/SAVPF
a=pcfg: c=1|3 p=1|2|3|4
a=pcfg: c=2 p=1
We have two potential configurations listed here. The first one
indicates that PCMU (payload type number 0 referenced by simcap
capability number 1) or G.729 (payload type number 18 referenced by
simcap 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 simcap capability number 2) can be supported with the
RTP/AVP profile only (transport protocol capability number 1).
4.3.4. The 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 simcap
capabilities, and the transport protocol (if included) capabilities
from the offer that were actually used by the answerer in his
offer/answer procedure.
The Actual Configuration Attribute ("a=acfg") is defined as follows:
a=acfg: <simcap-capability-list>
[<attribute-parameter-capabilities>]
[<transport-protocol-capability]
<simcap-capability-list> is defined by the following ABNF:
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simcap-capability-list = "c=" cap-list
cap-list = cap-num *("," cap-num)
cap-num = 1*3DIGIT ; defined in [RFC4234]
Each capability list is a comma-separated list of simcap capability
numbers where cap-num refers to simcap capability numbers and hence
MUST be between 1 and 255 (both included).
<attribute-parameter-capabilities> is defined by the following ABNF:
attribute-parameter-capabilities = "a=" capar-cap-list
capar-cap-list = att-cap-num *("," att-cap-num)
att-cap-num = 1*3DIGIT ;defined in [RFC4234]
Each attribute parameter capability 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 255 (both included).
<transport-protocol-capabilities> is defined by the following ABNF:
transport-protocol-capability = "p=" trpr-cap-num
trpr-cap-num = 1*3DIGIT ; defined in [RFC4234]
The trpr-cap-num refers to transport protocol capability numbers
defined above and hence MUST be between 1 and 255 (both included).
When a transport protocol capability is not included, the transport
protocol information from the media description ("m=" line) in the
offer is being used.
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=acfg: c=1 p=2
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It indicates that the answerer used an offer consisting of simcap
capability 1 from the offer (PCMU) and transport protocol capability
2 from the offer (RTP/AVPF).
4.4. 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.
4.4.1. Generating the Initial Offer
An offerer that wants to use capability negotiation extensions
defined in this document MUST include the following in the offer:
o one or more simcap capability descriptions (as defined in
[RFC3407] and extended above) for each of the capabilities.
o optionally, one or more attribute parameter capability attributes
(as defined in Section 4.3.1. ) if one or more alternative
attribute parameter values is to be negotiated.
o optionally, one or more transport protocol capability attributes
(as defined in Section 4.3.2. ) if one or more alternative
transport protocols is to be negotiated.
o one or more potential configuration attributes (as defined in
Section 4.3.3. which define the potential configurations supported
by the offerer.
Each of the potential configurations listed 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]).
4.4.2. Generating the Answer
When the answerer receives an offer with one or more valid potential
configuration information attributes present, it may use any of the
potential configurations as an alternative offer. A potential
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configuration information attribute is valid if all of the
capabilities (simcap, attribute capabilities and transport protocol)
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 capabilities. Furthermore,
if the offered potential configuration included attribute capability
parameters and/or transport protocol capabilities, those parameters
MUST be included in the actual configuration attribute in the answer
as well.
4.4.3. 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 was 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 ("cdsc" capabilities),
attribute capability parameters ("capar") and transport protocol
capabilities ("ctrpr")
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.
4.4.4. Modifying the Session
Potential configurations may be included in subsequent offers as
defined in [RFC3264, Section 8]. The procedure for doing so is
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similar to that described above with the answer including an
indication of the actual configuration used by the answerer.
5. Examples
TBD.
6. Security Considerations
TBD.
7. IANA Considerations
TBD.
8. 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 We are currently capping all capability numbers at 255. Is this a
concern, not least considering the limits apply to the session,
not just individual media streams.
9. Acknowledgments
Thanks to Francois Audet and Dan Wing for comments on this document.
10. Change Log
10.1. Changes since -00
o Added requirements to allow for alternative attribute values to be
negotiated at the session level.
o Removed requirements to support unicast/multicast as alternatives
and IPv4/IPv6 as alternatives.
o Updated section 3.6. on opportunistic encryption using a session
attribute
o Added new section 3.7. on best-effort secure real-time transport
protocol.
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o Updated solution to align with updated requirements. More
specifically
o Added minor extensions to simcap in new Section 4.2.
o Removed the "a=ctrad" attribute that supported transport
addresses as capabilities and updated the rest of the
attributes and procedures accordingly.
o Allowed for the "a=ctrpr" to be specified at the session level
as well.
o Updated semantics for the "a=pcfg" attribute to specify that
potential configurations are listed in order of preference.
o Defined a new attribute "a=capar" that enables the offerer to
determine which of several possible alternative attributes
from an offer was chosen by the answerer.
o Updated example in Section 4.1. to illustrate backwards
compatibility with a non-SRTP capable endpoint.
o Updated open issues section and in particular noted issue
around session level and media level parameter semantics
overlap.
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11. References
11.1. 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.
11.2. 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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Author's Addresses
Flemming Andreasen
Cisco Systems
Edison, NJ
Email: fandreas@cisco.com
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